Acetylene Hydrogenation in CI Bubble Column Slurry Reactor Ronald M. Heck' and Theodore G. Smith Department of Chemical Engineering, Uniuersity.of Maryland, College Park, M d . 20740
The hydrogenation of acetylene in a bubble column slurry reactor (BCSR), using Raney nickel catalyst with process water as the inert liquid medium, was studied a t several flow rates, temperatures, catalyst loadings, and hydrogen to acetylene feed ratios. The effect of these variables on selectivity, acetylene conversion, and product distribution was determined. The effect of process time on catalyst activity and selectivity was also investigated. Fractional conversion of acetylene to ethylene and ethane was strongly dependent on process time. This dependence was attributed t o polymer formation on the most active sites of the catalyst. After long process time, it i s believed that catalyst sites with sufficient activity t o permit polymerization are essentially covered with polymer. However, many of the remaining catalyst sites have sufficient activity t o allow the reaction, acetylene to ethylene to ethane, to proceed.
INTEREST
in studying the reaction of gaseous bubbles ia catalyst slurries has grown in recent years. One reactor used in studying this type of heterogeneous reaction, usually referred to as a bubble column slurry reactor (BCSR), consists of a vertical column containing a catalyst suspended in a liquid through which reaction gases are bubbled. The literature contains several studies of the synthesis of hydrocarbons in this type of reactor (Calderbank et al., 1963; Kolbel, 1961; Kolbel et al., 1964; Schlesinger et al., 1951). Many of the hydrogenation reactions studied in the BCSR have involved the hydrogenation of ethylene to form ethane (Calderbank et al., 1963; Farkas, 1964; Kolbel and Maennig, 1962; Slesser and Highet, 1966). Most investigators have tried to limit their studies t o simple reaction systems and thus the study of consecutive reaction in this type of reactor has not been reported. The study reported here was concerned v. ith the hydrogenation of acetylene over Raney nickel catalyst in a BCSR. The catalyst was suspended in process water, which acts as an inert liquid medium. The product of the hydrogenation of acetylene is ethylene, which may in turn be hydrogenated to ethane. Although the hydrogenation of acetylene has been studied in bench-scale batch reactors (Bond, 1962, Dupont, 1936; Farkas and Farkas, 1939; Sheridan, 1944) and in packed beds (Mars and Gorgels, 1964) and stirred tank reactors (Mann and Safo, 1968), there is no reported study of the hydrogenation of acetylene in a BCSR. Most of these studies considered steady-state yield with no indication of a significant catalyst line-out period. This study considered the effect
' Present address, Celanese Chemical. Co., P . 0. Box 9077, Corpus Christi. Tex. 78408
of reaction temperature, catalyst loading, reactant feed ratio and reactant feed' rate, and process time upon reactant conversion, product formation rate, and product distribution or selectivity. Particular attention is given t o the effect of process time on the catalyst selectivity. Apparatus and Procedure
A flow diagram of the apparatus is shown in Figure 1. T h e major portion consisted of a jacketed 3-inch-inside diameter glass column reactor 1 2 feet long. A t its base
was a glass reducer fitted with a 3-inch-diameter fritted glass disk and a side arm to allow aqueous feed t o be introduced to the reactor. At the top of the reactor a
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Figure 1. Flow diagram of experimental apparatus
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glass tee permitted a slurry temperature thermocouple t o be introduced into the system. Above the glass tee H S-foGt-high condenser was used to remove entrained liquid vapor from exiting product gases. The reactor temperature was controlled by circulating hot water through the jacket. The feed rates of the reaction gases were measured with rotameters and the effluent flow rate was measured with a wet-test gas meter. During an experimental run the slurry temperature, inlet and outlet temperatures of the reactor jacket, feed gas flow rates, inlet pressure and temperature, and the effluent gas flow rate were recorded. Gas holdup was determined by measuring slurry expansion. Effluent gas composition was determined by taking a gas sample at the effluent sampling tee with a 1000-pl syringe and injecting the sample into a calibrated F. & M. Model 700 dual-column chromatograph. The C ? compounds (acetylene, ethylene, and ethane) were determined using a )-inch-diameter 6-foot-long Porapak N column, while C , and C 6 gaseous compounds were determined using a Porapak P colump. Discussion of
@erultr
Operating Variables. Results were obtained on the effect of reaction temperatwe, catalyst loading, hydrogen to
acetylene feed ratio, and reaction gas feed rate upon conversion. product distribution, and reaction selectivity (defined as ClH4/C2&). Only two reaction products, ethylene and ethane, were detected in all of the experiments. All experiments reported h1,re were run under conditions which might be called “hydrogen-starved” with respect to the feed gas ratios and the relative solubilities of the feed gases in the reaction slurrv. At lhe two highest temperatures studied. 120” and 135”F, there was little change in conversion with temperature. I t is believed that a t these higher temperatures
the vapor pressure of the solution becomes significant and may adversely affect the mass transfer driving force from the gas to the liquid phase. Similar results were observed by Slesser and Highet (1966) during the hydrogenation of ethylene. The moles of ethylene and ethane in the product gas stream increased with increasing temperature, while selectivity remained essentially constant over the temperature range studied. Acetylene conversion increases with increasing catalyst loading up t o 1 gram per liter. The mole fraction of both ethylene and ethane in the product gas stream increased with increasing catalyst concentration. The hydrogen to acetylene feed ratio was studied a t 1 to 1 and 4 t o 1 ratios, with the total feed rate constant a t 0.91 gram per minute. Acetylene conversion markedly increased with increasing feed ratio. The selectivity (CIH,/ CLHL) increased slightly a t the lower feed ratio. The results a t lower feed ratios are attributed to the fact that the reaction was studied under hydrogen-starved conditions. At the lower rates of gas flow, the fraction of acetylene converted was higher, indicating the dependence of reaction rate on residence time. The conversion of acetylene to the consecutive reaction products, ethylene and ethane, was higher for each product with decreasing flow rate. Changes in gas flow rate had little effect upon selectivity. Effect of Reactor Process Time. Experimental measurements made over a period of several hours showed that acetylene and hydrogen conversion, ethylene and ethane formation, and reaction selectivity were dependent upon the duration of the run. Figure 2 shows the effect of process time on the formation of ethylene and ethane for run 58. Additional data for run 58 show that the conversion of acetylene decreases with process time and tends t o reach a constant value after 320 minutes. While hydrogen conversion also decreases with process time, it tends to reach a constant value after about 200 minutes. Reaction selectivity decreases with process time and tends
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Figure 2 . Effect of process time o n formation of ethylene a n d ethane Hydrogen
and acetylene flow rate. 666 cc per minute
Cotalyst loading. 0.526 g r o m per liter Temperature. 99’ F
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to reach a constant value after 320 minutes. This dependence of conversion and selectivity upon process time would seem to indicate a change in catalyst activity with run duration. Catalyst activity appears to decrease with increasing process time and the preference for ethane formation increases with increasing process time. I n an attempt to determine experimentally why this reaction was dependent upon process time, a series of experiments was made using the process conditions of run 58. Acetylene was bubbled through the liquid medium containing the catalyst so as to have initial conditions in which the system was saturated in acetylene; then the normal run was made (run 64). The liquid medium containing the catalyst was saturated with hydrogen and then a normal run was made (run 61). The same catalyst was used for consecutive runs. Run 65 (a normal run) was made, the reactor was drained and refilled, and run 66 was carried out using the catalyst from run 65. Pretreatment with acetylene promotes a reduction in the formation of ethane but no change in ethylene formation compared with the standard run (run 58). Hydrogen pretreatment of the slurry decreases formation of both ethylene and ethane compared with run 58. Figures 3 and 4 show the effect of using the same catalyst in two different runs. The mole fraction of ethylene in the product decreases in the second run (run 66) for the same process Lime, while the mole fraction of ethane increases. If the results of the second run (No. 66) are plotted as a continuation of the first run (No. 65), the composite curve is similar to the normal run (No. 58). The results of runs 65 and 66 indicate that the decrease in ethylene formation is due to an irreversible process such as polymer formation on the catalyst surface. However, as process time increases, the catalyst surface becomes more active in the formation of ethane. Previous studies on acetylene hydrogenation (Bond,
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Figure 4. Effect on ethane formation of using same catalyst in t w o different runs Hydrogen and acetylene flow rate. 660 cc per minute Catalyst loading. 0.533 gram per liter Temperature. 100" F
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1962; Dupont, 1936; Farkas and Farkas, 1939; Mann and Safo, 1968; Mars and Gorgels, 1064; Sheridan, 1944) have shown that polymer may form on the catalyst surface under the mildest reaction conditions. Although the overall mass balances for each experimental run in this study indicated no polymer accumulation during a run, this does not rule out the possibility of polymer formation. A recent study by Ozawa and Bischoff (1968) has shown that polymer formation as low as 10 - 4 gram of polymer per gram of catalyst may cause significant poisoning. Such a small quantity of polymer could not be measured in this work. However, the results from runs 65 and 66 and observations of this reaction by other workers (Bond, 1962; Dupont, 1936; Farkas and Farkas, 1939; Mann and Safo, 1968; Mars and Gorgels, 1964; Sheridan, 1944) indicate that polymer formation is the cause of the decrease in catalyst activity. It is believed that the following reactions occur a t the catalyst surface. -+
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Figure 3. Effect on ethylene formation of using same catalyst in i w o different runs Hydrogen and acetylene flow rate. 660 cc per minute Catalyst loading. 0.533 gram per liter Temperature. 100" F
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C?H2+ H? CZH? C2H4 + H2 CLHG C?H4*+ H*-+ P
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L
s
(1) (2) (3)
where P represents polymer formed by the polymerization of the activated ethylene complex on the catalyst surface. If Figure 2 is considered in the light of the proposed reaction scheme, it may be divided into three regions: short-term (20 to 80 minutes), intermediate-term (80 to 360 minutes), and long-term (360 to 440 minutes or longer). The short time represents the initial or startup conditions. The catalyst is most active during this period, with both ethylene and polymer formation at their maximum rates. I n the intermediate time range, the formation of ethane increases but the quantity of ethylene in the product gas continues to decrease. The decrease in acetylene and hydrogen conversion during the intermediate term range may be attributed to catalyst poisoning due to polymer formation. Previous experimental studies (Bond, 1962) indicate that the activation energy for polymer formation Ind. Eng. Chem. Process Des. Develop., Vol. 9,No. 4,1970
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(about 15 kcal per mole) is somewhat greater than the activation energy for ethylene and ethane formation (about 11 kcal per mole). If a distribution of site activity on the catalyst surface is assumed, it is possible for the most active sites to become poisoned initially by polymer formation. As the reaction proceeds and the most active sites are covered with polymer, the rate of polymer formation decreases because of a decrease in the number of sites with sufficient activity for polymer formation. A decrease in polymer formation will permit the ethylene previously used to form polymer to be available for ethane formation a t catalyst sites of lower activity. Accordingly, the quantity of ethane in the product stream should increase during this intermediate time period, as indicated in Figure 2. In the third time interval, a t long process times, the hydrogen and acetylene conversion and the ethylene and ethane formation approach a steady state. In this interval, polymer formation has subsided substantially, the poisoning effect approaching a maximum for the particular reaction temperature. Few catalyst sites with sufficient activity for polymer formation remain and thus the reaction of ethylene is biased toward the formation of ethane. Conclusions
The hydrogenation of acetylene in a bubble column slurry reactor is a function of the process variables: reactor temperature, feed ratio of reactant gases, catalyst loading, and reactant gas flow rate. The reaction is strongly dependent upon the length of a reaction run. The formation of ethane and ethylene in this type of reactor is believed to be affected by the formation of polymer of the form n(C2H4)on the Raney nickel catalyst. I t is believed that polymer forms only on the most active sites and a t long process times, when these sites are covered, polymerization is greatly reduced and the reaction is biased toward the
formation of ethane. All previous studies on the hydrogenation of acetylene fail to mention such a catalyst induction period. Acknowledgment
Raney nickel catalyst used in this study was donated by the W. R. Grace Co. Computer time was supplied by the Computer Science Center a t the University of Maryland under NASA Research Grant NSG-398. literature Cited
Bond, G. C., “Catalysis by Metals,” Academic Press, New York, 1962. Calderbank, P., Evans, F., Farley, R., Jepson, G., Poll, A., “Catalysis in Practice,” p. 66, Institute of Chemical Engineers, London, 1963. Dupont, G., Bull. SOC.Chim. Fr. 3; 1030 (1936). Farkas, E., Sc. D. thesis, Massachusetts Institute of Technology, Cambridge, Mass., 1964. Farkas, A., Farkas, L., J . Amer. Chem. SOC.61, 3396 (1939). Kolbel, H., Chem. Eng. Sci. 14, 151 (1961). Kolbel, H., Hammer, H., Meisl, V., 3rd European Symposium on Chemical Reaction Kinetics, p. 115, 1964. Kolbel, H., Maennig, H., 2. Elektrochem. 66, 744 (1962). Mann, R., Safo, S., private communication, November 1968. Mars, P., Gorgels, M., 3rd European Symposium on Chemical Reaction Engineering, p. 55, 1964. Ozawa, Y., Bischoff, K. B., IND. ENG. CHEM.PROCESS DES. DEVELOP 7,67,72 (1968). Schlesinger, M., Crowell, J., Leva, M., Storch, H., Ind. Eng. Chem. 43, 1474 (1951). Sheridan, J., J . Chem. SOC.1944, 373. Slesser, C., Highet, J., Brit. Chem. Eng. 11 (4), 247 (1966).
RECEIVED for review February 12, 1969 ACCEPTED May 15, 1970
Photochemical Decomposition Rates of Potassium Ferrioxalate in Cone-Shaped Reactor
P. R. Harris, M. C. Hawley, and M. H. Chetrick Department of Chemical Engineering, Michigan State University, East Lansing, Mich. 48823
THE
study is based on earlier work (Harris, 1964) which suggested improvements in experimentation and theory for scale-up of photochemical reactions. In the previous research, a cylindrical annulus with a linear source a t the center, aligned with cylinder axis, was used. I t was assumed that all radiation from the source was radial, but actually radiation is skewed from a linear source. Jacob and Dranoff (1966) tried to remedy this problem by putting spaced mica disks around the source. 540
Ind. Eng. Chem. Process Des. Develop., Vol. 9, No. 4, 1970
Use of a point source in a spherical geometry eliminated the earlier problem with the linear source, since no line can be skew to a point. Another experimental advantage was the increase of volume and the strong decrease of radiation intensity with an increase of reactor radius. The decomposition of potassium ferrioxalate (Hatchard and Parker, 1956) was the photochemical reaction used. Composition and other quantities were accurately determined for this system. Analysis was based on a continuous
