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Ind. Eng. Chem. Res. 2004, 43, 4060-4065
KINETICS, CATALYSIS, AND REACTION ENGINEERING Thermodynamics and Kinetics of the Dehydration of tert-Butyl Alcohol Maija L. Honkela,* Tuomas Ouni, and A. Outi I. Krause Helsinki University of Technology, Department of Chemical Technology, P.O. Box 6100, Espoo FIN-02015 HUT, Finland
The dehydration of tert-butyl alcohol to water and isobutene was studied using an ion-exchange resin catalyst at temperatures between 60 and 90 °C. A temperature-dependent equilibrium constant for the dehydration reaction was obtained that gave a reaction enthalpy of 26 kJ mol-1, in good agreement with values in the literature. Measured data were used for kinetic modeling of the reaction. The best model with physically meaningful parameters was of LangmuirHinshelwood type where isobutene does not adsorb on the catalyst. The activation energy for the reaction in this case was 18 kJ mol-1. Introduction Both the dehydration of tert-butyl alcohol (TBA) to isobutene and the reverse reaction have been studied widely. TBA production is of interest because of the use of TBA as a gasoline component [RON (research octane number) ) 109, MON (motor octane number) ) 91].1 TBA formed as a side product in 1,2-epoxypropane synthesis is used not only as a gasoline component2 but also in the production of isobutene for methyl tert-butyl ether (MTBE) and other high-octane gasoline components.3,4 Also, direct routes from TBA to ethers (without isobutene formation in between) exist.5-7 Gates et al.8 studied TBA dehydration on Amberlyst 15 ion-exchange resin catalyst and proposed a carbonium ion mechanism at low TBA concentrations. In this mechanism TBA and the proton of the catalyst form a tert-butyl cation. This cation can either react back to TBA or form isobutene at the same time as the proton is regenerated. At higher concentrations, the reaction was reasoned to proceed according to a concerted mechanism involving the participation of several active sites. In later studies, Gates and Rodriguez9 proposed a rate equation that takes into account the active sites that result from alcohol adsorption. In the presence of water, the reaction was of first order with respect to the TBA concentration, and with low water contents, the rates were represented by Langmuir-Hinshelwoodtype kinetics. Abella et al.10 also studied the kinetics of the dehydration of TBA in the liquid phase. However, they used an atmospheric-pressure system, and because the isobutene that was produced evaporated from the mixture, the reaction was considered irreversible. The hydration of isobutene on ion-exchange resins is different from the dehydration of TBA because a large amount of water is present on the catalyst. Gupta and Douglas,11 for example, carried out experiments in which water was present in large excess so that the * To whom correspondence should be addressed. E-mail:
[email protected]. Fax: +358-9-451 2622.
resin was fully swollen. They obtained first-order irreversible kinetics for the hydration reaction. Delion et al.12 applied various solvents in the hydration of isobutene with the aim of keeping the mixture in a single liquid phase. They tested p-dioxane, acetone, nitromethane, butylcellosolve (2-butoxyethanol), isopropyl alcohol, cyclohexanol, tetrahydrofurfurylic alcohol, and acetic acid and calculated solvent-dependent equilibrium constants for the reaction. Velo et al.13 obtained both equilibrium constants and kinetics for the hydration of isobutene. The kinetic equations were based on a carbonium ion mechanism in which isobutene forms a tert-butyl cation with the proton of the catalyst. They also concluded that TBA inhibits the hydration of isobutene more than water. In another study,14 they found that intraparticle diffusivity increased with temperature and decreased with TBA concentration. Diffusion has also been studied in other publications. TBA dehydration studies indicate that, when macroporous ion-exchange resins are used as catalysts, masstransport limitations do not exist.8,15 Mass transport seems to affect the rates of isobutene hydration, however.16 Studies in a trickle-bed reactor with an aqueous phase and isobutene as the gas indicated that intraparticle diffusion has a greater effect on the rate than does liquid-to-particle mass transfer.17 In trickle-bed reactors, both the wetting efficiency and mass transfer influence the total rate.18 On an acidic ion-exchange resin catalyst, isobutene reacts to form diisobutenes and higher oligomers. Moreover, when diisobutenes need to be produced selectively, polar components such as TBA, water, or methanol are added to the dimerization.19-21 This means that the dehydration of TBA (or the hydration of isobutene) might also occur in the dimerization of isobutene. Although TBA dehydration and isobutene hydration on ion-exchange resin catalysts have been studied widely, few studies have been carried out under isobutene dimerization conditions, i.e., in the liquid phase with
10.1021/ie049846s CCC: $27.50 © 2004 American Chemical Society Published on Web 06/05/2004
Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004 4061
low initial water content. The purpose of the present work was to study the equilibrium of TBA dehydration under these conditions and to construct a kinetic model for the reaction. Experimental data were used to obtain equilibrium constants and reaction enthalpies and to determine parameters for kinetic equations derived from various mechanistic assumptions. Experimental Section Reactor System. Experiments were carried out in a stainless steel batch reactor (80 cm3) in the liquid phase. The catalyst (1 g) was placed in the reactor in a metal gauze basket. The reactor included a magnetic stirrer and a mixing baffle. It was pressurized with nitrogen to about 1.3 MPa to keep the reaction mixture in the liquid phase, and it was held in a thermostated water bath with which various temperatures (60-90 °C) could be maintained. The mixing speed in the batch reactor was 1000 rpm so that no external mass-transfer limitations would occur. Samples were taken manually from the reactor via a sample valve. Analytical Methods. The samples taken from the reactor were analyzed with a Hewlett-Packard 5890 Series II gas chromatograph (GC) equipped with a DB-1 capillary column of length 60 m, film thickness 1.00 µm, and diameter 0.250 mm. A flame ionization detector was used. The products were quantified by treating the solvent as an internal standard. Water in the product could not be analyzed by GC, and the molar balance for isobutene was poor because of its high volatility. The analysis of TBA can be considered reliable, however, and the amounts of water and isobutene were accordingly calculated from the TBA molar balance. Chemicals and Catalyst. TBA (Merck Schuchardt OHG, >99%) was the reactant and isopentane (Fluka Chemika AG, G99%) the solvent. Isooctane (Fluka Chemika AG, G99.5%) was added to the reaction mixture to study the reliability of the sampling. The amount of water in the TBA was less than 0.1 wt %. The catalyst was a commercial acidic ion-exchange resin consisting of a styrene-divinylbenzene-based support to which sulfonic acid groups had been added as active sites. The surface area of the dried catalyst was measured to be 37 m2/g (BET analysis), the acid capacity 5.1 mmol/g (by titration22), and the particle size between 0.42 and 1.0 mm. Before use, the catalyst was dried overnight in an oven at about 100 °C. The water content of the dried catalyst was measured to be 1.7 ( 0.5 wt % (AMB 310 moisture analyzer). Intraparticle Resistances. The dried catalyst was sieved to different particle sizes and tested at 80 °C in a continuous flow system to study whether internal diffusion limitations existed. The reactor system used has been described in detail in our previous publication.21 The total flow of isopentane and 2 mol % of TBA was about 36 g h-1. The conversion of TBA at the steady state was the same (43-44%) in the experiments with catalyst particle sizes of 0.42-0.59, 0.59-0.71, and 0.71-1.0 mm. Unfortunately, the experimental setup did not allow for the testing of smaller particle sizes. The results suggest that internal diffusion limitations were not present. The unsieved catalyst was used in further experiments. Experiments. In this work, the dehydration of TBA was studied in the absence of isobutene with TBA as the sole reacting component in the initial reaction mixture. Experiments were carried out with TBA con-
Figure 1. Mole fractions as a function of time in an experiment at 60 °C.
tents of 2-18 mol %, with an isooctane content 1 mol %, and with isopentane as the solvent. The temperatures studied were 60-90 °C, and the pressure was kept at about 1.3 MPa. Several samples were taken from the reactor during each experiment. The reaction was carried out for about 6 h, during which time the system reached equilibrium. Two or more points were used in every experiment to determine whether equilibrium was reached, and in uncertain situations, the experiment was repeated. An example of molar fractions as a function of time in a typical experiment is presented in Figure 1. Under the conditions applied, isobutene did not dimerize appreciably (diisobutene content < 0.06 mol % at 90 °C). The isobutene dimerization rate clearly decreases with increasing TBA content.21 This explains the negligible diisobutene formation in these experiments with relatively high TBA contents. Because of the low TBA conversions, the isobutene and water contents were low and only one liquid phase was observed. Thermodynamics. Equilibrium constants Ka can be calculated with equation
Ka )
aIBaH2O aTBA
)
xIBxH2O γIBγH2O xTBA
(1)
γTBA
where ai is the activity of component i, xi is the corresponding mole fraction at equilibrium, and γi is the activity coefficient. Activity coefficients were calculated by the Dortmund modified UNIFAC method.23 Equilibrium constants can also be expressed in terms of the Gibbs free energy change for the reaction (∆rG)
(
Ka ) exp -
)
(
)
∆rG ∆rH ∆rS + ) exp RT RT R
(2)
where ∆rH is the enthalpy change and ∆rS the entropy change for the reaction and R is the universal gas constant. If the temperature range investigated is narrow, ∆rH and ∆rS can be assumed to be independent of temperature. Kinetic Modeling. The parameters of the kinetic models were determined using Kinfit software24 with the Levenberg-Marquardt optimization algorithm. In the optimization, various kinetic models were combined with an ideal batch reactor model, and the calculated compositions were compared with the measured ones. The temperature dependence of the rate constants was described by the Arrhenius equation
(
k ) F exp -
E RT
)
(3)
4062 Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004 Table 1. Reaction Rate Equations model
1
rate equation
-rTBA )
(
k(KaaTBA - aH2OaIB)
aIB +
2
-rTBA )
KH2O KTBA aTBA + a KIB KIB H2O
-rTBA )
)
2
LH (two sites)
k(KaaTBA - aH2OaIB) aTBA +
3
mechanism
KH 2O KTBA
LH (one site)
aH2O
k(KaaTBA - aH2OaIB) 1 + KH2OaH2O
Petrus et al.26,27
where F is the preexponential factor, E is the activation energy, and R is the universal gas constant. This equation was reparametrized to the form
[ (
k ) Fref exp -
E 1 1 R T Tref
)]
TBA + H+ S TBA‚H+ TBA‚H+ S H2O‚H+ + IB H2O‚H+ S H2O + H+
(4)
where Tref is the reference temperature and Fref and E are the parameters to be optimized. Tref was chosen to be 343 K (70 °C). The adsorption equilibrium parameters were assumed to be independent of temperature. In the tested models, the reaction on the surface of the catalyst was considered as the rate-determining step, and the active sites of the catalyst were assumed to be equivalent. Parameters for a model that took into account the different active sites that result from alcohol adsorption9 were determined, but they did not give satisfactory results. Furthermore, adsorbed components were assumed to occupy one surface site, and the reaction was assumed to proceed through carbonium ions. Because very low adsorption equilibrium constants (
