Simultaneous Isomerization and Etherification of Isoamylenes

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Ind. Eng. Chem. Res. 1999, 38, 4563-4570

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Simultaneous Isomerization and Etherification of Isoamylenes Juha A. Linnekoski,* Pa1 ivi Kiviranta-Pa1 a1 kko1 nen, and A. Outi Krause Laboratory of Industrial Chemistry, Helsinki University of Technology, Kemistintie 1, P.O. Box 6100, FIN-02015 HUT, Finland

Liisa K. Rihko-Struckmann Technology Center, Fortum Oil and Gas Oy, P.O. Box 310, FIN-06101 Porvoo, Finland

The etherification of isoamylenes (2-methyl-1-butene, 2M1B, and 2-methyl-2-butene, 2M2B) with methanol, ethanol, and n-propanol was studied using a commercial ion-exchange resin as a catalyst. The steady-state reaction rates for the formation of tert-amyl ethyl ether from isoamylenes and ethanol were measured in a continuous stirred tank reactor. At 333 K the reaction rate of the ether formation was measured to be zero order with respect to the ethanol (ETOH) and positive with respect to the olefin. Initial reaction rates for the simultaneous etherification and isomerization of 2M1B were measured in a batch reactor. Initial reaction rates measured at temperatures of 333 and 353 K showed that the different alcohols (methanol, ethanol, and 1-propanol) affected the isomerization rate but not the etherification rate. Measurements were also made with different initial ETOH/2M1B mole ratios. According to the results, etherification and isomerization rates are equal until the lowest mole ratio (0.2) is reached. It was also found that the reaction rates have a constant value at stoichiometric or higher ETOH/2M1B mole ratios. A new model was developed to explain the obtained results. Introduction Since 1970, methyl tert-butyl ether (MTBE, 2-methoxy-2-methylpropane) has been added to gasoline as an octane enhancer to replace lead compounds and to reduce emissions. The production of MTBE rose significantly through the early 1990s as a result of environmental legislation in the United States of America and several other countries that required oxygenates in the gasoline to reduce the emission of vehicles. Current world MTBE production capacity totals 26 000 000 tons/ year.1 Recently, MTBE has been detected in groundwater, and this might render some drinking water unpalatable. The solubility of MTBE in water has created a situation where other oxygenates are under consideration as alternatives to MTBE.2 In addition to MTBE, which has been the predominant oxygenate, the octane boosters of interest are tert-amyl methyl ether (TAME, 2-methoxy-2-methylbutane), ethyl tert-butyl ether (ETBE, 2-ethoxy-2-methylpropane), and tert-amyl ethyl ether (TAEE, 2-ethoxy-2-methylbutane).3-6 Higher ethers (TAME and TAEE) can be used to meet the amendments of the blend Reid vapor pressure (bRvp) levels and the limits for the olefin content of the reformulated gasoline. The water solubility of higher ethers (TAME, 1.15 g/100 g of water) is lower compared to MTBE (4.8 g/100 g of water).1 TAME and TAEE production utilizes two isoamylenes and thus reduces the olefin content of the light fluid catalyst cracking (FCC) gasoline. One advantage of TAEE is that ethanol, the other reagent, can be produced by fermentation from renewable resources, such as molasses, sugarcane, sugar, corn, or potatoes.7 TAME and TAEE can be made in a liquid-phase reaction of isoamylenes with methanol and ethanol, * To whom correspondence should be addressed. E-mail: [email protected]. Fax: 358-9-451 2622. Tel: 358-9-451 2619.

respectively. When using ion-exchange resins as the catalyst, only tertiary olefins react in the etherification reaction.8 From the three isoamylenes, 2-methyl-1butene (2M1B) reacts fastest and 2-methyl-2-butene (2M2B) reacts 2-5 times slower depending on the alcohol,9 whereas 3-methyl-1-butene gives no reaction.8 In addition to the etherification, isomerization of the olefin double bond occurs simultaneously. The equilibrium favors more stable 2M2B.10 Other side reactions are the olefin dimerization, followed in certain circumstances by olefins and dimer polymerization, olefin hydration, and alcohol condensation to ethers.11-13 Despite the growing interest in higher ethers and ethanol-based ethers, few studies have been published regarding the formation of TAEE. Rihko and Krause9 published a report on the reactivity of isoamylenes with ethanol. Rihko et al.10 and Kitchaiya and Datta14 have studied the reaction equilibrium in the synthesis of TAEE. Zhang et al.15 studied the formation of mixed tert-alkyl ethyl ethers, from isoamylenes and terthexylolefins. In the preceding publication,16 we tested three kinetic models for the formation of TAEE. From the tested models, the Langmuir-Hinshelwood type model best described the experimental results. This model is based on single-site adsorption of every component, with the surface reaction being the rate-limiting step. Etherification of isoamylenes with aqueous ethanol has been studied by Jayadeokar and Sharma17 and by Linnekoski et al.18 Liquid-phase studies on the equilibrium and kinetics of other ethanol-based ethers have been presented by Jayadeokar and Sharma,19 Vila et al.,20 Fite´ et al.,21 Izquierdo et al.,22 Zhang and Datta,23,24 Jensen and Datta,25 and Gomez et al.26 The mechanisms that are proposed for the etherification of tertiary olefins with alcohol using ion-exchange resins vary from heterogeneous Langmuir-Hinshelwood type models to homogeneous models. The first assumptions were presented by Ancillotti et al.,27,28 who

10.1021/ie9902481 CCC: $18.00 © 1999 American Chemical Society Published on Web 11/03/1999

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proposed that the etherification mechanism depends on the alcohol/olefin mole ratio. They stated that at stoichiometric or higher methanol/isobutene ratios the ionic mechanism predominates (quasi-homogeneous), and the protonation of the olefin by the solvated proton was proposed as the rate-determining step. At lower than stoichiometric alcohol/olefin ratios, the mechanism is a concerted one (quasi-heterogeneous), and undissociated sulfonic groups operate. Panneman and Beenackers29 also suggested a pseudohomogeneous model for the formation of MTBE at isobutene/methanol ratios between 0.1 and 1. Several researchers have presented heterogeneous or pseudoheterogeneous mechanisms for the etherification reactions.15,16,21,23,24,30-37 These heterogeneous models are usually Langmuir-Hinshelwood or Eley-Rideal type models. It is usually assumed that in LangmuirHinshelwood type models alcohol, olefin, and ether adsorb on the active sites and in Eley-Rideal type models alcohol and ether adsorb on the active sites. Information on the mechanism of the etherification reactions has also been obtained by studying the etherification with different alcohols. According to Macho et al.,38 Ancillotti et al.,27 Kaitale et al.,39 and Linnekoski et al.,40 the activity order of the lower alcohols in the etherification of isobutene is n-butanol ) n-propanol > ethanol > methanol. Ancillotti et al.27 and Kaitale et al.39 concluded that because the alcohols solvate the protons, which are the active sites on the catalyst, a different alcohol basicity can result in different solvated proton acidity and therefore different reactivity. However, the effect of alcohol on the acidity of the proton is not clear. Rys and Steinegger41 measured the value of the Hammett acidity function using several indicators and solvents. They found only small differences between the values of the Hammet acidity function (-H0) when ethanol (1.50-1.54) and methanol (1.49-1.50) were used as solvents. When propanols were used as solvents, the values were higher (1.97-2.00). Casey and Pietrzyk42 investigated the acidity of the protons in sulfonic acidic groups of the ion-exchange resins using a nuclear magnetic resonance spectrometer. They found that the acidity in the resin interior decreases in the order n-propanol > ethanol > methanol. The acidity of the solvated proton might change with the alcohol/olefin mole ratio.32 The changes in the acidity of the proton can be indicated by measuring the extent of ionization of an inert base using an acidity function41 or by using a resin solubility parameter.43 The most used acidity function is the Hammett function (Hammett and Deyrup44), which uses nitroaniline indicators. Another way to take into account the changes in the nature of the catalytic site has been introduced by Fite´ et al.43 They used the Hildebrand solubility parameter to characterize the influence of the reaction medium on the catalytic activity. The parameter is directly related to the swelling and to the accessibility of the active sites.43 In this work, we present a new kinetic model, which explains the experimental data better at low ETOH/ 2M1B mole ratios. This model was developed from the obtained results of the simultaneous isomerization and etherification of isoamylenes. Initial reaction rates were measured with different ethanol/olefin mole ratios. No previous data were found from the literature. New data on the effect of alcohols on the etherification and

isomerization of 2M1B are also shown. No similar data were found from the literature. Experimental Section The initial reaction rates for the isomerization of 2M1B to 2M2B and for the etherification of 2M1B and 2M2B with ethanol, methanol, and 1-propanol were measured in a batch reactor at 333 and 353 K. In addition, the effect of the initial ETOH/2M1B mole ratio on the isomerization and etherification rates was studied at three temperatures, 333, 343, and 353 K. The used mole ratios were 5, 2, 1, 0.2, and 0.5 at 333 and 353 K. At 343 K only mole ratios 2, 1, and 0.5 were used. The experiments with different mole ratios were done with and without solvent. S, the total number of experiments, was 28 including the repeated experiments. The initial reaction rates for etherification and isomerization were obtained from the slopes of the lines of formed TAEE and 2M2B (moles) plotted as a function of contact time (h × g of catalyst). To study the effect of ETOH and 2M1B mole fractions on the etherification rate, measurements were made in a continuous stirred tank reactor (CSTR). The ETOH mole fraction was constant, and the effect of olefin mole fraction on the etherification rate was measured and vice versa. Solvents were used to change the mole fraction of the other component. Reactors. The initial reaction rate experiments were carried out in a batch reactor (80 cm3 stainless steel). The reaction mixture was stirred magnetically. The catalyst (0.1-0.7 g) was placed in a metal gauze basket (60 mesh, 2 cm3), which also worked as a mixing baffle in the reactor. The temperature (333-353 K) was controlled within (0.2 K by immersing the reactor in a thermostated water bath. The pressure was kept constant at 0.7-0.8 MPa, with an accuracy of 0.03 MPa, to guarantee a liquid-phase operation at all temperatures. Samples were withdrawn from the reactor as a function of contact time (amount of catalyst × time). A detailed description of the experimental setup and procedure is given elsewhere.10 The measurements of the effect of ETOH and olefin mole fractions on the etherification rate were made in a CSTR (55.6 cm3 stainless steel). The reaction mixture was stirred magnetically. The catalyst was placed in a metal gauze basket (60 mesh, 2 cm3), which also worked as a mixing baffle in the reactor. The temperature was kept constant at 333 K within (0.2 K by immersing the reactor in a thermostated water bath. The pressure was kept constant at 0.7-0.8 MPa, with an accuracy of 0.03 MPa, to guarantee a liquid-phase operation at all temperatures. The feed and the reactor effluent were analyzed on-line with a gas chromatograph using an automated liquid sample valve. To guarantee a pulsefree flow, the reaction mixture was fed from a pressurized tank to the reactor with a pressure difference of 0.2-0.4 MPa over the liquid mass flow controller. The state of the system (flow, pressure, and temperature) was kept constant for 3-4 h, and repeated analyses were made of the reactor outlet to ensure that the steady state was reached. A Mettler PM 6000 balance was used to calculate the actual flow at the outlet of the reactor system. In our previous publication,16 we showed that no reaction occurs in the absence of the catalyst and the external diffusion has no effect at mixing speeds above

Ind. Eng. Chem. Res., Vol. 38, No. 12, 1999 4565 Table 1. Initial Reaction Rates for Etherification and Isomerization of 2M1B with Stoichiometric Alcohol/ Olefin Mole Ratios at 333 and 353 K. (Error of the Reaction Rates: (0.01 mol h-1 g-1) isomerization, mol h-1 g-1

etherification, mol h-1 g-1

alcohol

333 K

353 K

333 K

353 K

MEOH-99% ETOH-99% 1PROPA-99% ETOH-96%a

0.05 0.10 0.17 0.04

0.15 0.37

0.11 0.10 0.12 0.03

0.32 0.34

a

Linnekoski et al.18

5.0 s-1. Also it was found that the internal diffusion has no effect with particles smaller than 0.65 mm. Analytical Methods. A Hewlett-Packard gas chromatograph 5890 series II, equipped with a flame ionization detector, was used for the analysis. The compounds were separated in a 60 m × 0.258 mm glass capillary column HP-1, with a film thickness of 1.0 µm (HewlettPackard). All compounds were calibrated in order to obtain quantitative results. The reproducibility of the analysis was (3%. Catalyst. The catalyst used was a commercial, strong cation-exchange resin, Amberlyst 16 W (Rohm & Haas), which has an exchange capacity of 4.4 mequiv/g of dry catalyst and a cross-linking level of 12 wt %. The catalyst particles were pretreated as described in ref 16. The catalyst was sieved, and a particle size of 0.3-0.6 mm was used in the experiments because of the internal mass transfer.16 In batch reactor experiments, the catalyst was changed after each run. In the CSTR experiments, the catalyst was replaced after several runs. After the runs, the catalyst was dried and weighed. All rate calculations are based on the amount of dry catalyst. The amount of dry catalyst used in the experiments varied between 0.1 and 0.7 g. Reactants and Solvents. The reagents used were ethanol (ETOH, Alko Oy, >99.5 wt %), methanol (MEOH, Riedel-de Haen, >99.8 wt %), 1-propanol (1PROPA, Merck, >99.5 %), 2-methyl-1-butene (2M1B, Aldrich, 99 wt%), and a mixture of 2-methyl-2-butene and 2-methyl-1-butene (2M2B, Fluka Chemie, 90.7 wt%). In some experiments, the reagents were diluted with a mixture of unreactive hydrocarbons, 10 wt % isopentane (I-Pen, Fluka Chemie, p.a.), 45 wt % cyclohexane (CYHE, Merck, p.a.), and 45 wt % n-octane (nOCT, Fluka Chemie, p.a.). The solvent hydrocarbons were chosen to represent the typical hydrocarbons present in a light FCC gasoline fraction. Results and Discussion Initial Reaction Rates. In the formation of TAEE from 2M1B and 2M2B, three reactions proceed simultaneously, the etherification of 2M1B and 2M2B and simultaneous isomerization of these two isoamylenes. To get information about these reactions for the development of a new kinetic model, the initial reaction rates were measured for the etherification and isomerization of 2M1B with various alcohols and as a function of the ethanol/2M1B initial mole ratio. First the initial reaction rates were measured using different alcohols. The measured initial reaction rates are shown in Table 1 at two temperatures, 333 and 353 K. It can be concluded from the table that the initial reaction rates for the etherification are in the same range for pure alcohols, whereas larger differences are

found in the initial reaction rates for the isomerization at temperatures 333 and 353 K. The isomerization rate of the isoamylenes is twice as high with ethanol compared to methanol and more than 3 times higher with 1-propanol compared to methanol. Adding a small amount of water, 4 wt %, to the ethanol reduces both the isomerization and etherification rates to about 1/3 of that without water. At the same time, the initial hydration rate increases to about the same as the etherification rate 0.02 mol h-1 g-1, although the ethanol/water mole ratio is as high as 11.18 The initial reaction rates for the formation of TAME in Table 1 were compared to the values found in the literature. Ancillotti et al.27 obtained an etherification rate of 0.017 mol s-1 equiv-1 and an isomerization rate of 0.0081 mol s-1 equiv-1 at 333 K. These agree with the values calculated from Table 1 for the etherification, 0.0127 mol s-1 equiv-1, and for the isomerization, 0.0063 mol s-1 equiv-1. Not all of the results in Table 1 agree with the literature. The finding that all studied alcohols have the same reaction rate in the etherification of isoamylenes is contrary to the results obtained in the etherification of isobutene. In the etherification of isobutene, an activity order of n-butanol ) n-propanol > ethanol > methanol has been found.27,28,38,39 An explanation to this difference can be found from the solvation of the active sites. Alcohols solvate the protons and the acidity; that is, the activity of the proton changes with the alcohol. The more basic alcohols produce more acidic solvated protons, and more acidic protons increase the protonation of the alkenes. This means that both the isomerization and the etherification reaction rates should increase, which is in contrary to the results presented at Table 1. This difference is caused by the isomerization reaction. In the isomerization reaction more 2M2B is formed from 2M1B with more basic alcohols as shown in Table 1. From the two isoamylenes 2M2B is more stable and less reactive in the etherification reaction. As a result, because of the increased isomerization, there is less 2M1B, which is more reactive in the etherification, available for the etherification and thus the etherification reaction does not increase. According to Rihko and Krause,9 2M2B is 2-5 times slower in the etherification reaction depending on the alcohol. In the case of isobutene, there is no isomerization reaction to consume the protonated olefin, and it can all react with alcohol to produce ether. In our previous publication14 we tested three models in the formation reaction of TAEE. From the tested models, the Langmuir-Hinshelwood type model best described the experimental results. However, it was shown that this model did not explain the experimental results at low alcohol mole fractions. To develop a model that better explains the experimental data at low alcohol mole fractions, measurements were made with different ETOH/2M1B ratios at temperatures 333, 343, and 353 K. The results at 333 K are presented in Figure 1. According to the figure, initial reaction rates for etherification and isomerization increase as the ETOH/2M1B mole ratio decreases and the ratio of etherification and isomerization rates is equal until the lowest initial ETOH/2M1B mole ratio 0.2, where the isomerization rate increases significantly. At ETOH/2M1B mole ratio 0.2, the isomerization reaction rate is twice as high as the etherification reaction rate (see Figure 1). Identical results were obtained at 353 K, where the initial rates

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Figure 1. Initial reaction rates for the etherification and isomerization of 2M1B as a function of the ethanol/olefin mole ratio with and without solvent at 333 K.

Figure 2. Etherification rate as a function of the ethanol mole fraction at constant olefin mole fraction (0.3) at 333 K.

are equal at all other mole ratios except at the lowest mole ratio (0.2), and the reaction rate increases when the ethanol/olefin mole ratio is decreased. The only side reaction that was observed was the hydration of isoamylenes to tert-amyl alcohol (TAOH). A typical amount of TAOH was 0.5 mol % after 4-5 h of reaction time. Similar results were obtained in the formation reaction of TAME from 2M1B and methanol. The etherification rate is about 2.5 times higher than the isomerization rate at all other MEOH/2M1B initial mole ratios except at the lowest mole ratio, where the isomerization rate is faster than the etherification rate.45 Effect of Ethanol and Olefin Mole Fractions on the Etherification Rate. The effect of ethanol and olefin (2M1B + 2M2B) mole fractions on the etherification rate was studied by measuring steady-state reaction rates using a mixture of 2M1B and 2M2B as the reagent. First the effect of the ETOH mole fraction on the etherification rate was measured at constant olefin mole fraction (0.3). According to the results in Figure 2, the reaction is zero order with respect to ethanol. Figure 3 shows the results of the same measurements made at constant ETOH mole fraction and varying the olefin mole fraction. According to the results, the reaction order with respect to the olefin is positive. The positive dependency is quite general for the olefin, whereas zero or slightly positive dependency has been proposed for the alcohol. Ancillotti et al.27 found a positive order for isobutene and a zero order for methanol in the formation reaction of MTBE at stoichiometric or higher methanol/isobutene ratios. Positive dependencies for the olefin and alcohol have been presented by Caetano et al.36 and Refinger and

Figure 3. Etherification rate as a function of the olefin mole fraction at constant ethanol mole fraction (0.3) at 333 K.

Hoffmann30 in the formation of MTBE and by Oost and Hoffmann31 in the formation of TAME. Kinetic Mechanism for the Etherification and Isomerization. In the kinetic modeling the ratedetermining step for the etherification is usually assumed to be the protonation of the olefin or the reaction between the protonated olefin and the alcohol. The models in this work support are based on the latter assumption, i.e., for isomerization and etherification reactions, the surface reaction is the rate-limiting step. There are two major findings to support this assumption. First, the etherification rate depends linearly on the olefin (Figure 2) mole fraction and is zero order with respect to the ethanol (Figure 3) mole fraction. Second, the different alcohols (Table 1) only affect the isomerization rate and not the etherification rate. This means that the protonation of the olefins cannot be the ratelimiting step for the etherification but the surface reaction. On the other hand, alcohols can only affect the isomerization rate by solvating the active site and producing protons with different acidities or by competing for the adsorption with olefins on the active sites (protons). However, the results in Figure 1 indicate that the adsorption of the ETOH and isoamylenes cannot be competitive. If the adsorption were competitive, we would assume that at high initial ETOH/2M1B mole ratios the isomerization rate would decrease. Another fact that speaks against the competitive adsorption is the stronger polarity of the alcohol compared to olefins. As polar molecules, alcohol molecules prefer to adsorb stronger than the olefins and cover the catalyst surface. In our previous publication,16 we stated that the Langmuir-Hinshelwood type model best described the experimental results. The same Langmuir-Hinshelwood type model was also used in modeling the results in the study of simultaneous etherification and hydration of isoamylenes.18 In this model it is assumed that the adsorption of alcohol, olefin, and ether is competitive. However, the results in this study indicate that at initial ethanol/olefin mole ratios > 0.2 the mechanism might not be a competitive one for ethanol and olefins. A more reasonable mechanism might be the one proposed by Piccoli and Lovisi,34 where alcohol first adsorbs to the active site and solvates it. Olefins adsorb to the same active site and form carbocations with the solvated protons. The protonated olefin reacts with ethanol from the surrounding liquid phase. So, we have a modified Eley-Rideal type mechanism. Kinetic Model. A kinetic model was developed according to the modified Eley-Rideal type mechanism presented above.34 In this model, alcohol molecules adsorb to the catalyst and solvate the active site:

Ind. Eng. Chem. Res., Vol. 38, No. 12, 1999 4567

ETOH + S / ETOH[S]

(1)

k′1(aETOHa2M1B - aTAEE/Ke1)

2M1B adsorbs to the same site:

2M1B + ETOH[S] / 2M1B[S]ETOH

(2)

The adsorbed 2M1B can isomerize (eq 3) or react with ethanol to TAEE (eqs 4 and 5):

2M1B[S]ETOH / 2M2B[S]ETOH

(3)

2M1B[S]ETOH + ETOH / TAEE[S]ETOH (4) 2M2B[S]ETOH + ETOH / TAEE[S]ETOH (5) Finally ether and 2M2B desorb:

TAEE[S]ETOH / TAEE + ETOH[S]

(6)

2M2B[S]ETOH / 2M2B + ETOH[S]

(7)

The rate-limiting steps for the isomerization and for the etherification reactions are assumed to be the surface reactions, for isomerization the reaction of protonated 2M1B to 2M2B (eq 3), and for etherification the reaction between the protonated olefin and alcohol molecule (eqs 4 and 5). Using these assumptions and eqs 1-7, the rate equations for etherification (eq 8) and isomerization (eq 9) can be written as follows. In eq 8 a′ETOH is the activi-

rETHER ) k1Θ2M1Ba′ETOH - k2ΘTAEE + k3Θ2M2Ba′ETOH - k4ΘTAEE (8) rISOM ) k5Θ2M1B - k6Θ2M2B

(9)

ty of ethanol in the near vicinity of the active site and the Langmuir isotherm is used to describe it. The surface concentrations of the species Θi can be described with the Langmuir isotherm, where Ki is the adsorption equilibrium constant and ai is the activity of component i:

Θi )

Kiai 1+

∑Kiai

(10)

Using eq 10 in eq 8 and 9, we obtain the following equations for the etherification (eq 11) and isomerization (eq 12): rETHER ) k1KETOHKO(aETOHa2M1B - aTAEE/Ke1) (1 + KO(a2M1B + a2M2B) + KTAEEaTAEE)(1 + KETOHaETOH)

+

k3KETOHKO(aETOHa2M2B - aTAEE/Ke2) (1 + KO(a2M2B + a2M1B) + KTAEEaTAEE)(1 + KETOHaETOH)

(11) rISOM )

k5KO(a2M1B - a2M2B/Ke3) 1 + KO(a2M1B + a2M2B) + KTAEEaTAEE

rETHER )

(12)

In the development of the eqs 11 and 12, it is assumed that all free active sites (solvated by ethanol) are occupied. To reduce the number of parameters, we divide eqs 11 and 12 with the parameter KTAEE and assume that the term 1/KTAEE is small compared to other terms. This results in eqs 13 and 14. In equations

(K′O(a2M1B + a2M2B) + aTAEE)(1 + KETOHaETOH) k′3(aETOHa2M2B - aTAEE/Ke2) (K′O(a2M2B + a2M1B) + aTAEE)(1 + KETOHaETOH) k′5(a2M1B - a2M2B/Ke3) rISOM ) + K′O(a2M1B + a2M2B) + aTAEE

+

(13)

(14)

k′1 ) k1KETOHKO/KTAEE, k′3 ) k3KETOHKO/KTAEE, k′5 ) k5KO/KTAEE and K′O ) KO/KTAEE, it was also assumed that the adsorption constants of the olefins are equal, KO ) K2M1B ) K2M2B. Equilibrium Constants. The temperature dependency of the equilibrium constants can be expressed with eq 15.47 The equilibrium compositions have been

ln Kei )

-∆rH° ∆rS° + RT R

(15)

published by Rihko et al.10 In the present work, the UNIFAC estimates of activity coefficients were used to calculate the activities.48 A plot of ln Kei versus 1/T produces a straight line. The standard heats of the reactions (∆rH°) can be obtained from the slopes of these lines. From the intercept, we obtain the values for the standard entropy change of the reactions (∆rS°). The standard heat of the reaction for TAEE formation from 2M1B was -34.5 kJ mol-1, and that from 2M2B, -27.0 kJ mol-1. The standard entropy changes of the reactions were 80.1 and 76.5 J mol-1 K-1, respectively. Results. The model developed in this work was compared to the Langmuir-Hinshelwood type model, which best described the experimental results in our previous publication.16 The developed rate equations were coupled with the reactor model. The kinetic parameters were estimated with the KINFIT49 estimation program by minimizing with the LevenbergMarquardt method the weighted sum of residual squares (WSRS) between the experimental (yi,exp) and calculated (yi,est) compositions in the outlet of the reactor. Relative weight factors (wi ) 1/yi) were used for all compounds.

WSRS )

∑(yi,exp - yi,est)2wi

(16)

The rate equations were fitted to the experimentally measured rates of reaction for etherification and isomerization. The estimated parameters were the mean preexponential factors and activation energies. The temperature dependency of ethanol’s adsorption coefficient was taken from Zhang et al.,15 in order to reduce the number of fitted parameters. The temperature dependence of the reaction rate parameters was correlated using the Arrhenius equation. To reduce the correlation between the preexponential factor A0 and activation energy E, the equation was written in the form

k ) Ameane-zE

(17)

where Amean is defined as A0 exp(-E/RTmean) and z as 1/8.314(1/T - 1/Tmean). The temperature dependence of the ratio of the adsorption coefficients (K′O) was assumed to be close to zero.

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Ind. Eng. Chem. Res., Vol. 38, No. 12, 1999 Table 2. Values of the Estimated Parameters for a Noncompetitive Model (A) and for a Langmuir-Hinshelwood Type Model (B) (Rate Constants ki, mol h-1 g-1; Activation Energies and Adsorption Enthalpies Ei ) ∆Hi, kJ mol-1; Adsorption Coefficient KETOH, No Units) model A k1 E′1 k3 E′3 k5 E′5 KO ∆H′ETOH KETOH

Figure 4. Residuals of the TAEE formation (wt %) in the EleyRideal type model.

Figure 5. Residuals of the TAEE formation (wt %) in the Langmuir-Hinshelwood type model.

According to the statistics, the Langmuir-Hinshelwood type model (0.137) has a smaller mean WSRS than the modified Eley-Rideal type model (0.208) although the difference is small. The residuals between the experimental and estimated TAEE compositions (wt %) at the reactor outlet are shown for the two models in Figures 4 and 5. According to the figures, the modified Eley-Rideal type model has lower residuals at low ethanol mole fractions. The Langmuir-Hinshelwood type model has significantly larger residuals at low ethanol/alcohol mole fractions. Also, in our previous publication,16 the Langmuir-Hinshelwood type model could not predict the experimental etherification rates accurately at low ethanol content. The values of the estimated parameters are shown in Table 2. The parameters are well identified in both models, because the errors are quite small. Both models predicted the preexponential factors (Amean) quite well, having only a 10-20% standard error of estimates. The standard error of estimates for the apparent activation energies varied from 5 to 10%. For the K′O, the EleyRideal type model predicted a value of 0.22 with a standard error of 0.015. No significant correlations were found between the fitted parameters. The apparent activation energies obtained in this work were compared to those obtained in our previous study and those obtained by Zhang and Datta24 in the tert-hexyl ethyl ether synthesis. Zhang and Datta24 obtained an apparent activation energy of 87.3 kJ mol-1 for the etherification of R-olefin (2-methyl-1-pentene) with ethanol and an apparent activation energy of 103.2 kJ mol-1 for the etherification of β-olefin (2-methyl-2pentene) with ethanol. The values obtained in this work

a

model B

value

STD

value

STD

0.077 80 0.066 88 0.005 90 0.22 11a 27a

10% 7% 20% 16% 16% 7% 21%

0.81 87 0.38 107 0.17 76

3% 4% 10% 10% 4% 5%

11a 27a

Fixed parameters (Zhang et al.15).

are comparable to their values (Table 2). In our previous study, we obtained values of 76.8 kJ mol-1 for the etherification of R-olefin (2-methyl-1-butene) and 95.6 kJ mol-1 for the etherification of β-olefin (2-methyl-2butene) with ethanol using the Langmuir-Hinshelwood type model. In this work, the corresponding values are 87 and 107 kJ mol-1, which are a little higher than the previous values. One reason for the differences is the large standard deviations of the apparent activation energies in our previous study.16 For the isomerization, the apparent activation energies obtained in this study were 91 kJ mol-1 with the modified Eley-Rideal type model and 76 kJ mol-1 with the Langmuir-Hinshelwood type model. Both values are quite close to what has been presented in the literature. For the isomerization, Zhang and Datta24 obtained a value of 87.3 kJ mol-1 in the etherification of R-olefin (2-methyl-1pentene) with ethanol, and Linnekoski et al.,16 a value of 72.9 kJ mol-1 in the etherification of 2-methyl-2butene using a Langmuir-Hinshelwood type model. In conclusion, the modified Eley-Rideal type model developed in this study gives an equal fit between the experimental and estimated component compositions in the outlet compared to the Langmuir-Hinshelwood type model. In addition, it predicts the experimental results better at low alcohol contents. Conclusions The simultaneous isomerization and etherification of isoamylenes with various alcohols was studied in a batch reactor. Initial reaction rates for the isomerization and etherification of isoamylenes were measured with different initial ethanol/2-methyl-1-butene mole ratios. According to the results, the adsorption of ethanol and olefins is not competitive. Alcohol adsorbs first, and then olefins and ether adsorb to the same site. A new kinetic model was developed according to this mechanism. This model describes the experimental results better at low alcohol contents. Apparent activation energies were obtained, and they were comparable to the earlier obtained values. Acknowledgment The financial support for this work from the Nordic Minister Council and from the research foundation of the Neste Corp. is gratefully acknowledged.

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Notation ai ) activity Amean ) A0 exp(E/RTmean), mol h-1 g-1 A0 ) preexponential factor, mol h-1 g-1 E′ ) apparent activation energy, kJ mol-1 ∆rH° ) standard heat of the reaction, kJ mol-1 ∆HETOH ) adsorption enthalpy for ethanol, kJ mol-1 i ) ETOH, 2M1B, 2M2B, or TAEE ki ) rate constant, mol h-1 g-1 Kei ) equilibrium constant of reaction i Ki ) adsorption equilibrium constant for component i R ) 8.314, J mol-1 K1 ri ) rate of reaction for component i, mol h-1 g-1 Si ) vacant adsorption site [S] ) solvated proton ∆rS° ) standard entropy change of the reaction, J mol-1 K-1 T ) temperature, K Tmean ) mean temperature, 347 K wi ) weight factor xi ) mole fraction of component i Abbreviations CSTR ) continuous stirred tank reactor bRvp ) blend Reid vapor pressure FCC ) fluid catalytic cracking ETOH ) ethanol MEOH ) methanol 1PROPA ) 1-propanol 2M1B ) 2-methyl-1-butene 2M2B ) 2-methyl-2-butene MTBE ) methyl tert-butyl ether (2-methoxy-2-methylpropane) ETBE ) ethyl tert-butyl ether (2-ethoxy-2-methylpropane) TAEE ) tert-amyl ethyl ether (2-ethoxy-2-methylbutane) TAME ) tert-amyl methyl ether (2-methoxy-2-methylbutane) THEE ) tert-hexyl ethyl ether (2-ethoxy-2-methylpentane) WSRS ) weighted sum of residual squares OLEFIN ) olefins (2M1B and 2M2B) SOLVENT ) mixture of isopentane, cyclohexane, and n-octane ETHER ) etherification ISOM ) isomerization Greek Letter Θ ) fraction of surface covered by component I

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Received for review April 5, 1999 Revised manuscript received September 14, 1999 Accepted September 26, 1999 IE9902481