697
Ind. Eng. Chem. Res. 1989,28, 697-702
N,, = molar velocity, mmol-min-l m K = mass of catalyst, g pi = partial pressure, bar s = specific surface area, m2.g-l .P = temperature, K TR = reaction temperature, *C up = catalyst pore volume, m3.g-l xi = liquid mole fraction Xi = conversion, mol % yi = gas mole fraction Greek Symbols v = stoichiometric coefficient g = viscosity, Pa-s
Subscripts BP = o-tert-butylphenol cis-OL = cis-2-methylcyclohexanol H = hydrogen i = component i j = reaction step j MP = o-cresol (2-methylphenol) OL = 2-methylcyclohexanol ON = 2-methylcyclohexanone PH = phenol derivative tr-OL = trans-2-methylcyclohexanol 0 = initial condition Registry No. MP, 95-48-7; ON, 583-60-8; CS-OL, 7443-70-1; Tr-OL, 7443-52-9; BP, 88-18-6; Ni, 7440-02-0.
Literature Cited Bartok, M. Stereochemistry of heterogeneous metal catalysts; Wiley: New York, 1985. Carberry, J. Chemical and catalytic reaction engineering; McGrawHill: Cambridge, 1970. Fedoseenko, V. I.; Yursha, I. A.; Kabo, G. J. Dokl. Akad. Nauk BSSR 1983, 27, 926. Froment, G. F.; Bischoff, K. B. Chemical reactor analysis and design; Wiley: New York, 1979. Kabo, G. J.; Frenkel, M. L. Thermodynamics of diastereomeric transformations of alcohols with different carbon-skeleton structures. J. Chem. Thermodyn. 1983,15, 377. Kiperman, S. L. Some problems of chemical kinetics in heterogeneous hydrogenation catalysis. In Catalytic hydrogenation; Cerveny, L., Ed.; Elsevier: Amsterdam, 1986. Kut, 0. M.; Datwyler, U. R.; Gut, G. Stereoselective Hydrogenation of 2-tert-Butylphenol to cis-2-tert-Butylphenol. 2. Kinetics of the Liquid-phase Hydrogenation of 2-tert-Butylphenol over Nickel, Cobalt, and Noble Metal Catalysts. Ind. Eng. Chem. Res. 1988, 27, 219. NMR-Spectra, Sadler Research Laboratories, Philadelphia, 17117 M and 21229 M, 1975. Schumann, W. K. Stereoselektive Gasphasenhydrierung von 2-A1kylphenolen. Ph.D. Thesis 8524, ETH-Zurich, 1988. Van Krevelen, D. W.; Chermin, H. A. G. In Landolt-Bdrnstein Zahlenwerte und Funktionen aus Physik, Chemie, Astronomie, Geophysik, Technik, 6th ed.; Springer Verlag: Berlin, 1961; Vol. 11/4, p 18. Received for review J u n e 6, 1988 Accepted February 13, 1989
Influence of Reaction Parameters on the Stereoselectivity of the Nickel-Catalyzed Gas-Phase Hydrogenation of o -Cresol. 2. Kinetic Modeling W e r n e r K. S c h u m a n n , Oemer M. K u t , and Alfons B a i k e r * Department of Industrial a n d Engineering Chemistry, Swiss Federal Institute of Technology (ETH), 8092 Zurich. Switzerland
A modified Langmuir-Hinshelwood model was used to describe the gas-phase hydrogenation of o-cresol over a nickel on silica catalyst. To optimize the various estimated model parameters, a stepwise procedure, which includes differential and integral measurements in a plug-flow reactor, was used. The model is based on nonequilibrium adsorption of the intermediate product, a reaction order in hydrogen and substrate of one for each reaction step, competitive adsorption between hydrogen and organic substances, and thermodynamic restrictions for the reaction system. T h e estimated kinetic parameters were found to be consistent with imposed physical criteria. The kinetic model was also successfully applied to describe the kinetic data of the hydrogenation of o-tertbutylphenol. A critical analysis of the method used for the development of the kinetic model for these complex reaction systems is given. The effort which has been expended to search for kinetic models to describe the gas-phase hydrogenation of simple aromatics (such as benzene) has been reviewed by Kiperman (1986). Van Meerten and Coenen (1977) have shown that the kinetic data of the benzene hydrogenation can be fitted to various models based on different mechanisms. All of these models explained the observed kinetic behavior, including the conversion maximum at increasing temperatures. Chou and Vannice (1987) report that the observed kinetic behavior as well as results obtained by isotopic exchange and deactivation reactions can be described by including an auxillary reaction-the btnzene hydrogenation-to various carbonaceous surface species. To our knowledge, there has been no report in the literature dealing with the kinetic modeling of more complex 0888-5885/89/2628-0697$01.50/0
stereoselective hydrogenations of aromatics, such as alkylphenols, in the gas phase. Kut et al. (1988) reported on the influence of reaction conditions as well as the influence of various metal catalysts on the stereoselective liquid-phase hydrogenation of o-tert-butylphenol. The observed kinetic behavior was described by a modified Langmuir-Hinshelwood-typemodel based on nonequilibrium sorption of the intermediate product. In the preceding paper in this issue, we studied the influence of the reaction parameters on the stereoselectivity of the hydrogenation of o-cresol and o-tert-butylphenol in the gas phase (Schumann et al., 1989). A reaction pathway, which includes nonequilibrium sorption of the intermediate product and thermodynamic restrictions for the reaction system, was postulated. The aim of
0 1989 American Chemical Society
698 Ind. Eng. Chem. Res., Vol. 28, No. 6, 1989
this work is to develop a kinetic model which accounts for the observed kinetic behavior and describes the influence of the reaction parameters on the stereoselectivity of the reaction.
190
I I I
a
3
z = 2
I-
*
180
a
Evaluation of Kinetic Data The kinetic data were free from inter- and intraparticle mass-transfer influences, as has been shown in our previous work (Schumann et al., 1989). In the same work, a brief description of the experimental procedure was given. Differential Reactor Experiments. Reaction conditions were chosen such that low conversions were obtained. Hence, one can assume that the reaction rate is constant over the whole reaction zone. This mean value for the reaction rate is given by
I I
W
W
53
tL -I
5
170
x U
160
- 50
,
1
0
50
I
1
100
AXIAL POSITION
150
200
(2)
"[I Figure 1. Simulation of the axial temperature profile within the reactor bed dashed line, temperature of molten salt; continuous line, simulated temperature; symbols, measured temperature.
For the modeling of the kinetic data, the values of the reaction parameters were estimated as follows: The reaction temperature was taken as the temperature measured in the center of the catalyst bed. The partial pressure of the organic substances was approximated by the mean of the inlet and outlet pressures. Since hydrogen was always in excess and since only differential conversions were obtained, the inlet hydrogen pressure was used in the kinetic modeling. Experiments were carried out with various feed mixtures. As mentioned in part 1 of this study (Schumann et al., 1989), the thermodynamic equilibrium for the conversion of 2-methylcyclohexanone to 2-methylcyclohexanol is reached when the feed consists of a mixture of o-cresol and 2-methylcyclohexanol(60:40). As mean concentrations are used in this mode of analysis, the results of these experiments cannot be analyzed by this method. Integral Reactor Experiments. This mode of operation resulted in high conversions and significant axial temperature gradients in the catalyst bed. The differential rate equations for the individual components were integrated by using the computer program SDRIV3 (ETH-Zurich program library). This program enables the integration of nonstiff, first-order differential equations, combined with algebraic equations, by using the Adams method with variable order and path length. After each integration step, the partial pressures were readjusted by means of a mass balance. Since the reactor could no longer be assumed to be isothermal, the axial temperature profile within the catalyst bed had to be taken into account. Radial temperature profiles within the catalyst bed were assumed to be negligible. A stationary energy balance, assuming a one-dimensional, pseudohomogeneous model, gave the following equation for the axial temperature profile (Bird et al., 1960):
where 2=4 L $ = - TR -
Ts - To
( TR 1
This equation takes into account heat transfer due to conduction, flow, chemical reaction, and energy losses
cs-OL
t
~
= *
t r - O L
r
-
~
through the wall. The constants ml, m2, and m3 are products of physical and thermodynamic properties of the reacting system (Schumann, 1988). For each data point, these constants were estimated in separate regression calculations from the measured temperature profiles within the reactor bed. A simulation of a typical axial temperature profile is shown in Figure 1.
Kinetic Modeling Model Equation. Figure 2 shows the relevant steps of the o-cresol hydrogenation reaction. Thermodynamic considerations indicated that the reverse reaction of 2-methylcyclohexanone to 2-methylcyclohexanol is important but that the same reaction for the conversion of o-cresol to 2-methylcyclohexanone is only of minor significance (Schumann et al., 1989). The suitability of various kinetic models (all based on Langmuir-Hinshelwood or adapted Langmuir-Hinshelwood mechanisms) was investigated to describe the kinetic data (Schumann, 1988). The best results were obtained with a model that is based on the following assumptions: each reaction step is first order in hydrogen and organic substrate; competitive adsorption exists between hydrogen and organic substrate; there is nonequilibrium adsorption of the intermediate product. Therefore, the rates of the individual components are described by the following set of equations: -~PH=
r c s - o=~ ~
Ts, dimensionless temperature; To = 120 "C)
=
Figure 2. Proposed reaction pathway for the hydrogenation of o-cresol; (*) adsorbed species.
(O
