4586
Ind. Eng. Chem. Res. 1997, 36, 4586-4594
Liquid-Phase Synthesis of Ethanol-Derived Mixed Tertiary Alkyl Ethyl Ethers in an Isothermal Integral Packed-Bed Reactor Tiejun Zhang, Kyle Jensen, Prakob Kitchaiya, Cory Phillips, and Ravindra Datta* Department of Chemical and Biochemical Engineering, The University of Iowa, Iowa City, Iowa 52242-1219
Industrial production of mixed tertiary ethers by reacting methanol or ethanol with a mixed tertiary olefin stream is a distinct possibility. Consequently, the synthesis of mixed ethyl tertiary alkyl ethers from ethanol and mixtures of reactive iso-olefins in an isothermal integral packedbed reactor is studied. The rate expressions in terms of activities for the individual reaction rates in this family of ethers are taken from related work. These kinetic expressions are utilized here for comparison with experimental data obtained in a packed-bed reactor containing Amberlyst 15 ion-exchange resin catalyst for a mixed iso-C6 olefin feed stream as well as for a mixed C4, C5, and C6 iso-olefin stream. Etherification and isomerization conversions increase with space time at the lower temperatures; however, at the higher temperatures equilibrium limitations are approached. As a function of temperature, the conversions first increase and then decline due to thermodynamic limitations for these exothermic reactions. Good agreement is obtained between predictions and experiments. Introduction Oxygenated fuels have shown a steady growth during the nineties. Methyl tert-butyl ether (MTBE), produced by reacting a C4 stream containing isobutylene with methanol over an ion-exchange resin catalyst, is currently the dominant gasoline additive. Industrial synthesis of ethyl tert-butyl ether (ETBE), produced by substituting ethanol for methanol, has also commenced. Due to the immense growth of these oxygenates, however, the isobutylene supply may become limiting in the future. Consequently, higher tertiary ethers produced from a combination of methanol or ethanol with C5 and C6 tertiary olefins are of interest. There are substantial quantities of C5 and C6 olefins present (∼10.5 and 6.5%, respectively) in the fluid catalytic cracking (FCC) light gasoline stream (Ignatius et al., 1995), which could be utilized to form these ethers. Owing to the high olefin volatility, their conversion into the corresponding ethers instead of blending the olefins directly with gasoline is attractive. Furthermore, there is little reason to produce pure ethers from separated olefins, since a mixture of ethers resulting from the FCC light gasoline stream without separating the individual olefins should be equally effective for blending purposes. This paper is, hence, concerned with the synthesis of mixed ethers from ethanol and mixed olefin streams in an integral packed-bed reactor. As shown in Table 1, ethanol and isobutylene form ETBE, while tert-amyl ethyl ether (TAEE) is formed with ethanol from either of reactive C5 isomer, namely 2-methyl-1-butene (2M1B), or 2-methyl-2-butene (2M2B). There are three possible isomers of tertiary hexyl ethyl ethers (THEE) formed from ethanol, i.e., THEE1 from either 2-methyl-1-pentene or 2-methyl-2-pentene (2M1P or 2M2P), THEE2 from 2,3-dimethyl-1-butene or 2,3dimethyl-2-butene (2,3DM1B or 2,3DM2B), and THEE3 from cis- or trans-3-methyl-2-pentene (C3M2P or T3M2P) or 2-ethyl-1-butene (2E1B). These higher ethers also have desirable gasoline-blending properties. Thus, TAEE has a motor octane number (MON) of 112 and a research octane number (RON) of 105. The three isomers of the tertiary hexyl ethyl ether have RON ) * Author to whom correspondence may be addressed. Email:
[email protected]. S0888-5885(97)00099-7 CCC: $14.00
Figure 1. Ether synthesis reaction network. (a) ETBE, TAEE, THEE1, and THEE2 systems from the corresponding olefins (B and C), except isobutylene (B) which has no conformational isomer (C). (b) THEE3 (D) reaction system, including isomerization among C3M2P (B2), T3M2P (B3), and 2E1B (B1).
105.9 and MON ) 93.1 for THEE1, RON ) 104.9 and MON ) 91.9 for THEE2, and RON ) 101.4 and MON ) 91.5 for THEE3 (Milne, 1996). The etherification reaction occurs according to the Markownikoff addition; that is, the hydrogen from ethanol adds to the less-substituted carbon atom of the double bond. Furthermore, except, of course, for isobutylene, each iso-olefin also undergoes isomerization on the catalyst. Thus, ETBE, TAEE, THEE1, and THEE2 are synthesized as shown in Figure 1a, with either the R-olefin or the β-olefin reacting with ethanol. The isomerization of 2M1B and 2M2B follows pathway number 3 of Figure 1a, as do the olefins that form THEE1 and THEE2. Three conformational isomerization reactions are possible for the C6 olefins (i.e., 2E1B, C3M2P, and T3M2P) that form THEE3 as shown in Figure 1b. These etherification reactions are denoted by numbers 1, 2, and 3, whereas the isomerization reactions are labeled as 4, 5, and 6 in Figure 1b. Due © 1997 American Chemical Society
Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 4587
to the structural similarities of the olefins and a common alcohol involved, it seems reasonable to assume that there may be some systematicity in the mechanism, rate expressions, and the kinetics. Reaction Kinetics and Thermodynamics A recently published series of papers provides expressions for the equilibrium constants for ETBE (Jensen and Datta, 1995), TAEE (Kitchaiya and Datta, 1995), and the THEEs (Zhang and Datta, 1995b, 1996). Furthermore, kinetic expressions have been developed for liquid-phase synthesis of ETBE (Jensen and Datta, 1997), TAEE (Kitchaiya and Datta, 1997a), and THEEs (Zhang and Datta, 1995c, 1996) in terms of activities. Zhang and Datta (1995c, 1996) provided LangmuirHinshelwood-Hougen-Watson (LHHW) mechanismbased rate expressions for THEE synthesis, assuming that two active catalyst sites are involved in the ratedetermining step (rds) of etherification (A + B a D) and one site in the rds of isomerization (B a C). More recently, Jensen and Datta (1997) and Kitchaiya and Datta (1997b), through the use of partially deactivated catalysts to vary the total number of active catalyst sites, determined that a total of three sites are involved in the surface reaction rate-determining step of the etherification reaction, with two adsorbed ethanol sites reacting with one adsorbed tertiary olefin, while a total of two sites are involved in the surface rate-determining step of the olefin isomerization, with an adsorbed olefin reacting with a vacant site. Thus, the previous data are reanalyzed here to conform to the revised form of the rate expressions. These are then utilized in the analysis of the mixed ether systems. Based on the above, the overall mechanism, thus, is kA
A+S\ {k } A•S kB
kE
kI
for etherification and isomerization, respectively, where ksE ≡ kEKA2KBCt3, ksI ) kIKBCt2, KE ) KAKBkE/KDk-E, and KI ) kIKB/KCk-I. The ethanol adsorption equilibrium constant for ethanol on Amberlyst 15 was determined by independent adsorption experiments by Kitchaiya and Datta (1997a) as
1 )] [11R000(T1 - 303
KA ) 27 exp
(4)
Kitchaiya and Datta (1997a) have further shown that due to the strong adsorption of ethanol, except for xA e 0.04, the vacant sites fraction may also be neglected. For this case eqs 2 and 3 reduce to the power-law forms
rE ≈ krE and
(
aB aD aA K a
(
rI ≈ krI
aB
aA
2
aC -
)
(5)
)
(6)
2
E A
KIaA2
where krE ) ksE/KA3 and krI ) ksI/KA2. The activity coefficients in the above rate expressions are calculated by the UNIFAC method (Reid et al., 1987). The thermodynamic equilibrium constants, KE and KI, in the above rate expressions were determined previously (Jensen and Datta, 1995; Kitchaiya and Datta, 1995; Zhang and Datta, 1995b, 1996) and are given for reaction i as
(1b)
where the coefficients, λi, for the different reactions are given in Table 2. The Arrhenius parameters (Ai, Ei) for the rate constants of etherification (eq 2) and isomerization (eq 3) reaction are given in Tables 3 and 4, respectively, while those for the power-law forms, eqs 5 and 6, are also provided in these tables.
(1c)
-E
B•S + S \ {k } C•S + S
(3)
(1 + KAaA)2
ln Ki ) λi1 + λi2/T + λi3 ln T + λi4T + λi5T2 + λi6T3 (7)
-B
2A•S + B•S \ {k } D•S + A•S + S
ksI(aB - aC/KI)
(1a)
-A
B+S\ {k } B•S
rI )
(1d)
-I
k-D
D•S \ {k } D + S D
k-C
C•S \ {k } C + S C
(1e) (1f)
Applying the thermodynamic transition-state theory (TTST) (Connors, 1990) to each of the above elementary steps, i.e., assuming each step to possess a unique transition state, along with the assumption of pseudoequilibrium for the adsorption and desorption steps, and surface reaction as the rds, with the most abundant surface species assumption (MASSA) for ethanol (Kitchaiya and Datta, 1997a), yields the following rate expressions (Jensen and Datta, 1997; Kitchaiya and Datta, 1997a)
rE ) and
ksEaA2[aB - aD/(KEaA)] (1 + KAaA)3
(2)
Experimental Section Catalyst. Amberlyst 15, a cation-exchange macroreticular resin catalyst, was obtained from Sigma Chemical Co. and has the following properties: average particle diameter ) 0.74 mm, surface area ) 50 m2/g, porosity ) 30%, and exchange capacity ) 4.8 mequiv/g. The catalyst was first washed and then converted into the acid form by contacting with 1 N HNO3. The acidexchanged catalyst particles were washed with ethanol prior to drying overnight in a vacuum oven. Ground and sieved to obtain a granulometric fraction in the size range of 0.125-0.25 mm (average diameter ) 0.188 mm), the catalyst was mixed with cleaned and dried silicon carbide diluent to prevent temperature variations in the reactor bed. With this particle size, the internal diffusional limitations were judged to be eliminated under all the experimental conditions investigated (Zhang and Datta, 1995a). Materials. For the THEEs synthesis, the individual C6 olefins, 2E1B (purity > 97%), 2M1P (purity ) 98%), and 2,3DM2B (purity ) 98%) were obtained from TCI America. Isobutylene (Matheson, cp grade) and 2M2B
4588 Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 Table 1. Reactive C4-C6 Olefins and Ethers Formed with Ethanola
a
Only iso-olefins with the double bond attached to a tertiary carbon are reactive in etherification.
Table 2. Coefficients of Liquid-Phase Etherification and Isomerization Reaction Equilibrium Constant Correlations, ln Ki ) λi1 + λi2/T + λi3ln T + λi4T + λi5T2 + λi6T3, T in K reaction, i
λi1
λi2
λi3
λi4
λi5 × 105
λi6 × 108
EtOH + IB a ETBE EtOH + 2M1B a TAEE EtOH + 2M2B a TAEE 2M1B a 2M2B EtOH + 2M1P a THEE1 EtOH + 2M2P a THEE1 2M1P a 2M2P EtOH + 2,3DM1B a THEE2 EtOH + 2,3DM2B a THEE2 2,3DM1B a 2,3DM2B EtOH + 2E1B a THEE3 EtOH + C3M2P a THEE3 EtOH + T3M2P a THEE3 2E1B a C3M2P 2E1B a T3M2P C3M2P a T3M2P
10.387 22.809 26.779 -3.970 -71.4519 -84.4308 12.9789 -76.2082 -61.4243 -14.7839 -75.2419 -72.1147 -85.5225 -3.1272 10.2806 13.4078
4060.59 3136.3 2078.6 1057.7 7149.74 6870.67 279.07 6000.86 4660.20 1340.66 6296.29 5433.52 5730.14 862.77 566.15 -296.62
-2.8906 -5.8227 -6.5925 0.7698 11.0547 13.0318 -1.9771 12.5825 9.4324 3.1501 12.2972 11.4115 13.6057 0.8857 -1.3085 -2.1942
-0.019 15 0.0179 0.0231 0.0052 -0.040 06 -0.037 83 -0.002 23 -0.047 62 -0.027 28 -0.020 34 -0.041 53 -0.034 62 -0.038 08 -0.006 91 -0.003 45 -0.003 46
5.2859 -0.6395 -1.126 0.4865 3.8979 3.3181 0.5798 4.7885 2.7866 2.0019 3.7941 3.1287 3.2520 0.6654 0.5421 -0.1233
-5.3298 -1.672 -1.414 -0.258 -3.6812 -3.2602 -0.0421 -4.0210 -2.9684 -1.0526 -3.5474 -3.2044 -3.1902 -0.3430 -0.3572 -0.0142
(TCI > 95%), and a C6 olefin, were the feed olefins for the mixed ether synthesis. Other materials used were
ethanol (Pharmco, dehydrated, 200 proof) and silicon carbide (McMaster-Carr, size ) 0.25-0.45 mm).
Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 4589 Table 3. Arrhenius Kinetics Parameters for Ether Synthesis from Ethanol kri (eq 5)
ksi (eq 2)
reactive olefin
Ari × 10-12 (mol/(h‚g))
Eri (kJ/mol)
Asi × 10-11 (mol/(h‚g))
Esi (kJ/mol)
isobutylene 2M1B 2M2B 2M1P 2M2P 2,3DM1B 2,3DM2B 2E1B C3M2P T3M2P
15.78 2.75 3.89 2.62 119.0 12.5 8.87 6.54 1.66 3.03
87.2 85.6 89.5 86.9 100.9 93.3 96.2 89.9 89.6 90.9
74.18 3.12 4.4 2.97 134.85 14.17 10.05 7.41 1.88 3.43
60.4 54.9 58.8 56.1 70.1 62.5 65.4 59.2 58.9 60.1
Table 4. Arrhenius Kinetics Parameters for Olefin Isomerization kri (eq 6)
ksi (eq 3)
reaction
Ari × 10-12 (mol/(h‚g))
Eri (kJ/mol)
Asi × 10-11 (mol/(h‚g))
Esi (kJ/mol)
2M1B f 2M2B 2M2B f 2M1B 2M1P f 2M2P 2,3DM1B f 2,3DM2B 2,3DM2B f 2,3DM1B 2E1B f C3M2P 2E1B f T3M2P C3M2P f T3M2P
2.15 0.8 1.25 3.63 16.6 3.05 4.32 1.38
85.0 89.1 87.1 91.3 102.7 91.9 90.2 89.9
5.02 1.86 2.08 6.06 27.4 7.15 10.1 3.22
64.5 68.6 66.6 70.8 82.2 71.4 69.7 69.4
Reaction Kinetics Experiments. The liquid-phase reaction kinetics were studied in a stainless steel tubular packed-bed reactor (12 in. length, 3/8 in. diameter) system, similar to the reaction system described by Zhang and Datta (1995c). The temperature of the jacketed reactor was controlled by an external water bath (Fisher, model 9101). The pressure was maintained at 100 psig to ensure a liquid phase in the reactor. Depending upon the reaction temperature and the mode of operation, a varying amount (0.155-0.66 g) of the catalyst, uniformly diluted with inert silicon carbide grains, was carefully packed into the tubular reactor. The temperature variation along the packed bed of catalyst was thus maintained to be less than 1 K to simulate isothermal conditions. The reactor ends were filled with glass wool to prevent any catalyst loss from the reactor. The reactor was operated in the integral mode, i.e., the WHSV employed were such that at the reaction conditions high conversions (up to equilibrium values) were obtained. The olefin conversions at the reactor outlet were experimentally determined at different temperatures and WHSV and were utilized to test the predictions based on the rate expressions and kinetics parameters determined from operation in the differential mode, i.e., at low conversions. Steady state was monitored by periodical on-line analyses by means of an internal liquid sampling injector (Valco, CI4WE, size ) 0.2 µL) of the product composition in a gas chromatograph (GC) as described below. When no change was observed in the product composition, typically after about 2 h of start-up, steady state was deemed to have been achieved. Thereafter, an average of at least three measurements was taken to determine the steady-state product composition. Some of the runs were replicated to check the reproducibility of the results under a given set of experimental conditions. The maximum deviation in the conversions found in such runs was 5%. No discernible deactivation of the catalyst was observed under extended operation. The selectivity of the reactions studied under the reaction conditions
Figure 2. Comparison of experimental THEE2 synthesis rate data from Zhang and Datta (1995c) and the three-site etherification rate expression, eq 2, with Arrhenius parameters from Table 3.
was very high, with 99.5% of the mass being accounted for by the detected species. For the mixed C6 feed experiments, the reactant feed consisted of ethanol and three C6 olefins isomers (2M1P, 2,3DM2B, and 2E1B) without any solvent. The molar ratio of ethanol to each olefin was in the range of Ω ) 1.05-1.5. For the mixed C4, C5, and C6 olefin feed experiments, a mixture of 2M2B, C6 olefin, ethanol, and n-pentene diluent was fed independently to the reactor system described above by a positive displacement pump (Gilson Medical Electronics, Inc., model 302). In addition, isobutylene (Matheson, cp grade) was introduced to the system by a ice-cooled pump head in a manner described previously by Zhang and Datta (1995a). Each feed vessel was placed on a top-loading balance for gravimetric measurement (Zhang and Datta, 1995c). Analysis. The liquid mixture composition was determined by using a Perkin-Elmer AutoSystem GC equipped with a flame ionization detector (FID) and He (99.999%) as the carrier gas. Ethanol, olefins, ethers, and the solvent were separated using a Supelco capillary column (SPB-1, 0.25 mm i.d. × 60 m). The column was temperature programmed with a 10 min initial hold at 65 °C, followed by a 20 °C/min ramp up to 160 °C, and it was finally held at 160 °C for 5 min. Analysis with the column being held at 160 °C for longer periods indicated no elution of any dimerization products. The GC was calibrated for all the species, and the error in mole fraction was estimated to be 2%. Results and Discussion Single C6 Olefin Feed. In earlier work, Zhang and Datta (1995c) had utilized a rate expression based on a two-site model for the THEE synthesis along with an ethanol adsorption constant, KA ) 2.75. As discussed above, subsequent work (Jensen and Datta, 1997; Kitchaiya and Datta, 1997a) determined the number of sites to be three for etherification and two for isomerization. Furthermore, the adsorption equilibrium constant for ethanol was determined to be given by eq 4 (Kitchaiya and Datta, 1997a). For consistency, it was decided to check the new rate expressions for the THEE system. As shown in Figure 2, for THEE2 synthesis, the new expression, eq 2, fits the experimental data adequately for the etherification reaction. Similar results were found for isomerization. Mixed C6 Olefin Feed. A feed of mixed C6 olefins, namely 2M1P, 2,3DM2B, and 2E1B, in the absence of
4590 Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997
any solvent, and ethanol were fed to the reactor. Since all the three isomers of THEE are simultaneously formed in parallel from this mixed C6 olefin feedsteam, the following olefin conversions were calculated for the THEE system k from the experimentally measured molar composition of the product at the reactor outlet.
For THEE1 system (k ) 1): X1,E ) xTHEE1/(x2M1P + x2M2P + xTHEE1) X1,I ) x2M2P/(x2M1P + x2M2P + xTHEE1)
(8)
For THEE2 system (k ) 2): X2,E ) xTHEE2/(x2,3DM1B + x2,3DM2B + xTHEE2) X2,I ) x2,3DM1B/(x2,3DM1B + x2,3DM2B + xTHEE2)
(9)
For THEE3 system (k ) 3): X3,E ) xTHEE3/(x2E1B + xC3M2P + xT3M2P + xTHEE3) X3,IC ) xC3M2P/(x2E1B + xC3M2P + xT3M2P + xTHEE3) (10) X3,IT ) xT3M2P/(x2E1B + xC3M2P + xT3M2P + xTHEE3) The symbols for the various conversions given above are explained in the Nomenclature section. Furthermore, the individual molar ratios of ethanol to 2M1P, 2,3DM2B, and 2E1B are defined as Ω1, Ω2, and Ω3, respectively. The overall molar ratio of ethanol to all three olefins is then calculated from 3
1
1
∑ j)1Ω
) Ω
(11) j
Material Balance Equations. The etherification and isomerization conversions may be determined from the following sets of mass balance equations of the products:
( )
dFk ) (Rk) ) [ν]k(r)k dW
(k ) 1, 2, 3)
(12)
where (Rk) is the rate of production vector for the THEE system k, [ν]k is the stoichiometric coefficient matrix of the THEE system k, and (r)k is the reaction rate vector for the THEE system k. The rates of product formation may be written in terms of the rates of reactions according to the reaction stoichiometry in each individual ether system, i.e.,
νijri)k ∑ i)1
(Rj)k ) (
(k ) 1, 2, 3)
R (R1) ≡ RTHEE1 2M2P
) ( ) (
R (R2) ≡ RTHEE2 2,3DM1B
1
r1 + r2 ) -r 2 + r3
) )
r1 + r2 ) -r 2 1 - r3
( ) (
RTHEE3 (R3) ≡ RC3M2P RT3M2P
( )
dXk Ωk Ωk ) (Rk) ) [ν]k(r)k dτ Ω Ω
(13)
Thus, the rate of production vectors (Rk) are respectively defined for the three THEE systems (k ) 1, 2, 3) by
(
isomerization rate expressions are provided above along with the equilibrium and rate constants in Tables 2-4. The space time for the mixed C6 system is defined on the basis of the total feed rate of the three reactive olefins, τ ≡ W/Folefins ) W/(F2M1P + F2,3DM2B + F2E1B). Then, the mass balance equation from eq 12 may be written in the form
(k ) 1, 2, 3)
(17)
subject to the intial conditions
q
(
Figure 3. Effect of temperature. Experimental and predicted: (a) etherification and (b) isomerization conversions with Ω1 ) Ω2 ) Ω3 ) 3.15 (Ω ) 1.05) in the absence of any solvent. Open markers and dashed curves are for those at a space time τ ) 5.8 g‚h/mol and solid markers and curves are for those at τ ) 15.2 g‚h/mol.
(X1) ≡
r1 + r2 + r3 ) -r2 + r4 - r6 -r + r5 + r6 3
(15)
)
(16)
3
where the individual rates refer to the reactions in Figure 1 and the corresponding etherification and
(k ) 1, 2, 3)
(18)
The conversion vectors (Xk) are defined for the three THEE systems as
(14)
1
2
(Xk) ) 0 at τ ) 0
( )
( )
( )
X3,E X1,E X , (X2) ≡ 2,E , (X3) ≡ X3,IC X1,I X2,I X3,IT
(19)
The ethanol mole fraction is related to the above-defined conversions by
(
xEtOH ) 1 -
)
3
Xk,E
k)1
Ωk
∑
/Y
For the feed olefins, the mole fractions are
(20)
Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 4591
x2M1P ) (1 - X1,E - X1,I)/(Ω1Y) x2,3DM2B ) (1 - X2,E - X2,I)/(Ω2Y)
(21)
alcohol), Q, and the molar ratio of ethanol to the olefins, Ω, i.e.,
(1 -Q Q)
ΩI ) Ω
x2E1B ) (1 - X3,E - X3,IC - X3,IT)/(Ω3Y)
(26)
The molar ratio of ethanol to inert solvent, ΩI, may be expressed in terms of the mole fraction of the reactive olefins in the olefin stream (before being mixed with
If a pure olefin stream without any inert solvent is used to react with alcohol, then Q ) 1 and ΩI f ∞. The above mole fractions were utilized in the UNIFAC method for estimating the activity coefficients, and the activities were then used to compute each individual reaction rate from the rate constants with the preexponential factors and activation energies given in Tables 3 and 4. Differential equations represented by eq 17 were numerically solved by the Runge-Kutta method for the initial conditions represented by eq 18. Effect of Temperature. With a feed consisting of an equal fraction of 2M1P, 2,3DM2B, and 2E1B without any solvent, integral reactor experiments were conducted at space times τ ) 5.8 and 15.2 g‚h/mol, respectively. The molar ratio of ethanol to each of the three C6 olefins, thus, was 3.15, leading to an overall molar ethanol to olefins ratio of Ω ) 1.05. Theoretical predictions were also made as described above for these conditions. The results are plotted in Figure 3a for etherification and in Figure 3b for isomerization conversions as a function of temperature. It is seen that the etherification conversions are in the following decreasing order: 2M1P > 2E1B > 2,3DM2B, a result expected from the olefin stability and the order of variation of their reaction rate constants. Etherification and isomerization conversions are higher at higher space times at low temperatures, but approach the same thermodynamic conversions when the temperature is increased.
Figure 4. Effect of space time at a temperature of 333 K. Experimental and predicted: (a) etherification and (b) isomerization conversions with Ω1 ) Ω2 ) Ω3 ) 3.15 (Ω ) 1.05) in the absence of any solvent.
Figure 5. Effect of space time at 343 K. Experimental and predicted: (a) etherification and (b) isomerization conversions with Ω1 ) Ω2 ) Ω3 ) 3.15 (Ω ) 1.05) in the absence of any solvent.
For the olefins formed by THEE decomposition and isomerization
x2M2P ) X1,I/(Ω1Y) x2,3DM1B ) X2,I/(Ω2Y)
(22)
xC3M2P ) X3,IC/(Ω3Y) xT3M2P ) X3,IT/(Ω3Y) and the product THEEs mole fractions
xTHEEk ) Xk,E/(ΩkY)
(23)
while for any inert compounds in the feed stream
xI ) 1/(ΩIY)
(24)
where
Y≡1+
+ Ω
3
1
1
ΩΙ
Xk,E
∑Ω
k)1
(25)
k
4592 Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997
Figure 6. Effect of inert solvent. Predicted etherification conversions as a function of space time at 333 K with Ω1 ) Ω2 ) Ω3 ) 3.15 (Ω ) 1.05) as compared (a) between 50% and 100% and (b) between 10% and 100% olefins in stream. Solvent assumed is n-heptane.
Figure 7. Mixed ether production of ETBE, TAEE, and THEE1 at different temperatures. Predicted and experimental conversions: (a) etherification and (b) isomerization for equimolar olefin feed of isobutylene, 2M2B, and 2M1P (Ω ) 1.05).
The agreement between the experimental data and predictions is very good. Effect of Space Time. The effect of reactor space time was experimentally investigated at two different temperatures, 333 and 343 K, respectively. The feed molar ratio of ethanol to each individual C6 olefin was also equal to 3.15, leading to an overall molar ethanol to olefins ratio of Ω ) 1.05. No solvent was present in the feed. The experimental data are compared with the predictions as shown in Figure 4 at 333 K and in Figure 5 at 343 K, indicating very good agreement in each case. The conversions increase with space time, eventually approaching the equilibrium conversions at higher space times. Predicted Effect of Inert Solvent. In industry, the stream containing reactive olefins usually contains substantial amounts of other inert substances as well. Such inert species are not directly involved in the etherification or isomerization reactions, but will retard these reactions by diluting the reactants. Two different mole fractions of reactive olefins, i.e., Q ) 0.1 and 0.5 (ΩI ) 0.1167 and 1.05, corresponding to Ω ) 1.05), were used in calculations to simulate the effect of inert solvent on the etherification conversion of reactive olefins. Normal heptane was assumed to be the inert solvent in the calculations, and the molar ratio of ethanol to C6 olefins (2M1P, 2,3DM2B, and 2E1B) was assumed to be equal to 1.05. The etherification conversions are compared in Figure 6 as compared with the results for Q ) 1 (i.e., no inert solvent) at the temperature of 333 K. The presence of an inert solvent evidently decreases the etherification conversions of olefins at a given space time.
Mixed C4, C5, and C6 Olefin Feed. A similar procedure as above was followed in the analysis of the experiments with a mixed C4, C5, and C6 olefin feed. Effect of Temperature. For these mixed ethers synthesis experiments, a feed of isobutylene, 2M2B, and 2M1P was fed to the upflow reactor without any solvent in the feed stream for the simultaneous production of ETBE, TAEE, and THEE1. Integral experiments were conducted at a space time τ of 6.1 g‚h/mol and an equimolar olefin feed with the overall ethanol to olefin molar ratio of Ω ) 1.05. Using the procedure described above, the olefin conversions predicted agree reasonably well with experiments, as shown in Figure 7. Conversions of olefins to ethers, shown in Figure 7a, increase with increasing temperature until the conversion becomes equilibrium-limited and then decline for these exothermic reactions. It may be noted that the conversion of isobutylene to ETBE is substantially higher than that of other olefins to the corresponding ethers, followed by THEE1 and then TAEE. This order is in accord with the order of the corresponding equilibrium conversions as shown by Zhang and Datta (1996) in their Figure 5. Isomerization conversion of 2M2B and 2M1P, shown in Figure 7b, increases with increasing temperature. Effect of Space Time. Space time effects of the mixed olefin feed with Ω ) 1.05 at a temperature of 333 K are shown in Figure 8. The predicted conversions shown by the solid lines are in good agreement for both the etherification and the isomerization conversions of the feed olefins and approach equilibrium conversions at the higher space times.
Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 4593
Figure 8. Effect of space time on the olefin conversion. Predicted and experimental conversions: (a) etherification and (b) isomerization for equimolar olefin feed of isobutylene, 2M2B, and 2M1P (Ω ) 1.05).
Conclusions It is shown here that the developed rate expressions based on LHHW formalism, with kinetic parameters separately evaluated from experiments in differential reactors, are reliable in predicting the integral behavior of isothermal mixed ether systems, for example, for a system of simultaneous THEE1, THEE2, and THEE3 syntheses using a mixed C6 olefin feed stream. The effects of temperature and space time were experimentally investigated and theoretically simulated with good agreement. Feed mixtures of C4, C5, and C6 olefins were also studied for the simultaneous synthesis of ETBE, TAEE, and THEE and showed good agreement between predictions and experiments. Etherification and isomerization conversions increase with space time at lower temperatures; however, at higher temperatures equilibrium limitations are approached. It is thus concluded that a consistent set of expressions based on the same mechanism are applicable to the entire set of etherification and isomerization reactions for this family of reactions. The results of this paper are of interest since commercial production of mixed ethers for blending with gasoline is possible. Pure ethers are, of course, not necessary for blending purposes, and mixed ethers should suffice so long as the blend requirements of octane number, oxygen content, and RVP are met. Acknowledgment The funding provided for this work by the National Renewable Energy Laboratories and the Iowa Corn Promotion Board is gratefully acknowledged. Nomenclature A, B, C, D ) ethanol, R-olefin, β-olefin, ether, respectively Ai ) pre-exponential factor of reaction i, mol/(h‚g)
aj ) activity of species j, ≡ γjxj ) γjCCj Cj ) concentration of species j, mol/dm3 Ct ) total acid site concentration of Amberlyst 15, mequiv/g kB ) Boltzmann’s constant, 1.38 × 10-23 J/K Ei ) activation energy of reaction i, kJ/mol Fj ) molar flow rate of species j, mol/h h ) Planck’s constant, 6.626 × 10-34 J‚s ki ) rate constant of surface reaction (rds) of reaction i, mol/(h‚g) kri, ksi) effective rate constant of reaction i, mol/(h‚g) Ki ) liquid-phase thermodynamic equilibrium constant of reaction i KA ) adsorption equilibrium constant of A ) 27 exp[11 000(1/T - 1/303)/R] ri ) rate of reaction i, mol/(h‚g) (r)k ) reaction rate vector for the ether system k R ) gas constant, 8.314 J/(mol‚K) Rj ) rate of production of species j (Rk) ) rate of production vector for the THEE system k T ) temperature W ) weight of catalyst, g xj ) mole fraction of species j X1,E ) etherification conversion of 2M1P to form THEE1 X1,I ) isomerization conversion of 2M1P to form 2M2P X2,E ) etherification conversion of 2,3DM2B to form THEE2 X2,I ) isomerization conversion of 2,3DM2B to form 2,3DM1B X3,E ) etherification conversion of 2E1B to form THEE3 X3,IC ) isomerization conversion of 2E1B to form C3M2P X3,IT ) isomerization conversion of 2E1B to form T3M2P (Xk) ) conversion vector for the THEE system k Greek Letters κ ) transmission coefficient νij ) stoichiometric coefficient of species j in reaction i [ν]k ) stoichiometric coefficient matrix of the ether system k ν ) universal frequency τ ) space time of olefin, h‚g/mol Ωj ) molar ratio of ethanol to olefin j ΩI ) molar ratio of ethanol to inert I Subscripts 1, 2, 3 ) reactions 1, 2, 3, respectively A, B, C, D ) ethanol, R-olefin, β-olefin, ether, respectively i ) of reaction i j ) of species j k ) THEE system k. k ) 1 (THEE1); k ) 2 (THEE2); k ) 3 (THEE3) Abbreviations 2M1B ) 2-methyl-1-butene 2M2B ) 2-methyl-2-butene 2M1P ) 2-methyl-1-pentene 2M2P ) 2-methyl-2-pentene 2,3DM1B ) 2,3-dimethyl-1-butene 2,3DM2B ) 2,3-dimethyl-2-butene 2E1B ) 2-ethyl-1-butene C3M2P ) cis-3-methyl-2-pentene T3M2P ) trans-3-methyl-2-pentene ETBE ) ethyl tert-butyl ether IB ) isobutylene LHHW ) Langmuir-Hinshelwood-Hougen-Watson MASSA ) the most abundant surface species assumption MON ) motor octane number MTBE ) methyl tert-butyl ether rds ) rate-determining step RFG ) reformulated gasoline RON ) research octane number TAEE ) tert-amyl ethyl ether TSC ) transition-state complex TTST ) thermodynamic interpretation of the transitionstate theory
4594 Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 THEE ) tert-hexyl ethyl ether VOC ) volatile organic compounds WHSV ) weight hourly space velocity, h-1
Literature Cited Connors, K. A. Chemical Kinetics. The Study of Reaction Rates in Solution; VCH: New York, 1990; pp 187-243. Ignatius, J.; Jarvelin, H. Lindquist, P. Use TAME and Heavier Ethers to Improve Gasoline Properties. Hydrocarbon Process. 1995, 51, 51-52. Jensen, K.; Datta, R. Ethers from Ethanol. 1. Equilibrium Thermodynamic Analysis of the Liquid Phase Ethyl tert-Butyl Ether Reaction. Ind. Eng. Chem. Res. 1995, 34, 392-399. Jensen, K.; Datta, R. Ethers from Ethanol. 7. Transition-State Theory Analysis of the Kinetics of Liquid-Phase Ethyl tert-Butyl Ether Synthesis Reaction. Submitted for publication to Ind. Eng. Chem. Res. 1997. Kitchaiya, P.; Datta, R. Ethers from Ethanol. 2. Reaction Equilibria of Simultaneous tert-Amyl Ethyl Ether Synthesis and Isoamylene Isomerization. Ind. Eng. Chem. Res. 1995, 34, 1092-1101. Kitchaiya, P.; Datta, R. Ethers from Ethanol. 6. Kinetics of Simultaneous tert-Amyl Ethyl Ether (TAEE) Synthesis and Isoamylene Isomerization. Submitted for publication to Ind. Eng. Chem. Res. 1997a. Kitchaiya, P.; Datta, R. Ethers from Ethanol. 8. Examination of Kinetics and Mechanism of tert-Amyl Ethyl Ether Synthesis
Based on Partially Deactivated Catalysts and StructureEnergy Relationships. to be submitted to Ind. Eng. Chem. Res. 1997b. Milne, T. National Renewable Energy laboratory, Golden, CO, personal communication, 1996. Reid, R. C.; Prausnitz, J. M.; Pohling, B. E. The Properties of Gases and Liquids, 4th ed.; McGraw-Hill: New York, 1987. Zhang, T.; Datta, R. Integral Analysis of Methyl tert-Butyl Ether Synthesis Kinetics. Ind. Eng. Chem. Res. 1995a, 34, 730-740. Zhang, T.; Datta, R. Ethers from Ethanol. 3. Equilibrium Conversion and Selectivity Limitations in the Liquid-Phase Synthesis of Two tert-Hexyl Ethyl Ethers. Ind. Eng. Chem. Res. 1995b, 34, 2237-2246. Zhang, T.; Datta, R. Ethers from Ethanol. 4. Kinetics of the LiquidPhase Synthesis of Two tert-Hexyl Ethyl Ethers. Ind. Eng. Chem. Res. 1995c, 34, 2247-2257. Zhang, T.; Datta, R. Ethers from Ethanol. 5. Equilibria and Kinetics of the Coupled Reaction Network of Liquid-Phase 3-Methyl-3-Ethoxy-Pentane Synthesis. Chem. Eng. Sci. 1996, 51 (4), 649-661.
Received for review February 4, 1997 Revised manuscript received August 14, 1997 Accepted August 24, 1997X IE970099R X Abstract published in Advance ACS Abstracts, October 15, 1997.
