Ind. Eng. Chem. Res. 1996, 35, 1269-1274
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Kinetic Study and Modelization of n-Butenes Oligomerization over H-Mordenite L. M. Tiako Ngandjui and F. C. Thyrion* Chemical Engineering Institute, Louvain University, 1 voie Minckelers, B-1348 Louvain-la-neuve, Belgium
n-Butenes have been oligomerized at temperatures lower than 450 K in the liquid phase over H-mordenite in a continuously stirred tank reactor. The composition and molecular weight of the products are very dependent on reaction temperature. Products are essentially dimers, trimers, and tetramers. The complex reactions were modeled by taking into account only the type of carbonium ion; the initial rates showed that the oligomerization reactions followed a Rideal mechanism. The dimerization rate constants of butenes and oligomers decrease with increasing number of carbon atoms of the olefins, while the adsorption constants increase with the hydrocarbon chain length. A model for the conversion of butenes to liquid products over H-mordenite is proposed. This model fits the experimental data rather well, although some simplifying approximations were made to describe these complex reactions. Introduction In recent years, many research and development companies have developed process technology for the conversion of light olefins to gasoline and higher molecular weight distillates (Tabak et al., 1984, 1986; Garwood et al., 1983, 1988; Juguin et al., 1990, 1992, etc.). These new processes are based on zeolite catalysts. Although several zeolites are reported as being active for alkene oligomerization (Occelli et al., 1985; Heveling et al., 1988), by far the most promising industrial route of the oligomerization of light alkenes into liquid fuels is presently based over ZSM-5 and, for a lesser extent, over modified H-mordenite catalyst. Up to now, few people (Forni et al., 1975; Alberti, 1987; Quann et al., 1991) have studied the kinetics of the oligomerization reactions over zeolite catalysts. It is the goal of this study to investigate the oligomerization of normal butenes over H-mordenite. Some fundamental aspects of the kinetics of this reaction were examined, and a model was proposed to fit the experimental results. Experimental Section Catalyst Description and Activation. The catalyst from Zeocat was a highly dealuminated mordenite (HMOR(38)) in a protonic form with a Si/Al ratio of 38. It is nearly a polymorphic silica with a mordenite structure, the alumina content being around 1% (wt) and the sodium concentration around 100 ppm. The number of acidic sites is low, so that they are quite isolated from each other in the zeolite lattice, but these sites are assumed to be very strong. Another typical property of these catalysts is their porosity; besides the microporosity of the mordenite structure around 7 Å, there is a secondary porous system ranging from 50 to 200 Å. The experiments were realized with the white, freeflowing powder having a granulometry between 20 and 100 µm and a BET specific area of 500 m2 g-1. The calcined catalyst was activated at 773 K for 2 h under a flow of dry air in order to remove water and was kept inside a dessicator until use. Much care was * Author to whom correspondence should be addressed. Phone: 32 (10) 472327. Fax: 32 (10) 472321. E-mail: Thyrion@ PRCD.UCL.ac.be.
0888-5885/96/2635-1269$12.00/0
taken to avoid any contact between the catalyst powder and the atmospheric air. The catalyst was introduced into the reactor under a controlled nitrogen atmosphere in order to prevent any contact between the catalyst and air moisture. Apparatus and Procedure. n-Butenes were oligomerized in the liquid phase using n-pentane as the solvent in a 300 mL continuously stirred tank reactor installed in an oven regulated by a PID temperature controller. The flow sheet of the installation is shown in Figure 1. The alimentation tank containing n-butenes and n-pentane (solvent) was maintained under a 2 MPa helium pressure. This high pressure prevents any cavitation problem in the pump head. The liquid feed (butenes + n-pentane) was filtered across a 2 µm frit and pumped into the reactor by a piston-action HPLC pump via a heating coil. The catalyst was in suspension in the reaction mixture. To avoid catalyst withdrawing during sampling, a 0.5 µm filter was placed at the extremity of the dipping tube installed inside the reactor. The liquid effluent was cooled and sent to a liquid chromatography sampling valve and then via a back-pressure regulator to a knockout pot at room temperature. In order to check the flow, the pot was weighed continuously by using a balance equipped with a timer-recorder. The reaction was conducted in the liquid phase, and the reactor pressure was set at approximately 6 MPa. The starting procedure involved first the flushing of the reactor with nitrogen, then the introduction of a weighted amount of catalyst, the closing of the reactor, and finally the connection of the reactor to the installation. The reactor was first pressurized with nitrogen, the feed pump was then started, and the reactor temperature was raised to the required temperature. A thermodynamic study was realized to evaluate the temperature and the pressure required in order to maintain the reaction medium in the liquid phase. Results and Discussion Thermodynamics of Phase Equilibrium. It is well-known that the physical state of the reaction mixture has an influence on mass transfer and diffusion coefficients of reactants. To prevent any discontinuity in kinetic results, it is important that the kinetic data are obtained in the same phase (the liquid phase). © 1996 American Chemical Society
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Ind. Eng. Chem. Res., Vol. 35, No. 4, 1996
Figure 1. Experimental setup for n-butenes oligomerization.
This kinetic study was realized at a molar ratio of butenes over n-pentane situated between 0.5 and 1. For this feed composition the critical case is the one corresponding to a ratio of 1; that is the reason why the phase envelopes were calculated at this composition. As the reaction was carried out in a continuously stirred tank reactor at low conversion (
