4
Ind. Eng. Chem. Fundam. 1983, 22, 4-6
Hayden, J. G.; O'Connell, J. P. Ind. Eng. Chem. Process Des. Dev. 1975, 74, 20s. Knobler, C. M. I n "Chemical Thermodynamics", McGlashan, M. L., Ed.; The Chemical Society: London, 1978; Vol. 2, Chapter 7. McElroy, P. J.; Shannon, T. W.: Williamson, A. G. J. Chem. Thermodyn. 1980, 72, 37 1. O'Connell, J. M.: Rausnitz, J. M. Ind. Eng. Chem. Process Des. Dev. 1967, 6 , 245.
Reid, R. C.; Sherwood, T. K. "The Properties of Liquids and Gases"; McGrawHill: New York, 1958: p 21. Shannon, T. W. Ph.D. Thesis, University of Canterbury, New Zealand, 1976. Tsonopoulos. C. AIChE J. 1074, 2 0 , 263.
Received for review July 2, Accepted August 24,
1981 1982
Global Rates of Reaction in Trickle-Bed Reactors: Effects of Gas and Liquid Flow Rates Mordechay Herskowltz' and Shelomo Mosserl Department of Chemical Engineering, Ben Gurion University of the Negev, Beer Sheva, Israel
Global rates of reaction were measured in a batch-recycle trickle-bed reactor. The hydrogenation of a-methylstyrene to cumene on a Pd/A1,03 catalyst at atmospheric pressure and 40 O C was employed. The liquid and gas flow rates were varied over a wide range in the gas-continuous flow regime. The effect of gas and liquid rates on the rate of reaction is Significant. Calculations of the wetting efficiency were performed with intrinsic kinetic parameters measured in a stirred tank and the Goto and Smith correlation for the liquid-solid mass transfer. The results indicate that the wetting efficiency depends strongly on the liquid rate at high gas rates, and it is almost independent of liquid rate at low gas rates.
Trickle-bed reactors have been the subject of numerous experimental studies during the past decade. Recent studies (Morita and Smith, 1978; Herskowitz et al., 1979; Mata and Smith, 1981) have indicated that in the gascontinuous flow regime the liquid flow rate may have a significant effect on the global rate of reaction, especially at low liquid flow rate. The effect is twofold. Increasing the liquid flow rate enhances both the rate of liquid-solid mass transfer and the wetting efficiency of the catalyst particles (defined as the fraction of the particle external surface covered by flowing liquid). The wetting efficiency f, is less than unity a t liquid Reynolds numbers less than about 20. It has been reported in a number of studies (Herskowitz, 1978) that f decreases with decreasing liquid flow rate, but no reliable correlation off as a function of the liquid flow rate and other operating parameters has yet been proposed. If the particle is not completely covered by flowing liquid (f < l),gaseous reactants may be transferred from the gas to the particle surface through a very thin layer of stagnant liquid (called the gas-covered surface or the dry surface). The resistance to mass transfer on the gas-covered surface may be significantly lower than on the liquid-covered surface. This would enhance the global rate of reaction. As a result, the global rate of reaction may increase with decreasing liquid flow rate, even though the liquid-solid mass transfer rate decreases. Herskowitz et al. (1979) and Mata and Smith (1981) have measured global rates of reaction that decreased with increasing liquid flow rate, reached a minimum, and then increased with further increase in liquid flow rate. Clearly, this behavior is expected if the liquid-solid mass transfer resistance is important over the range of liquid flow rates. The effect of the gas flow rate on the global rate of reaction has also been investigated. Morita and Smith (1978) and Herskowitz et al. (1979) found this effect to be insignificant. However, the experiments were performed at relatively high liquid flow rates. Furthermore, the ac-
tivity of the catalyst was not high so that the effect of the gas flow rate would be expected to be insignificant. The objective of this work was to determine the effect of the gas and liquid flow rate on the global rate of reaction. The hydrogenation of a-methylstyrene on a Pd/ A1203catalyst was selected for this work. The reaction is first order in hydrogen, with cumene as the only product. At ambient temperature and atmospheric pressure a relatively active Pd/A120, catalyst could yield high intrinsic rates of reaction, so that the interphase mass transfer resistances would be important. The intrinsic kinetics was measured in a batch reactor for the liquid. Trickle-bed runs were performed in a batch-recycle reactor. Experimental Section The stirred tank reactor consisted of a 1-Ljacketed resin flask manufactured by Sovirel. A four-bladed turbine impeller powered by a variable-speed motor provided thorough agitation. The reactor operation was semibatch with the hydrogen being introduced through a dispersion tube and leaving through a condenser. The dispersion tube was located just below the impeller to improve the gasliquid mass transfer. The condenser was maintained at 0 "C so that liquid loss could be avoided. In the batch-recycle system the liquid was pumped from a reservoir (where the liquid was saturated with hydrogen) to the reactor, flowed concurrently with the gas (hydrogen), and then was separated from the gas and returned to the reservoir. A schematic diagram of the apparatus is shown in Figure 1. The liquid was fed to the reactor by a variable speed peristaltic pump (Heidolph). Fluctuations in the pump discharge were dampened with a surge tank. The distributor consisted of five 0.09-cm i.d., 1.1cm long capillary tubes (stainless steel) for the liquid and eight 0.05-cm holes for the gas, placed uniformly as shown in Figure 2. Several runs were performed with a similar distributor made of twelve tubes.
0196-4313/83/1022-0004$01.50/00 1983 American Chemical Society
Ind. Eng. Chem. Fundam., Vol. 22, No. 1, 1983 5
Table 11. Intrinsic Rate Data intrinsic rate eff tiveness const. diffusivity, cm2/s factor cm3/gs effec-
reacn rate mol/s.g 3.44 X
part. av diam, cm
