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Manufacture of hydroxylamine by reduction of nitric oxide in trickle-bed electrochemical reactors Bathia, Mahendra Liladhar 1978

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MANUFACTURE OF HYDROXYLAMINE BY REDUCTION OF NITRIC OXIDE IN TRICKLE-BED ELECTROCHEMICAL REACTORS by MAHENDRA LILADHAR BATHIA B. Tech (Hons)., Indian Institute of Technology, 1976 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE in THE FACULTY OF GRADUATE STUDIES (Department of Chemical Engineering) We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA June, 1978 © Mahendra Liladhar Bathia , 1978 I In presenting t h i s thesis in p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library shall make it f r e e l y 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 representatives. It is understood that copying or publication of this thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Chemical Engineering Department of ____________________ The University of B r i t i s h Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 Date June 30, 1978. ABSTRACT The manufacture of hydroxylamine by e l e c t r o l y t i c reduction of n i t r i c oxide was experimentally i n -vestigated. The experiments were carried out i n a two-compartment packed bed electrochemical reactor consisting of a cathodic bed of tungsten carbide and a lead plate anode. The compartments were separated by an anion exchange membrane which was supported by a bed of glass beads on the anode side. The reduction occurred as the n i t r i c oxide gas and sulphuric acid e l e c t r o l y t e t r i c k l e d down cocurrently over the tung-sten carbide bed. Sulphuric acid was also c i r c u l a t e d separately through the anode chamber. The e f f i c i e n c y of tungsten carbide p a r t i c l e s for n i t r i c oxide reduction was observed to drop between successive runs. When i n operation the a c t i v i t y re-mained f a i r l y stable for a period of twelve hours. The drop in a c t i v i t y between the runs appeared to be the r e s u l t of surface oxidation of the p a r t i c l e s by dissolved or free oxygen. Tungsten carbide neverthe-less gave better performance than a graphite bed electrode. The operating parameters studied over the s t a b i l i z e d portion of the a c t i v i t y of fresh tungsten Cii) carbide included catholyte flowrate and composition, catholyte recycle, gas flowrate and composition, particle size, bed dimensions and reactor pressure and temperature. In typical operation at atmospheric pressure, a current efficiency of 62% and a hydroxy-lamine concentration of 0.03 M were obtained under a current density of 213 A m . Doubling the pressure approximately doubled the current density under which similar current efficiencies were observed. The hy-droxylamine concentration decreased sharply with in-crease in catholyte flowrate. Increases in gas flow-rate produced only a moderate increase in current efficiency above a certain current density. In re-cycling catholyte, the hydroxylamine concentration was built up almost linearly with number of passes of catholyte through the reactor without appreciable drop of current efficiency. The rate of reduction of n i t r i c oxide to hydroxy-lamine appeared to be controlled by mass transfer. Appropriate mass transfer correlations for trickle-bed reactors were suitably corrected for the present system and applied to predict the theoretical limiting current densities. ( i i i ) TABLE OF CONTENTS Pag e ABSTRACT i i LIST OF TABLES v i i LIST OF FIGURES i x ACKNOWLEDGEMENTS x i i CHAPTER 1 INTRODUCTION AND,BACKGROUNDf 1 1.1 Hydroxylamine 1 1.2 Conventional Manufacturing Processes 2 1.3 Ele c t r o c h e m i c a l Reduction v s . Chemical Reduction 3 1.4 Previous Attempts at E l e c t r o l y t i c Production of Hydroxylamine from N i t r i c Oxide: L i t e r a t u r e Review.. .. 4 2 THEORY . . 8 2.1 Chemistry of E l e c t r o l y t i c Reduction 8 2.2 Oxidation of Hydroxylamine 13 2.3 Tungsten Carbide Packed Bed Electrode 15 2.4 Voltage Requirement 17 2.5 E f f e c t s of Mass Transfer 19 3-" OBJECTIVE 23 (iv ) Pag e CHAPTER 4 EXPERIMENTAL . . . . 25 4.1 Apparatus 25 4.2 Method 39 5 RESULTS AND DISCUSSIONS. 42 5.1 A c t i v i t y of Cathodic Bed 43 1. Tungsten Carbide 43 2. Graphite 52 5.2 Current E f f i c i e n c y and Hydroxy-lamine Concentration 54 5.3 E f f e c t of Catholyte Flow 56 5.4 E f f e c t of Gas Flow 61 5.5 E f f e c t of Gas Composition 65 5.6 E f f e c t of Catholyte Temperature .. 67 5.7 E f f e c t of Catholyte Recycle .... 69 5.8 E f f e c t of Bed Height 73 5.9 E f f e e t . of Bed Dimensions 76 5.10 E f f e c t of C e l l Pressure 78 5.11 E f f e c t of Catholyte Concentra-' t i o n 82 5.12 E f f e c t of WC P a r t i c l e Size 84 . 5.13 Use of P l a t e Electrode and Non-Conductive Bed 86 5.14 Losses of Hydroxylamine on Cathode 88 5.15 A l t e r n a t e C e l l Arrangement 89 5.16 Voltage Requirements . 91 (v) Pag e 5.17 Comparison of Observed E f f e c t s w i t h T h e o r e t i c a l P r e d i c t i o n s 91 5.18 Concentration and C r y s t a l l i -z a t i o n of a D i l u t e S o l u t i o n of Hydroxylamine 95 6 CONCLUSIONS 98 7 RECOMMENDATIONS 104 NOMENCLATURE 107 REFERENCES I l l APPENDICES 1 AUXILIARY EQUIPMENT SPECIFICATION ... 115 2 TABULATED EXPERIMENTAL RESULTS ANALYTICAL TECHNIQUE SAMPLE CALCULATION 118 3 THEORETICAL PREDICTIONS OF MASS TRANSFER AND LIMITING CURRENT DEN-SITY 151 4 RELEVANT PHYSICAL DATA 167 Cvi) LIST OF TABLES Pag e Table 1 Oxidation States of Nitrogen 9 2 Dimensions of Important Parts of the Apparatus Used 35 3 Summary of Typical Properties of the Tungsten Carbide Used 37 4 Summary of Typical Properties of the Anion Exchange Membrane Used 38 5 Ranges of Variables Covered 40 APPENDIX 2 A(l) A c t i v i t y of Tungsten Carbide 120 A(2) A c t i v i t y of Tungsten Carbide Using 5% H 2S0 4 as Catholyte 124 A(3) A c t i v i t y of Tungsten Carbide Over A Single Long Run 127 B(l) A c t i v i t y of Graphite (Untreated) 128 B(2) A c t i v i t y of Graphite (Pretreated) .. .. 129 C Effe c t of Catholyte Flow 130 D Effect of N i t r i c Oxide Flow 132 E Eff e c t of Gas Composition 134 F Eff e c t of Catholyte Temperature 135 G Effe c t of Catholyte Recycle ;136 H Effe c t of Bed Height . . ..138 ( v i i ) Page TABLE I Eff e c t of C e l l Pressure 139 J E f f e c t of WC P a r t i c l e Size 140 K Effe c t of Bed Width 141 L Use of Plate Electrode and Non-Conductive Bed . 142 M Effe c t of Catholyte Strength .. .... .. 143 N Losses of Hydroxylamine on Cathode 144 0 Alternate C e l l Arrangement 145 APPENDIX 3 A Hydrodynamics Calculations 165 B Mass Transfer Calculations 166 ( v i i i ) LIST OF FIGURES Page FIGURE 1 Schematic diagram of the apparatus for the manufacture of hydroxyla-mine 26 2(a) Arrangement of reactor for atmos-pheric operation 29 2(b) Arrangement of reactor for pressure operation 30 3 Photograph of the apparatus used for the manufacture of hydroxylamine 34 4 Scanning electronmicrographs of t y p i c a l tungsten carbide p a r t i c l e s .... 36 5 A c t i v i t y of tungsten carbide for n i t r i c oxide reduction (using 10% I^SO^ as catholyte 44 6 A c t i v i t y of tungsten carbide over a single long run (using 10% I-^ SO^  as catholyte 46 7 A c t i v i t y of tungsten carbide using 5% and 20% H 2S0 4as catholyte 49 8 Scanning electronmicrographs of the fresh and the oxidised tungsten carbide p a r t i c l e s 51 9 A c t i v i t y of graphite for n i t r i c oxide reduction 53 10, Ef f e c t of catholyte flow and current density on current e f f i c i e n c y and hydroxylamine concentration 57 11 Ef f e c t of catholyte flow on current e f f i c i e n c y and hydroxylamine concentration for a fixed value of current density 58 (ix) FIGURE Page 12 Effect of catholyte flow and current density on the rate of hydroxy-lamine production 59 13 Effect of n i t r i c oxide flow and current density on current e f f i -ciency and hydroxylamine concen-tration 62 14 Effect of n i t r i c oxide flow on current efficiency and hydroxy-lamine concentration for a fixed value of current density 63 15 Effect of gas composition on current efficiency and hydroxy-lamine concentration . . . . 66 16 Effect of catholyte temperature on current efficiency and hydroxy-lamine concentration 68 17 Accumulation of hydroxylamine with time in 1010 cm.3 (average) of re-cycling catholyte 70 18 Accumulation of hydroxylamine with time in 480 cm^  (average) of re-cycling catholyte 72 19 Effect of bed height on current efficiency for fixed values of current density 74 20 Effect of bed height on hydroxy-lamine concentration for fixed values of current density . . . 75 21 Effect of bed width and current density on current efficiency and hydroxylamine concentration 77 22 Effect of c e l l pressure and current density on current efficiency and hydroxylamine concentration 79 23 Effect of partial pressure of n i t r i c oxide in gas stream (containing n i t r i c oxide and nitrogen) on the rate of hydroxylamine production 81 (x) FIGURE Page 24 E f f e c t of H2SO4 concentration i n c a t h o l y t e on current e f f i -c i ency 83 25 E f f e c t of tungsten carbide p a r t i c l e size on current e f f i -c i ency and hydroxylamine concentration 85 26 Comparison of e f f i c i e n c i e s of a tungsten carbide packed bed, an i n e r t bed and a p l a t e electrode f o r n i t r i c oxide r e d u c t i o n 87 27 T y p i c a l voltage requirements i n the c e l l under d i f f e r e n t flow c o n d i t i o n s 92 28 E f f e c t of gas flow and l i q u i d flow on t h e o r e t i c a l l y p r e d i c t e d l i m i t -ing current d e n s i t y 94 29(a) C r y s t a l s of hydroxylamine sulphate obtained from a 3M s o l u t i o n of commercially a v a i l a b l e s a l t 97 29(b) C r y s t a l s of hydroxylamine sulphate obtained from a 0.4M s o l u t i o n produced i n r e c y c l e run of Figure 18 197 APPENDIX 3 A Flow p a t t e r n diagram f o r cocurrent g a s - l i q u i d flow on packed beds 154 APPENDIX 4 A S o l u b i l i t y of hydroxylamine sulphate i n aqueous s o l u t i o n s of s u l p h u r i c a c i d 169 B S o l u b i l i t y of n i t r i c oxide gas i n aqueous s o l u t i o n s of s u l p h u r i c a c i d 170 (xi) ACKNOWLEDGEMENTS The author wishes to express his thanks to Dr. Paul Watkinson, under whose supervision this work was conducted, for his advice and encourage-ment throughout the whole of-this work. Thanks are also due to Mr. Colin Oloman for his sincere interest and suggestions during the experimental phase of the work. Appreciation is extended to Dr. Norman Epstein for his suggestions and for his willingness to schedule classes in the evenings so as to accommodate long experimental runs. The author is also indebted to the faculty, staff, students and the workshop, who contributed in their own way towards the completion of this project. Financial support in the form of fellow-ship from The University of British Columbia is gratefully acknowledged. (xii) 1 CHAPTER ONE INTRODUCTION AND BACKGROUND 1.1 HYDROXYLAMINE [1,3] Hydroxylamine was discovered by Lossen in 1865 in the form of its salt, hydroxylammonium chloride. The isolation of free hydroxylamine was reported in 1891 by Lobry de Bruyn. Hydroxylamine, N^OH, is an interesting compound of nitrogen which in the past four decades has achieved considerable industrial importance. It is a colourless, hygroscopic crystalline solid. As shown by its formula, hydroxylamine is closely related to ammonia and exhibits many of the basic properties of ammonia. It is most commonly handled as one of its salts, since these are much more stable than the free base i t s e l f . The most fre-quently used hydroxylammonium salts in industrial and laboratory work are the chloride, (NH^OHJCl; sulphate, (NH3OH)2S04; and acid sulphate, (NH3OH)HS04. One of the largest commercial uses of hydroxylamine is captively in the synthesis of caprolactam, the raw material for nylon 6. In 1970, the output of this fibre intermediate exceeded a million tonne mark. It is widely useful in the transformation of organic compounds to de-rivatives, which in turn may be intermediates in phar-maceutical or other industrial syntheses of complex molecules. It is well-known as a laboratory chemical f o r the q u a l i t a t i v e and q u a n t i t a t i v e determination of carbonyl compounds as oximes. The o x i d a t i o n - r e d u c t i o n c a p a b i l i t i e s of hydroxylamine make i t e s s e n t i a l i n many d i f f e r e n t a p p l i c a t i o n s . An e x c e l l e n t account of the use-f u l p r o p e r t i e s and important a p p l i c a t i o n s i s found i n [1] . 1.2 CONVENTIONAL MANUFACTURING PROCESSES A l l the conventional manufacturing processes [1,2, 3] to date are n o n - e l e c t r o l y t i c . The important ones are described below. a) The c l a s s i c a l method f o r the production of hydroxylamine i s that a s c r i b e d to Raschig. I t c o n s i s t s of reduction of a n i t r i t e w ith b i s u l p h i t e and sulphur d i o x i d e at 0°C to produce hydroxylammonium disulphonate which i s then hydrolyzed to form hydroxylammonium sulphate. The o v e r a l l r e a c t i o n s , using sodium compounds, can be given as NaN02+ NaHS03+ S0 2 + HON(S0 3Na) 2 (1.1) HON(S0 3Na) 2+ 2H 20 + (NH^OH)HSO^-t- Na 2S0 4 (1.2) There have been innumerable m o d i f i c a t i o n s of t h i s basic process. Ammonium s a l t s are used almost e x c l u s i v e l y at present and t h i s r e -s u l t s i n ammonium sulphate being obtained i n large amounts as by-product. This c o n s t i t u t e s a major disadvantage to t h i s process because of low r e s a l e value of the sulphates. b) The h y d r o l y s i s of a primary n i t r o p a r a f f i n w i t h water and a strong a c i d , such as s u l p h u r i c or hy d r o c h l o r i c a c i d , at temperatures of 100-150°C, i s now used as a convenient method f o r the manu-fa c t u r e of hydroxylammonium s a l t s . The general r e a c t i o n i s RCH2N02+ H 20 + H 2S0 4 + (NH3OH)HS04+ RCOOH....(1.3) 3 Economics do not favour this process any better than the previous one and hence the Raschig s t i l l remains a more widely used process. c) The reduction of n i t r i c oxide with hydrogen i n acid solution can be made selective for the pre-paration of hydroxylammonium s a l t s , and this has been the p r i n c i p l e of the recently commercialized BASF hydroxylamine process. Reduction takes place in the presence of platinum-containing catalyst and ammonium sulphate i s the chief by-product. NO + 1.5H2+ 1.5H2S04+ NH3 -> (NH3OH)HS04 +0.5 (NH 4) 2S0 4 (1.4) The method of preparation, nature, a c t i v i t y and s e l e c t i v i t y of the catalyst dictate the success-f u l operation of the above reaction and this has been the subject of numerous German, Netherlands, Russian, Japanese, B r i t i s h and U.S. patents. 1.3 ELECTROCHEMICAL REDUCTION vs. CHEMICAL REDUCTION When the electrodes that are selective for the desired process are available at reasonable cost, an electrochemical reduction process may have advantage over chemical reduction processes.. This i s p a r t i c u l a r l y so in situations where e l e c t r i c a l power i s less costly than chemical reducing agents. However, electrochemical synthesis operations i n -volving gaseous reactants cannot be carried out at high rates in conventional electrochemical reactors owing to the heterogeneous nature of such operations. The problem i s more acute in cases where the s o l u b i l i t y of the gaseous reactant i s low in commonly used e l e c t r o l y t e s . The reduction of 4 n i t r i c oxide i s a case i n p o i n t . The volumetric r a t e s of such e l e c t r o l y t i c r e a c t o r s w i l l then be much lower i n comparison w i t h the volumetric r a t e s of chemical r e a c t o r s and hence l e s s a t t r a c t i v e than the l a t t e r . In order to compete, favourably i n t h i s regard and s t i l l have the advantages o u t l i n e d before, e l e c t r o -chemical r e a c t o r s f o r such synthesis must be designed w i t h electrodes which e x h i b i t large surface area per u n i t volume f o r the r e a c t i o n . 1.4 PREVIOUS ATTEMPTS AT ELECTROLYTIC PRODUCTION OF HYDROXYLAMINE FROM NITRIC OXIDE: LITERATURE REVIEW Hydroxylamine has been reported to have been pro-duced by e l e c t r o l y t i c r e d u c t i o n of n i t r i c oxide or n i t r i c a c i d . Considerably more informat i o n e x i s t s on the e l e c t r o l y t i c r e d u c t i o n of n i t r i c a c i d , mostly i n the patent l i t e r a t u r e , and an experimental p l a n t i n Poland has reported a y i e l d of 94% [ 1 ] . Two experimental st u d i e s have been reported on the e l e c t r o l y t i c r e d u c t i o n of n i t r i c oxide to produce hydroxylamine. Savodnik [4-6]and Shepelin [ 7 , 8 ] ' i n v e s t i g a t e d the p o s s i b i l i t y of c a r r y i n g out the e l e c t r o c h e m i c a l synthesis of hydroxylamine on a platinum e l e c t r o d e . They proved that i n the r e d u c t i o n of n i t r i c oxide by molecular hy-drogen on platinum, a coupled e l e c t r o c h e m i c a l r e a c t i o n c o n s i s t i n g of two p a r t i a l . r e a c t i o n s occurs, v i z . a cathode r e a c t i o n 2N0 + 6e" + 6H 30 + 2 N H 2 0 H + 6H 20 ( 1 . 5 ) 5' and an anode process 3H„ -•6e^ + 6H o0 + 6H T0 + (1.6) O v e r a l l : 2NO + 3H 2 (1.7) The r e a c t i o n s show that the components ( v i z . n i t r i c oxide and hydrogen) i n t e r a c t not wi t h each other but with the metal at the m e t a l - s o l u t i o n boundary where e l e c t r o n t r a n s f e r occurs. Savodnik et a l . c a r r i e d out experiments i n a three-electrode c e l l w i t h separate anode and cathode com-partments. A platinum wire and sheet were used as an-ode and cathode r e s p e c t i v e l y . The experiments were c a r r i e d out at 25°C i n a v i g o r o u s l y s t i r r e d s o l u t i o n of 3N s u l p h u r i c a c i d . I t was found that the r e a c t i o n i s c o n t r o l l e d by d i f f u s i o n w i t h respect to supply of the components to the platinum surface. Increases i n the rate of flow of the d i s s o l v e d gases to the c a t a l y s t surface, i . e . increases i n the process r a t e s were l i m i t e d by the speed of the s t i r r e r . Higher pressure could not be used because of t h e i r equipment design l i m i t a t i o n s . To determine whether the i n t e n s i f i c a t i o n of the process was p o s s i b l e , they turned to a svstem i n v o l v i n g the use of a p a r t i a l l y immersed e l e c t r o d e . Due to the appearance of a very t h i n l i q u i d f i l m on the metal above the meniscus , 1 thickness of the d i f f u s i o n l a y e r was 6 reduced and the stirring of the solution was no longer required. However, the reaction used to cease within 20-30 minutes from the start due to the drying of the acid, film owing to the accumulation of a salt-like product in i t and to d i f f i c u l t y in its removal into the bulk solution. This problem was solved by locally heating the electrode to 52-65°C. The film on this partially im-mersed locally heated electrode was in a state of dy-namic motion owing to evaporation of water from the film electrolyte. By further improvement of the sur-face characteristics of the electrode by hydrophobi-zation, etc., they were able to obtain a rate of 6.7x10 mole hydroxylamine/min per cm of catalyst surface. L.J.J. Janssen (9,10) investigated the effect of various parameters on the reduction of n i t r i c oxide at a flow-through mercury plated nickel electrode. He carried out his experiments in a thermostated two-compart-ment: •' c e l l with a mercury coated nickel gauze electrode serving as cathode and a platinum f o i l as anode. A porous glass diaphragm separated the anode and the cath-ode compartment and a 1M'(9.4%) ^ SO^ solution was c i r -culated in the cathode compartment. Some of the inter-esting observations made by him are: a) Apart from a substantial amount of NH2OH and H-2 > small amounts of nitrous oxide (N2O) and traces of ammonia are formed in the reduction of NO. 7 b) The r a t i o between the NH^OH and N-0 formation depends on the current density ana the flow-rate of the e l e c t r o l y t e through the cathode, but does not depend on the H2SO4 concentration in the investigated range from 0.25M (2.4%) to 2. OM (18.0%) or on temperature. c) The rate of reduction of n i t r i c oxide to NF^OH and N2O increases with increasing current density up to a maximum value, thereafter this rate decreases with increasing current density. d) At current densities much lower than the current density for which the rate of reduction of NO reaches i t s maximum, the reduction of NO i s affected by both the electrochemical parameters and by the transport of.NO to the electrode surface. However, at high current densities the reduction i s dominated by mass - transport of NO only. 8 CHAPTER TWO THEORY 2 .1 CHEMISTRY OF ELECTROLYTIC REDUCTION Nitrogen forms compounds having a l l the o x i d a t i o n states from - 3 to +5 [11]. The most important compounds i n the var i o u s o x i d a t i o n s t a t e s are l i s t e d i n Table 1. From t h i s t a b l e , hydroxylamine has an o x i d a t i o n number of -1 and n i t r i c oxide, +2 . C l e a r l y , the re-duction of n i t r i c oxide to hydroxylamine i s a 3 - e l e c t r o n t r a n s f e r process. A l s o , depending on the process con-d i t i o n s and the nature of electrode m a t e r i a l , thermo-dynamically i t i s p o s s i b l e to reduce n i t r i c oxide to any one (or more than one) of the compounds i n lower o x i -d a t i o n s t a t e than NO; v i z . N 20, N 2, NH2OH, N 2H 4 and NH^. Numerous experimental studies [4-10, 12-14] however, have shown that i n strong a c i d i c s o l u t i o n s (1-4M) and at various electrode m a t e r i a l s , n i t r i c oxide i s reduced p r i m a r i l y to hydroxylamine. Polarographic r e d u c t i o n studies [5, 9, 12, 13] of n i t r i c oxide d i s s o l v e d i n strong a c i d s o l u t i o n y i e l d two waves; the f i r s t of these waves corresponds to one e l e c t r o n r e d u c t i o n of NO while the sum of these two waves i s d i f f u s i o n c o n t r o l l e d and corresponds to 3 - e l e c t r o n reduction of NO. From Table 1, i t can be seen that one e l e c t r o n r e d u c t i o n of NO co r r e s -ponds to the formation of e i t h e r N 20 or H 2N 20 2 (or NOH). Of these, only the l a t t e r has been observed to be the intermediate. A c c o r d i n g l y s e v e r a l workers [5,10,12] 9 -TABLE 1 Oxidation States of Nitrogen (Ref. 11) Oxidation Number Compound Formula < Q i—i o 2 o I—I E—' U ZD Q m -3 -2 -1 0 + 1 + 2 + 3 +'4 + 5 Ammonia NH3 Hydrazine N 2H 4 Hydroxylamine NH2OH Nitrogen N2 Nitrous Oxide N 20 Hyponitrous A c i d H2 N2°2 or NOH N i t r i c Oxide NO Nitrou s A c i d HN02 Nitrogen Dioxide N0 2 Nitrogen Tetroxide N2°4 N i t r i c A c i d HN03 have proposed the f o l l o w i n g mechanism f o r the f o r -m a t i o n of NI^OH: NO + H + + e" -> NOH (2.1) NOH + 3H ++ 2e~ -»• NH 3OH + (2.2) Sum: NO + 4H ++ 3e" + NH 3OH + (2.3) From L a t i m e r [ I T ] , the s t a n d a r d p o t e n t i a l f o r r e a c t i o n (2.1) i s 0.71 and t h a t f o r r e a c t i o n (2.2) i s 0.496; so the s t a n d a r d p o t e n t i a l f o r the sum of these two r e a c t i o n s i s c a l c u l a t e d t o be 0.567. In s u l p h u r i c a c i d s o l u t i o n s the r e d u c t i o n of NO to NH2OH can the n be d e s c r i b e d by the f o l l o w i n g equa-t i o n s : At the cathode: NO + 4H + + 3e" + NH 30H + E Q = 0.567 ...(2.3) H 2 S 0 4 -> 2H + + S 0 4 = ^  E Q= 0 (2.4) 2NH 30H + + S 0 4 = -> (NH3QH)2'S04 E Q= 0 (2.5) O v e r a l l cathode: 2N0 + 6H + + 6e" + H 2 S 0 4 + '(NH.3OH) 2 S 0 4 E = 0.567 . .. (2.6) o At the anode: 3H 20 -> § 0 2 + 6H + + 6e" E Q= 1.229 ...(2 . 7) The s i g n c o n v e n t i o n f o r s t a n d a r d e l e c t r o d e p o t e n t i a l used here i s t h a t of I n t e r n a t i o n a l U nion o f Pure and A p p l i e d C h e m i s t r y . 11 Hydroxylamine obtained here i s in the form of i t s s a l t , hydroxylammonium sulphate (NH^OH^SO^, which, as mentioned in Section 1.1, i s more stable than the free base hydroxylamine. Masek [13] observed i n his polarographic studies that the f i r s t wave in the reduction of NO corresponds to s l i g h t l y less than a one-electron reduction and he introduces a mechanism which involves the formation of a dimer (NO) 2• Following the work of Masek, Janssen [10] proposes that the dimer (NO) 2 reacts with H + ions to form NOH. So i t seems that the actual path of re-duction to NOH may by i t s e l f be quite complex. Conse-quently, whatever l i t t l e information available i n l i t -erature on the ki n e t i c s of the reduction to NH20H appears to be based on fragmentary grounds. A l l the workers, however, have observed that the reduction of NO to NH20H is quite rapid and the rate of formation of NH20H i s con-t r o l l e d by. mass transfer. In weak acidic solutions [pH=5-7] and at mercury electrodes, NO i s reduced primarily to N20. Ehman and Sawyer [12]' propose the following mechanism which again involves the formation of intermediate NOH. NO + H + + e" NOH .......(2.1) 2NOH -> N 20 + H 20 (2.8) Overall: 2N0 + 2H+ + 2e~ N o0 + H„0 E = 1.59 (2.9) 2 2 o v J In very strong a c i d i c s o l u t i o n s (e.g. 8M H-^ SO^ ) NO i s reduced p r i m a r i l y to NH^: NO + 6H + + Se" + NH* + : H 20 E q = 0.88 (2.10) I t can be seen that the p o t e n t i a l s r e q u i r e d f o r the formation of N 20 and NH^ are higher than those r e -quired f o r the'formation of NH20H. A l s o , N 20 and NH^ are formed under d i f f e r e n t e l e c t r o l y t e c o n c e n t r a t i o n c o n d i t i o n s . However, even when the c o n d i t i o n s are such that the primary r e d u c t i o n i s to NH2OH, traces of N 20 and NH^ are normally observed because of non-uniform concen t r a t i o n and p o t e n t i a l gradient d i s t r i b u t i o n ex-i s t i n g i n a r e a l system. This i s e s p e c i a l l y so i n the present case, where a packed bed electrode was used. Traces of n i t r o g e n found i n the o u t l e t gas have been shown [10] to be present because of i m p u r i t i e s i n the c y l i n d e r NO (0.6% N 2 i n our case) and are not as a r e s u l t of re d u c t i o n process. F i n a l l y , since the e l e c t r o l y t e used f o r the r e -duc t i o n of NO i s an aqueous s o l u t i o n of s u l p h u r i c a c i d , the c u r r e n t e f f i c i e n c y f o r the major electrode r e a c t i o n i s often lowered due to the simultaneous decomposition of the water present [15]. This process r e s u l t s i n the hydrogen formation at the cathode: H 30 + +e" j H 2 + H 20. E Q = 0 (2.11) The above r e a c t i o n can very o f t e n become the domi-nant process i n an el e c t r o c h e m i c a l r e a c t o r unless s p e c i a l 13 attention i s given to design and operation. 2.2 OXIDATION OF HYDROXYLAMINE Oxides of nitrogen are subject to a number of de-composition and hydrolysis reactions. This requires that great care be taken i f meaningful concentrations of the product are to be achieved. In the present case, a chain of rapid reactions i s set fo r t h by the contact of oxygen with n i t r i c ox-ide of the system: 2N0 + 0 2 + 2N02 (2.12) N0 2 reacts with water to form HN02 and HN03 2N02 + H 2p + HN02 + HN03 (2.13) Nitrous acid (HN02) thus formed reacts immediately with NH20H according to NH20H + HN02 + N 20 + 2H20 (2.14) thus r e s u l t i n g i n a loss of NH2OH by oxidation to N 20. It should be noted that, in the absence of n i t r i c oxide, hydroxylamine (especially in the form of i t s salts) w i l l not be oxidized at ordinary temperatures. The loss of hydroxylamine as a r e s u l t of decom-position c a l l s for the need to have a two-compartment re-actor so that the oxygen generated at the anode does not 14 come in contact with the n i t r i c oxide on the cathode side. A porous diaphragm, which i s a commonly used c e l l dividing medium, has been found to be i n e f f e c t i v e i n the present case since i t would permit p a r t i a l d i f -fusion of oxygen. An ion exchange membrane, preferably of an anion s p e c i f i c type, would be expected to provide a complete separation ensuring that even the NH^OH* ions do not diffuse to anode compartment. Since a large excess of n i t r i c oxide gas i s used for the reaction, the decomposition reactions become p a r t i c u l a r l y c r i t i c a l when the product at the outlet of the c e l l i s analysed for NF^OH. Before doing so, the unreacted n i t r i c oxide must be stripped off the l i q u i d sample (acid e l e c t r o l y t e containing Nfi^OH) by an inert gas. F a i l i n g to do so invariably destroys most of the hydroxylamine as soon as the NO-electrolyte mixture comes in contact with the a i r , and the a n a l y t i c a l tech-nique used recognizes l i t t l e or no hydroxylamine. Ab-sence of such a precaution i s the reason why Bruno Piccone [16] obtained apparently inconsistent results and current e f f i c i e n c i e s , calculated on the basis of actual amount of NP^OH present i n a sample of product at the outlet of c e l l , i n the range of 0-10%. Nitrous acid and n i t r i c acid produced by NO^  (Equation (2.13)) not only destroy NH^OH but also may corrode the surface with which they are i n contact. This dictates expensive materials of construction for the experimental system. N i t r i c oxide by i t s e l f i s non-corrosive and most common metals of construction can be used [17]. An a i r - t i g h t c e l l with a s l i g h t positive pressure i s also of prime importance i f no atmospheric a i r i s to enter the reactor. F i n a l l y , i t should be r e a l i z e d that the oxidation of a f r a c t i o n of product i s inevitable because of im-p u r i t i e s in the n i t r i c oxide cylinders (0.1% N0 2 i n our case) and because of dissolved oxygen in the e l e c t r o l y t e . 2.3 TUNGSTEN CARBIDE PACKED BED ELECTRODE N i t r i c oxide can be reduced, to NK^OH on platinum, on amalgamated phosphor bronze or on mercury, cadmium or t i n covered phosphor bronze [9] . Definite experimental evidences are available i n l i t e r a t u r e for reduction of NO on a mercury cathode [10,12,13] and a platinum cath-ode [5-8] .. Because of the side reaction involving generation of hydrogen and corrosive nature of NO con-taining acid e l e c t r o l y t e , p r a c t i c a l cathodes should have high hydrogen overpotential and be inert towards elec-t r o l y t e . In recent years, tungsten carbide (WC) has been shown to have platinum-like behaviour as a catalyst [18-21], therefore i t was t r i e d out as a cathode i n this work. Although tungsten carbide i s by no means as 16 i n e r t a m a t e r i a l as platinum, t h i s disadvantage i s more than o f f s e t by i t s low cost compared to platinum. In the present work, graphite, was al s o t r i e d out as an electrode m a t e r i a l i n some of the i n i t i a l runs but was found to be l e s s than h a l f as e f f i c i e n t as tungsten c a r b i d e , because of the low hydrogen o v e r p o t e n t i a l of the former as compared to WC. To be commercially a t t r a c t i v e , an e l e c t r o l y t i c r e d u c t i o n process would need to employ current den-_ 2 s i t i e s of the order of 1000 Am . Owing to an extremely heterogeneous nature of NO re d u c t i o n (Section 1.3) be-cause of i t s low s o l u b i l i t y i n a c i d i c e l e c t r o l y t e s (Appen-dix" V). and the l a r g e thickness of the d i f f u s i o n l a y e r , the r e d u c t i o n of NO cannot occur w i t h high current d e n s i t y at p l a i n e l e c t r o d e s . High current d e n s i t y would r e q u i r e high r a t e s of mass t r a n s f e r ; t h i s i s d i f f i c u l t to achieve on a p l a i n surface f o r r e l a t i v e l y i n s o l u b l e gases. A packed bed wit h gaseous NO present and operating i n a t r i c k l e - b e d mode has the f o l l o w i n g advantages: 1) A packed bed provides large i n t e r f a c i a l area per u n i t volume so that the current d e n s i t y through the membrane ( s u p e r f i c i a l current density) i s s i g n i f i c a n t l y higher than that at the electrode surface. In t h i s way the s i z e and the c a p i t a l cost of the re a c t o r can be kept to a minimum. 2 ) The main r e a c t a n t , NO, can be stored i n a c i d s o l u t i o n i n clo s e p r o x i m i t y to the electro d e surface. The s o l u t i o n gets depleted of NO due to r e a c t i o n as i t goes down the bed. By p r o v i d -ing an excess of NO, the s o l u t i o n surrounding 17 the bed particles can always be kept saturated and thus the problem of low solubility overcome. This also reduces the activation overpotential of system. 3) The use of excess NO forces the electrolyte through the packed bed and keeps the liquid film on the particles relatively thin. This tends to minimize the diffusional resistances in the liquid phase. Packed bed electrodes, however, have the dis-advantage that uniformity of electrode potential cannot be maintained across the bed thickness. The uniformity of concentration is usually less easily maintained [22]. Both of these would tend to promote side reactions in the present case (Section 2.1). Moreover, as pointed out by Armstrong [22], the advantages of the packed bed electrode become decreasingly worthwhile as the bed thickness is increased, mainly because of ohmic drop through the bed. The active volume of the electrode is essentially confined to a narrow region nearest to the counter electrode and this volume is inversely propor-tional to the superficial current density. For the case of a two-phase mass transfer controlled reaction, the maximum thickness of the active region (in the direction of current) has been shown by Armstrong to be about 1 cm. 2.4 ' VOLTAGE REQUIREMENT The potentials given in equations (2.3)-(2.7) are the standard potentials, i.e. potentials when the activities of species taking part in the electrode re-actions are unity. The Nernst equation applied to these r e a c t i o n s at 25°C gives the e q u i l i b r i u m p o t e n t i a l s , E and E , which are the minimum cathode and anode C a voltages r e q u i r e d : [NH OH ] E = 0 . 567 + 0.0086 l n — r (2.15) [ N 0 ] [ H + ] 4 E a = 1.229+0.0064 l n { [ 0 2 ] [ H + ] 4 } (2.16) According to equation (2.15) the formation of hy-droxylamine i s favoured by an increase i n NO pressure or hydrogen i o n co n c e n t r a t i o n and a decrease i n hy-droxylamine c o n c e n t r a t i o n . The t o t a l c e l l voltage (V i ^ ) would in c l u d e the e f f e c t i v e p o t e n t i a l drop due. to bed matrices and e l e c t r o l y t e s ( I R p s ) , membrane- CIR^) a n d the over-p o t e n t i a l s f o r the main r e a c t i o n s as w e l l as side r e -a c t i o n s (n^) : V c e l l = E a + lEcl + E M + I R P s + I Rm ••••••(2.17) The use of an anion exchange membrane would i n -crease IR over a diaphragm because the former tends to prevent the n a t u r a l m i g r a t i o n of H + ions i n the d i r e c ^ t i o n of p o t e n t i a l g r a d i e n t , thus i n c r e a s i n g the e l e c t r i -c a l energy consumption. A l s o , a packed bed of high gas voidage (or low l i q u i d holdup) would tend to have a l a r g e r value of IRp S because of d i s c o n t i n u i t i e s i n the bed m a t r i x . An e l e c t r o l y t e having high a c i d c o n c e n t r a t i o n w i l l be expected to have a low i R p S -19 2 . 5 EFFECTS OF MASS TRANSFER Since the reduction of NO to N^OH has been r e -ported to be d i f f u s i o n c o n t r o l l e d (Section 1.4, 2.1), i t i s appropriate to give t h i s e f f e c t some considera-t i o n . The r e d u c t i o n of NO to Nf^OH can be v i s u a l i z e d to be occurring i n f o l l o w i n g steps: 1) Transfer of NO from gas bulk to g a s - l i q u i d i n t e r f a c e ; mass t r a n s f e r c o e f f i c i e n t k^ 2) Transfer of NO from g a s - l i q u i d i n t e r f a c e to bulk of l i q u i d ; mass t r a n s f e r c o e f f i c i e n t k^ 3) Transfer of NO from bulk of l i q u i d to l i q u i d -s o l i d i n t e r f a c e ; mass t r a n s f e r c o e f f i c i e n t kg 4) Reduction of NO on s o l i d s urface. The removal of product NF^OH from s o l i d surface to bulk of l i q u i d i s not considered here because of high s o l u b i l i t y of NH^OH s a l t i n a c i d e l e c t r o l y t e [Appendix 4]. The steps ( l ) - ( 3 ) can be lumped together under 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 (k ) , whereas the step (4) can be represented by Faraday's law. Under steady s t a t e , Moles of NFL OH formed ^ _ i (m2 of catalyst surfacloOO = h ^ O ? = *° So,.)''' t 2 ' 1 8 ) 20 where 1 Current d e n s i t y , A m Number of elect r o n s associated w i t h the formation of NFL OH 3 [Equation 2.3] F Faraday's constant = 96493 coul (gm equiv) 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 , m s" -1 -1 o l / ( l / k s + l / k L + l / k Q ) (2.19) C NG concentr a t i o n of NO i n bulk of gas, moi. m~3 b C^ Q = Concentration of NO on s o l i d s u r f a c e , s moi. m"3 For a given set of process and flow c o n d i t i o n s , as the current d e n s i t y i s increased, the surface con-c e n t r a t i o n of NO decreases due to r e a c t i o n u n t i l i t be-comes so small that i t can p r a c t i c a l l y be taken to be zero. This current d e n s i t y i s denoted by mass t r a n s f e r l i m i t e d c u r r e n t d e n s i t y or l i m i t i n g current d e n s i t y , Since the gas fl o w r a t e i n our system i s much higher compared to l i q u i d f l o w r a t e and a l s o since the gas i s p r a c t i c a l l y pure NO, the gas-phase r e s i s t a n c e can be considered n e g l i g i b l e compared to l i q u i d - p h a s e r e s i s t a n c e , and the g a s - l i q u i d i n t e r f a c e can be assumed to be satu-r a t e d w i t h gas (NO). From t h e o r e t i c a l c a l c u l a t i o n s of b (2.20) 21 k and k T (Appendix 3) to be discussed l a t e r , i t was found that f o r the lowest gas and the highest l i q u i d f l o w r a t e s used i n our system, k^ - 46 k^. This means that k Q i s p r a c t i c a l l y independent of k^, and the above assumption i s w e l l j u s t i f i e d : thus i L = 3F k Q C s (2.21) where -3 C s = S o l u b i l i t y of NO i n e l e c t r o l y t e , mol a m For f i x e d values of k and C , i f the a p p l i e d o s ^ r current d e n s i t y exceeds i ^ , the excess c u r r e n t i s wasted f o r the generation of hydrogen or side r e a c t i o n s , be-cause no ex t r a NO should t h e o r e t i c a l l y be a v a i l a b l e at the electrode surface above the l i m i t i n g current d e n s i t y . Hence the current e f f i c i e n c y f o r NH2OH production de-creases. I f higher current d e n s i t i e s are to be used, the i ^ has to be increased i f the excess current i s to be e f f e c t i v e l y u t i l i z e d . I t i s always d e s i r a b l e to have high l i m i t i n g current d e n s i t i e s so that the side r e a c t i o n s are minimized. The l i m i t i n g current d e n s i t y on an el e c t r o d e can be increased by i n c r e a s i n g k Q or C^. In a packed bed el e c t r o d e , t h i s can be achieved by i n c r e a s i n g gas or l i q u i d f l o w r a t e s or the pressure. This has been given adequate t h e o r e t i c a l c o n s i d e r a t i o n i n Appendix 3. The c a l c u l a t i o n of k„ i s based on the best a v a i l a b l e 22 e m p i r i c a l c o r r e l a t i o n s s u i t e d to our system and i t i s r e a l i z e d that the increase of gas f l o w r a t e i s not as e f f e c t i v e as the increase of l i q u i d f l o w r a t e f o r i n -creasing k Q and hence i ^ . Both the mass t r a n s f e r co-e f f i c i e n t and gas s o l u b i l i t y w i l l be expected to i n -crease w i t h the gas pressure, as shown i n Appendix 3. However, a decrease i n C g would be expected w i t h i n -crease i n temperature, but there was no observed e f f e c t of temperature on current e f f i c i e n c y f o r temperatures upto 40°C. Since the r e d u c t i o n of NO to NH2OH has been r e -ported to be quite r a p i d , the l i m i t i n g c u r r e n t d e n s i t y i s reached e a r l i e r and the e f f e c t of mass t r a n s f e r be-comes obvious at curren t d e n s i t i e s lower than those observed f o r other gas e l e c t r o l y t i c r e d u c t i o n processes. I t should be noted that at current d e n s i t i e s lower than l i m i t i n g , the side r e a c t i o n s are competing w i t h the main r e a c t i o n , whereas at current d e n s i t i e s higher than l i m i t i n g , these would be expected to be dominating. 23 CHAPTER THREE OBJECTIVE The aim of t h i s work was to develop a process f o r manufacturing hydroxylamine by e l e c t r o l y t i c r e d u c t i o n of n i t r i c oxide gas i n a c i d e l e c t r o l y t e . A packed bed cathode was s e l e c t e d as an e f f i c i e n t way to contact the gas and e l e c t r o l y t e because of the low s o l u b i l i t y of n i t r i c oxide i n a c i d e l e c t r o l y t e . Some experiments w i t h p l a t e e lectrodes were to be done to j u s t i f y t h i s choice. The research reported here includes the design and co n s t r u c t i o n of equipment to carry out the process and an experimental study of the e f f e c t s of important operating v a r i a b l e s . These v a r i a b l e s i n c l u d e the type of cathode p a r t i c l e (tungsten carbide or g r a p h i t e ) , p a r t i c l e s i z e , c a t h o l y t e composition and f l o w r a t e , gas composition and f l o w r a t e , bed height, width and depth, and c e l l pressure and temperature. Apart from t h i s , the s t a b i l i t y of the process over extended periods was to be i n v e s t i g a t e d . In a p r a c t i c a l process i t would be d e s i r a b l e to operate w i t h higher concentration of hydroxylamine i n e l e c t r o l y t e than can be achieved i n a s i n g l e pass i n a short c e l l . The behaviour of the c e l l i n r e c y c l e mode and w i t h concentrated s o l u t i o n s was th e r e f o r e of i n t e r e s t . A few t e s t s on the f e a s i b i l i t y of con c e n t r a t i n g d i l u t e 24 s o l u t i o n s o f t h e p r o d u c t were a l s o t o be c a r r i e d o u t . F i n a l l y , an a s s e s s m e n t was t o be made o f c u r r e n t l y a v a i l a b l e mass t r a n s f e r c o r r e l a t i o n s f o r t r i c k l e - b e d s f o r t h e i r a p p l i c a b i l i t y t o t h e p r e s e n t s y s t e m . 25 CHAPTER FOUR EXPERIMENTAL 4.1 APPARATUS A l i n e diagram of the apparatus used to study the e l e c t r o l y t i c r e d u c t i o n of n i t r i c oxide i s given i n Figure 1. The heart of the equipment i s the ele c t r o c h e m i c a l r e a c t o r , and except f o r the case i n which the c e l l rearrangement was s t u d i e d , t h i s con-s i s t e d of a two-compartment c e l l fed wi t h separate anolyte and c a t h o l y t e s o l u t i o n s . The anolyte i s pumped from a 5 l i t r e tank, passes through the anode chamber of the c e l l and i s r e c y c l e d to the tank. The ca t h o l y t e i s d e l i v e r e d from a 10 l i t r e tank, mixes with NO or/and N 2 from c y l i n d e r s and the mixture i s sent to the cathode chamber of the c e l l . The flows of N 2, NO, c a t h o l y t e and anolyte are monitored by r o -tameters and c o n t r o l l e d manually through needle v a l v e s . The c a t h o l y t e i s cooled by tap water c i r c u l a t i n g through a short length of ja c k e t t e d tube, to ensure that the temperature of the c a t h o l y t e at the i n l e t of the r e a c t o r remains reasonably constant. The pressure drops i n the NO and N 2 l i n e s are i n d i c a t e d by pressure gauges. Power f o r the c e l l i s obtained from a d i r e c t current power supply w i t h a maximum power output of 1.0 D D.C. POWER SUPPLY D SAMPLE FIGURE 1 CATHOLYTE SCHEMATIC DIAGRAM OF THE APPARATUS FOR THE MANUFACTURE OF HYDROXYLAMINE. SYMBOLS EXPLAINED ON NEXT PAGE 27 LEGEND FOR FIGURE 1 A Ammeter AC Absorption column: packed bed of r a s c h i g r i n g s AN Anode compartment: packed bed of glass beads BC Bubbling column CA Cathode compartment: packed bed of tungsten carbide (or graphite) D To d r a i n LT L e v e l l i n g tank N2 Nitrogen gas P Pressure gauge R Rotameter S To stack SC S t r i p p i n g column: packed bed of glas s beads T Temperature gauge V Voltmeter W Tap water 28 KVA and capable of e i t h e r voltage or cu r r e n t c o n t r o l up to 40 v o l t s or 25 amperes. The r e a c t o r used here i s shown s c h e m a t i c a l l y i n Figures 2(a) and 2(b). I t c o n s i s t e d of a re c t a n g u l a r shallow cathode chamber separated from a s i m i l a r anode chamber by an anion exchange membrane. The cathode chamber was packed with a bed of tungsten carbide p a r t i c l e s except f o r a few i n i t i a l runs where graphite was used. The anode chamber was charged w i t h a bed of gla s s beads, which c o n s t i t u t e d an i n e r t bed serving to support the membrane from sagging due to the weight of cathodic bed. The current was supplie d to the cathodic and the anodic beds by t h i n p l a t e s of 316 s t a i n l e s s s t e e l and a n t i m o n i a l lead r e s p e c t i v e l y , by p l a c i n g them at the extreme ends of the beds. Tests were a l s o made with an a l t e r n a t e c e l l arrangement i n which the anion exchange membrane was replaced by a t h i c k nylon f e l t p l a c e d f l a t against the anode feeder p l a t e and there was no bed i n between. The anolyte flow to the r e a c t o r was cut o f f i n t h i s case and the i n l e t and the o u t l e t of the anode com-partment were blocked. The cathode and the anode feeder p l a t e s w i t h beds i n between were then held p a r a l l e l to each other by p l a c i n g them between two compression p l a t e s . The compression p l a t e s were constructed out of p l e x i g l a s s (Figure 2(a)) or m i l d s t e e l channel (Figure 2 ( b ) ) , and were C A T H O L Y T E IN CATHOLYTE OUT ANOLYTE IN A N O L Y T E OUT (not to scale) Figure 2(a) ARRANGEMENT OF REACTOR FOR ATMOSPHERIC OPERATION a, i | 2.54 cm t h i c k p l e x i g l a s s compression p l a t e b, h | 0.16 cm t h i c k neoprene gasket c I 0.16 cm t h i c k s t a i n l e s s s t e e l cathode d I 0.25 cm t h i c k neoprene s l o t t e d gasket c o n t a i n i n g cathodic bed 0.178 mm (7 m i l s ) t h i c k anion exchange membrane 0.25 cm t h i c k neoprene s l o t t e d gasket con-t a i n i n g anodic bed 0.32 cm t h i c k antimonia lead anode a ,m Compression p l a t e , 0.635 cm channel b , l 0.16 cm t h i c k asbestos gasket c ,k 0.16 cm t h i c k s t a i n l e s s s t e e l 316 p l a t e d,j 0.16 cm t h i c k neoprene gasket e 0.16 cm t h i c k s t a i n l e s s s t e e l 316 cathode £ 0.25 cm t h i c k neoprene s l o t t e d gasket c o n t a i n i n g cathodic bed g 0.178 mm (7 m i l s ) t h i c k anion exchange membrane h 0.25 cm t h i c k neoprene s l o t t e d gasket c o n t a i n i n g anodic (glass beads) bed i 0.32 cm t h i c k a n t i m o n i a l lead anode. Figure 2(b) ARRANGEMENT OF REACTOR FOR PRESSURE OPERATION 3 1 b o l t e d along the v e r t i c a l edges. The mild s t e e l channel was selec t e d mainly f o r use at higher pressure. In t h i s case, a d d i t i o n a l 316 s t a i n l e s s s t e e l p l a t e s were used on e i t h e r side of the cathode and the anode feeder p l a t e s to f a c i l i -t a t e the welding of i n l e t and o u t l e t connectors to the c e l l . This would also prevent the m i l d s t e e l channel from undergoing excessive c o r r o s i o n due to contact w i t h s u l p h u r i c a c i d s o l u t i o n . Pressure was maintained i n the cathode compartment of the r e a c t o r by f i x i n g an i n - l i n e valve at the o u t l e t . This c e l l arrangement was based on a design by C. Oloman. The product c o n t a i n i n g unreacted n i t r i c oxide gas coming out of the cathode compartment was l e d v i a a three-way v a l v e to a s t r i p p i n g column where the ex-cess n i t r i c oxide was s t r i p p e d o f f the product mixture by n i t r o g e n gas. This ensures that the l i q u i d sample i s e s s e n t i a l l y f r e e of n i t r i c oxide before i t comes i n contact w i t h the a i r . F a i l i n g to do so would de-s t r o y most of the hydroxylamine i n the l i q u i d sample (Section 2.2). When the sampling was not done, i . e . when the c e l l was i n the process of a t t a i n i n g a steady s t a t e , the o u t l e t mixture from the cathode compartment was d i r e c t e d to a bubbling column f o l l o w e d by an ab-s o r p t i o n column to remove as much n i t r i c oxide as 32 p o s s i b l e by absorption i n water. As mentioned i n Section 2.2, n i t r i c oxide r e a c t s immediately w i t h oxygen of a i r to form n i t r o g e n d i o x i d e (Equation 2.12) which i s c h a r a c t e r i z e d by i t s brown fumes and i s harmful at concentrations above 0.25 ppm [23]. The unabsorbed gas coming out of the top of the absorption column was then vented o f f to the stack of the fumehood. Even when sampling was done, the mixture of s t r i p p e d NO and N 2 gas was l e d through t h i s treatment system before being f i n a l l y vented o f f , as shown i n Figure 1. The system as described operates w i t h a s i n g l e pass of c a t h o l y t e , and i s said to be operating i n a s i n g l e pass mode. With a l i t t l e m o d i f i c a t i o n , t h i s apparatus could be converted to a r e c y c l i n g mode. In order to achieve t h i s , the s t r i p p i n g column i s oper-ated c o n t i n u o u s l y and the l i q u i d product coming out at the bottom of the .column i s d i r e c t e d back to the ca t h o l y t e tank, as shown by dotted l i n e i n Figure 1. To e f f e c t a good mixing, n i t r o g e n gas i s bubbled through the c a t h o l y t e tank. This a l s o maintains an i n e r t environment above the c a t h o l y t e so that the hydroxylamine t h e r e i n does not get o x i d i z e d due to prolonged exposure to a i r . The e n t i r e apparatus except the gas c y l i n d e r s and the power supply was erected i n a fumehood w i t h a 33 s l i d i n g p l e x i g l a s s f r o n t f o r observation. A photo-graph of the apparatus i s shown i n Figure 3. The dimensions of important p a r t s of t h i s apparatus are l i s t e d i n Table 2. The s p e c i f i c a t i o n s of the parts of the apparatus are given i n Appendix 1. A l l p a r t s i n d i r e c t contact with the e l e c t r o l y t e are made of e i t h e r 316 s t a i n l e s s s t e e l , a n t i m o n i a l l e a d , p l e x i g l a s s , polypropylene, nylon, t e f l o n or g l a s s . The appearance of a t h i n b l u i s h f i l m i n the s t a i n l e s s s t e e l tube f i t t i n g s a f t e r prolonged operation i n d i c a t e d that some c o r r o s i o n d i d occur. The s t a i n l e s s s t e e l cathode feeder p l a t e was observed to have undergone hydrogen embrittlement and had to be replaced from time to time. The anode feeder p l a t e was also observed to be o x i d i s e d and the r u s t - l i k e f i l m thereon was scraped o f f a f t e r each operation. The p l a t e was replaced a l t o g e t h e r a f t e r a few c e l l operations. The tungsten carbide p a r t i c l e s used were of i r r e g u l a r shape wi t h sharp edges and they appeared to have an as-pect r a t i o c l o s e to u n i t y . This i s i l l u s t r a t e d i n the scanning electronmicrographs of t y p i c a l p a r t i c l e s shown i n Figure 4. Some of the t y p i c a l p r o p e r t i e s of the tung-sten carbide p a r t i c l e s are l i s t e d i n Table 3. Relevant t e c h n i c a l s p e c i f i c a t i o n s of the anion ex-change membrane are given i n Table 4. Figure 3 PHOTOGRAPH OF THE APPARATUS USED FOR THE MANUFACTURE OF HYDROXYLAMINE 35 TABLE 2 Dimensions of Important Parts of the Apparatus Used Cathode compartment Height : Width : Depth Packing s i z e 0.375 m 0.05 m 0.25 cm WC -.42+.18 mm -.59+.42 mm -2.0+.8 4 mm Graphite -.59+.42mm Anode compartment Height Width Depth Packing s i z e : 0.375 m : 0.05 m : 0.25 cm : Glass beads, average diameter 1.02 mm S t r i p p i n g column I.D. : 0.0153 m Height of column : 0.229 m Packing : Glass beads, average s i z e diameter 4 mm Bed height : 0.114 m Bubbling column I.D. : 0.037 m Height of column: 0.375 m Height of water column : 0.28-0.33 m Absorption column I.D. : 0.035 m Height of column : 0.434 m Packing s i z e : Raschig r i n g s , 6 mm Bed height : 0.305 m TABLE 3 Summary of T y p i c a l P r o p e r t i e s of Tungsten.Carbide Used PROPERTY SIZE RANGE, mm -.42+.18 - .59 + .42 -2.0 + 84 Average particle size, mm Bulk density (untapped), -3 g cm Bulk density (tapped), -3 g cm 0.298 5 7 .323 9.30 0.5075 8 . 299 9 .378 1.4205 8 .543 9 .383 Bed voidage (dense) Specific surface area of packing (assuming spherical particles), 2 -3 cm cm 0.405 119 .6 0.40 70.9 0 .40 25.3 _3 Actual density, g cm Specific resistance of solid,vohm-cm' -Chemical composition*, per cent • •Total W (minimum) Total C (typical) Free C (maximum) Combined C (minimum) 15': 63 53 . 0 93.1 6.2 0.05 6.13 Impurities: Ti (maximum) 0.20 Ta (maximum) o : i o Nb (maximum) 0.10 Fe (maximum) 0.20 Ni (maximum) 0.01 Co (maximum) 0.10 From manufacturer's literature Macro division of Kennametal Inc., Port Coquitlam, B.C. 38 TABLE 4 Summary of Typical Properties* of Anion Exchange Membrane Used Membrane thickness (mils) 7 Approx. density (g/m2) 202 Electrical resistance (ohm-cm2, A.C. measurement) 1.0 NaCI 9.0 Mullen Burst strength (minimum psi) 175 Water permeability (ml/hr/ft2/5 psi) Negligible Capacity (meq/g) 0.96 Dimensional stability (ability to rewet after drying) Good Chemical stability H2S04 HCl Up to 60°C Up to 5% Up to 4% ** Manufacturer's catalogue Number MA-3148 From manufacturer's literature Ionac Chemical Co., Birmingham, N.J. 39 4.2 METHOD The effects of a number of different variables on the concentration of hydroxylamine produced and current efficiency for the production of the same were ' studied. The ranges of these variables covered are listed in Table 5. To carry out these runs, the reactor was packed with the desired particles and fastened in a manner shown in.Eigure • 2(a)., or. 2(b)- '. The catholyte tank was charged with a solution of sulphuric acid of desired strength, and the anolyte tank with 10% H^ SO^ . The acid solutions were cooled to the room temperature and the water through the cooling jacket in the catholyte line was allowed to circulate. The three-way valve at the outlet of the reactor was set so that the product went to the treatment section and eventually to the drain until a steady state was established. Water flows through the bubbling column and the absorption column were adjusted. In the begin-ning only catholyte (without any gas mixed with it) and the anolyte were allowed to flow through the reactor at 3 -1 low flowrates (15-40 cm min ) for about 20 minutes to make sure that the beds were throughly wetted with acid solution before any current was passed. The electrode surface not wetted with acid solution catalyzes side reactions, possibly the formation of ammonia and N90 [8]. 40 'TABLE 5  Ranges of V a r i a b l e s Covered Current', density- 0-640 A m~2 Cathodic bed particles a) WC 0.3mm - 1.42mm dia b) Graphite (untreated) 0.51mm dia c) Graphite (pretreated) 0.51mm dia Effectiveness runs Up to 14 hours single run; Up to 4 runs using same bed. Cell dividing media a) Anion exchange membrane b) Nylon f e l t Catholyte strength 5-15% H 2S0 4 Catholyte flow rate 3-31 cm3 min 1 Recycling of catholyte a) Number of times recycled 9 times, 25 times b) Volume i n recycle ( i n i t i a l ) 1050 cm3, 500 cm3 Inlet temp, of catholyte 22-41°C Gas composition (volumetric) 100% NO, 0% N 2 to 18.7% NO, 81.3% N 2 Gas flow rate 94-1117 cm3 min" 1 Pressure i n the c e l l 119 kPa, 236 kPa Bed depth in the direction of current 0.25 cm, 0.6 cm Bed height 0-37.5.cm Bed width 3 cm, 5 cm Moreover, passage of current through dry sections of bed causes overheating in those sections and the voltage drop through the c e l l increases. After about 20 minutes, the gases NO and [if required) were introduced and their flow rates adjusted to the desired level. The valves downstream of the gas pressure gauges were kept f u l l y open so that the reading on the gauges indicated the pressure drops through the c e l l in the NO and N2 lines. The power supply was switched on almost simultaneously and the current through the c e l l was set with the power supply on the current control mode. Samples of the catholyte product were then taken at intervals of time until a constant hydroxylamine concentration indicated steady state operation of the c e l l . From i n i t i a l runs i t was determined that the c e l l took about 25 minutes to reach steady state after the current was switched-on.. Analysis for hydroxylamine was made by the standard titration with ferric ammo-nium sulphate in acid conditions [10, 24-27]. The de-tailed technique is given in Appendix 2. When operating in the recycling mode, the recycling was initiated only after a steady state was f i r s t established, and then the sampling was done from the catholyte tank at intervals of time. A total of 219 such runs were carried out. They are a l l tabulated in Appendix 2 and the results for the same will be discussed in Chapter 5. 42 CHAPTER FIVE RESULTS AND DISCUSSIONS The performance of the el e c t r o c h e m i c a l r e a c t o r described i n the previous chapter f o r the re d u c t i o n of n i t r i c oxide to hydroxylamine was analyzed on the bas i s of concent r a t i o n of hydroxylamine produced i n the r e a c t o r and the current e f f i c i e n c y f o r the pro-duction of the same. The hydroxylamine c o n c e n t r a t i o n , ^NH OH' w a s determined by a n a l y s i s of the product, while the current e f f i c i e n c y i s defined as Current e f f i c i e n c y for hydroxylamine product!on', n N H n H Faraday's law immediately enables one to c o r r e l a t e hydroxylamine conce n t r a t i o n to the current consumed f o r i t s formation: Current consumed f o r hydroxylamine production T o t a l a p p l i e d current I (5.1) I X NH o 0H Z X H o 0 H OF I. 1000 x 60 (5.2) where X M o n u ^ M o l a l c o n c e n t r a t i o n of hydroxylamine 2 produced, M ZNH OH = Number of e l e c t r o n s a s s o c i a t e d with 2^ * the p r o d u c t i o n of a molecule of NH2OH 3 [Equation 2.3] 3 -1 Q = Ca t h o l y t e flow r a t e (cm min ) F = Faraday's constant^ = 96493 c o u l (g equiv) S u b s t i t u t i o n of (5.2) i n t o (5.1) g i v e s XNH 2OH ZNH 2OH Q F nNH OH = A ....(5.3) 1 N H 2 U n 6.0 x 10 4 I Expr e s s i n g (5.3) a l s o amounts to saying t h a t Amount of NH20H produced nNH 20H " T h e o r e t i c a l amount of NH2OH produced assuming no other r e a c t i o n s occur ...(5.4) 5.1 ACTIVITY OF CATHODIC BED 1 Tungsten Carbide (Appendix 2, Tables A ( l ) - A ( 3 ) ) I t was r e a l i z e d a f t e r a few t r i a l runs that the a c t i v i t y of the e l e c t r o c a t a l y s t (tungsten c a r b i d e ) dropped r a p i d l y over time and t h i s might prove to be se r i o u s drawback to the otherwise a t t r a c t i v e performance of the tungsten c a r b i d e as cathode f o r the r e d u c t i o n of n i t r i c oxide to hydroxylamine. This behaviour i s t y p i -f i e d i n Fi g u r e 5 i n terms of the e f f i c i e n c y f o r ' t h e . 70 5 0 >-O z UJ o I'-LL. L J H 30| z UJ cr o • • -GENERATED H 2 ON CATHODE V o A • V 1st DAY 2nd DAY 3rd DAY 4th DAY 101 0 8 TIME (hrs.) Figure 5 ACTIVITY OF TUNGSTEN CARBIDE FOR NTTRTC OX THF. M l i m f l M (Using 10% H 2S0 4 as Catholyte) Catholyte flowrate 16.4 cm3 min -l WC particle size -.59 + .42 mm NO flowrate 221.5 cm3 min"! Current density .213.3 A m"2 45 production of hydroxylamine on tungsten carbide over time. The efficiency f a l l s from 61.51 on the f i r s t day of operation (i.e. when fresh) to 24% after 3 days of an average 8 hour operation. It can also be seen that after an i n i t i a l rise in the f i r s t two hours, the efficiency is more or less constant over the day, and the major drop occurs overnight. At the end of each day, water was flushed through the cathodic bed to wash off acidic solution, although i t was not ensured that a l l the acid was washed off. Continuous operation of 14 hours duration (Figure 6)shows that there is only a slight drop in efficiency near the end when the c e l l is in operation, and that most of the drop occurs between the two runs when the c e l l remains wetted in water or dilute acid. Fresh tungsten carbide always gave the same current e f f i -ciency on the f i r s t day, so the reduced efficiency on the successive days was not because of any operational variations. There is evidence in literature [18, 20, 21] of the tendency of WC to undergo oxidation with acidic electrolytes in the presence of dissolved oxygen. This results in the formation of an inactive species, tungsten oxide, on the surface of the electrocatalyst. The susceptibility of WC surfaces to undergo rapid deacti-vation due to oxidation was verified by shutting off the NO and switching c e l l polarity. Thus WC becomes the "Figure 6 ACTIVITY OF TUNGSTEN CARBIDE OVER A SINGLE LONG RUN (Using 10% H 2S0 4 as Catholyte) Catholyte flowrate 16.4 cm3 min" 1 WC p a r t i c l e -.59 + .42 mm NO fl o w r a t e 221.5 cm3 m i n _ l Current d e n s i t y 213.3 A rn"2 ON 47 anode and oxygen i s generated on i t . I t was observed that i n l e s s than h a l f an hour, the e f f i c i e n c y f o r NH2OH formation had dropped from 54% to 5%. In normal o p e r a t i o n some oxygen i s expected to enter the WC bed i n d i s s o l v e d form i n the c a t h o l y t e because no pre-c a u t i o n was taken to av o i d c o n t a c t of a i r with c a t h o l y t e while i n storage or i n o p e r a t i o n . The presence of reducing agent such as hydrogen has been r e p o r t e d to i n c r e a s e the a c t i v i t y of WC c a t a -l y s t [14, 18]. This was s u b s t a n t i a t e d i n t h i s work' i n an experiment where NO was shut o f f f o r two hours on the f o u r t h day of o p e r a t i o n which r e s u l t e d i n H 2 gen-e r a t i o n on the c a t h o d i c bed. A f t e r t h i s treatment the e f f i c i e n c y f o r hydroxylamine p r o d u c t i o n was r a i s e d by 42% ( F i g u r e .5', Table A ( l ) . 4 ) . In the present work, when the c u r r e n t e f f i c i e n c y was not near 100%, a major p o r t i o n of the l o s t c u r r e n t had been found to produce hydrogen. This would tend to l e s s e n the e f f e c t of d i s s o l v e d oxygen i n the c a t h o l y t e . The c o u n t e r a c t i n g nature of these two e f f e c t s probably s t a b i l i z e s the e f f i c i e n c y during the day wit h s l i g h t f l u c t u a t i o n s , although i n the i n i t i a l p e r i o d , where a ga i n of a c t i v i t y i s ndtedj the reducing nature of hydrogen seems to dominate. Even the s u s t a i n e d presence of a c o n s i d e r a b l e amount of NF^OH can i n c r e a s e the a c t i v i t y s l i g h t l y , due to the reducing nature of the l a t t e r . T h i s weak e f f e c t was observed i n one of the r e c y c l e runs. Since the o x i d a t i o n of WC surface has Been, r e -ported i n concentrated C.2N, 9.41) E^SO^ s o l u t i o n s ? i t was decided to t r y out a more d i l u t e s o l u t i o n (51) as c a t h o l y t e . Results f o r t h i s case are p l o t t e d i n Figure 7. I t can be seen that the e f f i c i e n c y on the f i r s t day i s lower than that i n case of 10% ; B^SO^, but the drop i n e f f i c i e n c y between the f i r s t and the second day i s a l s o lower. At the end of second day, no washing of the cathodic bed was done and a s l i g h t cathodic p o t e n t i a l was maintained overnight [Appendix 2, Table A(2).2] t i l l the s t a r t of the 3rd day. The bed was f l u s h e d i n t e r m i t t e n t l y w i t h a c i d i c s o l u t i o n to prevent overheating of bed due to the presence of stagnant s o l u t i o n w i t h i n i t . This improved the e f f i -c i ency s u b s t a n t i a l l y on the t h i r d day, as the e f f i -c iency was about the same as that i n the f i r s t two hours of the f i r s t day (Figure 7), although i t l a t e r s t a b i l i z e d at a l e v e l lower than that on the second day. Nevertheless, the drop i n the e f f i c i e n c y between the second and the t h i r d day was 72% l e s s than that observed between the second and the t h i r d day using 10% K^SO^ as c a t h o l y t e and maintaining no cathodic p o t e n t i a l at the end of the second day. Thus the drop i n e f f i c i e n c y observed between successive days of operation probably occurred as a r e s u l t of o x i d a t i o n of the WC surface i n the absence of 90 70 o UJ o LU r -5 50 cc cc CJ 30 CATHOLYTE CONC. O 1 St DAY 5 % 2nd DAY • 3rd DAY 20% © 1 St DAY 9 8 TIME (hrs.) Figure 7 ACTIVITY OF TUNGSTEN CARBIDE USING 5% AND 20% H2SO4 AS CATHOLYTE Cathodic potential maintained on bed at the end of the second day t i l l the start of the third day r,3 T T I T T I - 1 r \ , - ™ „ „ 4 - - J 1 1 7 7 I ™-~2 -1 Catholyte flowrate NO flowrate 16.4 221.5 cnr cm-mm min" Current density 213.3 A m WC-particle size -.59.+ .42.mm 4^ 1X5 current, i.e., in the absence of any reducing agent. The rate of oxidation, too, w i l l be lower during the day than when the c e l l is idle, because pressure in the c e l l w i l l prevent any free air from entering the system. Whatever air gets in to assist the oxidation is in dissolved form in the catholyte. When the c e l l is idle, air can easily get into the c e l l from leaks on the sides. A strikingly similar observation, viz. drop of activity at night, was observed by Shapelin [8] working on reduction of n i t r i c oxide on a specially prepared platinum cathode. Shapelin claimed that this was because of the oxidation of the catalyst by atmos-pheric oxygen when the c e l l was shut off, because the catalyst he used was strongly susceptible to oxidation too. A scanning electronmicrograph of the surface of the oxidised species (obtained at the end of the fourth day in Figure 5) clearly shows that the surface has a different appearance (Figure 8b) when compared with the surface of a fresh WC particle (Figure 8a). A chemical analysis of the surface constituents of the oxidised WC particles may help verify the theory outlined here. In order to maintain consistency in the experi-mental results, fresh WC was used for each day of operation and a l l the subsequent runs with WC were carried out over the stabilized portion of the acti-vity on the f i r s t day. Normally, the c e l l was allowed to operate at the conditions of Figure 5 at the start of each day before any sample was withdrawn and a . (b) Oxidised x 200 / Figure 8 SCANNING ELECTRONMICROGRAPHS OF THE FRESH AND THE  OXIDISED TUNGSTEN CARBIDE PARTICLES Size of p a r t i c l e s -.59 + .42 mm Oxidised species recovered from the end of operation on the f o u r t h day i n Figure 5 maximum of 12 hours of operation [Figure 6 ] was carried out before a batch was replaced. Moreover, as shown in Appendix 2, values for a variable under investigation were randomized to eliminate systematic bias due to slight deactivation. 2. Graphite [Appendix 2, Tables B(l), B(2)] Graphite behaves unsatisfactorily as a cathode with regards to reduction of n i t r i c oxide. This is depicted in Figure 9 and i t can be seen that, although the i n i t i a l efficiencies are 30-40%, they stabilize at values around 20% within about 2 hours, both in case of untreated and pretreated graphite. This is mainly because of the low hydrogen overpotential of graphite as compared to tungsten carbide, and most of the current goes to produce hydrogen. Obviously, from expression (5.1), the current efficiency for NF^ OH production w i l l be reduced. Since pretreated graphite has given better per-formance over untreated graphite in some of the processes using graphite cathodic beds [28-31], i t was decided to test pretreated graphite as electrocatalyst. The pre-treatment was carried out by boiling graphite parti-cles in dilute hydrochloric acid for a few hours. This serves to dissolve iron and other metallic im-purities present in trace amounts on the surface. Pre-treated graphite gave better efficiency in the f i r s t half hour; however, i t f e l l rapidly to a level slightly below that obtained with untreated graphite. 60 O UNTREATED A PRETREATED > 401 z: UJ o u_ u. UJ £ 201 r r O A 0 0 TIME (hrs.) Figure 9 ACTIVITY OF GRAPHITE FOR NITRIC OXIDE REDUCTION Catholyte flowrate NO flowrate Current density 3 -1 16.4 cm min 221.5 213.3 cm 3 min'l m -2 Graphite p a r t i c l e s i z e -.59+.42 mm Catholyte concentration 101 H2S0^ On OJ 54 Since the hydrogen overpotential is dependent on the physical state of the electrode material, i t appears that pretreatment altered the surface characteristics of graphite in such a fashion as to fa c i l i t a t e a further reduction of hydrogen overpotential. 5.2 CURRENT EFFICIENCY AND NH^ OH CONCENTRATION It is necessary to consider these two parameters separately before the effects of different variables on them is taken up. Theoretically, i t follows ffcom Faraday's law that outlet concentration of NH-OH (X,,1LJ should in-crease linearly with current (I) with a slope bf 4 Y 1 — - — T y p (Equation 5.3) i f the current efficiency ^NH2OH ^ for NH2OH production is 100%. In a real situation, however, i t is observed that the increase is far from linear because of losses of both current and hydroxy-lamine as a result of side reactions. The side re-actions were discussed in Sections 2.1-2.2, and the losses due to these can be classified as direct and in-direct: Direct Losses These refer to the loss of hydroxylamine after i t is once formed due to decomposition by reaction with 55 HN02 (Equation 2.12-2.14). I n d i r e c t Losses These r e f e r to losses i n p o t e n t i a l hydroxylamine formation because the current i s consumed f o r un-d e s i r e d side r e a c t i o n s . This current could otherwise have been used f o r production of NK^OH i n an i d e a l s i t u a t i o n . Such losses could be summarized as a) Generation of hydrogen (Equation 2.11) b) Reduction of NO to N 20 v i a intermediate NOH (Equation 2.8-2.9) c) Reduction of NO to NH^ (Equation 2.10) Because of the increase of i n d i r e c t losses w i t h c u r r e n t , the current e f f i c i e n c y of NH2QH production decreases, which otherwise would have been constant. This i s the trend normally observed i n the runs that f o l l o w . S p e c i f i c a l l y , generation of hydrogen was i d e n t i -f i e d to be a major source of l o s s ; e.g. when the current e f f i c i e n c y f o r NH20H production was 47%, that f o r hy-drogen production was c a l c u l a t e d to be 30%. The l a t t e r f o l lowed from an a n a l y s i s of the o u t l e t gas i n a gas chromatograph. I d e n t i f i c a t i o n of other by-products was considered beyond the scope of t h i s work. Gas a n a l y s i s was not c a r r i e d out on a r o u t i n e b a s i s . 5.3 EFFECT OF CATHOLYTE FLOW [Appendix 2, Table C] Effects of catholyte flowrate on current e f f i -ciency , NH2OH concentration and rate of NH2OH pro-duction, for a fixed gas flow rate, are shown in Figures 10-12. In Figure 10, current e f f i c i e n c y and NH2OH con-centration are plotted against s u p e r f i c i a l current density for three d i f f e r e n t values of catholyte flow rate. The s u p e r f i c i a l current density (or, diaphragm current density) referred to i s the current density based on the area of the cathodic bed projected on a plane perpendicular to the d i r e c t i o n of the current; i . e . , on the area of the diaphragm. ":Tt' -is observed that there i s no d e f i n i t e effect of catholyte flow rate on the current e f f i c i e n c y but the NH2OH concentration decreases with increases i n catholyte flow rate. In Figure IT, the current e f f i c i e n c y and NH2OH concentration are plotted against catholyte flow rate for a fixed value of current density. Current e f f i c i e n c y exhibits a broad maximum with catholyte flow rate. In Figure 12, the rate of NH20H formation i s plotted against current density for d i f f e r e n t cath-olyte flow rates. The rate of NH20H formation i s defined here as the product of catholyte flow rate and NH2OH concentration at the outlet of the c e l l , i . e . i t i s the cumulative rate over the length of 9 0 £ 70 > o z UJ o LL. U _ , w 5 0 h z UJ cr rr O 30 10 r CATHOLYTE FLOW, cm 3mirf 1 7.75 o 16.4 A A 23.75 .08 .06 z o a •04 g o z o o X o _ _ CM .02 x 0 100 200 300 400 CURRENT DENSITY ( A r r f 2 ) 500 .00 Figure 10 EFFECT • OF CATHOLYTE FLOW AND CURRENT DENSITY ON CURRENT EFFICIENCY AND HYDROXYLAMINE CONCENTRATION' ' o-i NO flow r a t e 221.5 cm' m m 58 Figure 11 EFFECT OF CATHOLYTE FLOW ON CURRENT EFFICIENCY AND HYDROXY LAM I NE CONCENTRATION FOR A "FIXED; VALUl-. : OF  CURRENT DENSITY _ 2 Current density 213.3 A m , NO flow r a t e 221.5 cm3 min" 59 0 100 200 300 400 CURRENT DENSITY (A m"2) F i g u r e 12 EFFECT OF CATHOLYTE FLOW AND CURRENT DENSITY ON THE RATE OF HYDROXYLAMINE PRODUCTION NO f l o w r a t e 3 221.5 cm min 60 r e a c t o r . Again, no sharp d i s t i n c t i o n i s observed i n the rates f o r d i f f e r e n t c a t h o l y t e flow r a t e s . This v i r t u a l independence of the rate of NF^OH production on c a t h o l y t e flow r a t e r e s u l t s i n a sharp decrease i n Nf^OH concent r a t i o n w i t h flow r a t e (Figure 11). Except f o r a s l i g h t increase i n current e f f i c i e n c y at low current d e n s i t i e s , the current e f f i c i e n c y and NF^OH concent r a t i o n p l o t s of Figure 10 f o l l o w the trends p r e d i c t e d i n Section 5.2. NH^OH concentrations increase w i t h current d e n s i t y as long as the electrode k i n e t i c s are c o n t r o l l i n g . Once the l i m i t i n g current d e n s i t y i s reached, i . e . when the mass t r a n s f e r of NO to WC surface becomes a c o n t r o l l i n g step, these curves tend to l e v e l out, as seen i n two out of three cases p l o t t e d . The approximate values of these observed l i m i t i n g current d e n s i t i e s are i n the. same range as those p r e d i c t e d i n Section 5.17 from mass t r a n s f e r c o n s i d e r a t i o n s . Moreover, as shown i n Section 5.17, an increase i n c a t h o l y t e flow rate w i l l be expected to increase mass t r a n s f e r of NO to the WC surface and hence, the l i m i t i n g current d e n s i t y f o r hydroxylamine formation. However, f o r some unknown reasons, an approximately reverse behaviour i s observed i n Figure 10. In the present context, although mass t r a n s f e r appears to be a c o n t r o l l i n g f a c t o r , i t f a i l s to e x p l a i n 61 a l l the observed e f f e c t s . A detailed analysis of the packed trickle-bed considering three dimensional current, voltage and concentration d i s t r i b u t i o n s , may help resolve this problem. Such an analysis was attempted in this program but in the absence of re-levant information i n the l i t e r a t u r e for packed bed electrodes operating in t r i c k l e - f l o w mode, i t was found to be too complex. 5.4 EFFECT OF GAS FLOW [Appendix 2, Table C,D ] Eff e c t of gas (NO) flow on current e f f i c i e n c y and outlet Nr^OH concentration, for a fixed value of catho-lyte flow rate, i s shown in Figures 13 and 14. Both current e f f i c i e n c y and NF^OH concentration increase- considerably for current densities above 200 - 2 3 - 1 Am by increasing the NO flow rate from 221.5 cm min 3 -1 to 450 cm min . By increasing the NO flow rate further 3 -1 by 2'36 cm min , the current density above which an i n -crease in current e f f i c i e n c y and NK^OH concentration occurs i s pushed up to a higher value. From these r e s u l t s i t appears that the reduction of NO to NK^OH i s mass transfer controlled above a certa i n current density. However, increasing the gas flow rate above a ce r t a i n l i m i t does not have any effect on current e f f i c i e n c y or NH?0H concentration u n t i l the current 0 100 200 300 400 500 600 CURRENT DENSITY (A m"2) Figure 15 EFFECT OF NITRIC OXIDE FLOW AND CURRENT DENSITY ON CURRENT EFFICIENCY AND HYDROXYLAMINE CONCENTRATION 3 -1 Catholyte flowrate 16.4 cm min to » 200 400 600 800 1000 1200 NO FLOW (cm3 min"1 ) Figure 14 EFFECT OF NITRIC OXIDE FLOW ON CURRENT EFFICIENCY AND HYDROXYLAMINE CONCENTRATION FOR A FIXED VALUE OF CURRENT DENSITY -2 Current density 213.3 A m _^ Catholyte flowrate 16.4 cm 3 min 64 d e n s i t y i s f u r t h e r increased. For example, i n -3 -1 creasin g the NO flow r a t e from 221.5 cm min to 3 -1 450 cm min increases the mass t r a n s f e r and pushes up the l i m i t i n g current d e n s i t y ; thereby the l e v e l l i n g out of NF^OH con c e n t r a t i o n appears at higher current d e n s i t i e s . The increase of NF^OH concent r a t i o n r e s u l t s i n an increase of current e f f i c i e n c y f o r Nf^OH pro-du c t i o n above that current d e n s i t y , because the losses due to side r e a c t i o n s can w e l l be assumed to be con-stant w i t h the gas flow r a t e . Once again, the general nature of the'current e f f i c i e n c y and NF^OH concentration p l o t s i s approximately p r e d i c t a b l e from d i s c u s s i o n s of Section 5.2. The e f f e c t of NO flow r a t e i s f u r t h e r i l l u s t r a t e d i n Figure 14' where the current e f f i c i e n c y and NH^OH conce n t r a t i o n have been p l o t t e d against a range of NO flow r a t e s f o r a f i x e d value of current d e n s i t y . The 2 current d e n s i t y s e l e c t e d was 213.3 A>m , i . e . the point where the e f f e c t of mass t r a n s f e r on current e f f i c i e n c y and NF^OH concent r a t i o n i n Figure 13 j u s t becomes ob-vi o u s . But even f o r t h i s s i t u a t i o n , there i s a value 3 -1 of gas flow r a t e ( i . e . 300 cm min approx.) above which the e f f e c t of gas flow r a t e i s not very pronounced. Once again, the values of observed l i m i t i n g current d e n s i t i e s are approximately i n the same range as those p r e d i c t e d from mass t r a n s f e r c o n s i d e r a t i o n s [Section 5.171. A l s o , the observed increase i n l i m i t i n g current 65 density with gas flow rate i s moderate, as predicted in Section 5.17. 5.5 EFFECT OF GAS COMPOSITION (Appendix 2, Table E) The effect of gas composition was studied primarily with the purpose of establishing the eff e c t of mass transfer. NO gas was mixed with an inert gas, v i z . N 2 in such proportions that the t o t a l gas flow rate re-mained constant. The resu l t s are plotted i n Figure 15, and i t can be seen that reducing the NO content of the gas from 100% to 20% ( i . e . , by 80%) reduces the current e f f i c i e n c y from 68% to 42% ( i . e . by only 38%). The corresponding reduction i n NH^OH concentration i s of the same order. If the reduction- of n i t r i c oxide was controlled by electrode k i n e t i c s , a proportional decrease of NK^OH concentration would be expected. Since the reductions are far from proportional, i t appears that the reduction of NO i s a process controlled by d i f f u s i o n of NO through catholyte surrounding WC p a r t i c l e s . These results are discussed further in Section 5.10 under the effect of c e l l pressure. Large-scale production of NK^OH by c a t a l y t i c re-duction of NO by BASF process (Section 1.2) involves the on-site production of NO by c a t a l y t i c oxidation of NH, [ 2 ] . The c a t a l y t i c oxidation of NH, introduces N-100 80 60 %N0 40 20 0 20 40 % N 2 60 80 VOLUMETRIC GAS COMPOSITION Figure 15 EFFECT OF GAS COMPOSITION ON CURRENT EFFICIENCY AND  HYDROXYLAMINE CONCENTRATION Total gas flowrate 646 cm3 min~l Current d e n s i t y = 213.3 A m"2 Catholyte flowrate 16.4 cm3 min"l 67 as impurity i n the NO produced. Our r e s u l t s suggest that presence of as high as 20% N 2 i n the NO gas r e -duces the current e f f i c i e n c y f o r NH2OH formation only m a r g i n a l l y , i . e . by 71. 5.6 EFFECT OF CATHOLYTE TEMPERATURE [Appendix 2, Table F] E f f e c t of c a t h o l y t e temperature was studied i n order to a s c e r t a i n i f there was any decomposition of hydroxy-lamine sulphate, e s p e c i a l l y f r e e NH^OH*, due to heating caused by high c u r r e n t s . S u b s t a n t i a l amounts of heat are generated by ap p l i e d c u r r e n t s ; as seen i n most of the tables of Appendix 2, c a t h o l y t e o u t l e t temperatures are higher where the currents are higher. Much higher tem-peratures would be encountered i n a commercial process where only a small f r a c t i o n of the e l e c t r i c a l heat gen-erated would be t r a n s f e r r e d from the c e l l stack. Under such c o n d i t i o n s the e l e c t r o l y t e temperature would r i s e to about 70-80°C i n the absence of c o o l i n g . Such an e f f e c t was simulated by h e a t i n g t h e c a t h o l y t e and insu-l a t i n g the l i n e s as f a r as p o s s i b l e . The r e s u l t s are shown i n Figure 16, where current e f f i c i e n c y and NH2OH concentration are p l o t t e d against temperature of c a t h o l y t e at the i n l e t of c e l l . Over the temperature range st u d i e d , there appears to be no e f f e c t on current e f f i c i e n c y and :NH"?0H con c e n t r a t i o n . Higher temperatures were intended >-o z L U O LU-LL. Ld L U tr tr ZD o 20 25 CATHOLYTE 40 30 35 INLET TEMPERATURE Figure 16 EFFECT OF CATHOLYTE TEMPERATURE ON CURRENT EFFICIENCY AND 45 (°C) HYDROXYLAMINE CONCENTRATION Catholyte flowrate Current density 3 . -1 23. 75 cm mm 213.3 A m2 NO flowrate 221.5 cm min ON 0 0 69 to be studied but because of losses of heat in the pump and the tube fi t t i n g s , catholyte temperature in the tank had to be maintained at 90-100^C. This caused excessive corrosion of the immersion heater used and the experiment had to be stopped short. 5.7 EFFECT OF CATHOLYTE RECYCLE [Appendix 2, Table G] It is observed that except in cases where low catholyte flows or high current densities were used, N^OH concentration at the outlet of the reactor in a single pass were normally below 0.05 M-. Since concentra-ting such dilute solutions for the purpose of crystal-lizing hydroxylamine sulphate could be expensive, i t was decided to test i f the NH^ OH concentration can be built up by prolonged contact of catholyte with electrode, i.e. by recycling. This can indeed be done, as seen in Figure 17, where N^OH concentration increases almost linearly with time, without appreciable drop of current e f f i -ciency. The average catholyte volume in recycle was 3 1010 cm and since the catholyte flow rate was 16.4 3 -1 cm min , i t took about an hour for one pass. The number of times the catholyte was recycled is shown along X axis (time), and this is not exactly proportional to time because of withdrawals of catholyte samples for analysis. The amount of catholyte held up in the lines 70 6 0 >-o ' Z UJ U. LU h-Z Ul cc ac O 1 0 0 / O CURRENT EFFICIENCY NH 20H CONCENTRATION A CONC. OF H 2 S 0 4 IN RECYCLING CATHOLYTE 1 0 0 4 6 TIME (hrs.) , 8 2 4 6 NO. OF TIMES RECYCLED 8 Figure 17 ACCUMULATION OF HYDROXYLAMINE WITH TIME IN  1010 cmJ (AVERAGE) OF RECYCLING CATHOLYTE Current d e n s i t y 213.3 Catholyte flowrare 16.4 NO flowr a t e 221.5 A m cm 3 cm 3 mm min' 71 was determined i n a separate t e s t and t h i s has been taken i n t o account f o r c a l c u l a t i o n of c a t h o l y t e volume. The weight percent f^SO^ i n r e c y c l i n g c a t h o l y t e was teste d from time to time and t h i s i s p l o t t e d on the same f i g u r e . As shown i n Figure 17, NF^OH concentrations upto 3 0.2 M can be b u i l t up l i n e a r l y m 9 hours i n a 1000 cm s o l u t i o n . Because of the r e s t r i c t i o n imposed on the length of a run (Section 5.1), a lower volume of catho-l y t e (average 480 cm ) was used i n the next r e c y c l i n g t e s t to see i f even higher concentrations of NH^OH were p o s s i b l e by extended contact. Results f o r t h i s are p l o t t e d i n Figure 18. In t h i s t e s t , the strength of H^SO^ i n the r e c y c l i n g c a t h o l y t e was not allowed to f a l l below 5% by frequent a d d i t i o n s of concentrated W^SO^ (see F i g u r e ) . A low H^SO^ ca t h o l y t e c o n c e n t r a t i o n , as to be discussed l a t e r , has c e r t a i n counteracting e f f e c t s on current e f f i c i e n c y which may mask the e f f e c t s due to r e c y c l i n g . I t can be seen i n Figure 18 that NH^OH con-c e n t r a t i o n increased almost l i n e a r l y again, and t h i s time concentrations upto 0.4 M were obtained i n 25 passes of 480 cm c a t h o l y t e . The drop i n e f f i c i e n c y a f t e r 25 passes was 35% of the i n i t i a l c urrent e f f i c i e n c y . CURRENT EFFICIENCY (%) NH 2OH CONCENTRATION (M) i 1 1 i i i Oi 0> ->J OO <£> Q H 2 S 0 4 CONCENTRATION (%) ZL 73 5.8 EFFECT OF BED HEIGHT [Appendix 2, Tables C,H] E f f e c t of cathodic bed height.was examined at four d i f f e r e n t current d e n s i t i e s i n s i n g l e pass mode of operation. The r e s u l t s are p l o t t e d i n Figures 19 and 20. Lower bed heights were simulated by packing the cathode chamber bottom upwards wi t h WC only upto the height under i n v e s t i g a t i o n . The cathodic and the an-odic current feeder p l a t e s were i n s u l a t e d above t h i s l e v e l . The r e s t of the top p o r t i o n of the cathode chamber was packed w i t h g l a s s beads of the same s i z e as WC below, i . e . -.59+.42 mm. This was done to support the membrane from sagging due to the weight of the anodic bed. As seen i n Figure 19, current e f f i c i e n c y normally increases as the bed height i s reduced. This c l e a r l y suggests the importance of g a s - l i q u i d flow d i s t r i b u t i o n i n the bed. Since there i s a bed of glass beads above the WC bed i n the case of lower bed h e i g h t s , longer lengths of t h i s bed s t a b i l i z e s the flow d i s t r i b u t i o n more before g a s - l i q u i d mixture enters the WC bed. As the i n l e t of the cathode chamber i s i n the centre of the width, i t appears that some height of the r e a c t o r i s i n e f f e c t i v e l y used due to m a l d i s t r i b u t i o n of gas-l i q u i d . As a r e s u l t , the lowermost p o r t i o n of the bed seems to be the most e f f e c t i v e . h/4 h/2 3h/4 BED HEIGHT (h = 0.375 m ) Figure 19 EFFECT OF BED HEIGHT ON CURRENT EFFICIENCY FOR FIXED VALUES OF CURRENT DENSITY Catholyte f l o w r a t e NO flow r a t e 3 -1 16.4 cm min n 221.5 cm3 min" BED HEIGHT (h = 0.375 m) F i g u r e 20 EFFECT OF BED HEIGHT ON HYDROXYLAMINE CONCENTRATION FOR FIXED VALUES OF CURRENT DENSITY 3 -1 Cathol y t e f l o w r a t e 16.4 cm min ^ NO f l o w r a t e 221.5 cm 3 min" In Figure 20, NH^ OH' concent r a t i o n i s p l o t t e d against bed height f o r f i x e d values of current den-s i t i e s . As one would expect, NK^OH conc e n t r a t i o n increases w i t h bed height. However, the increase i s not always p r o p o r t i o n a l because of reasons given i n the previous paragraph. 5.9 EFFECT OF BED DIMENSIONS [Appendix 2, Table C, K, A ( l ) ,A(3J1. Because of the importance of g a s - l i q u i d d i s t r i -b u t i o n r e a l i z e d i n the previous s e c t i o n , i t was d e c i -ded to see i f a smaller bed width (3 cm) minimized such e f f e c t s due to l e s s c h a n n e l l i n g and more e f f e c t i v e u t i l i z a t i o n of the sides of the bed. The sides of the bed were thought to be i n e f f e c t i v e l y u t i l i z e d be-cause of the presence of the i n l e t (1 cm dia.) i n the centre of the 5 cm wide bed. For the case of the smaller bed width, the gas and l i q u i d flow r a t e s were reduced proportionately so that the s u p e r f i c i a l v e l o c i t y remained the same as i n the 5 cm wide bed. The re-s u l t s are p l o t t e d i n Figure 21 and i t can be seen that the increase i n current e f f i c i e n c y and N^OH concentra-t i o n i s appreciable only at high current d e n s i t i e s . E f f e c t of bed depth i n the d i r e c t i o n of current was not examined e x p l i c i t l y but i t was r e a l i z e d during 150 250 350 450 550 CURRENT DENSITY (A rn"2) Figure 21 EFFECT OF BED WIDTH AND CURRENT DENSITY ON CURRENT EFFICIENCY AND HYDROXYLAMINE CONCENTRATION S u p e r f i c i a l v e l o c i t y of c a t h o l y t e 13.12 cm min 1 S u p e r f i c i a l v e l o c i t y of NO 177.2 cm min 1 78 the a c t i v i t y r uns ( T a b l e s A ( l ) , A ( 3 ) ) t h a t by r e -d u c i n g the bed depth from 0.6 cm (e.g. Table A ( 3 ) ) t o 0.25 cm (Table A ( l ) . l ) , t h e r e i s no change what-soever i n the s t a b i l i z e d c u r r e n t e f f i c i e n c y (e.g. F i g u r e s 5 and 6) and NH^OH c o n c e n t r a t i o n . A bed of s m a l l e r d e pth has lower v o l t a g e drops and r e q u i r e s l e s s e r amounts of e x p e n s i v e c a t a l y s t . 5.10 EFFECT OF CELL PRESSURE ['Appendix 2, Table C, E ,1] One of the most i n t e r e s t i n g e f f e c t s o b s e r v e d was t h a t of p r e s s u r e i n the c e l l ; the r e s u l t s are p l o t t e d i n F i g u r e 22. An average p r e s s u r e o f 236 kPa was m a i n t a i n e d i n the cathode compartment of the r e a c t o r by f i x i n g an i n - l i n e r e l i e f v a l v e a t the o u t l e t of the c e l l . The gas f l o w r a t e was a d j u s t e d such t h a t the moles of NO e n t e r i n g the r e a c t o r per minute remained the same as those used a t o r d i n a r y p r e s s u r e s , so t h a t a comparison can be made. Both c u r r e n t e f f i c i e n c y and o u t l e t NH2OH c o n c e n t r a t i o n i n c r e a s e s u b s t a n t i a l l y ait h i g h c u r r e n t d e n s i t i e s ; though the e f f e c t i s not observed below 125 A m . T h i s i s i n agreement w i t h the e f f e c t o f gas f l o w -"discussed i n S e c t i o n 5.4. P r o b a b l y a t low c u r r e n t d e n s i t i e s , enough NO i s a l r e a d y a v a i l a b l e a t the WC s u r f a c e so as t o consume a l l the c u r r e n t . r CELL PRESSURE (Average), kPa o A ® A 119 236 201 .00 100 200 500 600 300 400 CURRENT DENSITY (A m - 2 ) F i gure 2 2 EFFECT OF CELL PRESSURE AND CURRENT DENSITY ON CURRENT EFFICIENCY AND HYDROXYLAMINE CONCENTRATION Catholyte flowrate 16.4 cm3 min -* NO flowrate 9.2 x 10" 3 moi. m i n - J to At 425 Am , a pressure of 236 kPa increased the current e f f i c i e n c y by 50% over the e f f i c i e n c y at 119 kPa. The increase of NF^OH concentration was of the same order. Since the pressures as high as 1600 kPa have been s u c c e s s f u l l y used i n such r e a c t o r s [31], _ 2 i t appears that current d e n s i t i e s of 1000 A m , which are normally required f o r a process to be commercially a t t r a c t i v e , are p o s s i b l e f o r production of Nf^OH by t h i s process. T h e o r e t i c a l p r e d i c t i o n s (Section 5.17) show that by i n c r e a s i n g the c e l l pressure from 119 kPa to 236 kPa, both the 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 s and the s o l u b i l i t y of NO i n c a t h o l y t e are increased s u b s t a n t i a l l y , leading to an increase i n l i m i t i n g current d e n s i t y . In Figure 23, the rate of hydroxylamine pro-duction i s p l o t t e d against the p a r t i a l pressure of NO i n the gas stream. The p a r t i a l pressures and the co r r e s -ponding r a t e s are c a l c u l a t e d from gas composition t e s t s reported before [Section 5.5]. I t i s observed that the rate of NH^OH formation increases almost l i n e a r l y with p a r t i a l pressure of NO. 81 Figure 2 5 EFFECT OF PARTIAL PRESSURE OF NITRIC OXIDE IN GAS STREAM (CONTAINING NITRIC OXIDE AND NITROGEN) ON THE RATE OF HYDROXYLAMINE PRODUCTION Gas f l o w r a t e 646 cm 7 min Catholyte f l o w r a t e 16.4 cm Current d e n s i t y 213.3 A m mm 2 •1 1 5.11 EFFECT OF CATHOLYTE CONCENTRATION [Appendix 2, Tables A(1),A(2),M] Results of the t e s t s using 5% ^SO^ as c a t h o l y t e were discussed (Section 5.1) and reported (Figure 7) before. Results f o r 20% H^SO^ are also p l o t t e d on Figure 7. I t should be r e c a l l e d here that 10% ^SO^ was used as c a t h o l y t e i n a l l other runs. There appears to be some s c a t t e r of data when 20% U^SO^ was used; however, i f average values (value on the l i n e a r p o r t i o n of the curve f o r 5% and 10%) of current e f f i c i e n c i e s are taken f o r d i f f e r e n t concentra-t i o n s and p l o t t e d as. shown i n Figure 24, a good s t r a i g h t l i n e f i t i s observed. I t i s seen that current e f f i -c i ency f o r NH2OH production increases almost l i n e a r l y w i t h c a t h o l y t e c o n c e n t r a t i o n . T h i s , however, would be expected to l e v e l out at higher ^SO^ strengths because mainly NH^ rather than OT^OH r e p o r t e d l y forms i n strong H 2S0 4 (58%) s o l u t i o n s (Section 2.1). Since mainly N 20 i s formed i n d i l u t e H 2S0 4 s o l u t i o n s (e.g. pH=5), one would expect current e f f i c i e n c y to increase as the cat h o l y t e c o n c e n t r a t i o n i s increased away from t h i s lower l i m i t . Moreover, an increase i n c a t h o l y t e con-c e n t r a t i o n increases i t s c o n d u c t i v i t y , thereby g i v i n g a lower voltage drop and a lower p o t e n t i a l f o r hydrogen generation. On the other hand, the counteracting 83 D 5 10 15 20 CATHOLYTE CONCENTRATION (% H 2 S 0 4 ) F i g u r e 24 EFFECT OF H2SO4 CONCENTRATION IN CATHOLYTE ON CURRENT EFFICIENCY : Current d e n s i t y 213.3 Cat h o l y t e f l o w r a t e 16.4 NO f l o w r a t e 221.5 1 A m cm 3 min cm 3 m i n " l e f f e c t of reduced s o l u b i l i t y of NO i n c a t h o l y t e can come i n t o p l a y as the c a t h o l y t e concentration i s i n -3 3 creased. The s o l u b i l i t y of NO f a l l s from 0.06 cm /cm 3 3 of s o l u t i o n to 0.05 cm /cm of s o l u t i o n as the c a t h o l y t c o n c e n t r a t i o n i s increased from 10 to 201 [Appendix 4 ] . This l a t t e r e f f e c t does not appear to be dominating over the former two, probably because of the large excess of NO a v a i l a b l e throughout the r e a c t i o n . 5.12 EFFECT OF WC PARTICLE SIZE [Appendix 2, Table A ( T ) , J J In Figure 25, current e f f i c i e n c y and NH^OH con-c e n t r a t i o n are p l o t t e d against time f o r three d i f f e r e n t p a r t i c l e s i z e s . . I t i s seen that by reducing the par-t i c l e s i z e from -2.0+0.84 mm (average diameter 1.42 mm) to -0.59+0.42 mm (average diameter 0.51 mm), current e f f i c i e n c y increases from 211 to 62% i . e . by about 200% This i s mainly because of increased mass t r a n s f e r as a r e s u l t of increase i n s p e c i f i c surface area as the par^ t i c l e s i z e i s reduced [Table 3 ] . However, reducing the p a r t i c l e s i z e f u r t h e r to -0.42+0.18 mm (average d i a -meter 0.3 mm) produces a negative e f f e c t , as, although the i n i t i a l e f f i c i e n c i e s are the same, they appear to f a l l r a p i d l y f o r the smallest p a r t i c l e s i z e . .04 2 .03 o .02 .01 .00 h 80 > 60 40 LU £ 20 0 -A • •-WC PARTICLE SIZE , mm O - 42+ .18 A - . 5 9 + .42 • -2 .0+.84 0-A-0«=rA-^j A- -A •-0 2 3 TIME (hrs.) F i g u r e 25 EFFECT OF TUNGSTEN CARBIDE PARTICLE SIZE ON CURRENT EFFICIENCY AND HYDROXYLAMINE CONCENTRATION Cathol y t e f l o w r a t e 16 NO f l o w r a t e 221 Current d e n s i t y 213.3 A cm_ cm-m mm min 2 -1 -1 86 5.13 USE OF PLATE ELECTRODE AND NON-CONDUCTIVE BED [Appendix 2, Table C, L] These tests were carried out i n order to es-ta b l i s h the superiority of a conductive packed bed electrode over a plate electrode and a non-conductive packed bed for the reduction of a low s o l u b i l i t y gas. No bed was used in the cathode compartment when the effectiveness of plate electrode was studied. The stainless steel cathodic bed current c a r r i e r plate acted as the plate cathode and the catholyte and the NO gas were introduced in the c e l l from the bottom. In absence of a bed, this would be expected to give a better mass transfer than when introduced from the top. For the case of the non-conductive bed, glass beads (size -. 59+. 42 mm) were used in place of WC and the entry of catholyte and NO was as usual from the top of the bed. Results of these runs are plotted in Figure 26. The corresponding re s u l t s obtained from a WC bed operating under the same conditions are taken from a previous experiment (Figure 10,, Table C) and are plotted on the same figure for comparison. A WC bed i s c l e a r l y seen to be markedly superior, as both the NK^OH con-centration and current e f f i c i e n c i e s are an order of magnitude higher than those obtained from the other 87 80| 70 60 > o z UJ o t i -l l . UJ H z UJ rr rr o 50 40 r s 10 r CATHODE CONFIGURATION • PLATE CATHODE A A GLASS BEADS (-.59 + . 42 mm) ADJACENT TO PLATE o • WC (-.59+-.42 mm) ADJACENT TO PLATE .05 .04 03 < rr h-Z UJ o z o .02 ° X o X 01 .00 400 100 200 300 CURRENT DENSITY (Am" 2) Figure 26 COMPARISON OF EFFICIENCIES OF A TUNGSTEN CARBTDF. PACKF.T)  BED. AN INERT BED AND A PLATE ELECTRODE FOR NITRIC 0JQDE REDUCTION 3 Catholyte NO flow r a t e f l o w r a t e 16.4 221. 5 cm. cm-mm min -1 88 two. The reasons f o r t h i s were discussed before (Section 2.3). Although the highest e f f i c i e n c y given by the non-conductive bed i s 15% (at a low c u r r e n t d e n s i t y of 80 A m ), i t nevertheless gives b e t t e r performance than the p l a t e e l e c t r o d e . The reason f o r t h i s i s that the presence of the bed of gl a s s beads improves g a s - l i q u i d d i s t r i b u t i o n near the current c a r r i e r p l a t e where the r e d u c t i o n would how be expected to occur. 5.14 LOSSES OF HYDROXYLAMINE ON CATHODE [Appendix 2, Table N] I t would be i n t e r e s t i n g to see i f there i s any los s of hydroxylamine at a l l i f n e a r l y saturated s o l u -t i o n s of hydroxylamine are introduced i n the r e a c t o r . Such a c o n d i t i o n may a r i s e i n the f o l l o w i n g s i t u a t i o n s : 1) Extended r e c y c l i n g may lead to a heavy build-up of NH2OH. 2) In a commercial o p e r a t i o n , c e l l lengths of about 5 times the s i z e used i n t h i s program, would be expected [31]. In such cases sub-s t a n t i a l amount of NH2OH w i l l b u i l d up i n a s i n g l e pass. 3) In one r e c e n t l y proposed method f o r separation of s o l i d s a l t from NH^OH s o l u t i o n s , the mother l i q u o r i s r e c y c l e d to the r e a c t o r a f t e r c r y s t a l l i z i n g a part of the s a l t [ 3 2 ] , and adding make-up a c i d s o l u t i o n . High concentra-t i o n s of NH2OH would be expected i n such c a t h o l y t e . 89 Such conditions were simulated by the addition of about 0.356 g of hydroxylamine sulphate s a l t per 3 cm of catholyte solution. This corresponds to 0.143 g 3 of hydroxylamine per cm of solution or, 4.33 M hydroxy-lamine ; i . e . about 10 times the concentration ob-tained in our system after 25 passes. The s o l u b i l i t y of hydroxylamine sulphate under experimental conditions _ 3 i s about 0.44 g cm solution [Appendix 4] . The 3 -1 catholyte flow rate (7.63 cm min ) maintained was lower than usual so that the residence time of the con-centrated solution i s higher. The r e s u l t s are tabu-lated in Table N. It can be seen that there are n e g l i -gible losses of hydroxylamine due to side reactions even at high current densities. 5.15 ALTERNATE CELL ARRANGEMENT [Appendix 2, Table 0] A simple undivided c e l l which uses a porous dia-phragm rather than ion exchange membrane for packed bed e l e c t r o l y t i c reductions has recently been reported to give good re s u l t s [31] in the cathodic reduction of oxygen to hydrogen peroxide. According to t h i s c e l l arrangement, a porous diaphragm (e.g. nylon) i s placed f l a t against anode current feeder plate, without any inert bed i n between, and thus dispenses with the need for separate anolyte and catholyte c i r c u l a t i o n systems. 90 Because of the absence of the non-conductive bed of g l a s s beads and the ion-exchange membrane, such an arrangement would be expected to give very low voltage drops and hence low power consumptions. Results obtained using t h i s arrangement are tabu-l a t e d i n Table 0. I t can be seen that voltage drops have indeed reduced by 40-50% over a d i v i d e d c e l l operating under same co n d i t i o n s but current e f f i -c i e n c i e s and NH^OH concentrations are extremely low, mostly below 5% and 0.0016 M r e s p e c t i v e l y . The reason f o r t h i s i s obvious: Nr^OH i n our system i s much more s u s c e p t i b l e to o x i d a t i o n than hydrogen peroxide f o r which such a c e l l arrangement has been s u c c e s s f u l l y used [31], Oxygen generated at the anode d i f f u s e s through the diaphragm to the cathode side i n absence of an o u t l e t on anode side and immediately o x i d i s e s NK^OH to ^ 0 according to r e -a c t i o n s given i n Section 2.2. Even a c a t i o n exchange membrane w i t h a bed on the anode side was t r i e d out i n one of the i n i t i a l runs and t h i s was not found to be as e f f e c t i v e as anion membrane, probably because the former would permit NH^OH* to d i f f u s e through to the anode s i d e . 91 5.16 VOLTAGE REQUIREMENTS [Appendix 2, Tables C,D,I] Voltage required in the c e l l under d i f f e r e n t conditions has been plotted against current density in Figure 2,7. It can be seen that voltage drop i n -creases almost l i n e a r l y with current density in a l l cases. This however would not be expected to do so i n d e f i n i t e l y ; as seen in some of the cases plotted, i t would tend to taper off as the l i m i t i n g current density approaches; i . e . when there i s no more NO to be reduced at the WC surface. As i s also seen, an increase in catholyte flow has no eff e c t on voltage requirement, but increase i n gas flow or gas pressure increases voltage requirement considerably. Increase in gas flow i s expected to give larger void f r a c t i o n i n bed by reducing l i q u i d holdup. 5.17 COMPARISON OF OBSERVED EFFECTS WITH THEORETICAL  PREDICTIONS Mass transfer c o e f f i c i e n t s for the trickle-bed system were calculated using the most appropriate correlations available in l i t e r a t u r e for the present system. The c o e f f i c i e n t s were calculated from energy d i s s i p a t i o n i n the gas and l i q u i d streams; the l a t t e r followed from pressure loss and l i q u i d holdup-' e s t i -mates. The c a l c u l a t i o n scheme i s shown in d e t a i l in Appendix 3. The mass transfer c o e f f i c i e n t s and the o > U J O § 4 , o > CATHOLYTE NO PRESS. FLOW FLOW* (Avg.) cm 3 min"' c m 3 min"1 kPa o 16. 4 221 .5 119 A 23.75 221 .5 119 A 16.4 686.0 123 • 16.4 221 .5 236 0 200 400 CURRENT DENSITY (A m"2) Figure 27 600 TYPICAL VOLTAGE REQUIREMENTS IN THE CELL UNDER DIFFERENT FLOW CONDITIONS to N i t r i c oxide flowrates are at 101.3 kPa pressure and 20°C, 93 l i m i t i n g current densities (Equation 2,21'X have been calculated for d i f f e r e n t gas and l i q u i d flowrates used in our experiments and one pressure (236 kPa). The results for hydrodynamics (pressure loss, l i q u i d holdup, etc.) are presented in Table A, Appendix 3; whereas those for mass transfer and l i m i t i n g current density are presented i n Table B, Appendix 3. The results of Table B are plotted i n Figure 28. The following points are clear i n these re s u l t s : a) The pressure drops ( A P L Q ) observed are in the same range as those predicted. The observed pressure drops varied between 18 and 28 kPa (Tables C,D; Appendix 2);. they increased substantially with gas flowrate, but the increase with l i q u i d flow-rate was n e g l i g i b l e . In the predicted r e s u l t s , the increase in pressure loss i s substantial with both l i q u i d and gas flow and they are found to vary between 17 and 35 kPa. b) The l i q u i d holdup decreased with increase in gas flowrate. An ind i r e c t e f f e c t of this would be an increase in voltage requirement. In the experimental r e s u l t s , as discussed i n Section 5.16, voltage requirement increased with increase in gas flow. Also the l i q u i d holdup increases with increase in l i q u i d flow, as one would expect. c) The liquid-phase mass transfer ( k T , a ) and the l i q u i d - t o - s o l i d mass transfer (k sa) are con-t r o l l i n g f a c t o rs. The overall c o e f f i c i e n t (k 0a) i s v i r t u a l l y independent of the gas-phase mass transfer (kga). Although the mass transfer correlations in trickle-bed reactors are not well established, they seem to predict l i m i t i n g current densities well near the observed values after suitable corrections are made for the present system; 94 500 100 400 700 _ , 1000 1300 GAS FLOW (cm 3 min"1) 5 15 25 35 LIQUID FLOW (cm 3 min-' ) Figure 28 EFFECT OF GAS FLOW AND LIQUID FLOW ON THEORETICALLY PREDICTED LIMITING CURRENT DENSITY . [Append i TTT For varying gas flow: l i q u i d flow = 16.4 cm3 min" 1 For varying l i q u i d flow: gas flow = 221.5 cm3 min" 1 95 Approximately observed l i m i t i n g current den-s i t i e s vary between 300-500 A m-2 (Figures 10 and 13); those p r e d i c t e d vary between 280 and 465 A m-2 (Figure 28). S u b s t a n t i a l increase i n l i m i t i n g current d e n s i t y i s p r e d i c t e d w i t h the increase i n l i q u i d flow, whereas the i n -crease with gas flow i s moderate. Although the increase i n l i m i t i n g current d e n s i t y w i t h gas flow was observed i n our r e s u l t s (Section 5.4), the increase w i t h l i q u i d flow do not correspond to t h e o r e t i c a l r e s u l t s (Section 5.3). The l a t t e r i s not s u r p r i s i n g because no change of pressure l o s s was noticed w i t h increase i n l i q u i d f l ow, as discussed i n previous part of t h i s s e c t i o n . d) T h e o r e t i c a l l y p r e d i c t e d l i m i t i n g current den-s i t i e s increase by more than 1001 as the pressure i n the c e l l i s increased from 119 kPa to 236 kPa. This may have been magnified a l i t t l e because of n o n a v a i l a b i l i t y of s u i t a b l e c o r r e c t i o n f a c t o r s f o r s o l u b i l i t y of NO i n H2SO4 s o l u t i o n s . In-crease i n observed l i m i t i n g current d e n s i t y with pressure i s , however, quite s u b s t a n t i a l (Section 5.10). 5.18 CONCENTRATION AND CRYSTALLIZATION OF A DILUTE  SOLUTION OF HYDROXYLAMINE I t was observed that hydroxylamine concentrations at the o u t l e t of the r e a c t o r i n a s i n g l e pass were normally below 0.05 M. Concentrating such d i l u t e s o l u t i o n s f o r the purpose df c r y s t a l l i z a t i o n of s o l i d hydroxylamine sulphate could be expensive. Evaporation cost could nevertheless be reduced by i n c r e a s i n g NH2OH c o n c e n t r a t i o n i n the product by using lower c a t h o l y t e f l o w r a t e s , higher gas f l o w r a t e s , higher current den-s i t i e s , r e c y c l i n g , longer lengths of c e l l , e t c . A l l 96 of these r e s u l t in an increased c a p i t a l and/or operating costs, mostly because of reduced through-put or current e f f i c i e n c i e s . Decision of operating parameters of such a process appear to be an ideal problem of optimization. It was nevertheless decided to see i f d i l u t e solutions of hydroxylamine could be concentrated by careful evaporation, without appreciable loss of hydroxylamine due to decomposition. For this pur-pose, a 0.5 M solution of hydroxylamine was prepared with commercially available hydroxylamine sulphate, and evaporated c a r e f u l l y under vacuum to about 3 M NK^OH concentration. The actual amount of NK^OH l e f t in the concentrated solution was determined by analysis.' It was found that there was no loss of hydroxylamine whatsoever. This solution, when allowed to stand i n ai r for 1-2 days, produced c r y s t a l s of regular structure (Figure 29(a)). A 0.4 M NK^OH solution pro-duced in one of the recycle runs (Section 5.7) also gave c r y s t a l s of regular structure (Figure 29 (b,)). 97 Figure 29 (a) CRYSTALS OF HYDROXYLAMINE SULPHATE OBTAINED FROM A 5M SOLUTION OF COMMERCIALLY AVAILABLE SALT b) CRYSTALS OF HYDROXYLAMINE SULPHATE OBTAINED FROM A~0.4M SOLUTION PRODUCED IN RECYCLE RUN OF FIGURE 18 98 CHAPTER SIX CONCLUSIONS An investigation was made of a method to produce hydroxylamine by reduction of n i t r i c oxide in t r i c k l e -bed electrochemical reactors. Hydroxylamine was formed in an aqueous solution of sulphuric acid t r i c k l i n g down a packed bed electrode with cocurrent flow of n i t r i c oxide gas. Better performance was observed with a tungsten carbide bed than with a graphite bed, there-fore the tungsten carbide p a r t i c l e s were used as a cath-ode in this work. The parameters studied were catholyte flowrate and composition, gas flowrate and composition, tungsten carbide p a r t i c l e size, bed dimensions, and c e l l pressure and temperature. Both single pass and recycling of catholyte were studied. In t y p i c a l operation a 0.0314 M solution of hydroxylamine was produced at atmospheric pressure in a single pass i n a 0.375 m high, 0.25 cm deep and 5 cm wide cathode bed at a current density of 213.3 -7 3 - 1 A m , catholyte flowrate of 16.4 cm min and a current e f f i c i e n c y of 62.2% . The a c t i v i t y of tungsten carbide p a r t i c l e s for re-duction of n i t r i c oxide was found to drop between succes-sive days of operation. The drop in a c t i v i t y occurred as a r e s u l t of o x i d a t i o n of p a r t i c l e surface by f r e e or d i s s o l v e d oxygen i n c a t h o l y t e . When i n use, n e g l i -g i b l e o x i d a t i o n was observed because of the s i m u l t a -neous generation of hydrogen on the bed. C r i t e r i a were then e s t a b l i s h e d f o r operation with tungsten car-bide so that the drop i n a c t i v i t y d i d not i n t e r f e r e during the i n v e s t i g a t i o n of the e f f e c t s of other v a r i -ables. The e f f e c t s of d i f f e r e n t parameters on the hy-droxylamine c o n c e n t r a t i o n and current e f f i c i e n c y f o r i t s formation are summarized below. 1) There was no d e f i n i t e e f f e c t of c a t h o l y t e f l o w r a t e on current e f f i c i e n c y or the r a t e of hydroxylamine formation i n the range from 3 to 31 cm3 min" 1. Hydroxylamine concen t r a t i o n was observed to decrease sharply w i t h increase i n c a t h o l y t e f l o w r a t e . 2) Increase of a c i d c o n c e n t r a t i o n i n c a t h o l y t e from 5 to 20% produced an increase i n both c u r r e n t e f f i c i e n c y and hydroxylamine con-c e n t r a t i o n . 3) Increase i n gas f l o w r a t e produced an increase i n both current e f f i c i e n c y and hydroxylamine co n c e n t r a t i o n above a c e r t a i n current d e n s i t y . 4) Reduction of n i t r i c oxide content of the gas stream was found to produce a l e s s than pro-p o r t i o n a l decrease i n current e f f i c i e n c y and hydroxylamine co n c e n t r a t i o n . 5) Increase of c e l l pressure from 119 kPa to 236 kPa s u b s t a n t i a l l y increased both the current e f f i c i e n c y and hydroxylamine co n c e n t r a t i o n . Under a pressure of 236 kPa, a hydroxylamine co n c e n t r a t i o n of 0.07 M was obtained i n a s i n g l e pass of c a t h o l y t e , at a s u p e r f i c i a l current d e n s i t y of 625 Am"2, which c o r r e s - • ponded to a current e f f i c i e n c y of 44%. 100 6) Increasing the average tungsten carbide p a r t i c l e s i z e above 0.51 mm reduced the current e f f i c i e n c y and NH2OH c o n c e n t r a t i o n . 7) There was no observed e f f e c t of temperature i n the i n v e s t i g a t e d range from 22-41°C. 8) Recycling of c a t h o l y t e produced almost a l i n e a r increase i n NH 20H c o n c e n t r a t i o n with number of passes through the r e a c t o r , w i t h -out appreciable drop of current e f f i c i e n c y . In a 500 cm 3 c a t h o l y t e volume, a hydroxy-lamine c o n c e n t r a t i o n up to 0.4 M was b u i l t up a f t e r 25 passes. 9) Reducing the bed height corresponded to an increased current e f f i c i e n c y . Reducing bed width produced no appreciable changes at ordinary current d e n s i t i e s . Reducing the bed depth i n the d i r e c t i o n of current from 0.6 cm to 0.25 cm produced no change i n the average curren t e f f i c i e n c y . With increase i n current d e n s i t y , the current e f f i c i e n c y f o r hydroxylamine formation was observed to drop presumably due to the formation of undesired n i -trous oxide, ammonia and hydrogen. As a r e s u l t of side r e a c t i o n s again, the hydroxylamine c o n c e n t r a t i o n i n -creased l e s s than l i n e a r l y w i t h increase i n current den-s i t y ; a l i n e a r increase should be expected from Faraday's laws. The re d u c t i o n of n i t r i c oxide appeared to be mass t r a n s f e r c o n t r o l l e d . As a r e s u l t , mass t r a n s f e r l i m i t e d c urrent d e n s i t i e s were reached at r e l a t i v e l y low a p p l i e d current d e n s i t i e s . This produced a v i r t u a l l y constant hydroxylamine concentration even i f current density was increased further. The l i m i t i n g current den-s i t i e s were predicted t h e o r e t i c a l l y from mass trans-fer considerations. Although the mass transfer cor-r e l a t i o n s for trickle-beds are not well established, they seem to predict l i m i t i n g current densities resonably close to the observed values. Increasing gas flowrate produced an increase in current e f f i -ciency- onTLy . above a certa i n current density, which can now be recognized as l i m i t i n g current density under that s i t u a t i o n . This l i m i t i n g current density increased with gas flowrate, as expected from mass transfer considerations. Increase in catholyte flow-rate, however, apparently produced a decrease in observed l i m i t i n g current density, contrary to theo-r e t i c a l mass transfer predictions. The increase in hydroxylamine concentration and hence the current e f f i -ciency due to increase in pressure i s expected from mass transfer considerations. Also, an increase in p a r t i c l e size brings about a reduction i n surface area over which mass transfer takes place. This e f f e c t was observed only for p a r t i c l e size above 0.51 mm. Decreasing bed heights gave higher current e f f i -c iencies. In normal operation, the sides of the bed near the i n l e t appeared to be i n e f f e c t i v e l y u t i l i z e d . 102 However, reducing bed width to minimize such„ e f f e c t s produced only a marginal increase i n current e f f i -c i e ncy. A thinner bed (0-25 cm) i s p r e f e r r e d over a t h i c k e r bed (0.6 cm) because both of them give more or l e s s the same e f f i c i e n c y under same n i t r i c oxide-c a t h o l y t e loadings. An increase of a c i d co n c e n t r a t i o n i n the c a t h o l y t e produced l e s s n i t r o u s oxide and hence produced an increase i n current e f f i c i e n c y . F i n a l l y , the use of a conductive packed bed electrode f o r re-duction of n i t r i c oxide was j u s t i f i e d by proving that a conventional p l a t e electrode or a non-conductive packed bed give extremely poor r e s u l t s . In t h i s program r e l a t i v e l y d i l u t e hydroxylamine s o l u t i o n s were pro-duced even i n the r e c y c l i n g experiments. Such a process could p o s s i b l y be used f o r more concentrated s o l u t i o n s since losses of hydroxylamine from n e a r l y saturated s o l u t i o n s on the electrode were n e g l i g i b l e . I t appears that although the red u c t i o n of n i t r i c oxide i s mass t r a n s f e r c o n t r o l l e d , the observed e f f e c t s cannot be f u l l y explained by mass t r a n s f e r considera-t i o n s alone. A more thorough a n a l y s i s should consider v o l t a g e , current and concent r a t i o n d i s t r i b u t i o n s i n three-dimensional e l e c t r o d e . At present such an analy-s i s i s d i f f i c u l t to c a r r y out because s u i t a b l e cor-r e l a t i o n s f o r such d i s t r i b u t i o n s i n t r i c k l e - b e d e l e c -trodes are not yet e s t a b l i s h e d . 103 In sum this work shows that i t i s possible to produce hydroxylamine e l e c t r o l y t i c a l l y by reduction of n i t r i c oxide at useful current e f f i c i e n c i e s . A rough estimate of the cost of hydroxylamine pro-duced by this method gives a value which i s less than a quarter of the current market price. The process outlined here therefore does deserve further consideration. : CHAPTER SEVEN RECOMMENDATIONS There are two broad areas which should be con-centrated on in the next phase of experimental pro-gram on production of hydroxylamine on trickle-bed reactors: a) Improving the performance of tungsten carbide cathode. b) Increasing the current density and hydroxy-lamine concentration i n a single pass while maintaining a reasonable current e f f i c i e n c y . Making a more ef f e c t i v e use of tungsten carbide seems to be of primary importance because of the high cost of t h i s e l e c t r o c a t a l y s t . To t h i s end, a good starting point would be a rigorous insu l a t i o n of tung-sten carbide p a r t i c l e s from oxygen once i t i s put into use. Implementation of either of the following modi-f i c a t i o n on the process as outlined would be f u l l y j u s t i f i e d : a) Blanketing of catholyte tank with nitrogen. b) Stripping of catholyte with nitrogen before i t i s mixed with n i t r i c oxide. The c e l l should be made a i r - t i g h t as far as possible and a s l i g h t p o s i t i v e pressure of an inert gas should be maintained i n the c e l l when not i n operation. The regeneration of oxidized tungsten carbide should also be looked into. A surface characteri-zation of the oxidized particles to identify the un-desired compounds formed would be a logical step. Tungsten carbide anodic beds have been reported [18, 20,33] to have been reactivated by treatment with boiling alkaline solutions. An obvious outcome of this program is a strong effect of pressure in the c e l l . Higher pressures should certainly be tried out as a f i r s t step towards increasing the current density and hydroxylamine con-centration . Other factors which should be looked into on a long run are: a) Routine analysis of the outlet gas for N2O, N02 and H2 , and liquid for (NH4)2S04 and H2S04 to establish optimum conditions for operation. b) Investigation of other cathodic materials with high hydrogen overpotential. c) Simplification of c e l l design to eliminate anolyte circulation system: placing an anion exchange membrane fl a t against anode would prove to be a good starting point. Voltage requirements would be considerably reduced. d) Improving gas-liquid distribution in the re-actor by putting baffles along the edges. e) Modelling of electrochemical parameters of a trickle-bed electrode so as to be able to predict current efficiency under given pro" cess conditions. 106 I n v e s t i g a t i o n of these f a c t o r s , together w i t h an in-depth study of those a l r e a d y mentioned, would r a i s e the prospect of producing hydroxylamine e l e c t r o -l y t i c a l l y by t h i s method. NOMENCLATURE Typical Units 2 -3 Specific surface area of bed m m Empirical constant m _3 Concentration of NO in bulk of gas moi. m -3 Concentration of NO on solid surface moi. m -3 Solubility of NO in electrolyte moi. m -3 Density of electrolyte g cm Average particle diameter m Modified particle diameter m Diameter of a column having same perimeter as rectangular wall m 2 -1 Diffusivity of gas in the liquid m s Anode equilibrium potential volt Cathode equilibrium potential volt Standard electrode potential volt -3 Energy dissipation for gas flow W m -3 Energy dissipation for liquid flow W m Friction factor Faraday's constant coul (g equiv) -2 -1 Gas mass superficial flow rate kg m s Bed height m -2 Current density A m _2 Real limiting current density A m Superf c al limiting curren  density 108 Typical Units I Total applied current A ]"NH QJ_J Current consumed for hydroxylamine 2 production A IR Effective potential drop due to bed p matrices and electrolytes Volt IR Membrane potential drop Volt k Overall mass transfer coefficient m s 1 o kg Gas-phase mass transfer coefficient m s 1 k^ Liquid-phase mass transfer coefficient m s 1 kg Liquid-to-solid mass transfer co- , efficient m s K Henry's law constant mm Hg -2 -1 L Liquid mass superficial flowrate kg m s M Wall correction factor M^ Molecular weight of electrolyte P N Q Partial pressure of NO in gas mm Hg AP Pressure loss Pa AP^  Single phase liquid pressure loss Pa APg Single phase gas pressure loss Pa M'LG T W O phase pressure loss Pa 3 -1 Q Catholyte flowrate cm min Re Particle Reynolds number R^  Hydraulic radius m Sc Schmidt number t Depth of bed in the direction c rent u Superficial velocity u^ Superficial velocity of liquid ^ Superficial velocity of gas cur- . m m -1 s m -1 s m -1 s 109 V cell w NO NH2OH iNFLOH Cell voltage Width of bed Mole fraction of NO in electrolyte Molal concentration of hydroxylamine Number of electrons associated with formation of hydroxylamine Typical Units Volt m M Bed height m GREEK LETTERS a N f ) Volume of NO/Volume of electrolyte at 760 mm pressure g Liquid holdup per unit volume of void A Difference e Voidage of bed e' Bed voidage corrected for wall effect Random loose porosity e Q Empirical constant p Density kg m Density of gas kg m P L Density of liquid kg m p . Density of air kg m ^air 1 & pwat Density of water kg m Overpotentials Volt N^H OH Current efficiency for hydroxylamine "2 production 110 yG ywat Typical Units Viscosity PI Viscosity of gas PI Viscosity of liquid PI Viscosity of water PI -2 aT Surface tension of liquid N m -2 a ^ Surface tension of water N m wat <J>^  Lockhart-Martinelli parameter, / A PLG / A PL Lockhart-Martinelli parameter, I l l REFERENCES 1. Kirk-Othmer Encyclopedia ofi Chemical Technology, 2nd Edition, Vol. 11, 493-508, Interscience Publishers, N.Y. (1966). 2. Jockers, K. "Manufacture of Hydroxylamine by Catalytic Reduction of Nitric Oxide" Nitrogen, No. 50, 27-30, Nov./Dec. (1967). 3. Sneed, M.C. and Brasted, R.C. (eds.) Comprehensive In-on.ga.nlc Chemistry, Vol. 5, Chapter 1, 31-41, Van Nostrand, N.J. (1956). 4. Savodnik, N.N. et a l . "Kinetics of the Catalytic Synthesis of Hydroxylamine from Nitric Oxide and Hydrogen" Kinetika i Kataliz, 13, (6), 1520-1526, Nov.-Dec. (1972). 5. Savodnik, N.N. et al "Synthesis of Hydroxylamine on Platinum: I. Electrochemical Reduction of Nitric Oxide on a Platinum Electrode" Electrokhimiya, 7_, (3), 424-427, March (1970). 6. Savodnik, N.N. et a l . "Synthesis of Hydroxylamine on Platinum: II. Interaction of Hydroxylamine and Nitric Oxide.on Platinum" 7 (4), 583-585, April (1971). 7. Shepelin, V.A. "Selection of a Model for a New Method of Hydroxylamine Production" Zhurnal Prikladnoi Khimii, 47_ (4), 713-716, April (1974). 8. Shepelin, V.A. et a l . , "Investigation of a Model for a New Method of Hydroxylamine Production" Zhurnal Prikladnoi Khimii, '47 (5), 985-988 , May (1974). 9. Janssen, L.J.J. "Electrolytic Reduction of Nitric Oxide to Hydroxylamine" Extended abstracts of 23rd meeting of J.S.E. Stockholm, Sweden, 209-210, (1972). 10. Janssen, L.J.J. "Reduction of Nitric Oxide at a Flow-Through Mercury Plated Nickel Electrode" Electrochimica Acta, 21_, 811-815 (1976). 11- Latimer, W.H. The Oxidation States ofa the Elements and Their P o t e n t i a l s In Aqueous Solutions Chapter 7, p. 90-105, Prentice Hall, N.J., (1952). „ Ehman, D.L., and Sawyer, D.T. "Electrochemistry of Nitric Oxide and of Nitrous Acid at a Mercury Electrode" J. Electroanal. Chem. 16, 541-549 (1968). Masek, J. :Polarographic Reduction of NO0 Group" Z. Anal. Chem. 224-, 99-107 (1967). Chow, C. "Electrolytic Reduction of Nitric Oxide to Hydroxylamine" Unpublished Report, The University of British Columbia (1976). Pickett, D.J. EZe.ctSL0chzm4.cat Rcactoft Ve.6i.Qn , Elsevier Scientific Publishing Co., Amsterdam (1977). Picone, Bruno M. "Electrochemical Reduction of Nitric Oxide to Hydroxylamine" B.A. Sc. thesis, The University of British Columbia, (1977). Mathc6on Ga6 Vata Book, 4th Edition, 367-370, The Matheson Co. Inc. Ont. (1966) . Benda, K.V. et al EZ'cctfiochcmtcaZ 'Bckavi.bu.fi o{, Tungsten Cafibi.dc EZcctfiodc6, Electrocatalyst to Fuel Cell, U. of Washington Press (1971) . Kosolapova T. Ya. CaA.bi.de.6 - Vfiopcn.ti.c6, Vfiodu.cti.on and AppZi.cati.on, Plenum Press, N.Y. (1971). Ross, P.N. and Stoneheart P. "Surface Characterization of Catalytically Active Tungsten Carbide (WC)" Journal of Catalysis 39_, 298-301 (1975). Behret H. "Inorganic and Organic Non-noble Metal contain-ing Electrocatalyst for Fuel Cells" Proc. Symp. on Electrocatalysis, M. Brerter (Ed.), The Electrochem. Soc. (1974). Armstrong, R.D. et a l . "Factors in the Design of Electro-chemical Reactors" Nature, 219, 94, July 6 (1968). Government of Canada Ambient Air Standards, Canada Gazette Part II, 108_ (11), July 12 (1974). Hall, W.T. knaZytlcaZ Ckcml6tfiy Volume II, 9th Ed., 9th Printing, 564, J. Wiley and Sons, Inc., N.Y. (1958). Kolthoff, I.M. and Elving, P.T. (eds.) Tficti.6C on kn.atyti.caZ Chcml6tn.y Part II, Vol. 5, 288-290 , Interscience Publishers, N.Y. (1961). Vogel, A.I. Quant<Ltati.\)c Inofigantc KnaZy6i.6 3rd ed., 391, Longmans, Green and Co., London (1939). 113 27. Kolthoff, I.M. and Sandell, E.B. Textbook o& Qjuantatlve Inorganic Analysis 3rd Ed., 568, The McMillan Co., N.Y. (1952). 28. Oloman, C. "The Electroreduction of Oxygen to Hydrogen Peroxide on Particulate Electrodes" M.A.Sc. thesis, The University of British Columbia (1974). 29. Oloman, C. and Watkinson. A.P. "The Electroreduction of Oxygen to Hydrogen Peroxide on Fluidized Cathodes" Can. J. Chem. Eng. 5_3 , 268-273, June (1975). 30. Oloman, C. and Watkinson, A.P. "The Electroreduction of Oxygen to Hydrogen Peroxide on Fixed Bed Cathodes" Can. J. Chem. Eng. 5_4, 312-318, August (1976). 31. Oloman C. and Watkinson, A.P. "Hydrogen Peroxide Production in Trickle Bed Electrochemical Reactors" Accepted for publication, Journal of Applied Electro-chemistry (1978). 32. Korczynski, A. and Dylewski, R. "Electrochemiczna Synteza Siarczanu Hydrokoyloaminy" Przemyol Chemiczy 4_8 (32), 207-210 (1969). 33. Stockwell, B.D., Macro Division of Kennamental Inc., Port Coquitlam, B.C. Personal Communication (1978). 34. Charpentier, J.C. "Review Paper: Recent Progress in Two Phase Gas-Liquid Mass Transfer in Packed Beds" The Chemical Engineering Journal, 11_, 161-181 (1976). 35. Goto, S. and Smith, J.M. "Trickle-Bed Reactor Performance AIChE Journal, 21_ (4), 706-713, July (1975) . 36. Satterfield, C.N. "Trickle-Bed Reactors: Journal Review" AIChE Journal, 21 (2), 209-227, March (1975). 37. Charpentier, J.C. and Favier, M. "Some Liquid Holdup Data in Trickle-Bed Reactors for Foaming and Nonfoaming Hydrocarbons" AIChE Journal, 21 (6), 12 (6), 1213-1218, November (1975). 38. Ergun, S. "Fluid Flow Through Packed Columns" Chemical Engineering Progress, 4_8 (2), 89-94, February (1952). 39. Eastwood, J; Matzen, E.J.P; Young, M.J. and Epstein, N. "Random Loose Porosity of Packed Beds" British Chemical Engineering, 14- (11), 1542-1545, November (1969). 40. Sato, Y.: Hirose, T; Takahasi, F and Toda, M. "Pressure Loss and Liquid Holdup in Packed Bed Reactor with Co-current Gas-Liquid Down Flow". J. Chem. Eng. Japan, 6_ (2), 147-152 (1973). 114 41. R e i s s , L.P. "Cocurrent Gas-Liquid Contacting i n Packed Columns" I § EC Proc. Des. Dev. 6_ (4), 486-499-, October (1967) . 42. Perry, R.H. and C h i l t o n , C H . Chemical Engineers Handbook F i f t h E d i t i o n , McGraw H i l l , N.Y. (1973). 43. Comedy, A.M. § Hahn, D.A. A Dictionary o^ Chemical. Solubilities 2nd E d i t i o n , 614-617, MacMillan Co. N.Y. (1921). 44. S e i d e l l , A. Solubilities ofi lnon.ganic and Organic Compounds 2nd Ed., 461, D. Van Nostrand Co. (1919). 45. I n t e r n a t i o n a l C r i t i c a l Tables, Volume 3-276. 46. Schwarzkops, 'P." and K a i s s a r , R. Cemented Can.bi.dc 138, Macmillan Co., N.Y. (1960). 47. Weast, R.C. (ed.) Handbook o^ Chemistry and Physics 57th e d i t i o n , C.R.C. Press (1976). APPENDIX 1 AUXILIARY EQUIPMENT SPECIFICATION 116 Power Supply Sorenson, DCR 40-25 B 0-40V 0-25A Ammeters Central S c i e n t i f i c Co., D.C. Ammeter 0-10A Weston: D.C. Ammeter 0-15A Voltmeters Central S c i e n t i f i c Co., D.C. Voltmeter 0-1.5 vo l t s 0-15 vol t s Rotameters Catholyte: Gilmont size No.2, Model 3202-20 IS Teflon f l o a t . Anolyte : Brooks, Type 1510A Tube size 2-65, stainless steel f l o a t . NO : Brooks, Type 111 0 - 01-C-0000 Tube size R-2-25-A, stainl e s s steel f l o a t . N ? : Brooks, Type 1355V Tube size R-2-15-A, Teflon f l o a t . Pressure Gauges U.S. Gauge 29362 , 0-30 psig, stainless steel, obtained from Canadian Liquid A i r . Pumps Emerson E l e c t r i c Co., stainless steel gear pump Model ' F3-3HXEMF-2113 Tubing Imperial Eastman "Poly-Flow" and "Nylo-Seal", Fisher S c i e n t i f i c "Tygon"; 1/8" and 1/4". Valves Whitey, Forged body r e g u l a t i n g 316 s t a i n l e s s s t e e l , ORS'2 - 1/8" IRS4 - 1/4" Nupro, A d j u s t i b l e i n - l i n e r e l i e f , 316 s t a i n l e s s s t e e l , 4CA-3, 1/4" F i t t i n g s Swagelok compression tube f i t t i n g s 316 s.s. , 1/8" and 1/4". Anion Exchange Membrane Ionac Chemical Co., Type MA-3148 Tungsten Carbide Macro d i v . of Kennametal Inc., M a c r o c r y s t a l l i n e -.42 + .18 mm -.59 + .42 mm P a l l e t s -.2.0+ .84 mm Reagents NO - Canadian L i q u i d A i r , 99.0% pure < 50 ppm Ar- < 0.2% C0 2 < 0.6% N 2 < 0.1% NO2 < 300 ppm N 20 < N 2 - Canadian L i q u i d A i r , 99.99% pure H 2S0 4- A l l i e d Chemical, Assay 95.5 - 96.5% F e r r i c ammonium sulphate -F i s h e r Chemical, Assay 99.0 - 101.0% Hydroxylamine Sulphate Fisher Chemical, Assay 99.0% minimum Water -Laboratory, s i n g l e d i s t i l l e d water. APPENDIX 2 TABULATED EXPERIMENTAL RESULTS, ANALYTICAL TECHNIQUE AND SAMPLE CALCULATION 1 1 9 TABULATED EXPERIMENTAL RESULTS Unless otherwise mentioned, the following conditions apply unchanged to a l l the tabulated runs that follow: 1 ) Dimensions of cathode compartment: Same as in Table 2 . 2 ) Cathodic bed packing: Tungsten carbide, - . 5 9 + . 4 2 mm 3 ) Cell dividing medium: Anion exchange membrane 4 ) Strength of catholyte solution: 1 0 % H 2 S 0 4 5 ) Catholyte flow rate: 1 6 . 4 cmJ mm 6 ) Nitric oxide flow rate: •7 - 1 2 2 1 . 5 cmJ min 7) Nitrogen flow rate: n 3 - - 1 0 cm mm 8) Anolyte flow rate: 3 - 1 4 0 cm min 9 ) Strength of anolyte solution: 1 0 % H 2 S 0 4 A l l the gas flow rates are at 2 0 C and 1 0 1 . 3 kPa pressure. TABLE A ( l ) A c t i v i t y of Tungsten Carbide Strength of t i t r a n t (KMn04) = 0.0524N NO pressure gauge reading = 119 kPa (average Run No. Time Temperature Voltate Titrant required NH2OH Concentration Current efficiency In Out Hr. min °C °C Volt 3 cm M % 1 0.20 21.0 23.5 3.75 5.6 0.0293 58.05 2 0.50 22.0 24.5 3.75 5.9 0.0309 61.2 3 1.20 22.0 25.5 3.75 6.0 0.0315 62.2 4 2.05 22.0 25.5 3.7 5.9 0.0309 61.2 5 3.05 22.0 26.0 3.7 6.0 0.0314 62.2 6 4.20 22.0 26.0 3.75 6.0 0.0314 62.2 7 5.30 22.0 26.0 3.75 5.8 0.0304 60.1 8 6.30 22.0 26.5 3.7 6.0 0.0314 62.2 9 7.45 22.0 26.5 3.7 5.8 0.0304 60.1 10 8.45 22.0 27.0 3.7 6.0 0.0314 62.2 1. When Fresh Current density : 213.3 A m TABLE A ( l ) - (continued) 2. On the 2nd day Strength of t i t r a n t (KMnO ) = 0.0524N Current d e n i s t y = 213.3 A m" NO pressure gauge read i n g = 119 kPa (average) Run Time Temperature Voltage T i t r a n t r e q u i r e d NH 20H Concentration Current efficiency In Out No. Hr. min. °C °C V o l t 3 cm M % 11 0.20 19.0 21.5 3.8 5.3 0.0278 54.9 12 0.50 20.0 23.0 3.75 5.0 0.0262 51.8 13 1.30 20.0 23.5 3.75 5.2 0.0272 53.9 14 2.30 20.0 24 .0 3.7 5.0 0.0262 51.8 15 3.30 20.0 24 .0 3.7 6.0 0.0314 62 .2 16 4 .30 20.0 25.0 3.7 5.55 0.0291 57.5 17 5.30 20.5 26.0 3.7 5.35 0.0280 55.45 18 6.30 20.0 25.0 3.7 5.1 0.0267 52.9 19 7.30 20.0 25.0 3.7 5.4 0 .0283 56.0 TABLE A ( l ) - (continued) Strength of t i t r a n t (KMn04) = 0.0524N Current density =213.3 A n f 2 N 0 Pressure gauge reading = 119 kPa (average) Run Temperature T i t r a n t NH20H Current Time In Out Voltage required concentration efficiency No. Hr . min °C °C V o l t 3 cm M % 20 0.20 20.0 24.0 3.7 3.3 0.0173 34.2 21 0.50 20.0 24 .0 3.7 3.4 0.0178 35.2 22 1.30 20.5 25.5 3.6 3.8 0.0199 39.4 23 2.30 21.0 26.0 3.6 3.85 0.0202 39.9 24 3.30 21.0 26.0 3.6 3. 75 0.0196 38.9 25 4.45 21.0 27.0 3.6 3.8 0 .0199 39 .4 26 5.45 22.0 26.5 3.55 3.8 0.0199 39.4 27 6.45 22.0 26 .5 3.55 4.1 0.0215 42.5 TABLE A ( l ) - (continued) 4. On the 4th day Strength of t i t r a n t (KM11O4) = 0.0524N Current d e n s i t y = 213.3 A m" NO pressure gauge read i n g = 119 kPa (average) Time Temperature Voltage T i t r a n t required.. NH2OH c o n c e n t r a t i o n Current e f f i c i e n c y Run In Out No. Hr . min °C °C V o l t 3 • cm M a 0 28 0.30 20.0 24.0 3.6 2.0 0 .0105 20.7 29 1.00 21.0 24.5 3.5 2.2 0.0115 22.8 30 1.45 21.5 25.5 3.5 2.3 0.0121 23.8 31 2.45 22.0 26.0 3.45 2.3 0.0121 23.8 32 3.45 22.0 27.0 3.45 2.35 0.0123 24.4 4.00 • 6.15 Generated hydrogen at cathode by s h u t t i n g o f f NO 33 6.35 22.0 27.0 3.4 3.4 0.0178 35.2 34 7.05 22.0 27.0 3.4 3.2 0.0168 33 .2 35 7.45 22.0 27.5 3.4 3.3 0.0173 34.2 to OJ TABLE A(2) A c t i v i t y of Tungsten Carbide Using 5% H ?S0. as Catholyte When Fresh Current density = 213.3 A m Strength of t i t r a n t (KMn04) = 0.05025N NO pressure gauge reading = 119 kPa (average) Time Temperature Voltage T i t r a n t NH20H Current Run In Out required c o n c e n t r a t i o n e f f i c iency No. Hr. min °C °C V o l t 3 cm M % 36 0.20 20.0 22.5 3.1 4.9 0.0246 49 .5 37 0.50 21.0 24.0 3.1 4.9 0.0246 49.5 38 1.30 21.5 24.0 3.1 5.3 0.0266 53.55 39 2.10 21.0 24.0 3.1 5.3 0.0266 53.55 40 3.10 22.0 25.0 3.1 5.75 0.0289 58.1 41 4.10 22.0 25.5 3.1 5.8 0.0291 58.6 42 5.10 22.0 26.0 3.1 5.5 0.0276 55.6 43 6.10 22.0 26.0 3.0 5.7 0.0286 57.6 44 7.10 21.0 25.5 3.0 6.0 0.0301 60.6 45 8.00 22.0 26.0 3.0 5.65 0.0284 57.1 TABLE A(2) (continued) On the 2nd day Current density 213.3 A m Strength of t i t r a n t (KMnO^) = 0.05025N NO pressure gauge reading = 119 kPa (average) Time Temperature Voltage . Titrant NH7,0H • • Current Run In Out required •Li - - ' •concentration' ' e f f i c i e n c y No. Hr. min. °C °C Volt 3 cm M 0. 0 46 0.20 20.0 22.0 3.1 5.25 0 .0264 53.0 47 0.50 21.0 23.5 3.1 5.2 0.0261 52.5 48 1.30 22.0 24.0 3.1 5.15 0.0259 52.0 49 2.30 22.0 25.0 3.1 5.35 0.0269 54.05 50 3.30 22.0 26.0 3.1 5.8 0.0291 58.6 51 4.30 22.0 26.0 3.0 5.5 0.0276 55.6 52 5.30 22.0 26.0 3.0 5.0 0.0251 50.5 53 6.00 22.0 26.0 3.0 5.3 0.0266 53.55 54 6.45 22.0 26.0 3.0 4. 75 0.0239 48.0 55 7.15 22.0 26.0 3.0 5.05 0.0254 51.0 - 8:00 A s l i g h t cathodic ] potential ( 0 25A, 13.3A m"' I, ) was maintained on WC bed t i l l the start of 3rd day The bed was drained 24 .00 intermittently (every 2-3 hrs) with"acidic solution to prevent overheating TABLE A(2) (continued) 3. On the 3rd day Strength of t i t r a n t (KMh0 4) = 0.05025N Current d e n s i t y = 213.3 A m NO pressure gauge r e a d i n g - -." ll'9.:.'kPa (average) Run No. Time Temperature Voltage Titrant 'required NH20H concentration Current efficiency In Out Hr. min 0 C °C Volt 3 cm M 0. 0 56 0.25 21.0 23.0 3.0 5.8 0.0291 58.6 57 1.00 22.0 24.0 3.0 5.7 0.0286 57.6 58 1.45 22.0 25.0 3.0 5.5 0.0276 55.6 59 2.45 22.0 26.0 3.0 5.2 0.0261 52.5 60 3.45 22.0 26.0 3.0 4.9 0.0246 49 .5 61 4.45 22.0 26.0 3.0 5.1 0.0256 51.5 62 5.45 22.0 26.0 3.0 4.7 5 0.0239 48.0 63 6.35 22.0 26.0 3.0 •4.8 0.0241 48.5 64 7.00 22.0 26.0 3.0 4.7 0.0236 47.5 TABLE A(3) A c t i v i t y of Tungsten Carbide Over A Sin g l e Long Run - 2 Current d e n s i t y = 213.3 A m Cathodic bed depth = 0.6 cm Strength of t i t r a n t (KMn04) = 0.0526N NO pressure gauge reading • ="119 kPa (average) Run No. Time Temperature Voltage T i t r a n t r e q uired NH20H concentration Current e f f i c i e n c y In Out Hr. min °C °C V o l t 3 cm M .... . . . % . . . 65 0 . 3 0 2 7 . 0 3 1 . 0 4 . 1 5 4 . 8 0 . 0 2 5 2 4 9 . 9 66 1 . 0 0 2 7 . 0 3 2 . 0 4 . 1 5.1 0 . 0 2 6 8 5 3 . 1 67 1 . 3 0 2 7 . 0 3 3 . 0 4 . 1 6 . 4 0 . 0 3 3 7 6 6 . 6 68 2 . 1 5 2 7 . 5 3 3 . 5 4 . 0 6 . 0 0 . 0 3 1 6 6 2 . 4 69 3 . 1 5 2 8 . 0 3 4 . 0 4 . 0 6 . 1 0 . 0 3 2 1 6 3 . 5 70 4 . 1 5 2 8 . 0 3 4 . 0 3 . 9 5 5 .6 0 . 0 2 9 5 5 8 . 3 71 5 . 1 5 29 . 0 3 5 . 0 3 .9 6 . 2 0 . 0 3 2 6 6 4 . 5 72 6 . 1 5 2 9 . 0 3 5 . 0 3 .9 6 . 1 0 . 0 3 2 1 6 3 . 5 73 7 . 1 5 2 9 . 0 3 5 . 5 3 . 8 5 5 .9 0 . 0 3 1 0 6 1 . 4 74 8 . 4 5 3 0 . 0 3 7 . 0 3 .8 6 . 0 0 . 0 3 1 6 6 2 . 4 75 1 0 . 1 5 2 9 . 0 3 6 . 0 3 . 8 5 .9 0 . 0 3 1 0 6 1 . 4 76 1 2 . 0 0 29 . 0 3 5 . 5 3 . 8 5 .8 0 . 0 3 0 5 6 0 . 3 5 77 1 4 . 0 0 2 8 . 0 3 4 . 5 3 . 8 5 .6 0 . 0 2 9 5 5 8 . 3 TABLE B ( l )  A c t i v i t y of Graphite (Untreated) Size of particles = -.59+. 42 mm Cathodic--bed:-depth .-' = . 0 . 6 cm _2 Strength of t i t r a n t (Mn0 4) = 0.05N Current density = 213.3 A m NO pressure gauge reading = 319 kPa (average) Run No. Time Temperature Voltage T i t r a n t r e q u i r e d NH20H concentration Current e f f i c i e n c y In Out Hr. min °C ° C V o l t 3 cm M % 7 8 0 . 3 0 3 0 . 0 3 0 . 0 4 . 5 3 . 2 0 . 0 1 6 0 3 1 . 6 5 7 9 0 . 5 0 3 0 . 5 3 1 . 5 4 . 4 2 . 6 0 . 0 1 3 0 2 5 . 7 8 0 1 . 1 0 3 1 . 0 3 2 . 5 4 . 4 2 . 8 0 . 0 1 4 0 2 7 . 7 8 1 1 . 3 0 3 1 . 0 3 3 . 5 4 . 3 2 . 4 0 . 0 1 2 0 2 3 . 7 8 2 2 . 0 5 3 1 . 5 3 4 . 0 4 . 3 2 . 5 0 . 0 1 2 5 2 4 . 7 8 3 2 . 5 0 3 1 . 5 3 4 . 0 4 . 2 5 2 . 1 5 0 . 0 1 0 7 2 1 . 3 8 4 3 . 5 0 3 0 . 5 3 4 . 0 4 . 2 5 2 . 1 5 0 . 0 1 0 7 2 1 . 3 8 5 4 . 5 0 3 1 . 0 3 4 . 0 4 . 2 2 . 1 0 . 0 1 0 5 2 0 . 8 8 6 5 . 5 0 3 1 . 5 3 4 . 5 4 . 2 1.9 0 . 0 0 9 5 1 8 . 8 8 7 6 . 5 0 3 1 . 5 3 5 . 0 4 . 1 5 1 . 9 0 . 0 0 9 5 1 8 . 8 1—' OO TABLE B(2) A c t i v i t y of Graphite (Pretreated) Size of particles = -.59+.42 mm Current density = 213.3 A m~2 Cathodac bed depth =. 0.-6...cm Strength of titrant (KMP-O4) = 0.05N NO pressure gauge reading = 119 kPa (average) • Temperature Voltage Titrant NH20H Current Run Time In Out • required concentration efficiency No. Hr. min °C °C Volt 3 cm M % 88 0.25 3 1 . 5 30 .0 4.3 4.2 0 . 0 2 1 0 4 1 . 5 89 0.50 31.0 31.0 4.25 2.6 " 0 . 0 1 3 0 25.7 90 1 .15 3 0 . 5 3 1 . 5 4.2 2.1 0 . 0 1 0 5 20.8 9.1 1.45 3 0 . 5 3 1 . 5 4.2 1.8 0 . 0 0 9 0 17.8 92 2.15 3 0 . 5 32.0 4.15 1.8 0 . 0 0 9 0 17.8 93 2.45 30.5 32.0 4.1 1.7 0 . 0 0 8 5 16.8 94 3.40 30.5 32.0 4.1 1.8 0 . 0 0 9 0 17 .8 95 4.25 3 0 . 5 32.0 4 .05 1.7 0 . 0 0 8 5 16.8 96 5.25 32.0 33.0 4 .05 1.8 0 . 0 0 9 0 17 .8 97 6.25 32.0 33.0 4 .05 1.8 0 . 0 0 9 0 17.8 TABLE C  Effect of Catholyte Flow Strength of t i t r a n t (KMh04)= 0.05025N No pressure gauge reading = 119 kPa (average) Run No.-Catholyte flowrate Current Temperature Voltage Titrant required Current density NFLjOH concentration Current efficiency In Cut 3 . -1 cm mm A °C °C Volt 3 cm -? A m M '% 98 16 .4 4 21.0 25.0 3.4 6.3 213 .3 0.0317 62.7 99 16 .4 5 21.0 26.0 3.7 7.2 266.7 0.0362 57.25 100 16.4 3 21.5 25.0 3.0 5.5 160.0 0.0276 72.9 101 16.4 6 22.0 28.0 4.0 7.65 320.0 0.0384 50.7 102 16 .4 2 21.0 25.0 2.65 •3.8 106.7 0.0191 75 .5 103 16.4 7 22.0 30.0 4.35 7.75 373.3 0.0389 44.0 104 16.4 1 21.0 24.0 2.3 1.8 53.3 0.009 71.6 105 16 .4 8 21.0 30.0 4.65 8.7 426.7 0.0437 43.2 106 16 .4 0 21.0 24.0 1.0 0.05 0 0.0002 O-l o TABLE C (continued) Catholyte Temperature Titrant Current Mi 20H Current Run Flowrate Current In Out Voltage required density concentration efficiency No. 3 . -1 cm mm A °C °C Volt 3 cm A m"2 M % 1 0 7 2 3 . 7 5 4 2 2 . 0 2 6 . 0 3 . 4 3 . 8 2 1 3 . 3 0 . 0 1 9 1 5 4 . 7 1 0 8 2 3 . 7 5 5 2 2 . 0 2 7 . 0 3 . 7 4 . 2 2 6 6 . 7 0 . 0 2 1 1 4 8 . 4 1 0 9 2 3 . 7 5 3 2 2 . 0 2 5 . 0 3 . 0 3 . 0 5 1 6 0 . 0 0 . 0 1 5 3 5 8 . 5 1 1 0 2 3 . 7 5 6 2 2 . 5 2 8 . 0 4 . 0 4 . 2 3 2 0 . 0 0 . 0 2 1 1 4 0 . 3 1 1 1 2 3 . 7 5 2 2 2 . 0 2 4 . 0 2 . 7 2 . 4 1 0 6 . 7 0 . 0 1 2 1 6 9 . 1 1 1 2 2 3 . 7 5 1 2 2 . 0 2 4 . 0 2 . 3 1 . 2 5 3 . 3 0 . 0 0 6 6 9 . 1 1 1 3 7 . 7 5 4 2 0 . 0 2 6 . 0 3 . 3 1 3 . 1 5 2 1 3 . 3 0 . 0 6 6 1 6 1 . 8 1 1 4 7 . 7 5 5 2 0 . 0 2 7 . 0 3 . 6 5 1 5 . 4 2 6 6 . 7 0 . 0 7 7 4 5 7 . 9 1 1 5 7 . 7 5 3 2 0 . 0 2 6 . 0 2 . 9 5 1 0 . 9 1 6 0 . 0 0 . 0 5 4 8 6 8 . 3 1 1 6 7 . 7 5 6 2 0 . 0 2 8 . 0 3 . 9 1 7 . 4 3 2 0 . 0 0 . 0 8 7 4 5 4 . 5 1 1 7 7 . 7 5 2 2 0 . 0 2 5 . 5 2 . 6 7 . 6 1 0 6 . 7 0 . 0 3 8 2 7 1 . 4 1 1 8 7 . 7 5 1 2 0 . 0 , 2 4 . 5 2 . 2 5 3 . 6 5 3 . 3 0 . 0 1 8 1 6 7 . 6 1 1 9 3 1 . 0 4 2 2 . 0 2 5 . 0 4 . 0 5 3 . 0 2 1 3 . 3 0 . 0 1 5 1 5 6 . 4 1 2 0 2 . 7 5 4 2 1 . 5 2 7 . 0 4 . 0 3 6 . 2 5 2 1 3 . 3 0 . 1 8 2 2 6 1 . 5 TABLE D E f f e c t of N i t r i c Oxide Flow Strength of t i t r a n t (KMnOJ = 0.05025N NO Temperature NO Pressure Titrant required Current density NH20H concentratior Current efficiency Run flowrate Current In Out Voltage No. 3 . -1 cm mm A °C °C Volt kPa 3 cm A m"2 M % 121 450 4.5 21.0 25.5 4.3 119 7.35 240.0 0.0369 64 .9 122 450 3.25 21.5 25.0 3.65 119 5.7 173.3 0.0286 69 .7 123 450 5.75 22.0 28.0 4.7 120 9.15 306.7 0.046 63.3 124 450 2.0 22.0 25.5 2.9 120 3.5 106.7 0.0176 69 .6 125 450 7.0 22.0 30.0 5.1 120 10.95 373.3 0.055 62.6 126 450 0.75 22.0 25.5 2.25 122 1.0 40.0 0.005 53.0 127 450 8.25 22.0 32.0 .5.3 122 11.3 440.0 0.0568 54.5 128 450 9.5 22.0 34 .0 • 5.55 122 10.7 506.7 0.0538 44 .8 129 686 2.0 21.0 26.0 2.8 122 3.55 106.7 0.0178 70.6 130 686 8.25 22.0 32.5 5.15 124 11.5 440.0 0.0578 55.4 131 686 0.75 22.0 25.5 2.2 122 1.0 40.0 0.005 53.0 O J t o TABLE D  Ef f e c t of N i t r i c Oxide Flow (continued) Run NO "flow-rate Current Temperature Voltage NO Pressure ' Titrant required Current density NH;20H.eon-. centration Current efficiency In Out No. 3 .-1 cm mm A °C °C Volt kPa 3 cm A m"2 M % 132 686 9.5 22.0 34.0 5.4 124 12.35 506.7 0.0621 51.7 133 686 3.25 22.0 28.0 3.3 122 5.5 173.3 0.0276 67.3 134 686 7.0 22.0 32.0 4.75 123 10.85 373.3 0.0545 61.6 135 686 4.5 22.0 29.5 3.8 120 7.95 240.0 0.0399 70.2 136 686 5.75 22.0 30.0 4 .25 122 9.0 306.7 0.0452 62.2 137 686 10.2 22.0 36.0 5.5 125 12.6 544 .0 0.0633 49.1 138 94 4.0 21.5 27.0 4.0 119 4.9 213. 3 0.0246 48.7 139 904 4.0 21.5 27.5 3.9 125 7.5 213.3 0.0377 74.55 140 1117 4.0 22.0 28.0 3.85 129 7.2 213.3 0.0362 71.6 OJ OJ TABLE E E f f e c t of Gas Composition Current density = 213.3 Am"2 Strength of titrant (KMn04) = 0.05025N Total gas flow rate = 646 cm3 min" 1 Temperature Pressure NO • = N 2 •Titrant required NH20H: con-centration Current u c L S L j U I I i p U b X L J L U J ] Voltage efficiency Run NO N2 In Cut No Volume % Volume % °C °C Volt kPa cm3: M % 141 100 0 22.0 27.5 3.8 122 6.85 0.0344 68.4 142 83. 75 16.25 22.0 27.5 3.8 124 6.5 0.0327 64.6 143 66.25 33.75 22.0 27.5 3.8 125 6.0 0.0301 59 .6 144 48.8 51.2 22.0 27.5 3.8 131 5.4 0.0271 53.7 145 32.7 67.3 21.5 27.0 3.8 136 5.0 0.0251 49.7 146 18.7 81.3 21.0 27.0 3.8 138 4.0 0.0201 39.8 TABLE F E f f e c t of Catholyte Temperature Current density = 213.3 Am" Strength of titrant (KMnO,) = 0.05025N 3 - 1 Catholyte flow rate = 23.75 cm min NO pressure gauge reading = 119 kPa (average) Pun Catholyte Temperature Voltage Titrant required NH20I1 concentration Current efficiency No In Out °C °C Volt 3 cm M % 107 22.0 26.0 3.4 3.8 0.0191 54.7 147 34.0 30.5 3.6 3.95 0.0198 56.9 148 41.0 33.5 3.6 3.9 0.0196 56.1 l—1 O J Cn TABLE G  E f f e c t of C a t h o l y t e R e c y c l e 1. Recycling Volume ( i n i t i a l ) = 1050 cm Catholyte flow rate = 16.4 cm3 min" 1 Strength of t i t r a n t (KMnO.) = 0.05025N Current density = 213.3 A m WC p a r t i c l e size = -.42 + .18 mm Run No Number of times recycled Recycling time Recycling volume Temperature Voltage NO pressure Titrant required NH20H con-centration Current e f f i c i e n c y Tn Out. Hr. min 3 cm °C °C Volt . kPa 3 cm M % 149 0.55 0.35 1050 21.5 28 . 0 4.0 129 3.0 0.0151 54. 6 150 1.1 1.10 1040 21.5 28.0 4.0 129 6.0 0.03015 54. 0 151 1. 65 1.45 1034 21.0 28. 0 4.0 129 8.65 0.0435 51.7 152 2.2 2. 20 1023 20.5 28. 0 4.0 127 11.1 0.0558 49. 2 153 2.8 2.55 1017 20.0 27.0 4.1 125 14.0 0.07035 49.3 154 3.35 3.30 1011 20.0 27.0 4.1 125 16.3 0.0819 47.6 -155 4.1 4.15 1005 20.5 27.5 4.1 125 19.55 0.0982 46.7 156 4.7 4.55 994 21.0 28.0 4.0 125 22.35 0.1123 45.65 157 5.65 5. 50 988 21.5 28.0 4.0 125 26. 0 0.1306 44.5 158 6.7 6.50 982 20.5 27.5 4. 05 125 29.9 0.1502 43.4 159 7.7 7. 50 976 20. 5 27.5 4.05 124 33 . 8 0.1698 42.5 160 8.85 9.00 970 20.0 27.0 4.1 124 38 .05 0.1912 41.4 LA ON TABLE G (continued) Recycl i n g Volume ( i n i t i a l ) = 500 cm' 3 . -1 cm mm Catholyte flow rate = 16.4 _ 2 Current d e n s i t y = 213.3 A m NO pressure gauge reading = 119 kPa (average) Strength of t i t r a n t : Run 161-164:0.05263N Run 165-169:0.1279N Run No No of times " recycled Recycling time Hr. min Recycling volume |Catholyte strength cm % H2S04 Temperature In Or Out Voltage Volt T i t r a n t r e q u i r e d cm NH20H con-centration M Current efficiency 161 162 163 164 165 166 167 168 169 2.0 4.0 6.0 8.0 10 .5 10.6 11. 3 14.0 17.7 19.3 21.9 25.1 1, 2 3 4 5 5 5 6 00 00 00 00 10 15 35 55 8 . 40 9 .25 10.40 12.10 500 494 488 482 471 481 475 480 474 463 468 457 6.93 7.57 7 .12 21.5 22.0 22.0 22.0 Added 22.0 Added 22.0 22.0 Added 22.0 20.0 27 . 5 28.0 28 .0 28 .0 10 cm 3 cone 28.0 5 cm3 cone 28.0 28 .0 5 cm 3 cone 28 .0 27 .0 4.9 4.8 4.8 4.75 . H 2S0 4 4.7 , H 2S0 4 4.75 4.8 , H 2S0 4 4.8 4.9 9.0 17.0 0.0474 0.0895 47 44 24.65 0. 1297 42 .4 30.7 0. 1616 39 . 1 15 .05 0. 1925 35 .45 19.2- 0. 2456 34 .3 23.4 0 . 2993 32 .9 28.05 0. 3588 31 .6 31.9 0 . 4080 30 .8 CM TABLE H  E f f e c t of Bed Height Cathode bed height (h) = 0.375m Strength of t i t r a n t (KMnCO = 0.05263N current Current Temperature NO Titrant required NH20H con- Current Run Bed density In Out Voltage pressure centration efficiency No height A A m"2 °C °C Volt kPa 3 cm M' ' % 170 3h/4 3.0 213.3 20.0 23.5 3.55 119 . .4.93 0.02595 68. 4 171 3h/4 4.5 320.0 20.0 25.5 4.1 125 5.8 0. 0305 53. 7 172 3h/4 1.5 106. 7 20.0 23. 5 2. 75 115 2.95 0.0155 81.9 173 3h/4 6.0 426. 7 20.0 27.0 4.7 129 6.1 0 .0321 42.3 174 h/2 2.0 213.3 22.0 26. 0 3.55 112 3.4 0.0179 70.8 175 h/2 3.0 320.0 20.0 26.0 4.1 115 4.1 0.0216 56.9 176 h/2 1.0 106. 7 19. 5 23. 5 2.7 113 2.1 0.0111 87.45 177 h/2 4.0 426. 7 18. 0 25.5 4.8 119 4.9 0.0258 51.0 178 h/4 1.0 213 .3 18. 0 20. 5 2.9 112 1.75 0.0092 72.9 179 h/4 1.5 320.0 18. 0 21.0 3 .25 112 2.45 0. 0129 68.0 180 h/4 0.5 106. 7 18.0 20.0 2.4 110 1.0 0.0053 83.3 181 h/4 2.0 426.7 18. 0 21. 5 3.6 112 2.7 0.0142 56. 2 1 TABLE I E f f e c t of C e l l Pressure Average pressure i n c e l l = 236 kPa Strength of t i t r a n t (KMnO^) = 0.05348N Run Current Current Density Temperature Voltage T i t r a n t required NH20H con-c e n t r a t i o n Current efficiency In Out No A A m~2 °C °C V o l t 3 cm M . % 182 4 213.3 18.0 20.5 3.85 8.25 0.0446 87.3 183 6 320.0 17.0 22.5 4.9 9.9 0.053.1 70.0 184 8 426 .7 18.0 26 .0 5.9 11.95 0.0639 63.2 185 2 106.7 19.0 21.0 ' 2.7 3.5 0.0187 74.0 186 10 533.3 20.0 31.0 6.5 12 .4 0.0663 52.5 187 12 640.0 21.0 37.0 6.8 13 . 5 0.0722 47.6 TABLE J E f f e c t of WC P a r t i c l e Size Current d e n s i t y = 213.3 A m Strength of t i t r a n t : runs 188-192:0.05025N runs 193-196:0.05348N Particle size Time Temperature Voltage NO Titrant NH20H con- Current Run In Out pressure required centration" •'. efficiency No mm Hr .min °C °C Volt kPa 3 cm M - % 188 -.42+.18 1.05 20.0 25.5 4.15 136 6.3 0.0317 62.6 189 -.42+.18 1.40 20.0 26 .0 4.1 136 6 .15 0 .0309 61.1 190 -.42+.18 2 . 20 20 . 5 27.0 4.1 132 6.10 0.0307 60.6 191 -.42+.18 2.55 21.0 27 . 5 4.0 132 5.8 0.0291 57.65 192 -.42+.18 3.30 21.0 27.0 4.0 132 5 .55 0.0279 55.2 193 -2 .0+.84 1.00 20.0 23 . 0 3 .15 105 2 . 05 0.011 21.7 194 -2.0+.84 1. 35 20.0 23.5 3.1 105 2.0 0.0107 21.2 19 5 -2.0+.84 2.35 19.5 24 .0 3.1 105 2.0 0.0107 21.2 196 -2.0+.84 4.00 20 . 0 24.0 3.1 105 1.95 0 .0104 20.6 TABLE K E f f e c t of Bed Width Cathodic bed width = 3 cm Strength of titrant (KMnOJ = 0.05348N Catholyte flow rate (for same superficial velocity as in 5 cm wide bed) =16.4x^-=9.84cm min 3 3 -NO flow rate (for same superficial velocity as in 5 cm wide bed) = 221.5 x -F- = 132.9 cm mm Run No Current Current density Temperature Voltage NO pressure Titrant required NH2OH con-centration Current efficiency In Out A A m 2 °C °C Volt kPa 3 cm M % 197 2.4 213.3 17.0 21. 0 3.2 124 6.13 0.0328 64.8 198 3.6 320.0 18.0 23 . 5 3.7 127 7.8 0.0417 55.0 199 4.8 426. 7 19. 0 25.5 4.1 132 8.85 0.0473 46. 8 200 .1.2 106.7 19. 0 23. 5 2.55 119 3.7 0.0198 78.3 201 6.0 533.3 19.0 27.0 4.5 134 9.4 0.0503 39.8 TABLE L Use of Pl a t e Electrode and Non-Conductive Bed Strength of t i t r a n t (KMnO^) = 0.05263N Runs 202-204: Using only the current feeder p l a t e as p l a t e cathode, (without any bed i n cathode compartment). Runs 205-207: Using an i n e r t bed (glass beads, s i z e -.59+.42 mm) in.the cathode compartment. Run No Current Current density Temperature Voltage NO pressure T i t r a n t r e q u i r e d NH20H con-centration Current efficiency In Out A A m"2 °C °C V o l t s kPa 3 cm M % 202 203 204 4.0 7.0 1.5 213.3 373.3 80.0 20.0 20.0 20.0 24.0 27.0 24.0 4.8 6.5 3.2 108 108 108 0.25 0.3 0.25 0.0013 0.0016 0.0013 2.6 1.8 6.9 205 4.0 213.3 18.5 23. 0 4.0 122 0.55 0.0029 5.7 206 1.5 80.0 17.0 21.0 2.75 115 0.55 0.0029 15.3 207 7.0 373.3 20.0 28.0 5.6 122 0.8 0.0042 4.8 4^ TABLE M E f f e c t of Catholyte Strength Catholyte strength : 20% H 2S0 4 Strength of t i t r a n t (KMnO^) : 0.05348N Current density : 213.3 A m"2 Temperature Voltage NO T i t r a n t r e q u i r e d NH20H con-centration Current efficiency Run Time In Out pressure No Hr. min °C °C V o l t kPa 3 cm M % 208 6.15 20.0 26 . 0 3.7 132 6.1 0 . 0326 64. 5 209 7.00 21.0 26 .0 3.65 129 6.9 0.0369 73.0 210 7.25 21.0 26 .0 3.65 129 7.3 0.039 77 . 2 211 8.00 21.0 26.0 3.75 12 7 . 6.6 0 . 0353 69.8 212 8.30 21.0 26 .0 3.75 127 7.1 0 . 038 75.1 TABLE N Losses of Hydroxylamine on Cathode Catholyte composition: 4.33M (=143.1 g cm ) NH?OH i n 10% H ?S0 3 - 1 Catholyte flow r a t e : 7.63 cm min Strength of t i t r a n t (KMnO.) = 0.05348N Current NO Temperature • NH20H Titrant NH20H • NH20H Run Current density Voltage pressure In Out i n required* out Loss No A A m"2 V o l t kPa °C °C M 3 cm M % 213 ' 4 213.3 4.0 120 20.0 27.0 4.33 40.2 4.30 0.7 214 4 213.3 4.0 120 20.0 27.0 4.33 39.8 4. 26 1.6 215 8 426 .7 6.5 129 19. 0 33.0 4.33 39.0 4.17 3.7 For 5 cm of (5 cm sample d i l u t e d to 100 cm with water) TABLE 0 Al t e r n a t e C e l l Arrangement C e l l d i v i d i n g medium: Nylon diaphragm - - - - Strength'of t i t r a n t (KMnO.j* = 0.05348N Current Temperature NO T i t r a n t r e q uired NH2ON con-c e n t r a t i o n Current e f f i c i e n c y Run Current density In Out Voltage pressure No A A m"2 °C °C V o l t kPa 3 cm M % 216 4 213.3 18.0 20.0 •2.1 132 0.3 0.0016 3.2 217 6 320.0 20.0 23.0 2.2 134 0.3 0 .0016 2.1 218 2 106 .7 20.5 21.5 2.0 129 0.25 0.0013 5.3 219 8 426.7 22.0 27.0 2 . 2 136 0 . 2 0.0011 1.1 Ln 146 ANALYTICAL TECHNIQUE [10, 24-27] 3 A 5 cm sample of c e l l outlet liquor is collected in a measuring cylinder at the outlet of the stripping column (or withdrawn by a pipette from the catholyte tank in case of recycling mode) and is 3 transferred to a 250 cm conical flask. To this are 3 added 10 cm of cold saturated ferric ammonium sulphate solution* and 10 cm of 101 I-^ SO^  solution. The con-tents of the flask are heated to boiling and kept at this temperature for 5 minutes. Hydroxylamine re-duces ferric ion to ferrous ion in hot acid solution according to the following equation: 2NH2OH + 4 Fe + + + -> 4 Fe + + + N20 + 4H++ H20 ..(1) 3 The solution is diluted with 150 cm water and the amount of ferrous ion therein is determined immediately by titration with a 0.05N solution of potassium per-manganate, which has been standardized against a solution of ferrous ammonium sulphate. The equation for the re-action between permanganate and ferrous ion under acid condition i s : MnO~ + 5Fe + + + 8H+ -> 5Fe + + + + Mn + + + 4H20. . . . (2) _ A solution containing 420 g salt in 1000 cm of water was found adequate for the present case. 147 It can be easily seen from these equations that 1 cm3 of IM KMn04 - 1 cm3 of 5M F e + + = 1 cm3 of ^p-M NH2OH - | x 33.03 x YtTM S NH20H = 82.575 x 10"3 g NH2OH, where 33.03 is the molecular weight of NH2OH. Also, from equation (2), the equivalent weight of KMnO^  for reaction with ferrous ion is i t s molecular weight divided by 5. _3 So 1 cm3 of IN KMnO = 82.575 x 10 = 0 > 016515 g ' 4 b NH2OH. Alternately, i t can be seen from equation (1) that the equivalent weight of hydroxylamine for it s reaction with ferric ion is half i t s molecular weight. So hy-droxylamine concentration in moles per l i t r e (M) can be written as (Volume of KMnO. used) x (Normality of KMnO.) NH20H = — — i : _L_ ...(3) cone. (Volume of sample) x 2 The end point is determined by change of colour of the solution to pink. However, the end point is not very clear due to the yellow colour rendered to the solution by the excess ferric ammonium sulphate. Addition of a few drops of syrupy phosphoric acid before titration turns the solution colourless due to the formation of ferric phosphate, and the end point becomes very obvious. 148 Amounts of ferric ammonium sulphate and the sulphuric acid solutions to be added are dependent on a prior knowledge of the approximate amount of hydroxylamine present in the sample, and normally 10 3 cm of each are sufficient for every 0;05 mole per l i t r e of hydroxylamine to be analysed. Due to a wide variation in the procedure as out-lined in various cited references, i t was decided to test out the method given above against a known sample of A.R. grade hydroxylamine sulphate obtained from Fisher Chemicals. The latter was manufactured by conventional Raschig method. It was found that the amount of hydroxylamine predicted by this method was in-dependent of the amounts "of ferric salt and sulphuric acid solutions added, so long as both of them were in excess, i.e., above the amounts given before. Also, in a l l cases, the method predicted the amount of hy-droxylamine correctly within II of the actual amount present. 149 SAMPLE CALCULATION (a) Single pass made Run 98 (Table C) Catholyte flow r a t e Q Current Sample volume Strength of t i t r a n t T i t r a n t r e q uired From Equation ('3) , Product NH-OH co n c e n t r a t i o n , X 3 • -1 16.4 cm mm 4.0 A 3 5.0 cm 0.05025N 6.3 cm3 NH2OH 6.3: -x- 0.05025 5.0 x 2 From Equation (5 . 3) , Current e f f i c i e n c y , n = 0.0317 M 0.0317 x 3 x 16.4 x 96493  N H 2 ° H " 6.0 x 104 x 4 = 0.627 or 62.7% Current Current d e n s i t y ( s u p e r f i c i a l ) = A r e a o f c a t h o d e plate 4.0 0.375 x 0.05 = 213.3 A m Rate of NH20H production i s r e a d i l y obtained by m u l t i p l y i n g NH2OH concent r a t i o n w i t h c a t h o l y t e flow r a t e : Rate = 0.0317 x 16.4 x 10 -3 -4 . -1 5.2 x 10 moi mm (b) Recycle mode Run 151 (Table G) Recycling time Recycling volume Current Sample volume Strength of t i t r a n t T i t r a n t r e q u i r e d 1 hour 45 min 6300 sec 1034 cm 3 4.0 A 3 5.0 cm 0.05025N 8.6 5 cm3 From Equation (3), .p. j .... n u * 4 . - •"8.65 x 0.05025 Product NH20H concentration = 5 rj x 2 = 0.0435 M From Faraday's law, t h e o r e t i c a l Nl^OH concen t r a t i o n 6300 x 4 96493 x 3 x 1034 x I O - 3 = 0.0842 M From Equation (5.4), Current e f f i c i e n c y , n N H 0 H - 0.0842 0.517 or 51.7%. 151 APPENDIX 3 THEORETICAL PREDICTIONS OF MASS TRANSFER AND LIMITING CURRENT DENSITY 152 One of the prerequisites for successful modelling of mass transfer in trickle-bed electrochemical re-actors is a careful study of the hydrodynamics of the three phase system, and for this reason a knowledge of the f l u i d flow patterns, pressure losses and liquid holdup is of fundamental importance. The following assumptions are made to simplify the treatment. 1) Since the gas flow rate is much higher than the stoichiometric requirement (15-40 times), the decrease of gas flow rate due to con-sumption in reaction is considered negligible. Similarly/ the increase in flow rate due to hydrogen evolution is also neglected. 2) The physical properties (viz. density, vis-cosity, diffusivity) of the liquid and the gas do not vary appreciably as a result of reaction. 3) Since the size of the bed particles is small, they are assumed to be spherical. 4) The particles are completely wetted so that the effective surface area for mass transfer is approximately the same as the specific surface area of the bed [3^-36]. 5) The thickness of the liquid film surrounding the particles is small so that the surface area for mass transfer from gas to liquid is nearly the same as that from liquid to solid [34, 36 ], 153 HYDRODYNAMICS In terms of absolute and relative phase velocities, cocurrent flow used in our system is not limited by flood-ing phenomena and offers a great range of hydrodynamic patterns that must be determined before any mass transfer results can be presented. Flow patterns and the transitions from one form to another as a function of the phase flow rates have been described and represented diagrammatically by several authors [34]. Unfortunately, most of these representa-tions are confined to water-air system. In absence of other reliable information, the diagrammatical representa-tion of Charpentier and Favier [37], developed for various different gases and liquids, is found to be the most re-levant for the present case. This diagram is reproduced in Figure A:'. According to this diagram, for a l l combi-3 -1 nations of gas flow (94-1117 cm min ) and liquid flow 3 -1 (2.75-31.0 cm min ), the flow pattern in our reactor f a l l s in the trickling flow regime (shown by hatched re-gion). For some flowrates however, this f a l l s just within rlie trickling flow near the boundary line for pulsing flow; this is reflected in the energy dissipation calcu-lations. The correlations which are developed specifi-cally for trickling flow regime are used as far as possible in the following treatment. 154 Flow Pattern Diagram for Nonfooming Liquids. PC PL I 0 5 <>w.it T PL (Pwat \ 2 I + = | _ f \ Pwat Pair J «7. L Mwat * pL / Figure A FLOW PATTERN DIAGRAM FOR COCURRENT  GAS-LIQUID FLOW ON PACKED BEDS. Source: Ref. 37 The hatched region shows the regime i n which the flow p a t t e r n i n our rea c t o r f a l l s f o r a l l combinations of l i q u i d flow (2.75-31.0 cm 3 m i n - l ) and gas flow (94-1117 cm^ min - 1-) used. 155 Mass transfer properties in cocurrent flow are related to gas and l i q u i d energy d i s s i p a t i o n . Hence the c o r r e l a t i o n for pressure loss and the l i q u i d holdup are usually necessary, and are dealt with f i r s t . Pressure Loss, AP a) Single Phase Pressure Losses AP^, AP^ These are the pressure losses that would exist i f the l i q u i d and the gas were assumed to flow by them-selves at the same rates as those i n two-phase flows. The values of AP^ and AP^ are calculated by using the Ergun equation modified for the case of low depth rectangular bed. Because of low Reynolds number en-countered in t r i c k l i n g flow, Ergun's c o r r e l a t i o n i s expected to give good r e s u l t s . The wall ef f e c t i s expected to be appreciable because of large r a t i o (0.1-0.6) of p a r t i c l e to bed depth. In the development of the Ergun equation [38], the hydraulic radius (R^) i s introduced as a c h a r a c t e r i s t i c length of the packed bed. This would now be modified to account for the rectangular surrounding wall: 156 Volume of voids Volume of bed R = : _ (1) Wetted surface of packing + Wetted surface of wall Volume of bed Volume of bed or R= S (2) 6(1 - e') + 2(t + w) d p tw i.e., Rh= £' dP ( 3 ) 6(1 - e') M where (t + w) d M = 1 + 3 tw aJ) C4) t = depth of bed (in the direction of current), m w = width of bed, m e'= corrected bed voidage (to be defined later). I f the development used f o r Ergun equation i s employed using the h y d r a u l i c radius as i n equation ( 3 ) , an equation of the form of Ergun r e s u l t s w i t h d of Ergun equation replaced by d^: ' 2 Ap d 3 f = M -P- n = 150 + 1.75 Re (5) Re -= d^ u p / y ( W l (6) where d' = modified particle diameter = JL ; M is defined by Equation (4) . M 1 5 7 The. voidage e v used i n the equations (2) T(6) i s the a c t u a l bed voidage e (Table S ' l c o r r e c t e d f o r w a l l e f f e c t . In absence of any i n f o r m a t i o n on voidage c o r r e c t i o n f o r a bed p l a c e d a g a i n s t f l a t w a l l , the f o l l o w i n g c o n s e r v a t i v e estimate i s made on the b a s i s of the e m p i r i c a l c o r r e l a t i o n proposed i n [39] , f o r voidage c o r r e c t i o n i n c y l i n d r i c a l columns: Diameter (D ) of a c y l i n d r i c a l column having the same perimeter as the r e c t a n g u l a r w a l l = = 2(t + w) ( y ) For a c y l i n d r i c a l column having diameter D c, Equation (2) of [39] g i v e s e L = D^  + e o ^ c where = random loose p o r o s i t y e = e m p i r i c a l constant i n equation (8); s i g n i f i e s e at i n f i n i t e column diameter L C = e m p i r i c a l constant i n equation (8), m For a given p a r t i c l e s i z e , the v a l u e s of C and e Q are taken from Table II of [39]. The c o r r e c t e d voidage i s then d e f i n e d as e ' = — x e (9) e o 158 Equations (4)-(.9) permit c a l c u l a t i o n of AP L and AP G b) Two phase pressure l o s s , A P ^ The c o r r e l a t i o n of Sato et al [40], developed f o r t r i c k l i n g flow regime, i s used to c a l c u l a t e the two phase pressure l o s s from the s i n g l e phase pressure l o s s e s : <)• = 1.30 + 1.85X.' 0 , 8 5 (10) f o r 0.1 <x< 20 where • i = / A P L G / A P L C 1 1 ) X = /AP L/AP G C 1 2 ) L i q u i d Holdup, B The c o r r e l a t i o n of Sato et al [40] i s found appropriate f o r the c a l c u l a t i o n of l i q u i d holdup: 3 = 0.04 a 0 " 3 3 x ° " 2 2 (13) fo r 0.1 < x < 20 where a = 6(l-e')/d^ (14) 1 5 9 MASS TRANSFER a] Gas-Liquid Mass Transfer Coefficients k^a, k^a The gas-phase mass transfer c o e f f i c e i n t (k Ga) and the liquid-phase mass transfer c o e f f i c i e n t (k^a) are affected by both the gas flow rate and the l i q u i d flow rate. The relationships proposed by Reiss E'^l] are used for the calculations of k^a and k^a. It i s claimed by some authors [ 3 ^ 1 that Reiss's r e l a t i o n -ships are concerned with pulse flow and spray flow; but since the energy d i s s i p a t i o n values for l i q u i d flow and gas flow obtained in the present case are mostly above the range for which t r i c k l i n g flow re-lations are applicable, and are better f i t t e d by Reiss's c o r r e l a t i o n [ 3 ^ 1 , the l a t t e r i s considered appropriate. This apparent anomaly i s inexplicable in absence of s u f f i c i e n t information on gas-liquid mass transfer in t r i c k l i n g beds. A P T R E L - -r^- U L ( 1 5 ) k La = 0.0173 E ° - 5 (16) h " ~F- U G U71 k Qa = 2.0 + 0.069 E ° " 6 7 (18) where E^, energy d i s s i p a t i o n per unit,, . volume for l i q u i d and gas, .W m~3 160 tfL* ^G T' s u p e r f i c i a l v e l o c i t i e s of l i q u i d and gas, m s ~ l b) L i q u i d - t o - s o l i d Mass Transfer C o e f f i c i e n t , kga The relationship proposed by Sato [ 3 ^ ] is used for the c a l c u l a t i o n of kga. Lemay's relationship c i t e d i n the same, paper gives approximately the same value; however, generally recommended Goto and Smith cor-r e l a t i o n [ 3 5 ] tends to underpredict the values of kga by a factor of 3 due to low gas flow rates used i n their study. S a = 0.429 0.5 S r 0.33 a2„ 6 R e L S c L ^ a u L R e L = dp U L p L / y L C 1 - £ ' ^ ^ Sc L = V L / P L D L (20) D^, d i f f u s i v i t y of gas i n l i q u i d , i s calcu-lated by using Wilke and Cheng r e l a t i o n [ 4 2 ] . c) Overall Mass Transfer C o e f f i c i e n t , k Qa The o v e r a l l mass transfer c o e f f i c i e n t i s defined as k Q a k G a K^a MASS. TRANSFER LIMITED CURRENT DENSITY,. i L Mass transfer limited current density, or the limiting current density (Section 2.5) is calculated as follows: i T = 3 F k C (2.21) R where - Real limiting current density, i.e. R current density based on real surface area of packed cathode, A m~2 C - Solubility of NO in liquid under the conditions of experiment, Moi. m"3. Superficial limiting current density, i.e. the current density based on the area of current feeder plate can be readily calculated once the real current density is known: i T = i T a t (22) LS LR The solubility of NO in 101 H 2S0 4 at 760 mm., pressure and 18°C is 6.05 x 10 2 cm3/cm3 of solution [Appendix 4 ] . Since the solubility of NO in I^SO^ solutions is available only at 18°C P4.3-4 5] and also since the average temperature of catholyte in our system is around the same range, no attempt has been made to correct the solubility for temperature. The increase in solubility with pressure in the system is expected to be appreciable and the c o r r e c t i o n can be made by a p p l i c a t i o n of. Henry's law [45]: PNO = K XN0 C 2 3 ) where K = Henry's law constant, mm Hg N^O = P a r f i a l pressure of NO i n gas, mm Hg . t o t a l pressure, since the gas i s assumed to be pure NO XN0 = ^ o l e f r a c t i o n of NO i n s o l u t i o n . In absence of any information on the s p e c i f i c value of K f o r d i s s o l u t i o n of NO i n E^SO^ s o l u t i o n s , the f o l l o w i n g e m p i r i c a l r e l a t i o n suggested i n I n t e r -n a t i o n a l C r i t i c a l Tables [45] i s used: K - l " 3 2 4 0 0 d - • P N Q (24) where = volume of NO/volume of s o l u t i o n at 760 mm pressure 6.05 x 10 ~ 2d = Density of s o l u t i o n , g cm Mw= Molecular weight of s o l u t i o n . V 163 ' SUMMARY OF RESULTS With the help of p h y s i c a l p r o p e r t i e s given i n Appendix 4, the above equations are solved f o r d i f f e r e n t process flowrates under f o l l o w i n g c o n d i t i o n s : 1) Reactor dimensions as i n Table 2 2) Catholyte c o n c e n t r a t i o n 10% H^SO^ 3) No n i t r o g e n i n gas 4) Bed p a r t i c l e s i z e -.59 + .42 mm Results of c a l c u l a t i o n s are given i n Tables A and B. Results are discussed i n s e c t i o n 5.17. The l i q u i d and gas flow r a t e s s e l e c t e d are from those experimentally a p p l i e d [Appendix 2,Tables C and D]"so that a comparison can be made. The c a l c u l a t i o n s are repeated f o r a higher pressure (236 kPa) used [Appendix 2, Table I ]. In t h i s case, pressure l o s s i s not c a l c u l a t e d from g a s - l i q u i d flow r a t e s : pressure l o s s i n the re a c t o r i s s u b s t i t u t e d V'for A P ^ and c a l c u l a t i o n s of mass t r a n s f e r c o e f f i c i e n t s f o l l o w therefrom, as shown i n Tables A and B, Set 9. 164 Corrected p a r t i c l e diameter Cd^l and bed voidage Ce') : For p a r t i c l e s i z e - .59 + ,42 mm -3 Average p a r t i c l e diameter, d = 0.51.x 10 m From Table 5, bed voidage e =0.4 E m p i r i c a l constants f o r equation (8) from [ 3 9 1 C = 0.069 x 10"2 m e = 0.398 o Diameter of equivalent perimeter, equation (7) D = 3.342 x 10"2 m c From equation (8) , E l = 0.419 From equation (9), c o r r e c t e d bed voidage e' = 0.421 From equation (4) , w a l l c o r r e c t i o n f a c t o r M - = 1.123 Mo d i f i e d p a r t i c l e diameter dn -3 d' = J - = 0.452 x 10 m p IT From equation (14) s p e c i f i c surface area of bed a = 7686 m2/m3 Henry's law constant:(K): For the range of pressure used i n our system, the value of K (equation (24)) i s found to'be p r a c t i c a l l y independent of pressure and i t s v a l u e , c a l c u l a t e d from data of Appendix 4, i s common f o r the tabulated r e s u l t s : K = 1.1511 x 10 7 mm Hg Tabl e A HYDRODYNAMICS CALCULATIONS ScL(Eq'n 2 0 ) = 8 4 2 . 6 Set No Liquid Flowrate Gas Flowrate u L x - 1 0 2 - - i r\ 2 ReL ReG A PL APG APLG • U Q x 1 0 - -Eq'n(6) Eq'n ( 6 ) Eq'n ( 5 ) Eq'n(5) Eq'n ( 1 0 ) Eq'n ( 1 3 ) 3 . - 1 cm mm 3 . - 1 cm mm - 1 m s - 1 m s kPa kPa kPa -1 1 6 . 4 2 2 1 . 5 0 . 2 1 8 7 2 . 9 5 3 1 . 4 8 1 . 5 2 3 . 3 8 5 0 . 6 9 5 1 7 . 0 4 0 . 9 1 2 1 6 . 4 4 5 0 . 0 0 . 2 1 8 7 6 . 0 1 . 4 8 3 . 0 9 3 . 3 8 5 1 . 4 3 8 2 2 . 6 4 0 . 8 4 3 1 6 . 4 6 8 6 . 0 0 . 2 1 8 7 9 . 1 4 7 1 . 4 8 4 . 7 0 3 . 3 8 5 2 . 2 3 2 2 7 . 4 9 0 . 8 0 4 1 6 . 4 ' 9 0 4 . 0 0 . 2 1 8 7 1 2 . 0 5 3 1 . 4 8 6 . 2 3 . 3 8 5 2 . 9 8 9 3 1 . 5 9 0 . 7 8 5 1 6 . 4 1 1 1 7 . 0 0 . 2 1 8 7 1 4 . 8 9 3 1 . 4 8 7 . 6 6 3 . 3 8 5 3 . 7 5 3 3 5 . 3 8 0 . 7 6 6 7 . 7 5 2 2 1 . 5 0 . 1 0 3 3 2 . 9 5 3 0 . 7 1 . 5 2 1 . 5 8 4 0 . 6 9 5 1 0 . 7 3 0 . 8 4 7 2 3 . 7 5 2 2 1 . 5 0 . 3 1 6 7 2 . 9 5 3 2 . 1 4 1 . 5 2 4 . 9 3 9 0 v 6 9 5 2 1 . 8 7 0 . 9 5 8 3 1 . 0 2 2 1 . 5 0 . 4 1 3 3 2 . 9 5 3 2 . 7 9 1 . 5 2 6 . 4 9 3 0 . 6 9 5 2 6 . 3 8 0 . 9 8 9 1 6 . 4 2 2 1 . 5 - - -- ~ 1 3 4 . 6 1 Table B MASS TRANSFER CALCULATIONS Set Liquid Flowrate Gas E L EG k La V k ga k a o C s (corrected) Flow-rate Eq'n (15) Eq'n (17) Eq'n (16) Eq*n(18) Eq'n (19) Eq'n(21) Eq'n (23) Eq'n (2,21} Eq'n(2Z) NO 3 . -1 cm min 3 - -1 cm mm W T T T 3 W m~3 -1 s -1 s -1 s -1 s -3 Moi. m Am"2 . -2 A m 1 16.4 221.5 99.4 1342 0.1725 10.4 0.429 0.1216 3.154 14.44 277.6 2 16.4 450.0 132.0 3622 0.1988 18.7 0.465 0.1382 3.303 17.19 330.3 3 16.4 686.0 160.3 6705 0.2190 27.3 0.488 0.1503 3.432 19.43 373.3 4 16.4 904.0 184.2 10153 0.2348 35.4 0.501 0.1591 3.541 21.22 407.8 5 16.4 1117.0 206.3 14051 0.2485 43.5 0.514 0.1669 3.642 22.9 440.0 6 7.75 221.5 29.6 845 0.094 8.3 0.319 0.072 2.986 8.10 155.6 7 23.75 221.5 184.7 1722 0.2351 12.2 0.494 0.1572 3.282 19.43 373.4 8 31.0 221.5 290.7 2077 0.295 13.5 0.547 0.189 3.403 24.22 465.4 9 16.4 221.5 785.0 10599 0.4847 36.3 0.429 0.2261 6.287 53.53 1028.7 r~- rrr+ ON APPENDIX L\ . RELEVANT PHYSICAL DATA 168 Source; 17, 42, 47 The data given below are a l l at 20-25°C: Density of 101 H 2S0 4 1.064 x 10 3 Kg. n r 3 Density of NO (gas) 1.232 -kg-m"'3 Specific gravity of NO (air=l) 1.0366 Specific gravity of N 2 (air=l) 0.9674 Viscosity of 101 H 2S0 4 1.23 x 10"3 PI Viscosity of NO (gas) 0.0187 x 10"3 PI -3 Viscosity of water 1.002 x 10 PI Molar volume of NO at normal 5 3 boiling point 2.36 x 10 rn' (moi.) Diffusivity of NO into 101 H2SO4 (using Wilke-Cheng re- -9 2 - 1 rationship [42]) 1.372 x 10" m s -2 Surface tension of 10% H 2 S0 4 7.266 N m -2 Surface tension of water 7.197 N m Molecular weight of 10% H 2 S0 4 26.02 169 WATER 10% H 2 S0 4 (interpolated) 2 0 % H 2 S 0 4 30 % H 2 S 0 4 •5 5 15 TEMPERATURE (°C) Figure A SOLUBILITY OF HYDROXYLAMINE SULPHATE IN  AQUEOUS SOLUTIONS OF SULPHURIC ACID Source: Ref. 32 25 170 20 40 60 80 100 % H 2 S 0 4 FIGURE E SOLUBILITY OF NITRIC OXIDE GAS IN AQUEOUS SOLUTIONS OF SULPHURIC ACID Source: Ref. 4 5 

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