Kinetic Study for Oxidation of Ethylene over. Silver Catalyst under Stationary State*

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Kinetic Study for Oxidation of Ethylene over Silver Catalyst under Stationary State* by Akimi Ayame**, Hisao Kano**, Takatsugu Kanazuka** and Hiromu Baba*** Summary : A kinetic study for oxidation of

1 Kinetic Study for Oxidation of Ethylene over Silver Catalyst under Stationary State* by Akimi Ayame**, Hisao Kano**, Takatsugu Kanazuka** and Hiromu Baba*** Summary : A kinetic study for oxidation of ethylene was carried out using an integral reactor and a silver catalyst promoted by potassium sulfate under stationary state. Measurements were made under ordinary and higher pressures. Empirical rate equations were found to be: rc2h4o=k1p0.33cc2h4co21/2-k3p0.24cc2h4och2o-1/2 rco2=k2p0.32cc2h4co2, where P is the total pressure, and ki and Ci are the rate constant and mole fraction for component i, respectively. 1 Introduction gradual decline in the activity of a silver catalyst for ethylene oxidation could be attributed to the following factors; sintering of the silver, the formation of some oxidized state of silver and the formation of surface residues. Surface residues, silver glycoxide and its polymer, were recognized as being important not only in the earlier part of the reaction but also in the stationary state. The formation of surface residues was retarded in the presence of water, carbon dioxide and excess oxygen, and water was most effective. The amount of surface residues was quantitatively determined by the oxidative desorption technique. Consequently, the stationary activity can be regarded as being attained through a competition between deactivation by ethylene oxide and retardation of deactivation by water, carbon dioxide and oxygen. When the equilibrium is established between these two opposing influences, a stationary state for ethylene oxidation results. For such a catalytic reaction system, since the variation in the concentration of reactants and products causes the change in the stationary activity of the catalyst, kinetics will be complicated considerably, and it will be important to examine the effects of reactants and products on kinetics under each stationary state. * Received June 9, ** Department of Industrial Chemistr Institute y, Muroran 050) of Technology (Mizumoto-cho, Muroran, *** Department of Mechanical Engineering, Kitami Institute of Technology(Koen-cho, Kitami, 090) In the present investigation, an attempt was made to establish accurate rate equations under such a stationary state in which the change in the activity of silver catalyst could be neglected. 2 Experimental 2.1 Catalyst The catalyst used in this experiment had the following composition;ag2o:k2so4:al2o3= 15g:75mg:50g. It was reduced in a stream Norton Co.. The method of preparation of the catalyst is described in detail in the preceding paper1). 2.2 Preparation of Mixed Gas (Feed Gas) Ethylene was supplied by Japan Petrochemical Co., air by Hokkai-Sanso Co.. They were mixed per cent ethylene. 2.3 Apparatus A flow diagram is shown in Fig. 1. The 700mm long and settled in a salt bath (KNO3- NaNO2-NaNO3). An alumel-chromel thermocouple was inserted through the center of the reactor tube. The catalyst bed was located at the height of 300mm from the bottom of the tube. 50g of the catalyst was placed on top beads (Macroport, Norton Co.) on the support. The net catalyst bed was about 35mm in height. Since a considerable temperature difference through the catalyst bed resulted in the case of a higher pressure operation, the average temperature was assumed to be the same as the Bulletin of The Japan Petroleum Institute

2 Ayame, Kano, kanazuka and Baba: kinetic Study for Oxidation of Ethylene over Silver Catalyst Fig. 1 Flow Diagram Fig. 2 Results of Conditioning and Check Runs predetermined value of the control heater. The pressure and flow rate of feed gas were precisely controlled using regulating valves located at the inlet and outlet of the reactor. The pressure drop across the bed was negligible in the gauge pressure range from 0 to 15kg/cm2. Ethylene, ethylene oxide and carbon dioxide were analyzed by gaschromatography. 3 Results 3.1 Change of Catalyst Activity due to Changes in Reaction Conditions length of time. 12.8l/hr and P=1.0atm have been chosen, where Tm is the average temperature, F' is the flow rate of mixed reactant gas and P refers to the total pressure. Under these conditions, the catalyst activity was examined frequently for 1,600 hours (Fig. 2). It was apparent that at least 30 hours were required for stabilization, Volume 15, No. 2, November 1973 and that the catalyst Fig. 3 Typical Temperature Distribution Curves through Catalyst Bed activity remained unchanged over a considerably It has already been shown for the identical catalyst in a glasstubing reactor that if the temperature or flow rate is altered, the extent of On the other hand, when the operating pressure is raised at constant temperature and flow rate, the total conversion increases gradually and a constant conversion is attained within

3 Ayame, kano, kanazuka and Baba: Kinetic Study for Oxidation Table 1 Experimental Values under Ordinary and Higher Pressure 4 or 5 hours. Reverse results occur when pressure levels are reduced. From these facts, the conversion data corresponding to one set of reaction conditions were determined at about 5 hours after alternating the pressure levels. However, the variation in the total conversion mentioned above was found to be of the same order of magnitude as the experimental errors. 3.2 Temperature Distribution in the Catalyst Bed Because of the large heat of reaction, due particularly to the combustion of ethylene, a longitudinal temperature gradient was observed especially under high pressures. Some typical longitudinal temperature profiles in the bed are shown in Fig. 3, where Tb is the temperature of C2H4/hr). Therefore, the average temperature at the center of the catalyst bed was considered as the reaction temperature (Tm). 3.3 Kinetic Data Table 1 shows the results of the several kinetic runs made during the intervals indicated in Fig. 2 (A, B and C). For period A the temper- were measured at various pressures and contact times (W/F). For periods B and C, the temper- Bulletin of The Japan Petroleum Institute

of Ethylene over Silver Catalyst under Stationary State Table 2 Rate Constants in Eqs. (3) and (4) Table 3 Rate Constants in Eqs. (5) and (6) Fig. 4 Agreement between Calculated Values by Eqs.

4 of Ethylene over Silver Catalyst under Stationary State Table 2 Rate Constants in Eqs. (3) and (4) Table 3 Rate Constants in Eqs. (5) and (6) Fig. 4 Agreement between Calculated Values by Eqs. (1)-(2) or Eqs. (3)-(4) and Observed Values over Ag-Kieselguhr Catalyst7) respectively. Frequent checks indicated almost constant conversions within experimental errors, and hence the surface was assumed to be of the same state as that of the reference. The apparent rates of reactions (rc2h4o and rco2) were determined at each contact time by graphical differentiation9) of x or y versus W/F plots (for example, dotted curves in Fig. 5 and 6). Mole fraction Ci (i=c2h4, O2, etc.) was calculated from the equations established in a previous paper6). The calculated values of x and y, based on some set of rate equations were obtained by use of a modified Euler's method11). 3.4 Rate Equations Rate Eqs. (1) and (2), reported previously by Kano et al.6),7), are in good agreement with the kinetic data obtained for Ag-Kieselguhr catalyst at low conversion levels, but the agreement is poor at high conversion levels (Fig. 4, dotted line). rc2h4o=k1pc2h4po21/2-k3pc2h4o (1) rco2=k2pc2h4po2+k3pc2h4o (2) where k1, k2 and k3 are rate constants. When Eqs. (1) and (2) were applied to the kinetic data in Table 1, the data did not coincide with the equations; in particular, deviations of y were larger than those of x. Some modification was made on these equations based on the experimental data. As a result, it was found that the following rate equations well represented the observed data (solid curves at P=1atm in Fig. 5, 6, 7 and 8) and those of Ag-Kieselguhr catalyst7) (Fig. 4, solid line). rc2h4o= k1pc2h4po21/2-k3pc2h4o/ph2o1/2 (3) rco2=k2pc2h4po2 (4) k1 in Eqs. (3) and (4) was also estimated from the kinetic data under high total pressure (P) in Table 1, and it was found to decrease as P increased (Table 2). It is, therefore, impossible to represent the data under different pressures by the same rate equations, although good agreement was observed at each pressure level. The authors have attempted to represent their data with the following equations, assuming that the rate constant involves a term in total pressure. rc2h4o=(k1)1plcc2h4co21/2-(k3)1pncc2h4o/ch2o1/2 (5) rco2=(k2)1pmcc2h4co2 (6) (k1)p=(k1)1pl, (k2)p=(k2)1pm, (k3)p=(k3)1pn (7) where (k1)1, (k2)1 and (k3)1 are rate constants at one atm pressure, and l, m and n are constants. (k1)p, (k2)p, and (k3)p are rate constants at a fixed temperature and pressure. (ki)p was determined from the kinetic data and was tabulated in Table 3. l, m and n were determined from Eq. (7), (Table 3). Fig. 5, 6, 7 and 8 show the agreement between the observed and the calculated values by Eqs. (5) and (6) using (ki)1. It is evident that these equations give better agreement with the data, and the rates under high pressures can be estimated from the kinetic data available under ordinary pressure. Volume 15, No. 2, November 1973

5 Ayame, Kano, Kanazuka and Baba: Kinetic Study for Oxidation Fig. 7 Agreement between Calculated Values by Eqs. (3)-(4) or Eqs. (5)-(6) and Observed Values over Ag-K2SO4-Al2O3 Catalyst 4 Discussion 4.1 Pressure Effect It has been found that conversion of ethylene to ethylene oxide (x) and that to carbon dioxide (y) increase as the pressure increases at a given W/F, as shown Fig. 5 and 6. Consequently, at a given S. V. the increase in total pressure gives a higher space-time-yield (S. T. Y.) as shown in Fig Rate Equations Shishakov et al.12) and Kagawa et al.13) confirmed by means of electron diffraction that both adsorbed atomic oxygen, Oads-, and adsorbed molecular oxygen, (O2-)ads, exist on the surface of the catalyst. Kilty et al.14) supported the above findings by I. R. spectra of adsorbed O and O2. From these results, they have proposed the following mechanism; 1) ethylene oxide is formed from gaseous ethylene and (O2-)ads, 2) carbon dioxide and water are formed from gaseous ethylene and Oads-. Kenson et al.15) have also suggested that adsorption of ethylene oxide on the silver surface is the rate-determining Fig. 8 Space-Time-Yield vs. Space-Velocity step for its oxidation, and to Oads-in the present work) serves as an active site for the formation of ethylene oxide. If the intermediates in the combustion of both ethylene and ethylene oxide are to be identical, e. g. adsorbed ethylene oxide or acetaldehyde10), the reaction scheme could be represented as follows1),4); C2H4+(O2-)ads k1 C2H4O+Oads- (8) C2H4Oads (9) C2H4Oads (10) C2H4+Oadsk2' C2H4O+ek3' C2H4Oads+Oadsk4 2CH2Oads (11) Bulletin of The Japan Petroleum Institute

of Ethylene over Silver Catalyst under Stationary State CH2Oads+4Oads-(or 2(O2-)ads) (fast) (CO3-)aas+2OHads+4e-(or 2e-) (12) Here, the inhibition effects on adsorption of water, carbon dioxide and

6 of Ethylene over Silver Catalyst under Stationary State CH2Oads+4Oads-(or 2(O2-)ads) (fast) (CO3-)aas+2OHads+4e-(or 2e-) (12) Here, the inhibition effects on adsorption of water, carbon dioxide and oxygen to ethylene oxidation are assumed to be negligible. Assuming the adsorbed atomic oxygen to act as an active site in Eq. (8) and the inhibition effects of components other than water for adsorption oxide to be negligibly small because the adsorption is specificially retarded in the presence of water1),2),16) Eq. (3) is derived from Eqs. (8) and (10) as follows: ordinary and high pressures have been obtained in the present investigation. rc2h4o=k1pc2h4po21/2-k3pc2h4o/ph2o1/2 (13) The authors are grateful to Prof. Tatsuya Imoto and Dr. Yoshio Harano of Osaka City University equilibrium constant of water. The rate equation for the formation of carbon dioxide is derived assuming that the reaction rates of Eqs. (9) and (11) are of the same order, (14) Using a set of Eq. (14) and Eq. (3) or Eq. (13), the difference between the calculated and observed conversions was very large. Consequently, the concentration of C2H4Oads was assumed to be governed by Eq. (9), i. e. was obtained from Eq. (14). However, in the course of calculation of conversions based on Eqs. (3) and (4), it was observed that the value of k3pc2h4o/ph2o1/2 term was larger than that of k2'pc2h4po21/2 term in the large regions of W/F. Reasons for the occurrence of these conflicting results is not evident in the present work. The stationary activity (or catalyst surface) could be estimated to change with pressure of the system, because the constancy of ki in Eqs. (3) and (4) under different pressures was not attained. The transformation of Eqs. (3) and (4) to Eqs. (5) and (6), respectively, seems therefore, to be required. The inhibition effects of water, carbon dioxide and oxygen for ethylene oxidation have been neglected in the present work, because it is estimated that inhibiting actions to ethylene oxidation and the retarding actions to deactivation by ethylene oxide4) are attributable to the same behaviors of water, carbon dioxide and oxygen on the catalyst surface1),2). In the treatment of kinetics on this catalytic oxidation, there has, however, been no information for interaction between the inhibition effect and the retardation effect of each component. Even if it is possible to elucidate experimentally or theoretically some problems as described above, its rate equations will be much more complicated. It seems worthwhile that precise and simple rate equations applying to the kinetic data under Acknowledgement and to Prof. Haruo Kobayashi of Hokkaido University for their helpful suggestions. References 1) Ayame, A., Numabe, A., Kanazuka, T., Kano, H., Bull. Japan Petrol. Inst., 15, (2), 142 (1973). 2) Ayame, A., Kano, H., J. Chem. Soc. Japan, Chem. Ind. Chem., 1972, ) Ayame, A., Shibuya, Y., Yoshida, T., Kano, H., ibid., 1973, ) Ayame, A., Numabe, A., Watanabe, Y., Kano, H., ibid., 1973, ) Ayame, A., Suzuki, Y., Kano, H., ibid., 1973, ) Kano, H., Kanazuka, T., J. Chem. Soc. Japan (Ind. Chem. Sect.), 61, 1157 (1958). 7) Kano, H., Kanazuka, T., ibid., 65, 1 (1962). 8) Kano, H., Ayame, A., Memoirs of the Muroran Inst. of Tech., 4, 871 (1964). 9) Hougen, O. A., Watson, K. M., Ragatz, R. A., "Chemical Process Principles (Second Edition), Part 1", 7 (1954) John Wiley and Sons, Inc., New York. 10) Twigg, G. H., Proc. Roy. Soc. (London), A188, 92 (1946). 11) Lapidus, L., "Digital Computation for Chemical Engineers", 86 (1962) MacGraw-Hill Book Co. Inc., New York. 12) Vol, Yu. Ts., Shishakov, N. A., Izv. Akad. Nauk SSSR, Otd. Khim. Nauk, 1962, ) Kagawa, Sh., Tokunaga, H., Seiyama, T., J. Chem. Soc. Japan (Ind. Chem. Sect.), 71, 775 (1968). 14) Kilty, P. A., Rol, N. C., Sachtler, W. H. M., 5th International Congress on Catalysis, Preprint 67 (1972) Palm Beach, U. S. A. 15) Kenson, R. E., Lapkin, M., J. Phys. Chem., 74, 1493 (1970). 16) Ayame, A., Kano, H., Harano, Y., Imoto, T., 9th Kyusyu Meeting of Chem. Soc. Japan, Preprint 72, July, (1972). Volume 15, No. 2, November 1973

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