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Heterogeneous reaction kinetics of sulphur dioxide and airborne limestone particles Esplin, Gordon John 1988

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HETEROGENEOUS REACTION KINETICS OF SULPHUR DIOXIDE AND AIRBORNE LIMESTONE PARTICLES By GORDON JOHN ESPLIN B . S c , The U n i v e r s i t y of A l b e r t a , 1965 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCES i n THE FACULTY OF GRADUATE STUDIES Department of Chemical Engineering Ve accept t h i s t h e s i s as conforming to the req u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA February 1988 ©Gordon John E s p l i n , 1988 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. The University of British Columbia 1956 Main Mall Vancouver, Canada Department V6T 1Y3 DE-6(3/81) ABSTRACT The l i t e r a t u r e on a c i d r a i n provides evidence that l a r g e (>5 um) a l k a l i n e p a r t i c l e s i n the atmosphere, which derived from surface s o i l , play an important r o l e i n m i t i g a t i n g the e f f e c t of a c i d r a i n . Not only do they help to n e u t r a l i z e a c i d i t y but they a l s o are a source of n u t r i e n t s to the t e r r e s t r i a l and a q u a t i c environments. While ground-up limestone has been added to the f o r e s t s and the lakes of regions d e f i c i e n t i n a l k a l i n i t y , t h i s mode of a p p l i c a t i o n i s p r o h i b i t i v e l y expensive. An economic a l t e r n a t i v e method of d i s t r i b u t i o n would u t i l i z e the advective and turbulent d i f f u s i v e processes w i t h i n the troposhere i n order to supply limestone p a r t i c l e s of approximately 10 um diameter to the t e r r e s t r i a l and the a q u a t i c environments. However, wh i l e the p a r t i c l e s are being transported through the atmosphere they w i l l a l s o i n t e r a c t to some degree with the atmospheric p o l l u t a n t s ( S 0 2 > N0 x, photochemical o x i d a n t s , e t c . ) . While the two-phase chemical i n t e r a c t i o n of water d r o p l e t s and gaseous p o l l u t a n t s has been e x t e n s i v e l y s t u d i e d , l i t t l e i s known about the r e a c t i o n between limestone p a r t i c l e s and gaseous p o l l u t a n t s under ambient c o n d i t i o n s . As a f i r s t step i n understanding these processes l a b o r a t o r y experiments were conducted i n order to measure the r a t e of the heterogeneous r e a c t i o n between limestone p a r t i c l e s and sulphur d i o x i d e ( S O 2 ) gas i n c l e a n humid a i r . C a l c i t i c limestone (200-270 mesh T y l e r ) from Texada I s l a n d , B.C., was reacted w i t h 30-80 ppb S O 2 at room temperature (17-22°C) over a humidity range of 70-100%. A n c i l l a r y experiments were a l s o conducted to determine the maximum d i s s o l u t i o n r a t e of these limestone p a r t i c l e s i n an a c i d i c , aqueous environment. - i i i -The o v e r a l l r a t e ( r ) f o r the SO^ r e a c t i o n w i t h Texada I s l a n d limestone was determined to be approximately f i r s t - o r d e r w i t h respect to the SC>2 c o n c e n t r a t i o n and to be s t r o n g l y dependent upon the r e l a t i v e humidity: r = k ( S 0 2 ) A k - ° - 2 5 1.04 - R.H. k = r a t e constant (m/h) 3 r = S0 9 removal r a t e (moles/m -h) z 3 ( S 0 ? ) = gas-phase co n c e n t r a t i o n (moles/m ) 2 3 A = p a r t i c l e s urface area (m /m ) R.H. = r e l a t i v e humidity (dimensionless) L i m i t e d experiments w i t h p r e c i p i t a t e d dolomite i n d i c a t e d that i t r e a c t s w i t h SO2 somewhat f a s t e r than does the c a l c i t i c limestone. A study of the i n d i v i d u a l mass t r a n s f e r and r e a c t i o n processes i n d i c a t e d that the r a t e l i m i t i n g step f o r the o v e r a l l r e a c t i o n was the aqueous phase o x i d a t i o n of the b i s u l p h i t e i o n . Limestone d i s s o l u t i o n (determined e x p e r i m e n t a l l y ) , and estimated gas and aqueous phase d i f f u s i v e processes, were not r a t e l i m i t i n g . In a cl e a n humid atmosphere, f r e e of photochemical o x i d a n t s , a limestone a e r o s o l would react very s l o w l y w i t h SO2 (SO2 removal r a t e of about 3x10 X per hour). However, i n a humid p o l l u t e d atmosphere r i c h i n photochemical oxidants, s i m i l a r to that r e s p o n s i b l e f o r a c i d fog and a c i d r a i n , the aqueous phase o x i d a t i o n of d i s s o l v e d SO2 i s not expected to be r a t e l i m i t i n g . Under such c o n d i t i o n s b i s u l p h i t e d i s s o c i a t i o n : HS0 3 -» H + + SO^ - i v -Therefore i n humid, po l lu ted atmospheres l imestone aeroso l w i l l act as a "sink." fo r SO ,^, for so lub le photochemical oxidants, and probably a l so for HNO^ ( n i t r i c a c i d ) . Since the oxidants are considered to be phytotoxic while the other po l lu tant s are respons ib le fo r c rea t ing ac id r a i n we conclude that the deployment of a l imestone aeroso l may have a p o s i t i v e impact on the atmospheric environment, besides being b e n e f i c i a l to the aquat ic and t e r r e s t r i a l environments. I t i s s t rong ly recommended that fu r ther research be done i n th i s area i n order to bet ter quant i fy the rate processes, and perhaps a l so to i d e n t i f y p r a c t i c a l methods for inc reas ing the atmospheric concentrat ion of a l k a l i n e aeroso l in those geographic regions which are d e f i c i e n t . - V -TABLE OF CONTENTS PAGE ABSTRACT i i ACKNOWLEDGEMENTS i x 1.0 INTRODUCTION 1 1.1 Major O b j e c t i v e of the Research Program 1 1.2 Background 1 1.3 The Chemical System 5 1.4 Reaction System K i n e t i c s 15 1.5 Previous Research on Aerosol-S02 Reactions 18 2.0 RESEARCH METHODOLOGY 28 2.1 D e s c r i p t i o n of Apparatus 36 2.2 Experimental Procedures 42 2.3 Limestone D i s s o l u t i o n Experiments 54 3.0 RESEARCH RESULTS 56 3.1 Data Reduction 56 3.2 Summary of Re s u l t s 60 3.3 E f f e c t of Reactor Humidity 62 3.4 E f f e c t of S0 2 Concentration 64 3.5 E f f e c t of Reaction Extent 66 3.6 E f f e c t of Temperature 70 3.7 E r r o r A n a l y s i s 70 3.8 R e s u l t s of Limestone D i s s o l u t i o n Experiments 73 - v i -TABLE OF CONTENTS PAGE 4.0 DISCUSSION . 80 4.1 Gas-phase D i f f u s i o n (^g) 8 1 4.2 Phase E q u i l i b r i u m ( x p ) 82 4.3 S(IV) I o n i z a t i o n 82 4.4 Aqueous-phase D i f f u s i o n ( t ^ ) • • • • 82 4.5 Limestone D i s s o l u t i o n (.1^) 84 4.6 Aqueous Phase O x i d a t i o n (*ta_) 88 4.7 Aqueous Phase Reaction (x ) 89 4.8 Probable Rate L i m i t i n g Step 90 4.9 Comparison w i t h D o l o m i t i c Limestone Reaction 94 4.10 Comparison of Dry D e p o s i t i o n Rates 94 4.11 Atmospheric Reaction Rates 97 5.0 CONCLUSIONS AND RECOMMENDATIONS 100 6.0 APPENDICES 105 6.1 Nomenclature 105 6.2 B i b l i o g r a p h y 108 6.3 B.E.T. Surface Area Determination Using the Quanta-Sorb . I l l 6.4 Summary Tables of Experimental Runs: Limestone/S02 Reaction K i n e t i c s 117 - v i i -LIST OF TABLES TABLE PAGE 1 SUMMARY OF RELEVANT RESEARCH 26 2 TEXADA ISLAND LIMESTONE COMPOSITION 43 3 PRELIMINARY EXPERIMENTS 46 4 TEFLON RETENTION OF S0 2 48 5 SELECTED EXPERIMENTAL RESULTS (Limestone) 61 6 RESULTS OF LIMESTONE DISSOLUTION EXPERIMENTS 74 7 REACTION RATE OF LIMESTONE AND SULPHURIC ACID . . . . 77 8 CHARACTERISTIC TIMES (SECONDS) FOR LIMESTONE - S0 2 REACTION 83 - v i i i -LIST OF FIGURES FIGURE PAGE 1 HETEROGENEOUS REACTION OF SULPHUR DIOXIDE AND LIMESTONE AT AMBIENT CONDITIONS 6 2 BISULFITE-SULFITE AND BICARBONATE-CARBONATE DISTRIBUTIONS AS A FUNCTION OF pH 8 3 CONCENTRATION PROFILES 14 4 REACTOR SYSTEM 30 5 APPARATUS FOR SOj-LIMESTONE RXN KINETICS EXPTS . . . . 39 6 METRONICS DYNACALIBRATOR MODEL 450-53XQ MODIFICATION . 40 7 HUMIDIFICATION MODULE 41 8 SULPHATE ANALYSIS SCATTERGRAM 50 9 LIMESTONE/SULPHUR DIOXIDE REACTION RATE VERSUS RELATIVE HUMIDITY 65 10 NORMALIZED REACTION Vs. S 0 2 CONCENTRATION 67 11 NORMALIZED REACTION RATE Vs. REACTION PRODUCT . . . . 69 12 LIMESTONE DISSOLUTION IN ACID 75 13 CHARACTERISTIC TIMES FOR LIMESTONE - S0 2 REACTION . . 91 14 DRY DEPOSITION VELOCITY - SULPHUR DIOXIDE TO LIMESTONE PARTICLES (200-270 MESH) 95 ACKNOWLEDGEMENTS This work was supported by B.C. Research through t h e i r In-House Research Program. The able a s s i s t a n c e of Merv Aiken and Janet H a r r i s of B.C. Research i s appr e c i a t e d . The h e l p f u l advice and patience of Dr. A.P. Vatkinson and Dr. B.D. Bowen of the Department of Chemical Engineering of the U n i v e r s i t y of B r i t i s h Columbia i s acknowledged. - 1 -1.0 INTRODUCTION 1.1 Major O b j e c t i v e of the Research Program S o i l - d e r i v e d a l k a l i n e dust i n the atmosphere play s an important r o l e i n n e u t r a l i z i n g the e f f e c t s of a c i d r a i n and i n r e c y c l i n g calcium and magnesium n u t r i e n t s to the t e r r e s t r i a l and a q u a t i c environments. Despite the importance of t h i s m a t e r i a l i n m i t i g a t i n g a c i d r a i n l i t t l e research has been focused on the r e a c t i o n s which i t undergoes i n the atmosphere. The o b j e c t i v e of t h i s study t h e r e f o r e was to e x p e r i m e n t a l l y determine the r e a c t i o n r a t e between coarse (>5 um) limestone a e r o s o l and sulphur d i o x i d e gas under t y p i c a l atmospheric c o n d i t i o n s of c o n c e n t r a t i o n , temperature and humidity. 1.2 Background The term "Acid Rain" i s commonly used to d e s c r i b e the atmospheric d e p o s i t i o n of a c i d i c m a t e r i a l d e r i v i n g from the oxides of sulphur and n i t r o g e n . Excessive d e p o s i t i o n of these a c i d i c components i s known to cause s i g n i f i c a n t harm to our b i o p h y s i c a l environment. The attendant socio-economic co s t s of these impacts has s t i m u l a t e d c o n s i d e r a b l e research i n t o the causes and e f f e c t s of a c i d r a i n , and i n t o methods of a m e l i o r a t i n g t h i s problem. A c i d r a i n may have e x i s t e d i n a m i l d form as long as the e a r t h has had an oxygen c o n t a i n i n g atmosphere. N a t u r a l emissions of sulphur ( b i o g e n i c and v o l c a n i c ) are o x i d i z e d i n a l a r g e number of atmospheric r e a c t i o n s i n c l u d i n g the homogeneous gas phase r e a c t i o n w i t h the hydroxyl r a d i c a l and the heterogeneous aqueous phase r e a c t i o n w i t h hydrogen peroxide. The f i n a l o x i d a t i o n product of sulphur tends to form a s u l p h u r i c a c i d a e r o s o l which can be deposited as a c i d r a i n . N a t u r a l emissions of a c i d i c precursors are l a r g e l y balanced by a concurrent emission of a c i d n e u t r a l i z i n g components - ammonia from the decay of biomass and a l k a l i n e dust from wind-blown c r u s t a l matter. The - 2 -vet and dry d e p o s i t i o n of the o v e r a l l r e a c t i o n products, which i n c l u d e ammonium and calcium b i s u l p h a t e and sulphate, tend to c l o s e the f a m i l i a r n i t r o g e n and sulphur c y c l e s . The n a t u r a l d e p o s i t i o n i s t h e r e f o r e approximately n e u t r a l . ( I t may be s l i g h t l y a c i d i c or s l i g h t l y b a s i c , depending on the nature of the condensation n u c l e i . I t i s r a r e l y at pH 5.6, the a c i d i t y r e s u l t i n g from the e q u i l i b r i u m between atmospheric carbon d i o x i d e and pure water, s i n c e a pure water a e r o s o l does not n a t u r a l l y e x i s t ) . The n a t u r a l biogeochemical c y c l e s are d i s r u p t e d when anthropogenic emissions of a c i d precursors g r o s s l y exceed the n a t u r a l emission of n e u t r a l i z i n g components. This has happened i n regions where sulphur c o n t a i n i n g c o a l i s burned f o r i t s energy and where the automobile-d e r i v e d oxides of n i t r o g e n (N0 x) emissions are s i g n i f i c a n t . The r e s u l t i n g a c i d r a i n can be m i t i g a t e d by reducing emissions of the S0 2 and N0 x p r e c u r s o r s . Emission c u r t a i l m e n t i s brought about through government r e g u l a t o r y a c t i o n ; the present g o a l i s to reduce emissions by approximately 50%, at a cost of many b i l l i o n s of d o l l a r s per year to the Canadian and American economies. There i s , however, a growing body of evidence to i n d i c a t e that such c o s t l y measures w i l l not, by themselves, be adequate. A recent s t u d y ^ ^ of a c i d p r e c i p i t a t i o n i n the Adirondack. Mountains of New York concluded that h a l v i n g the input sulphate c o n c e n t r a t i o n would only r e s u l t i n a s h i f t of the pH from 4.20 to 4.35. I f an equ i v a l e n t decrease of hydrogen ions was assumed, then the pH would change from 4.20 to 4.52 and t h e r e f o r e would s t i l l be e x c e s s i v e l y a c i d ! Measurement of acid/base f l u x e s to an a c i d i c l a k e i n the same region showed that n e u t r a l i z a t i o n occurred mainly w i t h i n the t e r r e s t r i a l watershed; net a c i d input v i a p r e c i p i t a t i o n was estimated to be 583 e q u i v a l e n t s per hectare per year (equiv. ha~^y~*). Most of t h i s a c i d (2) input was r e t a i n e d w i t h i n the watershed^ . - 3 -I t would seem reasonable that t h i s a c i d input could be n e u t r a l i z e d with a s i m i l a r d e p o s i t i o n (583 equiv. ha~*y~*) ot an a l k a l i n e m a t e r i a l such as limestone dust. The c o n c e n t r a t i o n of a l k a l i n e a e r o s o l r e q u i r e d i n the atmosphere, to n e u t r a l i z e the a c i d i n p u t , can be roughly estimated u s i n g the r e l a t i o n s h i p : (dry d e p o s i t i o n r a t e ) = ( d e p o s i t i o n v e l o c i t y ) x (atmospheric c o n c e n t r a t i o n ) . Coarse atmospheric p a r t i c l e s (diameter g r e a t e r than about 5 microns) (3 4) have a d e p o s i t i o n v e l o c i t y of 1-2 cm/s ' , hence the r e q u i r e d atmospheric c o n c e n t r a t i o n of limestone, p a r t i c l e s i s computed to be 6 3 ug/m . T h i s c o n c e n t r a t i o n i s l e s s than that which normally e x i s t s i n r u r a l regions f o r coarse atmospheric p a r t i c l e s . Therefore i f the e x i s t i n g coarse atmospheric p a r t i c l e s were a l k a l i n e we could expect that any e f f e c t s of a c i d r a i n would l a r g e l y be ameliorated. There are i n f a c t examples of regions which have l a r g e emissions of a c i d r a i n precursors but which are n a t u r a l l y protected by s i m i l a r i l y l a r g e emissions of n e u t r a l i z i n g components. For instance A l b e r t a , which has high SC>2 emissions due to sour gas and o i l processing and c o a l - f i r e d power p l a n t s , does not have an a c i d r a i n problem. The coarse f r a c t i o n of i t s atmospheric p a r t i c u l a t e i s a l k a l i n e wind-blown dust d e r i v i n g from the u n d e r l y i n g predominantly Mesozoic sedimentary (limestone and sandstone) bedrock. The mid- and south-west USA a l s o has a l k a l i n e f u g i t i v e dust which, from recent measurements by A p p l i n et a l . ^ \ n e u t r a l i z e s the pH of r a i n f a l l . However, during extended r a i n f a l l s they found that the n e u t r a l i z a t i o n e f f e c t s g r a d u a l l y diminished as the suspended dust was washed from the atmosphere, thereby " y i e l d i n g more accurate values of the wet p r e c i p i t a t i o n pH". Chinese i n v e s t i g a t o r s ^ ^ measured sulphur d i o x i d e concentrations and r a i n f a l l a c i d i t y i n the southern and the northern provinces of China. Even though both regions have a high c o n c e n t r a t i o n of the a c i d r a i n - 4 -precursor SO^t derived from the use of c o a l f o r home he a t i n g , only southern Chinese provinces experience a c i d r a i n . I t i s observed that the s o i l i n the northern part of China i s of a l k a l i n e nature (pH 7-8) and that "ammonia i n l a r g e q u a n t i t y r e l e a s e s i n t o the a i r from a l k a l i n e s o i l s " . Khemani et a l . ^ ^ analyzed cloud and r a i n water samples i n the Pune region of I n d i a . They concluded that "the a l k a l i n e p r o p e r t i e s of s o i l p a r t i c u l a t e s have sustained high a l k a l i n e pH i n the cloud and r a i n water and have been r e s p o n s i b l e f o r c o n t r o l l i n g the a c i d r a i n i n I n d i a . D e f i c i e n c y of s o i l - o r i e n t e d components (Ca and Mg) and high c o n c e n t r a t i o n s of a c i d i c components (SO^ and NO^) are the main causes of a c i d r a i n i n the N.E. United S t a t e s , Europe and Scandinavian c o u n t r i e s " . Because of the i n c r e a s i n g l y l a r g e body of evidence as to the benevolent nature of l a r g e (>5 pm) a l k a l i n e p a r t i c u l a t e i n the atmosphere i t behooves our p o l i c y makers to r e c o n s i d e r r e g u l a t i o n s and g u i d e l i n e s which have been promulgated to reduce a l l f u g i t i v e dust emissions. Science must be used i n order to maximize the p o s i t i v e b e n e f i t s which our atmospheric, t e r r e s t r i a l , and a q u a t i c environments can r e a l i z e from these p a r t i c l e s , w h i l e at the same time minimizing p o t e n t i a l negative e f f e c t s . The p o s i t i v e b e n e f i t s are n e u t r a l i z a t i o n of environmental a c i d i t y , r e d u c t i o n of harmful sub-micron s i z e d sulphate p a r t i c u l a t e i n the atmosphere, and replenishment of the v i t a l calcium and magnesium n u t r i e n t s to the t e r r e s t r i a l and a q u a t i c environments. P o t e n t i a l negative e f f e c t s are an increase i n atmospheric haze and p o s s i b l e human h e a l t h e f f e c t s . However, these negative e f f e c t s can be obviated i f the a l k a l i n e p a r t i c l e s are l a r g e ( g r e a t e r than 5 microns) as i s t y p i c a l of s o i l d e r i v e d dust. These l a r g e p a r t i c l e s p l a y l i t t l e or no r o l e i n s c a t t e r i n g v i s i b l e l i g h t nor are they hazardous to human h e a l t h . In order to judge the environmental i m p l i c a t i o n s of changes i n the emission r a t e of e i t h e r a l k a l i n e p a r t i c u l a t e s or a c i d - r a i n precursors i t i s necessary to know t h e i r r e a c t i o n paths and r e a c t i o n k i n e t i c s . The - 5 -r e l e v a n t acid-base r e a c t i o n s which occur i n the t e r r e s t r i a l and the a q u a t i c environments have been e x t e n s i v e l y s t u d i e d by s o i l s c i e n t i s t s and a q u a t i c chemists. However, very l i t t l e i n f o r m a t i o n i s a v a i l a b l e i n the l i t e r a t u r e about the heterogeneous atmospheric r e a c t i o n between sulphur d i o x i d e and limestone p a r t i c l e s , except f o r the high con c e n t r a t i o n s and temperatures which are experienced i n the dry scrubbers used f o r SOj removal i n c o a l - f i r e d power p l a n t s . This atmospheric n e u t r a l i z a t i o n r e a c t i o n a l s o may be important. Mamane and fO\ N o l l ' used a scanning e l e c t r o n microscope equipped w i t h an x-ray energy d i s p e r s i v e analyzer to show that dry atmospheric mineral p a r t i c l e s are enriched i n sulphate. The sulphate was found to be a s s o c i a t e d w i t h c a l c i t e and c l a y minerals i n s i g n i f i c a n t amounts, about 2- -1 1.5 - 3% of the p a r t i c l e mass, or an average of 0.02 g S0^ g s o l i d . I f the coarse atmospheric p a r t i c l e s are a l k a l i n e and a l s o act as condensation n u c l e i d u r i ng the formation of fog or clouds, then i t i s apparent that they w i l l tend to n e t u r a l i z e aqueous phase a c i d i t y and t h e r e f o r e w i l l reduce the a c i d i t y of the fog or r a i n . The o b j e c t i v e of t h i s research w i l l be to measure the heterogeneous r e a c t i o n r a t e between limestone p a r t i c u l a t e and an a c i d gas (SO2) under t y p i c a l atmospheric c o n d i t i o n s of temperature, c o n c e n t r a t i o n , and humidity. 1.3 The Chemical System The r e a c t i o n between sulphur d i o x i d e (SO2) and s o l i d limestone (CaCO^) i n a i r r e q u i r e s the presence of water vapour i n order to proceed at room temperature^»*°\ Therefore the r e a c t i o n i s thought to occur w i t h i n a three-phase system which c o n s i s t s of a s o l i d p a r t i c l e surrounded by an adsorbed l a y e r of water molecules, which i n turn i s surrounded by a i r c o n t a i n i n g water vapour and sulphur d i o x i d e . A s i m p l i f i e d r e p r e s e n t a t i o n of t h i s chemical system i s shown i n F i g u r e 1, w h i l e the i n d i v i d u a l mass t r a n s f e r and r e a c t i o n steps are discussed below. FIGURE 1 - HETEROGENEOUS REACT!OH OP SULEMUR DIOXIDE ABO DD1ESTORE AT AMUEHT COHDITIOHS SOLID CaCO, REACTION (Thickness of th i s layer increases with tine) lpH>5 .6r CaSO3-2H20(ptn) C.** +S02~+S02~ 4 3 CalSO^lxISOjJyHjO (coprecipitatian) "Ash" inclui&d i n product layer AQUEOUS SURFACE FILM (Thickness of aqueous f i l m w i l l be a function of e l e c t r o l y t i c a c t i v i t y , p a r t i c l e radius, and humidity) H +HSO -» SO -nH,0 3 <- 2 2 i t -H + SO' 2- cat. HSO + lj 0. 3 * (SO (very slow unless catalyzed) accumulates i n the aqueous phase) H +S0 HSO + Ox 3 (Fast) ••Heavy metal ions -•••catalysts GAS FILM — H20 (Adsorption/ Condensation) — SO-Other Oxidants •OH etc.) BULK GAS Advection P a r t i a l Pressures (Atmospheres) CO2*330xl0 SO2=50xl0 H2O=30xl0 -3 I cn i - 7 -1. Sulphur d i o x i d e molecules d i f f u s e to the l i q u i d s u r f a c e where they form h y d r a t e s ^ * ^ : S0 2(g) + n H 20 £ S0 2 • nH 20 (1) *hs 2. The d i s s o l v e d S 0 2 i n turn d i s s o c i a t e s : S0 2 • n H 20 + H + + H S 0 3 + H 2 ° <2> * s l HSO~ £ H + + SOj" (3) K s 2 (12) S e i n f e l d v ' tabulates the above aqueous e q u i l i b r i u m constants at 298°K: K, =1.24 mol L" 1 a t m - 1 h s -2 -1 K - « 1.29 x 10 mol L s l p i K g 2 = 6.01 x 10"° mol L From Fi g u r e 2 i t i s observed that w i t h i n the pH range t y p i c a l of _2 atmospheric a e r o s o l s (pH 2-8) that the s u l p h i t e (SO^ ) and b i s u l p h i t e (HSO^) ions w i l l dominate the aqueous phase chemistry. With 50 parts per b i l l i o n (ppb) of S0 2 and 330 ppm of C0 2 i n the atmosphere the e q u i l i b r i u m pH of water i s computed to be 4.55. Carbon d i o x i d e , due to i t s much lower s o l u b i l i t y and lower d i s s o c i a t i o n constant, has very l i t t l e i n f l u e n c e upon t h i s value even though i t s atmospheric c o n c e n t r a t i o n i s much gr e a t e r then that of S0 2. 3. The hydrogen i o n d i f f u s e s to the p a r t i c l e s u r f a c e where i t d i r e c t l y a t t a c k s CaCO^, according to current t h e o r y ^ ^ ' : H + + CaC0 3 + C a 2 + + HC0~ (4) - 8 -- 9 -I f we d e f i n e a d i s s o c i a t i o n constant: K cc [ C a 2 + ] [ H C 0 3 ] [H +] then the s o l u b i l i t y data which i s a v a i l a b l e f o r one set of known c o n d i t i o n s can be r e a d i l y extended to other s i t u a t i o n s . ( I n the f o l l o w i n g text square brackets w i l l be used to denote m o l a r i t y , whereas 3 parentheses w i l l be used to denote concentrations i n u n i t s of mol/m ). For the simple C0 2~ water system the aqueous e q u i l i b r i u m constants at 298K are tabulated by S e i n f e l d ( 1 2 ) : [CO, . H,0] . K h c = PCO = 3.4 x 10 z mol L atm" 1 [ H + ] [ H C 0 - ] K c l = [C0 2 . H 20) " 4 ' 2 8 * 1 0 m 0 1 L [H +][C0 2 _ ] K „ = - = 4.69 x 10 1 1 mol L - i [HCO'] S o l u b i l i t y data f o r CaCO- i s s t r o n g l y dependent upon i m p u r i t i e s , c r y s t a l (16 ^  s t r u c t u r e , and upon pC0„. L i n k e v ' l i s t s a value of 0.056 g/L when i n -6 contact w i t h o r d i n a r y a i r , where pC0 2 ~ 330 x 10 atm. Using t h i s value and the above e q u i l i b r i u m constants i n the charge n e u t r a l i t y equation ( c a t i o n n o r m a lity = anion n o r m a l i t y ) : IH + ] +2[Ca 2 +]=[OH ]+[HC0 3 ]+2[C0 3 2 ] (5) - 10 -y i e l d s a limestone d i s s o c i a t i o n constant K = 139 mol L and an ' • cc e q u i l i b r i u m pH = 8.36. Using the above c a l c u l a t e d value of K c c i n the system of i n t e r e s t , limestone/water r e a c t i n g w i t h 50 ppb SO2, g i v e s an e q u i l i b r i u m s o l u t i o n pH = 7.99. Therefore the presence of 50 ppb SO2 has made the s o l u t i o n somewhat more a c i d i c , as expected. 2+ 2-4. The calcium i o n , Ca , r e a c t s w i t h s u l p h i t e , S0j , to form a r e l a t i v e l y i n s o l u b l e hydrate: H„0 C a 2 + + S 0 3 2 \ C a S 0 3 - 2 H 2 0 ( s ) (6) L i n k e * 1 6 ) g i v e s the s o l u b i l i t y of CaSO^ r^O as 0.0064 g/100 g H 20 at 30°C. This value can be used i n the d e f i n i t i o n of a s o l u b i l i t y constant: K c s 5 I C a 2 + H S 03 J = 1-68 x 10 7 (mol L " 1 ) 2 In the previous e q u i l i b r i u m system (pH=7.99) the f o l l o w i n g i o n conce n t r a t i o n s were c a l c u l a t e d : [HC0 3] = 4.7 X 10" -4 mol L" 1 [ C a 2 + ] = 3.0 X 10" -3 mol L" •1 [HS0 3] = 7.8 X 10" -2 mol L" •1 ISO- 2 -] = 4.6 X 10" •1 mol L" 1 I t can be seen that the ion product [Ca ][S0- ] = 1.38 x 10 i s much _7 g r e a t e r than the s o l u b i l i t y constant (K = 1.68 x 10 ). Therefore cs calcium s u l p h i t e p r e c i p i t a t i o n w i l l occur on the p a r t i c l e surface and s o l u t i o n e q u i l i b r i u m w i l l not be a t t a i n e d . - 11 -5. The bicarbonate i o n (HCO^ -) created during limestone d i s s o l u t i o n w i l l d i f f u s e towards the gas i n t e r f a c e . During t h i s t r a v e r s e i t can react i f s u f f i c i e n t a c i d i t y i s present: HC0 3" + H + £ H 2C0 3 (7) H 2C0 3 + H 20 + C0 2(g) (8) The e v o l u t i o n of gaseous C0 2 i s favoured at the a i r i n t e r f a c e where the pH i s depressed per equations 1 and 2. Therefore the abso r p t i o n of the a c i d gas S0 2 w i l l be balanced by an e v o l u t i o n of C0 2. (Bicarbonate w i l l accumulate w i t h i n the aqueous phase at high pH values (>7 according to Figure 2) and thereby quench the limestone d i s s o l u t i o n r e a c t i o n ) . 6. I f the limestone c o n t a i n s , as i m p u r i t i e s , metal c a t a l y s t s such as Fe or Mn then an o x i d a t i o n r e a c t i o n w i l l occur: cat . HS0 3 + W2 -> SO^ + H + (9) This r e a c t i o n augments the limestone /S0 2 r e a c t i o n f o r the f o l l o w i n g reasons: ( i ) a more a c i d i c s o l u t i o n phase increases the d i s s o l u t i o n r a t e of CaC0 3, ( i i ) H+ ac t s as a bicarbonate "sink." per equations 7 and 8, 2 ( i i i ) SO^ ac t s as an i r r e v e r s i b l e b i s u l p h i t e " s i n k " , 2 ( i v ) SO^ increases s o l u t i o n i o n a c t i v i t y which i n turn increases the thickness of the aqueous l a y e r which i s i n e q u i l i b r i u m w i t h the (12) gas-phase humidity l e v e l v . - 12 -A l a r g e r l i q u i d water content (aqueous f i l m t h i c k n e s s ) w i l l provide a gr e a t e r volume f o r aqueous phase r e a c t i o n s and w i l l t h e r e f o r e speed up the o v e r a l l r e a c t i o n i f one of the aqueous r e a c t i o n s i s r a t e l i m i t i n g (e.g., equation 9) 7. I f the gas phase (ambient a i r ) contains powerful oxidants such as ozone, hydrogen peroxide, hydroxyl or peroxy r a d i c a l s , e t c . , then the b i s u l p h i t e i o n may be o x i d i z e d : HSO~ + o x i d i z i n g agent -* SO 2" + H + (10) In p o l l u t e d systems t h i s may be the dominant path and may a l s o act as an important " s i n k " f o r photochemical oxidants which are known to be t o x i c to p l a n t l i f e . 8. Limestone i n e r t s (Si02» et c . ) plus the calcium s u l p h i t e / s u l p h a t e c o p r e c i p i t a n t s w i l l tend to form a product l a y e r which can reduce the mass t r a n s f e r r a t e between the s o l i d s urface and the aqueous f i l m . The p r e f e r r e d r e a c t i o n l o c a t i o n s may ther e f o r e tend to change from pores to su r f a c e p r o j e c t i o n s as the former become masked. 9. There w i l l be a pH gradient e s t a b l i s h e d between the s o l i d unreacted su r f a c e of the p a r t i c l e (approx. pH 8) out to the gas/aqueous-film i n t e r e f a c e . The pH of the l a t t e r s urface may be as low as pH 4.6 i f the SO2 - CO2 - H2O system i s near e q u i l i b r i u m at the i n t e r f a c e . The pH gradient i s expected to i n f l u e n c e where c e r t a i n r e a c t i o n s occur according to Figure 2 - e.g., s u l p h i t e p r e c i p i t a t i o n at the s o l i d s u r f a c e , carbon d i o x i d e e v o l u t i o n at the gas i n t e r f a c e , metal ion ca t a l y z e d b i s u l p h i t e o x i d a t i o n near the gas i n t e r f a c e , e t c . - 13 -According to the l i t e r a t u r e the r e a c t i o n product l a y e r i s commonly found to be hydrated sulphate (CaSO^H^O). But s i n c e the s u l p h i t e hemihydrate (CaSO^ • ZH^O) i s much l e s s s o l u b l e (by a f a c t o r of 1.1 x 10"^  at 20°C), i t i s expected to p r e c i p i t a t e f i r s t . T h is apparent anomaly may be explained w i t h the a i d of Figure 3. I n i t i a l l y the b i s u l p h i t e i o n can d i f f u s e d i r e c t l y to the high pH (8.36) surf a c e of the unreacted CaCO^. Under these c o n d i t i o n s the s u l p h i t e i s favoured and an i n i t i a l CaSO^ • 2H2O l a y e r forms. The s o l i d product l a y e r w i l l tend to form pronounced c o n c e n t r a t i o n g r a d i e n t s because of the r e l a t i v e l y low d i f f u s i v i t y i n s o l i d s compared to those experienced i n l i q u i d or i n gas f i l m s . The presence of an i n s o l u b l e CaSO^^H^O product l a y e r w i l l i n c r e a s e aqueous f i l m [H +] and [HS0-~], which decreases S0„ a b s o r p t i o n and which 2-a l s o decreases [SO- J. This negative feed-back, process w i l l be 2-enhanced by the o x i d a t i o n of S(IV) to S(VI) to form SO, , which f u r t h e r 2-lowers the pH of the aqueous f i l m and th e r e f o r e reduces the [SO- ] 2+ 2-according to Figure 2. The [Ca ] and [SO, ] w i l l accumulate u n t i l t h e i r s o l u b i l i t y product (1.88 x 10 ) i s exceeded, at which time they w i l l form CaSO^^H^O on the p a r t i c l e s u r f a c e . The pH of the aqueous f i l m w i l l be e n t i r e l y dependent upon the r a t i o of a k a l i n i t y a v a i l a b i l i t y (CaCO^ d i s s o l u t i o n and ion d i f f u s i o n ) to a c i d generation (S(IV) o x i d a t i o n ) . I f a k a l i n i t y a v a i l a b i l i t y i s r a t e l i m i t i n g then we would expect a pronounced r e d u c t i o n i n the o v e r a l l r e a c t i o n r a t e as the r e a c t i o n progressed. I f , on the other hand, the S(IV) o x i d a t i o n process i s ra t e l i m i t i n g then we would expect that the o v e r a l l r e a c t i o n r a t e would be f i r s t order w i t h respect to PSO2, and to be d i r e c t l y p r o p o r t i o n a l to the thickness of the aqueous f i l m and th e r e f o r e a f u n c t i o n of atmospheric humidity. - 14 -FIGURE 3 CONCENTRATION PROFILES - 15 -1.4 Reaction System K i n e t i c s The removal of S0 2 from the gas phase due to a chemical s i n k mechanism can be described by the SO, removal r a t e , r , which i s def i n e d as: Since heterogeneous systems which have complicated molecular d i f f u s i o n and chemical r e a c t i o n pathways are u s u a l l y r a t e - l i m i t e d by a f i r s t - o r d e r s u r f a c e mechanism i t i s common p r a c t i c e i n the l i t e r a t u r e to assume a p s e u d o - f i r s t - o r d e r expression of the form: where (S 0 2 ) i s the co n c e n t r a t i o n of S0 2 i n the gas phase (moles S02/m ), and A i s the sur f a c e area a v a i l a b l e f o r mass t r a n s f e r and r e a c t i o n I t should be noted that k i n equation 12 i s an o v e r a l l r a t e constant which i s based on the bulk SO^ co n c e n t r a t i o n and which contains a number of lumped f a c t o r s . I f we d e f i n e 2 S B s p e c i f i c s urface area of the a e r o s o l (m / g ) , and 3 B E a e r o s o l c o n c e n t r a t i o n (g/m ) r = -d (S0,)/dt, (moles SO,/m -h) (11) r = -d ( S 0 2 ) / d t = k A (S0 2) (12) 3 then A = SB (m/nT) and r = k SB (S0 9) (13) I f k (m/hr) has been measured then the r a t e of S0 2 removal, from the atmosphere to the a e r o s o l p a r t i c l e s , can be estimated. - 16 -For a p s e u d o - f i r s t - o r d e r r e a c t i o n the time constant 'x ', which i s the s time r e q u i r e d to reduce the SO^ c o n c e n t r a t i o n to a value of 1/e of i t s i n i t i a l c o n c e n t r a t i o n , i s given by the expression: T s (h) = ( k S B ) - 1 . (14) As an example, i f : k • r a t e constant = 1000 m h 2 -1 S - s p e c i f i c s urface area - 5 m g —6 —3 B • p a r t i c l e c o n c e n t r a t i o n « 20 x 10 g m then t » 10 hours. Since t h i s value i s s i m i l a r to the residence time s of coarse p a r t i c l e s i n the atmosphere we would have to conclude that the g a s - p a r t i c l e r e a c t i o n i s s i g n i f i c a n t . But i f x i s orders of magnitude s g r e a t e r then the t y p i c a l atmospheric residence time then we would conclude that the atmospheric r e a c t i o n i s not a s i g n i f i c a n t S 0 2 " s i n k " . Instead, a c i d n e u t r a l i z a t i o n would occur w i t h i n the t e r r e s t r i a l and a q u a t i c environments, which would r e c e i v e a steady input of S0 2 gas and a l k a l i n e p a r t i c u l a t e s v i a dry d e p o s i t i o n , or a c i d i c r a i n and p a r t i c u l a t e v i a wet d e p o s i t i o n . Other o v e r - a l l r a t e constants which appear i n the l i t e r a t u r e on aerosol-gas i n t e r a c t i o n s are the d e p o s i t i o n v e l o c i t y and the c o l l i s i o n e f f i c i e n c y or " s t i c k i n e s s f a c t o r " . The d e p o s i t i o n v e l o c i t y (cm s~*) i s an o v e r - a l l mass t r a n s f e r c o e f f i c i e n t which r e l a t e s the bulk gas phase c o n c e n t r a t i o n of a gaseous or p a r t i c u l a t e species to i t s f l u x to a macroscopic s u r f a c e , such as the s i d e of a b u i l d i n g or a f i e l d of grass. The s u r f a c e t h e r e f o r e a c t s as a " s i n k " f o r removing the d e p o s i t i n g species from the atmosphere. By d e f i n i t i o n , the d e p o s i t i o n f l u x F (mol -1 -2 h m ) i s given by: F = 36 V dC (15) - 17 -where C i s the bulk gas-phase c o n c e n t r a t i o n of the d e p o s i t i n g species (mol m ). I t i s important to s p e c i f y the c h a r a c t e r i s t i c s of the surf a c e when p u b l i s h i n g data on V^. The c o l l i s i o n e f f i c i e n c y , 0, i s defined as the f r a c t i o n of the t o t a l number of molecules that bombard a su r f a c e which a c t u a l l y are adsorbed. Here the d e f i n i t i o n of the su r f a c e area i s u s u a l l y , but not always, the microscopic area which can be measured using (e.g.) B.E.T. techniques. From J u d e i k i s ^ ^ ' w e have: (change i n con c e n t r a t i o n ) = (net f l u x to p a r t i c l e s u r f a c e s ) _ . 3 6 0 V AC ( 1 6 ) dt x where V x = u n i d i r e c t i o n a l molecular v e l o c i t y (7800 cm s~* f o r S0 2 at room temperature). Now the d e p o s i t i o n v e l o c i t y , V^, may be compared to the molecular v e l o c i t y i f the r e s p e c t i v e areas can be r e l a t e d u s i n g some s o r t of shape f a c t o r , y. Here y i s defined as the r a t i o of the microscopic area (used i n the d e f i n i t i o n of 0) to the macroscopic area (used i n the d e f i n i t i o n of V^). (This d i s t i n c t i o n i s necessary i f the data i n the l i t e r a t u r e on S0 2 - p a r t i c l e r e a c t i o n s are to be compared.) From equations 15, 16 and the d e f i n i t i o n of y one obtains V d = Y 0 V x (17) Vr1 < C M / S ) M D \ Therefore, f o r S0 2, 0 = ° 8 Q Q y K ™ } I t may be noted that the r e l a t i o n i n equation 17 w i l l not be c o r r e c t i f the bulk molecular c o n c e n t r a t i o n d i f f e r s s u b s t a n t i a l l y from the - 18 -molecular c o n c e n t r a t i o n which occurs at a di s t a n c e of one mean-free-path le n g t h from the a e r o s o l s u r f a c e . However, unless 0 - 1 t h i s w i l l not be the case and the equation w i l l be c o r r e c t . A more d e t a i l e d treatment of r e a c t i o n and d i f f u s i o n mechanisms w i l l be presented i n the D i s c u s s i o n s e c t i o n . 1.5 Previous Research on A e r o s o l - SOj Reactions (18 ) Urone v ' conducted batch r e a c t o r s t u d i e s on the r e a c t i o n between sulphur d i o x i d e and d i f f e r e n t powders. The r e a c t i o n s , conducted w i t h i n a 2 - l i t e r f l a s k , were g e n e r a l l y c a r r i e d out u s i n g about 10 ppm SO2 and 10-40 mg of powder i n dry a i r . The change i n (S09) was monitored both 35 w i t h c o l o r i m e t r y and w i t h S s c i n t i l l a t i o n counting. The r e a c t i o n r a t e between 30.3 mg of CaCO^ and 14 ppm of SO2 i n dry a i r was measured at about 0.2%/hr ( S 0 9 decrease). When the mixture was i r r a d i a t e d w i t h u.v. 15 2 r a d i a t i o n (5.4 x 10 photons/s-cm i n the 310-420 nm band, or about seven times the i n t e n s i t y of noonday s u n l i g h t i n the same wavelength region) the r e a c t i o n r a t e increased by approximately a f a c t o r of ten. (18) To convert the Urone r e s u l t s to a f i r s t order r a t e constant i t i s necessary to assume that the r e a c t o r i s p e r f e c t l y mixed, w i t h no w a l l r e a c t i o n s , and that the " f i n e l y ground powder" can be assigned an a r b i t r a r y s p e c i f i c s u rface area. Then V r l n < V C > (19) k ( m / h ) = —wst 3 Where: V r = re a c t o r volume (m ) W = mass of powder (g) S = s p e c i f i c t = time (h) 2 -1 S = s p e c i f i c s u rface area of powder (m g ) From t h i s w r i t e r ' s experience we may assume a s p e c i f i c s u r f a c e area of approximately 1.0 m /g f o r " f i n e l y - g r o u n d " powder. T h e i r data then converts to the r a t e constants presented i n Table 1 (k = 0.00015 m/h f o r 0% R.H., k = 0.029 m/h f o r 0% R.H. and u.v. i r r a d i a t i o n ) . Presumably - 19 -the u.v. r a d i a t i o n promotes the o x i d a t i o n of to SO^ and t h e r e f o r e the r e a c t i o n (CaCO^ + SO^ •+ CaSO^ + C0 2) can proceed i n the absence of water. (18) Urone ' a l s o experimented w i t h other p a r t i c u l a t e systems. At 0% R.H. the r e a c t i o n r a t e constant f o r CaO was about 0.15 m/h u n i r r a d i a t e d and 0.5 m/h u s i n g u.v. i r r a d i a t i o n . CaO i s a t y p i c a l component of the f l y ash which i s emitted from c o a l - f i r e d power p l a n t s . The highest r e a c t i o n r a t e constant measured was between S0 2 and F^O^ where k = 1.3 m/h ( a l l computations assume S = 1 m 2/g). (19) Corn and Cheng v ' s t u d i e d the r e a c t i o n of S0 2 w i t h i n s o l u b l e suspended p a r t i c u l a t e matter us i n g a laminar-flow bed packed w i t h p a r t i c l e - c o a t e d T e f l o n beads. The decrease i n the i n i t i a l S0 2 c o n c e n t r a t i o n of 14.4 ppm was measured w i t h a microcoulometer which had a claimed s e n s i t i v i t y of about 30 ng/sec. At room temperature and 20-95X R.H. no increase i n r e a c t i o n , above that of a dummy (blank) r e a c t o r , was observed between S0 2 and CaCO^ p a r t i c u l a t e . I t should be pointed out, however, that the gas flow (46 mL/min) and i n l e t S0 2 c o n c e n t r a t i o n (14.4 ppm) r e s u l t i n a computed S 0 2 flow (29.4 ng/s) which i s of s i m i l a r value to the claimed s e n s i t i v i t y of t h e i r microcoulometer! J u d e i k i s ^ * ^ used a laminar-flow, coated-surface t u b u l a r r e a c t o r to measure the heterogeneous r e a c t i o n r a t e between v a r i o u s urban a e r o s o l s and S0 2- The a x i a l S0 2 c o n c e n t r a t i o n p r o f i l e , decreasing from an i n i t i a l c o n c e n t r a t i o n of 3-100 ppm, was measured usi n g a mass spectrometer. These measurements were used to compute c o l l i s i o n e f f i c i e n c i e s ( f r a c t i o n of the g a s - s o l i d c o l l i s i o n s that are e f f e c t i v e i n removing S 0 2 ) , which were i n turn used to estimate atmospheric removal rat e s of S0« v i a gas-aerosol r e a c t i o n s . C a l c u l a t e d i n i t i a l S0„ removal 3 r a t e s (X/h), f o r an atmospheric a e r o s o l burden of 100 ug/m , v a r i e d from 0.09 ( L o u i s v i l l e f l y ash), 1 ( c h a r c o a l ) , 16 ( A l ^ ) , 19 (Mohave f l y ash), 21 (Fe 20.j), to 35£/h f o r MgO. No measurements were made f o r limestone p a r t i c l e s . - 20 -The research of J u d e i k i s et a l . ^ ^ ' ^ \ w h i l e of conceptual i n t e r e s t , i s d e f i c i e n t i n two important areas. Although the heterogeneous r e a c t i o n s under study were surface-phenomena c o n t r o l l e d , the e f f e c t i v e s u r f a c e area of the c y l i n d r i c a l l a y e r s of powder (deposited onto the su r f a c e of a Pyrex tube) were not measured. A l l computations appear to have been based upon the area of the bare tube. I f we d e f i n e the area enhancement f a c t o r : A e = Sap b (20) Where: S = s p e c i f i c s u rface area (B.E.T.) a = a c t i v e thickness of su r f a c e l a y e r = bulk d e n s i t y of deposited powder, E.g., f o r Mohave f l y ash where: S = 15.2 x 1 0 4 cm 2 a ~ 0.48 x 10~ 4 cm 3 P b ~ 2 g/cm then A g ~ 15, and the atmospheric SOj removal r a t e ( f o r the humidity c o n d i t i o n s of the experiment) would be reduced by a f a c t o r of A g down to about 1%/h. Secondly, t h e i r h u m i d i f i c a t i o n system c o n s i s t e d of a side-stream passing through a d i s t i l l e d - w a t e r bubbler. The r a t i o of the flow through the bubbler to the t o t a l flow was used to c o n t r o l the r e l a t i v e humidity (0-95%). No measurements appear to have been taken to v a l i d a t e the assumed humidity l e v e l s . Thus an R.H. of 50% i s t y p i c a l l y t a bulated f o r an experiment even though the t o t a l pressure w i t h i n the r e a c t o r was only about 50 Torr. This i s c l e a r l y impossible unless the water w i t h i n the simple bubbler was h e a r t i l y b o i l i n g at room temperature and at a pressure c l o s e to the r e a c t o r pressure. This mode of operation i s not apparent from t h e i r paper (a heater and a condensing c o i l would - 21 -have been r e q u i r e d ) . Therefore, we have to assume that the experiments were conducted at a much lower R.H. than those claimed and a l s o that the e f f e c t i v e s u r f a c e area was g r e a t e r than that used i n t h e i r c a l c u l a t i o n s . These conclusions are r e f l e c t e d i n Table 1 of t h i s r e p o r t , where the J u d e i k i s et a l . ^ ^ r a t e constant f o r Mohave f l y ash has been adjusted downwards by a f a c t o r of A g and where the reported R.H. has been l i s t e d as unknown. N e s b i t t et a l . ^ 2 ^ attempted to measure the k i n e t i c s of a low temperature moist limestone f l u e gas d e s u l p h u r i z a t i o n process by measuring, v i a TGA, the increase i n weight of a limestone p e l l e t which was exposed to a sa t u r a t e d (100% R.H.) gas stream at 35-43°C which contained 20,000 - 40,000 ppm S0 2. The r e s u l t s i n d i c a t e d a f i r s t - o r d e r process which was r e a c t i o n r a t e l i m i t e d and w i t h no s u l p h i t e measurable. Scanning e l e c t r o n microscopy showed that w i t h an average core r a d i u s of 0.4 um, that there was l i t t l e or no s u r f a c e p e n e t r a t i o n . The postulated mechanism f o r the modified dry limestone process was presented as: SO 2 + H 20 (ad) [H 2S0 3] -> H + + HS0 3 2H + + S0~ HS0 3 + W2 •* HS0 4 CaCO, + HSOT CaSO, + HCO-3 4 4 3 HC0 3 + H + •> H 20 + C0 2 (In t h i s r e a c t i o n the pH was such (pH = 2) that l i t t l e s u l p h i t e was 2 formed). The s u r f a c e area of the p e l l e t (guessed to be about 2.5 cm ), the measured weight change ( c o r r e c t e d f o r moisture pick-up v i a a blank/dummy run) of 54 x 10 -^ mol/h, and the S0 9 c o n c e n t r a t i o n of 1 3 mol/m y i e l d a c a l c u l a t e d f i r s t order r e a c t i o n r a t e of 0.22 m/h. The measured a c t i v a t i o n energy f o r t h i s r e a c t i o n was about 13 kcal/mole. - 22 -(21) Gauri et a l . looked at the e f f e c t of R.H. and g r a i n s i z e on the r e a c t i o n r a t e s of marble f o r high concentrations of S0 2. A porous p e l l e t , made by compressing marble powder, was reacted w i t h S0 2 (700-3000 ppm) and H 20 (50-90% R.H.) w i t h i n the flow chamber of an X-ray d i f f r a c t o m e t e r . They observed that at "normal" (40-80% R.H.) humidity the i n i t i a l r e a c t i o n product was s u l p h i t e (CaS0.j*%H 20) which then transformed p a r t i a l l y to sulphate (CaS0^'2H 20). The r e a c t i o n r a t e ( r a t e of formation of s u l p h i t e ) increased w i t h i n c r e a s i n g humidity and w i t h decreasing g r a i n s i z e . For a p e l l e t of Alabama white limestone about 25% (mole b a s i s ) was converted to the s u l p h i t e form i n 7 hours when exposed to 3000 ppm (0.12 moles/m ) of SO, at high humidity. I f the * 2 e f f e c t i v e s urface area of the porous p e l l e t i s assumed to be 1 m /g then the r a t e constant f o r the surface r e a c t i o n i s estimated to be about 0.002 m/h, i . e . , 0.25 r •= / moles S 0 2 jnole limestone-h. 0.036 moles S 0 2 133 g-h k — = 0 , 0 3 6 = 0.002 m/hr WSC, 133x1x0.12 K l i n g s p o r et a l . ' used a packed bed r e a c t o r (limestone p a r t i c l e s d i s p e r s e d i n quartz sand i n a 1:20 weight r a t i o ) to i n v e s t i g a t e the k i n e t i c s of the dry S0 2 - limestone r e a c t i o n at low temperatures (40-80°C). Separate experiments were undertaken i n a s p e c i a l l y designed g r a v i m e t r i c a d s o r p t i o n apparatus i n order to determine the amount of adsorbed water vapour on the limestone p a r t i c u l a t e s . The p a r t i c l e s used were 3-5 micron diameter Ignaberga limestone w i t h a shape f a c t o r ( r a t i o of B.E.T. surface area to surface area based on s p h e r i c a l p a r t i c l e s ) of -4 2 9.9 and an adsorbed monolayer c a p a c i t y of 3.7 x 10 g/m . The number of monolayers of water increased r a p i d l y above a r e l a t i v e humidity (R.H.) of about 80%, from about 2 at lower h u m i d i t i e s , to 3.2 at 84% - 23 -R.H., and to 4.6 at 92% R.H. The measured r e a c t i o n r a t e , determined u s i n g a packed-bed limestone/quartz sand r e a c t o r and an i n f r a r e d spectrophotometer to determine SC^ capture, a l s o increased e x p o n e n t i a l l y w i t h humidity w i t h no measurable r e a c t i o n below one monolayer of adsorbed water. The r e a c t i o n order w i t h respect to SC^ was determined to be zero at low humidity (R.H. <70%) but increased to about 0.8 at 92% R.H.. The r e a c t i o n r a t e was independent of the s u p e r f i c i a l bed v e l o c i t y over a range of 0-100 cm/s and was only weakly dependent upon temperature (Ea = 18.8 kJ/mol). Although a " l a r g e e f f o r t was focused on the t h e o r e t i c a l i n t e r p r e t a t i o n of the experiments ... a model, i n c l u d i n g a continuous change from r e a c t i o n c o n t r o l to d i f f u s i o n c o n t r o l , f a i l e d to f i t the experiments". T h e i r h e r o i c e f f o r t s may have been thwarted due to an apparent l a c k of c o r r e c t i o n f o r " w a l l e f f e c t s " - the a d s o r p t i o n / o x i d a t i o n of SOj on the extremely l a r g e s u r f a c e area of the quartz sand bed (when queried about t h i s the authors f a i l e d to r e p l y ) . However, by t a k i n g t h e i r experimental r a t e constants, f o r 1000 ppm SO2 and 70°C, at face value and by c o n v e r t i n g these to e q u i v a l e n t 1st order r a t e constants u s i n g the expression: k ( l s t order) = k'(order n ) / C 1 - n we c a l c u l a t e k = 0.02 m/h at 84% R.H. and k = 0.04 m/h at 92% R.H.. K l i n g s p o r et a l . ' concluded that "the r e a c t i o n occurs only on the e x t e r n a l s u r f a c e area and that no SO2 penetrates through micropores i n t o the bulk of the unreacted limestone. The i n t e r f a c e gas/unreacted limestone thus represents a defined r e a c t i o n zone. When the outer part of a limestone p a r t i c l e i s consumed then a more compact s t r u c t u r e with a sm a l l e r r e a c t i v e surface area i s exposed to Sto^' This causes a d r a s t i c drop i n the r e a c t i o n r a t e " . The formation of a l a y e r of i n e r t m a t e r i a l (12.5% by weight f o r Ignaberga limestone) could be expected to a l s o slow the r e a c t i o n r a t e . - 24 -Berresheim et a l . used a "dynamic multiphase r e a c t i o n system" c o n s i s t i n g of a v e r t i c a l , g l a s s pipe down which flowed a d i l u t e , humid gas/aerosol mixture at 25 L/min (Re = 118). The c o n c e n t r a t i o n g r a d i e n t of the 5-120 ppb ( p a r t s per b i l l i o n by volume) S0 2 was measured u s i n g an eight-way PTFE s w i t c h i n g v a l v e to m u l t i p l e x a coulometric S 0 2 monitor which had a 2a d e t e c t i o n l i m i t of 0.25 ppb. An S0 2 c o n c e n t r a t i o n g r a d i e n t corresponding to a p s e u d o - f i r s t - o r d e r removal r a t e of r> 36%/h (k^ > 0.4 h~ ) could be detected i n the r e a c t o r a f t e r a t o t a l r e a c t i o n time of 340 seconds (corresponding to the average plug-flow residence time). At R.H. > 96% both the S0 2 and. a e r o s o l w a l l l o s s e s became u n c o n t r o l l a b l e , t h e r e f o r e the r e l a t i v e humidity was c o n t r o l l e d below t h i s l e v e l by r e g u l a t i n g the temperature of two impingers. Aerosols -3 -2 were generated by n e b u l i z i n g 10 - 2 x 10 M s o l u t i o n s of s a l t s (MnSO^, CuSO^, F e C l j , e t c . ) and d r y i n g the r e s u l t i n g mist i n an 80 cm d i f f u s i o n dryer. The dry s a l t p a r t i c l e s (0.275 pm mass mean diameter) e n t e r i n g the r e a c t o r become "wetted a e r o s o l p a r t i c l e s " which r a p i d l y ( m i l l i s e c o n d s ) grew to an e q u i l i b r i u m s i z e depending upon chemical composition and r e l a t i v e humidity. During growth the s o l u b l e substance of the p a r t i c l e d i s s o l v e s i n the water f i l m and t o t a l d i s s o l u t i o n i s reached at a c r i t i c a l r e l a t i v e humidity which i s c a l l e d the deliquescence p o i n t ; above t h i s point f u r t h e r uptake of water d i l u t e s the s o l u t i o n l e a d i n g to a decrease i n mean molal a c t i v i t y of the d i s s o l v e d s a l t . The l i q u i d water content, L, of the a e r o s o l system was determined by the d i f f e r e n c e between the 3 a e r o s o l c o n c e n t r a t i o n (g/m ) i n humid a i r and i n dry a i r . The r e a c t i o n r a t e between S0 2 and a l l the heavy-metal a e r o s o l s was p s e u d o - f i r s t - o r d e r w i t h respect to S0 2 and showed a s i g n i f i c a n t increase w i t h r e l a t i v e humidity p a r t i c u l a r l y when c l o s e to the deliquescence point of the wetted p a r t i c l e s . The i n v e s t i g a t o r s showed that among the Mn(II) and Cu(I I ) a e r o s o l s which were i n v e s t i g a t e d that Mn(N0.j) 2 was the most e f f i c i e n t f o r the chemical removal of S0 2 at atmospheric background c o n d i t i o n s , e s p e c i a l l y i n haze and fog d r o p l e t s . The r e s u l t s i n d i c a t e that the c a t a l y t i c o x i d a t i o n of S(IV) i n such a e r o s o l systems may be as e f f i c i e n t as i s o x i d a t i o n by H 2 0 2 i n cloud water and that wetted p a r t i c l e s may th e r e f o r e act as very e f f i c i e n t s i n k s f o r atmospheric S0 9. - 25 -The above researchers e x t r a p o l a t e d t h e i r experimental data to a 3 3 h y p o t h e t i c a l atmospheric haze c o n t a i n i n g 55 ng/m of MnCNO^^t 0-9 ug/m l i q u i d water, L, (pH of 1.5) and a R.H. of 94% to estimate a maximum S0 2 removal r a t e of about 2.5%/h. I f t h i s p s e u d o - f i r s t - o r d e r r a t e (0.025 hr~*) i s converted to an equivalent f i r s t - o r d e r surface-dominated r e a c t i o n then the r a t e constant, k would be 4070 m/h and the molecular c o l l i s i o n e f f i c i e n c y would be 11.5%. Under these c o n d i t i o n s the l i q u i d f i l m d i f f u s i o n of SO- becomes s i g n i f i c a n t and may dominate. For the 3 co n d i t i o n s of the experiment - e.g., f o r MnSO, at 268 ug/m , L = 1.7 3 * mg/m and R.H. = 94%, the equivalent surface r a t e constant, k, i s c a l c u l a t e d to be 290 m/h and s o l u t i o n phase chemistry ( c a t a l y t i c o x i d a t i o n ) i s probably r a t e c o n t r o l l i n g . Jorgensen et a l . ^ * ^ used a r e a c t o r system s i m i l a r to that of K l i n s p o r ( 22) et a l . to evaluate sorbents and a d d i t i v e s f o r dry S0 2 removal. However, n i t r o g e n was used as the gas phase hence a d s o r p t i o n / r e a c t i o n on the s i l i c a sand would not be a f a c t o r . They found that at l e s s than 70% R.H. limestone was unr e a c t i v e unless a h i g h l y deliquescent a d d i t i v e such as C a C l 2 was added i n order to form a l i q u i d phase f o r the S0 2 r e a c t i o n w i t h CaCOj* The "promoted" limestone r e a c t i o n increased e x p o n e n t i a l l y w i t h r e l a t i v e humidity above the deliquescent point f o r C a C l 2 . For C a C l 2 promoted minus 325 mesh aragonite the r e a c t i o n r a t e at 70% R.H. was given as 0.4 moles/mole CaCO^/h. This i s e q u i v a l e n t to a p s e u d o - f i r s t - o r d e r surface-phenomenon r a t e constant, k, of about 0.08 m/h. No limestone r a t e data i s presented f o r higher h u m i d i t i e s . The authors use mass t r a n s f e r theory to show that gas f i l m d i f f u s i o n can probably be excluded as an o v e r a l l r a t e c o n t r o l l i n g step. Table 1 summarizes the p r e v i o u s l y described research, which p e r t a i n s to the heterogeneous r e a c t i o n of S0 2 w i t h limestone p a r t i c l e s . None of the e a r l i e r experiments were c a r r i e d out usin g atmospheric c o n d i t i o n s of S0 2 c o n c e n t r a t i o n , temperature, and humidity. Even when usin g a high S0 2 c o n c e n t r a t i o n the var i o u s researchers report a l a r g e divergence i n the (19) r e a c t i o n r a t e w i t h limestone, v a r y i n g from k = 0 (Corn and Cheng v ') to k ~ 0.22 m/h ( N e s b i t t et a l / 2 0 * ) . I t should be noted that some of t h i s nau l smus or K U V M R K S K M C S n S U W I U M W . SISTBI I M J M U M U T I U H I M B Z C X I ItTeTTWTrtOCT REMTlUal H O I O n B D H (Icon* et a l . (18) Batch raactor (flask containing powder and tha gas mixture; (SO.) 35 « via colorimetry of S s c i n t i l l a t i o n . Provisions for u.v. irradiation (SOj) - 10-20 ppa T a room tamp R.H. - 0% "Pinaly ground powdars" For CaCOj ( i f s a l m'/g) k a 0.00015 m/h (no u.v.) k a 0.021 m/h (u.v. irradiation) Com and Chang (19) laminar bad packad with particla coatad Taflon baa da. (SOj) via microcoulomoter (SOj) - 14.4 ppm T - 23«C R.H. - 20-95% "Powdered Limestone* *Ho raaction" (Instrument not sansitlva enough?) Hesbltt at a l . (20) Thermogravimetrlc analysia of a porous p a l l a t , with and without ( s t y (S0 2) - 20,000-40,000 ppm T - 35-43'C R.H. • 100% Limastona pallat S(BST) a 2 m2/g rc(SOt) - 0.4 im k - 0.22 m/h, "rata limitad by surfaca raaction* Judoikis at a l . (17) Laminar plug flow with coatad walls. <S02) gradient via suss spac. (SOj) - 3-100 ppm T - room tamp P • 50 - 100 Torr R.H. a ? Diffarant powdars (S - 15.2 m2/g via BET), Xn-situ surfaca araa not maasurad. For No have fly-ash k - 9 m/h (71 Limastona not tastad. Gauri at a l . (21) X-ray diffractometer flow-chaabar analysis for SOj/SO^ convarsion of porous pallat. (SOj) - 700-3000 ppm T - room tamp R.H. • 50-90* Pourous pallat of comprossad limastona, <3«vn. 'S' not maasurad. k - 0.002 m/h i f S a 1 n2/g Klingspor at a l . (22) Packad bad with limestone/quarti-sand packing. (SO^I via Infrarad spectrophotometry (SOj) a 50-4000 ppm T » 40-80» R.H. a 0-92* 3 - 5 im diamatar limastona. S(BBt) - 2.2 (m2/g) k - 0.02 m/h (B4% R.H.) k - 0.04 m/h (92% R.H.) (Ho correction aade for 'wall effects"). Berresheim at a l . (23) Laminar plug flow. (SOj) gradiant via microcoulometer. Catalytic oxidation of SO^ (SOj) a 5-120 ppb T a 25«C R.H. a 0-94% Mattad haavy-matal catalysts at varying chamical a c t l v l t l a s , ate. For ftaSO. aerosol at 261 jig/m3 with liquid water content 1.7 mg/m (R.H. a 94%) k a 290 m/h Jorgensen at a l . (10) Packad bad with limestone/quarts sand packing. SOj In humid (SOj) - 500-2000 ppm T - 54-«5«C R.H. a 0-70% Various commarcial limastona and alkalina powdars. Surfaca araa not maasurad. k a 0 for CaCOj (R.H. - 40-70%) unless deliquescent salt additives (CaCl^) used, k a o.Oi m/hr for 70% R.H. Oj a 0% with CaClj promotion. (Assumed S • lm2/g) - 27 -s c a t t e r may be a t t r i b u t a b l e to a r b i t r a r i l y a s s i g n i n g a s u r f a c e area of 2 1 in /g to those p a r t i c l e s whose surface area was not reported. Most researchers i n d i c a t e a strong dependence of the r e a c t i o n r a t e on r e l a t i v e humidity because the r e a c t i o n s r e q u i r e a l i q u i d f i l m on the s u r f a c e of the s o l i d phase i n order to proceed. L i t t l e or no r e a c t i o n occurs below a R.H. of about 70-80%. However, by adding a s t r o n g l y deliquescent s a l t such as C a C ^ as a promoter Jorgensen et al.^°^ showed that a s i g n i f i c a n t r a t e of r e a c t i o n (k = 0.08 m/h) could occur at a r e l a t i v e humidity of only 70%. Experiments w i t h a e r o s o l s other than limestone i n d i c a t e that the heterogeneous r e a c t i o n r a t e can indeed be f a s t . For example Berresheim (23) et a l . measured k = 290 m/h u s i n g an a e r o s o l composed of a s o l u t i o n of the heavy metal c a t a l y s t MnSO, and w i t h S09 and humidity c o n d i t i o n s (17) t y p i c a l of those i n the atmosphere, w h i l e J u d e i k i s et a l . recorded k - 9 m/h u s i n g f l y - a s h a e r o s o l , which i s u s u a l l y r i c h i n a l k a l i oxides, and a high (3-100 ppm) S0 9 c o n c e n t r a t i o n . From these r e s u l t s , and a l s o (24) f o l l o w i n g from the t h e o r e t i c a l a n a l y s i s of F r e i b e r g and Schwartz v , we t e n t a t i v e l y conclude that aqueous phase, or aqueous/solid phase, phenomena l i m i t the r a t e of r e a c t i o n between limestone p a r t i c l e s and SO2 gas. In the atmosphere, hydrogen peroxide (H 90 9) i s known (e.g. Berresheim et (23) a l . ') to r e a d i l y d i s s o l v e i n the aqueous phase of an a e r o s o l where i t r a p i d l y o x i d i z e s the s u l p h i t e to sulphate, thereby " d e b o t t l e n e c k i n g " the r e a c t i o n . I f the aqueous phase surrounds a p a r t i c l e of limestone then i t i s reasonable to assume that the r e s u l t i n g decrease i n s o l u t i o n pH would be l i m i t e d by a concurrent increase i n the r a t e of limestone d i s s o l u t i o n , s i n c e the limestone d i s s o l u t i o n i s a s t r o n g f u n c t i o n of pH. I f the c o n c e n t r a t i o n of o x i d a n t s , such as O2, ^02, hydroxyl and peroxy r a d i c a l s , are r e l a t i v e l y high and r e s u l t i n a r a p i d aqueous phase o x i d a t i o n of S(IV) to S ( V I ) , then the o v e r a l l r a t e l i m i t i n g step may w e l l be the a v a i l a b i l i t y of a l k a l i n i t y w i t h i n the d r o p l e t . I f a p a r t i c l e of limestone was the condensation n u c l e i f o r the fog or cloud d r o p l e t then the a l k a l i n i t y a v a i l a b i l i t y w i l l depend upon the r a t e of limestone d i s s o l u t i o n . - 28 -Based on the above observations i t i s apparent that there are no experimental data a v a i l a b l e on the a l k a l i n e p a r t i c u l a t e / S 0 2 system which are a p p l i c a b l e to the ambient atmosphere. Research i s re q u i r e d to measure the r e a c t i o n k i n e t i c s of t h i s system f o r v a r i o u s compositions and s i z e s of a l k a l i n e p a r t i c u l a t e s under d i f f e r e n t c o n d i t i o n s of r e l a t i v e humidity and s o l u b l e oxidant ( H 2 0 2 , ^3) or f r e e r a d i c a l (OH*, et c . ) c o n c e n t r a t i o n . T h i s t h e s i s w i l l d i s c u s s the f i r s t step i n t h i s research, which i s the i n v e s t i g a t i o n of the r e a c t i o n between limestone p a r t i c l e s and S0 2 ( c a . 50 ppb) i n humid a i r . Concurrently, a n c i l l a r y experiments were a l s o conducted i n order to measure the maximum r a t e of limestone d i s s o l u t i o n i n an aqueous system. Further research i s r e q u i r e d to extend t h i s study to i n c l u d e other types of a l k a l i n e s o i l p a r t i c l e s and to i n c l u d e the e f f e c t of atmospheric o x i d a n t s . 2.0 RESEARCH METHODOLOGY The ge n e r a l research procedure which was followed was to contact a l k a l i n e p a r t i c u l a t e , of known composition and morphology, w i t h a humid ( r e l a t i v e humidity 70%-100%) a i r stream which contained a low co n c e n t r a t i o n (ca. 50 parts per b i l l i o n ) of sulphur d i o x i d e gas. I d e a l l y the r e a c t i o n should be st u d i e d i n a r e a c t i o n system which simul a t e s the p a r t i c l e - g a s i n t e r a c t i o n s which occur i n the atmosphere. Therefore an "entrained-bed" r e a c t o r was designed wherein the p a r t i c u l a t e phase was kept i n suspension by an upward laminar flow of a i r c o n t a i n i n g traces (0-50 ppb) of S0 2. The r e a c t i o n r a t e was to be monitored by a n a l y z i n g the composition of the p a r t i c l e s over time, u s i n g an aqueous p a r t i c l e - s u r f a c e l e a c h / i o n chromatography procedure which was developed f o r t h i s purpose. The 1.2m x 40mm to 150mm diameter r e a c t o r was f a b r i c a t e d i n Pyrex g l a s s . I n i t i a l experiments d i s c l o s e d severe problems d u r i n g high humidity (R.H.>80%) op e r a t i o n . The suspended a e r o s o l tended to i r r e v e r s i b l y agglomerate and s i n k down to the s i n t e r e d - g l a s s a i r d i s t r i b u t i o n p l a t e , or to adhere to the r e a c t o r w a l l s and to react there. - 29 -Because of the above d i f f i c u l t i e s i t was then decided to operate the same system as a f l u i d i z e d - b e d r e a c t o r simply by i n c r e a s i n g the p a r t i c l e s i z e and mass so that a s t a b l e g a s - p a r t i c l e system would e x i s t w i t h i n the 40mm d i a . s e c t i o n of the r e a c t o r above the d i s t r i b u t i o n p l a t e . Experimentation i n d i c a t e d that 200-270 mesh ( T y l e r ) limestone p a r t i c l e s would f l u i d i z e without agglomerating. But extreme problems were again encountered w i t h contamination and w a l l r e a c t i o n s , and w i t h humidity c o n t r o l . (Humid a i r c o n t a i n i n g 50 ppb of S0 2 i s an extremely unstable system wherever a s o l i d s urface i s a v a i l a b l e f o r a d s o r p t i o n and r e a c t i o n ! ) The experiments d i d prove,, however, that the r e a c t i o n r a t e constant, k, was much gr e a t e r than that measured by other researchers, who used an S0 2 c o n c e n t r a t i o n from a thousand to a m i l l i o n times greater (Table 1 ) . Simple mass balance c o n s i d e r a t i o n s i n d i c a t e d that a l l of the S 0 2 e n t e r i n g the r e a c t o r was unexpectedly r e a c t i n g . Therefore i t was not p o s s i b l e to estimate the S0 2 c o n c e n t r a t i o n g r a d i e n t w i t h i n the bed and to compute a r e a c t i o n r a t e constant, but only to a s s i g n a lower l i m i t to the value of the r a t e constant. For a bed mass of 2g t h i s was estimated to be 0.5 m/h. (A lower bed mass could not be maintained i n a homogeneous f l u i d i z e d s t a t e . ) The next e v o l u t i o n i n the design of the experimental system was to r e t u r n to the concept of a p a r t i c u l a t e phase dis p e r s e d w i t h i n the gas phase. The limestone dispersed i n quartz sand packed beds of K l i n g s p o r et a l / 2 2 * and Jorgensen et a l . ^ ° \ and the limestone-coated T e f l o n bead packed bed r e a c t o r of Corn and Chen v ' were r e j e c t e d because of t h e i r l a r g e r e a c t o r surface areas. Previous experiments by the author had shown that S0„ r e a d i l y adsorbs under high humidity c o n d i t i o n s ; a l s o ( 2 6 ) s i l i c a i s known to promote S0 2 o x i d a t i o n . The r e a c t o r design chosen f i n a l l y f o r t h i s research c o n s i s t s of a small amount of limestone powder (ca. 10 mg) dispersed between two T e f l o n f i l t e r membranes, which i n turn are held w i t h i n a s o l i d T e f l o n f i l t e r h o l d e r . The maximum r e a c t i o n r a t e constant measurable w i t h t h i s system depends upon the r a t i o of the gas-phase v o l u m e t r i c flow r a t e to the - 30 -Q R E A C T O R • • thickness (em) Af • area . ( c r n") Q • gas flow (cm 3** 1) P A R T I C L E S D p « average diamatar (em) /Op* danslty (gem* 9) Z p a avaraga distanca batwaan particla aurfacaa (cm) Nps numbar W at total mass (g) S • apacific aurfaca araa (m2g"1) FLOW R e < 0.1 FIGURE 4 REACTOR SYSTEM (Not to scale) - 31 -s o l i d phase surface area. The use of approximately 10 mg of limestone p a r t i c l e s w i t h i n the r e a c t o r s , as a lower l i m i t on area, was chosen because i t provides a dispersed "monolayer" of p a r t i c l e s as shown i n Figure 4 and because a l e s s e r amount would increase the r e l a t i v e e r r o r encountered during weighing ope r a t i o n s . A dispersed p a r t i c l e phase more a c c u r a t e l y simulates atmospheric c o n d i t i o n s than say a packed bed, and o f f e r s the f u r t h e r advantage of being simple to model. I f the r a t e of d i f f u s i o n of SOg molecules w i t h i n the r e a c t o r i s much gr e a t e r than the r a t e of advection through the r e a c t o r l e n g t h then i t may be assumed that the co n c e n t r a t i o n of SO2 w i t h i n the r e a c t o r i s f a i r l y uniform and that the r e a c t o r behaves as a CSTR. This assumption w i l l now be v a l i d a t e d . (12) S e i n f e l d v ' d e r i v e d an expression f o r the t r a n s i e n t c o n c e n t r a t i o n f i e l d surrounding an i s o l a t e d p a r t i c l e , which can be s i m p l i f i e d to the form: C ( Z > t ) a exp I _ " I (21) f — ) I 4D t J where: Z = d i s t a n c e from p a r t i c l e s urface (cm) 2 -1 D = d i f f u s i o n c o e f f i c i e n t (cm s ) o t = time (s) C = bulk c o n c e n t r a t i o n The " c h a r a c t e r i s t i c time" ( f j g ) to a t t a i n steady s t a t e i s then simply: Zmax = k p S X (seconds), (22) g - 32 -where ^ m a x i s one-half the average d i s t a n c e between p a r t i c l e s , ( Z m a x = Zp/2). For the c o n d i t i o n s of our r e a c t o r (a monolayer of p a r t i c l e s as shown i n Figure 4) i t can be shown that Z = P - D , ( » ) P o where: A f = r e a c t o r c r o s s - s e c t i o n a l area (cm ) Dp at p a r t i c l e diameter (cm) and N p, the number of p a r t i c l e s i n the r e a c t o r , i s given by where: V = t o t a l mass of p a r t i c l e s (g) _3 pp = p a r t i c l e d e n s i t y (g cm ) S u b s t i t u t i n g i n t o equations 22-24 the f o l l o w i n g t y p i c a l values f o r our experiments: _3 pp = 2.7 g cm D = 65 x 10" A cm P -2 W = 10 1 g 2 A r = 4n cm (exposed area of 47 mm diameter T e f l o n f i l t e r s ) D = 0.126 c m 2 s _ 1 8 we o b t a i n N = 26,000 p a r t i c l e s P Z _ = 78 x 10 4 cm max and t h e r e f o r e T . = 1.2 x 10 4 s. dg - 33 -Therefore F i c k i a n d i f f u s i o n e s t a b l i s h e s the st e a d y - s t a t e c o n c e n t r a t i o n f i e l d w i t h i n about 1/10 m i l l i s e c o n d . This value w i l l now be compared w i t h the residence time ( T r g ) f ° r S0 2 molecules w i t h i n the r e a c t o r . The average gas v e l o c i t y , u r , i n the i n t e r - p a r t i c l e pores can be estimated from the vol u m e t r i c f l o w - r a t e , Q, d i v i d e d by the i n t e r - p a r t i c l e area: u = ^ (25) N n D P P A -r 4 3 -1 where: Q • gas v o l u m e t r i c flow r a t e (cm s ) Therefore a lower l i m i t on the S0 2 residence time ( T rg)» assuming only minor l o s s e s , i s given by: x = (seconds) (26) u r where: a = r e a c t o r bed t h i c k n e s s , cm (e q u i v a l e n t to the diameter of the l a r g e s t p a r t i c l e s ) . S u b s t i t u t i n g t y p i c a l values i n t o equations 20 and 21: Q = 10 cm 3 s " 1 A = 4 n cm 2 r N p = 26,000 p a r t i c l e s D = 65 x 10" 4 cm a = 74 x 10 cm gi v e s u = 0.8 cm s * r _ 2 and t h e r e f o r e , t = 0.9 x 10 s rg - 34 -(This value f o r T , 0.9 x 10 s, can a l s o be obtained by assuming a p a r t i c l e d e n s i t y of 2.9 g/cm i n order to c a l c u l a t e a r e a c t o r v o i d f r a c t i o n of 0.963. The v o i d volume d i v i d e d by the flow, Q, then g i v e s the r e a c t o r mean residence time). I t i s apparent that T = 0.9 x 10" 2 » T. = 1.2 x 10" 4s r g dg and t h e r e f o r e a f a i r l y uniform SO2 c o n c e n t r a t i o n , equal to C should e x i s t w i t h i n the r e a c t o r . Under the above flow c o n d i t i o n s the average p a r t i c l e Reynolds Number, Re, i s c a l c u l a t e d to be 0.03, and t h e r e f o r e the flow i s i n the "creeping flow" regime. In p r i n c i p l e both the r a t e of reactant (SO2) l o s s and product (sulphate and s u l p h i t e s ) formation can be measured over a period of time by u s i n g s e v e r a l i d e n t i c a l r e a c t o r s o p e r a t i n g i n p a r a l l e l . The change i n gas phase co n c e n t r a t i o n across each bed can be monitored u s i n g an S0 2 a n a l y z e r , w h i l e the change i n reactant c o n c e n t r a t i o n can be followed by s e q u e n t i a l l y s w i t c h i n g out each r e a c t o r and then a n a l y z i n g the limestone p a r t i c l e s f o r t h e i r i n d i v i d u a l r e a c t i o n products. I f the analyses are accurate then the r a t e of l o s s of r e a c t a n t s w i l l be r e l a t e d to the r a t e of formation of r e a c t i o n products v i a stochiometry and a mass balance. R e f e r r i n g to Figure 4 and assuming a p s e u d o - f i r s t - o r d e r r e a c t i o n w i t h respect to SO2, we can equate the l o s s of SO2 from the gas stream to the r a t e of the surface r e a c t i o n : 0.0036 Q (C. - C ) = k C V S v 1 o' o (27) - 35 -or, 0.0036 Q (Ci - C Q) (28) VS C where k = f i r s t - o r d e r r a t e constant (m h~*) 3 -1 Q = r e a c t o r flow r a t e (cm s ) _3 C. = gas phase c o n c e n t r a t i o n e n t e r i n g r e a c t o r (mol m ) 1 _3 C Q = gas phase c o n c e n t r a t i o n w i t h i n and l e a v i n g r e a c t o r (mol m ) V = mass of p a r t i c u l a t e (g) 2 -1 S « s p e c i f i c s u r f a c e area of p a r t i c u l a t e s (m g ) This expression (equation 28) should be v a l i d f o r an o u t l e t SO2 c o n c e n t r a t i o n down to about 10% of the i n l e t c o n c e n t r a t i o n . Reaction r a t e s g r e a t e r than t h i s may i n v a l i d a t e the assumption about c o n c e n t r a t i o n u n i f o r m i t y w i t h i n the bulk of the r e a c t o r . The maximum r a t e constant that can th e r e f o r e be measured (corresponding to 90% SO2 conversion) can be estimated from equation 28 f o r t y p i c a l r e a c t o r c o n d i t i o n s . Let Q = 10 c m 3 s _ 1 V = 0.01 g S « 0.3 m 2g - 1 Then, k = ° - 0 0 3 6 x 1 0 x 9 = 100 m h" 1' max 0.01 x 0.3 This upper l i m i t i s seen (Table 1) to be orders of magnitude gre a t e r than the values observed by other researchers f o r limestone and hence we conclude that our r e a c t o r design should be of adequate s e n s i t i v i t y . - 36 -In the I n t r o d u c t i o n i t was suggested that under c e r t a i n c o n d i t i o n s the o v e r a l l r e a c t i o n r a t e may be l i m i t e d by the limestone d i s s o l u t i o n r a t e . This p o s s i b i l i t y was i n v e s t i g a t e d by measuring the r a t e of d i s s o l u t i o n i n a s t i r r e d v e s s e l c o n t a i n i n g d i l u t e H^SO^. These a n c i l l a r y experiments w i l l be presented i n Sections 2.3 (Procedures) and 3.8 ( R e s u l t s ) . 2.1 D e s c r i p t i o n of Apparatus The apparatus used to study sulphur d i o x i d e - limestone r e a c t i o n k i n e t i c s i s i l l u s t r a t e d i n Figure 5. A l l m a t e r i a l i n contact w i t h humid SO2 was constructed of T e f l o n s i n c e t h i s p l a s t i c was (27) reported^ to minimize w a l l e f f e c t s . A modified Metronics permeation-tube oven and a h u m i d i f i c a t i o n module were used to generate 4-6 l i t r e s per minute of sat u r a t e d a i r , c o n t a i n i n g 0-100 ppb ( p a r t s per b i l l i o n , v/v) of SO2, at a pressure somewhat g r e a t e r than atmospheric. The modified Metronics " D y n a c a l i b r a t o r " ( F i g u r e 6) u t i l i z e d a pump capable of 10 L/min to pass room a i r through an accumulator to an a c t i v a t e d carbon scrubber, a Linde 5A molecular-sieve a i r p u r i f i e r , and a p o l i s h i n g f i l t e r . The r e s u l t i n g pure ( f r e e of SO2) "zero" a i r was then f l o w - r e g u l a t e d and h u m i d i f i e d . The a i r r e t u r n i n g from the h u m i d i f i c a t i o n module was s p l i t i n t o two streams w i t h one stream passing through a temperature-regulated oven c o n t a i n i n g an SO2 permeation tube. The permeation r a t e of the tube, which was nominally 800 ng/min at 30°C, was c o n t r o l l e d by r e g u l a t i n g the oven temperature. The SO2 - dosed stream then mixed w i t h the balance of the h u m i d i f i e d zero a i r and passed to the r e a c t o r manifold v i a a pressure r e d u c t i o n v a l v e . The h u m i d i f i c a t i o n module (Figu r e 7) needed to withstand a pressure of up to 70 kPa and to be fr e e of a l l sulphur and hydrocarbon contaminants. A l l l i n e s used were s t a i n l e s s s t e e l w i t h T e f l o n and - 37 -s t a i n l e s s s t e e l connections. The glassware and pipes were cleaned w i t h an acetone r i n s e , n i t r i c a c i d r i n s e and d i s t i l l e d water r i n s e . D i s t i l l e d water was heated to a regulated temperature and then pumped to the top of a column which was packed w i t h g l a s s beads. There i t flowed down through the packing, through a s e a l , and then back to the heating f l a s k . Water l o s s was manually compensated f o r v i a an elevat e d water r e s e r v o i r . A pressure-balancing l i n e a l s o connected the hea t i n g f l a s k to the r e s e r v o i r . Zero a i r entered the bottom of the h u m i d i f i c a t i o n column, v i a a tee, and then passed up through the column packing, c o u n t e r - c u r r e n t l y to the down-flowing hot water. The hot, h u m i d i f i e d a i r stream was then cooled to room temperature i n an a i r - c o o l e d heat exchanger/condenser which c o n s i s t e d of 3m of 1cm d i a . s t a i n l e s s - s t e e l tubing. A water separator and a quartz-wool demister removed l i q u i d water. The water s a t u r a t e d zero a i r then returned to the modified Metronics D y n a c a l i b r a t o r . Downstream of the Metronics system a l l components were f a b r i c a t e d from T e f l o n or from Pyrex. The pressure of the humid, dosed a i r e n t e r i n g the Pyrex r e a c t o r manifold was regulated w i t h a simple bubbler, which a l s o served as a flow i n d i c a t o r . Flow to the four i d e n t i c a l r e a c t o r s passed through T e f l o n plug-valves which were used to i s o l a t e the r e a c t o r s d uring s e q u e n t i a l reaction-product sampling. The r e a c t o r s c o n s i s t e d of T e f l o n 47 mm f i l t e r h olders (Cole-Parmer J-6623-10) w i t h i n which the solid-phase a l k a l i n e p a r t i c u l a t e was c a r e f u l l y d i s p e r s e d , i n t o a s i n g l e l a y e r w i t h a v o i d f r a c t i o n of about 0.95, between two Z i t e x membrane f i l t e r s (Cole-Parmer J-6623-51). Z i t e x i s a porous form of T e f l o n made from very f i n e f i b e r s . The mean pore s i z e i s 5-10 microns w h i l e the nominal thi c k n e s s i s 140 microns. This m a t e r i a l was chosen because of i t s i n e r t n e s s and i t s high gas p e r m e a b i l i t y . The f l o w - r a t e through the r e a c t o r s was regulated w i t h c a l i b r a t e d rotameters (Cole-Parmer J-3218-17). - 38 -The gas-phase S0 2 c o n c e n t r a t i o n before and a f t e r the r e a c t o r s was p e r i o d i c a l l y measured by connecting the appr o p r i a t e T e f l o n l i n e ( F i g u r e 5) i n t o the sample port of the c a l i b r a t e d TECO (Thermo E l e c t r o n Corp.) Model 43A Pulsed Fluorescent S0 2 Analyzer. Pulsed u l t r a v i o l e t l i g h t passes through a r e f l e c t i o n mode o p t i c a l f i l t e r system to a measurement chamber where i t e x c i t e s S0 2 molecules. As these e x c i t e d molecules r e t u r n to the ground s t a t e they emit a c h a r a c t e r i s t i c fluorescence w i t h i n t e n s i t y p r o p o r t i o n a l to the con c e n t r a t i o n of S0 2 molecules i n the sample. The f l u o r e s c e d l i g h t then passes through a second f i l t e r to. a p h o t o m u l t i p l i e r d e t e c t i o n tube. The TECO S 0 2 Analyzer provides a 90% response i n 1.5 minutes at a sampling r a t e of 0.5 L/min, and a usable range of 1 ppb to 5000 ppb. C a l i b r a t i o n was done before each run usin g a zero-gas generator (pressure-swing molecular s i e v e ) and a permeation tube c a l i b r a t o r . Some (max. 5%) quenching of the e x c i t e d S 0 2 molecules takes place because of the presence of water molecules. This negative i n t e r f e r e n c e e f f e c t s both the r e a c t o r i n l e t and o u t l e t S0 2 c o n c e n t r a t i o n s to approximately the same degree and th e r e f o r e tends to be c a n c e l l e d out. Other i n s t r u m e n t a l procedures f o r l o w - l e v e l S 0 2 a n a l y s i s i n c l u d e microcoulometry and GC-FPD (gas chromatography w i t h flame-photometric d e t e c t i o n ) . Microcoulometry i s prone to i n t e r f e r e n c e s from any oxidants which may be introduced i n t o the r e a c t i o n system, whereas the a v a i l a b l e GC-FPD instrument was very S 0 2 - s p e c i f i c but was found to be "noisy" below 50 ppb S0 2. The r e l a t i v e humidity w i t h i n the r e a c t o r s was v a r i e d by c o n t r o l l i n g the r a t i o of the absolute pressure of the permeation-tube o v e n / h u m i d i f i c a t i o n system to the absolute pressure of the r e a c t o r s (measured by a manometer on the end of the ma n i f o l d ) . This form of humidity c o n t r o l , known as the Two-Pressure Method, was used by (25) Honeywell as a primary humidity standard (Amdur et a l . ' ) • The Honeywell system enclosed a l l apparatus w i t h i n temperature regulated Figure 5 APPARATUS FOR S02-LIMESTONE RXN KINETICS EXPTS. n i j i n Sample Port | TECO Model 43 A PULSED FLUORESCENT S0 2 ANALYZER (0-50 PPB) CALIBRAT. GAS Chart Recorder (Optional) 1 ZERO AIR MODULE 4 - 6 L/mln 80-t00\ R.H. Presure Control Valve 100% R.H. Rotameters (0-1 L/min) Needle Valves Teflon Reactors V7 Manometer A H20) -ED Back-Press Control (H2O Bubbler) MODIFIED METRONICS DYNACALIBRATOR jHUMIDIFICATION MODULE (PERMEATION OVEN) Figure 6 METRONICS DYNACALIBRATOR MODEL 450-53XQ MODIFICATION HUMIDIFICATION MODULE FM2 Supply Inlet ° » | PUMP | | ACCUMULATORf-Bypass Loop Scrubber 2 Molecular Sieve 5A Scrubber 1 Activated Charcoal Filter ^ 7 Blocked < Spanl Control Valve H><0—j I F C V 2 SVrWsolenold ^ Control Valve DPR2 ' 1 Differential Pressure Regulator Spanl Flowmeter "6.0" Flow -» Restriction Valve FMI ? PERMEATION OVEN Flowmeter T-27°C Figure 7 HUMIDIFICATION MODULE Temperature Gauge Circulating Pump (10-20 mL/mln.) Magnetic Stirrer/ Hot Plate Cooling Coil 1cm die. K 3m S.S. Demister , (Quartz Wool) Saturated Air ^ Stream T - 42 -water-baths. They were thus able to maintain uniform c o n d i t i o n s throughout t h e i r t e s t . The reacted p a r t i c l e s were analyzed f o r s u l p h i t e and sulphate by an o u t s i d e l a b o r a t o r y . T h e i r Dionex i o n chromatograph i s b a s i c a l l y an ion-exchange column followed by a l i q u i d - c o n d u c t i v i t y d e t e c t o r . D i f f e r e n t ions have unique e l u t i o n times which are a s c e r t a i n e d by a p p r o p r i a t e c a l i b r a t i o n procedures. The instrument has a lower l i m i t of d e t e c t i o n f o r S0^~ of 0.01 mg/L i n a 10 mL sample. This i s e q u i v a l e n t to 0.1 ug (100 ng) of S0~~. Other options f o r sulphate a n a l y s i s were barium sulphate t u r b i d i m e t r y w i t h a d e t e c t i o n l i m i t of 50 ug, Technicon c o l o r i m e t r y (5 ug), and a Leybold-Heraeus CSA302 Carbon/Sulphur Analyzer (200 ug). C l e a r l y the Dionex I.C. i s the most s e n s i t i v e instrument a v a i l a b l e f o r sulphate a n a l y s i s . For t h i s reason i t i s widely used i n a c i d r a i n r e l a t e d s t u d i e s . 2.2 Experimental Procedures 2.2.1 P a r t i c l e C h a r a c t e r i s t i c s The m a j o r i t y of the experiments were c a r r i e d out u s i n g a commercially a v a i l a b l e limestone - Texada I s l a n d limestone. Some experiments were a l s o done usi n g p r e c i p i t a t e d dolomite (London Drug L a b o r a t o r i e s , Winnipeg, Manitoba). The limestone was predominantly calcium carbonate (34.9% by weight calcium per Table 2 ) , whereas the p r e c i p i t a t e d dolomite contained 24.1% calcium and 11.5% magnesium (by weight). The limestone was analyzed by plasma emission spectroscopy to determine trace - 43 -TABLE 2 - TEXADA ISLAND LIMESTONE COMPOSITION (200-270 mesh" T y l e r ) A. A n a l y s i s by Plasma Emission Spectroscopy (ICAP) Trace Elements A r s e n i c As <100 Boron B <4. B e r y l l i u m Be <0.4 Bismuth B i <80 Cadmium Cd <2. Cobalt Co 5. Chromium Cr <2. Copper Cu 9. Mercury Hg" <40 Molybdenum Mo <10 N i c k e l Ni <4. Lead Pb <20 Antimony Sb <40 Selenium Se <40 Thorium Th <20 Uranium U <100 Vanadium V 3. Zinc Zn <4. R e s u l t s i n ug/g Major Elements Aluminum A l 880 Barium Ba <20 Calcium Ca 349000 Iro n Fe 122. Potassium K <2000 Lit h i u m L i <400 Magnesium Mg 2000 Manganese Mn 69. Sodium Na 31000 Phosphorus P <800 S i l i c o n S i 1100 Strontium Sr 753. Titanium T i <20 Zirconium Zr <40 R e s u l t s i n ug/g B. A n a l y s i s by Leybold-Heraeus CSA302 Carbon/Sulphur Analyzer Sulphur 790 ug/g i n t r i p l i c a t e Carbon 117,500 ug/g i n t r i p l i c a t e C. Bulk Density - (200-270 cut) 1.23 g/cm3_ D. S p e c i f i c Surface Area (B.E.T.) - 0.323 m /g. - 44 -i m p u r i t i e s , and a l s o by a Leybold - Heraeus Carbon/Sulphur Analyzer i n order to measure any sulphur i n t e r f e r e n c e . The s o l i d m a t e r i a l s were ground and then c l a s s i f i e d by dry screening. The f r a c t i o n i n the s i z e range of 270-200 mesh ( T y l e r ) , or 55-74 microns ( s i e v e opening), were r e t a i n e d f o r the experiments. A previous i n v e s t i g a t i o n had shown that t h i s p a r t i c l e s i z e range r e s u l t e d i n a reasonable compromise between p e r m e a b i l i t y , s u r f a c e area, and agglomeration e f f e c t s . S p e c i f i c s u r f a c e area was determined u s i n g a B.E.T. a n a l y s i s (This procedure i n c l u d e s a l l the micropores and cracks w i t h i n a p a r t i c l e which are a v a i l a b l e f o r chemical r e a c t i o n ) . A mixture of n i t r o g e n and helium i s passed across the powder at l i q u i d n i t r o g e n temperature. The amount of n i t r o g e n adsorbed i s then p r o p o r t i o n a l to the s u r f a c e area. D e t a i l s of t h i s procedure are included i n Appendix 6.3. The r e s u l t s of the B.E.T. s u r f a c e area analyses were: 2 Texada I s l a n d limestone - 0.323 m /g 2 London Drugs dolomite - 0.716 m /g 2.2.2 E x p l o r a t o r y Experiments E x p l o r a t o r y runs showed that there were severe problems w i t h S0 2 l o s s to the s u r f a c e of the apparatus and contamination of the l i m e s t o n e / f i l t e r sandwich w i t h r e a c t i o n products from the w a l l of the " i n e r t " T e f l o n r e a c t o r . Four p r e l i m i n a r y experiments were done usi n g Texada I s l a n d limestone (200-270 mesh) and concentrations of approximately 50 ppb S0 2 ( i n a i r ) . Flow r a t e s of 60, 600 and 1020 mL/min were i n v e s t i g a t e d w i t h samples taken at one, two and four hours i n t o each r e a c t i o n . The r e a c t i o n s were run at approximately 85% r e l a t i v e humidity and at ambient pressure and temperature. R e s u l t s are presented i n Table 3. - 45 -A l l four r e a c t i o n s shoved increased absorbance of S0» w i t h time (Table 3 ) . The t o t a l amount of SO^ r e t a i n e d was c a l c u l a t e d u s i n g the flow r a t e and the c o n c e n t r a t i o n d i f f e r e n c e ( C J - C q ) across the r e a c t o r , and was checked by i o n chromatography of the d i s t i l l e d water leachate of the f i l t e r s and CaCO^. The ion chromatography r e s u l t s were un c o r r e l a t e d w i t h the amount of Sf^ removed from the gas stream and hence were probably unduly a f f e c t e d by sampling and a n a l y t i c a l e r r o r s . (This i s discussed f u r t h e r i n the t e x t . ) I t was discovered that an empty (no f i l t e r s , no limestone) sample chamber system a l s o r e t a i n e d s i g n i f i c a n t amounts of SC^. Further experiments were done to i n v e s t i g a t e the extent of, and to attempt to e s t a b l i s h some c o r r e c t i o n f a c t o r s f o r , t h i s r e t e n t i o n of SC^. These r e s u l t s are presented i n Table 4. I n experiment No. 5 three hours of exposure to 50 ppb SO2 at 630 mL/min were needed f o r the T e f l o n apparatus to pass about 92% of the incoming S02* This s a t u r a t e d c o n d i t i o n d i d not l a s t long however, as a f t e r two days i d l e the apparatus again adsorbed SO2 and needed to be r e c o n d i t i o n e d (another four hours). When l e f t overnight running c o n t i n u o u s l y at 50 ppb and 630 mL/min the next day the SO2 passed had r i s e n to about 98%. An experiment (#6) was done to check the SO2 a b s o r p t i o n of the Z i t e x f i l t e r membranes. Between zero and four f i l t e r membranes were used i n each of the four p a r a l l e l r e a c t o r s . The same r e s u l t s were found as i n the other c o n d i t i o n i n g experiments ( i . e . 90% a f t e r three hours and 98% a f t e r o v e r n i g h t ) . The number of f i l t e r membranes w i t h i n the f i l t e r h older d i d not a f f e c t the amount of SO2 r e t a i n e d . Therefore the SO2 must be r e t a i n e d on the f i l t e r holder. The SO2 was then turned o f f and zero a i r run through the apparatus. The SO2 given o f f by the apparatus f e l l q u i c k l y ( i n approximately 38 minutes) to 2 ppb. This i m p l i e s that the mechanism f o r S0O r e t e n t i o n i s more of a r e a c t i o n on the surface of - 46 -TABLE 3 PRELIMINARY EXPERIMENTS EXPERIMENT # 1 2 3 4 Flow r a t e * 630 mL/min 630 mL/min 60 mL/min 1020 mL/min CaC0 3 BO mg 10 mg -10 mg -10 mg Temperature (°C) 18 17 17 17 R e l a t i v e Humidity 92% 92% 83% 91% 2_ pg SO^ r e t a i n e d IC TECO IC TECO TECO TECO 1 hour 23 7 4 4 0.6 5 2 hours 36 11 4 6 1.4 14 4 hours 20 19 4 7 2.7 17 6 hours 16 27 - - - -empty - - 34 - 2.6 -Notes; I.C. - Ion chromatography analyses of the l i m e s t o n e / f i l t e r sandwich. TECO - SO2 co n c e n t r a t i o n d i f f e r e n c e as measured by the TECO S0 9 Analyzer. * 60 mL/min r e s u l t s i n an i n t e r s t i t i a l v e l o c i t y and p a r t i c l e Reynolds Number of 0.08 cm s~* and 0.004, r e s p e c t i v e l y . 1020 mL/min r e s u l t s i n an i n t e r s t i t i a l v e l o c i t y and p a r t i c l e Re of 1.7 cm s~* and 0.08, r e s p e c t i v e l y . - 47 -the T e f l o n than an absorption i n t o i t . A f t e r being flushed w i t h zero a i r f o r approximately three hours the SO^ was reconnected and immediately the (SO^) passed was 91% and rose to 96% i n Vh hours and to 100% when l e f t running overnight. No s i g n i f i c a n t d i f f e r e n c e was noted when the f i l t e r s were removed. I t was concluded that the f i l t e r s do not r e t a i n a s i g n i f i c a n t amount of S02> Even though both the f i l t e r s and the f i l t e r holder were made from T e f l o n , there must be a s i g n i f i c a n t d i f f e r e n c e i n the r e a c t i v i t y of d i f f e r e n t types of T e f l o n . Experiment No. 8 used n i t r o g e n as an SQ 2 d i l u e n t r a t h e r than a i r s i n c e i t was b e l i e v e d that the SO2 r e t e n t i o n was due to a s u r f a c e c a t a l y z e d o x i d a t i o n of S02 to SO^. The r e s u l t s (Table 4) show that S02 r e t e n t i o n q u i c k l y , but not immediately, drops to a low value. Therefore absorption/permeation and s u r f a c e o x i d a t i o n are both s i g n i f i c a n t SO2 l o s s mechanisms, w i t h surface o x i d a t i o n being the dominant one ( i . e . compare experiments #5 and #6 w i t h #8). Based on these r e s u l t s the experimental procedure used (as discussed i n S e c t i o n 2.2.3) was to c o n d i t i o n the T e f l o n r e a c t o r s , and a s s o c i a t e d plumbing, f o r at l e a s t 24 hours us i n g the same gas phase mixture as used du r i n g the subsequent experiments. As a f u r t h e r p r e c a u t i o n the four r e a c t o r s were operated i n p a i r s , w i t h an empty r e a c t o r always being used to o b t a i n a blank c o r r e c t i o n f o r the a c t i v e f i l t e r . Experiments w i t h SO2 a d s o r p t i o n and d e s o r p t i o n on v a r i o u s f i l t e r media by Byers et a l . ^ 2 6 ^ a l s o found problems with SO2 l o s s on the f i l t e r h o l d e r s . They concluded that a l l the m a t e r i a l s s t u d i e d ( T e n i t e p l a s t i c , T e f l o n , s t a i n l e s s s t e e l , and aluminum) would be s a t i s f a c t o r y f o r sampling SO2 i f a long c o n d i t i o n i n g time i s not o b j e c t i o n a b l e . For short sampling times and low peak concentrations a p l a s t i c holder w i t h a membrane f i l t e r was recommended. - 48 -TABLE 4 TEFLON RETENTION OF so 2 EXPERIMENT # 5 6 7A 7B 8 Co n d i t i o n s : No F i l t e r s 0-4 F i l t e r s 0-4 F i l t e r s 0-4 F i l t e r s 0-4 F i l t e r s Time % so 2 passed Flushed w i t h zero a i r , SO, outgassing A f t e r zero a i r f l u s h , X SO2 passed N 2 matrix X SO2 passed 5 min. 34 60 7 ppb 91 75% 30 66 75 3 98 94 60 74 83 1 96 98 90 74 89 0 96 93 120 80 84 0 - 98 150 94 80 0 - 98 180 96 90 0 - -240 98 98 - - -1100 - - - 100 -1260 - 98 - - -Notes: A l l experiments run at 630 mL/min, room temperature, ambient pressure, S0 9 ~ 50 ppb, R.H. ~ 85%. - 49 -The r e s u l t s of the ion chromatograph analyses i n d i c a t e no c o r r e l a t i o n between the measured sulphate i n the l i m e s t o n e / f i l t e r sandwich and the e q u i v a l e n t gas phase SC^ l o s t as determined by TECO analyses. Figure 8 presents a s c a t t e r diagram comparing the two independent methods of sulphate a n a l y s i s . Although extreme precautions were taken to preclude contamination, the problem of apparent sulphate contamination of the "sandwich" was not r e s o l v e d . The s c a t t e r can be a t t r i b u t e d to s e v e r a l f a c t o r s : l i m e s t o n e / f i l t e r contamination from the r e a c t i o n products on the s u r f a c e of the T e f l o n r e a c t o r (as p r e v i o u s l y d i s c u s s e d ) , contamination from handling, a n a l y t i c a l e r r o r s (both I.C. and TECO), and from the sulphur present i n the unreacted limestone. The l a t t e r source would depend upon how much of the limestone sample d i s s o l v e d during the d i s t i l l e d water l e a c h i n g operation and t h e r e f o r e should be l o o s e l y c o r r e l a t e d w i t h the amount of limestone which reacted. Since no such c o r r e l a t i o n i s obvious i n Figure 8 we conclude that the e r r o r s are random and are due to problems w i t h the i o n chromatograph. (The I.C. analyses were done by an o u t s i d e l a b o r a t o r y . ) The i o n chromatograph r e s u l t s were ther e f o r e not used i n computing the r e a c t i o n r a t e constants. 2.2.3 Run Procedure E x p l o r a t o r y experiments (presented i n S e c t i o n 2.2.2) had shown that the task of measuring the r e a c t i o n r a t e between a l k a l i n e p a r t i c u l a t e , and a very humid a i r stream c o n t a i n i n g 50 ppb of sulphur d i o x i d e , was extremely d i f f i c u l t . Under c o n d i t i o n s of high humidity the sulphur d i o x i d e adsorbed onto n e a r l y a l l s u r f a c e s . There i t e i t h e r reacted on the s u r f a c e or permeated i n t o the m a t e r i a l ( i f i t was T e f l o n ) . This behaviour was s t u d i e d by running a s e r i e s of experiments u s i n g empty r e a c t o r s or r e a c t o r s c o n t a i n i n g v a r y i n g numbers of f i l t e r s . The time dependent change i n SO2 c o n c e n t r a t i o n across the r e a c t o r s , as measured by the TECO, showed that the SO2 l o s s decreased to an acceptable l e v e l (0-10%) a f t e r approximately 24 hours. - 50 -FIGURE 8 SULPHATE ANALYSIS SCATTERGRAM 30-1 ©- Experiment number ® © ® 10, T " S 20) Tr-io IS © © - r -20 —r-25 -1 30 MICROGRAMS SO. (l.C.) - 51 -S i m i l a r l y , experiments with the h u m i d i f i c a t i o n module showed that i t took an extended period of operation f o r i t to " l i n e - o u t " to some semblance of a stea d y - s t a t e c o n d i t i o n . A s i g n i f i c a n t improvement i n performance was obtained by changing to forced-convection heat t r a n s f e r on the c o o l i n g c o i l ; t h i s m o d i f i c a t i o n dampened the f l u c t u a t i o n s i n temperature caused by changes i n l a b o r a t o r y a i r movement. I d e a l l y , the e n t i r e apparatus would have been i n s t a l l e d i n a l a r g e , constant temperature bath. The run procedure which evolved from .the i n i t i a l experiments and which gave the most c o n s i s t e n t r e s u l t s was as f o l l o w s : 1. The system was conditioned w i t h the d e s i r e d c o n c e n t r a t i o n of S0 2 and humidity by passing the gas through the empty (without p a r t i c l e s or f i l t e r s ) r e a c t o r s f o r a period of approximately 3 days. Steady-state o p e r a t i o n was confirmed by a n a l y z i n g the SOj co n c e n t r a t i o n which e x i t s from an empty r e a c t o r ; t h i s c o n c e n t r a t i o n should be i d e n t i c a l to that of the SOj which e x i t s from the r e a c t o r bypass l i n e ( F i g u r e 5 ) . 2. The s h u t - o f f values to the T e f l o n r e a c t o r s were then close d so that a l l of the flow passed through the bypass l i n e to the TECO and through the bubbler. 3. The four r e a c t o r s were then d i v i d e d i n t o p a i r s (e.g. A to C, and B to D) and each p a i r was subsequently t r e a t e d i d e n t i c a l l y w i t h one r e a c t o r h o l d i n g limestone and the other r e a c t o r being run as an empty reference. 4. The empty r e a c t o r s were disconnected i n p a i r s , and weighed: (a) bottom h a l f , (b) (a) plus f i l t e r 1, (c) (b) plus powder (approximately 10 mg), (d) (c) plus f i l t e r 2. - 52 -Care was taken to in s u r e that the powdered reactant was evenly spread over the surface of the T e f l o n membrane f i l t e r ( Z i t e x ) and that the m a t e r i a l and the f i l t e r s were not touched. The r e a c t o r s were then reassembled and reconnected to the manifold, but w i t h the s h u t - o f f valves s t i l l c l o s e d . 5. Two d r y i n g tubes were f i l l e d w i t h D r i e r i t e and s i l i c a - g e l , t ared, and then connected i n s e r i e s . These tubes were used to g r a v i m e t r i c a l l y determine the absolute humidity of the a i r stream which flowed through the r e a c t o r s . 6. A f t e r a l l p reparations were completed the experimental run was commenced by f u l l y opening the r e a c t o r s h u t - o f f valves and by a d j u s t i n g the flow r a t e s using the c a l i b r a t e d rotameters. 7. Approximately every 15 minutes the r e a c t o r e x i t S 0 2 concentrations were monitored by connecting the ap p r o p r i a t e T e f l o n l i n e to the sampling port of the TECO S0 2 Analyzer. The r e a c t o r s were sampled i n a l t e r n a t e order (eg. ACBDACBD—). For a r e a c t o r flow r a t e of l e s s than 0.5 L/min i t was necessary to c o l l e c t the e x i t flow i n a p l a s t i c sample bag (approx. 2% l i t e r s ) , s i n c e the TECO S0 2 analyzer r e q u i r e s a continuous flow of at l e a s t 0.5 L/min. When the bag was f u l l (40 minutes @ 60 mL/min) the bag was then connected to the sampling port of the a n a l y z e r . 8. Approximately every hour the r e a c t o r humidity was monitored by connecting the tared drying-tubes to the r e a c t o r b y p a s s - l i n e and a d j u s t i n g the humid a i r flow w i t h a c a l i b r a t e d rotameter. This a i r stream was d r i e d f o r about 45 minutes, and then the weight gain of the drying-tubes was determined. Simultaneously, the l a b o r a t o r y a i r temperature was measured. The r e l a t i v e humidity was then computed as: R.H. = 100 (h/h*) - 53 -where h i s the absolute humidity, c a l c u l a t e d by d i v i d i n g the weight g a i n by the volume of a i r passing through the d r i e r , and h* i s the s a t u r a t i o n humidity, obtained from a psychrometric t a b l e , corresponding to the ambient room temperature. 9. A f t e r the d e s i r e d r e a c t i o n time had elapsed, the f i r s t r e a c t o r p a i r was valved o f f , disconnected, disassembled, weighed, and the limestone "sandwich" removed f o r a n a l y s i s . The empty r e a c t o r s were then reassembled, reconnected, and exposed to the gas stream i n order to be c o n d i t i o n e d f o r the next experiment. 10. The limestone sandwich (reacted limestone powder sandwiched between two 47 mm T e f l o n [ Z i t e x ] f i l t e r membranes) was t r a n s f e r r e d to a c l e a n beaker f o r a n a l y s i s . ( A l l glass-ware, e t c . was cleaned by s e q u e n t i a l l y washing w i t h acetone, d i s t i l l e d water, n i t r i c a c i d , and d i s t i l l e d water, and then d r y i n g i n an oven.) The sandwich was soaked overnight i n 20.0 mL of d i s t i l l e d water. The beaker was covered w i t h P a r a f i l m to minimize contamination. The next day the beaker was sonnicated f o r one hour, and the l i q u i d was then f i l t e r e d i n t o an acid-washed p l a s t i c b o t t l e u s i n g a s y r i n g e f i t t e d w i t h a 0.45 micron M i l l i p o r e f i l t e r . The samples, and blanks, were analyzed by an o u t s i d e l a b o r a t o r y f o r s u l p h i t e / s u l p h a t e w i t h a Dionex ion-chromatograph. Selected samples were a l s o analyzed f o r calcium v i a AAS (atomic a b s o r p t i o n spectrophotometry) i n order to determine whether a l l of the s u l p h i t e / s u l p h a t e product was removed from the p a r t i c l e s . I t was assumed that i f the calcium to ( s u l p h i t e and sulphate) r a t i o was much g r e a t e r (10 times) than s t o i c h i o m e t r i c then i t would be safe to say that a l l of the product was recovered, s i n c e the s o l u b i l i t y of the product i s g r e a t e r then that of the parent calcium carbonate. - 54 -2.3 Limestone D i s s o l u t i o n Experiments Experiments were a l s o conducted i n order to measure the maximum d i s s o l u t i o n r a t e of the 200-270 Mesh ( T y l e r ) limestone p a r t i c l e s i n a w e l l mixed, s l i g h t l y a c i d i c , aqueous environment. T h i s i n f o r m a t i o n w i l l be used i n S e c t i o n 5.0 where the r a t e c o n t r o l l i n g step i n the o v e r a l l r e a c t i o n , l i m e s t o n e - p a r t i c l e s and humid a i r and S0 2, i s determined. Procedure Approximately 2-1/2 times the s t o i c h i o m e t r i c amount of ground-up (200-270 mesh) limestone was added to d i l u t e s u l p h u r i c a c i d (pH - 2.8) and s t i r r e d . The r e s u l t i n g increase i n pH w i t h respect to time was recorded. Auto S t i r 1. An H^SO^ stock, s o l u t i o n of 2.80 pH was made up by d i l u t i n g concentrated H^SO^. 2. I n t o 100 ml a l i q u o t s , i n 150 ml beakers, were added: #1 - 20 mg Texada I s l a n d limestone of 200 - 270 mesh #2 - 10 mg of same #3 - 20 mg of dolomite of 200 - 270 mesh 3. The contents i n each beaker were s l o w l y s t i r r e d ( s e t t i n g 2, Magnestir) w h i l e the pH was recorded every 5-10 minutes us i n g a Digi-phase pH meter, f o r a period of 1-1/4 hours. 4. The pH values of the 3 samples were again measured 4 days l a t e r . 5. The pH values of the 3 samples were again measured a f t e r 30 minutes treatment i n an u l t r a s o n i c bath (Haver). - 55 -U l t r a s o n i c Bath Treatment 1. 20 mg p o r t i o n s of the same limestone and dolomite were placed s e p a r a t e l y i n 100 mL of the same stock s o l u t i o n and placed i n the Haver u l t r a s o n i c bath. 2. The pH of each sample was again taken every 5-10 minutes f o r a period of approximately 2 hours. The beakers were s w i r l e d 3 times d u r i n g the 2 hours to ensure mixing. - 56 -3.0 RESEARCH RESULTS A t o t a l .of 23 experimental runs were made i n order to measure the o v e r a l l heterogeneous r e a c t i o n r a t e between limestone p a r t i c l e s and S0 2 i n humid a i r . Some of these were f o r purposes of developing the procedure (these " e x p l o r a t o r y " experiments were discussed i n Sec t i o n 2.2.2). 3.1 Data Reduction As p r e v i o u s l y d e s c r i b e d , the con c e n t r a t i o n of S 0 2 e n t e r i n g and l e a v i n g the r e a c t o r s were measured w i t h a TECO pulsed f l u o r e s c e n t S 0 2 a n a l y z e r . I f the r e a c t o r i n l e t c o n c e n t r a t i o n ( r e f e r to Figure 5) d i f f e r e d s i g n i f i c a n t l y (>1 ppb) from the o u t l e t c o n c e n t r a t i o n of the empty reference r e a c t o r ( s ) , then S0 2 was being l o s t to the r e a c t o r body ( T e f l o n f i l t e r housing). I t was assumed that t h i s l o s s (S) was e q u a l l y d i s t r i b u t e d between the f r o n t h a l f and the back h a l f of the r e a c t o r . Therefore the co r r e c t e d i n l e t and o u t l e t c o n c e n t r a t i o n s were computed r e s p e c t i v e l y as: These c o r r e c t e d values f o r co n c e n t r a t i o n were then s u b s t i t u t e d i n t o equation 28 i n order to determine a p s e u d o - f i r s t - o r d e r r a t e constant: Cj ( c o r r e c t e d ) = (raw) - % 6 C ( c o r r e c t e d ) « C (raw) + % S o o (29) (30) 0.0036 Q (C.-C ) k = m h -1 (28) WSC o - 57 -3 -1 where: Q = v o l u m e t r i c f l o w - r a t e through the r e a c t o r cm s W = mass of p a r t i c u l a t e s i n the r e a c t o r g 2 -1 S = B.E.T. s p e c i f i c s urface area of the p a r t i c u l a t e s m g The c o r r e c t e d values f o r the i n l e t and o u t l e t S0 9 c o n c e n t r a t i o n can a l s o be used to compute a d e p o s i t i o n v e l o c i t y V. (cm s ) f o r SO, d e p o s i t i n g - 2 - 1 onto limestone p a r t i c l e s . The f l u x , F, (mol m h ) of S0 2 to the s u r f a c e i s given by 0.0036Q(C.-C ) p <31> VS' where: S' = the s p e c i f i c s urface area based on the "macroscopic" e x t e r n a l area of the p a r t i c l e s as was described i n S e c t i o n 1.4. (By convention V^, which i s a measure of the r a t e of d e p o s i t i o n of an a i r p o l l u t a n t to a " s i n k " , i s based upon the macroscopic area of a s u r f a c e , as opposed to the microscopic p a r t i c l e area which i n c l u d e s p a r t i c l e cracks and pores and which i s normally used i n the study of heterogeneous r e a c t i o n k i n e t i c s . ) I f we base the p a r t i c l e area on that of spheres w i t h the same diameter, then S' may be computed: ' 6 2 - 1 S = _ 2 _ ( m z g ) (32) D p P P where: D = mass (volume) averaged diameter of p a r t i c l e s (m) P _3 p = p a r t i c l e d e n s i t y (gm ) - 58 -P r e v i o u s l y we defined a shape f a c t o r , y, as the r a t i o between the microscopic s p e c i f i c area and the macroscopic s p e c i f i c area. Y = — (33) S Combining equations 28, 31, and 33 y i e l d s the f o l l o w i n g expression f o r the d e p o s i t i o n f l u x , F, i n terms of the p s e u d o - f i r s t - o r d e r r a t e constant, k: F . YC 0k, mol m~2 h " 1 (34) By combining equations 15 and 34 the d e p o s i t i o n v e l o c i t y , V\j, (cm s -*) can now be computed as V, = — (35) ° 36 The S(>2 c o l l i s i o n e f f i c i e n c y , which was defined as the f r a c t i o n of the S(>2 molecules which bombard a p a r t i c l e s urface and " s t i c k " , was derived as: 0 = — — (18) 7800 By s u b s t i t u t i n g equation 35 i n t o equation 18 the c o l l i s i o n e f f i c i e n c y can be computed from the value of k, the p s e u d o - f i r s t - o r d e r r a t e constant: 0 = 3.6 x 10 6 k (36) - 59 -Example C a l c u l a t i o n A subset ( t = 30 min) of the data from the 420 min. run of experiment 16 w i l l be used i n order to i l l u s t r a t e the above computations. A r e a c t o r c o n t a i n i n g 9.8 mg of 200-270 mesh ( T y l e r ) Texada I s l a n d limestone p a r t i c l e s was exposed to 630 mL/min of humid (R.H. = 85%) a i r at 17°C c o n t a i n i n g 42 ppb of S02« The e x i t SO2 c o n c e n t r a t i o n from the r e a c t o r was 34 ppb, wh i l e that from a p a r a l l e l , but empty, "dummy" r e a c t o r was 40 ppb. A B.E.T. a n a l y s i s of the p a r t i c l e s i n d i c a t e d a s p e c i f i c surface 2 -1 area, S, of 0.323 m g Using equations 29 and 30 to c o r r e c t the data: (42-40) C. ( c o r r e c t e d ) = 42 = 41 ppb 2 (42-40) C ( c o r r e c t e d ) = 34 + = 35 ppb 0 2 (These con c e n t r a t i o n s can be expressed as ppb s i n c e equation 28 uses a r a t i o of co n c e n t r a t i o n s . ) From equation 28 the p s e u d o - f i r s t - o r d e r r a t e constant, at 30 minutes, i s computed: . 0.0036 x 630/60 x (41-35) „ n .-1 k. = 5 = 2.0 m h 9.8 x 10~3 x 0.323 x 35 Next, the e f f e c t i v e e x t e r n a l s p e c i f i c s urface area, S', of the p a r t i c l e s i s computed usi n g equation 32: S' = (65.4 x 10 6 ) x (2.9 x 10") = 0.032 m2 g 1 - 60 -The shape f a c t o r , y» i s then computed usi n g equation 33: 0.323 1 f t  Y " 6^032 = 1 0 The d e p o s i t i o n v e l o c i t y , V^, can now be computed usi n g equation 35: „ 10. x 2.0 n K , -1 = ^ = 0.57 cm s F i n a l l y , the c o l l i s i o n e f f i c i e n c y , 0, i s computed usi n g equation 36: 0 = 3.6 x 10~ 6 x 2.0 = 7.2 x 10" 6 3.2 Summary of R e s u l t s Because of the l a r g e amount of raw data generated d u r i n g the experiments only run summaries are presented i n t h i s r e p o r t ; they are i n c l u d e d as Appendix 6.4. Selected experimental r e s u l t s are f u r t h e r summarized i n Table 5. This data base i s used to evaluate the e f f e c t of S0 2 c o n c e n t r a t i o n , the humidity, and the r e a c t i o n product accumulation upon the value of the p s e u d o - f i r s t - o r d e r r a t e constant. For each experimental run r e p r e s e n t a t i v e k i n e t i c data are shown f o r d i f f e r e n t times during the run. However, f o r those runs where only the average humidity over the e n t i r e experiment was a v a i l a b l e , then only the run-averaged k i n e t i c data i s l i s t e d (as determined from the average c o r r e c t e d i n l e t and o u t l e t r e a c t o r S0 2 c o n c e n t r a t i o n s ) . The r e s u l t s of some experiments were not included i n the data base of Table 5 due to experimental d i f f i c u l t i e s and hence data u n r e l i a b i l i t y . A l l experiments (3, 11, 20, 23) which were conducted at a very low flow r a t e (0.060 L/min) experienced excessive w a l l l o s s e s (ca.50%) of the S0 2 w i t h i n the sampling bags. (The TECO S0 2 a n a l y z e r r e q u i r e s a flow of 0.5 L/min, t h e r e f o r e at a r e a c t o r flow of only 0.060 L/min the sample f i r s t had to be accumulated w i t h i n a 2Hi l i t e r p l a s t i c ( T e d l a r ) sampling - 61 -TABLE 5 - SELECTED EXPERIMENTAL RESULTS EXPERIMENT RUN TIME HUMIDITY FLOW REACTOR k vd 0xlO 6 PARTICULATE NUMBER (•In) (X) (L/ain) S02(ppb) (a/h) (cm/s) 1 75 96 0.63 16 1.5 0.42 5.4 Limestone 1 150 82 0.63 20 1.2 0.34 4.4 Limestone 1 315 73 0.63 24 0.83 0.23 3.0 Limestone 2 15 100 0.63 16 7.4 2.1 27. Limestone 2 45 100 0.63 25 4.8 1.3 17. Limestone 2 195 71 0.63 39 0.26 0.073 0.94 Limestone 10 run ave. 83 1.02 57 0.77 0.22 2.8 Limestone 12 run ave. 71 0.63 26 0.78 0.21 2.8 Limestone 13 run ave. 92 0.63 58 1.2 0.33 4.3 Limestone 14 run ave. 96 0.63 42 1.9 0.53 6.8 Limestone 15 run ave. 83 0.63 73 0.69 0.19 2.5 Limestone 16 30 85 0.63 35 2.0 0.57 7.2 Limestone 16 240 94 0.63 30 4.8 1.3 17. Limestone 16 420 92 0.63 32 3.4 0.94 12. Limestone 17 30 98 0.63 29 3.6 1.0 13. Limestone 17 240 85 0.63 38 1.0 0.28 3.6 Limestone 18 90 95 0.63 31 5.2 1.4 19 Limestone 19 30 97 1.02 46 2.7 0.75 9.7 Limestone 19 210 90 1.02 49 1.60 0.44 5.8 Limestone 19 330 90 1.02 51 0.91 0.25 3.3 Limestone 21 run ave. 91 0.63 34 2.7 1.7 9.7 Dolomite 22 run ave. 83 1.02 45 2.5 1.6 9.0 Dolomite Notes on Table 5 - Representative data, taken at different times during each experimental run, are shown in Table 5. For those runs where only the average humidity, over the entire run, was determined then run-averaged kinetic data is listed. - 62 -bag.) S i m i l a r i l y , experiments (4, 9) v h e r e i n the r e a c t o r s were not preconditioned and where the concentrations could not be c o r r e c t e d v i a reference r e a c t o r s , were a l s o excluded. The remaining experimental data, as presented i n Table 5, are thought to be reasonably r e l i a b l e i n l i g h t of the experimental d i f f i c u l t i e s encountered when d e a l i n g w i t h ppb (23) l e v e l s of i n humid a i r . Only Berresheim et a l . p r e v i o u s l y attempted to examine - a e r o s o l r e a c t i o n s under these c o n d i t i o n s . They were forced to l i m i t t h e i r r e a c t o r humidity to 94% otherwise t h e i r w a l l l o s s e s became excessive. 3.3 E f f e c t of Reactor Humidity An examination of the data i n Table 5 i n d i c a t e s a s t r o n g r e l a t i o n between r e a c t o r humidity and the magnitude of the p s e u d o - f i r s t - o r d e r r a t e constant, k. This i s s i m i l a r to the humidity e f f e c t observed by other researchers. (22) K l i n g s p o r et a l . derived an expression f o r the a d s o r p t i o n of water vapour onto limestone p a r t i c l e s . They showed that the number of monolayers of water increased r a p i d l y above a r e l a t i v e humidity of about 80%. At high humidity l e v e l s t h e i r expression reduces to an "adsorption isotherm" of the form: where: V y = k x / <k 2 - R.H.) (37) _2 V w = s u r f a c e water adsorbed per u n i t area (mg m ) k j , k 2 = constants I f i t i s assumed that an aqueous phase r e a c t i o n c o n t r o l s the o v e r a l l r e a c t i o n r a t e , r , then the p s e u d o - f i r s t - o r d e r r e a c t i o n r a t e (equation 12) becomes: r = k' A (S0 2) f x (R.H.), (38) - 63 -where: -1 -3 r = r e a c t i o n r a t e of SO™ mol h m z _l k' = r a t e constant m h 2 -3 A = surface area m m _3 (SO2) = SO2 c o n c e n t r a t i o n mol m fx = ( k 2 - R.H.)" 1 d i mensionless Equating equations (12) and (38): r = k A (S0 2) = k' A ( S 0 2 ) f j y i e l d s k = k' f x (39) The values of the constants k' and k 2 ( i n f^) were obtained by usi n g the data base of Table 5 i n an i t e r a t i v e procedure - a value of k 2 was assumed and f ^ was computed f o r each experimental data p o i n t . A l i n e a r r e g r e s s i o n ( l e a s t mean squares curve f i t t i n g ) of k versus f ^ then y i e l d e d k'. The curve was forced through the o r i g i n (per equation 39). This procedure was repeated u n t i l the best f i t was obtained (the c o r r e l a t i o n c o e f f i c i e n t was maximized). The best f i t ( c o r r e l a t i o n c o e f f i c i e n t = 0.92) was obtained w i t h the f o l l o w i n g values f o r k' and k2: k' = 0.248 m h " 1 k 2 = 1.04 Therefore equation (39) f o r the p s e u d o - f i r s t - o r d e r r a t e constant becomes: k = 0.248 (40) 1.04 - R.H. - 64 -Equation 40 i s p l o t t e d i n Figure 9 along w i t h the data p o i n t s from Table 5. ( A l s o shown f o r i n t e r e s t , but not i n c l u d e d i n the a n a l y s i s , are r e p r e s e n t a t i v e values f o r d o l o m i t i c limestone.) Considering the experimental d i f f i c u l t i e s encountered i n o b t a i n i n g the data the 2 c o r r e l a t i o n ( r = 0.92) i s q u i t e good. T y p i c a l e r r o r bars are shown; these w i l l be f u r t h e r discussed i n S e c t i o n 3.7. I f the a d s o r p t i o n isotherm of equation 37 i s d i r e c t l y p r o p o r t i o n a l to the volume of water which e x i s t s on the surface of the p a r t i c l e s at high humidity l e v e l s then, s i n c e the S 0 2 - p a r t i c l e r e a c t i o n r a t e i s d i r e c t l y p r o p o r t i o n a l to t h i s isotherm per equation 40, i t must be concluded that the r e a c t i o n r a t e i s p r o p o r t i o n a l to the volume of water present on the p a r t i c l e s . This c o n c l u s i o n would i n d i c a t e that some s o r t of aqueous phase phenomena i s the o v e r - a l l r a t e c o n t r o l l i n g step. 3.4 E f f e c t o f S 0 2 Concentration The data were a l s o examined to determine whether or not the assumption of a p s e u d o - f i r s t - o r d e r r e a c t i o n , w i t h respect to S0 2 c o n c e n t r a t i o n , was j u s t i f i a b l e . I f n i s the r e a c t i o n order, then r = k A (S0 2) = k' f j A ( S 0 2 ) n , (41) = (SO,,)"" 1 k' f x and l o g k' - l o g ( k f ^ 1 ) = (1-n) l o g (S0 2) (42) Equation (42) was used i n conjunction w i t h the data set of Table 5 to s t a t i s t i c a l l y determine the best value of the r e a c t i o n order 'n'. Figure 10 shows a p l o t of - l o g (k f ^ 1 ) versus l o g ( S 0 2 ) . The best f i t ( c o r r e l a t i o n c o e f f i c i e n t = 0.14) i s f o r a slope of 0.18 or n = 0.82. While such a low c o r r e l a t i o n c o e f f i c i e n t (0.14) i s not s t a t i s t i c a l l y s i g n i f i c a n t i t i s assumed that the r e a c t i o n i s approximately f i r s t - o r d e r - 65 -o a 3 UJ (A a o 5 o ce UJ o M GC 3H 2H © © - Calcific Limaaton* [~n~| -Dolomdtk: Lima-tone a - Experiment number Over bar - Run average Accuracy (± 20") •atlmatet shown .( 0 2 5 y \104 -RH J 22 •Bt 4 y -® © © —r-85 —T-70 75 80 90 95 I 100 RELATIVE HUMIDITY (l00 R.H.) FIGURE 9 LIMESTONE/SULPHUR DIOXIDE REACTION RATE VERSUS RELATIVE HUMIDITY - 66 -(n=l) w i t h respect to the S0 2 c o n c e n t r a t i o n . Other researchers have a l s o concluded that the o v e r - a l l r e a c t i o n i s approximately f i r s t - o r d e r w i t h respect to S0 9 c o n c e n t r a t i o n . I f the r e a c t i o n products (Ca S0 3 • % H 20 and Ca SO^ • 2 H 20) and the i n s o l u b l e i n e r t s formed a dense product l a y e r which l i m i t e d the o v e r a l l r e a c t i o n r a t e , then we would expect the r e a c t i o n r a t e to be a strong f u n c t i o n of the r e a c t i o n extent. The p s e u d o - f i r s t - o r d e r r a t e constant would then have to be modified to i n c o r p o r a t e the e f f e c t s of p a r t i c l e "ageing". F o l l o w i n g a procedure s i m i l a r to that used to examine the humidity f u n c t i o n ( f ^ ) we can assume an ageing f u n c t i o n , f 2 ( q ) , where q i s the amount of product formed per u n i t mass of p a r t i c l e and i s t h e r e f o r e p r o p o r t i o n a l to the degree of limestone u t i l i z a t i o n . 3.5 E f f e c t of Reaction Extent L e t : r=kA(S0 2) = k'A(S0 2) f± (R.H.) f 2 ( q ) , ( 4 3 ) Then: ( 4 4 ) I f k f^ i s p l o t t e d against q then the form of f 2 , i f f 2 i s s i g n i f i c a n t , should become obvious. Now d e f i n e q as the t o t a l product formed i . e . o (45) where F' i s the r a t e of product formation 'ug S0 4' jng-min. F' = 1.6 x 10 J kC S o ( 4 6 ) - 6 7 -NORMALIZED REACTION Vs. S02 CONCENTRATION - 68 -Equation 45 can be i n t e g r a t e d n u m e r i c a l l y u s i n g the Trapezoid r u l e to o b t a i n N (47) i = l where q^ = q (t=o) = 0 At = r e a c t i o n i n t e r v a l (minutes) N = t o t a l number of time i n t e r v a l s The data base of Table 5 (run averages excluded) was used i n conjunction w i t h equation (47) to generate Figure 11. This graph i n d i c a t e s that the amount of r e a c t i o n product (q) does not s t r o n g l y i n f l u e n c e the humidity normalized r e a c t i o n r a t e ( k f ^ - ). A l i n e a r r e g r e s s i o n of the data y i e l d s a c o r r e l a t i o n c o e f f i c i e n t of only 0.27. _3 For a t y p i c a l sulphate production r a t e (F') of 3x10 ug S0^ per mg limestone per minute the degree of limestone conversion i s c a l c u l a t e d to be only about 0.1% a f t e r 6 hours. This corresponds to a sulphate surface l a y e r , f o r an average 60 um diameter sphere, of only 0.01 um (100A) t h i c k . Apparently t h i s i s not enough to s i g n i f i c a n t l y decrease the o v e r a l l r e a c t i o n r a t e of sulphur d i o x i d e to r e a c t i o n product, at l e a s t d u r i n g the 6-7 hour experimental runs. This r e s u l t can be compared w i t h that of Mamane et a l . who used scanning e l e c t r o n microscopy, equipped w i t h an X-ray energy d i s p e r s i v e a n a l y z e r , to analyze l a r g e (>5 ym) a i r b o r n e p a r t i c l e s which were c o l l e c t e d at a r u r a l s i t e i n eastern United S t a t e s . They found that mineral p a r t i c l e s of 15 ym diameter had a f a i r l y uniform sulphate l a y e r of about 0.2 ym t h i c k n e s s . The sulphate was found to be a s s o c i a t e d with c a l c i t e and c l a y minerals i n s i g n i f i c a n t amounts, about 1.5-3% of the 2_ p a r t i c l e mass, or an average of 0.02 g SO, /g s o l i d . C a l c i t e s were - 69 -Experiment number Best fit curve (r2*0 27) ^ Slope«OH © __ © _ _ * @© © © © T 1 1 1 1 r •1 -2 -3 -4 -5 -6 REACTION PRODUCT(ffL°ie8tone) FIGURE 11 NORMALIZED REACTION RATE Vs. REACTION PRODUCT - 70 -found to have more elemental sulphur than c l a y s , by almost a f a c t o r of two. Newly emitted p a r t i c l e s , and p a r t i c l e s such as q u a r t z , d i d not show sulphate enrichment. The authors point out the f a c t that l a r g e p a r t i c l e s (>5 ym) remain i n the atmosphere f o r a much longer d u r a t i o n then t h e i r Stokes' Law s e t t l i n g v e l o c i t y would suggest, and have been observed to t r a v e l d i s t a n c e s of hundreds and thousands.of k i l o m e t e r s (without resuspension). The l a r g e r degree of sulphate conversion measured by Mamane et a l . may t h e r e f o r e be a t t r i b u t a b l e to a l a r g e r r e a c t i o n time as w e l l as due to a f a s t e r r e a c t i o n r a t e r e s u l t i n g from the presence of powerful oxidants i n the atmosphere such as ozone, hydrogen peroxide, hydroxyl and hydroperoxyl f r e e r a d i c a l s , e t c . (Berresheim et a l / 2 3 * ) . 3.6 E f f e c t o f Temperature The e f f e c t of temperature on r e a c t i o n r a t e was not i n v e s t i g a t e d . N e s b i t t et a l / 2 0 * presented an a c t i v a t i o n energy of 54 kJ/mol, whereas (22) K l i n g s p o r et a l . ' report a value of about 23 kJ/mol. In general the most s i g n i f i c a n t e f f e c t of temperature w i l l be i n d i r e c t ; temperature c o n t r o l s the vapour pressure of water and t h e r e f o r e the ambient r e l a t i v e humidity. The r e l a t i v e humidity i n turn c o n t r o l s the r e a c t i o n r a t e as shown by equation 40. 3.7 E r r o r A n a l y s i s The l a r g e s t experimental e r r o r s are thought to be those a s s o c i a t e d w i t h the c o n t r o l and measurement of the r e l a t i v e humidity w i t h i n the r e a c t o r s . O p e r a t i o n a l problems with the humidity module included keeping the column packing p r o p e r l y i r r i g a t e d but not flooded, and i n m a i n t a i n i n g a constant condenser temperature. Because of s u b s t a n t i a l and frequent room temperature f l u c t u a t i o n s the l a t t e r parameter was d i f f i c u l t to c o n t r o l . During a s i x hour experimental run the humidity of the a i r e x i t i n g the r e a c t o r s , as determined g r a v i m e t r i c a l l y w i t h a d r y i n g tube, v a r i e d between 70% and 105%. The standard d e v i a t i o n of the - 71 -humidity during a run v a r i e d from over 10% d u r i n g e a r l y runs (1,2) to about 5% during l a t e r runs (16-22). The expected e r r o r i n humidity measurements was estimated u s i n g standard indeterminate e r r o r a n a l y s i s procedures. The f o l l o w i n g measurement ac c u r a c i e s were assumed: Time ( t ) ± 0.5 min f o r 30-360 min Weight (W) ± 1 mg f o r 90-100 g Weight d i f f e r e n c e ± 1.4 mg f o r 5-400 mg Flow (Q) ± 30 cm 3/min f o r 500-700 cm 3/min Temperature (T) + 0.5°C f o r 17-22°C Saturated humidity ± 0.5 f o r 17-22 mg/L S i m i l a r i l y , the e r r o r i n determining k v i a equation 28, 0.0036 Q (C.-C ) k (28) K = W S C o was estimated u s i n g the f o l l o w i n g a d d i t i o n a l values f o r the measurement e r r o r s : Cone. (C) ± 1 ppb f o r 20-70 ppb Cone, d i f f e r e n c e ± 1.4 ppb f o r 2-30 ppb S p e c i f i c area (S) ± 5% i n B.E.T. analyses This a n a l y s i s i n d i c a t e d an expected e r r o r of ±5.8% i n the measurement of r e l a t i v e humidity, w i t h the m a j o r i t y of t h i s e r r o r stemming from flow measurement e r r o r s (estimated to be about ±5% with a c a l i b r a t e d rotameter). The e r r o r i n the measured value of the p s e u d o - f i r s t - o r d e r r a t e constant k i s seen to be (equation 2 8 ) s e n s i t i v e to the value of ( C J - C Q ) . T y p i c a l c a l c u l a t e d values are shown as v e r t i c a l e r r o r bars i n Figure 9. Although the s c a t t e r i n the data p o i n t s w i t h i n t h i s f i g u r e appears to be - 72 -e x c e s s i v e , the c o r r e l a t i o n between the measured value of k and the humidity f u n c t i o n f ^ , where f ^ = k 2 - R.H., i s e x c e l l e n t w i t h a c o r r e l a t i o n c o e f f i c i e n t of 0.92. L i n e a r r e g r e s s i o n a n a l y s i s procedures gave the best f i t f o r t h i s e m p i r i c a l f u n c t i o n : k' k = - = k' f 1 k 2 - RH w i t h k' = 0.25 m h" 1 k 2 = 1.04 ± 0.01 The range of e r r o r i n k' was estimated from a p l o t of k versus f ^ : the value of the slopes of two s t r a i g h t l i n e s which incor p o r a t e d 90% of the data p o i n t s were 0.41 and 0.13. Therefore: k' - 0 25 + 0 , 1 6  K " - 0.12 (The 2 a e r r o r bounds are somewhat asymetric because the l i n e a r r e g r e s s i o n curve was forced through the o r i g i n i n order to s a t i s f y a simple form f o r the c o r r e l a t i o n : k = k ' f j . I t was f e l t that a more complicated expression was not j u s t i f i e d given the l a r g e amount of "n o i s e " i n the experimental data.) Although the e r r o r bounds on the value of k are q u i t e l a r g e (approximately ± 50%) they are r e l a t i v e l y " p r e c i s e " when compared to the experimental r e s u l t s obtained by others f o r atmospheric chemical processes. The reported value f o r k (uncatalyzed aqueous phase 0 (29) o x i d a t i o n of S ( I V ) , f o r i n s t a n c e , i s shown v ' to vary by three orders of magnitude between researchers. Figure 12 i n Sect i o n 4.8 shows the e f f e c t of the u n c e r t a i n t y i n the value of k on the value of x J r a ( c h a r a c t e r i s t i c time f o r the aqueous phase r e a c t i o n of S ( I V ) ) . The - 73 -e r r o r band-width i s seen to be minor when compared to the range of s c a l e i n the processes i n v o l v e d , and i n s i g n i f i c a n t when compared to the var i a n c e i n T ( r e c i p r o c a l of the l i t e r a t u r e values of k ). o o 3.8 R e s u l t s of Limestone D i s s o l u t i o n Experiments The r e s u l t s of the limestone d i s s o l u t i o n experiments are presented i n Table 6 and p l o t t e d as Figure 12. I t i s apparent that Texada I s l a n d limestone at an i n i t i a l c o n c e n t r a t i o n of 200 mg/L (2 1/2 x s t o c h i o m e t r i c ) r a p i d l y n e u t r a l i z e s the, a c i d i t y and that there i s l i t t l e e f f e c t caused by the mode of mixing. D o l o m i t i c limestone, on the other hand, appears to i n i t i a l l y d i s s o l v e at a lower r a t e which may be due to the f a c t that t h i s m a t e r i a l was d i f f i c u l t to wet. I t was formed from p e l l e t i z e d p r e c i p i t a t e d dolomite and t h e r e f o r e may co n t a i n occluded a i r . In t h i s case u l t r a s o n i c a g i t a t i o n appeared to s i g n i f i c a n t l y increase the r a t e of d i s s o l u t i o n . For the o v e r a l l r e a c t i o n : HoS0,+CaC0» -» CaSO, + H„0+C0 o, a f i r s t - o r d e r 2 4 3 4 Z 2 r e a c t i o n may be defined which i s based on the co n c e n t r a t i o n of H^SO^ a n c* the s u r f a c e area of the limestone: r e a c t i o n r a t e , r ' , (moles/hour) = - V f dC/dt = k" C V S (48) where 3 V = v e s s e l volume (m ) r 3 C = con c e n t r a t i o n of R^SO^mol/m ) k" = r a t e constant (m/h) W = mass of limestone (g) 2 S = s p e c i f i c s urface (m /g) - 74 -TABLE 6 - RESULTS OF LIMESTONE DISSOLUTION EXPERIMENTS AUTO-STIR ULTRASONIC BATH SAMPLE 1 2 3 4 5 TIL T IL DOLOMITE T IL DOLOMITE TIME pH TIME pH TIME pH TIME pH TIME pH Apr. 9/87 1312 2.80 1421 2.80 1550 2.80 1317 2.80 1322 2.80 13 3.04 22 2.94 51 2.80 19 3.60 24 3.00 18 4.17 25 3.15 54 2.88 26 4.33 28 3.09 20 4.54 27 3.23 58 3.00 31 4.69 32 3.21 23 4.84 31 3.34 1611 3.40 36 5.05 37 3.47 26 4.97 34 3.42 16 3.60 42 5.32 43 3.85 28 5.09 38 3.50 25 4.18 47 5.46 48 4.52 30 5.15 42 3.56 30 4.58 57 5.67 58 5.15 34 5.28 46 3.58 35 4.92 1406 5.80 1407 5.39 38 5.37 1516 3.93 40 5.15 16 6.00 17 5.68 43 5.51 37 4.18 45 5.35 26 6.12 27 5.80 47 5.55 50 5.49 50 5.64 55 5.61 54 5.73 1700 5.75 59 5.80 1403 5.91 06 5.94 16 6.11 20 6.21 4-DAY DURATION ULTRASONIC Apr. 13 1300 7.63 1300 6.95 1300 7.26 Apr. 13 1450 7.83 1450 7.22 1450 7.60 1450 6.48 1450 6.17 TIL - Texada I s l a n d Limestone 80 70 6-0 5-0 4-0 3-0 FIGURE 12 LIMESTONE DISSOLUTION IN ACID 200 mg/L LIMESTONE WITH AUTO-STIR fSAMPLE #l ) 200 mg/L LIMESTONE WITH ULTRA-SOUND ( SAMPLE #4 ) 200 mg/L DOLOMITE WITH AUTO-STIR (SAMPLE #3) 200 mg/L DOLOMITE WITH ULTRA-SOUND (SAMPLE #5) 100 mg/L LIMESTONE WITH AUTO-STIR (SAMPLE #2) / -r— 10 ' T -I S - 1 -20 1 25 I 30 35 I 40 45 55 I 60 I 65 —r— 70 —r— 75 -J— 80 —T 85 I 90 50 STIRRING TIME (MINUTES) - 76 -For H oS0, we have 2 C = (H +) and ln[H+] = -2.303 pH (49) ( R e c a l l that square brackets denote m o l a r i t y whereas parenthesis denote 3 c o n c e n t r a t i o n as mol/m .) Therefore, i n the case of a l a r g e excess limestone f o r which W (and S) do not change s i g n i f i c a n t l y over the d u r a t i o n of the d i s s o l u t i o n experiment, equation 48 reduces to: Equation 50 can be used to estimate a p s e u d o - f i r s t - o r d e r limestone d i s s o l u t i o n r a t e constant, k", from simple a c i d n e u t r a l i z a t i o n experiments. The t i m e - d e r i v a t i v e of the s o l u t i o n pH was measured d i r e c t l y from Figure 12 to compute, u s i n g equation 50, the p s e u d o - f i r s t - o r d e r r e a c t i o n r a t e constant k" as a f u n c t i o n of r e a c t i o n time. These values of k" are tabulated i n Table 7. The r e s u l t s are c o n s i s t e n t between experiments - f o r Texada I s l a n d limestone (Samples 1, 2, 4) the i n i t i a l r e a c t i o n r a t e of 0.5 - 0.8 m/h decreases by an order of magnitude as the surface of the p a r t i c l e s a c q u i r e on i n e r t "ash" l a y e r from non-soluble i m p u r i t i e s . (Reaction product, calcium sulphate, i s dispersed i n the bulk of the l i q u i d . ) k" = 2.303V r dpH (50) VS dt - 77 -TABLE 7 - REACTION RATE OF LIMESTONE AND SULPHURIC ACID (k", m/h) SAMPLE TIL t l SAMPLE TIL #2 DOLOMITE #3 TIL #4 V, Mass (g) S (m 2/g) V r ( m 3 ) x l 0 6 2.303 Vr/WS 0.020 0.323 100 0.0356 0.010 0.323 100 0.0713 0.020 0.716 100 0.0161 0.020 0.323 100 0.0356 S t i r r e d S t i r r e d S t i r r e d U l t r a s o n i c TIME (MIN.) k" k" 1 2 4 8 16 32 64 155 346 444 492 507 508 508 0.48 0.48 0.48 0.48 0.085 0.046 0.040 0.60 0.37 0.18 0.078 0.047 0.039 0.042 0.85 0.25 0.17 0.064 0.046 0.040 TIL = Texada I s l a n d Limestone k" (m/h) = 2.303 V dpH q -VS dt ug SO^ product mg limestone - 78 -F o l l o w i n g the a n a l y s i s p r e v i o u s l y used i n Secti o n 3.5, to determine the e f f e c t of r e a c t i o n product accumulation on the value of the r e a c t i o n r a t e constant, we can d e r i v e a s i m i l a r expression f o r the limestone -s u l p h u r i c a c i d system. ug s o 4 D e f i n i n g F' = 5-3 j — as p r e v i o u s l y , then from equation (48) 6 mg limestone-min * J ' M ' F' = 1.6 x l ( r k" S (H 2S0 4) (51) where (R^SO^) = moles/m = % (H +) 3 k „ 3 So (H 2S0 4) = 500 l o g 1 (-pH) (52) and (52) reduces to: F' = 8.0 x 1 0 5 k" S l o g " 1 (-pH) (53) R e c a l l that: q [ y g S ° 4 ) = f F ' dt Ug Limestone J J - 79 -This i n t e g r a l was solved n u m e r i c a l l y using the Trapezoid r u l e (equation 47), f o r the f i r s t limestone sample (TIL #1). The r e s u l t s are included as the e x t r a column i n the bottom of Table 7. I t can be seen that the limestone p a r t i c l e s very r a p i d l y react to about 0.5 mg S0^ per mg of limestone. Since the limestone i s 87% CaCO^ and s i n c e the maximum t h e o r e t i c a l r e a c t i o n w i t h CaCO^ i s 0.96 mg S0^ per mg of CaCO^, i t i s seen that the r e a c t i o n went to about 60% completion before being quenched by a l a y e r of i n e r t m a t e r i a l . For pure d o l o m i t i c limestone the r e a c t i o n r a t e i s seen (Table 7) to remain more constant presumably because l e s s i n s o l u b l e "ash" i s formed from limestone i m p u r i t i e s . The r e s u l t s , however, may be misleading because of the aforementioned problems w i t h w e t t i n g of the powder. Figure 12 shows that a f t e r about 20-40 minutes of c o n d i t i o n i n g the dolomite r e a c t i o n r a t e does temporarily i n c r e a s e . The long-term increase i n the s o l u t i o n pH probably a t t r i b u t a b l e to the c o n t i n u a l r e l e a s e of a l k a l i n i t y (HCO-j) from the unreacted p a r t i c l e cores. - 80 -4.0 DISCUSSION The object of t h i s research i s to determine the r a t e of r e a c t i o n between coarse p a r t i c l e s of limestone and sulphur d i o x i d e i n a humid atmosphere. The previous chapters have shown that t h i s r e a c t i o n i s approximately f i r s t - o r d e r w i t h respect to the c o n c e n t r a t i o n of SO2 i n the atmosphere (Sect. 3.4) and that i t i s , f o r atmospheric h u m i d i t i e s of up to 100%, d i r e c t l y p r o p o r t i o n a l to the amount of water adsorbed onto the surface of the p a r t i c l e (Sect. 3.3). The r a t e of the heterogeneous r e a c t i o n was defined as: r = k S B (S02) (13) where 3 r = moles SO2/111 -h k = p s e u d o - f i r s t - o r d e r r a t e constant (m/h) 9 S = s p e c i f i c s u r f a c e area of p a r t i c l e s (m /g) 3 B = c o n c e n t r a t i o n of p a r t i c l e s i n the atmosphere (g/m ) 3 (S0-) = SO- c o n c e n t r a t i o n (moles/m ) The research found that k = 0.248 (40) 1.04-R.H. where R.H. = r e l a t i v e humidity, (0.7 < R.H. < 1.0) This chapter w i l l now examine the r a t e s of some of the i n d i v i d u a l mass t r a n s f e r and r e a c t i o n steps which are lumped together i n the p s e u d o - f i r s t - o r d e r constant of equation 40. This process should allow r a t e l i m i t i n g processes to be i d e n t i f i e d and t h e r e f o r e i n d i c a t e atmospheric c o n d i t i o n s wherein the S0 2 / p a r t i c l e r e a c t i o n i s s i g n i f i c a n t . Much of t h i s a n a l y s i s w i l l f o l l o w the procedures of (12) S e i n f e l d v , which i n turn were derived from the work of Schwartz and - 81 -F r e i b e r g 1 , . T h e i r a n a l y s i s uses the concept of a " c h a r a c t e r i s t i c time", which i s simply the r e c i p r o c a l of the r a t e constant f o r a f i r s t - o r d e r homogeneous process. 4.1 Gas-Phase D i f f u s i o n (r, ) The c h a r a c t e r i s t i c time (^g) f° r K a s phase d i f f u s i o n w i t h i n the experimental r e a c t o r was given by equation 22: z 2 _ (22) T d g " 4 D g where Z = one-half the average d i s t a n c e between p a r t i c l e s max r D = d i f f u s i o n c o e f f i c i e n t g (12) For atmospheric r e a c t i o n s S e i n f e l d ' recommends the form: D 2 T , (seconds) = — - — (54) Q g 16 D g For SC>2 d i f f u s i n g to 10 ym diameter atmospheric p a r t i c l e s : D = 10 x 10" 4 cm P 2 -1 D = 0.126 c m s g and hence, T , = 5 x 10~^s dg Therefore g a s - f i l m d i f f u s i o n i s r a p i d and e q u i l i b r i u m i s e s t a b l i s h e d i n about one microsecond. - 82 -4.2 Phase E q u i l i b r i u m ( t p ) The c h a r a c t e r e s t i c time (x^) r e q u i r e d to achieve i n t e r f a c i a l e q u i l i b r i u m when no chemical r e a c t i o n s are o c c u r r i n g w i l l depend upon the volume of the aqueous f i l m , the d i f f u s i v i t y of the d i s s o l v e d gas w i t h i n t h i s f i l m , and upon the s o l u b i l i t y of the gas w i t h i n the f i l m . Schwartz and ( 28 ) F r i e b e r g v ' computed x^ f o r S0 2 d i f f u s i n g through a i r to s m a l l water d r o p l e t s at 25°C. Th e i r values are l i s t e d i n Table 8 as a f u n c t i o n of aqueous phase pH (which determines S0 9 s o l u b i l i t y ) . For pure d r o p l e t s -5 -1 (no s o l i d core) x^ i s i n the order of 10 to 10 seconds f o r pH 4 -pH 6. These are conservative values f o r x^; a s o l i d core w i l l reduce the aqueous phase and hence reduce the time acquired to reach e q u i l i b r i u m . 4.3 S(IV) I o n i z a t i o n The c h a r a c t e r i s t i c times f o r the f i r s t and second d i s s o c i a t i o n of S ( I V ) : T a l : H 2 S 0 3 "* H + + H S 0 3 T A 2 : HS0~ -» H + + SO 2" are presented i n Table 8 as estimated by Schwartz and F r e i b e r g ^ 2 * ^ . The f i r s t d i s s o c i a t i o n time (x .) i s very r a p i d w i t h a t y p i c a l value of 3 x -7 10 seconds, w h i l e the second d i s s o c i a t i o n time ( t 9 ) i s pH dependent -5 -3 and ranges from 10 to 10 seconds f o r pH 4 to pH 6. 4.4 Aqueous-phase D i f f u s i o n ( * j a ) The a p p r o p r i a t e expression f o r the c h a r a c t e r i s t i c time f o r aqueous phase d i f f u s i o n can be derived from the s o l u t i o n f o r t r a n s i e n t d i f f u s i o n through a s l a b ( 3 0 ) , T - 4 A  d a " n2D a TABLE 8 - CHARACTERISTIC TIMES (SEOCNDS) PGR LDfZSKl« - 9^ REACTION SOLUriCN GAS FILM PHASE BOQTLIB. S0 2 KNEZATICN pH X. X X - T, ^ (No reactions) (First) (SecM) 3 SxHT 7 lO"7 3xlO - 7 4 5xl(T7 lO'5 3xlO~*7 io-5 5 5xlO - 7 lO"3 3xlO - 7 6 5xl ( f 7 lO"1 3xl(T7 io-3 7 5xlO - 7 lO1 3xlO - 7 io-2 AUBDUS LIMESTONE CAIX3T£ S(IV) AQUEOUS DTFFUSIGN mSSOLUTKN DISSCOJnJCN CKmATKN PHASE RXN. Td(l) Tnl(2) X ra IO"7 3xlO - 7 AxKT8 lO-lO 3 8xl0 - 3 IO"7 3xlO - 5 4X10-6 lO-lO 3 8xl0 - 2 IO"7 8xl(T 3 8X10"4 10-103 2x10° IO"7 2x10° 4xl0 - 2 10-103 4X101 IO"7 4x10° lO-lO 3 4X103 Notes: - x p, x^, x ^ from Schwartz and Freiberg v for a water droplet with no solid core. - t — i / i x from the limestone dissolution curves of Figure 11 (14) - from the calcite dissolution data of Plurmer and Parkhurst rd(2) f 2 9 ) - X q measured by other researchers ' for "roncatalytic" oxidation of S(TV) - x ^ detennined experimentally i n this study - 84 -where -5 2 -1 D & = aqueous d i f f u s i v i t y ~ 10 cm s a = thickness of the aqueous l a y e r (cm) I f we assume that the thickness of the l i q u i d f i l m i s equ i v a l e n t to 100 monolayers of H^ O ( f u r t h e r discussed i n the next c h a p t e r ) , then a ~ 2.8 x 10~ 6 cm and T d a ~ 10 -^ seconds. Only when the limestone p a r t i c l e s act as g i a n t cloud condensation n u c l e i _3 w i t h i n a cloud system ( l i q u i d water content (L) approximately 1 g m and _3 d r o p l e t s of 5-50 ym diameter), or perhaps w i t h i n fog (L - 0.1 g m and d r o p l e t s of ~ 0.5 - 10 ym) v , would the aqueous l a y e r d i f f u s i o n be s i g n i f i c a n t . 4.5 Limestone D i s s o l u t i o n (*"r{1) The limestone d i s s o l u t i o n r a t e was measured using the procedures documented i n Se c t i o n 3.8. The r a t e data can be used to compute a c h a r a c t e r i s t i c time f o r limestone d i s s o l u t i o n QO. 3 B a s i s : 1 m atmosphere. Rate of accumulation (moles h"*1) = r a t e of d i s s o l u t i o n , i . e . , 3600 L d [ C a 2 + ] _ TOOT ^ T E — S B r p ( 5 6 ) - 85 -where _3 L = l i q u i d water content of atmosphere (g m ) [ C a 2 + ] = calcium i o n c o n c e n t r a t i o n (mol L~ ) -3, B = limestone c o n c e n t r a t i o n i n the atmosphere (g m ) 2 -1 S = s p e c i f i c area of limestone (m g ) -2 -1 r = limestone d i s s o l u t i o n r a t e (mol m h ) (12) F o l l o w i n g S e i n f e l d v ' we d e f i n e a time constant ( c h a r a c t e r i s t i c time) f o r limestone d i s s o l u t i o n : •3 1 d[Ca2+J d t [Ca 2 + J (57) where equation 56 d e f i n e s the r a t e of d i s s o l u t i o n . The d e f i n i t i o n of T r^ r e q u i r e s an a p p r o p r i a t e expression f o r [ C a 2 + ] . A s u i t a b l e calcium i o n c o n c e n t r a t i o n would be that value which i s equal to the e q u i l i b r i u m c o n c e n t r a t i o n of unreacted d i s s o l v e d S(IV) species ( [ S ( I V ) ] * ) s i n c e t h i s i s the " d r i v i n g f o r c e " f o r the limestone d i s s o l u t i o n r e a c t i o n . So by d e f i n i t i o n , f o r T r < j , we have [ C a 2 + ] = [ S ( I V ) ] * (58) where [ S ( I V ) ] * = H S(IV) 'SO, (59) and H S(IV) hs . s i s i s2 1+ + [H +] [ H + ] 2 (60) = a modified Henry's law c o e f f i c i e n t . - 86 -The values of the constants, at 25°C, were l i s t e d i n the i n t r o d u c t o r y s e c t i o n (1.3). The limestone d i s s o l u t i o n r a t e ( r p ) was expe r i m e n t a l l y determined ( S e c t i o n 3.8), under c o n d i t i o n s wherein aqueous phase r e a c t i o n s would not be r a t e l i m i t i n g , and was defined there as r p = k» (H 2S0 4) (61) _3 where (H^SO^) i s the con c e n t r a t i o n (mol m ) i n the aqueous phase, and where k" ~ 0.6 m h" 1. L e t t i n g (H 2S0 4) = 0.5 (H +) = 500 [H +] then r p (mol n f V 1 ) = 500 k" [ H + J . (62) Noting that L_ g H2° = 1 0 4 a SB = 2 m where a = average thickness (cm) of the aqueous l a y e r , we o b t a i n , through s u b s t i t u t i o n s , T r < J = 3.6 x 10 4 a H * ( I V ) P S 0 2 r P ° r ' T r ( J = 7 2 a H S ( I V ) P S 0 2 (63) k" [H +] From equation 63 i t i s observed that T^^ i s d i r e c t l y p r o p o r t i o n a l to the thick n e s s of the aqueous f i l m on the p a r t i c l e s u r f a c e , which i n turn i s a f u n c t i o n of humidity, e l e c t r o l y t i c a c t i v i t y , and p a r t i c l e r a d i u s . - 87 -K l i n g s p o r et a l . ' i n d i r e c t l y measured the monolayers of water adsorbing onto a limestone p a r t i c l e i n the absence of SC^. They ranged from about 2 l a y e r s at 50% humidity, to about 5 l a y e r s at 90% humidity. Under s a t u r a t e d humidity c o n d i t i o n s the aqueous f i l m t hickness w i l l r a p i d l y i n c r e a s e to some higher value which, f o r t h e i r study, was indeterminate. For l a r g e p a r t i c l e s of limestone, of diameter i n the order of 10 um, the curvature ( K e l v i n ) e f f e c t i s i n s i g n i f i c a n t and the p a r t i c l e behaves as a (12) f l a t s u r f a c e v . The s o l u t i o n a c t i v i t y e f f e c t on vapour pressure can be approximated by Raoult's law: PAsoln - &A X A PA ( 6 4 ) e where = vapour pressure of pure water x. = mole f r a c t i o n of water A = a c t i v i t y c o e f f i c i e n t For the limestone - S(IV)-S(VI) system the s o l u t i o n s t r e n g t h i s c o n s t r a i n e d due to the l i m i t e d s o l u b i l i t y of CaSO^ and CaSO^; th e r e f o r e the s o l u t i o n a c t i v i t y e f f e c t i s expected to be minimal. Because of the d i f f i c u l t y i n a s s i g n i n g a d e f i n i t e value to the thickness of the adsorbed water f i l m we s h a l l a r b i t r a r i l y , and perhaps c o n s e r v a t i v e l y , assume that i t i s equivalent to 100 monolayers of H-0 (77 I molecules. I f a monolayer i s 2.80A t h i c k v , then the aqueous f i l m i s 2.8 x 10' 6 cm t h i c k . T his value f o r the f i l m thickness (280A) was used i n c o n j u n c t i o n with equations 60 and 63 to compute the c h a r a c t e r i s t i c time ( t r ( j ) limestone to e q u i l i b r a t e i n the aqueous phase w i t h atmospheric SO2 (50 ppb). Table 8 i n d i c a t e s that the c h a r a c t e r i s t i c time constant f o r t h i s process becomes s i g n i f i c a n t at higher s o l u t i o n pH v a l u e s , ranging from 3x10" seconds at pH 3 to about 2 seconds at pH 6. - 88 -Plummer and Parkhurst (14) measured the d i s s o l u t i o n r a t e of s e m i - o p t i c a l grade c r y s t a l s of Iceland spar i n d i l u t e s o l u t i o n s as a f u n c t i o n of pH, pCOj, and temperature. At atmospheric c o n d i t i o n s (pCX^ ~ 0, T=25°C) t h e i r data i n d i c a t e (Table 8) a f a s t e r d i s s o l u t i o n r a t e than what we determined f o r limestone p a r t i c l e s . At a pH g r e a t e r than about 5.4 they found that the dominant d i s s o l u t i o n mechanism changes from H + a t t a c k to a pH independent HjO s o l u t i o n process. 4.6 Aqueous Phase O x i d a t i o n (*"0) The r a t e of the n o n c a t a l y t i c o x i d a t i o n of S(IV) to S(VI) by 0« i n the (12) l i q u i d phase i s reported to be n e g l i g i b l e v . However, most aqueous systems w i l l c o n t a i n t r a c e amounts of d i s s o l v e d metal c a t i o n s which may d r a m a t i c a l l y enhance the r a t e of r e a c t i o n . (29) F i n l a y s o n - P i t t s and P i t t s v ' p l o t t e d the r e s u l t s of v a r i o u s experimental s t u d i e s of the p s e u d o - f i r s t - o r d e r r a t e constant k Q, as a f u n c t i o n of pH, where k i s defined f o r an aqueous system: They show that not only i s there no agreement as to the absolute values of k , but a l s o that there i s no agreement as to the e f f e c t of the - 3 - 1 -1 -1 s o l u t i o n pH. The value of k Q v a r i e d between about 10 s to 10 s over a pH range of 2 to 10. I f we d e f i n e : k s o 1 d [SO 2 ] (65) dt x = k " 1 o o then the corresponding T ranges from 10 seconds to 1000 seconds. - 89 -Since the Texada I s l a n d limestone which was used i n our research contained a measurable amount of metal c a t a l y s t s (122 ppm Fe and 69 ppm Mn) i t i s probable that some mild c a t a l y t i c e f f e c t was present. 4.7 Aqueous Phase Reaction ( x ) r s The c h a r a c t e r i s t i c time f o r the aqueous phase chemical r e a c t i o n can be de f i n e d : - T 1 = 1 d I S < I V " (66) r a [S(IV)] dt Thi s time constant w i l l be determined from our experimental r e s u l t s of Se c t i o n 3 and then compared w i t h the c h a r a c t e r i s t i c times l i s t e d i n Table 8. In t h i s way we may determine the r a t e l i m i t i n g step i n the limestone - sulphur d i o x i d e - water r e a c t i o n . F o l l o w i n g a procedure s i m i l a r to that employed i n S e c t i o n 4.5 f o r limestone d i s s o l u t i o n we have, assuming pseudo-steady s t a t e f o r d i s s o l v e d S ( I V ) : L d [S(IV)] k (SO,) B S _ i (67) 1000 dt 3600 From equation 59 [S(IV)J = H S ( I V ) pS0 2 (59) f 121 Now at 25°C ( S 0 2 ) = 40.90 pS0 2 ( S e i n f e l d 1 J ) so that at e q u i l i b r i u m : [S(IV)] (SO,) = 40.90 (68) H S(IV) - 90 -I f equation 68 i s s u b s t i t u t e d i n t o equation 67, and the l a t t e r i s rearranged to the format of equation 66, then the c h a r a c t e r i s t i c time f o r the aqueous phase r e a c t i o n of S(IV) becomes: T- 1 = U - A k 5 B (69) r a * L H S ( I V ) 4 L R e c a l l i n g that 10 a = — = thickness (cm) of the aqueous f i l m on the SB p a r t i c l e , ve get: 4 * % a = a H S ( I V ) ( 7 Q )  r a 11.4 k where a = f u n c t i o n of humidity, assumed to be about 100 monolayers or 2.8 x 10~ 6 cm. H s(iv) = ^ u n c t ^ o n °f PH P e r equation 68 . . 0.248 / n . and k = , per equation 40, i s 1.04 - RH the p s e u d o - f i r s t - o r d e r r a t e constant f o r the limestone - S O 2 - H 2 O system. 4.8 Probable Rate L i m i t i n g Step A comparison of the va r i o u s c h a r a c t e r i s t i c times that were estimated i n s e c t i o n s 4.1-4.7 can be made w i t h the a i d of Figure 13. I t i s apparent that at values of pH l e s s than 6 the r a t e l i m i t i n g step i s that of S(IV) aqueous phase o x i d a t i o n , w h i l e at higher pH values that the limestone d i s s o l u t i o n r a t e may a l s o become s i g n f i c a n t . - 91 -1 j 1 1 r 3 4 5 6 7 Aqueous Phase pH FIGURE 13 CHARACTERISTIC TIMES FOR LIMESTONE - S 0 2 REACTION - 92 -In our experiments aqueous phase S(TV) must be the r a t e l i m i t i n g step, w i t h m i l d c a t a l y s i s probably o c c u r r i n g because of the presence of trace amounts of d i s s o l v e d Fe and Mn. The r e a c t i o n product on the surface of the reacted p a r t i c l e s , as measured by the d i s t i l l e d water l e a c h / i o n chromatography procedure described i n Sect i o n 2.2.3, contained only calcium sulphate, w i t h no calcium suphite d e t e c t a b l e . Because of the e f f e c t of pH on s u l p h i t e a v a i l a b i l i t y ( F i g u r e 2), t h i s observation would i n f e r that the e f f e c t i v e pH of the surfa c e aqueous f i l m must be l e s s than. 6 and there f o r e that limestone d i s s o l u t i o n i s not r a t e l i m i t i n g . Another argument i n favour of S(IV) o x i d a t i o n being the r a t e l i m i t i n g step i s the f a c t that the o v e r a l l r e a c t i o n r a t e of SO^ and limestone was determined to be p r o p o r t i o n a l to the volume of l i q u i d water a v a i l a b l e f o r an aqueous phase r e a c t i o n ( S e c t i o n 3.3) and was found to be r e l a t i v e l y i n s e n s i t i v e to the extent of the r e a c t i o n and t h e r e f o r e to the thickness of any product l a y e r ( S e c t i o n 3.5). As was discussed i n the I n t r o d u c t i o n ( S e c t i o n 1.3, Figure 3) i t i s l i k e l y that a t h i c k f i l m of i n s o l u b l e CaSO^H^O forms next to the s u r f a c e of the unreacted CaCO^, there f o r e almost immediately e s t a b l i s h i n g a pH gradient between the aqueous phase and the limestone s u r f a c e . This f i l m i n i t i a t e s a d e l i c a t e feed-back mechanism which l i m i t s the thickness of the f i l m . I f the s u l p h i t e f i l m thickens then the limestone d i s s o l u t i o n and a s s o c i a t e d ion d i f f u s i o n processes w i l l slow down. Assuming that aqueous phase S(IV) o x i d a t i o n i s r a t e l i m i t i n g then the pH of the aqueous phase w i l l decrease because of a c i d generation ( i n the l i m i t i t could decrease to a pH where SO2 i s no longer d i s s o c i a t e d to b i s u l p h i t e , approximately pH 1). However, below about pH 6 the s u l p h i t e form of S(IV) i s converted to the s o l u b l e b i s u l p h i t e form and the r e f o r e the thickness of the CaS02*2H20 f i l m w i l l be eroded u n t i l an e q u i l i b r i u m between the limestone d i s s o l u t i o n r a t e and the S(IV) o x i d a t i o n r a t e i s maintained. - 93 -Since CaSO^ i s r e l a t i v e l y s o l u b l e (pK = 3.7), Ca and S O ^ ' w i l l accumulate i n the aqueous phase u n t i l the s o l u b i l i t y l i m i t , which i s pH independent, has been reached at which time p r e c i p i t a t i o n w i l l occur throughout the aqueous phase. Because of the presence of v a r y i n g q u a n t i t i e s of i n s o l u b l e i m p u r i t i e s o r i g i n a t i n g from the parent limestone i t i s probable that CaS04«2H20 c r y s t a l growth can be i n i t i a t e d throughout the s o l u t i o n . Such a c r y s t a l growth mechanism, w i t h i n the aqueous phase in s t e a d of on the s o l i d i n t e r f a c e , would e x p l a i n why the o v e r a l l r a t e constant, k, i s not a strong f u n c t i o n of the extent of the limestone r e a c t i o n (at l e a s t not i n i t i a l l y ) . At t h i s time i t i s not p o s s i b l e to e s t a b l i s h a value f o r the pH of the aqueous f i l m . In general i t w i l l be a n o n t r i v i a l f u n c t i o n of the S(IV) o x i d a t i o n r a t e , of L ( l i q u i d water content of the a e r o s o l ) , and of the thickness and p e r m e a b i l i t y of the CaSO.j-2H20 f i l m . With S(IV) o x i d a t i o n being r a t e l i m i t i n g the r a t e of o x i d a t i o n ( r Q ) w i l l be equal to the o v e r a l l r a t e of the SO2 r e a c t i o n and a l s o to the r a t e of limestone d i s s o l u t i o n . Since the conc e n t r a t i o n of d i s s o l v e d oxygen i s approximately constant r w i l l be zero order w i t h respect to oxygen. 3 ° Therefore, u s i n g 1 m a e r o s o l as the b a s i s , : r (mol m V 1 ) = 3 6 0 0 L k o [S(IV)]= 3.6 Lk H* T x n pSO, (71) 0 1000 z = k (S0 2 ) S B (from equation 67) = 500 k f [H +] S B = r (mol m - 3h - 1) (72) where k^ i s the r a t e constant f o r limestone d i s s o l u t i o n through a CaS0 3-2H 20 f i l m . Note that k f>k" ( r a t e constant f o r limestone d i s s o l u t i o n i n fi^SO^ per equation 61). The previous three equations c o n t a i n 5 unknowns ( r Q , L, k Q, [ H + ] , and k^) , t h e r e f o r e we must conclude that the value of [H +] i n the aqueous phase i s , at t h i s time, indeterminant. I f k^ i s determined through - 94 -a p p r o p r i a t e experimentation then [H +] can be computed from the r e a c t i o n r a t e (equation 72). S i m i l a r i l y i f e i t h e r k Q or L can be independently measured then the other can be estimated u s i n g equation 71. Since k^>k" and a l s o s i n c e k^ w i l l decrease at higher pH values where s u l p h i t e formation i s favoured ( F i g u r e 2 ) , a reasonable estimate f o r the c h a r a c t e r i s t i c time f o r limestone d i s s o l u t i o n i n a s o l u t i o n c o n t a i n i n g both H*2S04 and d i s s o l v e d S0 2 i s represented by the dotted l i n e i n Figure 13. Where t h i s dotted l i n e crosses the l i n e r e p r e s e n t i n g the e x p e r i m e n t a l l y determined S(IV) r e a c t i o n i s a "best guess" f o r the pH of the aqueous phase i n the experimental a e r o s o l system. The determination of the values of k^, k Q, and L were beyond the scope of t h i s study. 4.9 Comparison vith Dolomite Limestone Reaction The r e a c t i o n of S0 2 i n humid a i r w i t h d o l o m i t i c limestone was found to be s l i g h t l y f a s t e r (approximately a f a c t o r of 2 according to Figure 9) than that w i t h c a l c i t i c limestone even though the d i s s o l u t i o n r a t e i n an aqueous s o l u t i o n i s an order of magnitude l e s s ( F i g u r e 12 and Table 7). (13) This d i f f e r e n c e i s explained by K o h l v ' who r e p o r t s that any c a t i o n (such as Mg + + or Na + ) which forms a s o l u b l e s u l p h i t e or sulphate w i l l i n c r e a s e the c o n c e n t r a t i o n of a l k a l i n e species and, t h e r e f o r e , increase the r a t e of S0 2 removal. However, under c o n d i t i o n s where limestone d i s s o l u t i o n i s the r a t e l i m i t i n g step, the d o l o m i t i c limestone w i l l appear to be r e l a t i v e l y u n r e a c t i v e . 4.10 Comparison of Dry Deposition Rates The e f f e c t of r e l a t i v e humidity upon the dry d e p o s i t i o n r a t e of S0 2 on limestone p a r t i c u l a t e i s shown i n Figure 14. The d e p o s i t i o n v e l o c i t y f o r a damp limestone s u r f a c e increases from e s s e n t i a l l y zero at a low r e l a t i v e humidity to about 1.5-2 cm/s at s a t u r a t i o n . - 95 -20-1 2 16-e o o mi UI > i UI o 10-0-5-© (n^-C-fcltte limestone J n j - Dolomitic limestone n - Experiment number 00 l 1-75 ® © 05J ® © ® © © © —I 1 r— 80 85 90 RELATIVE HUMIDITY <%) 95 —1 100 FIGURE 14 DRY DEPOSITION VELOCITY — SULPHUR DIOXIDE TO LIMESTONE PARTICLES (200-270 mesh) - 96 -S e i n f e l d v ' reviews the three separate dry d e p o s i t i o n r e s i s t a n c e s : aerodynamic, s u r f a c e , and t r a n s f e r r e s i s t a n c e . The t o t a l r e s i s t a n c e to dry d e p o s i t i o n i s the sum of these three components; one of these components i s u s u a l l y r a t e c o n t r o l l i n g . The bulk aerodynamic and the surf a c e g a s - f i l m r e s i s t a n c e s are f u n c t i o n s of wind speed and sur f a c e roughness. He th e r e f o r e parameterizes the d e p o s i t i o n v e l o c i t y as a f u n c t i o n of these two v a r i a b l e s and the t r a n s f e r r e s i s t a n c e . The l a t t e r r e s i s t a n c e i s a f u n c t i o n of moisture. For instance when a surface i s wet, such as a f t e r a heavy d e w f a l l , uptake of S0 2 w i l l be e f f i c i e n t u n t i l the pH of the sur f a c e water becomes s u f f i c i e n t l y a c i d i c to impose a chemical l i m i t on the r a t e of abso r p t i o n of gaseous S02« (4) Sehmel v ' presents a comprehensive summary of ex p e r i m e n t a l l y determined dry d e p o s i t i o n v e l o c i t i e s f o r S0 2- A t y p i c a l value f o r calcareous s o i l i s g i ven as 1.1 cm/sec, e x t e r i o r stucco - 1.8 cm/sec, and adobe c l a y s o i l - 0.92 cm/sec. I t i s noted that these values are c o n s i s t e n t w i t h our ex p e r i m e n t a l l y determined d e p o s i t i o n v e l o c i t y f o r i n d i v i d u a l limestone p a r t i c l e s ( F i g u r e 14). The SO- p a r t i c l e c o l l i s i o n e f f i c i e n c y , 0, was computed f o r s e l e c t e d —6 experiments (Table 5 ) . The value of 0 ranged from about 3 x 10 to 2.7 x 10 _5. J u d e i k i s et a l . ^ 1 ^ measured values of 0 f o r adobe s o i l (8.4 x I O - 5 ) and e x t e r i o r stucco (23 x I O - 5 ) usin g a high (3-100 ppm) co n c e n t r a t i o n of S 0 2 > The d i f f e r e n c e i n values i s probably because J u d e i k i s used a pr o j e c t e d surface area r a t h e r than the more c o r r e c t B.E.T. area. Nevertheless, the extremely low value f o r 0 does i n d i c a t e that the major r e s i s t a n c e to S0 2 d e p o s i t i o n i s the t r a n s f e r r e s i s t a n c e . Processes which enhance the removal of deposited S0 2 w i l l a l s o increase the c o l l i s i o n e f f i c i e n c y , 0, the d e p o s i t i o n v e l o c i t y , V^, and the o v e r a l l r e a c t i o n r a t e constant, k. - 97 -4.11 Atmospheric Reaction Rates Coarse limestone p a r t i c l e s w i l l r e a ct only s l o w l y w i t h S 0 2 i n a c l e a n , humid atmosphere. R e c a l l from equation 13 that the r a t e of r e a c t i o n of 3 (moles/h/m ) i s given by d (S0 2) ( 1 3 ) r = d z k S B (S0 2) U J ' Thus, T g (h) = (k S B ) _ 1 (14) where * t g i s the time required to reduce ( S0 2) to e _ 1 of i t s o r i g i n a l value. From our research we found that at room temperature (20°C): . 0.25 .-1 / / n x k = , m h (40) 1.04-R.H. where R.H. i s the r e l a t i v e humidity of the atmosphere. Assuming v a l u e s : R.H. = 0.95 (95%) S = 1 m 2g _ 1 B = 20 x 1 0 _ 6 g n f 3 then k = 2.8 in h A T = 2 x 10 hours, s - 98 -Obviously under these c o n d i t i o n s the limestone a e r o s o l w i l l not s i g n i f i c a n t l y impact upon the amount of SO2 i n the atmosphere. The s u r f a c e of the p a r t i c l e s w i l l only s l o w l y convert to CaSO^^HjO, w i t h a pH of approximately 6. In a humid atmosphere which i s h e a v i l y p o l l u t e d w i t h photochemical oxidants (0-, H«0«, 0H>, e t c . ) the aqueous phase o x i d a t i o n of S(IV) may (12 22) no longer be the r a t e l i m i t i n g s t e p v ' . Under these c o n d i t i o n s an examination of Figure 13 shows that b i s u l p h i t e d i s s o c i a t i o n i s s i g n i f i c a n t . I f the c h a r a c t e r i s t i c time f o r t h i s r a t e l i m i t i n g step i s taken as 10 _ seconds at pH 3 then an e q u i v a l e n t value f o r k m a x (the p s e u d o - f i r s t - o r d e r r a t e constant f o r SO2 removal) can be estimated by r e a r r a n g i n g equation 70: 4 * 10 a H g ( I V ) max 11.4 x r a w i t h pH = 3, t » 1 0 - 6 seconds r a H S ( I V ) = ^ (equation 60) = 2.8 x 10 6 cm (100 monolayers) then k = 3.7 x 1 0 4 m h _ 1 max Assuming atmospheric c o n d i t i o n s : R.H. = 100% 1 2 -1 S = 1 m g B = 20 x 1 0 - 6 g n f 3 then from equation 14 x (minimum) can be estimated: x (min) = 1.4 hours. - 99 -Therefore we t e n t a t i v e l y conclude that i n a p o l l u t e d atmosphere a limestone a e r o s o l can i n f a c t be a s i g n i f i c a n t " s i n k " f o r atmospheric SO and ^2^2' I n a s ^ m ^ a r manner i t would be expected to remove and n e u t r a l i z e h i g h l y s o l u b l e HNO-j molecules which are mainly generated by a homogeneous gas phase r e a c t i o n ^ 2 ^ : OH- + N0 2 -» HN0 3 ( 6 6 ) In h e a v i l y p o l l u t e d clouds SO- conversion r a t e s of up to 100%/hour have (12 22) been observed ' , th e r e f o r e our estimate of about 45%/hour conversion f o r a limestone a e r o s o l system may not be u n r e a l i s t i c . The major d i f f e r e n c e w i l l be that i n the l a t t e r system any p r e c i p i t a t i o n o c c u r r i n g w i l l be r a p i d l y n e u t r a l i z e d , whereas i n the former system the p r e c i p i t a t i o n w i l l be a c i d i c . - 100 -5.0 CONCLUSIONS AND RECOMMENDATIONS The l i t e r a t u r e on a c i d r a i n provides evidence that the presence of l a r g e (>5 um), atmospheric a l k a l i n e p a r t i c u l a t e which d e r i v e from wind-blown dust play an important r o l e i n m i t i g a t i n g the e f f e c t of a c i d r a i n . Not only do they help to n e u t r a l i z e a c i d i t y but they a l s o are a source of n u t r i e n t s to the t e r r e s t r i a l and aquatic environments. Geographic regions where the s o i l i s not a l k a l i n e are a d v e r s l y impacted by a c i d r a i n . I t i s thought that t h i s negative impact may be ameliorated by i n c r e a s i n g the c o n c e n t r a t i o n of a l k a l i n e p a r t i c u l a t e i n the atmosphere. Such a s t r a t e g y , when used i n c o n j u n c t i o n w i t h SOj and N0 x emission abatement programs, could reduce a c i d r a i n from being a major g l o b a l problem. While ground-up limestone has been added to f o r e s t s and lak e s u s i n g a i r c r a f t , t h i s process i s p r o h i b i t i v e l y expensive. An economical method of limestone d i s t r i b u t i o n would invoke the advective and turbulent d i f f u s i o n processes w i t h i n the troposhere i n order to supply limestone p a r t i c l e s of approximately 10 ym to the t e r r e s t r i a l and the a q u a t i c environments. However, w h i l e the p a r t i c l e s are transported through the atmospheric environment they w i l l a l s o i n t e r a c t to some degree w i t h the atmospheric p o l l u t a n t s (SO2, N0 x, photochemical o x i d a n t s , e t c . ) . L i t t l e i s known about such heterogeneous g a s - a l k a l i n e p a r t i c l e i n t e r a c t i o n s under ambient c o n d i t i o n s of con c e n t r a t i o n and temperature. As a f i r s t step i n understanding the atmospheric chemical behaviour of coarse limestone p a r t i c l e s l a b o r a t o r y experiments were conducted i n order to measure the r a t e of r e a c t i o n between these p a r t i c l e s and SO2 gas. C a l c i t i c limestone (200-270 mesh T y l e r ) was reacted w i t h 30-80 ppb S0 2 at a temperature of 17-22°C and a r e l a t i v e humidity of 70-100%. The r e a c t i o n r a t e of limestone w i t h SO2, H 20 CaC0~ + S0 o + %0- • CaSO, + C0-, - 101 -was found to be approximately f i r s t order w i t h respect to ( S 0 2 ) , independent of the r e a c t i o n product c o n c e n t r a t i o n , and s t r o n g l y dependent upon r e l a t i v e humidity, R.H. The r a t e of the o v e r a l l r e a c t i o n above can be represented as a p s e u d o - f i r s t - o r d e r process: r = k (S0 2) A -1 -3 where r = S0„ removal r a t e (mol h m ) k = p s e u d o - f i r s t - o r d e r r a t e constant (m h ) _3 (S0«) = S0 9 c o n c e n t r a t i o n (mol m ) 2 - 3 A = p a r t i c l e s u rface area ( m m ) Li n e a r r e g r e s s i o n a n a l y s i s of the experimental data y i e l d e d a 2 r e l a t i o n s h i p ( r =0.92) between humidity and the r a t e constant: 1 / / U N 0-25 k (m/h) = 1.04 - R.H. where R.H. i s the r e l a t i v e humidity. This equation i s s i m i l a r to an H 20 ads o r p t i o n isotherm which represents the volume of adsorbed water when the r e l a t i v e humidity approaches u n i t y . Therefore we conclude that the heterogeneous r e a c t i o n r a t e i s p r o p o r t i o n a l to the amount of aqueous phase present on the p a r t i c l e s u r f a c e . The e f f e c t of temperature i s f e l t mainly through i t s i n f l u e n c e on the s a t u r a t i o n pressure of water vapour and ther e f o r e on the number of monolayers of water a c c r e t i n g on the surface of the p a r t i c l e s . (Other i n v e s t i g a t o r s have found Ea to be q u i t e low; values of 23 to 54 kJ/mol have been reported.) A n c i l l a r y experiments were a l s o c a r r i e d out i n order to measure the maximum d i s s o l u t i o n r a t e of the same limestone p a r t i c l e s i n d i l u t e H 2S0 4 (pH 2.8). The very f a s t r a t e of limestone d i s s o l u t i o n showed that such p a r t i c l e s would r a p i d l y n e u t r a l i z e a c i d i t y i n an aq u a t i c environment. - 102 -A mechanistic study of c h a r a c t e r i s t i c times f o r the v a r i o u s d i f f u s i o n and r e a c t i o n processes which take place w i t h i n the three phase system i n d i c a t e s that the o v e r a l l r e a c t i o n i s r a t e l i m i t e d by the aqueous phase o x i d a t i o n of d i s s o l v e d S ( I V ) . Although i t was not p o s s i b l e d u r i n g the experiments to measure the thickness of the aqueous l a y e r ( i n the order of 10-100 monolayers) or i t s a c i d i t y , i t i s expected that the presence of a CaS0 3«2H 20 b a r r i e r would l i m i t the f i l m pH to about 5.6-6.0. At t h i s pH the r a t e constant f o r the aqueous phase S(IV) o x i d a t i o n r e a c t i o n i s -2 -1 about 3 x 10 s , which i s comparable to the range of values reported i n -3 -1 the l i t e r a t u r e (k Q=10-10 s ) f o r noncatalyzed S(IV) o x i d a t i o n . I f adequate s o l u b l e photochemical oxidants are a v a i l a b l e then the r a t e l i m i t i n g step i s expected to become the d i s s o c i a t i o n r e a c t i o n : HS0" -» H + + S 0 2 _ and the maximum o x i d a t i o n r a t e would be l i m i t e d then to k =10^ s ^ at o about pH 3. I n the atmosphere these d i f f e r e n t r a t e s can be compared by e s t i m a t i n g e q u i v a l e n t S0» removal r a t e s . In a humid " c l e a n " atmosphere c o n t a i n i n g z 3 no photochemical oxidants the limestone a e r o s o l (assuming 20 ug/m 2 c o n c e n t r a t i o n and a surface area of 1 m /g) would remove SO- at a r a t e of -3 only 3 x 10 X per hour and t h e r e f o r e would have l i t t l e e f f e c t upon the atmospheric S0 2 c o n c e n t r a t i o n . Instead the p a r t i c l e s would be removed to the earth's s u r f a c e v i a dry d e p o s i t i o n , as would the SO- gas (they both have s i m i l a r d e p o s i t i o n v e l o c i t i e s of about 1 cm s ). There n e u t r a l i z a t i o n r e a c t i o n s w i l l occur and the S0 2 a c i d i t y would be exchanged and emitted as C 0 2 > In a p o l l u t e d humid atmosphere, such as those which g i v e r i s e to " a c i d f o g " , the same atmospheric con c e n t r a t i o n of limestone p a r t i c l e s could remove up to about 45% S0 2/hour and i n a d d i t i o n a l s o scavenge HNO^ molecules and s o l u b l e photochemical oxidants. While the aqueous phase - 103 -surfa c e of the p a r t i c l e would be q u i t e a c i d i c (pH 3) d u r i n g such a r a p i d r e a c t i o n , i t would r a p i d l y be n e u t r a l i z e d upon removal from the p o l l u t e d a i r mass. S i m i l a r i l y , i n p o l l u t e d cloud systems with a high l i q u i d water content, the p a r t i c l e s w i l l act as g i a n t cloud condensation n u c l e i (CCN) and w i l l tend to n e u t r a l i z e the i n - c l o u d formation of a c i d i t y . An SO2 removal r a t e of up to 100%/h has been measured i n p o l l u t e d c l ouds, where the a v a i l a b i l i t y of H2O2 g e n e r a l l y l i m i t s the extent of the r e a c t i o n . I f cloud d r o p l e t pH i s elevated because of limestone a l k a l i n i t y then 0^ o x i d a t i o n w i l l a l s o be s i g n i f i c a n t . P r e c i p i t a t i o n from the cloud system w i l l not be a c i d i c i f adequate limestone CCN are present. The dry d e p o s i t i o n v e l o c i t y of SO2 onto c a l c i t i c limestone p a r t i c l e s was exp e r i m e n t a l l y determined to be i n the approximate range of 0-1 cm/s and to i n c r e a s e w i t h i n c r e a s i n g humidity. The l i t e r a t u r e r e p o r t s s i m i l a r values measured f o r calcareous and adobe c l a y s o i l s . Therefore we conclude that a l k a l i n e s o i l p a r t i c l e s can act as a " s i n k " f o r SO2 both i n the t e r r e s t r i a l environment and to a l e s s e r extent i n the atmospheric environment. In c o n c l u s i o n i t has been shown that limestone p a r t i c l e s are not very r e a c t i v e w i t h SO2 i n the atmosphere unless the atmosphere i s humid. Under humid and h e a v i l y p o l l u t e d c o n d i t i o n s the limestone a e r o s o l may act as a s i g n i f i c a n t s i n k f o r atmospheric p o l l u t a n t s . Because of t h i s i t i s important to b e t t e r understand the r e a c t i o n mechanisms and k i n e t i c s . I t i s t h e r e f o r e recommended that f u r t h e r s t u d i e s be done u s i n g a l a r g e v a r i e t y of a l k a l i n e p a r t i c u l a t e and i n c o r p o r a t i n g t r a c e concentrations of H2O2 and 0^ « Since r e l a t i v e humidity has been shown to be extremely important, e x t r a precautions should be taken to i n s u r e a constant, c o n t r o l l e d temperature environment f o r the experiments. - 104 -E x i s t i n g measures taken by government agencies to reduce wind-blown s o i l dust, which c o n s i s t s mainly of l a r g e (>5 um) p a r t i c l e s , may be counter-productive to the p r o t e c t i o n of the environment and t h e r e f o r e should, i n some s i t u a t i o n s , be discouraged. - 105 -6.0 APPENDICES 6.1 Nomenclature Symbol D e s c r i p t i o n U n i t s A A e A r B C C o D g D P Ea F F' H H S(IV) k, k',k" kl» k 2 K K K c l c2 cc K cs Thickness of l a y e r A e r o s o l surface area per u n i t volume of gas phase Area enhancement f a c t o r (equation 20) Reactor Area A e r o s o l c o n c e n t r a t i o n i n gas phase Concentration SO2 c o n c e n t r a t i o n i n r e a c t o r D i f f u s i o n c o e f f i c i e n t , gas phase P a r t i c l e diameter A c t i v a t i o n energy Humidity f u n c t i o n (equation 38) Reaction product f u n c t i o n (equation 43) D e p o s i t i o n f l u x to a surface Surface r e a c t i o n r a t e Henry's Law constant M o d i f i e d Henry's Law c o e f f i c i e n t (equation 60) Reaction r a t e constants Constants (equation 37) Rate constant f o r limestone d i s s o l u t i o n through a s u l p h i t e f i l m (equation 72) Rate constant f o r the aqueous phase o x i d a t i o n of S(IV) (equation 65) D i s s o c i a t i o n constant f o r bicarbonate=4.28x10 D i s s o c i a t i o n constant f o r c a rbonate=4.69xl0 - 1 1 D i s s o c i a t i o n constant f o r calcium carbonate (equation 4) S o l u b i l i t y constant f o r calcium s u l p h i t e (equation 6) cm 2 -3 m m -7 cm g m -3 -3 mol m ppb 2 -1 cm s cm kJ mol -1 mol h m^ 2 Mg SO, mg - 1 min mol L atm mol L - 1 a t m - 1 .-1 m h m h -1 s mol L mol L~ mol L -1 -1 -1 ,2.-2 mol L - 106 -Symbol D e s c r i p t i o n U n i t s he hs K s l K s 2 S o l u b i l i t y constant f o r C0 2 @ 25°C=3.4xl0 - 2 S o l u b i l i t y constant f o r S0„ at 25°C = 1.24 -2 D i s s o c i a t i o n constant f o r bi s u l p h i t e = l . 2 9 x 1 0 -8 D i s s o c i a t i o n constant f o r sulphite=6.01x10 i • —1 . _ 1 mol L atm mol L _ 1 a t m - 1 mol L~* i .-1 mol L L L i q u i d water c o n c e n t r a t i o n -3 g m N p Number of p a r t i c l e s P s o 2 P a r t i a l pressure of S0 2 atm q Reaction product (equation 45) Q r Volumetric gas flow through r e a c t o r Reaction r a t e 3 -1 cm s i -3.-1 mol m h r o r p Rate of aqueous phase o x i d a t i o n (equation 71) Limestone p a r t i c l e d i s s o l u t i o n r a t e i -3 u - 1 mol m h mol m h R Gas constant Re Reynolds number dimensionless R.H. R e l a t i v e humidity S S p e c i f i c s urface area of p a r t i c l e s by B.E.T. a n a l y s i s 2 -1 m g S' S p e c i f i c s urface area of p a r t i c l e s from equation 32 2 -1 m g t time T Temperature C° u r Gas v e l o c i t y w i t h i n r e a c t o r -1 cm s V d D e p o s i t i o n v e l o c i t y -1 cm s V r Reactor volume 3 m V x U n d i r e c t i o n a l molecular v e l o c i t y -1 cm s W Mass of a e r o s o l or powder g Z Distance from p a r t i c l e s urface cm Z p Average i n t e r p a r t i c l e d i s t a n c e (equation 23) cm a P r o p o r t i o n a l i t y Y Shape f a c t o r dimensionless 8 Loss of S0 2 to re a c t o r housing ppb pb Bulk d e n s i t y -3 g cm PP P a r t i c l e d e n s i t y -3 g m - 107 -- 108 -6.2 B i b l i o g r a p h y 1. Chen, C.V., A.H. Johannes, S.A. G h e r i n i , R.A. G o l d s t e i n , E.R. A l t v i c k e r , and L. Gomez. 1987. "Expected pH f o r H a l v i n g S u l f a t e i n Adirondack Rain", ASCE Environment Engineering, 113(5):979-993. 2. Schafran, G.C., and C.T. D r i s c o l . 1987. "Comparison of T e r r e s t r i a l and Hypolimnetic Sediment Generation of A c i d N e u t r a l i z i n g Capacity f o r an A c i d i c Adirondack Lake", Environmental Science and Technology, 21(10):988-993. 3. Steen, B. 1986. "Dry D e p o s i t i o n of 4-50 pm Dolomite P a r t i c l e s on Veg e t a t i o n , F l a t Surfaces and D e p o s i t i o n Gauges", Atmospheric Environment 20(8):1597-1604. 4. Sehmel, G.A. 1980. " P a r t i c l e and Gas Dry D e p o s i t i o n : A Review", Atmospheric Environment 14:983-1011. 5. A p p l i n , K.R. and J.M. Jersak. 1986. " E f f e c t s of Airborne P a r t i c u l a t e Matter on the A c i d i t y of P r e c i p i t a t i o n i n C e n t r a l M i s s o u r i " , Atmospheric Environment 20(5):965-969. 6. Zhao, D. and B. Sun. 1986. "Atmospheric P o l l u t i o n from Coal Combustion i n China", J o u r n a l of the A i r P o l l u t i o n C o n t r o l A s s o c i a t i o n 36(4):371-374. 7. Khemani, L.T., G.A. Momin, M.S. Naik, P.S.P. Rao, P.D. S a f a i and A.S.R. Murty. 1987. "Influence of A l k a l i n e P a r t i c u l a t e s On pH of Cloud and Rain i n I n d i a " , Atmospheric Environment 21(5):1137-1145. 8. Mamane, Y. and K.E. N o l l . 1985. " C h a r a c t e r i z a t i o n of Large P a r t i c l e s At a R u r a l S i t e In The Eastern United S t a t e s : Mass D i s t r i b u t i o n and I n d i v i d u a l P a r t i c l e Analyses", Atmospheric Environment 19 ( 4 ) : 611-622. 9. Boynton, R.S. 1966. Chemistry and Technology of Lime and Limestone, John Wiley &.Sons. 10. Jorgensen, C , J.C.S. Chang and T.G. Brna. 1986. " E v a l u a t i o n of Sorbents and A d d i t i v e s For Dry SO, Removal", EPA/600/D-86/025 (NTIS:PB86-160272). 1 11. T r a v i s , E.0. and M.J. Matteson. 1984. "Mass Tra n s f e r of S u l f u r Dioxide to Condensing Aqueous A e r o s o l s " , P a r t i c u l a t e Science and Technology, 2:57-67. 12. S e i n f e l d , J.H. 1986. Atmospheric Chemistry and Ph y s i c s of A i r P o l l u t i o n , John Wiley & Sons. 13. K o h l , A.L. ( E d i t o r ) . 1985. "Gas P u r f i c a t i o n " Fourth E d i t i o n , Gulf P u b l i s h i n g Co. (Houston, Texas). - 109 -14. Plummer, L.N. and D.L. Pairkhurst. 1979. " C r i t i c a l Review of the K i n e t i c s of C a l c i t e D i s s o l u t i o n and P r e c i p i t a t i o n " , A.C.S. Symp Ser 93:537-573. 15. Chan, P.K. and G.T. Ro c h e l l e . 1982. "Limestone D i s s o l u t i o n : E f f e c t s of pH, C0 7, and B u f f e r s Modeled by Mass T r a n s f e r " , A.C.S. Symp Ser. 188:75-987 16. J u d e i k i s , H.S. and T.B. Stewart. 1976. "Laboratory Measurement of S0~ De p o s i t i o n V e l o c i t i e s on Selected B u i l d i n g M a t e r i a l s and S o i l s " , Atmospheric Environment 10:769-776. 17. J u d e i k i s , H.S., T.B. Stewart, and A.G. Wren. 1978. "Laboratory Studies of Heterogeneous Reactions of SO,", Atmospheric Environment 12:1633-1641. Z 18. Urone, P., H. Lutsep, CM. Noyes and J.F. Parcher. 1968. " S t a t i c Studies of Sulphur Dioxide Reactions i n A i r " , Environmental Science and Technology 2(8):611-618. 19. Corn, M. and R.T. Cheng. 1972. " I n t e r a c t i o n s of Sulphur Dioxide w i t h I n s o l u b l e Suspended P a r t i c u l a t e Matter", J o u r n a l of the A i r P o l l u t i o n C o n t r o l A s s o c i a t i o n 22:870-875. 20. N e s b i t t , F.L., C C . Shale and G.W. Stewart. 1978. " K i n e t i c Study of a Low Temperature Flue Gas D e s u l p h u r i z a t i o n Process Using Moist Limestone:, U.S. Dept. of Energy Report METC/RI-78/10. 21. G a u r i , K.L., R. P o p l i and A.C S a r n i a . 1982. " E f f e c t of R e l a t i v e Humidity and G r a i n S i z e on the Reaction Rates of Marble at High Concentrations of S0»", D u r a b i l i t y of B u i l d i n g M a t e r i a l s , 1:209-216, E l s e v i e r S c i e n t i f i c P u b l i s h i n g Co. (Netherlands, 1983). 22. K l i n g s p o r , J . , H.T. K a r l s s o n and I . B j e r l e . 1983. "A K i n e t i c Study of the Dry SO2 - Limestone Reaction at Low Temperature", Chemical Engineering Communications 22:81-103. 23. Berresheim, H. and W. Jaeschke. 1986. "Study of Metal Aerosol Systems as a Sink f o r Atmospheric SO,", J o u r n a l of Atmospheric Chemistry 4:311-334. 24. F r e i b e r g , J.E. and S.E. Schwartz. 1981. "Oxidation of S0 2 i n Aqueous D r o p l e t s : Mass-Transport L i m i t a t i o n i n Laboratory Studies and the Ambient Atmosphere", Atmospheric Environment 15(7):1145-1154. 25. Amdur, E.J. and R.W. White. 1963. "Two-pressure R e l a t i v e Humidity Standards", Humidity and Moisture, V o l . 3:445-454 (Fundamentals and Standards) E d i t e d by A. Wexler and W.A. Wildhack (Reinhold P u b l i s h i n g Corp., N.Y., 1965). - 110 -26. Byers, R.L. and J.W. Davis. 1970. " S u l f u r Dioxide Adsorption and Desorption on Various F i l t e r Media". J o u r n a l of the A i r P o l l u t i o n C o n t r o l A s s o c i a t i o n 20(41):236-238. 27. Slowik, A.A. and E.B. Sansome. 1974. " D i f f u s i o n Losses of S u l f u r Dioxide i n Sampling Manifolds". J o u r n a l of the A i r P o l l u t i o n C o n t r o l A s s o c i a t i o n 24(31):245-247 28. Schwartz, S.E. and J.E. F r e i b e r g . 1981. "Mass-Transport L i m i t a t i o n to the Rate of Reaction of Gases i n L i q u i d Droplets : A p p l i c a t i o n to Ox i d a t i o n of SO- i n Aqueous S o l u t i o n s " , Atmospheric Environment 15(7):1129-11447 29. F i n l a y s o n - P i t t s , B.J. and J.N. P i t t s , J r . 1986:Atmospheric Chemistry :Fundamentals and Experimental Techniques. John Wiley & Sons 30. Bennett, CO. and J.E. Myers. 1962 Momentum, Heat, and Mass Tr a n s f e r , McGraw-Hill Book Company Inc. - I l l -APPENDIX 6.3 B.E.T. SURFACE AREA DETERMINATION USING THE QUANTA-SORB - 112 -SURFACE AREA DETERMINATION USING THE QUANTA-SORB Introduction; When a flow of a mixture of nitrogen and helium Is passed across a solid, the nitrogen will adsorb on the surface of the solid i f the tempera-ture is lowered to liquid nitrogen temperatures. Desorption of N2 occurs when the sample warms up. The surface area of the solid may be calculated once the quantity of nitrogen adsorbed is known as well as the cross-sectional area occupied by one molecule of nitrogen. To obtain a multipoint BET iso-therm, Hz and He are mixed 1n various proportions and the amount of adsorp-tion and desorption of N2 measured at each ratio. Flowsheet and Procedure SURFACE AREA BY GAS ADSORPTION (Quantasorb Instrument) Ng adsorbs on solid at liquid N2 temps -desorbs when liquid N2 is removed thermal conductivity detector measures difference in 2 flows by thermal conductivity - recorder shows imbalance in the 2 streams, calibrated by injection of aliquot of pure Ng into sample stream. - 113 -1. The weighed sample must first be degassed, either by heating while flowing Hz and He across i t , or by cycling adsorption and desorption 3-6 times at P/P0 * 0.3 (by cooling with liquid nitrogen and warming by removal of same). 2. Switch power on and set to 150 amps. Wait for a straight base line to be established on the recorder and then zero the baseline using the coarse and fine adjust controls. Balance the Integrator to give a straight line. 3. Establish an adsorbate (N*) to carrier gas (He) ratio and immerse sample in liquid N2 until a straight base line Is achieved. I.e. Adsorbate flow Carrier flow 1.0 cm 10 cm 2 cm 9 cm 3 cm 8 cm 4 cm 7 cm 5 cm 6 cm 4. Remove the liquid N* and a desorbing signal will appear. 5. Calibrate the signal. Introduce a known amount (e.g. 1.0 ml) of N2 to produce a peak close to the sample peak 1n height. This eliminates any detector linearity problems. (The instrument 1s calibrated using known allquots of pure nitrogen to give peaks on the recorder roughly equivalent 1n size to the sample peak. The "counts" on the Integrator are then used to calculate the exact number of mis of nitrogen adsorbed or desorbed by the sample, for each different P/Po value i e. }°A lCOmlK l0r l*m?r r x mis In aliquot - mis N. adsorbed ' * e * 15.2 "counts" for N9 aliquot ^ 2 c by sample. - 1 1 4 -6. Repeat for successively higher P/P0 ratios. Ifcen P a d - x p ( <v 7 6 3 m ) now a d + n o w c a r r i e r and X • weight of gas adsorbed, determined using calibration volume of N 2 and relative area of peaks. Using al l the values calculated on the multipoint BET Data Sheet, a BET graph may be drawn: Po Then X N • ^ - J - J g • weight of N 2 for monolayer coverage S.A. " X M (6.02xl02 3)-Area of 1 molecule in M„ - mass of 1 molecule Fi l l out the chart, and find the value of the surface area of the hematite sample. H U L T I P C I S I E . £ . T . SURFACE AREA M I A SHEET RUB Ro« Operator AKMft*PO SanmlB GROOM? i>l K g - > T t » J C f Mo.te^UM.") Sample Weight fW) »>*»fcg7- m OUAMTACHROME CORPORATION ill Um Cm t m i * «.». ||MI • ftrnm 11* UI-OHO 1 Adsorbate 1 j almoin Carrier V P Signal Area (A) Calibration Area (A)cal X " T A & , * C X [ ( P O / P ) - ^ i X [ ( P o / p ) - l J I 21-0 227.247 O.Z9« 3.H3Z 2. ^ 32 6.213 | .8o7ri©"' 4./»93xlO~* ; X X 15.5 6B.0 Ifc7. 750 0.219 A.WO 5.007 6.157 U « * I 0 ~ * \-feM5xJ05 X X X 13.0 C50 127.150 O.le^ 6.139 5.155 4.37-* 6.335 I.W3xto"^ l.2lSx«03 17 25-25 4*0 2.6b.M • 0.312 1.1 X l.92t 5 5 1 3 C.l Ol • 18? 5/4 O.Z4o 4.1 Co t .735 |.7H *ID"* 5.617X10^ 1.313/uo3 • VII • nn • 760 Calibration Volume, Vc« Ambient Pressure', pa» ".^V...«BHg Sanple Pressure(above ambient),Pa- jniHg Total Pressure.Pt- Pa+P8- .T.C.?;.?. jmlHg Vapor Preosurf (above ambienth Pg"....?/?f.JtnHg Saturated Vapor Pressure, Pp" Pg* Pa" 780 ,nmHg Ambient Temperature, T •,.".*.'..........*K "Molecular Weight of Adsorbate, Ma-.*....j# g Adsorbate Cross Sectional Area, A<;8" lY?.'. 0... r Calibration Gas Weight, Xe- . 2.3l47<lo 6.235xl0HT 5 23 PLOT 1 Vs P/PQ Ttrtal Surfaca AreafB^*-*^6-02*10 ) A « x[(Po/ P)-l] H. Slo p e ^ S ) . . * ^ . 0 " 9 f M t ^ ( g ) . «t Intercept.d).. AW.?.... » 3 • I _ s • »Por Ife.St- lfa(3.li63 x 103) n 2 - 116 -- 117 -APPENDIX 6.4 SUMMARY TABLES OP EXPERIMENTAL RUNS: LIMESTONE/SO,, REACTION KINETICS - 118 -EXPERIMENT SUMMARY RUN NO. 1 DATE 9/30/86 Reaction Conditions Time (Minutes) Ave 15 75 150 315 Comments Temperature (°C) Relative Humidity (%) Flow Rate (L/min) 18 85 0.63 - 96 82 73 (Std.dev.=12%) (S0 2), Corrected Cj (ppb) C Q (ppb) 41 20 43 42 20 22 43 26 Bed Material Bed Mass (mg) Bed Area (m2 x 103) TIL 92 29.7 Reaction rate constant, k (m/hr) 1.3 1.5 1.2+0.2 0.83 Deposition Velocity, Vj (cm/s) 0.36 0.23 Collision efficiency, 4> x 10^ 0.47 0.30 Notes: - No preconditioning - Co corrected using blanks of Run No. 6 (similar run conditions). - TIL: Texada Island limestone RUN NO. 2 - 119 -EXPERIMENT SUMMARY DATE 10/13/86 Reaction Conditions Time (Minutes) Ave 15 45 195 Comments Temperature (°C) Relative Humidity (X) Flow Rate (L/min) 17 85 0.63 -100 -100 -71 large fluctuations (S0 2), Corrected C 1 (ppb) C Q (ppb) 39 23 42 29 42 41 V Bed Material Bed Mass (mg) Bed Area (m2 x 103) TIL 11 3.55 Reaction rate constant, k (m/hr) 7.4+1.2 4.8 0.26 Deposition Velocity, Vj (cm/s) 2.1 1.3 0.073 Collision efficiency, 4> x 10^ 2.7 0.094 Notes: - No preconditioning - Co corrected using blanks of Run No. 6 (similar run conditions). - 120 -EXPERIMENT SUMMARY RUN NO. 3 DATE 10/22/86 Reaction Conditions Time (Minutes) Ave Comments Temperature (°C) 17 Relative Humidity (%) 83 Flow Rate (L/min) 0.060 (S0 2), Corrected * C f (ppb) C (ppb) L i t t l e difference between blanks and reactors! Bed Material TTL Bed Mass (mg) 10 Bed Area (m2 x 103) Reaction rate constant, k (m/hr) Deposition Velocity, V, (cm/s) Collision efficiency, $ x 10 5 Notes: - No preconditioning - Reactor effluent collected in a plastic bag prior to (S0 9) analysis. - 121 -EXPERIMENT SUMMARY RUN NO. 4 DATE 10/24/86 Reaction Conditions Time (Minutes) Ave 30 80 200 Comments Temperature (°C) 17 Relative Humidity (%) 91 Highly variable Flow Rate (L/min) 1.02 ( S O 2 ) , Corrected Not corrected and Cj (ppb) 30 56 58 highly variable. C Q (ppb) 30 26 50 Bed Material TIL Bed Mass (mg) 10 Bed Area (m2 x 103) 3.23 Reaction rate constant, k (m/hr) Deposition Velocity, V. (cm/s) Collision efficiency, 4» x 10 5 Notes; - No preconditioning - 122 -EXPERIMENT SUMMARY RUN NO. 9 DATE 11/14/86 Reaction Conditions Time (Minutes) Ave 30 120 Comments Temperature (°C) Relative Humidity (%) Flow Rate (L/min) 17 92 0.63 Ave. over 180 min. (S0 2), Corrected C. (ppb) C Q (ppb) 65 61 63 56 67 65 Bed Material Bed Mass (mg) Bed Area (m2 x 103) TTL 9.7 3.13 Reaction rate constant, k (m/hr) 0.79 1.5 0.37 Deposition Velocity, Vj (cm/s) 0.22 0.42 0.10 Collision efficiency, • x 10^ 0.28 0.54 0.13 Notes; - No preconditioning RUN NO. 10 - 123 -EXPERIMENT SUMMARY DATE 11/18/86 Reaction Conditions Time (Minutes) Ave 20 200 360 Comments Temperature (°C) Relative Humidity (%) Flow Rate (L/min) 17 83 1.02 Ave. over 360 min. (S0 2), Corrected C. (ppb) C Q (ppb) 60 57 50 46 74 73 81 80 Large variations i n (S0 2). Bed Material Bed Mass (mg) Bed Area (m2 x 103) TIL 12.9 4.17 Reaction rate constant, k (m/hr) 0.77+0.37 1.3 0.20 0.18 Deposition Velocity, Vj (cm/s) 0.22 0.36 Collision efficiency, • x 10"* 0.28 0.47 Notes: - Apparatus conditioned for 24 hours. - 124 -EXPERIMENT SUMMARY RUN NO. 11 DATE 11/20/86 Reaction Conditions Time (Minutes) Ave 60 155 234 Corrments Temperature (°C) Relative Humidity (%) Flow Rate (L/min) 17 85 0.06 Variable 88 min. ave. (S0 2), Corrected Cj (ppb) C Q (ppb) 47 35 47 32 45 46 34 35 Bag samples* Bed Material Bed Mass (mg) Bed Area (m2 x 103) TTL 9.9 3.20 Reaction rate constant, k. (m/hr) 0.39 0.53 0.36 0.35 Deposition Velocity, V d (cm/s) 0.11 Collision efficiency, <M 10 5 0.14 Notes: - Apparatus conditioned for 72 hours * Large wall losses (30-40%) in reference samples; these were used to correct the reactor samples. Results not considered reliable. - 125 -EXPERIMENT SUMMARY RUN NO. 12 DATE 11/25/86 Reaction Conditions Time (Minutes) Ave 10 60 150 250 Comments Temperature (°C) Relative Humidity OS) Flow Rate (L/min) 17 71 0.63 Humidity averaged over 260 min. (S0 2), Corrected C. (ppb) C Q (ppb) 28 26 34 31 23 21 37 36 19 18 Highly variable. Bed Material Bed Mass (mg) Bed Area (m2 x 103) TTL 11.6 3.75 Reaction rate constant, k (m/hr) .78 0.98 0.96 0.28 0.56 Deposition Velocity, Vj (on/s) 0.27 Collision efficiency, <J> x 10^ 0.35 Notes; - Apparatus conditioned for 8 days with SC^. - Polycarbonate mnrnbrane f i l t e r s used within the reactor. - S0 2 source - strength (Dynacalibrator) increased at t=100 minutes. - 126 -EXPERIMENT SUMMARY RUN N O . 13 DATE 11/27/86 Reaction Conditions Time (Minutes) Ave 10 140 240 Comments Temperature (°C) Relative Humidity (X) Flow Rate (L/min) 17 92 0.63 Average humidity over 240 minutes (S0 2), Corrected C 4 (ppb) C Q (ppb) 64 58 63 56 70 58 67 52 Variable Bed Material Bed Mass (mg) Bed Area (m2 x 103) TIL 10.5 3.39 Reaction rate constant, k (m/hr) 1.2 1.4 0.50 1.3 Deposition Velocity, Vj (cm/s) 0.39 Collision efficiency, • x 10"* 0.50 Notes: - Apparatus conditioned with S02 for 10 days. - Polycarbonate membrane f i l t e r s used. - 127 -EXPERIMENT SUMMARY RUN NO. 14 DATE 12/1/86 Reaction Conditions Time (Minutes) Ave Comments Temperature (°C) Relative Humidity (%) Flow Rate (L/min) 17 96 0.63 Single measurement over 225 min. (S0 2), Corrected C. (ppb) C D (ppb) 50 42.5 Fairly Constant over 240 min. Bed Material Bed Mass (mg) Bed Area (m2 x 103) TIL 10.8 3.49 Reaction rate constant, k (m/hr) 1.9 No decrease over 240 min. Deposition Velocity, Vj (cm/s) 0.53 Collision efficiency, 4>x 10 5 0.68 Notes: - Apparatus conditioned with S09 for 14 days. - 128 -EXPERIMENT SUMMARY RUN NO. 15 DATE 12/2/86 Reaction Conditions Time (Minutes) Ave 30 240 Corrments Temperature (°C) Relative Humidity (%) Flow Rate (L/min) 17 83 0.63 Single measurement over 236 min. (S0 2), Corrected C. (ppb) C D (ppb) 78 73 73 67 81 77 Bed Material Bed Mass (mg) Bed Area (m2 x 103) TIL 11.6 3.75 Reaction rate constant, k. (m/hr) 0.69 0.90 0.52 Deposition Velocity, Vj (cm/s) 0.19 0.25 0.15 Collision efficiency, <frx 10 5 0.25 Notes: - Apparatus conditioned with S0 2 for 15 days. - Average over 240 minutes. - 129 -EXPERIMENT SUMMARY RUN NO. 16 DATE 12/4/86 Reaction Conditions Time (Minutes) Ave 30 240 420 Conments Temperature (°C) Relative Humidity (Z) Flow Rate (L/min) 17 89 0.63 85 94 92 (std.dev.-4.7Z) (S0 2), Corrected Cj (ppb) C D (ppb) 42 33 41 35 42 30 41 32 Bed Material Bed Mass (mg) Bed Area (m2 x 103) TIL 9.8 3.16 Reaction rate constant, k (m/hr) 3.3 2.0±0.6 4.8 3.4 Deposition Velocity, Vj (cm/s) 0.92 Collision efficiency, 4>x 10 5 1.2 Notes: - Apparatus conditioned with S0 2 for 16 days; humidity module conditioned overnight. - 130 -EXPERIMENT SUMMARY RUN NO. 17 DATE 12/5/86 Reaction Conditions Time (Minutes) Ave 30 240 Corrments Temperature (°C) Relative Humidity (%) Flow Rate (L/min) 17 85 0.63 98 85 Highly variable (std.dev.=13%) (S0 2), Corrected C. (ppb) C 0 (ppb) Al 35 AO 29 42 38 Bed Material Bed Mass (mg) Bed Area (m2 x 103) TIL 12.3 3.92 Reaction rate constant, k (m/hr) 1.6 3.6 1.0 Deposition Velocity, Vj (cm/s) O.AA 1.0 0.28 Collision efficiency, 4>x 10 5 0.58 1.2 0.36 Notes: - Average over 240 minutes. - R.H. varied 69% to 99% - apparatus modified for subsequent runs. - 131 -EXPERIMENT SUMMARY RUN NO. 18 DATE 12/11/86 Reaction Conditions Time (Minutes) Ave 90 Comments Temperature (°C) Relative Humidity (X) Flow Rate (L/min) 21.5 86 0.63 95 (Variable:std.dev.= 5.8%) (S0 2), Corrected Cj (ppb) C Q (ppb) 40 30 42 31 Steady over 360 minutes Bed Material Bed Mass (mg) Bed Area (m2 x 103) TTL 10 3.23 Reaction rate constant, k (m/hr) 3.9 5.2 Deposition Velocity, Vj (cm/s) 1.1 1.5 Collision efficiency, 4>x IO 5 1.4 Notes: - Humidity varied 80% - 95% over the run. - Temperature fluctuated 20.7°C - 22.1°C. - 132 -EXPERIMENT SUMMARY RUN NO. 19 DATE 12/15/86 Reaction Conditions Time (Minutes) Ave 30 210 330 Comments Temperature (°C) Relative Humidity (%) Flow Rate (L/min) 21.3 93 1.02 97 90 90 std.dev.=4.7% (S0 2), Corrected C. (ppb) C Q (ppb) 54 48 54 46 54 54 49 51 Bed Material Bed Mass (mg) Bed Area (m2 x 103) TTL 12.2 3.94 Reaction rate constant, k. (m/hr) 1.9 2.7 1.6+0.5 0.91 Deposition Velocity, Vj (cm/s) 0.54 0.76 Collision efficiency, 4>x 10 5 0.68 Notes: - Humidity varried 85% - 100% over run; standard deviation=4.7% - Temperature varried 20.6°C - 22.0°C. - 133 -EXPERIMENT SUMMARY RUN NO. 20 DATE 12/16/86 Reaction Conditions Time (Minutes) Ave Comments Temperature (°C) Relative Humidity (%) Flow Rate (L/min) 21.8 92.6 0.060 (S0 2), Corrected C. (ppb) C D (ppb) Bag sampling. Very large losses to bag walls (50-60%) Bed Material Bed Mass (mg) Bed Area (m2 x 103) TIL 10.9 3.52 Reaction rate constant, k (m/hr) 0.40 Deposition Velocity, V d (cm/s) 0.11 Collision efficiency, <j»x 10 5 0.14 Notes: - Humidity varied 89% - 96%; standard deviation=2.2% - Temperature 21.3°C - 22.1°C. - High S0 2 losses to sample bags, results not considered reliable. - 134 -EXPERIMENT SUMMARY RUN NO. 21 Date 12/18/86 Reaction Conditions Time (Minutes) Ave 30 120 330 Carmen ts Temperature (°C) Relative Humidity (X) Flow Rate (L/min) 21.1 91 0.630 20.6 89 21.3 21.7 88 90 std.dev.=2.4% (S0 2), Corrected C A (ppb) C Q (ppb) 51 34 49 31 51 52 34 38 Bed Material Bed Mass (mg) Bed Area (m2 x 103) Dolomite 9.9 7.09 Reaction rate constant, k (m/hr) 2.7 3.1 2.7 2.0 Deposition Velocity, Vj (cm/s) 1.7 Collision efficiency, 4»x 10 5 0.97 Notes: - 135 -EXPERIMENT SUMMARY RUN NO. 22 DATE 12/19/86 Reaction Conditions Time (Minutes) Ave 30 60 120 240 Confluents Temperature (°C) 21.6 Relative Humidity (X) 83 85 83 82 86 std.dev.=3.6Z Flow Rate (L/min) 1.02 (S0 2), Corrected Cj (ppb) 60 58 59 59 60 C D (ppb) 45 38 42 47 52 Bed Material Dolomite Bed Mass (mg) Bed Area (m2 x 103) 11.3 8.1 Reaction rate constant, k. (m/hr) 2.5 2.0 3.1 1.9 1.2 Deposition Velocity, V d (cm/s) 1.6 Collision efficiency, 4>x 10 5 0.90 Notes; - Humidity varied 77% - 87%; standard deviation=3.6% - Temperature varied 21.1°C - 22.0°C. I - 136 -EXPERIMENT SUMMARY RUN NO. 23 DATE 12/22/86 Reaction Conditions Time (Minutes) Ave Comments Temperature (°C) 21.6 Relative Humidity (%) 93 std.dev.=6.8% Flow Rate (L/min) 0.060 (S0 2), Corrected Cj (ppb) Very large losses C 0 (ppb) i n sample bags. Bed Material Dolomite Bed Mass (mg) 13.0 Bed Area (m2 x 103) 9.31 Reaction rate constant, k (m/hr) Deposition Velocity, V, (cm/s) Collision efficiency, 4>x 10 5 Notes: - Bag sampling - large losses (50-60%) - Humidity varied 83% - 98%. - Temperature varied 21.0°C - 22.2°C. - Results not considered reliable. 

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