CHEMICAL. ENGINEERING FACTORS IN THE PREPARATION OF UREA FROM CAR BONYL SULFIDE AND ANHYDROUS AMMONIA Dissertation Presented in Partial Fulfillment of the Requirements for the Doctor of Philosophy Degree in the Graduate School of The Ohio State University By FELICE JOSEPH CELLI, B. Ch. E. , M. Sc. The Ohio State University 1953 Approved by: L. Ker Herndon, Adviser TABLE OF CONTENTS Page Acknowledgement 1 Statement of Problem 2 Scope of Problem 3 Indications Batch System 6 Flow System 8 Historical 10 Manufacture of Urea from COz and NH3 Once Through Process 13 Solution Recycle Process 14 Hot Gas Recycle Process 14 Oil Slurry Process (Pechiney) 1 6 Preparation of Urea from COS and NH3 17 Theoretical 19 Raw Materials Ammonia 24 Carbonyl Sulfide 25 Absolute Alcohol 28 Analysis of Product 29 Experimental Program 33 i BATCH SYSTEM Page Equipment 34 Effect of Order of Condensation of Reactants on Yield of Urea 36 Effect of Reactants Weight Charge / Reactor-Volume on Yield 39 Effect of Reaction Time on Yield 43 Ratio of Reactants vs. Yield 47 Effect of Temperature on Yield No Supplementary .Liquid Phase 52 Ethanol as Reacting Medium 56 Effect of CS2 on Yield 6 0 SUMMARY 64 FLOW SYSTEM Reactor Design 6 6 Heat Transfer Fluid for Reactor 69 Flowmeter 71 Calibration of Flowmeters Ammonia 7 8 Nitrogen 79 Carbonyl Sulfide 81 Recovery of Urea from Catalyst 8 6 Operational Procedure 8 8 Li Page Nature of Catalyst vs. Yield 91 Ratio of Reactants vs. Yield Effect of Temperature on Yield 101 Effect of Space Velocity on Yield 104 SUMMARY Temperature - Yield Space-Velocity - Yield 1 0 6 Space-time-yield vs. Space Velocity 111 Catalyst Loading 115 Catalyst Loading vs. Space Velocity 120 Catalyst Life 123 Material Balance 125 SUMMARY 126 APPENDIX Sample Data Sheet - Batch System 128 Sample Data Sheet - Batch System 129 Sample Data Sheet - Flow System 13 0 AUTOBIOGRAPHY 131 ILL ACKNOWLEDGEMENT The author wishes to extend his appreciation to Dr. L.Kermit Herndon, Professor of Chemical Engineering, for his advice and criticism during the course of this work and also for his extramural professional guidance. STATEMENT OF PROBLEM The problem was to investigate the effect of the process variables, using both batch and flow processes, on the yield of urea when the latter is prepared according to the following reaction COS + 2 NH3 = CO(NH2 ) 2 + HZS carbonyl sulfide ammonia urea hydrogen sulfide SCOPE OF PROBLEM The problem of the preparation of urea from carbonyl sulfide and anhydrous ammonia was divided into two phases -- a batch system investigation and a flow system investigation. Equip­ ment was designed and constructed to carry out these studies. In the batch system the variables studied were as follows: 1.) The effect of the ratio of the weight or volume of charge to the reactor volume on the yield of urea. The range was from 0.25 gram- mole of carbonyl sulfide and 0. 50 gram-mole of ammonia to 1.25 gram-moles of carbonyl sulfide and 2.50 gram-moles of ammonia per 245 ml. of reactor volume. 2. ) The order of condensation of the reactants into the autoclave and its effect on the yield of urea. 3.) Effect of the time of reaction on the yield. 4.) Effect of the temperature of the reaction on the yield of urea. The temperature range was from 30° to 143°C. a. Using a liquid reacting medium, absolute ethanol. b. Using no liquid reacting medium but condensing the gases directly into the autoclave. 5.) The effect of the molar ratio of reactants, NH3 /COS, on the yield. The range was from 0 to 300 molar per cent escess ammonia and 0 to 163 molar per cent excess carbonyl sulfide. 6 .) The effect of carbon disulfide on the yield. The range of carbon disulfide was from 0 to 18 parts per 1 0 0 parts of carbonyl sulfide. Carbon disulfide was an impurity in the raw carbonyl sulfide before purification. In the flow system the process variables studied and their effect on the yield of urea were as follows: 1 .) Nature of catalyst. Those catalysts employed were activated charcoal, bone charcoal, wood charcoal, activated alumina, anhydrous calcium sulfate, anhydrous calcium chloride, a.nd glass wool. 2.) Effect of temperature. The temperature range was from 25° to 193°C. for three of the above catalysts. 3.) Effect of space velocity, from 1. 56 min. - 1 to 7. 50 min. at S. T.P for activated charcoal as the catalyst. 4.) The effect of varying both space velocity and temperature respecti vely through the ranges given in (2j and (3) above. 5.) Space-time-yield versus space velocity at constant temperature as given in( 2 ). 6 .) Ratio of reactants from 4. 0 moles ammonia per mole of carbonyl sulfide to 0 . 5 mole of ammonia per mole of carbonyl sulfide, i. e. , up to 100 molar per cent excess ammonia and up to 300 molar per cent molar excess carbonyl sulfide. 7.) Catalyst life. 8 . ) Catalyst loading versus yield a. At constant space velocities and variable time. b. At constant time and variable space velocities. -6 INDICATIONS BATCH SYSTEM 1. When absolute ethanol is used as a supplementary liquid phase with the carbonyl sulfide and ammonia reactants, a yield of urea of 76 per cent may be realized at a temperature of 76°C. If the temperature is raised the yield drops to 60 per cent at 110°C.. and from this point follows the same increase with increase in tem pera­ ture as when no supplementary liquid phase is used. From ilO°C. the maximum yield of 65 per cent is reached at approximately the melting point of urea. Using ethanol at this low temperature is equivalent to using a 40 molar per cent excess ammonia or 90 molar per cent excess carbonyl sulfide. Another big advantage is that the corrosion is greatly reduced at a low temperature and pressure. 2. When no supplementary liquid is introduced with the reactants the yield increases steadily with temperature and reaches a maximum of 64 per cent in the vicinity of the melting point of urea, 132°C. 3. The time for the reaction of stoichiometric quantities of carbonyl sulfide and ammonia to reach completion, using 93°C. as the basis, is approximately 1 2 0 minutes. 4. At 105°C. using an excess of carbonyl sulfide of 100 molar per cent a maximum yield of urea of 78 per cent may be obtained. 5. At 105°C. using ammonia in excess of 150 molar per cent a maximum yield of 8 8 per cent of urea may be obtained. Ammonia has a greater driving force than does the carbonyl sulfide in the for­ mation of urea from these two reactants. 6 . In the range of ratios of reactants weight charge to volume of reactor studied, 0.10 to 0.48 gr. per ml. of reactor volume, no effect was noted on the yield of urea. 7. The order of condensing the reactant gases into the autoclave has no effect on the yield of urea. 8 . Carbon disulfide, a possible impurity of carbonyl sulfide, does not decrease the yield of urea but rather increases it slightly. The product, however, is oily and would have to be purified before it could be sold on the market. 9. Excess ammonia is more corrosive than excess carbonyl sulfide on 18-8 stainless steel 316 reactor. FLOW SYSTEM 1. Since the urea remains on the catalyst, catalyst loading and de­ activation play one of the leading factors in the flow system. 2. Catalyst loading increases with space velocity at equivalent gas throughput. 3. Activated charcoal gave the best yields of any catalyst used. Activated alumina and bone charcoal followed. 4. Increase in space velocity results in a decrease in yield at all tempe iqtur e s. 5. Using activated charcoal and stoichiometric quantities of reac­ tants the best yields at all space velocities is at 125°C. 6 . The yield of urea drops off sharply above 125°C. (Urea decom­ poses above its melting point (132. 7°C)). 7. The highest space-time-yields are obtained at 125°C. The data show that there no major side reactions in the formation of urea from carbonyl sulfide and ammonia. 8 . Ammonia has a greater driving force in the formation of urea from carbonyl sulfide and ammonia than does the carbonyl sulfide. 9. At 1 2 9 °C. and space velocity of 2.41 min. a molar excess of ammonia of 50 per cent raises the yield of urea from 56 to a maximum of 75 per cent. 10. At 129°C. and space velocity of 2.41 min. a molar excess of carbonyl sulfide of 100 per cent raises the yield from 56 to a maximum of 65 per cent.- 11. At 121 °C. and space velocity of 2.41 min. an excess of carbo­ nyl sulfide has essentially no effect on yield. 12. At 121°C. and space velocity of 2.41 min. an excess of am­ monia of 80 molar per cent raises the yield from 20 to 36 per cent maximum. 13. A greater excess of ammonia is needed to attain a maximum yield of urea as the temperature drops from the 125-129°C. range. 14. The catalyst, activated charcoal, may be re-used. Its activity does not decrease with re-use after having been subjected to extrac­ tion process. -/< ? - HISTORICAL It was in the year 1828 that Wohler, on checking a mixture from an experiment he had performed and set aside four years earlier, found some crystals had developed.