etbe

Ind. Eng. Chem. Res. 1994,33, 581-591

Kinetics of the Liquid-Phase Synthesis of Ethyl tert-Butyl Ether (ETBE)

Introduction

581

Carles Fit6, Montserrat Iborra, Javier Tejero,. Jose F. Izquierdo, and Fidel Cunill Chemical Engineering Department, Faculty of Chemistry, University of Barcelona, Martt i FranquBs, 1, 08028-Barcelona, Spain

The liquid-phase addition of ethanol to isobutene to give ethyl tert-butyl ether (ETBE) on the ion exchange resin Lewatit K2631 has been studied. Rate data were obtained free of mass transfer influence in a continuous differential reactor operated at 1.6 MPa and 40-90 "C, measuring steady- state conversions <lo% for ethanol-isobutene and ethanol-isobutene-ETBE feeds. The reaction is highly temperature-sensitive. Isobutene has an enhancing effect on the rate whereas ethanol has an inhibitor one. Besides, low ETBE concentrations enhance the reaction but as chemical equilibrium is approached the ether shows an inhibitor effect, as expected. As alcohol-olefin-ether mixtures behave nonideally, kinetic analysis has been performed by using the UNIFAC liquid-phase activities of isobutene, ethanol, and ETBE. The best kinetic model stems from an Eley-Rideal mechanism in which the ethanol, adsorbed on one center, reacts with the isobutene from solution to give the ether adsorbed on one center. Surface reaction is the rate-limiting step. Two additional centers take part in this step.

The use of oxygenated fuels has allowed lead to be phased out from gasolines and has been proposed as a means for reducing carbon monoxide and reactive evap- orative and exhaust emissions. The Clean Air Amend- ments of 1990 have increased the severity of the emissions limits of vehicles and require the manufacture of reformulated gasolines (Peeples, 1991; Potter, 1991). Although reformulated gasoline specifications will not take effect until 1995, gasolines must already contain 2.7 wt % oxygen during the winter in the CO nonattainment areas of US. Moreover, to reduce volatile organic compounds (VOC) emissions, the maximum blending Reid vapor pressure (bRvp) of all US gasoline is set a t 9.0 psi. In Europe, unleaded gasoline has developed slowly and only because of tax incentives. However, under pressure from envi- ronmentalists, European gasoline is almost sure to be reformulated by the year 2000. As a result, oxygenates demand will increase enormously in the coming years.

Of all the oxygenates, tertiary ethers are preferred by refiners to lighter alcohols, among other reasons, because of their lower bRvp and lower vaporization latent heats and their full fungibility in the petroleum refining and distribution systems. Currently, the most common ether is methyl tert-butyl ether (MTBE) because low methanol prices make MTBE a more atractive option than ethyl tert-butyl ether (ETBE) (Anderson, 1988). Nevertheless, some of the US states and a few of the European Community countries are planning to use ETBE produced from bioethanol as an alternative to MTBE (Oxy-Fuels News, 1992a,b). In the US, it is thought that ETBE economics will become more attractive in 1995 when Federal reformulated gasoline specifications will encourage lower bRvp oxygenates such as ETBE. Furthermore, a cost-competitive process for obtaining bioethanol from cellulosic biomass appears possible by 2000 (Lynd et al., 1991). On the other hand MTBE plants can easily be revamped into ETBE. So, it seems that MTBE producers are mainly interested in the flexibility of MTBE/ETBE plants (Anderson, 1988; Forestiere et al., 1991). Another factor that could reduce ethanol cost is the possibility of using azeotropic ethanol (96.5 wt %) (Bakas et al., 1990; Cunill et al., 1993) or subazeotropic ethanol (80-85 wt % )

0888-5885/94/2633-0581$04.50/0

(Jayadeokar and Sharma, 1992) as a raw material for ETBE production.

Besides the fact that ETBE has a lower bRvp (4 psi) than MTBE (8-10 psi), which would allow ETBE to be successfully used to obtain gasoline with bRvp less than 7.8 psi in some US locations during the summertime, ETBE has an slightly higher blending octane number. Moreover, ETBE is produced from renewable ethanol, while MTBE is obtained from methanol derived from natural gas. That means ETBE would reduce the dependence on methanol and oil importations and create additional markets for grain products and agricultural wastes. Of course, the ETBE option is clearer in countries like India, where ethanol is considerably cheaper than methanol (Jay- adeokar and Sharma, 1992).

From a standard pollutants emissions standpoint, ETBE parallels MTBE, showing a sizeable reduction in CO, a smaller reduction in unburned hydrocarbons, and a nonsignificant effect on NO, (Corbett, 1991; Wise, 1992; Davis, 1992). As for toxic emissions, however, both ethers increase aldehyde emissions. To sum up, looking at the emissions data as a whole, the effect of using oxygenates is positive.

In spite of the increasing interest in ETBE, there is only limited information about the product, which is mostly devoted to its behavior as gasoline component (Kaitale et al., 1987; Iborra, et al., 1988; ARCO, 1989; Furey and Perry, 1990; Forestiere et al., 1991). Much more scarce is the information available on reaction catalysts (Evans and Edlund, 1936; Macho et al., 1982; Tau and Davis, 1989), thermodynamics of ETBE synthesis (Rock, 1992; Vila et al., 19931, and kinetics (Ancillotti et al., 1977; Kaitale et al., 1988; Jayadeokar and Sharma, 1992). In this work, a kinetic study of the reaction for obtaining .ETBE by addition of ethanol to isobutene has been carried out. As the alcohol-C4 hydrocarbon-ether system is highly com- plex (Ancillotti et al., 1977), pure isobutene was used instead of Cq cuts in order to gain a clear insight into the influence of reactant concentration on the reaction rate.

Experimental Section

Materials. Ethanol HPLC (<0.2 5% water, 99.5 76 pure) was supplied by Romil Chemicals Ltd. (Shepshed, U.K.)

0 1994 American Chemical Society

582

Table 1. Physical Properties of Lewatit K2631

Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994

skeleton structure active group nominal crosslinking degree, % moisture,' max % mean bead diameter,b mm uniformity coefficient,b porosity, vol/vol % mean porous diameter, 8, surface area: mz-g-1 exchange capacity: meqg-1 temperature operation, max. O C

styrene-divinylbenzene macroporous sulfonic 18 58-60 0.73 1.5 25 650 36 4.83 120

a Determined by Karl Fischer titration. b Determined in our laboratory by sieving. c Determined by the method of Fisher and Kunin (1955). Determined by the BET method.

and was dried to less than 0.05 7% water over 3-A molecular sieves (Fluka, Buchs, Switzerland). Isobutene (99 '% pure) was obtained from Carburos Metllicos (Barcelona, Spain) and used without further purification. ETBE was pre- pared from ethanol and isobutene and was purified by extracting the alcohol with water and distillation up to 99% pure (GC). The catalyst was the ion-exchange resin Lewatit K2631, before SPC 118, from Bayer AG (Le- verkusen, Germany). Its physical properties are given in Table 1.

Apparatus. Figure 1 shows the continuous upflow packed-bed microreactor (15-cm length; 4.4-mm i.d.1 191 where the reaction was studied. The setup was made from stainless steel.

Ethanol, or ethanol-ETBE mixtures, stored over 3-.A molecular sieves was fed from an atmospheric tank [l] placed 2 m over the pump suction valve to ensure a reliable pump operation. For the same reason, the isobutene was fed from a pressurized bottle [21, thus avoiding vaporization during the suction. Plugging of the pump suction valves was avoided by placing 2-pm filters [31 in the pump suction lines.

A positive displacement pump, Minipump (Dosapro, Pont-Saint-Pierre, France), duplex-type 141, accuracy f

I ,,pa] .

0.3% flow rate a t pressures over 0.7 MPa, dosed inde- pendently with ethanol, or ethanol-ETBE blends, and isobutene, to the reactor. Each feeding line was equipped with a pulsation damper 151 to supply an almost constant flow, a safety valve 161 to release an excessive buildup of pressure caused by incorrect operation, and a check valve 171 to prevent liquid contamination owing to a pump failure. The damper consisted of a 150 cm3 cylinder with a membrane (viton for isobutylene, and poly(ethy1ene- propylene) for ethanol) for sealing the gas against theliquid side. The damper operating pressure was adjusted before the experiments with compressed nitrogen to 70% of the operating pressure.

Before the feed was brought in the reactor, it was homogenized in a mixer (3/8 in. 0.d.; 18-cm length) [81 packed with 1-mm Pyrex spheres, and preheated in a preheater coil [lo]. The reactor temperature was controlled by immersing the reactor in a thermostatic bath [ l l l , thereby ensuring a temperature constancy of 10.1 K. Temperature readings of the reacting mixture were made with three NiCr/NiAl thermocouples (accuracy h0.5 K) placed at 2.5, 7.5, and 12.5 cm from the reactor inlet. The catalyst bed was placed between two 10-ym stainless steel frits. Two sampling valves Valco 4-CL4WE [121 allowed to take samples from the reactor inlet and outlet streams to analyze. To avoid plugging of the sampling valves 2-pm filters 131 were placed in both streams. The reactor operating pressure (about 1.6 MPa) was controlled by a BP-3 back-pressure regulator (GO Inc., Whittier, CA) U31.

Analysis. The sampling valves injected 0.2 pL into an HP 5890A GLC apparatus equipped with TCD. A 6 m X 3.2 mm 0.d. GLC column packed with Chromosorb 101 (80/100 mesh) separated ETBE, ethanol, isobutene, water, and possible reaction byproducts (diisobutene, diethyl ether, and tert-butyl alcohol). The column was temperature programmed with a 4-min initial hold at 130 OC followed by a 25 OC/min ramp up to 200 "C and held for

7,

2M Pa

Figure 1. Experimental setup. Key: 1, atmospheric tank; 2, pressurized bottle; 3, 2-pm filter; 4, pump; 5, pressure damper; 6, safety valve; 7, checkvalve; 8, mixer; 9, reactor; 10, thermostatic bath; 11, preheater coil; 12, sampling valve; 13, back-pressure regulator; 14, gas chromatograph.

l 8 'r

a 2 M P a

Table 2. Operating Conditions

6 -

4 -

2 -

Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994 583 r (mol/Q h )

catalyst particle size, mm catalyst mass (dry), g inert particle size, mm dilution ratio, (vol of diluent/

vol of catalyst) temperature, O C

pressure, MPa LHSV, h-l total flow rate, mol-h-l r(I/Et) isobutene concentration, mo1.L-1 ethanol concentration, mo1.L-l ETBE concentration, mo1.L-l isobutene conversion, %

xo.10 0.03-0.5 0.1 < d, < 0.16 2-9

40-90 1.6 >60 2-3.1 0.5-2 1.86-7.45 2.11-9.24 1.69-4.11 <lo

1 2 min. The carrier gas (helium, 99.998% pure (SEO, Barcelona, Spain)) flow rate was 30 cm3/min.

Procedure. The catalyst was crushed and sieved, then dried at 110 "C for more than 14 h, and stored in a desiccator over concentrated sulfuric acid (98 wt % ). The water content in the resin, measured by Karl Fischer titration, was less than 2.9 wt % .

Catalyst samples (d, < 0.10 mm) were diluted in quartz (0.1 < d, < 0.16 mm) to obtain an isothermal bed and a good contact pattern between reactants and catalyst, avoiding back-mixing and channeling. The remaining reactor volume was filled up with quartz of the same size to avoid bed expansion. Before the experiment, the catalyst bed (5-14-cm length) was preheated in the reactor a t the operating temperature in the presence of an ethanol stream of 70 cm3.h-l for 30 min, to ensure that an ethanol amount greater than 10 times the reactor volume was passed through it. As a result, water content in the resin was reduced to less than 1.6 wt % according to Fit6 et al. (1992).

The experiments were carried out in the liquid phase far from limitation by chemical equilibrium at the operating conditions given in Table 2. Steady-state conversions lower than 10 % were measured for ethanol-isobutene, and ethanol-isobutene-ETBE feeds. When the reactor behaved differentially, the temperature radial and axial gradients were lower than 1 "C, and byproducts were not detected in the reactor effluent. Moreover, according to the dilution criterion of van de Bleck et al. (1969), observed rates deviated less than 5 % from isothermal intrinsic ones. As a result, with consideration of reaction stoichiometry, the reaction rate was computed from isobutene conversion by the relationship

In each kinetic run, reactants flow rates were maintained for 3-4 h, and repeated analyses of reactor inlet and outlet streams were made to assure that the steady state was reached. Reported rates are accurate to within &5%. Afterward, new experimental conditions were tested. Since the activity level of resins drops irreversibly on changing feed polarity, the catalyst activity level was controlled by repeating the first run in the series. The catalyst was replaced by a fresh one after two or three runs.

Results and Discussion

Preliminary experiments were made at the highest temperature explored (89.7 "C) to assure that rate data were generated free of mass transport processes. Blank runs performed with the reactor packed with quartz showed that the reaction does not occur in the absence of

0 10 20 30 40 50 60 70 80 90 100 110 120 130

LHSV (l/h)

r (mol /h .a )

e-

r ( l / E t ) - 1.04 I 0 2 4 6 8 IO 12 14 16 18 20 22 24

l/dp (l/mm) Figure 2. Influence of mass transport on the rate a t 89.7 O C : (A) external mass transport influence (r(I/Et) = 1.04; 0.08 < d, < 0.1 mm; WHSV = 2255 h-1); (B) internal mass transport influence (series 1: r(1IEt) = 1.04; 89.7 O C ; LHSV = 118 h-1; WHSV 2255 h-1. series 2: r(I/Et) = 2.91; LHSV = 82 h-l; WHSV = 3860 h-1). The shaded area points out the limits of the experimental error.

catalyst. Runs carried out by maintaining feed composition and WIF ratio, but using different catalyst mass, stated the absence of external mass transport effects a t the operating conditions given in Table 2. As Figure 2A shows, reaction rate does not depend on flow rate for LHSV 1 60 h-l (u 1 2.5 X ms-l). At LHSV < 60 h-l data show a first-order relationship between rate and the LHSV, disclosing that external mass transfer is the rate-limiting step.

Internal mass transfer influence on the rate was studied by performing experiments where, a t the same flow rate and feed composition, catalyst samples of different particle size were used. Data show that the medium polarity determines the largest particle size working in a kinetic regime. As can be seen in Figure 2B, rates were unchanged, within the limits of the experimental error, for particles smaller than 0.25 mm at r(I/Et) = 1.04 and for particles smaller than 0.10 mm at r(1IEt) = 2.91. This behavior can be explained because the ethanol permeates easily, and as a result, the resin swells to a large extent in the presence of a large amount of the alcohol. On the contrary, when isobutene is in excess the resin hardly swells a t all. Figure 2B also shows that a t r(I/Et) 2.91 the rate increases for a

584

Table 3. Experimental Conditions and Obtained Rates for ETBE Synthesis

Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994

T, OC r(I/Et) CI, mol/L C E ~ , mol/L CE, mol/L 71 YEt YE a1 aEt UE r, mol/h.g 89.7 0.40

0.50 0.55 0.60 0.80 1.04 1.19 1.39 1.59 1.79 1.99 2.91 0.87 0.44 1.17 1.08 0.54 1.82

79.5 0.50 0.56 0.62 0.70 0.80 1.00 1.19 1.39 1.59 1.79 1.99 2.06 0.85 0.43 1.71 1.08 0.54 1.82

59.5 0.50 0.55 0.62 0.71 1.00 1.19 1.39 1.51 1.65 1.79 1.99 0.87 0.44 1.74 1.08 0.54 1.82

39.4 0.83 1.00 1.19 1.39 1.59 1.79 1.99

3.68 4.17 4.38 4.57 5.21 5.76 6.04 6.34 6.58 6.76 6.93 7.45 2.74 1.86 3.39 4.12 3.04 5.10 4.25 4.48 4.76 5.03 5.31 5.80 6.17 6.47 6.72 6.92 7.10 7.15 2.92 2.04 4.20 4.32 3.02 5.39 4.41 4.66 4.94 5.26 6.04 6.44 6.75 6.92 7.09 7.23 7.41 2.48 2.07 4.01 4.47 3.27 5.62 5.84 6.28 6.70 7.04 7.34 7.54 7.74

9.24 8.37 8.00 7.66 6.53 5.56 5.06 4.55 4.12 3.77 3.48 2.56 3.07 3.78 2.65 3.63 4.72 2.56 8.53 8.12 7.65 7.19 6.66 5.82 5.17 4.65 4.22 3.86 3.56 3.47 3.13 3.83 2.14 3.74 4.91 2.71 8.85 8.42 7.94 7.39 6.06 5.39 4.85 4.58 4.29 4.03 3.72 2.57 3.28 2.11 3.97 5.05 2.86 7.04 6.30 5.61 5.05 4.59 4.21 3.88

3.33 3.68 3.02 2.04 2.39 1.77

3.30 3.65 2.77 1.99 2.46 1.63

4.11 4.11 3.19 2.05 2.47 1.69

sample with 0.16 < d, < 0.25 mm. This rise in rate may be caused by the presence of simultaneous intraparticle heat and mass transfer resistances. A similar fact was reported by Rehfinger and Hoffmann (1992), who computed a temperature intraparticle gradient higher than 0.5 K in the ion-exchange-catalyzed addition of methanol to isobutene in the liquid phase.

Kinetic experiments were carried out a t the average temperatures 89.7, 79.5, 59.5, and 39.4 O C . The r(I/Et) range explored was 0.4-2.9 at 89.7 O C , and 0.5-2.0 at the other temperatures. A t each temperature three experimental series were performed. In the first one pure ethanol was fed, and in the other two ethanol containing 29 and

2.26 2.07 1.99 1.93 1.72 1.56 1.48 1.41 1.35 1.31 1.27 1.16 1.28 1.39 1.22 1.33 1.51 1.19 2.10 2.02 1.92 1.83 1.74 1.60 1.50 1.42 1.36 1.32 1.28 1.27 1.29 1.40 1.16 1.35 1.54 1.21 2.16 2.07 1.97 1.87 1.63 1.53 1.45 1.41 1.37 1.34 1.30 1.21 1.34 1.16 1.38 1.56 1.23 1.80 1.67 1.56 1.48 1.41 1.36 1.32

1.11 1.15 1.17 1.20 1.30 1.42 1.50 1.60 1.71 1.81 1.94 2.34 1.82 1.59 2.00 1.72 1.46 2.16 1.15 1.18 1.21 1.25 1.30 1.40 1.51 1.61 1.72 1.82 1.92 1.96 1.84 1.61 2.35 1.73 1.45 2.15 1.16 1.19 1.22 1.26 1.42 1.53 1.64 1.70 1.78 1.86 1.96 2.08 1.80 2.46 1.74 1.48 2.19 1.34 1.43 1.54 1.66 1.77 1.89 2.00

1.15 1.24 1.10 1.17 1.31 1.07

1.15 1.24 1.06 1.18 1.33 1.07

1.14 1.19 1.07 1.20 1.35 1.08

0.643 0.688 0.705 0.721 0.763 0.793 0.807 0.820 0.830 0.838 0.844 0.867 0.384 0.278 0.457 0.561 0.451 0.646 0.698 0.717 0.737 0.755 0.772 0.798 0.815 0.827 0.836 0.844 0.850 0.852 0.403 0.300 0.535 0.579 0.446 0.670 0.719 0.738 0.757 0.777 0.815 0.831 0.842 0.848 0.853 0.858 0.863 0.327 0.293 0.499 0.589 0.473 0.679 0.817 0.835 0.851 0.859 0.867 0.872 0.877

0.793 0.769 0.759 0.750 0.721 0.696 0.683 0.670 0.657 0.647 0.647 0.598 0.610 0.646 0.585 0.638 0.679 0.585 0.770 0.760 0.748 0.736 0.724 0.703 0.687 0.674 0.663 0.653 0.643 0.640 0.617 0.649 0.552 0.645 0.686 0.599 0.773 0.763 0.752 0.739 0.710 0.695 0.683 0.677 0.671 0.664 0.656 0.582 0.625 0.558 0.658 0.691 0.616 0.731 0.716 0.703 0.693 0.684 0.676 0.670

0.418 0.488 0.367 0.244 0.308 0.200

0.407 0.474 0.321 0.233 0.316 0.179

0.510 0.516 0.367 0.235 0.308 0.180

1.46 1.56 2.07 2.17 3.20 3.32 3.27 3.50 3.62 4.20 4.57 4.73 0.905 0.587 1.15 2.42 2.09 3.12 0.797 0.848 0.839 1.11 1.19 1.10 1.27 1.30 1.49 1.47 1.42 1.60 0.580 0.405 0.808 1.26 1.25 1.45 0.141 0.149 0.175 0.175 0.211 0.254 0.263 0.297 0.287 0.298 0.317 0.416 0.333 0.269 0.244 0.244 0.301 0.030 0.026 0.032 0.033 0.032 0.034 0.035

46 mol 76 ETBE, respectively, was fed. Series in the presence of ETBE were not carried out a t 39.4 O C because an excessive buildup of pressure was produced caused by the swelling of catalyst, since the catalyst mass used was very high in order to get reasonable rates a t that low temperature.

Great care was taken over controlling the presence of water in the system because of its inhibitor effect on the rate (Buttersack, 1988, Tejero et al., 1988; Cunill et al., 1989; Cunill et al., 1993). Besides, water seriously affects the catalytic activity level of the resin since ita presence changes the medium polarity and, as a result, the catalyst structure. So, to keep the level of water in a minimum

Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994 585 7 ( m e l , h g )

1 5 .

0 0 5 I I5 2 2 5 3 0 0 5 I 15 2 2 5 3 0 0 5 I I 5 2 2 5 3

r( l /El) r ( l /El ) r ( l / E l )

Figure 3. Rate vs r(I/Et): (A) experiments carried out without ETBE in the feed [XEt = 01; (B) experiments with XEt = 29% in the feed; (C) experiments with XEt = 46% in the feed. The solid line is the fit to data given by model 5.2: (0) 89.7 "C; (A) 79.5 OC; (*) 59.5 "c; (0) 39.4 "C.

0 2 I b 8 10

c I (mol/l)

Figure 4. ETBE synthesis at 79.5 "C: (A) rate vs isobutene concentration; (B) rate vs ethanol concentration: (C) rate vs ETBE concentration. The solid line is the fit to data given by model 5.2: (0) X E ~ = 0; (*) XEt = 29%; (A) Xp = 46%.

was of the main importance to measure coherent rates within the limits of the experimental error. Ethanol is highly hygroscopic and it is difficult to preserve it anhydrous, so that water was held into a very low level, by drying the alcohol over 3-w molecular sieves (Carton et al., 1987), and finally by percolating the ethanol through the bed (Fit6 et al., 1992). Once the steady state was reached, water adsorbed in the catalyst was at equilibrium with the water in the feed. As this was <0.03 wt %, we can expect that the amount of water adsorbed in the resin was also very low, and we assumed that its effect on the rate is negligible.

Table 3 shows the experimental conditions and the obtained rates upon a dry catalyst mass basis (see also Figure 3). In line with Benson (1960) we should expect to find the bimolecular reactions in solution about 5 times faster than in the gas phase. Despite of Benson refered to homogeneous reactions, such a trend could be expected for heterogeneous catalytic reactions. In this way, measured rates are more than 25 times faster than those obtained for the gas-phase addition of ethanol to isobutene on Lewatit SPC 118 (Iborra et al., 1990). On the other hand, Ancillotti et al. (1977) showed in a previous work that liquid-phase syntheses of MTBE and ETBE had similar rates. However, they used industrial samples of Amberlyst 15 as a catalyst and their results are probably influenced by diffusion processes. Measured rates are higher than those reported by Ancillotti et al. (1977) by 30%, and it deserves to be shown up that our data are similar to those reported by Rehfinger and Hoffmann (1990) for MTBE synthesis, a work in which the presence of mass transport effects can be discarded. From rate data we can draw the following conclusions:

1. Reaction rate increases with r(I/Et), in line with the behavior of isobutenemethanol and isobutene-n-butanol systems at olefin/alcohol molar ratios of about 1:1 (An- cillotti et al., 1977).

2. Ethanol inhibits the reaction, whereas isobutene enhances it. As Figure 4 (parts A and B) shows, reaction rate decreases on increasing ethanol concentration, but it increases with that of isobutene. The same trend has been observed at the other temperatures.

3. In each series, reaction rate decreases on increasing ETBE concentration (Figure 4C). However, a t 89.7 "C, rates in the presence of ether are lower than those obtained in the absence of ETBE. At 79.5 "C, the curve with lower ETBE content shows rates similar to those obtained for the reaction free of ether. Finally, a t 59.5 "C, both curves in the presence of ether show higher rates than the curve obtained in the absence of ether. The presence of ETBE increases the rate of decomposition of the ether, a t the same time as ethanol and isobutene concentrations in the feed decrease. These factors explain the decrease in the net reaction rate observed in each series. On the other hand, it is possible that the rate has a maximum which shifts to lower ethanol and isobutene concentrations with decreasing temperatureg.

4. Temperature highly influences the rate. It decreases by 2.5 times when temperature decreases 10 "C.

It is well known that alcohol-olefin liquid mixtures behave nonideally (Colombo et al., 1983; Gicquel and Torck, 1983; Rehfinger and Hoffmann, 1990; Izquierdo et al., 1992; Vila et al., 1993). As a consequence, the analysis of the ethanol-isobutene-ETBE system should be made as a function of activities rather than concentrations of involved substances. The activity of component i is related

586 Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994

to its molar fraction in the mixture by means of

ai = xiyi

where the activity coefficients Ti depend on temperature and composition (yi = 1 when Ti = 1). Estimates of activity coefficients were made through the UNIFAC predictive method (Reid et al., 1987). The parameters needed for the use of UNIFAC, i.e. group volumes, group surface areas, and group interaction parameters, were taken from the tables published by Fredeslund et al. (19771, Skjold- Jorgensen et al. (19791, Almeida et al. (1983), and Tiegs et al. (1987). As can be seen in Table 3, ethanol, isobutene, and ETBE activity coefficients are clearly > l . The estimates for isobutene and ethanol quite agree with calculated and experimental values for the isobutenemethanol system (Colombo et al., 1983). As expected, activity coefficients rise with decreasing molar fractions, and the ether behaves more ideally than the alcohol and olefin. It is worth noting the stronger dependence of rate on activities than on concentrations, within the range within the range explored, as Figures 4 and 5 show, which emphasizes the nonideal behavior of the mixtures.

Prior to formulating kinetic models for the reaction, the response surface of the rate as a function Of ai, aEt, and U E was analyzed. Analysis of the response surface allows us to outline the main features of kinetic models that best fit rate data, which spares substantial effort in their formulation and fit. Thus, following the surface response methodology (Tejero et al., 1989; Iborra et al., 1992), a polynomial as a function of U I , aEt, and U E was fitted to data, a t each temperature, by a multivariable least squares method. Table 4 shows the lower order polynomial, whose fit is significant. Statistical analysis of the terms of polynomials leads us to outline the following three points.

1. Since a second-order polynomial fits data in a statistically significant fashion, models to test should be at the very least of second order.

2. As CI, CEt, and CE are not independent variables because no diluents were added in the feed, U I , UEt and U E are reciprocally dependent. As a result, all the terms of the polynomials have a similar statistical significance (in fact, some interactive terms are actually more significant than terms in which a single activity appears), and it is difficult to analyze the influence of activities on the rate.

3. The influence of U I , UEt and U E terms on the rate changes with temperature. At 89.7 "C, the order of significance is UI > aEt > UE; a t 79.5 "C, UI > UE > aEt; a t 59.5 "C, aEt > U E > U I ; and at 39.5 "C, aEt > ar. Moreover, at some temperatures second-order terms appear more significant than first-order ones. As a consequence, in order to analyze the influence of U I , UEt, and QE on r , the terms ai and ai2 were considered jointly. Thus, a t 89.5 "C, a1 enhances the reaction rate whereas aEt and U E highly inhibit it. At 79.5 and 59.5 "C, a1 enhances the reaction rate, UEt inhibits the reaction except for low ethanol activities, and U E enhances the reaction for low ETBE activities, but inhibits it for high activities (close to chemical equilibrium). Finally, a t 39.4 "C, a1 enhances the reaction rate, and UEt inhibits it except for low ethanol activities.

I t has been firmly stated that several sulfonic groups take part in the rate-limiting step of ion-exchanger- catalyzed reactions. In this way, Wesley and Gates (1974) reported that up to seven active centers could take part to stabilize the reaction intermediate in benzene propylation. Ancillotti et al. (1977) pointed out a third-order dependence of the rate for obtaining MTBE on the concentration of sulfonic groups for isobutene-methanol equimolar mixtures and a fourth-order one when isobutene is in excess. However, Rehfinger and Hoffmann (1990) reported a second-order dependence. Discrepancy is Iikely due to the concentration range explored. Likewise, gas- phase addition of methanol to isobutene to give MTBE shows a third-order dependence (Tejero et al., 1989), whereas ethanol addition to give ETBE shows a fourth- order one (Iborra et al., 1990).

On the other hand, whereas ethanol adsorption on the resin is beyond all doubt, isobutene adsorption has not been considered in the open literature, emphasizing ita very low adsorption equilibrium constant. In fact, MTBE synthesis has been frequently described in terms of an Eley-Rideal (ER) mechanism in which isobutene from solution reacts with the methanol adsorbed on the resin (Gicquel and Torck, 1983). One might reasonably regard this picture as an oversimplification useful to explain the behavior of the olefin-alcohol mixtures when alcohol is in excess. However, it is difficult to explain diisobutene

Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994 587

Table 6. Equation Type Tested for Liquid-Phase Synthesis of ETBE (k+ = k&& for a LHHW Mechanism and k& for a ER Mechanism)

model type mechanism type equation observations 1 LHHW temverature-devendence of KEt is coherent

k*(aIaEt - a$K) temperature-dependence of KEt is coherent KI < 0 at 89.7 and 79.5 O C

r = 2 LHHW, ER (1 + KEtaEt + KEaE)"

3 LHHW k*(aIaEt - a d K ) temperature-dependence of KEI is coherent KE < 0 at 59.5 O C

r = (1 + KIaI + KEtaEt)"

formation as byproduct without acknowledging that isobutene adsorption takes place, even if weakly.

From an eclectic standpoint, and considering the features of the surface response outlined above, we can assume that the best kinetic model for the reaction could stem from the Langmuir-Hinshelwood-Hougen-Watson (LHHW) mechanism

Et + a + Et*u

I + a + I . a

Et-a + 1.a + (n - 2)a + E.a + (n - 1)a

E*a + E + u

in which the adsorbed ethanol and isobutene react to yield ETBE or, otherwise, from the following ER mechanism in which isobutene from solution reacts with the adsorbed ethanol.

Et + a + Et-a

Et-a + I + (n - 1)a =, E-a + (n - 1)a

E.u+E + One or more supplementary active sites take part in the surface reaction step, in both mechanisms. Considering this step as the rate-determining one, the basic LHHW and ER kinetic models would be, respectively, as follows:

kKEt( aIaEt - 2) r =

Such models, with n 2 2, fulfil the response surface features indicated above. The enhancing effect of isobutene on the rate, would be because isobutene influences mainly the driving force of eqs 1 and 2. The inhibitor effect of ethanol, except for low activities, would be because ethanol influences mainly the adsorption term and, a t low activities (low values for the adsorption term), the driving force. The fact that ETBE enhances the rate at low activities, whereas it inhibits the reaction at high activities,

temperature-dependence of KEt is not coherent convergency is difficult for n > 2 KEt < 0 for n even

temperature-dependence of k* is coherent

could be explained because the presence of ETBE involves a decrease in a1 and aEt, so that a simultaneous decrease in the driving force and in the adsorption term is seen. At low UE the decrease in the adsorption term is higher, and an increase in reaction rate is observed. When ether concentration is high, and therefore aE, the decrease in the driving force is higher, and as a result, reaction rate decreases. Finally, these models with n 1 2 can describe appropriately the presence of a maximum in rate, a t least relative to aEt.

Models of the type indicated in Table 5 were fit to data by using the nonlinear least-squares method of Box- Kanemasu (Beck and Arnold, 1977), with n ranging from 2 to 7. The thermodynamic equilibrium constant for the reaction, K, was estimated by means of (Vila et al., 1993) In K = 1140 - 14580T' - 232.9(1n 2') + 1.087T-

(1.114 X 10-3)p + (5.538 X lO-')P The sum of the squares of lack of fit for model type 1

worsened on increasing n. The adsorption parameter for ethanol was always positive, and its temperature-dependence was coherent. However, negative adsorption parameters for isobutene at 89.7 and 79.5 "C and for ETBE at 59.5 "C were found. This fact could be due to the nature of the experimental design. As mentioned above, activities of compounds were reciprocally dependent, and as a result, correlation between parameters was too high ( ~ 1 ) . It is worth noting that there was no possibility to obtain data close to chemical equilibrium, due to the very low rates, which would have allowed us to obtain adsorption parameters with physicochemical meaning on enlarging the range of activities explored. Model types 2 and 3 had higher sums of squares of lack of fit than model type 1, as could be expected, but show the same trends. As a result, the influence of ethanol on the adsorption term is much higher than those of isobutene and ETBE, in agreement with their decreasing order in polarity.

As rates were measured free of control by diffusion, it is likely that all the accesible sites were attached to reactants. Ethanol permeates more easily than isobutene and ETBE. So, most of the active centers were occupied by the alcohol, the fraction of free sites being negligible. That is why it was striking that negative adsorption parameters for ethanol were found at 79.5 and 59.5 "C for model type 4. This incoherence can be only due to mathematical problems in the fit. Therefore, model type 5 was tested with the aim of finding a model with a


Table 6. Kinetic Models of Type 5 Tested for ETBE Liauid-Phase Synthesis

r calc (mol1h.g)

102k', model equation T, "C mol/h.g s d 2

5.1

5.2

5.3

5.4

5.5

5.6

89.7 79.5 59.5 39.4

89.7 79.5 59.5 39.4

89.7 79.5 59.5 39.4

89.7 79.5 59.5 39.4

89.7 79.5 59.5 39.4

89.7 79.5 59.5 39.4

temperature-dependence coherent.

289 110 2.34 2.58

194 74.3 1.62 1.79

127 49.2 11.0 1.24

82.1 31.9 7.33 0.855

52.1 20.2 4.79 0.589

32.5 12.4 3.07 0.405

5.990 0.910 0.188 2.20 x 10-5

3.49 0.829 0.154 1.91 x 106

4.43 1.13 0.130 2.71 X 10-5

7.16 1.84 0.119 4.55 x 10-8

11.8 2.94 0.125 7.35 x 10-5

18.2 4.38 0.148 1.11 x lo"'

It converges easily, because there is only an adjustable parameter, the apparent rate coefficient k', which is the product of the following constants for the LHHW and the ER mechanism, respectively:

(3) k' = k*KE;n = k K I K Et -(n - l)

k' = k*KE;n = k K E t -(n - l) (4)

As Table 6 shows, a t 89.7,79.5, and 39.4 O C the sum of squares of lack of fit has a minimum for model 5.2 (three active centers in the surface reaction step), whereas at 59.5 OC, model 5.4 (five centers in this step) has the minimum sum of squares. As it is not likely that the mechanism changes at an intermediate temperature of the range explored, we assume that model 5.2 is the best. Besides, as Figure 6 shows, residuals corresponding to 79.5 "C are random. The residuals of the rest of temperatures have a similar trend.

Figure 7 shows the dependence of k' with temperature. The fit of the Arrhenius law to k'values gives the expression

k' = 4.7 X 10" exp - - ( ' O F ) , (5)

Model 5.2 can be regarded as a coherent temperature- dependence one. But it remains to be seen whether it is able to predict the behavior of an integral reactor. In an earlier work (Cunill et al., 1993) an experiment performed in a batch reactor at 40.5 "C and r(I/Et) = 0.76 was reported. An isobutene conversion of about 82% was achieved on an industrial sample of Lewatit K 2631 in a 200 min run. As Figure 8 shows, conversions computed from model 5.2 are lower only by 5%, in good agreement with data within the limits of the experimental error, seeing that the presence of concentration and temperature gradients can be neglected in line with the modified Weisz- Prater criteria for testing intraparticle transport limitations (Mears, 1971; Rase, 1977). The discrepancy can be due to some inaccuracy of the model because of the

1.6 - 1.4 - 1.2 -

1 -

0.8 -

0.6 - 0 .4 -

I

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

r (mol/h.g) Figure 6. Residuals of model 5.2 at 79.5 OC.

in k' 1 1

2.5 2.7 2.9 3.1 3.3 3.5

(1/~)*103 ( K-l Figure 7. Temperature dependence of k'.

impossibility of measuring reliable rate data in the differential reactor at conversions >60%, and also to the fact that integral run was performed in a different setup, and with a different procedure.

From eq 5, an apparent activation energy of 86.1 kJ/ mol was obtained. This value is itself rather high, in line with the high temperature-sensitivity of the reaction. As can be seen in Table 7, this value is higher than those of Ancillotti et al. (1977) and Kaitale et al. (1988). These used industrial samples of catalyst, so that their results are probably influenced to some degree by mass transport limitations. On the other hand, comparing with the apparent activation energy of MTBE synthesis (Table 8), one can observe that the E'value is similar to that reported by Rehfinger and Hoffmann (19901, a work in which the presence of mass transport effects can be discarded.

Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994 589 Table 7. ADDarent Activation Enerav for ETBE Synthesis and ComDarison with Literature Data ~~ ~

reference E', kJ-mol-l catalyst observations this work 86.1 K 2631 d, < 0.10 mm Ancillotti et al. (1977) 73.8 Amberlyst 15 industrial samples Kaitale e t al. (1988) 49.6 Dowex M-32 industrial samples (0.4 < d, < 1.2 mm)

Table 8. Activation Energy for MTBE Synthesis catalyst E', kJ.mol-1 E, kJ.mol-1 reference

p-toluenesulfonic acid

sulfuric acid

methyl sulfuric acid Amberlyst 15 77.7

92.4

X I experimental

0 0,l 0.2 0.3 0,4 0,5 0.6 0.7 0.8 0.9 1

XI model Figure 8. Experimental conversion against time in a batch reactor (r(I/Et) = 0.76; 40.5 OC; 500 rpm; catalyst mass 10 g; reactor volume 260 cm3) and predicted values by model 5.2.

Table 9. Vaporization and Gas-Phase Adsorption Enthalpies for Isobutene and Ethanol

A " a ( P . ) O , AH"o, ' compound kJ mol-' kJ mol-' reference ethanol -44.0 Kabel and Johanson (1962)

isobutene -72.6 Iborra et al. (1990) 40.6 Gallant (1968)

18.8 Gallant (1968) The activation energy, E, can be computed from E' and

the liquid-phase adsorption enthalpies of isobutene and ethanol. From eqs 3 and 4, the following expressions can be found for the LHHW and the ER mechanism, respectively:

E = E' - + (n - l).AHoa,E, (6)

(7) As liquid-phase adsorption enthalpies for isobutene and

ethanol were not found in the literature, these quantities were computed from the values of Table 9 by means of

Gas-phase adsorption enthalpy of ethanol was deduced from chemisorption data of Kabel and Johanson (1962).

E = E' + (n - l).AHoa,,,

m o a , i ( l ) = m o a , i ( g ) + mov,i

101.5 104.1 91.2 85.7

104.6 91.2 71.1 82.0 82.4

Evans and Halpern (1952) Ancillotti et al. (1977) Gicquel and Torck (1983) Csikos et al. (1980) Al-Jarallah et al. (1988) Gicquel and Torck (1983) Ancilotti et al. (1977) Gicquel and Tork (1983) Rehfinger and Hoffmann (1990)

Isobutene chemisorption data were not found in literature, so that adsorption enthalpy for isobutene was obtained from gas-phase synthesis of ETBE kinetic data (Iborra et al., 1990). Despite the fact that the liquid-phase adsorption enthalpy for isobutene is less reliable than that of ethanol, it is useful to obtain the order of magnitude of activation energy for liquid-phase ETBE synthesis.

An activation energy of 133 kJ/mol was obtained from equation 6 assuming that the reaction follows a LHHW mechanism, whereas a value of 79.3 kJ/mol was obtained from eq 7 assuming that the reaction follows an ER mechanism. As true activation energy values were not found in the literature, there has not been able to check the obtained values. However, it is useful to compare these values with those of MTBE synthesis. As Table 8 shows, the first value is higher than those of homogeneous MTBE synthesis, whereas the second are only a bit smaller. In spite of the fact that it is about different reactions, the value of 133 kJ/mol seems unlikely. So, we can conclude that the most probable mechanism for the reaction would be an ER one, in which isobutene from solution reacts with adsorbed ethanol. However, from an eclectic standpoint it must be recognized that despite the fact that ETBE synthesis occurs mainly through an ER mechanism under the experimental conditions, the contribution of an LHHW mechanism might gain importance on increasing r(I/Et).

Conclusions

The liquid-phase addition of ethanol to isobutene to give ethyl tert-butyl ether (ETBE) has been studied on the ion exchange resin Lewatit K2631 at 1.6 MPa and 40-90 OC. Rate data show that the reaction is highly temperature sensitive, and isobutene has an enhancing effect on the rate whereas ethanol has an inhibitor one. Low ETBE concentrations enhance the reaction but as chemical equilibrium is approached the ether shows an inhibitor effect, as expected. Alcohol-olefin-ether mixtures behave nonideally. So, kinetic analysis has been carried out by means of the UNIFAC liquid-phase activities of isobutene, ethanol, and ETBE. The best kinetic model could be deduced from a ER mechanism and also from a LHHW one. In the ER mechanism, isobutene from solution reacts with the ethanol, adsorbed on one center, to give the ether adsorbed on one center. In the LHHW mechanism, the ethanol and isobutene, each one adsorbed on one center, react to give the ether adsorbed on one center. The rate-determining step of both mechanisms is the surface reaction, and up to three active centers take part in this step. From the activation energy values it can be concluded that the most probable mechanism is the Eley-Rideal one.


Acknowledgment The authors wish to express thanks for financial support

to CAICYT, Spain (Project PB87-0509) and to the refining company Repsol Petroleo S.A.

Nomenclature ai = activity of compound i, dimensionless. ci = concentration of compound i, mo1.L-1 d, = particle diameter, mm E = ethyl tert-butyl ether E = activation energy, kJmmol-1 E' = apparent activation energy, kJ-mol-' Et = ethanol F = total molar flow rate, mo1.h-1 Fi = molar flow rate of compound i, mo1.h-1 I = isobutene k = rate coefficient, mol.(h.g)-l k' = apparent rate coefficient, mol.(h.g)-l k' = apparent rate coefficient, mol.(h.g)-l K = thermodynamic equilibrium constant for the reaction,

Ki = adsorption equilibrium constant of compound i, di-

n = number of active centers that take part in the surface

r = intensive reaction rate, mol.(h.g)-l r(I/Et) = isobutene/ethanol molar ratio in the feed Sd* = sum of squares of lack of fit T = temperature, K u = superficial velocity based upon cross-sectional area of

W = catalyst mass, g Xi = degree of conversion of reactant i, dimensionless xi = molar fraction of substance i, dimensionless yi = activity coefficient of compound i, dimensionless AXi = degree of conversion change of reactant i, dimensionless AHoa,i = adsorption enthalpy of compound i, kJ.mol-l AHov,i = vaporization latent heat of compound i, kJ-mol-1 u = active center

Subscripts

a = adsorption E = ethyl tert-butyl ether Et = ethanol g = gas phase I = isobutene i = inlet 1 = liquid phase o = outlet v = vaporization

Abbreviations

dimensionless

mensionless

reaction step

empty reactor, m d

bRvp = blending Reid vapor pressure ER = Eley-Rideal ETBE = ethyl tert-butyl ether LHHW = Langmuir-Hinshelwood-Hougen-Watson LHSV = liquid hourly space velocity, h-1 MTBE = methyl tert-butyl ether VOC = volatile organic compounds WHSV = weight hourly space velocity, h-1

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Received for review April 20, 1993 Revised manuscript received October 4, 1993

Accepted November 7, 1993'

Abstract published in Advance ACS Abstracts, January 15, 1994.

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