An Investigation of the Reaction i > ci>:' Between Sulfur Dioxide and Oxygen in a Fluidised Catalyst Bed By Robert W. Bell »■ and Howard W. Strauss A THESIS Submitted in partial fulfillment of the requirements for degrees of BASTER OF SCIENCE Department of Chemical Engineering The Rice Institute August^ 19h9 ;-v. v', ACKB0M3GEMKJ Tiie authors wish to express their appreciation to all those who have aided in this investigation# In particular* they wish to thank the follot/ing: Dr. G* T. McBride, Jr., for his continued interest and advice* Professor A. J. Hartsook for the opportunity of carrying out this investigation. The Pan American Refining Corporation and Dr. R. H. Eric© for the financial assistance which made this work possible* Dr. John T. Smith and the Shell Oil Company for a spectrographic analysis 13 TABLE OF CONTENTS Page I. SUMMARY i II. INTRODUCTION g A. Purpose of Investigation g B. Theory of Heterogeneous Reactions g 1. Effects of Mass Transfer g 2. Adsorption of a Reactant Rate Controlling g 3* Desorption of a Product Rate Controlling 7 L» Surface Reaction Rate Controlling f $m Effects of Temperature 8 C. Work of Other Investigators 8 III. APPARATUS AHD PROCEDURE ■ * 13 A. Apparatus £3 B« Materials jg G. Procedure 16 1. Catalyst Preparation 15 2* Run Technique • 18 3* Analyses 19 a. Sulfur Dioxide 19 b. Oxygen 19 c* Sulfur Trioxide ■ 20 IV. EXPERIMENTAL RESULTS 21 A. Preliminary Data 21 B. Investigation Proper 20 V. INTERPRETATION OP DATA 43 lli i! flags A. Differential Rate Equations Developed for the Theory 48 V of Activated Adsorption. 1. Adsorption of Sulfur Dioxide Rate Controlling. 48 2. Adsorption of Oxygen Rate Controlling. Si 3. Desorption of Sulfur Trioxide Rate Controlling. 62 i*. Surface Reaction Rate Controlling. GS 5. Effects of Mass Transfer. 66 B. Empirical Equations 67 C. Integrated Equations 67 VI. CONCLUSIONS 62 VII. APPENDICES 65 A. Summary of Experimental and Calculated Data 64 1* Original Data 6$ 2* Calculated Reaction Rates 76 B. Details of Apparatus 60 1. Purification of Gases 61 2» Metering of Gases 61 3* Preheater 81 iio Reactor 8$ 5* Solids Separator 07 6. Photograph of Apparatus 88 C. Chemical Analysis of Silica Gel 60 D» Sulfur Dioxide Analysis Technique gg E. Calculation of Conversion gg F. Reaction Equilibrium Constant Versus Temperature Go Nomenclature II. Literature Cited I. Location of Original Data INDEX TO FIGURES No* Title Page 1. Schematic Flow Diagram 14 2. Typical Temperature Gradients in Empty Reactor 17 3* Typical Temperature Gradients in Loaded Reactor IS It* Conversion Gradients in Empty Reactor 25 $» Conversion Versus Time 30 6. Conversion Versus Reciprocal Space Velocity - Series A 52 7» Conversion Versus Reciprocal Space Velocity - Series B gg 8* Conversion Versus Reciprocal Space Velocity - Series C 34 9» Reaction Rate Versus Reciprocal Space Velocity at U00°C gg 10, Reaction Rate Versus Reciprocal Space Velocity at i*5G°C 37 11. Reaction Rate Versus Reciprocal Space Velocity at £Q0°C gg 12* Product Composition Versus Reciprocal Space Velocity - 39 Series A at )400°C 13. Product Composition Versus Reciprocal Space Velocity - 40 Series A at ii50°C lit. Product Composition Versus Reciprocal Space Velocity - 41 Series A at 5>00°C 1$. Product Composition Versus Reciprocal Space Velocity - 42 Series B at itOO°C 16* Product Composition Versus Reciprocal Space Velocity - 45 Series B at bS0°0 17. Product Composition Versus Reciprocal Space Velocity - 44 Series B at 500°C 18. Product Composition Versus Reciprocal Space Velocity - Series C at iiG0°C ■trl No. Title Page 19. Product Composition Versus Reciprocal Space Velocity - 40 Series C at itSO°C 20. Product Composition Versus Reciprocal Space Velocity - m Series C at £00°C 21. Reaction Rate Versus Reciprocal Temperature - Series A ss 22. Reaction Rate Versus Reciprocal Temperature - Series R m 23. Reaction Rate Versus Reciprocal Temperature - Series C m 21* • Orifice Calibration, Oxygen 0-1 82 2£. Orifice Calibration, Oxygen 0-3 as 26. Orifice Calibration, Nitrogen 8-3 84 2?. Reactor Desigd Details 86 28. Photograph of the Apparatus 89 29. Reaction Equilibrium Constant Versus Temperature 99 yii IMDEX TO TABLES Ho. Title Rige 1. Silica Gel Particle Size Distribution IS II. Original Investigation of Catalytic Activity of Empty 2S Reactor and Unplatinised Silica Gel III. Comparison of Reactor Wall Activity at Beginning and End 24 of Investigation Proper 17. Conversion Gradient in Empty Reactor 26 7. Investigation of Effect of Reducing the Reactor Wall 20 tilth hydrogen ■ VI. Effect of Time on Conversion 31 VII. Average Feed Compositions 29 VIII. Initial Compositions and Reaction Rates 00 IX. Constants for the Surface Reaction as the Rate Controlling gg Step X. Summary of Experimental Data - Series A 33 XI. Summary of Experimental Data - Series B @© XII. Summary of Experimental Data - Series C fg XIII. Reaction Bates and Partial Pressures at ijOQ0C fg XIV. Reaction Rates and Partial Pressures at h$0°G 77 XV. Reaction Rates and Partial Pressures at 5GG°C 73 XVI. Reaction Rates at Constant Product Composition 70 I. SUMMARY Date are presented on the rate or oxidation of sulfur dioxide on a platinized silicia gel catalyst in a batch fluidised bed* The variables investigated were feed composition, space velocity, and temperature* The pressure was approximately one atmosphere. The data indicate that the rate of mss transfer was a substantial portion of the total resistance to reaction. The lack of suitable data for isolating the respective effects of mss transfer and chemical re¬ action prevented a correlation of the data* An apparent activational energy of 23,500 calories per granwaol was established from reaction rate data alone* This compares with a value of 20,000 calories per graia-mol reported by other investigators. II. IRTEODUGTJOH A» Purpose of Investigation It was the purpose of this investigation to study the kinetic rela¬ tionships in the oxidation of sulfur dioxide to sulfur trioxide over an active platinum catalyst by employing a wide-range of temperature, feed composition, and space velocity* In view of the highly exothermic na¬ ture of the reaction, a batch fluidised system was chosen for the pur¬ pose of maintaining isothermal conditions. B. Theory of Heterogeneous Reactions It is a generally accepted hypothesis that when catalysed by a solid, a gas phase chemical reaction actually occurs on the surface of the catalyst and involves the reaction of molecules or atoms which are chemically adsorbed on the active centers of the surface. In this re¬ spect the catalyst functions to increase the rate of reaction through its ability to adsorb the reactant gases in such a form that the activa¬ tion energy necessary for reaction is reduced below that required for the uncatalysed reaction. The development of rate equations based upon this theory of activa¬ ted adsorption has been adequately summarized by Hougen and batson (1). The succeeding equations have been taken from the work of these authors. If the chemical change in a heterogeneous reaction occurs between molecules or atoms on the active centers of a catalyst,the overall pro¬ cess of converting reactant gases to product gases may be classified in¬ to five separate steps (1): a 1® The mass transfer of reactants from the main gas phase to the surface of the catalyst® 2® The activated adsorption of the reactants on the active centers of the catalyst® 3® The surface reaction of adsorbed reactants to fora adsorbed products. h» The desorption of the product® 5® The mass transfer of products from the surface of the cat¬ alyst to the Gain gas phase® The rate at which each of these steps occurs is important in deter¬ mining the overall rate, but it is generally sufficient to assume that the rate is controlled by one of the above steps, all others being considered at equilibrium® 1. Effects of Mass Transfer The rate of mass transfer of the reactant and product molecules to and from the surface of the catalyst is de¬ termined by the flow characteristics of the system such as mass velocity of the main fluid stream, catalyst particle 3ise, and the diffusions! characteristics of the fluid. In the batch fluid catalytic process the velocity of i flow is limited because of catalyst carryover® The mass velocities, in general, are comparable to those in a fixed- bed system operating at low mass velocity® Such a situa¬ tion is particularly conducive to large mass transfer effects, especially if the rate of the surface reaction is high® The rate of transfer of a component from the main fluid stream to the catalyst interface my be expressed (2) by the equations rA = ApG^PA (i) a(BTU)J^ Pgf where = rate of mass transfer of component A, mols per unit mass of catalyst per unit time Ap - gross external area of the catalyst per unit mass G = mass velocity per total unit cross-section ^pA = Difference in partial pressure of component A in main gas phase and at the catalyst surface, % z Mean molecular freight of the gases = log-mean partial pressure of gases other than component A in the gas film a s interfacial area per unit volume of packing HTU s height of a transfer unit Values of a(HTU) have been correlated as function of ir the diffusing component for relatively large particles (3S its 5)s but no data are available for the particle size range generally encountered in fluid-bed work, thereby rendering the calculation of interfacial partial pressures impossible® • It is, in general, sufficient to compare reaction rates at two or more different values of oass velocity at constant apace velocity to determine if the rate of mss transfer is a significant portion of the resistance to reaction* In the operation of a bench scale fluid unit* however* the varia¬ tion in reaction rate resulting from differences in cat¬ alyst-gas mixing characteristics at different mass veloci¬ ties obscures the mass transfer effects* Although it has been reported (6) that gas back-mixing is not likely to.