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Ind. Eng. Chem. Res. 1996, 35, 2500-2507
Etherification of FCC Light Gasoline with Methanol Liisa K. Rihko Neste Oy, Technology Center, P.O. Box 310, FIN-06101 Porvoo, Finland
A. Outi I. Krause* Laboratory of Industrial Chemistry, Helsinki University of Technology, FIN-02150 Espoo, Finland
The etherification with methanol of C5 and C6 compounds present in FCC light gasoline was studied. The catalyst used in the experiments was a cation-exchange resin, and the temperature range was 323-353 K. The initial etherification rates were measured in a batch reactor. The reactivities of the alkenes were compared by calculating the rate constants. The rate constant of 1-methylcyclopentene was the highest for the C6 compounds. The effects of thermodynamic limitations on the etherification of FCC light gasoline were studied in a plug flow reactor. The thermodynamic limitations were significant for 1-methoxy-1-methylcyclopentane above 323 K and for the formation of the other C6 methyl ethers at temperatures above 333 K. The experimental equilibrium constants were calculated at temperatures of 343 and 353 K and the values compared with the published values. Introduction During the last few years the high demand for tertiary ethers as gasoline components has greatly expanded research and also commercial interest with regard to ethers other than methyl tert-butyl ether (MTBE). Although MTBE is still the major ether produced for the gasoline pool, research is focusing on the heavier ethers, e.g., ethyl tert-butyl ether (ETBE), tert-amyl methyl ether (TAME), and tert-amyl ethyl ether (TAEE). The supply of isobutene and isoamylenes may become limited, and therefore it is important to explore alternative sources of tertiary alkenes. Recent publications have presented a new process for manufacturing oxygenates, not only from isobutene or isoamylenes but also from heavier tertiary alkenes (Jakkula et al., 1995; Ignatius et al., 1995). Substantial quantities of C5 and C6 unsaturated compounds are present in the light gasoline produced in the fluid catalytic cracking units of refineries. Some of these compounds can be etherified with alcohols. In the etherification reaction catalyzed with cation-exchange resins, the only reactive compounds are those with the double bond attached to a tertiary carbon atom. Of the C5 alkenes, the reactive ones are, therefore, only 2-methyl-1-butene (2M1B) and 2-methyl-2-butene (2M2B). 3-Methyl-1-butene is not reactive under the same conditions (Ancillotti et al., 1977; Krause and Hammarstro¨m, 1987). Among the C6 compounds of FCC gasoline, the number of reactive compounds is higher. There are seven C6 alkenes (e.g., 2-methyl-1pentene, 2M1P) and one cycloalkene which can be etherified to form four different ethers (Krause et al., 1984; Pescarollo et al., 1993). The list of the reactive C5 and C6 compounds, the respective ethers, and the nomenclature used for them in this paper are presented in Table 1. The studies of the etherification of 2M1B and 2M2B with methanol can largely be found in the literature. Both the kinetics (Ancillotti et al., 1977; Muja et al., 1986; Randriamahefa et al., 1988; Rihko and Krause, 1995; Oost et al., 1995a) and the reaction equilibria (Safronov et al., 1989; Rihko et al., 1994; Piccoli and * Author to whom correspondence is addressed. e-mail:
[email protected].
Lovisi, 1995, Oost et al., 1995b) are studied. The experimental etherification investigations of C6 compounds are very few. Some physical properties of the respective ethers were published in 1936 (Evans and Edlund, 1936). Krause et al. (1984) initially studied the etherification of 2M1P, 2M2P, 23DM1B, c-3M2P, and t-3M2P with methanol using a packed-bed integral reactor. Recently, studies of the reaction kinetics and the equilibria of four reactive C6 alkenes (2M1P, 2M2P, 23DM1B, and 23DM2B) with ethanol have been published (Zhang and Datta, 1995a,b). Recently, a study of the calculated chemical equilibria in the etherification of light FCC gasoline with methanol was reported by Wyczesany (1995). The data which were not available from thermodynamic tables were calculated using two essentially different group contribution methods (The Benson and the Yoneda group contribution methods). In particular, the standard Gibbs free energies of formation were estimated for C7 alkenes and C5-C7 methyl ethers (i.e., ethers originating from C5-C7 alkenes and methanol). No experimental studies of the formation of the cyclic ether, 1M1McP, can be found in the literature. The initial rates of etherification of C6 compounds have not been published earlier. Here we report on an experimental study of the initial rates and thermodynamic limitations for the reactions of methanol with the C5 and C6 compounds which are present in FCC gasoline and are reactive in the etherification reactions. The experimental equilibrium constants for the formation of C5 and C6 methyl ethers at temperatures of 343 and 353 K are also presented. Experimental Section In the first stage of the study, the initial rates for the formation of the ethers were measured in a batch reactor. The relative reaction rate constants of C6 compounds and the activation energy of the formation of the ether were calculated based on measurements of the initial rate of reaction. In the second stage, the effect of temperature and space velocity on the etherification conversion of the reactive compounds with methanol was studied in a plug flow reactor. Apparatus. Batch Reactor. The volume of the stainless steel batch reactor was 80 cm3. The reaction
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Ind. Eng. Chem. Res., Vol. 35, No. 8, 1996 2501 Table 1. Reactive C5 and C6 Alkenes and the Corresponding Ethers Formed in the Etherification of FCC Light Gasoline with Methanol reactive alkene
abbreviation
2-methyl-1-butene 2-methyl-2-butene 2-methyl-1-pentene 2-methyl-2-pentene cis-3-methyl-2-pentene trans-3-methyl-2-pentene 2-ethyl-1-butene 2,3-dimethyl-1-butene 2,3-dimethyl-2-butene 1-methylcyclopentene
2M1B 2M2B 2M1P 2M2P c-3M2P t-3M2P 2E1B 23DM1B 23DM2B 1McP
mixture was stirred magnetically, and the temperature was controlled within (0.25 K by immersing the reactor in a thermostated water bath and measuring the temperature inside the reactor. Experiments were carried out at temperatures between 323 and 353 K. The pressure was set at 0.7 MPa. The catalyst (about 1 g of dry catalyst) was placed in a metal gauze basket. The samples from the reaction mixture were taken manually via an ice-cooled sample valve at the top of the reactor. An experiment took 6-9 h, depending on the reaction temperature. Flow Reactor. The flow reactor experiments were carried out in a continuous plug flow reactor (stainless steel, i.d. 21 mm) with a catalyst volume of 33 cm3. The temperature was varied from 323 to 353 K at 10 K intervals, and the LHSV (liquid hourly space velocity, cm3 of feed cm-3 of cat h-1) was 1.1, 2.2, or 4.3 h-1. The experiments were performed in random temperature sequence, and the catalyst load was changed to a fresh one as the space velocity was changed. The temperature was regulated automatically by measuring the temperature inside the reactor and controlling the temperature of water circulating in the reactor jacket. The pressure was kept between 0.5 and 0.7 MPa in order to guarantee a liquid-phase operation at every temperature. The pulse-free flow (25-75 g h-1) of the feed was controlled by a liquid mass flow controller. The feed and the reactor effluent were analyzed on-line with a gas chromatograph using an automated liquid sample valve. Several samples were taken from the reactor effluent under one set of conditions in order to guarantee steady state operation. One experiment took about 4 h after 2 h of stabilization time. Catalyst and Chemicals. A commercial, macroreticular strong cation-exchange resin in H+ form (Amberlyst 16, Rohm & Haas) was used as the catalyst. The cross-linking level of the resin was 12 wt % and the exchange capacity 4.4 mequiv/g of dry catalyst, measured with the method described in ASTM D 2187 (1991). The catalyst was treated with methanol at room temperature prior to the experiments in order to remove water from the catalyst pores. Feed. The light gasoline of a fluid catalytic cracking unit was used as the hydrocarbon feed. The molar ratio of methanol to reactive C5 and C6 compounds was 1:1. A distribution of the hydrocarbon groups present in the feed can be found in Table 2. The methanol was from Merck (p.a., >99.8 wt %). TAME (>97.2 wt %) and 2M2MP (98.5 wt %) required for the chromatograph calibration were supplied by Yarsintez, Russia. Analysis. The products were analyzed with a Hewlett-Packard gas chromatograph 5890 Series II equipped with an FI detector, using HP 3396A or Chemstation Program from Hewlett-Packard as an integrator. The compounds were separated in a glass
ether formed
abbreviation
2-methoxy-2-methylbutane
TAME
2-methoxy-2-methylpentane
2M2MP
3-methoxy-3-methylpentane
3M3MP
2,3-dimethyl-2-methoxybutane
23DM2MB
1-methoxy-1-methylcyclopentane
1M1McP
Table 2. Distribution of Hydrocarbons Present in the FCC Light Gasoline group alkanes alkenes (reactive) alkenes (nonreactive) cyclic compounds (nonaromatic) aromatics
C4 wt %
C5 wt %
C6 wt %
C7+ wt %
25.1 5.9 69.0
48.9 26.1 22.1 2.9
45.8 21.7 15.4 14.9 2.2
34.7 24.2 30.0 11.1
capillary column DB-1 (length 60 m, film thickness 1.0 µm, column diameter 0.254 mm, J & W Scientific). For quantitative analysis, the gas chromatograph was calibrated with an external calibration standard. 2M2MP was used as a model compound for the C6 methyl ethers in the calibration, and the response for the other C6 methyl ethers (3M3MP, 23DM2MB, and 1M1McP) was assumed to be the same as that for 2M2MP. Results and Discussion Initial Rates of Etherification. In our earlier study of the kinetics of TAME splitting, we compared kinetic equations derived for three mechanisms (Rihko and Krause, 1995). The experimental results for the TAME splitting were found to be best described by the kinetic equations for a mechanism, in which the ether and alcohol adsorb on the catalyst active site and the isoamylene reacts from the bulk liquid phase. The kinetic equations were written in terms of activities, because of the significant nonideality of the liquid phase. The rate of reaction was proportional to the surface coverages of the adsorbed compounds (ΘM and ΘT) and the activities of the compounds (a1B and a2B) reacting from the bulk liquid phase. The overall rate of TAME according this mechanism was as follows:
rT ) k1ΘMa1B - k2ΘT + k3ΘMa2B - k4ΘT
(1)
Using the equilibrium constants, Kj, determined earlier (Rihko et al., 1994), we obtained the final form of the equation for the net rate of TAME by this mechanism:
rT )
(
)
(
)
KT aMK1B KT aMK2B -k2 aT 1 - K1 - k4 aT 1 - K2 KM aT KM aT KT a + aM KM T (2)
(
)
In this work, we investigated the formation of the ethers. Therefore, eq 2 is written in the form:
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2502 Ind. Eng. Chem. Res., Vol. 35, No. 8, 1996
(
k1aMa1B 1 rT )
)
(
)
aT aT + k3aMa2B 1 K1aMa1B K2aMa2B KT a + aM KM T (3)
(
)
In the initial stage of the reaction the ether concentration was assumed to be very small compared to the concentrations of the other compounds and eq 3 reduces to eq 4.
rT,0 ) k1a1B + k3a2B
(4)
Here we assumed a similar mechanism for the etherification of reactive C6 compounds as presented above for TAME reactions. Using similar assumptions, we obtained the equations for the initial rates of C6 methyl ethers:
r2M2MP,0 ) k1Pa1P + k2Pa2P
(5)
r3M3MP,0 ) kcisacis + ktratr + k2Ea2E
(6)
r23DM2MB,0 ) k231Ba231B + k232Ba232B
(7)
r1M1McP,0 ) k1M1McPa1McP
(8)
In earlier studies, the rate of etherification of methanol with 2M1B has been found to be higher than that with 2M2B (Ancillotti et al., 1977; Krause and Hammarstro¨m, 1987). However, because of the very low concentration of an individual reactive C6 compound in the FCC light gasoline, in this study we took C6 compounds forming the same ether together and estimated the reaction rate constant to be equal for all the alkenes forming the same ether. The mole fraction of each compound in the feed can be found in Table 3. Using the simplifications presented above, we obtained the equations for the initial rate of reactions as follows:
Table 3. Mole Fractions of the Reactive C5 and C6 Alkenes and the Measured Initial Rate of Formation of the Corresponding Ethers reactive alkene 2M1B 2M2B 2M1P 2M2P c-3M2P t-3M2P 2E1B 23DM1B 23DM2B 1McP a
mole fraction initial rate of of the alkenes ether formation confidence in the feeda (×10-2) (mmol g-1 h-1) interval (95%) 1.64 8.89 0.63 1.84 0.96 1.64 0.12 0.30 0.38 1.51
2.666
(0.045
0.352
(0.015
0.337
(0.014
0.073
(0.007
0.439
Methanol mole fraction in the feed 17.8 ×
(0.034 10-2.
Figure 1. Ether concentrations as a function of contact time: temperature, 333 K; pressure, 0.7 MPa; batch mass, 46.1 g; TAME (b); 2M2MP (×); 3M3MP (4); 23DM2MB (0); 1M1McP (O).
TAME formation, because the kinetics of TAME is much more precisely known than that of the C6 methyl ethers. As an example, for the ratio of the initial reaction rates forming 2M2MP and TAME, we obtained the following expression:
r2M2MP,0 ) k2M2MP(a1P + a2P)
(9)
r2M2MP,0 k2M2MP(a1P + a2P) ) rT,0 kT(a1B + a2B)
r3M3MP,0 ) k3M3MP(acis + atr + a2E)
(10)
where the activity, a, of a compound i is:
r23DM2MB,0 ) k23DM2MB(a231B + a232B)
(11)
ai ) γixi
r1M1McP,0 ) k1M1McPa1McP
(12)
Assuming the activity coefficients of similar compounds (2M1P, 2M2P, 2M1B, and 2M2B) to be approximately equal under the reaction conditions, we could further simplify eq 15.
Consequently, using assumptions similar to those used for the alkenes forming TAME, the initial rate was
rT,0 ) kT(a1B + a2B)
(13)
The kinetic analysis was performed on the initial rates expressed as mol g-1 of dry catalyst h-1. As an example of the formation of the ethers, Figure 1 presents the concentration of each ether as a function of time at 333 K. The calculation of initial rates was made by regression from the slopes of initial straight lines of experimental ether concentration as a function of contact time. The initial rates of formation of the ethers are presented in Table 3. Because of the significantly different mole fractions of the reactive C6 alkenes, the reactivity could not be compared by simply comparing the initial formation rates. The comparison was made more accurate by investigating the ratio of the reaction rate constants. As a reference rate constant, we selected the one for
r2M2MP,0 k2M2MP(x1P + x2P) ) rT,0 kT(x1B + x2B)
(14)
(15)
(16)
Finally, for the ratio of the reaction rate constants we obtained the expression:
k2M2MP r2M2MP,0(x1B + x2B) ) kT kT,0(x1P + x2P)
(17)
In Figure 2, the initial reactivities of the C6 alkenes compared to that of TAME-forming alkenes (eq 17) at temperatures of 323, 333, 343, and 353 K are presented. The initial reactivity of 2M1B and 2M2B was about twice than that of the respective C6 alkenes, 2M1P and 2M2P, at the studied temperatures. The reactivity of
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Ind. Eng. Chem. Res., Vol. 35, No. 8, 1996 2503 Table 4. Activation Energies Calculated for the Formation of TAME and C6 Methyl Ethers
Figure 2. Relative initial rate constant (eq 17) of the ether formation reactions at temperatures of 323, 333, 343, and 353 K: TAME (9); 2M2MP (0); 3M3MP (%); 23DM2MB (*); 1M1McP (!).
ether formed
Eact (kJ/mol)
confidence interval (95%)
TAME 2M2MP 3M3MP 23DM2MB 1M1McP
86.4 92.7 85.0 94.6 77.8
(5.1 (9.9 (5.9 (39.8 (13.9
reaction, e.g., 81.1 kJ/mol (Randriamahefa et al., 1988) and 89.5 kJ/mol (Oost et al., 1995a). Flow Reactor Experiments. The aim of the flow reactor experiments was to examine the conversions of reactive C6 compounds forming the C6 methyl ethers under steady-state conditions. Parts a-e of Figure 4 show the conversions of the compounds to the respective ether as a function of temperature. Equation 18 presents, as an example, the conversion calculated for the formation of 2M2MP:
conversion (%) )
Figure 3. Reaction rate constant as a function of reciprocal temperature: 2M2MP (b); 3M3MP (×); 23DM2MB (0); 1M1McP (O).
the alkenes forming 3M3MP is approximately the same as those forming 2M2MP. The reactivity of the alkenes forming 23DM2B is the lowest of all the reactive C6 alkenes. The reason for the very low reactivity of 23DM1B and 23DM2B is probably the steric hindrance, since both of the alkenes have a bulky spatial structure. The reactivity of 1McP is considerably higher than that of the other reactive C6 compounds. At lower temperatures, 323 and 353 K, the reactivity of 1McP was even higher than that of TAME-forming alkenes, 2M1B and 2M2B. The isomerization reactions between the alkenes (e.g., 2M1B to 2M2B) have been investigated in our earlier studies in detail (Krause and Hammarstro¨m, 1987). However, in this study the isomerization reaction could not be measured quantitatively, because the feedsgasoline fractionscontained all the reactive isomers. Temperature Dependence and the Activation Energies. In Figure 2, one can observe that the ratios of the initial rate of reactions varied with temperature. For example, the relative initial rate constant for 2M2MP was 0.505, 0.563, 0.571, and 0.626 at temperatures of 323, 333, 343, and 353 K, respectively. This indicates the different activation energies for the various etherification reactions. The temperature dependence of the rate of etherification was estimated with the Arrhenius equation. In Figure 3, plots of the reaction rate constants for the C6 methyl ethers as a function of reciprocal temperature in logarithmic scale are presented. From the slope, we determined the activation energies of the etherification reactions which are presented in Table 4. The high confidence interval for the activation energy of 23DM2MB was due to experimental difficulties in analyzing very low concentrations of the ether. The measured values are in the same range as presented in the literature for the TAME formation
n2M2MP,product n2M1P,feed + n2M2P,feed
(18)
The conversions for the formation of the other ethers were calculated respectively, by summing up the relevant alkenes in the denominator. Etherification Conversion of Isoamylenes. In the case of isoamylene etherification to TAME, the conversion was highest (71.2%) at a temperature of 333 K with a space velocity 2.2 h-1. At higher temperatures, the etherification conversion declined in the same way as observed in the experiments by Krause and Hammarstro¨m (1987). The effect of thermodynamic equilibrium was confirmed by also measuring the conversion with a lower space velocity 1.1 h-1, at temperatures 343 and 353 K. At temperatures above 333 K, no changes could be seen in the etherification conversion as a function of space velocity, so we concluded that the thermodynamic equilibria controlled the reaction significantly at these temperatures. The measured conversions in the flow reactor experiments at temperatures of 343 and 353 K (Table 5) are typical of those obtained as equilibrium conversions of isoamylene in equilibrium measurements. The equilibrium conversion of isoamylenes to TAME by Safronov et al. (1989) was 62%, by Muja et al. (1986) 72%, by Rihko et al. (1994) 68%, and by Piccoli and Lovisi (1995) 65% at a temperature of 343 K. Only the experimental results by Oost et al. (1995b) deviate clearly. They obtained equilibrium conversions as low as 29% in their measurements at equimolar isoamylene/methanol ratio at 343 K. Furthermore, in the equilibrium measurements they reported a lower conversion of methanol at 333 K than at 343 K, which might indicate that the equilibrium state was not fully reached during their measurements. On the other hand, the thermodynamic calculation of Wyczesany (1995) showed the equilibrium conversion of 2M1B and 2M2B to be as high as 92.7% at 340 K and 88.8% at 355 K. These deviate clearly from the experimentally measured equilibria. Etherification Conversion of C6 Compounds. The conversion of the compounds forming C6 methyl ethers clearly differs from that of isoamylenes. The highest conversion of the alkenes forming 2M2MP, 3M3MP, and 23DM2MB was obtained at 333 K, using the space velocity 2.2 h-1. The conversion of the alkenes forming 2M2MP was 60.2% at that temperature. For comparison, the conversion of 2M1B and 2M2B was 71.2% at the same temperature. The maximum conver-
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2504 Ind. Eng. Chem. Res., Vol. 35, No. 8, 1996
Figure 4. Etherification conversions of reactive C5 and C6 compounds with methanol as a function of temperature: LHSV 1.1 (×), 2.2 (O), and 4.3 h-1 (0); TAME (a); 2M2MP (b); 3M3MP (c); 23DM2MB (d); 1M1McP (c). Table 5. Etherification Conversions of the Reactive C5 and C6 Compounds at 343 and 353 K: Equilibrium Conversions of the Respective Compounds at 355 K Calculated from the Results by Wyczesany (1995) ether formed TAME 2M2MP 3M3MP 23DM2MB 1M1McP a
conversion at 343 K
standard dev.
conversion at 353 K
standard dev.
conversion at 355 K by Wyczesany (1995)a
conversion at 355 K by Wyczesany (1995)b
65.9 59.8 29.4 55.8 28.2
1.1 4.3 0.6 8.1 4.9
57.6 51.8 23.5 47.6 22.4
0.6 7.8 0.0 9.4 1.7
88.9 96.0 70.6 94.9 85.9
91.3 96.6 70.2 91.3 56.2
Thermodynamic values estimated by the Benson method.
a
Thermodynamic values estimated by the Yoneda method.
sion of alkenes forming 3M3MP (34.9%) was considerably lower than those forming 2M2MP. The conversions of 23DM1B and 23DM2B to 23DM2MB (59.6%) were in the same range as measured for 2M2MP. The initial rate of formation for cyclic ether (1M1McP) was observed to be much faster than for the other C6 methyl ethers. This could be seen in the batch reactor experiments (Figure 2). The same effect could also be clearly seen in the flow reactor experiments. In the case of formation of 1M1McP, the conversion declined over the whole temperature range studied. The highest conversion (40.2%) was obtained at a temperature of 323
K. At 353 K the conversion was 22.1%, independent of the space velocity. From this, one can conclude that the etherification reaction of 1McP was kinetically fast and was significantly limited by thermodynamic equilibria at all the temperatures studied. Equilibrium Conversions. As stated earlier, we concluded that the thermodynamic equilibria controlled the etherification reactions at temperatures of 343 and 353 K in our flow reactor experiments, since no differences were found in the conversions using the space velocities 1.1, 2.2, and 4.3 h-1. The conversions obtained at temperatures of 343 and 353 K are presented in Table
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Ind. Eng. Chem. Res., Vol. 35, No. 8, 1996 2505 Table 6. Equilibrium Constants for Etherification Reactions at Temperatures of 343 and 353 K: Experimental Ka and K Calculated from the Thermodynamic Data and Equilibrium Constants K at 355 K Calculated from the Results by Wyczesany (1995) reaction
Kaa at 343 K
standard dev.
Kaa at 353 K
standard dev.
K(calc) at 353 K
K at 355 K by Wyczesany (1995)a
2M1B + MeOH f TAME 2M2B + MeOH f TAME 2M1B f 2M2B 2M1P + MeOH f 2M2MP 2M2P + MeOH f 2M2MP 2M1P f 2M2P c-3M2P + MeOH f 3M3MP t-3M2P + MeOH f 3M3MP t-3M2P f c-3M2P 23DM1B + MeOH f 23DM2MB 23DM2B + MeOH f 23DM2MB 23DM1B f 23DM2B 1McP + MeOH f 1M1McP
34.6 3.54 9.77 19.0 2.63 7.23 1.94 1.18 1.65 3.30 1.18 2.82 0.72
0.15 0.01 0.05 0.93 0.01 0.26 0.03 0.001 0.008 0.01 0.03 0.09 0.01
21.0 2.35 8.96 10.73 1.79 5.99 1.24 0.79 1.57 2.50 0.82 3.05 0.48
1.8 0.004 0.11 1.10 ≈0 0.40 ≈0 0.001 0.001 ≈0 0.003 0.04 ≈0
6.054 0.671 9.018 4.334 0.815 5.318 0.0496 0.0306
145.6 16.6 8.8 447.3 47.2 9.5 14.3 6.4 2.2 182.9 40.1 4.6
a
1.094 0.1952 5.606
Activity coefficients calculated by the UNIFAC method.
5. The average and the standard deviation values in Table 5 have been calculated based on the three experiments with different space velocities at temperatures of 343 and 353 K. Additional material was available (Table SIII) from the paper by Wyczesany (1995), from which one could calculate the respective equilibrium conversions for the formation of C6 methyl ethers. For comparison, the equilibrium conversions at 355 K by Wyczesany (1995) are also presented in Table 5. If we compare the calculated (Wyczesany, 1995) and the experimental equilibrium conversions (Table 5), we can see that in the calculation the highest conversions were obtained in the production of 2M2MP and 23DM2MB. The same phenomenon was observed in the experiments. The deviations between the calculated and the experimental values were, however, significant. The lowest conversion was calculated for 1McP, 56.2% using the Yoneda estimation method and 85.9% using the Benson estimation method. Experimentally, we also obtained the lowest value for 1McP (22.4%) compared to the conversions of the other C6 compounds, but the measured value was remarkably lower than the calculated one. Experimentally, Zhang and Datta (1995b) obtained equilibrium conversion as high as 59.6% in the formation of the 2-ethoxy-2-methylpentane at 353 K. The higher conversion of 2M1P and 2M2P with ethanol, compared to that with methanol (51.8%, Table 5) is unexpected. Higher equilibrium conversion of isobutene has been observed with methanol rather than with ethanol (Colombo et al., 1983; Izquierdo et al., 1992; Vila et al., 1993). The equilibrium conversion of 2M1B and 2M2B with methanol has been found to be clearly higher than that with ethanol (Rihko and Krause, 1993; Rihko et al., 1994). Equilibrium Constants. On the basis of the equilibrium results obtained in the flow reactor experiments at temperatures of 343 and 353 K, we also made preliminary calculations of the equilibrium constants for the etherification reactions at these temperatures. The activity coefficients of the compounds were estimated by the UNIFAC method (Fredenslund et al., 1977) and the experimental equilibrium constant (Ka) was calculated as shown in eq 19. The experimental
Ka )
∏(γi)v ∏(xi)v ) ∏(ai)v i
i
i
(19)
Ka values are presented in Table 6. Table 6 shows the averages and the standard deviation values of Ka based on the three experiments with different space velocities
Table 7. Thermodynamic Data (298 K, 0.103 25 MPa, Gas Phase) Used in the Calculation of K in Table 6 by TRC Thermodynamic Tables (1986) ether
∆Gf (kJ mol-1)
∆Hf (kJ mol-1)
S (kJ mol-1 K-1)
TAME 2M2MP 3M3MP 23DM2MB 1M1McP
-104.0 -97.5 -87.4 -92.8
-298.7 -320.5 -314.5 -319.1
0.3980 0.4392 0.4256 0.4280
at temperatures of 343 and 353 K. The respective values from the results by Wyczesany (1995) at a temperature of 355 K are also presented in the table. For comparison, we also evaluated the equilibrium constants, K, from the thermodynamic data using eq 20.
K ) exp(-∆rG/RT) ) exp(-∆rH/RT + ∆rS/R)
(20)
Table 7 presents the thermodynamic data (TRC Thermodynamic Tables, 1986) which we used in the evaluation. The measured equilibrium constants for the formation of TAME are in good agreement with the experimental values presented in the paper by Rihko et al. (1994). However, the differences between the measured and the calculated equilibrium constants are significant for the etherification reactions. Also, the differences in the calculated equilibrium constants by us and by Wyczesany are very large, as can be seen in Table 6. The calculated values by Wyczesany are higher than the measured ones, but our calculated values are considerably smaller than the measured ones. The equilibrium constant and the calculated equilibrium conversion are highly sensitive to the thermodynamic data used in the calculations (Rihko et al., 1994). Therefore, already a small difference in the estimated thermodynamic values used in the calculations (especially in the Gibbs free energy of formation) causes a significant change in the equilibrium conversion. As was stated in Rihko et al. (1994), an increase of 1% in the value of ∆fH of TAME changed the value of K1 from 10.5 to 36.3. This looks like the main reason for the significant differences in the calculated values by us and by Wyczesany. The methods for the estimation of thermodynamic values have limitations, which partly may cause the differences in the calculated values. The calculated equilibrium constants for the isomerization reactions are in good agreement with the experimental ones, as can be seen in Table 6. Ether Concentration in the Gasoline Product. The mole fractions of the reactive C5 and C6 compounds
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2506 Ind. Eng. Chem. Res., Vol. 35, No. 8, 1996 Subscripts
Figure 5. Amount of the ethers (wt %) as a function of reaction temperature in the product stream in the flow reactor experiments: TAME (9); 2M2MP (0); 3M3MP (^); 23DM2MB (*); 1M1McP (!).
in the FCC light gasoline were very different (Table 3). The C5 alkenes, isoamylenes, were present at the highest concentrations. They represented about 70% of the total amount of reactive C5 and C6 compounds. Because the conversion of C5 alkenes was also generally higher than that of reactive C6 compounds, TAME was the most dominant of all the ethers present in the product in the flow reactor experiments. In Figure 5, the amounts of different ethers in the products can be seen. The highest conversion of the reactive C6 compounds was obtained at 333 K. The overall conversion (the conversion of all the reactive C6 compounds) was 45.3% at that temperature. Conclusions TAME is the main ether compound formed in the etherification of FCC light gasoline with methanol. The heavier etherss2-methoxy-2-methylpentane, 3-methoxy-3-methylpentane, 2,3-dimethyl-2-methoxybutane, 1-methoxy-1-methylcyclopentaneswere analyzed in considerably lower concentrations. In the kinetic study, the formation of 1-methoxy-1-methylcyclopentane was found to be twice as fast as the formation of other C6 methyl ethers. The activation energies for the formation of C6 methyl ethers were between 78 and 94 kJ/mol, depending on the ether. The equilibrium constants for the etherification reactions of unsaturated C6 compounds with methanol were lower than those of isoamylenes. Nomenclature ai ) activity of a component i Eact ) activation energy, kJ/mol ∆fH ) enthalpy of formation, kJ/mol ki ) reaction rate constant for a reaction i (i ) 1-4), eqs 1-4, mol g-1 h-1 kj ) reaction rate constant for a component j, mol g-1 h-1 Ki ) adsorption equilibrium constant for a component i Kj ) reaction equilibrium constant for a reaction j ni,feed ) amount of a component i in the feed, mol ni,product ) amount of a component i in the product, mol ri ) rate of reaction, mol g-1 h-1 ri,0 ) initial rate of reaction, mol g-1 h-1 T ) temperature, K xi ) mole fraction of a component i Greek Letters γi ) activity coefficient for a component i Θi ) fraction of surface covered by component i νi ) stoichiometric coefficient of a component i
M ) methanol 1B ) 2-methyl-1-butene 2B ) 2-methyl-2-butene 1P ) 2-methyl-1-pentene 2P ) 2-methyl-2-pentene cis ) cis-3-methyl-2-pentene tr ) trans-3-methyl-2-pentene 2E ) 2-ethyl-1-butene 231B ) 2,3-dimethyl-1-butene 232B ) 2,3-dimethyl-2-butene 1McP ) 1-methylcyclopentene T ) 2-methoxy-2-methylbutane (TAME) 2M2MP ) 2-methoxy-2-methylpentane 3M3MP ) 3-methoxy-3-methylpentane 23DM2MB ) 2,3-dimethyl-2-methoxybutane 1M1McP ) 1-methoxy-1-methylcyclopentane
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Zhang, T.; Datta, R. Ethers from Ethanol. 4. Kinetics of the LiquidPhase Synthesis of Two tert-Hexyl Ethyl Ethers. Ind. Eng. Chem. Res. 1995b, 34, 2247-2257.
Received for review January 19, 1996 Accepted May 13, 1996X IE960041X
X Abstract published in Advance ACS Abstracts, July 1, 1996.
