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Electro synthesis of propylene oxide in a bipolar trickle bed reactor Manji, Aminmohamed 1985

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E L E C T R O S Y N T H E S I S OF P R O P Y L E N E O X I D E I N A B I P O L A R T R I C K L E BED REACTOR By AMINMOHAMED MANJI B . A . S c , The U n i v e r s i t y o f B r i t i s h C o l u m b i a , 1981 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE i n THE FACULTY OF GRADUATE STUDIES . (The Depar tment of Chem i ca l E n g i n e e r i n g ) We a c c ep t t h i s t h e s i s as c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1985 ® Aminmohamed M a n j i , 1985 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f the requirements f o r an advanced degree a t the U n i v e r s i t y o f B r i t i s h Columbia, I agree t h a t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and study. I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e copying o f t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the head o f my department o r by h i s o r her r e p r e s e n t a t i v e s . I t i s understood t h a t copying or p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l gain s h a l l not be allowed without my w r i t t e n p e r m i s s i o n . Department of 6ifefrA.lCftL 1^)^ . The U n i v e r s i t y o f B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date *~2,\ i i -A B S T R A C T The synthesis of propylene oxide by e l e c t r o l y s i s of d i l u t e sodium bromide so lut ion with propylene gas was invest igated in an e l e c t r o -chemical b ipo lar reactor cons is t ing of six para l l e l f ixed beds of graphite p a r t i c l e s separated by polypropylene f e l t diaphragms. One reactor was operated in s ing le pass and recyc le mode with two phase co-current flow of propylene and sodium bromide so lut ion through the beds of graphite p a r t i c l e s . The maximum pressure in the system was 2.22 atmospheres abso lute . The e f f e c t s of s u p e r f i c i a l current density* (413 - 2687 A/m 2 ), e l e c t r o l y t e (sodium bromide so lu t ion) concentrat ion (0.2 M and 0.5 M), e l e c t r o l y t e flow rate (100 and 300 cm /min), propylene gas flow rate (100/1000/1500/2000 cm3/min at STP), reactor out le t temperature (30° and 60°C), bed thickness (s ix beds - 8.57/4.29/3.07 cm) and d i f f e r e n t carbon types (Union Carbide and U l t ra Carbon) on the space time y i e l d and s e l e c t i v i t y for propylene oxide were measured. Depending on the cond i t ion for s ing le pass runs, the space-time y i e l d for propylene oxide was between 5.5 kg/hr m 3 and 97.2 kg/hr m 3 , and the s e l e c t i v i t y was between 54.5% and 87.3%. The current e f f i c i e n c y and the s p e c i f i c energy consumption var ied from 14.0 to 58.2 percent and 5.7 to 59.9 Kwh/kg of propylene oxide. The space-time y i e l d for propylene oxide increased with decreasing bed th i ckness . For the recyc le mode the space-time •Super f i c i a l current density = appl ied current/electrode area . - i i i -y i e l d decreases with t ime. The highest space time y i e l d obtained a f te r one hour of operation was 127.8 kg/hr m 3 with a s e l e c t i v i t y of 95.9%. The current e f f i c i e n c y fo r hydrogen, oxygen and dibromopropane was also determined. - iv -T A B L E O F C O N T E N T S Page ABSTRACT i i TABLE OF CONTENTS iv LIST OF TABLES v i LIST OF FIGURES v i i ACKNOWLEDGEMENTS ix CHAPTER 1 - INTRODUCTION 1 CHAPTER 2 - BACKGROUND 8 2.1 Previous Work 8 2.2 E lectro Synthesis of Propylene Oxide 11 2.3 Electrochemical Ce l l 16 2.4 B ipolar Reactor 18 CHAPTER 3 - OBJECT 25 CHAPTER 4 - APPARATUS, METHODS AND ACCURACY 26 4.1 Appartus 26 4.2 Method 34 4.3 Accuracy 38 CHAPTER 5 - RESULTS AND DISCUSSIONS 42 5.0 General Considerat ions 42 5.1 Prel iminary Experiments 46 5.2 Fac tor ia l Experiments 47 5.2.1 Reactor Outlet Temperature 53 5.2.2 Current 57 5.2.3 Sodium Bromide Concentration 59 5.2.4 E l e c t r o l y t e Flow Rate 60. 5.2.5 Propylene Gas Flow Rate 62 - V -Page 5.3 pH E f fec t s 63 5.4 Further Invest igat ion 65 5.4.1 E f f ec t of Current and Propylene Gas Flow Rate 65 5.4.2 E f fec t of Bed Thickness 70 5.4.3 E f f e c t of D i f f e ren t Graphite Types 73 5.4.4 E l e c t r o l y t e Recycle 73 5.5 Commercial Potent ia l 77 CHAPTER 6 - CONCLUSIONS 82 CHAPTER 7 - RECOMMENDATIONS 84 REFERENCES 85 APPENDICES 1. Potent ia l Drop Through a Bed of Graphite at Various Currents 87 2. A n c i l l a r y Equipment 89 3. C a l i b r a t i o n Curves 92 4. Tabulated Experimental Resu l ts , Ana ly t i ca l Technique and Sample Ca lcu la t ions 98 5. Costing Exercise '. 121 6. Overpotential for Oxygen and Bromine Over Graphite 130 7. Source Table Result ing from Five Factor Analys is 133 8. Data Rep l i ca t ion 134 - vi -LIST OF TABLES Page Table 1 Dimension of an Electrochemical B ipolar Reactor 31 Table 2 Range of Experimental Var iab les 39 Table 3 Estimates of Experimental Accuracy 41 Table 4 L i s t of Indendent and Dependant Var iab les 48 Table 5 Fac to r ia l Experiment Results 49 Table 6 Fac to r ia l Experiment Results 50 Table 7 Fac to r i a l Experiment Results 51 Table 8 The S ign i f i cance of the E f fec t of Operating Var iab les on the Space Time Y i e l d , Current E f f i c i e n c y , S e l e c t i v i t y and S p e c i f i c Energy Consumption 52 Table 9 E f f e c t of Propylene Concentration on the Real L imi t ing Current Density for Propylene 55 Table 10 The E f f ec t of Temperature on Voltage Requirement and S p e c i f i c Energy Consumption for Propylene Oxide 56 Table 11 Current that goes into Producing Propylene Oxide, Dibromopropane and the Total Bromates 58 Table 12 The E f fec t of E l e c t r o l y t e Flow rate and Concentration on the Mass Transfer C o e f f i c i e n t and the Real L imit ing Current Density for the Bromide Ion 61 Table 13 The E f fec t of Propylene Gas Flow Rate on the Overal l Mass Transfer C o e f f i c i e n t for Propylene at Varying E l e c t r o l y t e Flow Rate 64 Table 14 The E f fec t of D i f fe rent Carbon Types on the Space Time Y ie ld for Propylene Oxide 74 Table 15 Experimental Condit ions Used for the Scale up of Propylene Oxide Reactors 79 Table 16 Fixed Capita l Investment Cost for Propylene Oxide Capacity of 10,000 Short Tons Per Year 80 Table 17 Operating Cost 81 - v i i -LIST OF FIGURES Page F igure 1 Combination of C h l o r - a l k a l i E l e c t r o l y s i s and Propylene Oxide Manufacture 5 Figure 2 A Simple Electrochemical Ce l l 17 Figure 3 Ver t i ca l Cross Section of a Bipolar Reactor 20 Figure 4 The E f fec t of the Degree of Bed Compression on the Potent ia l Drop at 10 Amperes 21 F igure 5 P lo t of E lectrode Potent ia l Versus Distance on a Matrix E lectrode Bed 23 Figure 6 Apparatus for the Synthesis of Propylene Oxide 27 Figure 7 Photographs of the Apparatus for the Synthesis of Propylene Oxide 28 F igure 8 Side E levat ion and Top E levat ion of a Bipolar Reactor 29 Figure 9 Dimensions of an Electrochemical Bipolar Reactor 32 Figure 10 Electrochemical Reactor Used for the Synthesis of Propylene Oxide 33 F igure 11 Scanning Electronmicrograph of Typical Union Carbide Graphite P a r t i c l e s . . 35 Figure 12 Scanning Electronmicrographs of Typica l U l t r a Carbon Graphite P a r t i c l e s 36 F igure 13 E f f e c t of Current and Gas Flow Rate on Space Time Y i e l d . . . . 66 Figure 14 E f fec t of Current and Gas Flow Rate on Energy Consumption 68 F igure 15 S e l e c t i v i t y and Current E f f i c i e n c y as a Function of Applied Current 69 Figure 16 E f fec t of Bed Thickness on Space Time Y i e l d . . . 71 Figure 17 Ef fect of Bed Thickness, Current and Propylene Gas Flow Rate on the Space Time Y ie ld for Propylene Oxide 72 - v i i i Page Figure 18 Propylene Oxide Space Time Y ie ld Var ia t i on During Recycle Condit ion 75 F igure 19 Proposed Process for the Manufacture of Propylene Oxide 78 ACKNOWLEDGEMENTS My s incere thanks to Professor Col in Oloman for his advice and help in th i s work. Also acknowledged are the s t a f f of the workshop and stores for the i r help and adv ice . F i n a l l y , I would l i k e to thank the National Science and Engineering Research Council of Canada which financed th is p ro jec t . - 1 -CHAPTER 1 INTRODUCTION Propylene oxide is an organic chemical used as a bu i ld ing block for other chemicals and indus t r i a l products. Some of the products manufactured from propylene oxide are: a) Propylene glycol which i s widely used as an ed ib le solvent for f l avours . b) Cosmetics, fumigants, paints and surfactants such as a lky l polyoxy propylene. c) In recent years large quant i t ies of propylene oxide have been consumed in the production of polyurethanes. The present l i s t pr ice for propylene oxide reported in the Chemical Marketing Reporter i s $1.05 per kilogram [ 1 ] . According to K.H. Shimmrock [2] the production capacity of propylene oxide in 1978 was estimated at approximately 1 to 1.1 m i l l i o n metr ic tons per year each in the United States and in Western Europe, and approximately 0.2 to 0.3 m i l l i o n metric tons per year in the rest of the world excluding the Eastern Block. Propylene oxide is produced e i the r by the hydroperoxide or the ch lorohydr in process. The former process uses an organic hydroperoxide to epoxidize propylene; an organic alcohol i s a coproduct. Oxirane, a d i v i s i o n of Arco Chemicals, has units at Bayport and Channelview, Texas. The chlorohydrin process involves the react ion of propylene with c h l o r i n e and water to produce propylene ch lo rohydr in , fol lowed by - 2 -dehydrochlor inat ion with lime or caust ic to give propylene oxide and a s a l t . Dow Chemicals Company has plants at Freeport , Texas and Plaquemine, Lou is iana . 1.1 Hydroperoxide Process Th is process i s based on the fo l lowing react ions RH + 0 2 •ROOH 0 M / \ ROOH + CH3CH=CH2 •CH 3 CH-CH 2 + ROH At present ethylbenzene and isobutane are being used i n d u s t r i a l l y as the s t a r t i n g mater ia l s ; thus i f isobutane is used, the oxidized product i s t e r t - b u t y l hydroperoxide: (CH 3 ) 3 CH + 0 2 • ( C H 3 ) 3 COOH Some t e r t - b u t y l alcohol i s a lso formed. The next step i s the epoxidat ion of propylene in the presence of a metal c a t a l y s t : 0 M A (CH 3) 3C00H + CH3CH=CH2 : • CH 3CH-CH 2 + (CH 3 ) 3 C0H Approximately 3 kgs. of tert -butanol i s produced per kilogram of propylene ox ide . With ethyl-benzene as the s ta r t ing m a t e r i a l , ethyl - 3 -benzyl alcohol i s the coproduct. The alcohol i s then dehydrated in the presence of ca ta lys t to produce styrene. Approximately 2.5 kgs of styrene is produced per kilogram of propylene oxide. M' C 6 H 5 C H 2 C H 2 0 H _ • C 6 h , 5 C H = C r l 2 + H 20 2. The Chlorohydrin Process The present commercial process i s based on mixing propylene and ch lo r ine in equal molar amounts with an excess of water to form a d i l u t e so lu t i on of propylene ch lorohydr in . CH 3 CH=CH 2 + C l 2 + H 20 •CH 3CH0HCH 2C1 + HC1 The chlorohydr in is then treated with a s l u r r y of calcium hydroxide to form propylene oxide and an e f f luent water stream of calcium ch lo r ide 0 2CH3CH0HCH2C1 + Ca(0H) 2 • 2CH 3CH-CH 2 + CaCl 2 + 2H 20 Some of the disadvantages of t h i s process are: a) the ch lor ine value is l os t as calcium ch lor ide (2.1 tons of calcium ch lor ide i s produced which .is contained in at least 43 tons of waste water per ton of propylene ox ide) . - 4 -b) calcium ch lor ide has a l im i ted market and therefore causes disposal problems. c) requires the handling of tox ic molecular c h l o r i n e . A v a r i a t i o n of the above process would be a combination of a c h l o r - a l k a l i e l e c t r o l y s i s with propylene oxide manufacturing p lant . A s i m p l i f i e d flow diagram for th i s process is shown in Figure (1) . This process i s present ly operated by Dow Chemical at Stade (Germany) producing 250,000 tons of propylene oxide per year . In t h i s process c h l o r i n e is produced by e l e c t r o l y s i s , simultaneously caust ic soda i s a lso produced. Therefore, i t would be reasonable from a technological point of view to replace the calcium hydroxide used in the dehydro-ch lor inat ion stage with caust ic soda so lut ion and to re-use the sodium ch lo r ide so lut ion produced. This would do away with the burden on waste water, amounting to approximately 43 tons of 5-6 percent calc ium ch lor ide so lut ion per ton of propylene oxide. A drawback of t h i s process res ides in the fact that br ine leaving the sapon i f i e r i s very d i l u t e . To maintain the water balance, approximately 37 tons of lean br ine have to be removed per ton of propylene oxide. A b r i e f desc r ip t ion of other processes for the manufacture of propylene oxide, i r r e s p e c t i v e of t h e i r f e a s i b i l i t y on a commercial sca le i s contained in reference [ 2 ] . A de ta i l ed account of the propert ies of propylene oxide i s given in reference [ 3 ] , In recent years considerable e f f o r t has gone into a search for a more economical route to propylene oxide to replace the present conventional processes. The hydroperoxide process produces large amounts of co-product such as tert-butanol that can be manufactured by - 5 -Hcl AUXILIARY AGENTS H 2 PROPYLENE H 2 0 SALT BRINE SATURATION AND TREATMENT ELECTROLYSIS Cl 2* BRINE TREATMENT CHL'OROHYORINATION SAPONIFICATION CHLORINATED COMPOUND CRUDE PROPYLENE OXIDE ELIMINATION OF OILUTE BRINE FIGURE 1: COMBINATION OF CHLOR-ALKALI ELECTROLYSIS AND PROPYLENE OXIDE MANUFACTURE - 6 -other economical routes and the chlorohydrin process produces large amounts of unwanted by-products such as calcium c h l o r i d e . One p o s s i b i l i t y i s an electrochemical route in which the propylene i s converted to propylene halohydrin by react ion with halogen generated in s i t u by anodic oxidat ion of a metal ha l ide s o l u t i o n . The propylene halohydrin i s converted to propylene oxide by react ion with the hydroxyl ion generated at the cathode. The general react ion scheme when sodium bromide is used as the e l e c t r o l y t e i s [ 4 ] : Anode: 2Br _ e l e c t r i c a l energy B r 2 + 2e" C 3 H 6 + H 20 + B r 2 *.C 3 H 6 Br0H + HRr Cathode: 2H20 + 2e- 20H- + H 2 C 3H 6BrOH + 0H- *~C 3 H 6 0 + H 20 + Br" Overal1: C 3 H 6 + H 20 e l e c t r i c a l energy * C 3 H 6 0 + H 2 In p r i n c i p l e , only water, propylene and e l e c t r i c a l energy are consumed in the formation of propylene oxide and hydrogen. The sodium bromide - 7 -so lu t ion is continuously oxid ized and regenerated within the c e l l for fur ther use, although losses of bromine may be caused by the formation of hypobromite, bromate, bromite, bromine gas and dibromopropane. The advantage of t h i s electrochemical route, which obviates the production of waste calcium ch lor ide encountered in the conventional chemical process, has long been recognized but attempts to implement i t by various people employing d i f f e r e n t reactor designs have not proved to be very e f f e c t i v e because of low space time y i e l d s for propylene ox ide , although the s e l e c t i v i t y in these reactors for propylene oxide is h igh. The object of th i s thes is is to invest igate the f e a s i b i l i t y of producing propylene oxide with high space time y i e l d and high s e l e c t i v i t y by e l e c t r o l y z i n g an aqueous so lut ion of sodium bromide while passing both the propylene gas and the aqueous so lu t ion through the e lectrodes of a b ipo lar f ixed bed reac tor . - 8 -CHAPTER 2 BACKGROUND 2.1 Previous Work Numerous experimental studies on the e lec t ro -synthes i s of organic compounds have been reported in the l i t e r a t u r e over the past several y e a r s . The bulk of t h i s material i s summarized in references [ 5 ] , [6] and [ 7 ] . The present d iscuss ion i s concerned only with previous work d i rec ted at the synthesis of propylene oxide by e l e c t r o l y s i s of a metal hal ide s o l u t i o n . Beck [8] studied the synthesis of propylene oxide in a c a p i l l a r y gap c e l l with distances between electrodes of up to 0.5 mm. In his experiments, propylene gas dispersed in a d i l u t e sodium bromide e l e c t r o l y t e i s supplied through a central hole in a p i l e of e lectrode d i s cs and flows r a d i a l l y outwards between the d i s c s . The gap between e lectrodes was made small to enable low bromide concentrat ions to be handled with low ohmic l o s s e s . A current e f f i c i e n c y of 73 percent for propylene oxide was reported at a current density of 0.1 kA/m2 with the sodium bromide concentrat ion of 0.5 weight percent and an e lectrode gap of 0.5 mm. The c e l l voltage of 3.0 vo l t s corresponded to an energy consumption of 3.8 Kwh/kg of propylene oxide. The maximum space time y i e l d reported based on e lectrode gap is 162.2 kg/hr m . The e lect rode th ickness var ies from 3 to 30 mm. In 1977 Ghoroghchian et a l . [9] employed an electrochemical pump c e l l , which is s im i la r to the c a p i l l a r y gap ce l l used by Beck, to - 9 -synthesize propylene oxide. As in the c a p i l l a r y gap c e l l , the sodium bromide so lut ion and the propylene gas are fed through a central hole and flows r a d i c a l l y outwards in the gap between the rotat ing e lec t rodes . The rotat ional motion increases turbulence and thus helps to re -es tab l i sh saturat ion as propylene i s consumed. Production rate for propylene oxide of up to 0.661 kg/hr m 3 based on tota l pump volume was obtained at an e lectrode rotat iona l speed of 3000 rev/min and the e l e c t r o l y t e temperature of 15°C. The current e f f i c i e n c y was high and reached a maximum (100%) at an e lectrode gap of 0.25 mm, which corresponds to an energy consumption of 2.69 Kwhr/kg. This c e l l i s not e a s i l y scaled up for indus t r i a l production of propylene oxide. A b ipo lar flow ce l l was used by King et a l . [10] to study the production of propylene oxide. The c e l l consisted of v e r t i c a l rows of e l e c t r i c a l l y conductive rods, separated from one another by a small gap. The e l e c t r o l y t e was fed to the top rods , flowed downwards over the v e r t i c a l rows and was co l l ec ted from the bottom rods for r e c i r c u l a t i o n . The gaseous reactant , propylene, was passed up the the space between the v e r t i c a l rows, in continuous contact with the e l e c t r o l y t e f i l m . - The c e l l was operated at atmospheric pressure. The current e f f i c i e n c y for propylene oxide in th i s c e l l was between 35-76 percent and the energy consumption was estimated in the range of 3-9 Kwh/kg. The temperature var ied from 5-32°C. The general conclusions were that by increasing the temperature and the current at uncontrol led pH the current e f f i c i e n c y for propylene oxide decreases. Boussoulengas et a l . [11] have studied the synthesis of propylene oxide in a b ipo lar t r i c k l e tower. The c e l l consisted of a tower packed - 10 -with several layers of carbon raschig r ings each separated by a p l a s t i c screen. A mixture of sodium bromide so lut ion and propylene gas t r i c k l e s down over the packing as a th in f i l m . When a potent ia l i s appl ied across the tower, b i p o l a r i t y i s induced in each element of packing. Current e f f i c i e n c y for propylene oxide of up to 97.5 percent was reported at a conversion of propylene of 74.8 percent with 39 layers of Raschig r ings and an e l e c t r o l y t e temperature of below 10°C. This corresponds to a space time y i e l d of 6.48 kg/hr m 3 and an energy consumption of 2.92 Kwh/kg. One of the l i m i t a t i o n s of t h i s process i s that the tower is suscept ib le to f looding and th i s d r a s t i c a l l y reduces e l e c t r i c a l e f f i c i e n c y . In the synthesis of propylene oxide, one of the major l i m i t a t i o n s i s the r e l a t i v e low s o l u b i l i t y of propylene in the metal ha l ide s o l u t i o n . To overcome t h i s problem, the system can be operated at e levated pressures. Bejerano et a l . [12] studied the electrochemical formation of propylene oxide in a ce l l containing a set of b ipo la r graphite rods, using an aqueous sodium bromide s o l u t i o n , at pressures of up to 5 atmospheres. Their general conclusions were that by increas ing the e l e c t r o l y t e flow and the pressure, the current e f f i c i e n c y fo r propylene oxide increases , but by increas ing the appl ied voltage and the e l e c t r o l y t e concentrat ion the current e f f i c i e n c y for propylene oxide decreases. At a pressure of 5 atm., with an e l e c t r o l y t e flow rate of 1.3 x 1 0 _ l t m3/s and a temperature of 30°C the space time y i e l d was 156.6 3 kg/hr m with a corresponding energy consumption of 9.14 Kwh/kg. Fleischmann et a l . [13] have studied the synthesis of propylene oxide using a b ipo lar packed bed c e l l . The c e l l consisted of a packed - 11 -bed made up of a mixture of conducting and non-conducting particles. The conducting particles become bipolar by using dilute electrolyte in the cell and applying sufficient voltage gradient between the contact electrodes so as to overcome the resistance drop in the electrolyte. Glass particles coated with graphite were used as the conducting medium, and the non-coated particles as the non-conducting medium, all particles having a diameter of 0.5 mm. The energy consumption of such a cell was found to be high, in the range of 43-52 Kwh/kg of propylene oxide. Robertson et a l . [14] designed an electrolysis cell consisting of a rolled up electrode and separators known as the Swiss Roll cell and studied the synthesis of propylene oxide in this c e l l . Specifically, with an anode area of 0.16 m , a current efficiency for propylene oxide of 81 percent was obtained with a power consumption of 2.83 Kwhr/kg. Recently, Alkire et a l . [15] have investigated the fe a s i b i l i t y of synthesizing propylene oxide by alternating current electrolysis in a batch monopolar c e l l . They found that the current efficiency for propylene oxide was independent of sodium bromide concentration but dependent upon the current frequency. Specifically, the current efficiency was 22 percent at a frequency of 1 Hz and a current density of 1750 A/m2. The current efficiency decreased to 2 percent at 40 Hz. None of the above reactor designs have been applied to the commercial production of propylene oxide. 2.2 Electro Synthesis of Propylene Oxide In the electrochemical production of propylene oxide, propylene and a metal halide electrolyte are introduced in the vicinity of the - 12 -anode. The e l e c t r o l y t e is oxidized e lec t rochemica l ly at the anode to generate f ree halogen which reacts with the propylene in the presence o f water to form the halohydrin de r i va t i ve of propylene. Simultaneously, water i s reduced at the cathode forming hydroxyl ions and hydrogen gas. The hydroxyl ions produced react with the halohydrin of propylene to form propylene oxide within the c e l l . The react ion sequence i s descr ibed in the fol lowing review. I n i t i a l l y , bromide ions are discharged at the anode. The sto ich iometry of th i s react ion is [16]: E l e c t r i c a l B r 2 + 2e" * ^.2Br~ E Q = +1.08 vo l ts (1) Energy S i m i l a r l y at the cathode, hydrogen and hydroxyl ions are produced. The stoichiometry is [16] : E l e c t r i c a l 2H 20 + 2e~ ' ^20H- + H 2 E = -0.828 (2) Energy The bromine generated hydrolyzes i n the v i c i n i t y of the anode forming hypobromous acid as fo l lows: B r 2 + H 20 *" ^ HOBr + H + + Br" (3) - 13 -with the equi l ibr ium constant for th is react ion [19] , K = 9.6 x 10" where x [H0Br][H+][Br-] [WD The hypobromite formed in Step (3) from the electrogenerated bromine reacts with propylene to form propylene bromohydrin CH3CH=CH2 + HOBr • CH3CH0HCH2Br (4) The propylene bromohydrin reacts with the hydroxyl ion generated at the cathode to produce the desired propylene oxide OH Br 0 1 1 A CH3CH CH 2 + OH" — C H 3 C H - C H 2 + H 20 + Br" (5) The overal l reaction i s E l e c t r i c a l Energy CH3CH=CH2 + H 20 ^CH 3 CH-CH 2 + H 2 (6) The desired ce l l reaction u t i l i z e s intermediates from both the anode and the cathode. Since the halogen species does not appear in the overa l l r e a c t i o n , other halogens can be used, however iodine reaction with - 14 -propylene is slower than bromine [17] , a lso the formation of propylene oxide from bromohydrin is more rapid than from chlorohydrin [ 1 8 ] , There are a var ie ty of poss ib le loss react ions suggesting that the overa l l performance of the c e l l depends on the balance between e lec t rochemica l , chemical and transport rate processes. The bromine formed can be hydrolyzed to form hypobromous acid HOBr • H + + OBr- (7) which can further be hydrolyzed to form anions that can d isproport ionate to form bromate 2H0Br + OBr" B r 0 3 " + 2Br" + 2H + (8) Bromates are also formed by anodic oxidat ion of the hypobromite 60Br- + 3H 20 * ^ 2 B r 0 3 - + 4Br" + 6H + + 3/2 0 2 + 6e~ (9) Both react ions 8 and 9 are s i g n i f i c a n t at high concentrat ion of hypobromite. Under good mixing c o n d i t i o n s , the t rans fe r of bromine to the cathode where i t may be reduced provides another poss ib le loss r e a c t i o n . Anode: 2Br" ^ B r 2 + 2 e " (oxidat ion) (10) Cathode: B r 2 + 2e~ 2Br" (reduct ion) (11) - 15 -Another undesirable react ion at the anode is the generation of oxygen. 2H 20 • 4H+ + 0 2 + 4e- (12) Side react ions invo lv ing propylene also occur since the carbonium intermediate i s ava i lab le for nucleophi les other than water. The major by-product i s dibromopropane Br Br I I CH3CH=CH2 + B r 2 • CH 2 CH CH 2 (13) The formation of propylene bromohydrin (PBH) and dibromopropane (DBP) proceed v ia the same c a t i o n i c intermediate PBH Br 0H-+ I , CH3CH CH 2 (14) DBP It i s obvious from Equation 14, that the achievement of a high s e l e c t i v i t y for propylene oxide would require low bromide concentrat ion and high hydroxyl ion concentrat ion . However, the pH value must not exceed an optimum value otherwise the sapon i f i ca t ion of the propylene oxide to the glycol would become predominant 0 OH OH A • I I CH 3CH-CH 2 + H 20 •CH 3 CH-CH 2 (15) - 16 -Reactions 8, 9, 11, 12, 13 and 15 are damaging to the current e f f i c i e n c y for propylene oxide. Extensive work has been done on the hydro lys is of bromine, the ana lys is and the formation of hypobromite and bromate [19 ,20] , The react ion of bromohydrin formation has been studied by Goguslavskaya [21] , Atkinson and Bell [22] and De La Mare [ 2 3 ] . 2.3 Electrochemical C e l l F igure 2 represents an idea l i zed electrochemical c e l l . It cons is t e s s e n t i a l l y of two e l e c t r i c a l l y conducting electrodes immersed in a bath of e l e c t r i c a l l y conducting l i q u i d known as the e l e c t r o l y t e . The general ized electrode react ion can be written as fo l lows: Anode: A ^ — ; y A+ + e" (16) Cathode: B + e - <^'">B- (17) The overa l l react ion i s A + B *===?A+ + B- (18) The e lectrodes are connected outside the bath to the terminal of a dc power supply. When an emf of a s u f f i c i e n t magnitude is a p p l i e d , e lectron t rans fer occurs between the e lectrode and the l i q u i d , r e s u l t i n g in a flow of e l e c t r i c i t y in the external c i r c u i t and chemical react ions at each e lec t rode . By convention, the current flow external to the c e l l - 17 -D. C POWER SUPPLY ANODE OXIDATION ELECTROLYTE CATHODE REDUCTION FIGURE 2: A SIMPLE ELECTROCHEMICAL CELL - 18 -i s f rom t h e anode t o t h e c a t h o d e . The anod i c r e a c t i o n i s c a l l e d t h e o x i d a t i o n r e a c t i o n , t h a t i s , t he l o s s o f one or more e l e c t r o n w i t h a c o r r e s p o n d i n g i n c r e a s e i n t h e o x i d a t i o n s t a t e o f t he s p e c i e s . The c a t h o d i c r e a c t i o n i s t h e o p p o s i t e - a r e d u c t i o n r e a c t i o n or a g a i n o f e l e c t r o n w i t h a d e c r e a s e i n t h e o x i d a t i o n s t a t e o f t h e r e a c t a n t . The ma jo r c h a r a c t e r i s t i c o f t h e c e l l i s t he p o t e n t i a l d i f f e r e n c e between the anode and the c a t h o d e . E l e c t r o d e p o t e n t i a l s a r e based on a r e f e r e n c e e l e c t r o d e s . The p r i m a r y r e f e r e n c e i s t he hydrogen e l e c t r o d e w i t h gaseous hydrogen a t a p r e s s u r e o f 1 a tmosphe r e . R e a c t a n t s more r e a d i l y o x i d i z a b l e than hydrogen have a n e g a t i v e p o t e n t i a l w i t h r e s p e c t t o i t ; t h o s e more r e a d i l y reduced a re p o s i t i v e w i t h r e s p e c t t o h y d r o g e n . The p o t e n t i a l s o f v a r i o u s e l e c t r o d e r e a c t i o n w i t h r e ga r d t o hyd rogen a r e l i s t e d i n r e f e r e n c e [ 1 6 ] , 2.4 Bipolar Reactor E l e c t r o c h e m i c a l r e a c t o r s emp l oy i ng s o l i d p l a t e b i p o l e s have been used i n t h e i n d u s t r y to produce compounds such as sod ium c h l o r a t e and a d i p o n i t r i l e . However , f o r c e r t a i n e l e c t r o c h e m i c a l p r o c e s s e s , t h e s e s o l i d b i p o l e r e a c t o r s a re i n a d e q u a t e . Such p r o c e s s e s i n c l u d e t h o s e i n wh i c h t h e r e a c t i v e s p e c i e s a r e i n r e l a t i v e l y low c o n c e n t r a t i o n o r i n t e r p h a s e mass t r a n s f e r i s s low and where i n t i m a t e c o n t a c t between anode and c a t hode p r odu c t i s n e c e s s a r y f o r p r o c e s s e f f i c i e n c y . A p a r t i c u l a r d i f f i c u l t y w i t h s o l i d p l a t e b i p o l e e l e c t r o d e i n such a p p l i c a t i o n s i s t h e i r r e l a t i v e l y low s p e c i f i c e l e c t r o d e a r e a , wh i ch - 19 -a l l o w s o n l y low space t ime y i e l d s and g i v e s h i g h c a p i t a l c o s t f o r p r o c e s s p l a n t . S e v e r a l a u t h o r s ( [ 1 0 ] , [ 1 1 ] , and [ 1 3 ] ) have u t i l i z e d d i f f e r e n t b i p o l a r e l e c t r o d e r e a c t o r c o n f i g u r a t i o n s t o overcome the prob lem men t i oned above , but t h e i r r e a c t o r s s u f f e r f rom l i m i t a t i o n s such as e x c e s s i v e v o l t a g e , h i g h energy c o n s u m p t i o n , low space t ime y i e l d s and d i f f i c u l t i e s w i t h s c a l i n g up t o i n d u s t r i a l c a p a c i t i e s . In t h i s work , a b i p o l a r e l e c t r o c h e m i c a l r e a c t o r c o n s i s t i n g o f po rous c a r bon m a t r i c e s and hav i ng a c o c u r r e n t f l o w o f t h e e l e c t r o l y t e and a gas was emp loyed . A s c h e m a t i c d i ag ram o f a v e r t i c a l c r o s s s e c t i o n o f t h e e l e c t r o c h e m i c a l b i p o l a r r e a c t o r used f o r the s y n t h e s i s o f p r o p y l e n e o x i d e i s shown i n F i g u r e 3 . The r e a c t o r i s c ompr i s ed o f s i x e l e c t r i c a l l y c o n d u c t i n g m a t r i x e l e c t r o d e beds o f g r a p h i t e p a r t i c l e s sandw i ched between two monopole e l e c t r o d e s . The m a t r i x beds a d j a c e n t t o t h e monopo le e l e c t r o d e s a r e i n e l e c t r i c a l c o n t a c t w i t h t hem. Each monopole e l e c t r o d e has an i n l e t and an o u t l e t open ing t h r ough wh i ch t h e f e ed e n t e r s and the p r odu c t l e a v e s t h e r e a c t o r . The e n t i r e a s semb ly i s compressed so t h a t each m a t r i x e l e c t r o d e bed a c t s as a s i n g u l a r ' e l e c t r i c a l l y c o n d u c t i n g body wh i ch has a h i g h s u r f a c e a rea and can a l l o w a l i q u i d and a gas to pass t h r o u g h i t . Compress i on o f g r a p h i t e p a r t i c l e s i n t h e m a t r i x e l e c t r o d e beds r e s u l t s i n h i g h e r c o n d u c t i v i t i e s t han when t h e p a r t i c l e s a re l o o s e , as i l l u s t r a t e d by F i g u r e 4 and Append i x 1. The m a t r i x e l e c t r o d e beds a re s e p a r a t e d f rom each o t h e r by means o f an i o n c o n d u c t i n g d iaphragm wh i ch was t r e a t e d w i t h a w e t t i n g agent so - 20 -LECTRODE FIGURE 3: VERTICAL CROSS SECTION OF A BIPOLAR REACTOR - 21 -THE EFFECT OF THE DEGREE OF BED COMPRESSION ON THE POTENTIAL DROP AT 10 AMPERES FIGURE 4 Condit ions Graphite P a r t i c l e S ize = 1.168 - 1.68 mm Cross Section Area = 19.05 x 2.54 cm 2 - 22 -that i t will retain electrolyte over gas. The holes are cut in the diaphragm to provide an inlet and an outlet passageway at the bottom and the top ends of each of the matrix electrode beds. The diaphragm was held in place by means of insulating (four Durabla and one neoprene rubber) gaskets which also form the side walls of the reactor (Figure 3). The neoprene rubber provides the resiliency necessary to sufficiently compress the matrix electrode beds to the degree needed for higher matrix electrode bed conductivities. When a potential is applied across the monopole electrodes and a mixture of appropriate conductivity passed between the monopole electrodes through the matrix electrode beds, each of the matrix electrode beds between the monopoles will act as a bipole electrode. Figure 5 represents a plot of electrode potential versus distance across the bipole matrix electrode bed. For a given electrochemical reaction, because of the variation of potential over distance, the matrix electrode bed has three distinct areas: the anode region, the cathode region and the unreactive region between the anode and the cathode region. In the unreactive region, the electrode potential is insufficient to drive either the anode or the cathode reaction. The advantages of this reactor are: a) The electrode matrix bed has high specific electrode area which gives high transfer rates for the bromide ion to the anode surface and can sustain high superficial current densities for dilute reactant species. - 23 -PLOT OF ELECTRODE POTENTIAL VERSUS DISTANCE ON A MATRIX E L E C T R O D E 0 © DISTANCE FIGURE 5 - 24 -I t i n c r e a s e s t h e p o s s i b i l i t y o f i n t i m a t e m i x i n g between the anode and t he ca thode p r odu c t s w i t h i n each s e p a r a t e b i p o l e m a t r i x . In the ca se o f t he p r o d u c t i o n o f p r o p y l e n e o x i d e t h i s i s i m p o r t a n t because bo th t he anode and t he ca thode p r oduc t t a k e p a r t i n t h e r e a c t i o n . T h i s r e a c t o r g i v e s h i g h gas l i q u i d i n t e r f a c i a l a r ea and mass t r a n s f e r r a t e s f o r p r o p y l e n e i n t o t h e e l e c t r o l y t e s o l u t i o n and a l s o d e c r e a s e s t h e e f f e c t i v e e l e c t r o l y t e c o n d u c t i v i t y wh i ch p e r m i t s t h e use o f t h i n e l e c t r o d e beds f o r h i gh space t i m e y i e l d s . - 25 -CHAPTER 3 OBJECT The object of this work is to experimentally investigate the possibility of producing propylene oxide with high space time yield and high selectivity in an electrochemical bipolar trickle bed reactor. The effect of the following variables are to be studied on the space time yield and the selectivity of propylene oxide: 1. Superficial current density 2. Sodium bromide electrolyte concentration 3. Electrolyte flow rate 4. Propylene gas flow rate 5. Reactor outlet temperature 6. Matrix electrode bed thickness 7. Different graphite types (Union Carbide and Ultra Carbon). The work involves an i n i t i a l experimental assessment of the effect of the f i r s t five variables mentioned above on the space time yield for propylene oxide. Further exploration of the variables that have the greatest positive effect* on the space time yield for propylene oxide were then investigated. •Positive effect means by increasing the independent variable the dependent variable also increases. - 26 -CHAPTER 4 APPARATUS, HETHOD AND ACCURACY 4.0 One e l e c t r o c h e m i c a l r e a c t o r was used i n t h i s wo rk . I t was a l a b o r a t o r y u n i t made to o p e r a t e i n a c o n t i n u o u s manner w i t h t h e sod ium b rom ide e l e c t r o l y t e f eed r a t e r a n g i n g from 100 t o 300 cm 3 per m inu t e and the p r o p y l e n e gas f eed r a t e r a n g i n g from 100 t o 2000 c m 3 pe r m i n u t e a t s t a n d a r d t e m p e r a t u r e and p r e s s u r e . T h i s r e a c t o r was b u i l t t o o p e r a t e a t a t m o s p h e r i c p r e s s u r e . The r e a c t o r a l ong w i t h the a n c i l l a r y equ ipment w i l l be d e s c r i b e d s e p a r a t e l y . 4.1 Apparatus F i g u r e 6 r e p r e s e n t s a l i n e d i ag r am o f t h e equ ipment used t o s t u d y the s y n t h e s i s o f p r o p y l e n e o x i d e in a b i p o l a r f i x e d bed r e a c t o r , and F i g u r e 7 shows pho tog raphs o f t h i s a p p a r a t u s . The r e a c t o r employs two monopo l a r e l e c t r o d e s c o n s t r u c t e d f rom p l a t i n i z e d t i t a n i u m 1/8 o f an i n c h t h i c k and i s cha rged w i t h g r a p h i t e p a r t i c l e s r a n g i n g i n s i z e f rom 1.168 t o 1.68 mm. The r e a c t o r i s f ed from t he bot tom w i t h sod ium b romide e l e c t r o l y t e and p r o p y l e n e gas f l o w i n g c o c u r r e n t l y t h r o u g h t h e m a t r i x e l e c t r o d e beds wh ich a re s e p a r a t e d by p o l y p r o p y l e n e f e l t d i a p h r a g m s . F i g u r e 8 shows t he s i d e e l e v a t i o n and t op e l e v a t i o n o f a b i p o l a r r e a c t o r . The e l e c t r o l y t e i s pumped from a 40 l i t r e t a n k . The e l e c t r o l y t e l e a v i n g the f eed tank d i v i d e s i n t o two s t r e a m s ; one i s r e c y c l e d back t o t he f eed t ank t h r o u g h a c o n s t a n t t e m p e r a t u r e ba th w h i l e P R O P -Y L E N E G A S W E T G A S M E T E R D. C . P O W E R S U P P L Y E L E C T R O L Y T E F E E D T A N K KSLSi) C I R C U L A T O F : — W A T E R -B I P O L A R R E A C T O R A B S O R P T I O N T O W E R £2* £2> S E P A R A T O R B ro FIGURE 6: APPARATUS FOR THE SYNTHESIS OF PROPYLENE OXIDE • . . . - 28 -FIGURE 7: PHOTOGRAPHS OF THE APPARATUS FOR THE SYNTHESIS OF PROPYLENE OXIDE - 29 -O U T L E T PORT -g- N P T I. COMPRESSION P L A T E 2,STAINLESS S T E E L P L A T E 3.INSULATING G A S K E T ( 0 - 3 2 cm) 4. E L E C T R 0 D E 5. G A S K E T ( 0 - 3 2 cm) 6. DIAPHRAGM (0-32 cm) I •g cm = I cm INLET PORT -g NPT FIGURE 8: SIDE ELEVATION AND TOP ELEVATION OF A BIPOLAR REACTOR - 30 -t h e o t h e r i s f ed t o t he b i p o l a r r e a c t o r . The p r o p y l e n e gas i s d e l i v e r e d f rom a p r o p y l e n e gas c y l i n d e r a t a l i n e p r e s s u r e of 22 .7 pounds per s qua r e i n c h a b s o l u t e . The gas m ixes w i t h t he e l e c t r o l y t e b e f o r e e n t e r i n g t h e r e a c t o r near t he anode . The f l o w s o f the sodium bromide e l e c t r o l y t e and the p r o p y l e n e gas t o the r e a c t o r a r e measured by r o t a m e t e r s and c o n t r o l l e d m a n u a l l y by n eed l e v a l v e s . The m i x t u r e l e a v i n g t he r e a c t o r can be r e c y c l e d i n t o t h e f eed tank o r passed t h r o u g h a s i n g l e pass heat exchanger and i n t o t h e s e p a r a t o r . The l i q u i d i s d i s p o s e d o f t o t he d r a i n from t h e bot tom o f t h e s e p a r a t o r and the gases wh i ch l e a v e from the top are passed t h r o u g h a s i n g l e pass c o n d e n s e r . The c o n d e n s a t e , i s c o l l e c t e d and a n a l y s e d . The uncondensed gases e n t e r t he a b s o r p t i o n tower a t t h e bot tom and wa te r i s fed from t he top o f t h e t o w e r . The l i q u i d l e a v i n g the bot tom o f t h e tower i s sampled b e f o r e b e i n g d i s p o s e d o f to t h e d r a i n . F low r a t e o f gases l e a v i n g the top o f t h e t owe r i s measured and a sample o b t a i n e d f o r a n a l y s i s . Power f o r t he r e a c t o r i s s u p p l i e d by two d i r e c t c u r r e n t power s u p p l y c onnec t ed i n s e r i e s w i t h a maximum power ou t pu t o f 2.25 kW and c a p a b l e o f e i t h e r c u r r e n t o r v o l t a g e c o n t r o l up t o 30 ampere o r 75 v o l t s . A l l p a r t s i n c o n t a c t w i t h the e l e c t r o l y t e a re made o f e i t h e r s t a i n l e s s s t e e l , n y l o n , g l a s s , p o l y e t h y l e n e o r p l e x i g l a s s . Some c o r r o s i o n was obse rved around the s t a i n l e s s s t e e l p a r t s . The s p e c i f i c a t i o n s o f t he p a r t s f o r t h i s a p p a r a t u s a r e g i v e n i n Append i x 2 . Some o f t h e i m p o r t a n t d i m e n s i o n s o f t h e r e a c t o r a re g i v e n i n T a b l e 1 and F i g u r e 9 . Pho t og r aphs o f t h e r e a c t o r a r e shown i n F i g u r e 1 0 . - 31 -T a b l e 1 D imens i on s o f an E l e c t r o c h e m i c a l B i p o l a r R e a c t o r Comp r e s s i o n p l a t e s 22 . 9 cm l o n g x 10 . 2 cm w ide ( i r o n ) Number o f ee l 1s 5 Monopo le e l e c t r o d e s 38 cm l ong x 5.1 cm w ide ( p l a t i n i z e d t i t a n i u m ) P a r t i c l e s Range 1.168 - 1.68 mm ( g r a p h i t e ) S u p e r f i c i a l a r ea per c e l l 48 . 4 cm 2 E l e c t r o d e bed t h i c k n e s s Range 3.1 - 8 .6 cm D iaphragm 22 .9 l o ng x 5.1 cm w ide ( P o l y p r o p y l e n e f e l t ) Type and we igh t o f p o l y p r o p y l e n e f e l t N a t i o n a l f e l t 510 g /m 2 E o o OJ OJ 5-1 cm • 2-5 cm* 6 u THICKNESS ELECTRODE *0-32 cm GASKET « 0-32 cm DIAPHRAGM »0-32 cm MONOPOLE ELECTRODE B) NEOPRENE RUBBER GASKET C) POLYPROPYLENE FELT DIAPHRAGM FIGURE 9.: DIMENSIONS OF THE ELECTRODE, GASKET AND DIAPHRAGM FIGURE 10: ELECTROCHEMICAL REACTOR USED FOR THE SYNTHESIS OF PROPYLENE OXIDE - 34 -The graphite used was obtained from two d i f f e r e n t s u p p l i e r s , Union Carbide and U l t ra Carbon. P a r t i c l e s ranging in s ize from 1.168 - 1.68mm were obtained by s e i v i n g . This material was pretreated by soaking in 1 molar hydrochlor ic acid overnight , washing in d i s t i l l e d water, soaking in approximately 1 molar sodium hydroxide for 8 hours and then r ins ing several times in d i s t i l l e d water. Scanning electronmicrographs of typ i ca l p a r t i c l e s , shown in Figure 11 and 12, i l l u s t r a t e that the graphite p a r t i c l e s were of i r r e g u l a r shape with a rough, porous, p i t ted sur face . The ent i re apparatus was i n s t a l l e d in a fume hood for safety reasons which included both explosion danger and t o x i c i t y of propylene, dibromopropane and propylene oxide. 4.2 Method F i r s t the e l e c t r o l y t e , gas and water rotameters were c a l i b r a t e d . The c a l i b r a t i o n curves are presented in Appendix 3. The e l e c t r o l y t e so lut ion was prepared from 99 percent pure sodium bromide (BDH Company) and f i l t e r e d , deionized water. The water was f i r s t f i l t e r e d in the AMF Cuno f i l t e r , model number 1 Mi and then deionized in a Calgon Corporation car t r idge de ion i ze r . Propylene oxide concentration was monitored on a g a s - l i q u i d chromatograph (Varian Model 1400) with a flame ion iza t ion detec tor . A 12 f t . column was used. The packing was Carbopak C 80/100 mesh coated with 0.2% CW 1500. The gas phase c a r r i e r was helium. A microsyr inge was used to in jec t a ten m i c r o l i t e r sample. Analys is of propylene was ca r r i ed out at 80°C. Ca l i b ra t i on samples were prepared by p ipet t ing a - 35 -x 26 x 300 FIGURE 11: SCANNING ELECTRONMICROGRAPHS OF TYPICAL UNION CARBIDE GRAPHITE PARTICLES FIGURE 12: SCANNING ELECTRONMICROGRAPHS OF TYPICAL ULTRA CARBON GRAPHITE PARTICLES - 37 -known volume of propylene oxide (BDH Company) into a 100 ml volumetric f lask containing d i s t i l l e d water. Several samples of varying concentrat ion were prepared. The c a l i b r a t i o n curve is shown in Figure D of Appendix 3. Experimentation consisted of a ser ies of runs in which measurements were made on the sodium bromide e l e c t r o l y t e and propylene gas feed r a t e , the e l e c t r o l y t e temperature entering the reactor , the pH of the e l e c t r o l y t e leaving the bottom of the separator , the e l e c t r o l y t e concentrat ion , the water flow rate to the absorption tower, the reactor out le t temperature, the current to the c e l l , the voltage drop across the c e l l , the gas flow rate from the absorption tower, the volume of dibromopropane produced, the tota l volume and concentration of propylene oxide so lut ion co l l ec ted and the concentrat ion of propylene oxide so lu t ion leaving from the bottom of the absorption tower. To do the experiments, the e l e c t r o l y t e feed tank was f i r s t charged respec t i ve ly with 0.2 molar and 0.5 molar sodium bromide s o l u t i o n . This so lu t ion was c i r c u l a t e d through a c i r c u l a t o r to control i t s temperature to the desired l e v e l , thereby c o n t r o l l i n g the reactor out le t temperature. The e l e c t r o l y t e flow rate through the c e l l was set between 100 and 300 cm per minute and the propylene gas flow rate to the ce l l was a lso adjusted between 100 and 2000 cm 3 per minute (STP). The current through the ce l l was then set with the power supply in the current control mode. The analys is for propylene oxide co l l ec ted in the separator and leav ing the bottom of the absorption tower was made on a g a s - l i q u i d chromatograph (Varian Model 1400). The hypobromite, bromite and bromate - 38 -c o n c e n t r a t i o n i n t h e s o l u t i o n l e a v i n g the s e p a r a t o r bot tom were a n a l y z e d u s i n g t h e s t a n d a r d i d i o m e t r i c t i t r a t i o n w i t h ammonium s u l p h a t e ( 2 4 ) . The t e c h n i q u e i s d e s c r i b e d i n s e c t i o n 4 . 1 o f Append ix 4 . The d i b r omop ropane forms a s e p a r a t e phase i n t h e p r o p y l e n e o x i d e s o l u t i o n t h e r e f o r e was e a s i l y s e p a r a t e d w i t h a m i c r o s y r i n g e o r p i p e t . The d i s s o l v e d d i b romopropane in t h e s o l u t i o n l e a v i n g the s e p a r a t o r bot tom was d e t e r m i n e d by e x t r a c t i n g w i t h e t h e r . T h i s was done a t an e l e c t r o l y t e c o n c e n t r a t i o n o f 0.5 M and the v a l u e o b t a i n e d was presumed t o remain c o n s t a n t i n c o n t a c t w i t h u n d i s s o l v e d d i b r omop r opane . The gas l e a v i n g t he a b s o r p t i o n tower i s sampled t h r ough a t ee w i t h a P r e s s u r e Lok ( P r e c i s i o n Samp l i ng C o r p o r a t i o n ) gas t i g h t s y r i n g e and a n a l y z e d u s i n g a gas ch romatog raph ( H e w l e t t P a c ka r d Model 5710 A Gas Ch romatog raph ) w i t h a t he rma l c o n d u c t i v i t y d e t e c t o r . The gas phase c a r r i e r was A r g o n . The gas ch romatog raph used two co l umns ; one c o n t a i n i n g m o l e c u l a r s i e v e wh i ch s e p a r a t e s h y d r o g e n , o x y g e n , n i t r o g e n , methane and c a rbon monox ide and a n o t h e r c o n t a i n i n g Porapak Q wh i ch s e p a r a t e s c a rbon d i o x i d e . Up t o f i v e c u b i c c e n t i m e t e s o f gas samples were i n j e c t e d . A t o t a l o f 167 runs were p e r f o r m e d . The ranges o f t h e o p e r a t i n g v a r i a b l e s c o v e r ed in t h e s e s t u d i e s a re g i v en i n Tab l e 2 . Sample c a l c u l a t i o n s a r e shown i n Append i x 4 . T a b l e 0 o f Append ix 8 shows d a t a r e p l i c a b i l i t y . 4.3 A c c u r a c y T a b l e 3 c o n t a i n a l i s t o f t h e q u a n t i t i e s measu r ed , t h e method used and an e s t i m a t e o f t h e a c c u r a c y f o r each me thod . Tab l e 2 Range o f E xpe r imen t a l V a r i a b l e s R e a c t o r O u t l e t Tempera tu re E l e c t r o l y t e F low Gas F low C u r r e n t El e c t r o l y t e C o n c e n t r a t i o n E l e c t r o d e Bed T h i c k n e s s I n l e t P r e s s u r e P a r t i c l e S i z e Degrees C e n t i g r a d e cm 3 /m in cm 3 /m in STP Amps M o l e s / 1 i t r e cm A tm . A b s o l u t e mm 30-60 100-300 100-2000 2-13 0 . 2 - 0 . 5 3 . 0 7 - 8 . 5 7 1 . 41 -2 . 22 1 . 1 6 8 - 1 . 6 8 - 40 -The maximum degree of uncertainty fo l lows: Space time y i e l d Propylene balance Dibromopropane balance Current e f f i c i e n c y Propylene oxide concentrat ion Rate of propylene oxide production Propylene oxide s e l e c t i v i t y Propylene conversion Propylene oxide y i e l d Total moles of hypobromite, bromiti in the ca lcu lated quant i t ies are = ± 3.5% = ± 8.5% = ± 6.5% = ± 4% = ± 3% = ± 3.5% = ± 15% = ± 6.5% = ± 8.5% ; and bromate = ± 2% - 41 -Table 3 Estimates of Experimental Accuracy Quantity Range Method Accuracy Current 2-13 Amps Ammeter ± 3% Graphite P a r t i c l e S ize 1.168-1.68 mm Standard s ieve -Bed Dimensions Thickness Height Width 3.07-8.57 cm 19.1 cm 2.5 cm Vernier gauge Mi l l imeter scale M i l l imeter scale ± .01 cm ± .1 cm ± .1 cm Propylene Gas Flow 100-2000 cm3/min Rotameter ± 5% E l e c t r o l y t e Flow 100-300 cm3/min Rotameter ± 2% Water Flow 190 cm3/min Rotameter ± 2% Reactor Outlet Temperature 30-60°C Temperature gauge ± 1°C Pressure 1.41-2.22 Atm Marsh pressure gauge ± .1 Atm pH 7-12 pH meter ± .05 pH Product Volume 50-775 ml Graduated cy l inder ± 2% Gas Flow from Tower 100-2000 cm3/min Wet test meter ± 3% Product Temperature 80-85°C Mercury thermometer ± .5°C Dibromopropane Sol u b i l i t y .014-.016 cm3 per 150 cm3 of so lut ion Extract ion with ether ± 5% Voltage 14-65 vo l ts Voltmeter ± .5 vo l t Iodine and Thiosulphate T i t r e 1-50 ml 50 ml burette ± .05 ml - 42 -C H A P T E R 5 R E S U L T S AND D I S C U S S I O N 5.0 General Considerations In t h e e l e c t r o c h e m i c a l p r o d u c t i o n o f p r o p y l e n e o x i d e , t h e e f f e c t i v e n e s s o f t h e r e a c t o r i s measured by f o u r f i g u r e s o f m e r i t wh i c h a r e d e f i n e d as f o l l o w s : (a) Space t ime y i e l d (STY) = P r o p y l e n e o x i d e p r o d u c t i o n r a t e ' J v ' T o t a l r e a c t o r vo lume % r . • mr\ Ra t e o f p r o d u c t i o n o f t h e d e s i r e d s p e c i e s (b) C u r r e n t e f f i c i e n c y (CE) = -K—r -e — J — r - = r v c — A — = — J — K — = — v ' Ra te o f p r o d u c t i o n o f t h e d e s i r e d s p e c i e s i n s t o i c h i o m e t r i c e q u i v a l e n c e t o t h e t o t a l c u r r e n t i n t h e p r o ce s s CE = ^ f ( c ) S p e c i f i c ene rgy consumpt i on (SE) = Energy r e q u i r e d to p roduce a k i l o g r a m o f t he d e s i r e d p r o d u c t . SE = K J /Kmo l e S E = 3bUU FCL m Kwh/kg (d) S e l e c t i v i t y (SEL) = P r o p y l e n e o x i d e y i e l d K ' P r o p y l e n e consumpt i on where N = mo les o f d e s i r e d p r o d u c t F = F a r a d a y ' s number - 43 -Z = Sto ich iometr ic number of e lectrons t rans fer red per mole of product I = Current V = Potent ia l drop MW = Molecular weight of des ired product. The f igures of merit l i s t e d above are determined by the design and operat ing condit ions of the reactor through t h e i r e f fec ts on the mass t r a n s f e r , react ion ra tes , mixing between anode and cathode, reactor vo l tage , potent ia l d i s t r i b u t i o n and the extent of current by-pass across the b ipo le beds. The major e lectrode react ions are the bromide ion oxidat ion to bromine at the anode (react ion 1) and the reduction of water to hydroxyl ion at the cathode (react ion 2). Both these rates increase with current dens i t y . The react ion rate is defined as: r = ZFi where r = react ion rate Z = s to ich iometr i c number of e lectrons t ransferred per mole of product i = current density The bromine generated reacts with the water present near the anode to generate hypobromous acid (react ion 3) which is then transported to the bulk of the s o l u t i o n . Here i t reacts with the d isso lved propylene to produce propylene bromohydrin ( react ion 4). Improved transverse mixing should ra ise the space time y i e l d and the current e f f i c i e n c y as i t - 44 -brings together propylene bromohydrin and hydroxy! ion to form propylene oxide (react ion 5) . However, the t ransfer of bromine to the cathode where i t may be reduced is a lso promoted (react ion 11) but depends on the rate of bromine react ion with propylene. Improved transverse mixing i s probably attained by increased gas flow rate and e l e c t r o l y t e flow r a t e . With higher gas flow ra tes , the e f f e c t i v e conduct iv i ty of the mixture in the reactor decreases thereby lowering the ionic current by-pass. Thus, the higher gas flow rates should have a pos i t i ve e f f e c t on the space time y i e l d and current e f f i c i e n c y for propylene ox ide . The increased gas flow rate would also reduce the e f f e c t i v e bed th ickness of the e lec t rode . Act ive bed thickness under mass t rans fer control i s defined as [27] : T = (2K'AV/i*a)0«5 where x = maximum act ive thickness of a three dimensional e lectrode (cm) K' = e f f e c t i v e conduct iv i ty of the e l e c t r o l y t e (mho/cm) AV = t o l e r a b l e potent ia l d i f f e rence between competing react ion on the e lectrode ( v o l t s ) a = s p e c i f i c surface area (cm 2/cm 3) i* = real current density (A/cm ) With reduced act ive bed thickness the potent ia l gradient r i ses in the ac t ive reg ion . Higher potent ia l gradients favour the generation of other by-products such as oxygen and bromates, which reduces the current e f f i c i e n c y for propylene ox ide . - 45 -Increased e l e c t r o l y t e c o n d u c t i v i t y , which is re lated to concentrat ion , would reduce space time y i e l d due to higher ion ic by-pass c u r r e n t s . Temperature a lso a f fec ts the e l e c t r o l y t e conduct iv i ty . The conduct iv i ty increases with increasing temperature. The r e l a t i v e rates of react ion are also af fected by temperature. Increasing the temperature should ra ise the rates of r eac t i on , however the rates of some react ion increase more than others . Thus, the increasing temperature could have e i ther a pos i t i ve or a negative e f fec t on the space time y i e l d for propylene oxide. High gas flow rates and e l e c t r o l y t e flow rates would increase mass t r a n s f e r rates of propylene and the bromide ion . Thus, the increased gas flow rate and e l e c t r o l y t e flow rate ra ises the space time y i e l d fo r propylene oxide i f the process is l im i ted by the mass t rans fer of propylene or bromide i on . The process can also be l imited by chemical k i n e t i c s . At low current dens i t i es the process would probably be l im i ted k i n e t i c a l l y whereas at high current d e n s i t i e s the process i s poss ib ly l im i ted by propylene t rans fer into the s o l u t i o n . Improved mass t ransfer and transverse mixing would also prevent the formation of l o c a l i z e d pH regions which in turn would increase the space time y i e l d . S p e c i f i c energy consumption for propylene oxide is a f fected by e l e c t r o l y t e conduc t i v i t y , which determines the c e l l p o t e n t i a l , and the current e f f i c i e n c y of the desired product. The e f f e c t i v e e l e c t r o l y t e conduc t i v i t y i s determined by three f a c t o r s : the gas flow r a t e , the e l e c t r o l y t e temperature and sodium bromide concentrat ion . With higher - 46 -gas flow rates the c e l l potent ia l required to dr ive the current i s l a r g e r , thereby increasing the s p e c i f i c energy consumption for propylene ox ide . High temperature would y i e l d a lower ce l l voltage and may give a reduced s p e c i f i c energy consumption for propylene oxide. Increased sodium bromide concentrat ion should y i e l d a higher s p e c i f i c energy consumption for propylene oxide due to higher ion ic by-pass cur ren t , which reduces the current e f f i c i e n c y for propylene oxide, and increases the p o s s i b i l i t y of dibromopropane production rate v ia react ion 14. F i n a l l y , the r e l a t i v e react ion rates and mass t ransfer rates of propylene and bromide ion a f fec t the s e l e c t i v i t y for propylene ox ide . The r e l a t i v e react ion rates are determined by temperature, concentrat ion of various reactants and the k ine t i c parameters. With increased gas and e l e c t r o l y t e flow r a t e s , which e f f e c t s the mass t ransfer ra tes , the s e l e c t i v i t y for propylene oxide would vary depending on other fac tors such as current . At high cur rent , the s e l e c t i v i t y would probably decrease. The above d iscuss ion ind icates cur rent , temperature, e l e c t r o l y t e flow r a t e , gas flow rate and e l e c t r o l y t e concentration would be expected to a f f e c t the f igures of mer i t . 5.1 Pre l iminary Experiments P r i o r to the main inves t iga t ion of the e lect rosynthes is of propylene oxide using a b ipo lar reac tor , prel iminary studies were undertaken to determine the l i m i t a t i o n s of the equipment. Ear ly experiments were done to determine the minimum reactor ou t l e t temperature which could be attained at a propylene gas flow* of 100 - 47 -cm3/min and an e l e c t r o l y t e flow rate of 100 cm 3/min. This value was determined to be 30°C. The maximum flow capaci ty of the reactor was 2000 cm 3 per minute* of propylene with 300 cm 3 per minute of e l e c t r o l y t e . At these flow r a t e s , the pressure drop in the system was 1.22 atmospheres. Raising the gas flow above 2000 cm 3 per minute or the e l e c t r o l y t e flow over 300 cm 3 per minute ra ises the pressure drop in the system to the extent that the flows could not be cont ro l led at the desired ra te . Increasing the propylene flow beyond 2000 cm3 per minute flooded the absorption tower. I n i t i a l l y , a s ta in less steel e lectrode was employed as the cathode but i t was found to be unsuitable as i t corroded and p i t t e d . The s t a i n l e s s steel cathode was replaced with a p l a t i n i z e d t itanium cathode. 5.2 Factorial Experiments The l i s t of independent var iab les and dependent var iab les i s i l l u s t r a t e d by Table 4. Results obtained from a f a c t o r i a l experiment performed at two leve ls are presented in Tables 5, 6 and 7. In determining which of the f i v e independent var iab les mentioned in Table 4 had the largest pos i t i ve e f fec t on the space time y i e l d for propylene oxide, the technique of analys is of variance was u t i l i z e d (25) . The source tab le resu l t i ng from t h i s ana lys is i s i l l u s t r a t e d in Table N of Appendix 7 and summarized in Table 8. The pr inc ipa l features of these resu l t s are discussed below. *A11 gas flows given at STP. - 48 -T a b l e 4 L i s t o f Independent and Dependent V a r i a b l e s Independen t V a r i a b l e s L e v e l s R e a c t o r o u t l e t t e m p e r a t u r e (°C) 30 and 60 C u r r e n t (Amps) 2 and 5 E l e c t r o l y t e c o n c e n t r a t i o n (M) 0 .2 and 0 .5 E l e c t r o l y t e f l o w r a t e ( c m 3 / m i n ) 100 and 300 P r o p y l e n e gas f l o w r a t e ( cm 3 /m i n ) STP 100 and 1000 Dependent V a r i a b l e Space t ime y i e l d f o r p r o p y l e n e o x i d e S e l e c t i v i t y f o r p r o p y l e n e o x i d e C u r r e n t e f f i c i e n c y f o r p r o p y l e n e o x i d e C u r r e n t e f f i c i e n c y f o r d i b r omop ropane C u r r e n t e f f i c i e n c y f o r hydrogen and oxygen C u r r e n t e f f i c i e n c y f o r b r o m a t e s , b r om i t e and h y p o b r o m i t e S p e c i f i c ene rgy c on sump t i on f o r p r o p y l e n e o x i d e Table 5 Factorial Experiment Results Reactor Outlet Temperature °C 30 60 Current (Amps) Current (Amps) 2 5 2 5 Concentrator l (M)* Concentration (M) Concentration (M) Concentration (M) 0.2 0.5 0.2 0.5 0.2 0.5 0.2 0.5 Electrolyte Flow (cm3 /min) o o r> c E ro E o 3 o [Z VJ ro ID o o o STY kg/h m3=12.4 A SEL % = 72.2 Current e f f i c i e n c y PO % = 47.3 DBP % = 18.3 Hydrogen % = 59.8 Oxygen % » 4.9 14.8 H 70.1 56.5 24.1 62.6 0.0 30.5 I 85.8 46.6 7.7 54.2 2.2 32.5 P 83.4 49.9 9.9 62.4 1.1 5.9 Q 54.5 22.6 18.8 60.8 3.9 11.8 X >66.6 45.2 22.7 63.4 0.0 32.1 Y 86.9 49.3 7.5 52.2 2.9 32.0 FI 82.2 49.1 10.7 50.1 0.0 Electrolyte Flow (cm3 /min) o o 7.4 B 77.5 28.2 9.1 40.2 3.7 11.1 G 78.0 42.6 12.0 47.7 2.9 16.2 J 85.2 24.7 4.3 63.2 5.3 21.9 0 87.3 33.8 4.9 30.8 3.1 4.8 R 65.9 18.2 9.4 38.1 3.7 5.3 W 59.8 20.5 13.8 25.2 2.2 14.6 Z 71.3 22.3 8.9 37.0 3.3 22.9 E l 85.8 35.1 5.9 39.8 1.9 Electrolyte Flow (cm3 /min) o o c E ro E i j X o uT VI ro O o o o 11.5 C 72.3 44.0 16.8 58.5 4.8 12.3 F 73.5 47.3 23.2 53.5 0.0 21.2 K 81.8 32.4 7.2 52.9 4.9 30.3 N 86.1 46.3 7.5 47.8 2.2 5.4 S 58.5 20.8 14.9 39.8 9.5 8.5 V 61.8 32.7 20.0 37.7 0.0 18.8 Al 80.0 28.8 7.2 51.8 5.6 26.3 01 83.2 40.3 8.2 45.0 2.1 Electrolyte Flow (cm3 /min) o o 1—• 5.5 D 67.1 20.9 10.3 35.0 4.3 11.3 E 81.7 43.3 9.7 41.0 3.4 13.4 L 81.7 20.5 4.6 33.5 4.8 18.7 M 85.3 28.7 4.9 35.4 7.5 4.0 T 63.9 15.5 8.7 29.9 3.4 6.3 U 68.7 24.0 10.9 34.6 3.3 12.8 Bl 80.8 19.7 4.7 51.5 8.5 13.6 Cl 82.3 20.8 4.5 28.6 2.0 *Sodium bromide concentration. E l e c t r o l y t e F l o w cm / n i n 100 G a s F l o w c n r / m i n 100 1000 o o OH O 00 CO o « in 300 G a s F l o w c n 3 / m i n 100 1000 7* m z -» o U3 O ro o o o IV o LTt O to 00 O O IV 3 ro o o s - 05 -Table 7 Factorial Experiment Results Reactor Outlet Temperature °C 30 60 Current (Amps) Current (Amps) 2 5 2 5 Concentration (M) Concentration (M) Concentration (M) Concentration (M) 0.2 0.5 0.2 0.5 0.2 0.5 0.2 0.5 Electrolyte Flow cn3 /n1n o o c m E u I uZ 3 o o o A Current e f f i c i e n c y BrO" « 4.4 Br0 2" » 2.3 BrO," ' 6.2 Total -12.9 H 8.2 0.0 11.5 19.7 I 4.4 2.2 25.7 32.3 P 2.5 0.1 1.7 4.3 Q 3.6 2.3 5.3 11.2 X 6.2 0.0 3.0 9.2 Y 5.9 3.8 27.2 36.9 Fl 1.8 .2 .8 2.8 Electrolyte Flow cn3 /n1n o c B 4.0 10.4 31.2 46.6 G 14.9 .7 14.3 29.9 J 6.0 2.9 45.6 54.5 0 13.8 2.9 24.5 41.2 R 4.5 0.0 7.5 12.0 W 6.2 3.9 5.2 15.3 Z 11.0 7.7 46.4 65.1 El 7.5 4.1 9.3 20.9 Electrolyte Flow cn3 /n1n o © 1-t C E 5 » o C (/> ID © o © C 2.1 2.4 8.5 13.0 F 3.2 2.4 4.5 10.1 K 3.5 1.6 21.9 27.0 N 4.0 1.3 12.4 17.7 S 1.5 .8 1.3 3.6 V 2.7 0.0 4.4 7.1 A l 3.0 0.6 15.5 19.1 01 4.5 1.8 4.2 10.5 Electrolyte Flow cn3 /n1n c o ft D 6.0 2.4 16.9 25.3 E 3.4 0.8 5.1 9.3 L 4.2 3.5 48.2 55.9 M 4.9 1.0 27.5 33.4 T 3.0 4.0 4.5 11.5 U 3.0 .2 2.6 5.8 Bl 3.8 2.2 29.8 35.8 Cl 3.9 1.8 16.1 21.8 Table 8 The Signif icance of the Effect of Operating Variables on Space Tine Y i e l d , Current E f f i c i ency , Se lec t iv i ty and Speci f ic Energy Consumption Dependent Variable Independent Variable Propylene Oxide Current E f f i c i ency Space Time Yield - Se lect iv i ty Current E f f i c iency Specif ic Energy Conspt. Dibrono-propane Hydrogen Oxygen Hypobromite Bromlte Bromate Tenperature (T) . (.) * (-) * (-) *(-) Current (A) * (+) * ( + ) * ( + ) * (-) * (•) E lectrolyte Cone. (C) * ( + ) * ( » ) • (-) * (+) * (-) * (-) * (-) E lectrolyte Flow Rate (I) * (-) * (•) * (-) * ( + ) Gas Flow Rate (G) * ( + ) * (•) * (-) * ( + ) * (*) ' (-) * H » (-) A x L * A x G * * * A x C * * T x A * * ' S t a t i s t i ca l s ignif icance level • 0.05 or lower, (+) or (-) Indicate that increasing the independent variable (Increase) (decrease) the dependent var iab le . - 53 -5.2.1 Reactor Out le t Temperature The results in Tables 5 to 8 and Table N of Appendix 7 indicate the following features: a. Temperature has a negative effect* on the space time yield, selectivity and current efficiency for propylene oxide. b. Temperature also has a negative effect on the current efficiency for bromates. The f i r s t effect can be explained in part by reduced solubility of propylene in the electrolyte solution with increasing temperature. A graphical illustration of the relation between temperature and solubility of propylene in water is contained in Figure F of Appendix 4. As the solubility of propylene decreases, the formation of propylene oxide becomes limited by i t , thereby reducing the space time yield for propylene oxide. This negative effect of temperature was also found by King et a l . [ 1 0 ] . The variation in the limiting current density, which is defined as ZFKCb L " T~±T where K = mass transfer coefficient C D = bulk concentration *Negative effect means by increasing the independent variable the dependent variable decreases. - 54 -t = t r a n s p o r t number = r e a l l i m i t i n g c u r r e n t d e n s i t y w i t h c o n c e n t r a t i o n o f p r o p y l e n e i s i l l u s t r a t e d in T ab l e 9 . The mass t r a n s f e r c o e f f i c i e n t was c a l c u l a t e d from the model p r e s en t ed i n r e f e r e n c e [ 2 7 ] , The l i m i t i n g c u r r e n t d e n s i t y f o r p r o p y l e n e can be i n c r e a s e d by o p e r a t i o n o f t h e r e a c t o r a t e l e v a t e d p r e s s u r e . B e j a r a n o e t a l . [ 1 2 ] o p e r a t e d t h e i r r e a c t o r a t p r e s s u r e s o f up t o 5 a tmospheres and d e t e r m i n e d t h e space t ime y i e l d f o r p r o p y l e n e o x i d e t o be 156.6 k g / h r m 3 . The c o n d u c t i v i t y o f t h e e l e c t r o l y t e s o l u t i o n r i s e s w i t h h i g h e r t e m p e r a t u r e r e s u l t i n g i n a l o w e r c u r r e n t e f f i c i e n c y and h i g h e r s p e c i f i c ene rgy consumpt i on f o r p r o p y l e n e o x i d e . T h i s i n c r e a s e d s p e c i f i c ene rgy c on sump t i o n o c c u r s even though the v o l t a g e r e qu i r emen t t o d r i v e c u r r e n t t h r o u g h t h e r e a c t o r i s l owe r at a h i g h e r t e m p e r a t u r e . T ab l e 10 i l l u s t r a t e s t h i s . The above s u g g e s t s t h a t t he reduced c u r r e n t e f f i c i e n c y and h i g h e r s p e c i f i c ene rgy consumpt i on f o r p r o p y l e n e o x i d e i s due t o i n c r e a s e d bypass c u r r e n t a s s o c i a t e d w i t h i n c r e a s e d s o l u t i o n c o n d u c t i v i t y . T h i s i s a l s o s u p p o r t e d by the f a c t t h a t t e m p e r a t u r e does no t have a v e r y l a r g e e f f e c t on t he c u r r e n t e f f i c i e n c y f o r d i b r o m o p r o p a n e , h y p o b r o m i t e , b r o m i t e and b r o m a t e s . I n c r e a s i n g t e m p e r a t u r e a l s o a f f e c t s t h e r e l a t i v e r a t e s o f t h e c h e m i c a l and e l e c t r o c h e m i c a l r e a c t i o n s . - 55 -T a b l e 9 E f f e c t o f P r o p y l e n e C o n c e n t r a t i o n on the Real L i m i t i n g C u r r e n t D e n s i t y f o r P r o p y l e n e Tempe ra tu r e °C S o l u b i l i t y wt % C o n c e n t r a t i o n M L i m i t i n g C u r r e n t D e n s i t y ( A /m 2 ) 30 2.4 x l O " 2 5.7 x l O " 3 50 .7 60 8 . x l O " 3 1.9 x 1 0 - 3 16 .9 C o n d i t i o n s E l e c t r o l y t e f l o w = 300 cm /m in P r o p y l e n e f l o w = 1000 cm 3 /m in (STP) Mass t r a n s f e r c o e f f i c i e n t = 4.61 x 1 0 " 5 m/sec - 56 -T a b l e 10 The E f f e c t o f Tempera tu re on V o l t a g e Requ i rement and S p e c i f i c Energy Consumpt ion f o r P r o p y l e n e Ox ide I l l u s t r a t i o n f o r boxes G and W o f T a b l e s 5 Tempera tu re °C 30 60 V o l t a g e r e q u i r e m e n t 17 V 14V S p e c i f i c ene rgy c o n s u m p t i o n 7.4 Kwhr/kg 12 .6 Kwhr/kg C u r r e n t e f f i c i e n c y f o r p r o p y l e n e o x i d e 42.6% 20.5% - 57 -5.2.2 Current The f ind ings in Table 5 to 8 and Table N of Appendix 7 i l l u s t r a t e the fo l lowing features : a . Sup e r f i c i a l current density has a p o s i t i v e e f fec t* on the space time y i e l d , s e l e c t i v i t y and s p e c i f i c energy consumption for propylene oxide. b. The bromate current e f f i c i e n c y increases with higher cu r ren ts . c . Sup e r f i c i a l current density has a negative e f fec t on the current e f f i c i e n c y for dibromopropane. Higher s u p e r f i c i a l current dens i t i es resu l t in increased bromine generation rates (react ion 1) which can match the propylene gas absorp-t i o n process to form propylene bromohydrin by react ion 4. Box A and I of Tables 5 and 6 indicate that the current e f f i c i e n c y for propylene oxide decreases together with the current e f f i ency for dibromopropane. However, the current that went into the formation of dibromopropane for both cases remains approximately the same (75.6 A/m2 and 79.5 A/m 2 ). The above suggests that although the current e f f i c i e n c y for propylene oxide decreases, the extra current does go into producing propylene oxide and the bromates as indicated by Table 11. Table 7 shows a steep r i s e in the tota l bromate formation with an increase in the s u p e r f i c i a l current densi ty ind i ca t ing that the formation takes place through the e lectrochemical rather than chemical route (react ion 9 ) . - 58 -T a b l e 11 C u r r e n t t h a t Goes i n t o P r o d u c i n g P r o p y l e n e O x i d e , D ib romopropane and T o t a l Bromates S u p e r f i c i a l D ib romopropane T o t a l P r o p y l e n e c u r r e n t d e n s i t y b romates o x i d e (A /m 2 ) ( A /m 2 ) ( A /m 2 ) ( A /m 2 ) 413 A /m 2 75 .6 53 .3 195 .3 1033 A /m 2 79 . 5 333.7 481 .4 - 59 -From the data of Appendix 6 i t would be expected that, by increasing the superficial current density, the oxygen current efficiency would decrease. However, analysis (Table 8) indicates that current had no effect. 5 . 2 . 3 S o d i u m B r o m i d e C o n c e n t r a t i o n The data of Tables 5 to 8 and Table N of Appendix 7 show the following effects: a. Concentration has a positive effect on the space time yield and current efficiency for propylene oxide. b. The current efficiency of dibromopropane increases with increasing concentration. c. Concentration has a negative effect on the current efficiency for oxygen, bromite and bromate. d. Propylene oxide specific energy consumption decreases with increasing concentration. Although, i t would be expected that the current efficiency for propylene oxide would decrease with increasing electrolyte concentration due to higher current by-pass this was not observed. The higher current efficiency for propylene oxide can be attributed to lower total bromate and oxygen current efficiency with increasing electrolyte concentration. The current that previously went into generating the bromates and oxygen now goes into the generation of propylene oxide and dibromopropane thereby increasing their current efficiency. This - 60 -c o n t r a d i c t s t h e r e s u l t s of B e j a r a n o e t a l . [12] . They showed t h a t t h e c u r r e n t e f f i c i e n c y f o r p r o p y l e n e o x i d e f o l l o w e d the expec t ed t r e n d , t h a t i s , a d e c r e a s e w i t h i n c r e a s i n g e l e c t r o l y t e c o n c e n t r a t i o n . However , t h e i r r e s u l t s i n d i c a t e d t h a t i n c r e a s e d e l e c t r o l y t e c o n c e n t r a t i o n had a n e g a t i v e e f f e c t on the bromate f o r m a t i o n wh i ch i s i n agreement w i t h t h e r e s u l t s i n T ab l e 7 . The l owe r s p e c i f i c ene rgy c on sump t i on f o r p r o p y l e n e o x i d e can be a s c r i b e d i n p a r t t o t h e l o w e r v o l t a g e r e q u i r e d t o d r i v e t h e c u r r e n t t h r o u g h t he r e a c t o r as t h e e l e c t r o l y t e c o n c e n t r a t i o n and hence the h i g h e r c o n d u c t i v i t y i n c r e a s e s . The c o n d u c t i v i t y o f 0 .2 M sodium b rom ide s o l u t i o n i s 0 .0183 mho/cm and t h a t o f 0 .5 M s o l u t i o n i s 0 .0423 mho/cm. 5.2.4 E l e c t r o l y t e Flow Rate T a b l e s 5 t o 8 and T a b l e N o f Append ix 7 i l l u s t r a t e the f o l l o w i n g f e a t u r e s : a . The e l e c t r o l y t e f l o w r a t e has a p o s i t i v e e f f e c t on t he space t ime y i e l d and c u r r e n t e f f i c i e n c y f o r p r o p y l e n e o x i d e . b. The s p e c i f i c energy consumpt i on d e c r e a s e s w i t h i n c r e a s e d e l e c t r o l y t e f l ow r a t e . c . Hypob rom i te c u r r e n t e f f i c i e n c y i n c r e a s e s w i t h i n c r e a s e d e l e c t r o l y t e f l ow r a t e . T a b l e 12 shows t h a t w i t h i n c r e a s e d e l e c t r o l y t e f l o w r a t e the mass t r a n s f e r c o e f f i c i e n t [ 27 ] f o r t h e b romide i on and p r opy l e ne i n c r e a s e s s u g g e s t i n g improved mass t r a n s f e r r a t e s . W i th i n c r e a s e d e l e c t r o l y t e Tab l e 12 The E f f e c t o f E l e c t r o l y t e F low Rate and C o n c e n t r a t i o n on the Mass T r a n s f e r C o e f f i c i e n t and the Real L i m i t i n g C u r r e n t D e n s i t y f o r t he Bromide I o n . E l e c t r o l y t e F low Rate (cm /min) 100 300 Gas F low Rate ( cm 3 /m in ) Gas F low Ra te ( c m 3 / m i n ) 100 1000 1500 2000 100 1000 1500 2000 Cone. (M) Cone. (M) Cone. (M) Cone. (M) Cone . (M) Cone . (M) Cone . (M) Cone . (M) .2 .5 .2 .5 .2 .5 .2 .5 .2 .5 .2 .5 .2 .5 .2 .5 K (m/s) 6 .92 x 1 0 - 5 7.83 x l O " 5 8.17 x l O " 5 8.54 > c l O " 5 9 .58 x l O " 5 1.07 > < I O - 4 1.11 ) < I O - 4 1.15 ! < 1 0 - " i L ( A /m 2 ) 2671 6678 3022 7556 3153 7884 3296 8241 3698 9245 4130 10326 4285 10712 4439 11098 i L S ( A /m 2 ) 4910 12277 5556 13891 5796 14494 6059 15150 6798 16996 7593 18983 7878 19693 8161 20403 i|_ = r e a l l i m i t i n g c u r r e n t d e n s i t y i|_5 = l i m i t i n g s u p e r f i c i a l c u r r e n t d e n s i t y - 62 -flow rate the mixing condit ion within the reactor poss ib ly improves. The e f f e c t i v e conduct iv i ty of the mixture also increases due to higher e l e c t r o l y t e hold up. Improved mass t rans fer rates of propylene and bromide ion , and mixing condit ions could poss ib ly account for the increased space time y i e l d and current e f f i ency for propylene oxide. The s p e c i f i c energy consumption decreases due to higher current e f f i c i e n c y for propylene oxide and due to increased e l e c t r o l y t e so lut ion temperature entering the reactor with increased e l e c t r o l y t e flow rate to contro l the reactor out let temperature. 5.2.5 Propylene Gas Flow Rate The fol lowing features were observed in Tables 5 to 8 and Table N of Appendix 7: a. Propylene gas flow has a pos i t i ve e f fec t on the space time y i e l d and current e f f i c i e n c y for propylene oxide. b. The gas flow rate has a negative e f fec t on the s p e c i f i c energy consumption for propylene oxide. c . Dibromopropane and hydrogen current e f f i c i e n c y increase with increasing gas flow r a t e . d . Propylene gas flow has a negative e f fec t on the hypobromite, bromite and bromate current e f f i c i e n c y . Increasing propylene gas flow rate poss ib ly increases the transverse mixing of reactants within the reactor producing high space time y i e l d , and current e f f i c i e n c y for propylene oxide. This could a lso explain the increased current e f f i c i e n c y for dibromopropane. Increased - 63 -gas flow rate also improves the overa l l mass t rans fe r c o e f f i c i e n t s for propylene as demonstrated in Table 13. The improved mass t rans fe r c o e f f i c i e n t suggests higher mass t rans fe r rates producing larger space time y i e l d s for propylene oxide. With increasing gas flow rate the e f f e c t i v e mixture conduct iv i ty decreases. High space time y i e l d and current e f f i c i e n c y for propylene oxide can also be a t t r ibuted to t h i s fact due to reduced current by-pass associated with a lower e f f e c t i v e mixture c o n d u c t i v i t y . The reduced total bromate current e f f i c i e n c y can poss ib ly be explained in part by the improved mixing condit ions in the reac to r . These condi t ions decrease the high local concentrations of hypobromite, which in turn reduce the bromate formation by react ions 8 and 9. The lower s p e c i f i c energy consumption for propylene oxide is probably due to high current e f f i c i e n c y for propylene oxide. Increased hydrogen current e f f i c i e n c y can be explained in terms of l ess by-pass current with high gas flow ra tes . Improved mass t r a n s f e r for propylene which suppresses the concentrat ion of hypobromous ac id can also explain the higher current e f f i c i e n c y for hydrogen. 5.3 pH E f f e c t s Although the e f f ec t of pH was not studied separately some mention of i t should be made as i t is a very important v a r i a b l e . The formation of propylene oxide is dependent on the sapon i f i ca t i on of propylene bromohydrin, which is favoured by basic condit ions ( reac t ion 14). Therefore , the current e f f i c i e n c y of propylene oxide is Tab l e 13 The E f f e c t o f P r o p y l e n e Gas F low Rate on 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 f o r P r o p y l e n e at V a r y i n g E l e c t r o l y t e F low Ra te E l e c t r o l y t e Flov v Rate ( c m 3 / m i n ) 100 300 Gas F low Rate ( c m 3 / m i n ) * Gas F low Rate ( c m 3 / m i n ) * 100 1000 1500 2000 100 1000 1500 2000 K Q ( m / s e c ) 1.54 x 1 0 - 5 1.94 x l O " 5 2.10 x 10~ 5 2.24 x l O " 5 3.96 x l O " 5 4 .61 x l O " 4 4 .83 x l O " 4 5.01 x \0-h i L S ( A /m 2 ) 31 . 1 39 .2 42 . 5 4 5 . 3 80 . 1 9 3 . 2 97 . 7 101 . 3 ij_s = L i m i t i n g s u p e r f i c i a l c u r r e n t d e n s i t y Tempera tu re = 30°C C o n c e n t r a t i o n = 5.7 x 1 0 " 3 M *A11 gas f l ow s g i v en at STP - 65 -a f u n c t i o n o f t h e pH o f t h e e l e c t r o y t e . I f t h e pH i s t o o h i g h , t h e p r o p y l e n e o x i d e can be h y d r o l y z e d t o p r o p y l e n e g l y c o l , and a t low pH t h e f o r m a t i o n o f d i b romopropane i s f a v o u r e d ( r e a c t i o n 1 4 ) . T h i s s u g g e s t s t h a t t h e r e i s an optimum pH f o r t he f o r m a t i o n o f p r o p y l e n e o x i d e . In t h e p r e s e n t s t u d y , d u r i n g the c ou r s e o f t h e e l e c t r o l y s i s , t h e r e was a r a p i d r i s e i n t h e pH v a l u e w i t h i n t h e r e a c t o r . The r e a c t o r o u t l e t pH f o r a l l t h e e x p e r i m e n t a l runs ranged from 9.70 t o 1 1 . 5 8 . 5.4 Further Invest igat ion Data i n T ab l e N o f Append i x 7 show t h a t s u p e r f i c i a l c u r r e n t d e n s i t y and p r o p y l e n e gas f l ow r a t e have t he ma jo r e f f e c t on the space t i m e y i e l d , s e l e c t i v i t y and c u r r e n t e f f i c i e n c y f o r p r o p y l e n e o x i d e . F u r t h e r e x p e r i m e n t s were c a r r i e d ou t to see how f a r t h e i r e f f e c t c o u l d be e x t e n d e d . 5 . 4 .1 E f f e c t o f Current and Propylene Gas Flow R a t e T y p i c a l r e s u l t s o b t a i n e d f rom t he s e e x p e r i m e n t a l runs a r e p r e s e n t e d i n T a b l e J o f Append ix 4 and i l l u s t r a t e d i n t he graph o f F i g u r e 1 3 . As seen f rom t h e g r a p h , t he space t ime y i e l d f o r p r o p y l e n e o x i d e i n c r e a s e d w i t h c u r r e n t a p p l i e d t o t he r e a c t o r , went t h r ough a maximum, t hen d e c r e a s e d as t h e c u r r e n t was i n c r e a s e d f u r t h e r . I n c r e a s i n g p r o p y l e n e gas f l o w had t h e same e f f e c t as r a i s i n g t h e c u r r e n t ; t he space t i m e y i e l d f o r p r o p y l e n e o x i d e i n c r e a s e s , r ea ches a maximum, and t hen f a l l s . These r e s u l t s i n d i c a t e t h e r e i s a maximum s u p e r f i c i a l c u r r e n t d e n s i t y f o r a g i v e n c o n d i t i o n beyond wh i ch t h e space t i m e y i e l d d e c r e a s e s . T h i s i s due t o i n c r e a s e d r a t e o f b romine g e n e r a t i o n , wh i ch l e a d s to t h e - 66 -EFFECT OF CURRENT AND GAS FLOW RATE ON SPACE TIME YIELD CD CO CO * X H \ o c n "* QJ ° . — I 0 QJ CD QJ <N u ro C L CO CD CD C u r v e Number 1 2 3 (ias Flow Rate cc /mtn 1000 1500 2000 pH Range 11.40-11.46 11.45-11.60 11.27-11.38 0.0 2.0 4.0 6.0 Current FIGURE 13 8.0 (Amps) 10.0 12.0 Condi t ions Graphite P a r t i c l e Size Graphite Type Temperature Concentrat ion L iqu id Flow Pressure Bed Thickness Area 1.168 - 1.68 mm Union Carbide 28-35°C 0.5 M 300 cc/min 1.4 - 2.2 atm 8.57 cm 4.84 x l O - 3 m 2 - 67 -deplet ion of d isso lved propylene in the s o l u t i o n . The excess bromine generated 1s then free to par t i c ipa te in various loss reactions reducing the space time y i e l d for propylene oxide. Increasing propylene gas flow had the same e f f e c t as increasing super f i c i a l current dens i ty . I n i t i a l l y the space time y i e l d for propylene oxide increases probably due to improved mixing and mass t ransfer in the reactor . However, a fur ther increase in the gas flow rate resu l t s in a lower residence time for the mixture in the reactor which could account for low space time y i e l d s for propylene ox ide . Increased gas flow rate could also reduce transverse mixing in the reactor producing low space time y i e l d s for propylene oxide. Figure 13, 14 and Table J of Appendix 4 show that the s p e c i f i c energy consumption increases together with the space time y i e l d for propylene oxide. This occurs unt i l the space time y i e l d reaches a maximum. Beyond th i s point there is a trade o f f between space time y i e l d and s p e c i f i c energy consumption. The specif ic: energy consumption increases although the space time y i e l d for propylene oxide decreases. The e f f ec t of s u p e r f i c i a l current density on the current e f f i c i e n c y for propylene oxide at a propylene gas flow rate of 1500 cm3/min is presented in the graph of Figure 15. This f igure also i l l u s t r a t e s the e f f e c t of s u p e r f i c i a l current density on the dibromopropane and oxygen current e f f i c i e n c y . One of the features observed from t h i s graph i s the decreasing current e f f i c i e n c y for propylene oxide and dibromopropane with increasing s u p e r f i c i a l current dens i ty . The amount of oxygen formed, however, i nc reases . Decreasing current e f f i c i e n c y for propylene - 68 -EFFECT OF CURRENT AND GAS FLOW RATE ON SPECIFIC ENERGY CONSUMPTION cn \ JO. o ZX cr a — i o * +-J CD a . CO CO cr a O CD CD* CN cn i _ cu c LxJ CD CD *~ —i <J_ — i u OJ CL CD CO C3 Curve Number 1 2 3 Gas Flow Rate c c/Mln 1000 1500 2000 0.0 2.0 4.0 6.0 8.0 Current (Amps) 10.0 12.0 FIGURE 14 Condit ions Graphite P a r t i c l e Size Graphite Type Temperature Concentrat ion L iqu id Flow Pressure Bed Thickness Area 1.168 - 1.68 mm Union Carbide 28-35°C 0.5 M 300 cc/min 1.4 - 2.2 atm 8.57 cm = 4.84 x 10- rn - 69 -SELECTIVITY AND CURRENT EFFICIENCY AS A FUNCTION OF APPLIED CURRENT FIGURE 15 Condit ions Graphite P a r t i c l e Size = 1.168 - 1.68 mm Graphite Type = Union Carbide Temperature = 28-35°C Concentrat ion = 0.5 M L iqu id Flow = 300 cc/min Pressure = 1.4 - 2.2 atm Bed Thickness = 8.57 cm Area = 4.84 x 10" 3 m 2 - 70 -o x i d e and i n c r e a s i n g c u r r e n t e f f i c i e n c y f o r oxygen i s t h e r eason f o r h i g h e r ene rgy c o n s u m p t i o n . S i m i l a r e f f e c t s a re obse rved a t p r o p y l e n e gas f l o w r a t e s o f 1000 and 2000 c m 3 / m i n . 5.4.2 Effect of Bed Thickness Hav i ng i n v e s t i g a t e d the e f f e c t o f s u p e r f i c i a l c u r r e n t d e n s i t y and p r o p y l e n e gas f l o w r a t e , t he e f f e c t o f bed t h i c k n e s s on the space t i m e y i e l d f o r p r o p y l e n e o x i d e was t hen e x p l o r e d . R e s u l t s a re p r e s e n t e d i n T ab l e K o f Append ix 4 and shown i n g r a p h i c a l form i n F i g u r e 16 . T h i s f i g u r e shows how the space t ime y i e l d f o r p r o p y l e n e o x i d e i s a f f e c t e d by t he r e a c t o r bed t h i c k n e s s . I t c l e a r l y shows t h a t the space t i m e y i e l d f o r p r o p y l e n e o x i d e i n c r e a s e s w i t h d e c r e a s i n g r e a c t o r bed t h i c k n e s s . T ab l e K i n d i c a t e s t h a t t h e p r o p y l e n e o x i d e p r o d u c t i o n r a t e , c u r r e n t e f f i c i e n c y and s e l e c t i v i t y d e c r e a s e i n i t i a l l y , then remain a p p r o x i m a t e l y c o n s t a n t . From t h e s e r e s u l t s , i t appea r s t h a t t he by -pa s s c u r r e n t i n c r e a s e s as t h e r e a c t o r bed t h i c k n e s s i s l o w e r e d . In o r d e r to max im i ze t he space t ime y i e l d f o r p r o p y l e n e o x i d e e x p e r i m e n t a l runs were done a t a s m a l l e r r e a c t o r bed t h i c k n e s s w i t h i n c r e a s i n g c u r r e n t and p r o p y l e n e gas f l o w r a t e . F i n d i n g s f rom t h e s e e x p e r i m e n t s a re i l l u s t r a t e d i n t h e g raph o f F i g u r e 17 and T a b l e L o f Append i x 4 . F i g u r e 17 shows how the i n c r e a s i n g s u p e r f i c i a l c u r r e n t d e n s i t y and p r o p y l e n e gas f l o w r a t e a t s m a l l e r r e a c t o r bed t h i c k n e s s a f f e c t t h e space t ime y i e l d f o r p r o p y l e n e o x i d e . One f e a t u r e i s c l e a r l y seen i n t h i s f i g u r e : t h e space t ime y i e l d f o r p r o p y l e n e o x i d e i n c r e a s e s - 71 -EFFECT OF BED THICKNESS ON SPACE TIME YIELD FIGURE 16 Condit ions Graphite P a r t i c l e Size = 1.168 - 1.68 mm Graphite Type = Porous Temperature = 29-32°C Concentrat ion = 0.5 M Current = 5 amps Pressure = 1.4 - 2.2 atm Bed Thickness = Thickness of 5 beds inc luding diaphragms - 72 -EFFECT OF BED THICKNESS, CURRENT AND PROPYLENE GAS FLOW RATE ON THE SPACE TIME YIELD FOR PROPYLENE OXIDE CD C3 CD CO cn R CO •—I cu —I >- CD CU CJ CD C L CO C N Gas Bed Flow 1000 1500 2000 Thickness cc/mln cc/m1n cc/mln 3.07 cn 1 2 3 8.57 cm 4 5 6 0.0 2.0 4.0 6.0 Current 8.0 (Amps) 10.0 12.0 Condi t ions FIGURE 17 Graphite P a r t i c l e Size Graphite Type Temperature Concentrat ion L iqu id Flow Pressure Area 1.168 - 1.68 mm Union Carbide 28-35°C 0.5 M 300 cc/min 1.4 - 2.2 atm = 4.84 x 10- rn - 73 -with decreasing bed th i ckness . The higher space time y i e l d for propylene oxide with smaller reactor bed thickness can be ascribed to the unequal drop in the ra t io of reactor volume and propylene oxide production ra te . 5.4.3 E f f e c t o f D i f f e r e n t Graphite Types Findings from experiments done with d i f f e r e n t graphite samples are presented in Table 14. Changing the graphite sample from Union Carbide to U l t ra Carbon y i e l d s lower space time y i e l d , current e f f i c i e n c y , s e l e c t i v i t y and production rate for propylene oxide. These resu l t s are probably due to higher porosi ty of the U l t r a Carbon graphite as i l l u s t r a t e d by Figure 11. The highly porous surface creates dead space where l o c a l i s e d low pH regions caused by poor mass t rans fer c h a r a c t e r i s t i c s can develop, al lowing the undesirable react ions to take place thereby reducing the s e l e c t i v i t y and space time y i e l d for propylene oxide. The lower space time y i e l d for propylene oxide i s a lso supported by the fact that the current e f f i c i e n c y for dibromopropane inc reases . 5.4.4 E l e c t r o l y t e Recycle F i n a l l y , recyc le runs were done. The ca l cu la t ions are presented in Appendix 4. The resu l ts are i l l u s t r a t e d in Figures 18 and Tables G, H and I of Appendix 4. The in te res t ing feature of Figure 18 is that the space time y i e l d for propylene oxide f a l l s with t ime. The decrease in space time y i e l d for propylene oxide could be due to the r i s i n g pH of the recyc l ing e l e c t r o l y t e s o l u t i o n , as seen in Tables G, H and I - 74 -T a b l e 14 The E f f e c t o f D i f f e r e n t Carbon Type on the Space Time Y i e l d f o r P r o p y l e n e Ox i de Carbon Type Un ion C a r b i d e U l t r a Carbon STY k g / h r m 3 32 . 5 25 .6 <— PO Produced c m 3 / h r 16 .2 12 .8 •1— o o C . E . (PO) % 49 .9 39 . 4 ro o r — C . E . (DBP) % 9 .9 10.2 o E o S e l e c t i v i t y % 8 3 . 5 79 . 5 o C O 3 o STY kg /h r m 3 21 . 9 18 . 6 LT PO Produced c m 3 / h r 11 .0 9 .3 ro C O « 3 C c C . E . (PO) % 33 . 8 2 8 . 6 E <j C J 3 t — C . E . (DBP) % 4 .9 6 .2 3 C Se l e c t i v i t y % 8 7 . 3 8 2 . 3 • LT STY k g / h r m 3 3 0 . 3 18.9 PO P roduced c m 3 / h r 15 .1 9 . 5 £ c • I— c: C . E . (PO) % 4 6 . 3 29 .1 E c C . E . (DBP) % 7 .5 8.7 ro E S e l e c t i v i t y % 86 .1 77 . 0 c U c ,— 3 STY kg /h r m 3 18.7 11 .1 1 i PO P roduced c m3 / h r 9 .4 5.5 (/" c C . E . (PO) % 28 . 7 17 .0 cr. C . E . (DBP) % 4 .9 6 . 0 S e l e c t i v i t y % 85 . 4 73 . 8 C o n d i t i o n s : C o n c e n t r a t i o n = 0.5 M C u r r e n t = 5A Tempera tu re = 28 - 34°C Bed T h i c k n e s s = 8.57 cm P r e s s u r e = 1.4 - 2 .2 a t m . - 75 -PROPYLENE OXIDE SPACE TIME YIELD VARIATION DURING RECYCLE CONDITION FIGURE 18 Condit ions Graphite P a r t i c l e Size = 1.168 - 1.68 mm Graphite Type = Union Carbide Temperature = 29-36°C Pressure = 1.4 - 2.2 atm E l e c t r o l y t e Cone. = 0.5 M Propylene Gas Flow Rate = 1500 cm2/min - 76 -o f Append i x 4 . The r i s i n g pH o f t h e e l e c t r o l y t e s o l u t i o n c o u l d r e s u l t i n t h e p r o p y l e n e o x i d e be i ng c o n v e r t e d to p r o p y l e n e g l y c o l by r e a c t i o n 15 o r t h e m o l e c u l a r b romine b e i n g c o n v e r t e d t o hypob rom i t e wh i ch i s t hen o x i d i z e d t o bromate by r e a c t i o n s 8 and 9 . The f o r m a t i o n o f h y p o b r o m i t e , b r o m i t e and bromates a r e damaging to the c u r r e n t e f f i c i e n c y f o r p r o p y l e n e o x i d e . These r e s u l t s a l s o i n d i c a t e t h a t maybe t he optimum pH i n t h e s e e x p e r i m e n t s i s l e s s than o r equa l t o 1 0 . 4 5 . The second p o s s i b l e r eason f o r t h e d e c r e a s e i n the space t ime y i e l d f o r p r o p y l e n e o x i d e c o u l d be a t t r i b u t e d t o the r i s i n g e l e c t r o l y t e t e m p e r a t u r e . P r o p y l e n e s o l u b i l i t y i n wa te r d e c r e a s e s w i t h i n c r e a s i n g t e m p e r a t u r e ( F i g u r e E, Append ix 4 ) . The f a l l i n the pe r f o rmance o f p r o p y l e n e o x i d e f o r m a t i o n c o u l d a l s o be due t o c a r r y ove r o f t h e c o r r o s i o n p r o d u c t s f rom t h e equ ipment and i m p u r i t i e s p r e s e n t i n t he e l e c t r o l y t e wh i ch can c a t a l y z e t h e homogenous p r o d u c t i o n o f t h e bromide ion from t h e h ypob r om i t e i o n by r e a c t i o n 19 . M 20B r - 2Br~ + 0 2 (19) M can be c o b a l t , n i c k e l o r ch rom ium. T h i s r e a c t i o n i s p a r a s i t i c as i t r educes t h e a v a i l a b i l i t y o f t h e OBr- i on wh i ch r e a c t s w i t h p r o p y l e n e to produce p r o p y l e n e b r omohyd r i n by r e a c t i o n 4 . The c o n c e n t r a t i o n o f b romates i n c r e a s e w i t h t i m e as seen i n T a b l e I o f Append i x 4 . The r e s u l t s o f t h i s e x p e r i m e n t i n d i c a t e the p o s s i b i l i t y f o r r a i s i n g t he space t ime y i e l d f o r p r o p y l e n e o x i d e by d e t e r m i n i n g t h e opt imum pH - 77 -o f t h e e l e c t r o l y t e s o l u t i o n , by c o n t r o l l i n g the t e m p e r a t u r e and by u s i n g equ ipment c o n s t r u c t e d o f n o n - c o r r o d i n g m a t e r i a l such as t i t a n i u m and T e f l o n . 5.5 Commercial Potential A v e r y s i m p l e c o s t i n g e x e r c i s e was unde r t aken t o d e t e r m i n e the commerc i a l p o t e n t i a l f o r a 10 ,000 s h o r t t ons per y e a r p r o d u c t i o n p l a n t f o r p r o p y l e n e o x i d e . A l i n e d i ag ram f o r the p roposed p l a n t i s i l l u s t r a t e d i n F i g u r e 19 . U s i n g t h e b e s t r e s u l t s f rom t he e x p e r i m e n t s done ( T a b l e 15) and assuming t h a t the r e a c t o r s c a l e s up l i n e a r l y , i t was c a l c u l a t e d t h a t 36 r e a c t o r s hav i ng an e l e c t r o d e a rea o f 1 m 2 w i t h 40 c e l l s w i l l be r e q u i r e d . The c a l c u l a t i o n s a r e p r e s e n t e d i n Append ix 5. The f i x e d c a p i a l i n v e s tmen t c o s t i s shown i n Tab l e 16 and the o p e r a t i n g c o s t i n T ab l e 17 . From T a b l e 17 i t can be seen t h a t t h e c o s t o f p r o p y l e n e o x i d e i s c a l c u l a t e d a t $3 .97 per k i l o g r a m mak ing the p r o c e s s e c o n o m i c a l l y u n j u s t i f i a b l e as t h e p r e s e n t p r i c e o f p r o p y l e n e o x i d e i s $1 . 05 per k i l o g r a m . - 78 -HYDROGEN AND OXYGEN WATER PROPYLENE RECYCLE PROPYLENE SEPARATION PROCESS ASORPTION TOWER PROPYLENE OXIDE COOLER SODIUM BROMIDE SOLUTION 1 w CELL HOUSE t PROPYLENE DISTILLATION COLUMN BY PRODUCTS FIGURE 19: PROPOSED PROCESS FOR THE MANUFACTURE OF PROPYLENE OXIDE - 79 -T a b l e 15 E x p e r i m e n t a l C o n d i t i o n s Used f o r t he S c a l e Up o f P r o p y l e n e Ox ide R e a c t o r Bed t h i c k n e s s * = 3.07 x l O " 2 m P r o p y l e n e gas f l o w r a t e = 1.5 x 1 0 - 3 m 3 /min E l e c t r o l y t e f l o w r a t e = 3.0 x 1 0 _ 1 + m 3 /m in C u r r e n t ( s u p e r f i c i a l C u r r e n t d e n s i t y ) = 8A (1650A/m 3 ) E l e c t r o l y t e c o n c e n t r a t i o n = 0.5 M R e a c t o r o u t l e t t e m p e r a t u r e = 30°C V o l t a g e per c e l l = 6V Space t ime y i e l d = 127 .8 k g / m 3 hr C u r r e n t e f f i c i e n c y f o r p r o p y l e n e o x i d e = 56.5% S e l e c t i v i t y f o r p r o py l e n e o x i d e = 95% S p e c i f i c ene rgy consumpt i on = 9.8 Kwhr/kg *Bed t h i c k n e s s = t h i c k n e s s f o r f i v e beds i n c l u d i n g d i a p h r a g m s . - 80 -Table 16 Fixed Capi ta l Investment Cost for Propylene Oxide Plant having a Capacity of 10,000 Short Tons Per Year Ce l l House Cost ing (36 r e a c t o r s , 40 c e l l s / r e a c t o r , 1 m 2 /ce l l ) Item Cost $ M Compression p lates $ 270.0 E lec t rodes 72.0 Gaskets 360.0 Graphite 22.8 Diaphragms 28.8 Reactor assembly 156.0 Total reactor cost 909.6 R e c t i f i e r 1,740.0 Total c e l l house cost (4 time reactor $5,378.4 cost + r e c t i f i e r cost) The to ta l plant cost i s estimated at $22.0 M - 81 -T a b l e 17 O p e r a t i n g Cos t Co s t I tem $ M/day $ /kg Power ($0 .04/Kwh) 15 .4 0 .56 Raw m a t e r i a l - p r o p y l e n e ( $ 0 . 4 4 / k g ) - sodium bromide ( $ 1 . 0 / k g ) 12 .0 73 .5 0 .44 2 .66 L abou r 2.0 0 .07 T o t a l o p e r a t i n g c o s t 102 .9 3 .73 C a p i t a l c o s t * 6.5 0 .24 T o t a l p r o p y l e n e o x i d e c o s t 109 .4 3.97 * P l a n t d e p r e c i a t e d over 10 y e a r s ( s t r a i g h t l i n e d e p r e c i a t i o n t e c h n i q u e ) . - 82 -C H A P T E R 6 C O N C L U S I O N S P r o p y l e n e o x i d e was s y n t h e s i z e d i n a b i p o l a r e l e c t r o c h e m i c a l r e a c t o r c o n s i s t i n g o f two monopole e l e c t r o d e s c o n s t r u c t e d from p l a t i n i z e d t i t a n i u m and f o u r b i p o l e beds o f g r a p h i t e p a r t i c l e s s e p a r a t e d by p o l y p r o p y l e n e d i a p h r a g m s . In t h e o p e r a t i o n o f t h e b i p o l a r r e a c t o r i n a s i n g l e pass mode, t h e f o l l o w i n g v a r i a b l e s had a ma j o r e f f e c t on the space t ime y i e l d f o r p r o p y l e n e o x i d e . a . P r o p y l e n e gas f l o w r a t e ( i n c r e a s i n g t he f l o w r a t e r a i s e s t he space t ime y i e l d f o r p r o p y l e n e o x i d e ) . b . S u p e r f i c i a l c u r r e n t d e n s i t y ( i n c r e a s i n g the c u r r e n t d e n s i t y a l s o r a i s e s t he space t ime y i e l d f o r p r o p y l e n e o x i d e ) . c . E l e c t r o d e bed t h i c k n e s s ( d e c r e a s i n g the bed t h i c k n e s s r a i s e s t h e space t ime y i e l d f o r p r o p y l e n e o x i d e ) . The e l e c t r o l y t e c o n c e n t r a t i o n and f l o w r a t e had a v e r y sma l l p o s i t i v e e f f e c t on t h e space t ime y i e l d f o r p r o p y l e n e o x i d e . I n c r e a s e d t e m p e r a t u r e had a n e g a t i v e e f f e c t on the space t ime y i e l d f o r p r o p y l e n e o x i d e . The s m a l l e r r e a c t o r bed t h i c k n e s s , c o n s i s t i n g o f Un ion C a r b i d e g r a p h i t e and p r o p y l e n e gas f l o w r a t e o f 1500 cm /min (STP) a t a s u p e r f i c i a l c u r r e n t d e n s i t y o f 2282 A / m 2 gave the bes t r e s u l t s f o r a - 83 -s i n g l e pass o p e r a t i o n . At t h e s e c o n d i t i o n s the space t ime y i e l d f o r p r o p y l e n e o x i d e was de t e rm ined t o be 97 .2 k g / h r m 3 w i t h a c u r r e n t e f f i c i e n c y o f 24.3% and a s e l e c t i v i t y o f 72 .0%. Fo r the r e c y c l e mode t he bes t space t ime y i e l d f o r p r o p y l e n e o x i d e was o b t a i n e d a f t e r one hour o f o p e r a t i o n . T h i s v a l u e was c a l c u l a t e d t o be 127 .8 kg /h r m 3 . The c o r r e s p o n d i n g c u r r e n t e f f i c i e n c y and s e l e c t i v i t y were 56.5% and 95 .9%. As i t s t a n d s , t he p r o c e s s i s not e c o n o m i c a l l y j u s t i f i a b l e as t h e c o s t o f p r o p y l e n e o x i d e was d e t e r m i n e d t o be a p p r o x i m a t e l y $4 . 0 per k i l o g r a m . T h i s s ugge s t s t h a t f u r t h e r work i s r e q u i r e d wh i ch shou l d be d i r e c t e d a t i n c r e a s i n g t he space t i m e y i e l d and s e l e c t i v i t y f o r p r o p y l e n e o x i d e w h i l e m a i n t a i n i n g a r e a s o n a b l e c u r r e n t e f f i c i e n c y . Work s h o u l d a l s o be d i r e c t e d a t r e d u c i n g t h e bromide l o s s e s wh i ch impose a ma jo r c o s t f a c t o r . In c o n c l u s i o n , t h i s work shows t h a t i t i s p o s s i b l e t o p roduce p r o p y l e n e o x i d e w i t h a f a i r l y h i g h space t i m e y i e l d and s e l e c t i v i t y i n an e l e c t r o c h e m i c a l b i p o l a r t r i c k l e - b e d r e a c t o r . - 84 -CHAPTER 7 RECOMMENDATIONS The f o l l o w i n g recommendat ions a r e sugges t ed f o r f u t u r e work i n t h e s y n t h e s i s o f p r o p y l e n e o x i d e : a . The p r o c e s s s hou l d be i n v e s t i g a t e d a t a c o n t r o l l e d e l e c t r o l y t e pH. b. To i n c r e a s e p r o p y l e n e s o l u b i l i t y i n t he e l e c t r o l y t e , o p e r a t i o n o f t he r e a c t o r a t e l e v a t e d p r e s s u r e s s hou l d be c o n s i d e r e d . c . S m a l l e r g r a p h i t e p a r t i c l e s s h o u l d be t e s t e d . d . D i f f e r e n t d i aph ragm and membrane m a t e r i a l s s hou l d be t e s t e d . e . 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A . , 1950 , Chem i ca l E n g i n e e r s ' Handbook, T h i r d E d i t i o n , McGraw H i l l . - 87 -APPENDIX 1 POTENTIAL DROP THROUGH A BED OF GRAPHITE AT VARIOUS CURRENTS Table A The Effect of Degree of Bed Compression on the Potent ia l Drop Union Carbide U l t r a Carbon Degree of Potential Degree of Potenti al Compression Bed Thickness Current Drop Compression Bed Thickness Current Drop (cm) (Amps) (volts) (cm) (Amps) (vo l t s ) 1.0 .06 1.0 .05 Loose 1.02 5.0 .25 Loose 1.02 5.0 .20 10.0 .50 10.0 .40 1.0 .02 1.0 .02 Med i urn 0.94 5.0 .15 Med i urn 0.94 5.0 .10 10.0 .30 10.0 .20 1.0 .02 1.0 .02 Tight 0.91 5.0 .11 Tight 0.91 5.0 .10 10.0 .24 10.0 .18 P a r t i c l e Size = 1.168 - 1.68 mm. Cross Sectional Area = 48.4 cm 2 . - 89 -APPENDIX 2 ANCILLARY EQUIPMENT - 90 -1. Ammeter T r i p l e t , Model number 4 2 0 , D .C . amperes . 2 . A b s o r p t i o n Tower One tower 24 i n c h e s l o n g and 1 i n c h i n d i a m e t e r f i l l e d w i t h 1/4 i n c h b e r l s a dd l e r i n g s . 3 . C i r c u l a t o r C o l o r a t e m p e r a t u r e b a t h , Model number 34375 . 4 . Condense r 2 f t o f 1/4 i n c h s t a i n l e s s s t e e l tube e n c l o s e d by 1/2 i n c h c oppe r t u b e . 5 . Feed Tank 50 l i t r e s o f Na lgene p o l y e t h y l e n e t a n k . 6 . . F i t t i n g s Swagelok 316 S . S . and n y l o n . 7 . Heat Exchange r 2 f t o f 1/4 i n c h s t a i n l e s s s t e e l t ube e n c l o s e d by 1/2 i n c h coppe r t u b e . 8 . Mi x e r T a l b o y s E n g i n e e r i n g C o r p . Model Number 104 . 9 . pH Me t e r C o l e Parmer I n s t r u m e n t s C o . , Model Number 5986-00 Combined e l e c t r o d e . 1 0 . Power Supp l y S o r e n s e n , S . R . L . 20-25 S o r e n s e n , D .C .R . 40-25B 1 1 . P r e s s u r e Gauge Marsh mas te rgauge t y p e 100-3SS 316 S . S . 1 2 . Pumps E a s t e r n D - l l c e n t r i f u g a l pump 316 S . S . 1/8 H.P. m o t o r . - 91 -1 3 . Reagen t s P r o p y l e n e - Canad ian L i q u i d A i r L t d . , C P . g rade 99.0% p r o p y l e n e . Sodium Bromide - BDH L t d . , 99.0% p u r e . Water f i l t e r e d and d e i o n i z e d . 14 . Ro t ame t e r s E l e c t r o l y t e B r o o k s , tube s i z e R-6-15-A F l o a t S . S . Gas B r o o k s , tube s i z e R-2-15-D F l o a t S . S . and g l a s s Water B r o o k s , tube s i z e 4-65 F l o a t S . S . 1 5 . S e p a r a t o r 1 f t . l o ng and 3.5 i n c h e s i n d i a m e t e r ( S . S . ) w i t h S . S . f l a n g e on both e n d s . 1 6 . Thermometers F i s h e r S c i e n t i f i c C o . , U . S . A . , Range -10 to 110°C. 17 . T ub i ng P o l y - f 1 o w - 1 / 4 i n ch and s t a i n l e s s s t e e l 1/4 i n c h t u b e . 1 8 . V a l v e s Wh i tey and Nupro 1/4 i n c h S . S . 1 9 . Wet T e s t Me t e r P r e c i s i o n wet t e s t me t e r , MFD by P r e c i s i o n S c i e n t i f i c C o . , C h i c a g o , U . S . A . - 92 -APPENDIX 3 CALIBRATION CURVES - 93 -Table B C a l i b r a t i o n of the E l e c t r o l y t e Rotameter Float Elevat ion Flow - cm3/min 1 29 4 135 7 237 10 338 13 438 15 499 Table C C a l i b r a t i o n of the Propylene Gas Rotameter Flow - cm3/min Float Elevat ion Del ivery Pressure = 8 psig 1 101 3 291 5 505 7 700 9 883 11 1,081 13 1,299 15 1,495 Table D C a l i b r a t i o n of the Water Rotameter Float Elevat ion Flow - cm3/min 0.5 32 1.0 63 2.0 126 3.0 190 4.0 252 - 94 -a CD L O CD CD e CD u u QJ +—' ft) or o CD CD CN CD CD i i i i 1 1 1 1 r ROTAMETER-BROOKS, TUBE SIZE R-6-15-A, FLOAT S.S [-ELECTROLYTE CONCENTRATION - 0.5 M _ / -O I 0 . 0 3.0 6.0 F loat 9.0 E leva t ion 12.0 15.0 FIGURE A: CALIBRATION CURVE FOR THE ELECTROLYTE FLOW - 95 -3-° 6.0 9.0 12.0 15.0 F loa t E levat ion FIGURE B: CALIBRATION CURVE FOR THE PROPYLENE GAS FLOW - 96 -ro o CD <_) CO QJ +-< ro o or e? CN O CD CO CD CD 1 1 I I I 1 1 ROTAMETER-BROOKS, TUBE SIZE 4 .65, FLOAT S.S. -TEMPERATURE - 20°C 0.0 1.0 2.0 3.0 F loat E leva t ion 4.0 FIGURE C: CALIBRATION CURVE FOR THE WATER FLOW - 97 -DATE: 25 MAY 1984 0-2 0.4 0.6 0.8 1.0 Propylene Oxide In Water vol. % FIGURE D: CALIBRATION CURVE FOR PROPYLENE OXIDE - 98 -APPENDIX 4 TABULATED EXPERIMENTAL RESULTS, ANALYTICAL TECHNIQUE AND SAMPLE CALCULATIONS - 99 -4.0 Propylene Oxide Upon l e a v i n g the c ondense r t he p r odu c t m i x t u r e was d e l i v e r e d to a 500 m l . beake r where the p r o p y l e n e o x i d e s o l u t i o n was s e p a r a t e d from t h e g a s . The gas f l o w s to the a b s o r p t i o n tower where some p r o p y l e n e and a l l t h e p r o p y l e n e o x i d e i s abso rbed i n t h e water f l o w i n g c o u n t e r c u r r e n t t o t h e gas f l o w . The gas l e a v i n g the t op o f t he tower i s sampled t h r o u g h a t e e b e f o r e b e i n g passed t h r ough t he wet t e s t m e t e r . Sample C a l c u l a t i o n 3 E l e c t r o l y t e f l ow = 300 cm /m in Gas f l o w = 1000 cm 3 /m in (STP) R e a c t o r o u t l e t t e m p e r a t u r e = 30°C Run t ime = 10 min C u r r e n t = 8 amps V o l t a g e = 34 v o l t s Bed t h i c k n e s s (5 beds + d i aph r agms ) = 8.57 x 10" m Carbon t ype = Un ion C a r b i d e E l e c t r o l y t e c o n c e n t r a t i o n = 0.5 M E l e c t r o d e a rea = 4 .84 x 1 0 - 2 m 2 Water f l o w to t he a b s o r p t i o n t owe r = 190 cm /min Gas f l o w r a t e l e a v i n g the tower = 1091 cm 3 /m in - 10U -a . Space Time Y i e l d Sjw _ P r o d u c t i o n r a t e R e a c t o r volume R e a c t o r volume = 4 .15 x 10 - 1 * m 3 S a m p l i n g f l a s k Volume = 242 m l . % p r o p y l e n e o x i d e = 0.82 A b s o r b t i o n t owe r Water f l o w r a t e = 190 cm 3 /m in % p r o p y l e n e o x i d e = .063 P r o d u c t i o n r a t e - < 2 4 2 cm 3 ) ( y i n / h r ) ( . 82 /100 ) + ( . 0 6 3 / 1 0 0 ) ( 1 9 0 c m 3 / m i n ) ( 6 0 m i n / h r ) = 19 .1 cm 3 o f p r o p y l e n e o x i d e / h r S l y = (19-1 c m 3 / h r ) ( l m 3 / l x l 0 6 c m 3 ) ( 8 3 1 kg /m 3 ) ( 4 . 1 5 x l O " 4 m 3 ) STY = 38 . 2 k g / h r m 3 b. Dibromopropane balance (F igure E) L e a v i n g S e p a r a t o r B (S t r eam 4) Vapour p r e s s u r e o f d i b romopropane a t room t e m p e r a t u r e = 5 mm Hg S t r eam 4 D ib romopropane in s t ream 4 = ( ^ Q m ^ H j j g ) (10 min) (1091 cm 3 /m i n ) = 71 .8 cm o f gas - 1 0 1 -S E P A R A T O R A COOLER i »- TO T H E A B S O R P T I O N T O W E R S E P A R A T O R B FIGURE E - 102 -1 mo le o f gas a t room t e m p e r a t u r e o c c u p i e s 24,400 c m 3 . m o l e c u l a r we i gh t o f d i b r omopropane = 201 .9 g/mole d e n s i t y o f d i b r omopropane = 1.9 g / cm 3 ( 7 1 . 8 c m 3 o f g a s ) ( 2 0 1 . 9 g/mo le) ( 24 ,400 cm 3 o f g a s / m o l e ) ( 1 . 9 g/cm 3 ) = 0 .31 cm o f l i q u i d d i b romopropane i n s t r e am 4 S t ream 3 Volume o f d i b r omop ropane as a s e p a r a t e phase = 0 .20 cm 3 Amount o f d i s o l v e d i n t he p r o p y l e n e o x i d e s o l u t i o n = ( 0 . 2 5 g DBP) ( g 2 m 3 ) (1 g H ? 0) (1 c m 3 DBP ) 100 g H 2 0 c m 3 H 2 0 1.9 g DBP = 0.12 cm 3 T o t a l d i b r omop ropane i n steam 3 = 0 .32 cm 3  S t r eam 2 T o t a l d i b r omopropane in steam 2 - ( - 0 1 5 c " 3 D B P ) ( 1 0 m i n ) 150 cm 3 min = 0.30 cm 3 S t r e am 1 Steam 1 = Steam 2 + Steam 3 + Steam 4 T o t a l d i b r omop ropane p roduced = 0 .93 cm - 103 -c . P r o p y l e n e B a l a n c e ( i ) Mo les o f p r o p y l e n e e n t e r i n g (gas f l o w a t STP) = (1000 cm 3 /m in ) (10 m in) a 0 # 4 1 g m Q l e ( 24 , 400 cm 3 /mo le) ( i i ) p r o p y l e n e consumed t o p roduce d i b romopropane and p r o p y l e n e o x i d e _ (0 -93 cm 3 o f DBP ) ( 1 . 9 g / c m 3 ) ( 201 .9 g/mole) , (19 .1 c m 3 / h r ) ( l h r / 6 0 m i n ) ( 1 0 m i n ) ( . 8 3 g/cm 3) ( 58 .0 g /mo l e ) = .054 mo les ( i i i ) p r o p y l e n e d i s s o l v e d i n s e p a r a t o r A u n d e r f l o w ( F i g u r e E) The s o l u b i l i t y o f p r o p y l e n e a t 83°C = .0032 g/lOOg o f wa te r p r o p y l e n e d i s s o l v e d i n t h e e l e c t r o l y t e = (300 cm 3 /m i n ) (10 m i n ) ( ' 0 0 3 2 9 P ) ( l J J i ^ ) ( 1 m o 1 e P ) 100 g H 2 0 c m 3 H 2 0 42 g P = .0023 mo les ( i v ) p r o p y l e n e d i s s o l v e d i n s e p a r a t o r B. = ( 9 2 c m 3 ) C 0 4 4 9 P ) ( ! 9 H ? ° ) ( 1 M O 1 E P ) 100 g H 2 0 cm 3 H 2 0 42 g P = .0010 moles ( v ) p r o p y l e n e d i s s o l v e d i n t he tower u n d e r f l o w - M q n c m 3 w 1 n _ i r i w . 0 4 4 q P w l q H o O w l mo le P\ " ( 1 9 ° mTn-){1° m i n ) ( 100 g H 2 0 ) ( l c M ) ( 42 g P } = .0198 moles - 104 -SOLUBILITY OF PROPYLENE IN WATER AT 1 ATM. ABSOLUTE CM * X cn n —i o CO •0 20.0 40.0 60.0 80.0 Temperature (Degrees Centigrade) FIGURE F - 105 -(v i) Propylene leaving in the gas. Since the GC only detects N 2 , H 2 , 0 2 , CHit, CO, and C02 the fol lowing technique was used. F i r s t 1 ml of nitrogen gas i s injected into the gas chromatograph to determine i t s p u r i t y . Next 4 ml of gas sample i s in jec ted . F i n a l l y 1 ml of nitrogen gas and 4 ml of the gas sample are combined and analyzed. Nitrogen (1 ml) 1 ml of gas analyzed has 99.6% nitrogen and .4% oxygen. Gas Sample (4 ml) 4 ml of gas sample contained 93.578% hydrogen, 3.634% oxygen and 2.788% n i t rogen . Combined Sample (5 ml) 5 ml of the combined gas sample contained 37.694% hydrogen, 2.109% oxygen and 60.197% n i t rogen . Let P = propylene, H = hydrogen, 0 = oxygen, N = nitrogen A l l units are in ml For the gas sample x = m l . of gas sample detected H/x = .93578, 0/x = 0.0364, N/x = .02788 (1) 4.0 - x = P For nitrogen gas sample injected = 1 ml . - 106 -N = 0 .996 ml 0 = 0.004 ml F o r the combined sample l e t Z = ml o f sample d e t e c t e d . N b a l a n c e : 0.60197 Z = .996 ml + N i n gas sampl 0 b a l a n c e : 0 .02109 Z = .004 ml + 0 i n gas sampl H b a l a n c e : 0.37694 Z = H i n gas sample Z = 1 ml o f n i t r o g e n gas sample + x ml o f gas sample From E q . (3) .60197 (1 + x) = .996 + N From E q . (1) N/x = .02788 S u b s t i t u t e E q . (7) i n t o E q . (6) .60197 (1 + x) = .996 + .02788 x S o l v i n g E q . (8 ) g i v e s x = ( .996 - . 6 0 1 9 7 ) / ( . 6 0 1 9 7 - .02788) x = 0 .686 ml - 107 -amount o f p r o p y l e n e l e a v i n g i n the gas = 4.0 - 0.686 = 3.314 ml = (3.314/4.0)(100) = 82.85% moles o f p r o p y l e n e l e a v i n g i n gas = ' a?o.G.\ 0.370 moles (1091 cm 3 /min ) (10 min) ( .8285) (24400 cm 3/mole) Total moles in - 0.416 moles Total moles out = 0.447 moles % di f ference = 7 .45% d . Propylene oxid^ y i a l d - propylene o x i d e produced propylene reed prooylene oxide y i e l d = ( 1 9 a c m 3 / h r ) ( l hr/60 min)(10 min)(.83 a/< T^07iriolTT7jDrrFlTO = .1095 or 10,95% propylene consumed in the production e. Propylene conversion = o f propylene oxide and dibromopropane propylene 'feel] .054 moles_ Pes propylene conversion =• *l!^ ,JI!2JL!=L 0.416 mol = 0.1298 or 12.98% f. Propylene oxide s e l e c t i v i t y = P r o p y l e n e oxide y i e l d propylene conversion , = .1095/ . 12% - 108 -= .8436 o r 84.36% g . C u r r e n t e f f i c i e n c y and s p e c i f i c ene rgy consumpt i on c u r r e n t e f f i c i e n c y o f p r o p y l e n e o x i d e - ( .0456 m o l e s ) ( 2 ) ( 9 6 5 0 0 c o u l / e q . ) '" (10 mm) (60 s e c / m i n ) ( 8 A m p s / c e l l ) (5 c e l l s ) = 0.3667 o r 36.67% S p e c i f i c ene rgy consumpt i on o f p r o p y l e n e o x i d e SF - ( 2 6 . 8 ) ( Z ) ( V )  5 L (MW)(CE) MW = m o l e c u l a r we i gh t CE = c u r r e n t e f f i c i e n c y V = v o l t a g e d r o p / c e l l Z = number o f e l e c t r o n s SF - ( 2 6 . 8 ) ( 2 ) ( 3 4 V/5)  b t (bB g / m o l e ) ( . 3 6 6 / ) SE = 17 . 1 kwhr /kg S i m i l a r l y t h e c u r r e n t e f f i c i e n c y and t he s p e c i f i c energy consumpt i on f o r h y d r o g e n , n i t r o g e n , oxygen and d i b r omop ropane were d e t e r m i n e d . - 109 -T ab l e E Compound C u r r e n t E f f i c i e n c y % S p e c i f i c Energy Consumpt ion kwhr/kg Oxygen 3.6 3150.6 Hydrogen 58 .7 1553.7 P r o p y l e n e o x i d e 36 .7 85 . 7 D ib romopropane 7.1 127.1 4.1 H y p o b r o m i t e , B r o m i t e a n d B r o m a t e On l e a v i n g t he r e a c t o r t he l i q u i d was c o l l e c t e d i n a 150 ml beake r and a n a l y z e d f o r h y p o b r o m i t e , b r o m i t e and bromate as f o l l o w s : a . F i r s t a 0 .1 N a r s e n i o u s o x i d e s o l u t i o n was p r epa r ed (26) and t i t r a t e d w i t h a s t a n d a r d 0.1 N s o l u t i o n o f i o d i n e t o d e t e r m i n e t h e e x a c t n o r m a l i t y . 0.01 N sodium t h i o s u l p h a t e s o l u t i o n was a l s o p r e p a r e d . b . To a 5 ml o f m i x t u r e c o n t a i n i n g the h y p o b r o m i t e , b r o m i t e and b r oma t e , was added 3 to 4 grams o f s o l i d p o t a s s i um i o d i d e , f o l l o w e d by 10 ml o f 4 N s u l p h u r i c a c i d . The s o l u t i o n was d i l u t e d to t w i c e i t s vo l ume , and t he l i b e r a t e d i o d i n e was t i t r a t e d w i t h t h i o s u l p h a t e u s i n g s t a r c h as t h e i n d i c a t o r . c . 5 ml o f t h e m i x t u r e was added t o a f l a s k c o n t a i n i n g an e x c e s s o f ammonium s u l p h a t e and about 1 gram o f sod ium b i c a r b o n a t e . A f t e r 10 m i n u t e s , 3 to 4 grams o f s o l i d p o t a s s i um i o d i d e and 10 ml o f 4 N s u l p h u r i c a c i d was added . The s o l u t i o n was l e t s t and f o r 5 m i nu t e s b e f o r e i t was d i l u t e d to t w i c e i t s volume and t i t r a t e d w i t h t h i o s u l p h a t e . - 110 -d . To 5 ml o f m i x t u r e , a known ex ce s s o f a r s e n i o u s o x i d e was added (4 m l ) . A f t e r 5 m i n u t e s , 4 t o 5 grams o f sod ium b i c a r b o n a t e was added . The s o l u t i o n was t i t r a t e d w i t h i o d i n e u s i n g s t a r c h as t he i n d i c a t o r . e . 5 ml o f m i x t u r e was added t o a f l a s k c o n t a i n i n g an ex ce s s o f ammonium s u l p h a t e and about 1 gram o f sod ium b i c a r b o n a t e . A f t e r 10 m inu t e s a known e x c e s s o f a r s e n i o u s o x i d e was added (4 ml) and the s o l u t i o n was l e t s tand f o r 5 m i nu t e s b e f o r e t i t r a t i n g w i t h i o d i n e . The e q u a t i o n s f o r the r e a c t i o n s a r e : F o r (b) BrO" + 21 " + 2 H + • I 2 + B r " + H 2 0 (1) B r 0 2 " + 4 1 " + 4 H + 2 I 2 + B r " + 2 H 2 0 (2) B r 0 3 ~ + 6 1 " + 6H + • 3 I 2 + B r " + 3H 2 0 (3 ) Fo r ( c ) pH = 8-9 4BrO" + 2HH^ ^ 4Br~ + N 2 + 4H 2 0 (4 ) B r 0 2 - + N H ^ ^ no r e a c t i o n (5 ) B r 0 2 " + 4 1 " + 4 H + • 2 1 2 + + B r " + 2H 2 0 (6 ) B r 0 3 ~ + N H 4 + • no r e a c t i o n (7 ) B r 0 3 " + 6 1 " + 6H + • 3 I 2 + B r " + 3H 2 0 (8) F o r (d ) B rO" + A s 0 3 " 2 • A s 0 3 - 3 + B r " (9) B r 0 2 " + 2 A s 0 3 " 2 ^ 2 A s 0 4 " 3 + B r " (10) B r 0 3 " + 3 A s 0 3 " 2 »- no r e a c t i o n (11) - I l l -Fo r (e) pH=8-9 4BrO" + 2NH„ + pH = 8-9 4 B r " + N 2 + 4 H 2 0 (12) B r 0 2 " + NHL>+ • no r e a c t i o n (13) B r 0 2 ~ + 2 A s 0 3 _ 2 ^ B r " + 2AS0 , , - 3 (14) B r 0 3 - + N H ^ »- no r e a c t i o n (15) B r 0 3 " + A s 0 3 " 2 no r e a c t i o n (16) Sample C a l c u l a t i o n (Box T , T a b l e 7) ( i ) T o t a l h y p o b r o m i t e , b r o m i t e , bromate p r e s en t i s c a l c u l a t e d f rom p r o c e d u r e (b) and Eq . ( 1 ) , ( 2 ) , and ( 3 ) . Ave rage volume o f t h i o s u l p h a t e used = 3.25 ml mo les o f S 2 0 3 2 ~ used = ( .01 m o l e s / L ) ( 3 . 2 5 L / 1 0 0 0 ) ( 1 0 5 ymo l e s /mo l e ) = 32 .5 ymoles 2 S 2 0 3 2 " + I 2 ^ 21 - + S 4 0 6 2 " (17) mo les o f I 2 = 32 .5 pmo les /2 = 16 .25 ymo l e s . From E q u a t i o n s ( 1 ) , (2) and (3) t he r a t i o o f I 2 : B r O " , B r 0 2 " , B r 0 3 " = 2 : 1 . mo l e s o f B r O " , B r O ? ~ , B r 0 3 ~ = 8 . 13 ymoles ( i i ) T o t a l b r o m i t e and bromate i s c a l c u l a t e d from p r o c edu r e (c ) and E q u a t i o n s (4) to ( 8 ) . - 112 -Average volume of thiosulphate used = 1.75 ml moles of S 2 S0 3 _ used = 17.5 umoles moles of I 2 = 8.75 umoles Ratio of I 2 : B r 0 2 _ , B r 0 3 " = 5:2 moles of BrO?~, B r O q " = 3.50 umoles ( i i i ) Total hypobromite and bromite is ca lculated from procedure (d) and Equation (9) to (11) . Average volume of iodine used = 3.7 ml A s 0 3 2 - + I 2 + H 2 0 A s 0 4 3 - + 21" + 2H + (18) moles of A S O 3 2 - present i n i t i a l l y = (0.1012 N)(1M/4N)(4.0 L/1000)(10 6 ymol es/mole) = 101.2 umoles moles of I 2 used = = (3.7 L /1000) ( . l N)(1M/2N)(10 6 umoles/mole) = 185.0 umoles moles of As0 3 - unreacted from Equation (18) = 185.0 umoles/2 =92 .5 umoles moles of A s 0 3 2 _ reacted = 101.2 - 92.5 = 8.7 umoles Ratio of A s 0 3 2 - : B r O " , B r 0 2 _ from Equations (9) and (10) = 3:2 moles of Br0~, BrQ 2 " = 5 . 8 umoles - 113 -( i v ) Mo les o f b r o m i t e i s d e t e rm ined from p r o cedu r e (e) and E q u a t i o n s (12) t o ( 1 6 ) . A v e r age volume o f i o d i n e used = 3 .8 ml mo les o f A s 0 3 2 - i n i t i a l l y p r e s e n t = 101.2 ymoles mo les o f I 2 used = 190 ymo les mo les o f A s 0 3 2 " u n r e a c t ed = 95 ymoles mo les o f A s 0 3 2 _ r e a c t e d = 6 .2 ymoles R a t i o o f A s 0 3 2 - : B r 0 2 " = 2:1 mo l e s o f B r 0 2 " = 3.1 y m o l e s . Mo les o f h ypob r om i t e = i - i i = 8 . 13 - 3.50 = 4 .63 ymo les mo l e s o f bromate = i - i i i = 8 . 1 3 - 5 . 8 = 2 .33 ymoles mo les o f b r o m i t e = 3.10 ymoles C u r r e n t e f f i c i e n c y f o r b r o m i t e ( 3 . 1 0 x I P ' 6 mo l e s / 5 m l ) ( 1 0 0 m l / m i n ) ( 4 ) ( 9 6 5 0 0 c o u l / m o l e ) (60 s e c / m i n ) ( 1 0 amps) 0 .04 o r 4.0% S i m i l a r l y t h e c u r r e n t e f f i c i e n c y f o r hypob rom i t e and bromate was d e t e r m i n e d . Tab l e F Compound ymo les C u r r e n t E f f i c i e n c y % Hypob rom i t e 4 . 63 3.0 B r o m i t e 3 . 10 4 .0 Bromate 2 . 33 4 . 5 - 114 -4.2 Recycle F o r the r e c y c l e mode the m i x t u r e l e a v i n g the r e a c t o r was c i r c u l a t e d back to the feed t a n k . The gas l e a v i n g t he feed tank was ana l y z ed f o r p r o p y l e n e o x i d e and the amount was de t e rm ined to be n e g l i g i b l e . A f t e r one hour o f o p e r a t i o n t he m i x t u r e was passed t h r ough the sys tem ( h ea t e x c h a n g e r , s e p a r a t o r and c onden se r ) a t t he same f l o w r a t e as t he r e c y c l e f l o w . The t e c h n i q u e d e s c r i b e d in S e c t i o n 4 .0 was employed t o d e t e r m i n e t h e space t ime y i e l d . The same p r o c e d u r e was used to d e t e r m i n e the space t ime y i e l d a f t e r 2, 3 and 4 hous o f o p e r a t i o n . F i r s t Hour R e a c t o r bed t h i c k n e s s = 8 .57 cm E l e c t r o l y t e f l ow r a t e = 300 cm 3 /m in I n i t i a l vo lume = 26 L C u r r e n t = 8 A Tempe ra tu r e = 30 - 36°C E l e c t r o l y t e C o n c e n t r a t i o n = 0 .5 M P r o p y l e n e gas f l o w r a t e = 1500 cm 3 /m in (STP) T o t a l vo lume r e c y c l e d = (300 c m 3 / m i n ) ( 6 0 m i n / h r ) ( l L/1000 c m 3 ) ( l hr) = 18 L Assume a l l t he p r o p y l e n e o x i d e was produced d u r i n g the r e c y c l e . U s i n g the same a n a l y t i c a l t e c h n i q u e d e s c r i b e d i n 4 . 0 the p r o p y l e n e o x i d e p r oduced was d e t e rm i ned t o be 2 4 . 3 c m 3 / h r . - 115 -Volume o f p r o p y l e n e o x i d e in feed tank = (24 .3 c m 3 / h r ) ( 2 6 L/18 L ) ( l h r ) = 35 . 1 cm 3 Space t ime y i e l d i n the f i r s t hour = ( 35 .1 c m 3 / h r ) ( . 8 3 1 g / c m 3 ) ( l kg/1000 g) ( 4 . 15 x l O - 4 m 3 ) = 70 .3 k g / h r m 3 Second Hour T o t a l produced i n = amount produced + amount p roduced 2 hours i n the 1s t hour i n the 2nd hour a Aga i n u s i n g the t e c h n i q u e s o f 4 .1 the t o t a l p r o p y l e n e o x i d e p roduced was d e t e r m i n e d to be = 37 . 0 c m 3 / h r . T o t a l volume a f t e r 1 hour = 20 l i t . T o t a l p r o p y l e n e o x i d e produced in t he second hour = ( 37 .0 - 24 .3 ) c m 3 / h r = 12.7 c m 3 / h r C o n c e n t r a t i o n o f p r o p y l e n e o x i d e in feed tank = (12 .7 c m 3 / h r ) ( 2 0 L/18 L ) ( l h r ) = 14 .1 cm 3 Space t ime y i e l d = 28 .3 k g / h r m 3 Th i rd Hour T o t a l p r o p y l e n e o x i d e produced = 4 9 . 3 c m 3 / h r T o t a l volume a f t e r 2 hours = 14 L Space t ime y i e l d = 19.2 k g / h r m 3 - 116 -F o u r t h Hour T o t a l p r o p y l e n e o x i d e produced = 62 .5 c m 3 / h r T o t a l vo lume a f t e r 3 hours = 8 L Space t ime y i e l d = 1 1 . 8 k g / h r m 3 Tab l e G Time Space Time Y i e l d pH ( h o u r s ) k g / h r m 3 1 70 . 3 10 .58 2 28 . 3 10 .65 3 19 .2 10.70 4 11 .8 10.99 T a b l e H Time Space Time Y i e l d pH ( h o u r s ) k g / h r m 3 1 41 .4 10 .45 2 20 .6 10 .55 3 13 .1 10 .63 . C o n d i t i o n f o r Tab l e H: Bed T h i c k n e s s = 8.57 cm E l e c t r o l y t e f l o w r a t e = 300 cm 3 /m in I n i t i a l vo lume = 14 L C u r r e n t = 5A Tempe ra tu r e = 29 - 32°C E l e c t r o l y t e C o n c e n t r a t i o n = 0 .5 M P r o p y l e n e gas f l o w r a t e - 1500 cm /min (STP) - 117 -Tab le I Time Space Time Y i e l d PH To t a l Bromates i n ( hou r s ) kg /h r m 3 umoles 1 127.8 10.52 53.4 2 44 . 3 10.96 70 .5 C o n d i t i o n f o r Tab le I: Bed T h i c k n e s s = 3.07 cm E l e c t r o l y t e f l ow r a t e = 300 cm 3 /min I n i t i a l volume = 14 L C u r r e n t = 8A Tempera ture = 30 - 32°C E l e c t r o l y t e C o n c e n t r a t i o n = 0.5 M P r opy l e ne gas f l ow r a t e = 1500 cm /min (STP) Table J The Ef f e c t of Current and Propylene Gas Flow on the Space Tine Y i e l d and, Energy Consumption for Propylene Oxide and Current E f f i c i e n c i e s Current pH Gas Flow Space Time Energy Current E f f i c i e n c y - % S e l e c t i v i t y Ainps cm3/min Y i e l d Consumption Percent (STP) kg/hr m3 Kwhr/kg Propylene Di bromopropane Oxygen Hydrogen Oxide 2 11.45 1000 14.8 5.9 56.5 24.1 0.0 62.6 70.1 5 11.42 1000 32.6 10.0 49.9 9.9 1.1 62.4 83.5 8 11.40 1000 38.0 17.3 36.4 7.0 3.6 56.9 83.9 11 11.46 1000 26.6 40.1 18.6 7.0 4.5 58.3 72.5 5 11.50 1500 38.0 9.2 58.2 12.9 1.6 59.4 81.8 8 11.53 1500 54.2 13.1 51.9 12.1 3.0 62.1 81.1 11 11.60 1500 60.7 17.4 42.3 8.6 3.8 58.0 83.1 13 11.45 1500 44.8 31.2 26.4 8.4 4.9 56.9 76.0 2 11.35 2000 12.4 7.0 47.6 29.6 _ _ 61.8 5 11.27 2000 28.5 12.7 43.6 13.9 2.9 56.3 75.8 8 11.38 2000 24.2 31.1 23.2 7.7 5.5 60.9 75.2 Conditions: Graphite P a r t i c l e Size = 1.168 - 1.68 m Liquid Flow Rate = 300 cc/min Graphite Type = Union Carbide Pressure = 1.4 - 2.2 atm Temperature = 23 - 35°C Red Thickness = 8.57 cm Concentration = 0.5 M •o ci n r> o H -j -i o* c o to to ot -» -> 3 3 W D 1 O " O ul 7 0 t • • C 3 3 D» to d o I 00 3 r\j ^ - - 1 • cr at —•• rt 3 rr> 3 3 WOT! —I SO o -c -o O O "O "O -• -> Df cr o o -i *o ID o *< 3 — H O U -~-0 3 3 -j ID (6 o xi o —< O* X -•- 3 — ID (ft CL —• to C L E l e c t r o l y t e F low cn3/r»1n 100 G a s F l o w o n /m1n 100 N n n R n n cr, \o *-* cn ro • • cn» o • 0 3 ( D * PO • O W H O 00 fr* » * Q 3 O O »* 00 - J ro >* o 3 1000 n n n H fl it co ro co O J ro • o oo • -o • UQ • • * CT» CO 00 "^ J O J »* ** O 3 7** \ r r zr 3 n n n n R H >JvO iO N 00 O J * • CTi • CO • cO CO • O0 • -t* y*. K£> O X » * Q 300 G a s F l o w cm /m1n 100 u n n B M n oo • J > O J ro • • cO • • • O J r o oo o * o •9* O J ** zr ~o CTi »~* cn ro • O J • -O CTv • CO O oo co zr zr -j ~> 3 N J M ^ H» yi w ro O • 00 • O J • • rjO * VJO • - J o »* o cn •&+ o 3 7*-1000 COT3 < oo <I> ZC C C O — 1 o ro —* -j -» -< —» a. n -J - i -o c o ft) iD -J 3 —l ZI 3 O n> zr r t r> a —*• < e o ro r& o 3 ooZJ *< cx —• fD V> U» o ~o oo O -o n II n H H R R R CO •—' CO x» • O J 4 * 00 O J •—» • co ro CD • • * cn J> w x «JO ro tn cn x o *« O * ~ X Q 3 CO 3 zr O zr -J -i H X N R H R ^ 1 M <*) H J > CT\ • O J > • J > • oo * • ro * ai ro oi J> co >< X ^ C j ^ zr zr »—*•—» ro co cn ro o o oo • ro • • • • O J • OS O 00 f * >* O J >« »t g _^ zr zr ^ -» 3 ro -X* o r o n N CO a. 3 IV CL -s o 10 3 to o - 6IT -Table L Effect of Bed Thickness, Superficial Current Density and Propylene Gas Flow Rate on the Space Time Yield for Propylene Oxide Bed Thickness - cm 3.07 (volume = 1.49 x lO"") 8.57 (volume • 4.15 x 10-'' m3) Current Araps Current Amps 5 8 11 2 5 8 11 13 Space time y i e l d (kg/hr m3) = 56.9 pH « 10.73 S e l e c t i v i t y % «= 69.5 79.1 10.98 74.4 86.5 11.03 75.5 12.4 11.35 61.8 28 .5 11.27 75.8 24.2 11.38 75.2 - -o C Current E f f i c i e n c y c E CM Propylene oxide » 31.3 Dibromopropane = 13.ft PO Prod, rate » 10.2 27.2 9.3 14.2 21.6 7.1 15.5 47.6 29.6 5 .6 43.6 13.9 14.3 23.2 7.7 12.3 : -m 6 i S o uZ V ) IV o o r—* 63.4 10.98 73.5 34.9 12.6 11.35 85.2 10.92 71.2 29.3 11.8 15.3 97.2 10.83 72.0 24.3 9.5 17.4 - 38.0 11.50 81.8 58.2 12.9 19.0 54.2 11.60 81.1 51.9 12.1 27.0 60.7 11.60 83.1 42 .3 8.6 30.3 44.8 11.45 76.0 26.4 8.4 22 .3 52.1 10.03 61.7 10.87 56.2 10.76 14.8 11.45 32.6 11.40 38.0 11.40 26.6 11.46 O c o 72.6 28.7 10.8 9.3 67.1 21.2 10.4 11.0 57.8 14.0 10.2 10.1 70.1 56.5 24.1 7.4 83 .5 49.9 9.9 16.2 83.9 36.4 7.0 19.0 72 .5 18.6 7.0 13.3 -Conditions: Graphite partial Size = 1.168 - 1.68 mm Liquid Flow » 300 cc/m1n Graphite type = Union Carbide Pressure • 1.4 - 2.2 atm. Temperature = 28-35°C Area • 48.4 cm Concentration = 0.5 M - 121 -APPENDIX 5 COSTING EXCERCISE - 122 -Stream factor Space time y i e l d Monopole e lectrode area Reactor volume Bed thickness Graphite density Poros i ty S p e c i f i c surface area of carbon Graphite p a r t i c l e size Temperature Weight of graphite in the reactor Applied current Appl ied voltage Current densi ty Voltage/cel l E l e c t r o l y t e Cone. E l e c t r o l y t e flow rate Gas flow rate G/L Ratio CE propylene oxide Production rate = 0.9 = 127.8 kg/hr/m3 (reactor vol) = 4.84 x 10" 3 m 2 = 1.5 x lO"*4 m3 = 3.07 x 10" 2 m = 1.92 x 10 3 kg/m3 = 0.4 = 2400 m 2/m 3 = 1.168 - 1.68 mm = 30°C = 0.172 kg = 8A = 30V = 1650 A/m3 = 6V = 0.5 m = 3.0 x 1 0 - 4 m3/min = 1.5 x l O " 3 m3/min (8 psig and 20°C) = 5 = 56.5% = (STY) (Reactor Vol) = (127.8 kg/hr m3) (1.5 x 10"1* m 3) = 1.92 x 1 0 - 2 kg/hr - 123 -Capacity = 10,000 short tons/yr _ 2.0 x 10 7 l b s / y r " (2.2 lbs /kg)(365.25 day /yr)(24 hrs/day)(0.9) = 1.15 x 10 3 kg/hr Reactor Vol = 1.5 x 10 _ l* m3 Production rate = 1.92 x 1 0 - 2 kg/hr Reactor Vol = x Production rate = 1.15 x 10 3 kg/hr x = 9.0 m3 S i m i l a r l y the gas flow was determined. R e a c t o r D i m e n s i o n s Elec t rode length = 1 m Electrode width = 1 m Electrode area = 1 m2 per c e l l bed thickness = 3.07 x 1 0 - 2 m Therefore 40 b ipo la r c e l l s bed thickness = 0.25 m Reactor Vol = (1 m 2 ) ( .25 m) = 0.25 m3 No. of reactors required = 9 m 3/0.25 m3 = 36 reactors each cons i s t ing 40 c e l l s C o s t o f a S i n g l e R e a c t o r Compression plates (constructed from iron) inc luding bolts and plumbing 2 2 cost = $2,500/m . Approximately 3 m is r equ i red . - 124 -C o s t c p = (3 m 2)($2,500/m 2) = $7,500 Monopole electrodes constructed from p l a t i n i z e d titanium cost $ l , 0 0 0 / m 2 . Cost E = (2 m 2 ) ($l ,000/m 2 ) = $2,000 Diaphragm cost = (40)(1 m 2)($20/m 2) = $800. Wt. of graphite required = (1.92 x 10 3 kg/m 3 )(0.6)(0.25 m3 288 kg. Graphite cost = (288 kg)($2.20/kg) = $634. Neoprene gasket cost = ($100/m2)(100 m2) = $10,000. S ing le reactor material cost = $20,934. No. of reactors required = 36 Total reactor material cost = $753,624. Parameters for a large scale reactor Electrode area = 1 m 2 Current dens i ty = 1650 A/m 2 E l e c t r o l y t e cone. = 0.5 m Current = 1650 A V o l t a g e / c e l l = 6V Reactor v o l . =0 .25 m3 Poros i ty = 0.4 - 125 -No. of c e l l s / r e a c t o r = 40 Bed thickness for 40 c e l l s = 0.25 tn G/L r a t io = 5 Applied voltage = 240 V Propylene gas flow rate = 2 . 5 m 3/min E l e c t r o l y t e flow rate = 0.5 m 3/min CE of propylene oxide = (29.4 c m 3 / h r ) ( l hr)(.831 g /cm 3 ) ( l m/58 g) _ (0.4212 moles)(2)(96500 coul /eq . ) " (3600 sec) (40 V) = 56.5% CE of dibromopropane = (1.93 c m 3 / h r ) ( l hr)(1.9 g /cm 3 ) ( l m/201.9 g) = (.0182 moles)(2)(96500 coul /eq . ) (3600 sec) (40 V) = 2.44% S e l e c t i v i t y = 95.9% Consumpt i on o f p r o p y l e n e (1500 cm 3/min)(60 m i n / h r ) ( l hr) 24030 OT3/nole 3.75 moles Consumption = (.4212 + .0182)/3.75 = 11.73% Propylene oxide y i e l d = 11.23% - 126 -R e c t i f i e r C o s t R e c t i f i e r cost = (100 + ^g j j 0 ) 1.44 x I0k KW = $1.74 x 10 6 This cost includes transformers, voltage regula tors , c o n t r o l s , switch gears and r e c t i f i e r s . O p e r a t i o n C o s t Power Cost = $.04/Kwhr Power requiement for 24 hrs = (480 V)(30,000 A)(24 hrs) 1UUU watts/kw) 3.46 x 10 5 Kwhr = 3.84 x 10 3 Kwhr n = 0.9 Cost = $15,400/day Raw Mater ia l s Cost Propylene = $0.20/1b NaBr = $1.00/lb Propylene required = (2.5 m /min)(36)(60 min/hr)(24 hr/day) = 1.30 x 10 5 m 3/day = 1.30 x 10 8 l i t / d a y PV = nRT (1 atm) (1.30 x 10 8 l i t / d a y ) = (n)(.082 j ^ g " ^ '(293 k) n = 5.4 x 10 6 mol es/day Consumption + losses = 12%, recycle = 88%. Make up/day = ( .12)(5.4 x 10 6 moles/day) = 6.47 x 10 5 moles/day - 127 -RECTIFIER 4 8 0 V-2 R E A C T O R S IN S E R I E S 1= 3 0 . 0 0 0 A U - - T J P R O P O S E D LAY OUT FOR T H E R E A C T O R S FIGURE G - 128 -l b s r e q u i r e d = (6 .47 x 1 0 5 m o l e s / d a y ) ( 4 2 g / m o l e s ) ( l l b / 4 5 4 g) = 6.0 x l O 4 l b s / d a y Cos t p r o p y l e n e = ( $ 0 . 2 0 / l b ) ( 6 x I0h l b s / d a y ) = $1 . 20 x 1 0 4 / d a y T o t a l B r " l o s t as h y p o b r o m i t e , b r o m i t e and bromate i n t he 1s t hou r . ( 1 8 L) ( 5 3 - 4 ( 5 ffi) m ° 1 e S ) < 1 0 0 ot3/1U) " 0 A 9 Z m o l « To t a l B r " l o s t as d i b r omop ropane , 0 . 15 cm 3 DBP» ,18 ,000 c m 3 ) ( 1 . 9 g /cm 3 * n 1 7 . . n o n ( 150 cm 3 > ( ( 201 .9 g/moTe) ) = - 0 1 7 m o 1 e s o f R a t i o B r " :DBP = 2 : 1 Mo l e s o f B r - l o s t as DBP = .034 mo les T o t a l B r " l o s t per hour = 0.226 mo les o r 5.42 m o l e s / d a y . Mo l e s e n t e r i n g the r e a c t o r / d a y = (300 c m 3 / m i n ) ( 6 0 m i n / h r ) ( 2 4 h r / d a y ) ( 0 . 5 m o l e s / L ) ( l L/1000 cm 3 ) = 216 m o l e s / d a y Make up s t r eam = ( 5 . 4 2 / 2 1 6 ) ( 1 0 0 ) = 2.5% - 129 -gram moles entering c e l l house per day = (0.5 moles/L)(500 L/min)(36 reactors)(60 min/hr)(24 hr/day) = 1.3 x 10 7 moles/day Make up stream = (.025)(1.3 x 10 7 moles/day) = 3.24 x 10 5 moles/day Cost of NaBr per day = (3.24 x 10 5 moles/day)(102.9 g/mole) (1 lb/453.6 g)($1.00/1 b) = $7.35 x 10*7day Labour 3 s h i f t s at 4 people / sh i f t 0 $15/hr Cost/day = (3)(4)(8)($15/hr) = $l ,440/day « $2 )000/day - 130 -A P P E N D I X 6 O V E R P 0 T E N T I A L FOR OXYGEN AND BROMINE OVER G R A P H I T E - 131 -The two competing reactions at the anode are: (a) B r 2 + 2e" ^ 2Br _ E 0 = 1.08 vo l t s (b) 0 2 + 4H + + 4e~ « " ^ 2H 20 E 0 = 1.23 vol t s Using Nernst equation the revers ib le potential for react ion (a) at a concentrat ion of 0.5 M was determined. S i m i l a r l y the revers ib le potent ia l for react ion (b) at an average pH (pH = 10.64) obtained in the experiments was determined. The Nernst equation for react ion a and b at 2 5 ° C are: E = 1.08 - .03 log a 3 [ B r 2 T E b = 1.23 - .015 log [ H + ] - l + The overpotent ia l for oxygen at 2 5 ° C over graphite was ca lcula ted using the Tafel Equation given in reference [28] overpotential = .4 + .17 log i where i i s in A/m 2 The bromine overpotential over graphite was obtained from reference [29] - 132 -Table M Overpotential of Oxygen and Bromine Over Graphite Oxygen: £n = 1.23 vo l t s E , = 0.59 vo l t s AGr = 113.9 KJ/mole Current Density (A/m 2) Overpotential (V) Total potential (V) 10 0.57 1.16 100 0.74 1.33 1,000 0.91 1.50 5,000 1.03 1.62 10,000 1.08 1.67 Bromine: E Q = 1.08 vo l t s E b = 1.10 vo l t s AGr = 212.3 KJ/mole Current Density (A/m 2) Overpotential (V) Total potential (V) 100 0.002 1.102 1,000 0.027 1.127 5,000 0.160 1.260 10,000 1.33 1.430 - 133 -APPENDIX 7 SOURCE TABLE RESULTING FROM FIVE FACTOR ANALYSIS Table N Source Table Resulting from Five Factor Analysis of Variance Dependent Variahle Independent Variable Propylene Oxide Current E f f i c i ency Space Time Yield Se lec t i v i t y Current E f f i c iency Sped f ic Energy Conspt. Dibromo-propane Hyd rogen Oxygen Hypobromite Bromite Bromate T 0.26 {-) 427.8 (-) 685.4 (-) 381.1 (-) A 5.86 ( + ) 1758.3 (+) • 336.7 (+) 562.0 (-) 1436.1 (+) C 0.49 (+) 744.0 (*) 875.7 (-) 37.2 (•) 60.5 (-) 26.7 (-) 1303.7 (-) L 0.39 (+) 350.5 (+) 89.1 (-) 72.9 (+) G 1.65 (+) 1804.5 (+) 379.5 (-) 300.8 (+) 1815.0 (+) 46.5 (-) 22.2 (-) 896.9 (-) A x L 0.19 A x G 0.47 83.4 222.0 A x C 20.6 636.6 T x A 399.8 99.4 Residual (D.F.) 0.40 (24) 647.3 (29) 551.8 (26) 302.1 (26) 65.9 (26) 2249.3 (30) 111.5 (30) 119.4 (29) 108.4 (29) 848.0 (25) Total 9.71 2833.4 4536.0 2082.5 1069.9 4064.3 172.0 304.5 157.3 5724.4 T • temperature A • current C • concentration L • e lect ro ly te flow rate G • propylene gas flow rate A x L - F i r s t order Interaction. It measures the extent to which the ef fect of one factor , in this case current , depends upon the value of the other factor , e lectro lyte flow rate. O.F. • degrees of freedom The numbers reported above are the sums of squares. - 135 -APPENDIX 8 DATA REPLICATION - 136 -T a b l e 0 Data R e p l i c a t i o n C u r r e n t Space Time Y i e l d f o r P r o p y l e n e Ox i de k g / h r m 3 5 38 .49 A 37 .53 B 8 55 .81 52 .59 11 63 .67 57 .73 13 43 .40 46 .20 C o n d i t i o n s : G r a p h i t e p a r t i c l e s i z e = 1.168 - 1.68 mm G r a p h i t e t y p e = Un i on C a r b i d e Tempe ra tu r e = 28-35°C C o n c e n t r a t i o n = 0 .5 M L i q u i d f l o w r a t e = 300 cm /m in Gas f l o w r a t e = 1500 cm 3 /m in a t STP P r e s s u r e = 1.4 - 2.2 atm Bed t h i c k n e s s = 8.57 cm 

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