Znd. Eng. Chem. Res. 1995,34, 2811-2816
2811
Framework for Correlating Composition Dependent Equilibrium Conversion in Methyl tert-Butyl Ether Formation by Ion-Exchange Catalysts Michael Ladisch,* Paul Westgate3 Richard Hendrickson, and Mark Brewer Laboratory of Renewable Resources Engineering, Department of Agricultural Engineering, Purdue University, 1295 Potter Center, West Lafayette, Indiana 47907-1295
Catalyst performance for the reaction of methanol and isobutylene (IB) to form methyl tertbutyl ether can be assessed based on maximum conversion. The equilibrium conversion attainable for this reaction is of practical interest since separation of products from reactants downstream of the reactor can be simplified as the extent of conversion increases. A framework is presented by which different catalysts can be compared on an internally consistent basis for different temperatures, isobutylene concentrations, and/or methanollisobutylene mole ratios. An equilibrium expression which accounts for the presence of nonreacting components is presented to correlate the effect of methanoHB ratios and IB concentrations with observed conversions for different catalysts. Assumptions inherent in this method are discussed and data for several types of ion-exchange catalysts illustrate use of this framework.
Introduction The reaction of methanol (MeOH) with isobutylene (IB) to form methyl tert-butyl ether (MTBE)is a reversible reaction of the type MeOH
+ IB * MTBE
(1)
Over the range of temperatures (40-80 "C) and methanol/IB molar ratios ( 21)typical for MTBE formation, the selectivity is nearly 100%. This reaction is of significant practical interest since MTBE is a widely used nonleaded octane booster in gasoline as a consequence of the provisions of the Clean Air Act Amendment of 1990. Greater demand is likely in the future because of provisions which will take effect on Jan 1, 1995. Reactor configurations for manufacturing MTBE include tubular non-isothermal reactors, two-stage adiabatic reactors, boiling point reactors, and combinations of simultaneous reaction and distillation systems (Miller and Piel, 1989;Jones et al., 1989). Reactioddistillation separates product from reactants in the presence of catalyst with the objective of driving the reversible MTBE reaction t o higher conversions (Jacobs and Krishna, 1993; Jones et al., 1989). These processes, designed for a wide range of feed compositions and operating conditions, give different conversion end points if equilibrium conversions are attained. Both kinetic and equilibrium effects are associated with the methanol concentration in an MTBE reaction mixture. The Helfferich model (1954) presents the liquid reaction mixture as consisting of a bulk phase away from the catalyst (in which no reaction occurs) and a second phase associated with the catalyst in which the reaction occurs and reactants and product are in equilibrium with the external liquid. This model justifies the use of homogeneous kinetics t o describe heterogeneous catalysis by ion-exchange resins such as those used for MTBE formation (Pitochelli, 1976). In the case of MTBE, the reaction can be approximated as
* To whom correspondence should be addressed.
Current address: W. R. Grace & Co.-Conn, Washington Research Center, 7379 Route 32,Columbia, Maryland 210444098. +
first order with respect to IB, and zero order with respect to methanol, since methanol sorbs into the resin, surrounds the activated sites, and solvates the protons (Ancillotti et al., 1977, 1978). Catalyst activity (as measured by rate of reaction) decreases with increasing methanol up to a bulk phase concentration of 3 m o m where a leveling-off effect occurs. A methanol concentration of 3 mol& corresponds to about 17 wt % methanol in a typical olefin mixture. Ancillotti and his co-workers carried out studies with Amberlyst 15 and, in 1978, were the first of the early researchers to point out the experimental observation that the alcohol is "strongly in excess in the liquid pores owing to a selective swelling", as confirmed by high heats of immersion and by NMR and IR studies. They also found that the higher reactivity of methanol "has to be attributed to its preferential adsorption on the resin rather than its greater nucleophilicity". They suggested that the Hammett-type acidity function is appropriate for describing catalyst acidity because the hydrogen ion concentration in the sorbed liquid approaches 3 equiv/L (Ancillotti et al., 1977, 1978). Rehfinger and Hoffmann (1990) present plots based on their kinetic model which show that equilibrium conversions at a given temperature are expected to increase with increasing isobutylene concentration as well as methanollisobutylene ratio. For example, a t 60 "C and a stoichiometric methanol/IB ratio, about 87% conversion would be expected at 10 mol % concentration of isobutylene (in the C4 fraction) compared to 90% conversion for 20% isobutylene (data from Rehfinger and Hoffmann (19901, Figure 14). Similarly, it is predicted that an increase in the methanollisobutylene mole ratio from 1 to 1.5 increases the equilibrium conversion of isobutylene from about 92 to 97.5% a t 60 "Cfor a feed containing 45 mol % isobutylene in the Cd fraction. At temperatures below 60 "C, larger methanol/ isobutylene ratios have little effect on the predicted equilibrium conversions. At 38 "C, conversions vary less than 1.5%, from 98 to 99.5, for methanollisobutylene ratios of 1.1-5.0 and 45 mol % isobutylene (data from Rehfinger and Hoffmann (19901, Figure 15). The relationship between methanol/IB ratio and temperature is readily explained, recognizing that MTBE formation is exothermic. Decreases in temperature
0888-588519512634-2811$09.00/00 1995 American Chemical Society
2812 Ind. Eng. Chem. Res., Vol. 34, No. 8, 1995
therefore have the general effect of increasing equilibrium conversion. While decreases in temperature and increases in methanol concentration can increase the maximum conversion, reactor design and catalyst selection must account for the accompanying decrease in reaction rates if close to equilibrium conversion is to be achieved. The literature on MTBE catalysts reports conversions for a range of compositions, as well as reactor configurations. The outlet temperature of a given reactor is usually readily identified, and therefore its potential effect on the equilibrium conversion is easily assessed. However, estimation of equilibrium conversion is more complicated when both temperature and feed compositions differ from one case t o the next. This paper presents a framework by which equilibrium conversions can readily be extrapolated from one condition to another and thereby facilitate correlation of maximum conversions for catalysts evaluated at different conditions. Conversions are calculated for a range of K, values a t several different reactant mixtures to generate characteristic curves which are useful for such comparisons.
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