2224
I n d . Eng. Chem. Res. 1988,27, 2224-2231
Fan, L. T.; Shenoi, S.; Gharpuray, M. M.; Lai, F. S. "Chemical and Process System Engineering. Applications of Fuzzy Set Theory: Tutorial Note". Paper presented a t the Summer School on Advances in Chemical Engineering Mathematics, Hannover, West Germany, 1985. Frenklach, M. "Computer Model of Infinite Reaction Sequences: a Chemical Lumping". Chem. Eng. Sci. 1986, 40, 1843. Haag, W. 0.;Lago, R. M.; Rodewald, P. G . "Aromatic Light Olefins and Mechanistic Pathways with ZSM-5 Zeolite Catalyst". J . Molec. Catal. 1982, 17, 161. Hoerl, A. E.; Kennard, R. W. "Ridge Regression: Biased Estimation for Nonorthogonal Problems". Technomet 1970, 12, 55. Hoffmann, U.; Hofman, H. "Reaction Engineering. 8. Kinetics of Multicomponent Multireaction Systems and Simplification of Their Description". Znt. Chem. Eng. 1977, 17, 414. Hosten, L. H.; Froment, G. F. "Kinetic Modelling of Complex Reactions". In Recent Advances in the Engineering Analysis of Chemically Reacting Systems; Doraiswamy, L. K., Ed.; Wiley: New York, 1984. Iordache, 0. "Polystochastic Models in Chemical Engineering"; VNU-Science Press: Utrecht, Netherlands, 1987. Corbu, S. "A Stochastic Model of Lumping". Chem. Iordache, 0.; Eng. Sci. 1987, 42, 125. Jardine, N.; Sibson, K. Mathematical Taxonomy; Wiley: New York, 1971. Li, G. "A Lumping Analysis in Mono- or/and Bimolecular Reaction Systems". Chem. Eng. Sci. 1984, 29, 1261. Liu, L.; Tobias, R. G.; McLaughlin, K.; Anthony, R. G. "Conversion of Methanol to Low-Molecular-Weight Olefins with Heterogeneous Catalysts". In Catal. Conuers. Synth. Gas. Alcohols Chem. Herman, R. G., Ed.; Plenum: New York, 1984. Luss, D.; Golikeri, S. V. "Grouping of Many Species Each Consumed by Two Parallel First-Order Reactions". AZChE J. 1975,21,865. Maria, G.; Muntean, 0. 'Model Reduction and Kinetic Parameters Indentification for the Methanol Conversion to Olefins". Chem. Eng. Sci. 1987, 42, 1451. Mihail, R.; Straja, S.; Maria, G.; Musca, G.; Pop, G. "Kinetic Model
for Methanol Conversion to Olefins". Znd. Eng. Chem. Process Des. Dev. 1983, 22, 532. Mihail, R.; Straja, S.; Maria, G.; Musca, G.; Pop, G . "Reply to Comments on "Kinetic Model for Methanol Conversion to Olefins" with Respect to Methane Formation at Low Conversion". Znd. Eng. Chem. Res. 1987,26,637. Mole, T.; Whiteside, J. A. "Conversion of Methanol to Ethylene over ZSM-5 Zeolite in the Presence of Deuterated Water". J. Catal. 1982, 75, 284. NegoitH, C. V.; Ralescu, D. A. "Applications of Fuzzy Sets t o Systems Analysis"; Birkhauser: Basel, 1975. Ono, Y.; Mori, T. "Mechanism of Methanol Conversion into Hydrocarbons over ZSM-5 Zeolite". J . Chem. SOC., Faraday Trans. 1 1981, 77, 2209. Peereboom, M. "Approximate Lumping Applied to the Isomerization of Methylcyclohexenes". Znd. Eng. Chem. Res. 1987, 26, 1663. Perot, G.; Carmerais, F. X.; Guisnet, M. "Carbon-13 Tracer Study of the Conversion of Dimethyl Ether into Hydrocarbons on Silica-Alumina and HZSM-5 Zeolite". J.Molec. Catal. 1982,17,255. Too, J. R.; Nassar, R.; Fan, L. T. "Simulation of the Performances of a Flow Chemical Reactor by Markov Chains". In Residence Time Distribution Theory in Chemical Engineering; Verlag Chemie: Weinheim, 1982. Van den Berg, J. P.; Wolthuizen, J. P.; Van Hooff, J. H. C. "The Conversion of Dimethyl Ether to Hydrocarbons on Zeolite HZSM-5-The Reaction Mechanism for Formation of Primary Olefins". In Proc. 5th Znt. Conf. on Zeolites; Rees, L. V. C., Ed.; Heyden: London, 1980; pp 649-660. Wei, J.; Kuo, J. C. W. "A Lumping Analysis in Monomolecular Reaction Systems". Znd. Eng. Chem. Fundam. 1969,8, 114. Zhu, K.; Chen, M.; Yan, W. "An Engineering Model of a Network of Multicomponent, Reversible Reactions". Znt. Chem. Eng. 1985, 25, 542. Received for reuiew March 1, 1988 Revised manuscript received July 18, 1988 Accepted July 30, 1988
Inhibition by Product in the Liquid-Phase Hydration of Isobutene to tert -Butyl Alcohol: Kinetics and Equilibrium Studies Enric Velo, Luis Puigjaner, and Francesc Recasens* Department of Chemical Engineering, ETS Enginyers Industrials de Barcelona, Universitat Politecnica de Catalunya, Diagonal 647, 08028 Barcelona, S p a i n
Intrinsic rates of isobutene hydration t o tert-butyl alcohol on Amberlyst-15 particles were measured to establish a rate equation in a solvent-free, liquid-phase system. T h e ranges of temperature and concentration are those likely to be found in a multiphase reactor, such as trickle bed or slurry type. Although inhibition by water may also be present, the effect of TBA is far more significant. Thus, the alcohol is found to inhibit the rate more than expected from the values of the equilibrium constant for the hydration reaction, determined in separate experiments. Various rate expressions, including that derived from the accepted hydration mechanism, are tested t o account for product inhibition. Extreme care has been put in ascertaining the effects of internal and external mass-transfer resistances, by use of suitable derived criteria. Introduction tert-Butyl alcohol ( T B A ) is an important oxygenated octane enhancer that is used to replace toxic lead additives in gasoline (O'Sullivan, 1985). For other oxygenates, such as methyl tert-butyl ether, the technology is already well established. For TBA, however, processes have been filed (Franz et al., 1975; Matsuzawa et al., 1973; Moy and Rakow, 1976), but only a few seem to be in commercial operation (O'Sullivan, 1985; Huls, 1983). A high-yield, catalytic route to TBA is based on the direct hydration of isobutene (iB) contained in refinery C4 streams. The synthesis reaction 0888-5885/88/2627-2224$01.50/0
CH3
H3C\ ,C=CHz
4-
H20
e CH3-C-OHI
H3C
(1)
I
CH3
catalyzed by acid, is exothermic and reversible at low temperatures (50-90 "C) and highly selective toward the desired product. Reaction 1 involves a multiphase mixture of a hydrocarbon, an aqueous phase, and a solid catalyst. Successful processes depend on how the problem of the limited miscibility of the components is overcome. Existing pro-
0 1988 American Chemical Society
Ind. Eng. Chem. Res., Vol. 27, No. 12, 1988 2225 cesses use either a cosolvent (Delion et al., 1986) or excess TBA (Huls, 1983) to achieve miscibility. Three-phase catalytic contactors, such as trickle-bed reactors (Satterfield, 1975; Herskowitz and Smith, 1983) operating with liquid water and gaseous butene, offer an interesting alternative not yet fully studied. Cosolvent would not be necessary, and relatively low operating pressure could be used. Besides, excess water would shift the equilibrium favorably, and low dissolved iB would reduce byproduct formation by competing reactions of higher order (Levenspiel, 1972). On the other hand, the low solubilities of gaseous iB in aqueous mixtures (Leung et al., 1987a) and the interphase mass-transfer resistances associated with trickle-flow operation (Leung et al., 1987b) may retard the global rate substantially. Furthermore, early rate measurements on ion-exchange catalysts show that both intrinsic kinetics and pore diffusion affect the global rate (Gupta and Douglas, 1967). Previous Work. Amberlyst-15 (A-15), a preferred catalyst for the process, is a sulfonic acid macroporous ion-exchange resin. In preliminary studies (Leung et al., 1986), the hydration reaction was found to be linear in dissolved iB, with significant internal diffusion limitations even for a small bead size (0.45 mm) and low temperatures (30 "C). Using two pellet sizes allowed us to separate the intrinsic first-order rate constant and the effective diffusivity of iB within the particles over a wide temperature range (Leung et al., 1986). Also, the measurement of point rates in a laboratory trickle-bed reactor showed that the wetting efficiency of the catalyst by the liquid flow, as well as the liquid-to-particle mass-transfer coefficient, affected the rate (Leung et al., 1987b). In these prior studies, the reaction was first order since little iB was present kmol/m3). TBA is known to reduce the intrinsic rate since reaction 1 is strongly reversible (Delion et al., 1986). But in contrast, the presence of TBA in the liquid enhances the solubility of gaseous iB exponentially (Leung et., 1987a). This is most important when considering the optimal operation of a multiphase reactor, where a liquid mixture consisting of water and TBA is contacted with the gas. In this regard, Clceres et al. (1988) studied the autocatalytic effect brought about by the enhanced solubility of iB on the optimal operation of a trickle-bed reactor for the process. It is known (Levenspiel, 1972) that reactor-size minimization by product recycle is best for autocatalytic reactions. In the presence of high TBA concentrations, the direct rate would be increased exponentially due the presence of more dissolved iB, whereas the effect of TBA in accelerating the reverse reaction is only linear. Depending on the relative importance of kinetics on the global rate in a trickle-bed reactor, an optimal compromise between liquid recycle and temperature can be found that minimizes the bed-size requirement for a given production (CBceres et al., 1988). The Catalyst. In our studies we use A-15 beads as a catalyst whose pores bad been previously filled with water. The catalyst is a styrene-divinylbenzene copolymer with sulfonic acid functional groups. The particles are composed of agglomerates of gel-type microparticles surrounded by a macroporous matrix (Pitochelli, 1975). Cumulative pore-volume distribution, average pore diameter, and surface area as well as other pertinent characteristics are available in the literature (Kun and Kunin, 1967; Dooley et al., 1982; Leung et al., 1986). We reasoned (Leung et al., 1986) that because of the small size of the microparticles, with Thiele modulus of about the reaction is not retarded much by diffusion within the
L Figure 1. Apparatus: 1, reactor; 2, reservoir; 3, condenser; 4, thermostated bath; 5 and 6, pumps; 7, wet gas meter; 8, rotameter; 9 and 10,sampling septa; 11, throttling valve; 12, bypass valve; 13, back-pressure valve.
microspheres (Ihm et al., 1988). Therefore, the measured values of the effective diffusivity corresponded to the macropore region. Presently liquid-phase kinetic work on macroporous ion-exchange particles is scarce. Thus, Gupta and Douglas (1967) separated the first-order kinetic constant and the diffusivity of iB on particles of Dowex 50, a gel-type ion-exchange resin. For the reverse reaction (dehydration of TBA), Heath and Gates (1972) and Gates and Rodriguez (1973) studied the catalysis for both geltype and macroporous resins. They found that with excess water the reaction is catalyzed by the hydrated protons freely present about the macropores rather than by localized HS03 groups. Recent studies postulate a micropore-macropore model with different diffusion rates in each region (Ihm et al., 1988). Using a series of macroporous ion-exchange catalysts, Dooley et al. (1982) found that the rates of certain gas-phase reversible reactions (reesterifications) could well be described by a model accounting for diffusion in the macropores and Langmuir adsorption followed by surface reaction in the swollen microparticles. For the case of liquid-phase hydration of linear olefins, Petrus et al. (1984, 1986) studied kinetics and equilibria over macroporous ion-exchange catalysts similar to A-15. For the purpose of evaluating a trickle-bed process for TBA, it is first necessary to have an intrinsic kinetic expression for the hydration reaction in a solvent-free liquid system. The aim of this work is to provide such kinetic information for the range of temperatures and TBA concentrations likely to be found in a trickle-bed reactor. Furthermore, in order to use thermodynamically consistent rate expressions, equilibrium measurements, over the appropriate temperature range, are also necessary. These are also provided here.
Experimental Methods Apparatus. Figure 1 shows the apparatus where rate and chemical equilibrium determinations were made. The reactor, 1, was an AIS1 316 stainless steel tube, 1.21 cm in diameter, that contained the catalyst packing between mesh screens. The reactor was operated liquid-full with recirculation. The liquid, resaturated with iB,was recycled upflow through the catalyst bed. The reactor temperature was controlled by means of water from a thermostat bath, 4, available through pump 6. Temperature readings were made with a Pt-100 resistor inserted into the catalyst. The
2226 Ind. Eng. Chem. Res., Vol. 27, No. 12, 1988 Table I. ODerating Conditions catalyst particle size," mm catalyst mass (dry), kg liquid flow rate, m3/s temperature, K pressure, kPa total liquid vol, m3 iB flow to reservoir, m3/s TBA concn,b kmol/m3 iB concn, kmol/m3 per pass conversion of iB, %
0.124 (4.0-6.5) X (0.56-5.6) X lo4 303-333 iO0-250 3 x 10-3 8.3 X lO* 0-3.2 (1.2-38) X 10-20
"Effective wet size of a dry-sieved fraction, 100-115 mesh. = 1.2-2.2 kmol/m3 in equilibrium runs.
recirculation of process liquid is made through a jacketed reservoir, 2 (AIS1 316, 0.003 m3), where iB is bubbled abundantly to ensure saturation. The reservoir was operated at the same temperature of the reactor. During a reaction run, the dissolved iB concentration could be held constant by proper adjustment of the gas flow to the sparger. The reservoir exit was connected to the atmosphere through a reflux condenser, 3, and a back-pressure valve, 13. Exit flow of gas was measured with a wet gas meter, 7. By use of chilled water (2 "C) in the condenser, the carryover of TBA was prevented. Process liquid was circulated with a pump, 5. The line was equipped with a calibrated rotameter, 8, to control liquid flow. This was adjusted by a throttling valve, 11, and a bypass valve, 12. During a run, component concentrations were measured by analysis of samples taken at locations 9 and 10. Precise measurements of liquid flow were made with a stop watch. The liquid flow rates were selected as to obtain 10-20% per pass conversions of iB. Because per pass conversions and operating times were small, TBA accumulation due to reaction could be neglected. Measured operating conditions are given in Table I. In equilibrium runs, the reservoir was fdled with aqueous solutions of known TBA concentration. The reservoir was first saturated with iB, and then the iB flow was stopped and the system was isolated from the atmosphere. Liquid recirculation through the catalyst was then started until constant iB concentration a t all points of the apparatus was reached. At this time, equilibrium concentrations were measured. Analytical Procedures. Rate Measurements. The concentrations of iB were about a thousandth of those of TBA (see Table I). Consequently, the rate of TBA accumulation was very small; thus, it could not be used to measure reaction rate. In contrast, iB concentrations at inlet and outlet of the catalyst bed could be accurately measured by gas chromatography (GC). Usually 10-12 replicate analyses of dissolved iB in the reactor streams were made accurately ( f l % ) . For conversions below 10-20%, the reactor behaves differentially (Massaldi and Maym6, 1969; Vatcha and Dadyburjor, 19861, and the reaction rate can be calculated simply as (2) r = QL(CB;- C,,)/m Gas chromatographic measurements were made with a Shimadzu GC-8A apparatus with flame-ionization detector. For analysis of TBA, isopropyl alcohol was used as the internal standard. For the analysis of iB, an external standard was employed. This consisted of distilled water saturated with iB at 303 K and atmospheric pressure, whose concentration is known precisely (Kazanskii et al., 1959; Leung et al., 1987a). Samples for analysis were directly withdrawn at locations 9 and 10 (see Figure 1)with 10-pL gas-tight syringes. Chemicals. Isobutene with a stated purity of 99% from Linde A.G. (West Germany) was used. tert-Butyl and
Table 11. Properties of A m b e r l y ~ t - 1 5 ~ ~ ~ grade 15 dry strongly acidic, macroporous type functionality HSOC physical form beads 393 max operating temp, K moisture content, %
