730
Ind. Eng. Chem. Res. 1995,34,730-740
Integral Analysis of Methyl tert-Butyl Ether Synthesis Kinetics Tiejun Zhang and Ravindra Datta* Department of Chemical and Biochemical Engineering, The University of Iowa, Iowa City, Iowa 52242
Kinetics of the liquid phase synthesis of methyl tert-butyl ether (MTBE) from methanol and isobutylene were studied experimentally in an isothermal integral packed-bed reactor over a commercial ion exchange resin catalyst, Amberlyst 15, under conditions corresponding to those encountered in industrial practice. The reaction equilibrium was also studied experimentally and an expression for the equilibrium constant is proposed. The intrinsic kinetics were determined under conditions free of diffusional influence. The resulting effective activation energy, 85.4 kJ/mol, is about the average of those reported in the literature. Next, intraparticle diffusional limitations were investigated for compositions close to the stoichiometric feed and the results indicate that, with Amberlyst 15 catalyst of average diameter 0.74 mm, the reaction is substantially limited by intraparticle diffusion resistance at temperatures above 333 K and at isobutylene conversions closer to the equilibrium.
Introduction The use of methyl tert-butyl ether, or MTBE, as a gasoline additive has been growing sharply during the past decade, in particular since the passage of the Clean Air Act Amendments of 1990. Actually, in 1993, MTBE was the second largest volume organic chemical produced in the US.,only behind ethylene (Reisch, 1994), which represents a phenomenal rise from virtual nonexistence barely two decades ago. Addition of MTBE to the base gasoline can substantially enhance the research and motor octane numbers (Marceglia and Oriani, 1982). Consequently, it has effectively replaced the addition of leaded compounds for the same purpose. Another significant advantage of adding MTBE to gasoline is the increased oxygen content of the blended fuel, which is extremely desirable for the reduction of carbon monoxide emissions. As compared t o ethanol, the other major oxygenate gasoline additive at the present time, MTBE is preferred by refiners owing to its better blending characteristics as a result of its relatively moderate polarity. Commercially,MTBE is synthesized from a combination of methanol and isobutylene in the liquid phase at temperatures of 313-353 K and pressures of 100-300 psig in the presence of acidic cation exchange catalysts such as Amberlyst 15 (Ladisch, 1990). The reaction typically takes place in a packed-bed reactor, and is moderately exothermic and thermodynamically limited. The equilibrium conversion limitations increase quickly with temperature, as does the catalyst deactivation due to loss of sulfonic acid from the resin. Therefore, a major concern of reactor design is reaction temperature control by removal of heat of the reaction (Brockwell et al., 1991) either through reactor walls or between stages. While conversions of isobutylene, which is the limiting reactant, over 90% are readily realized in a single stage, reaction in two stages is usually required to achieve an overall conversions approaching 99% (Bitar et al., 1984). Alternatively, catalytic (or reactive) distillation is also industrially practiced to continuously remove the formed MTBE from the reaction zone, thus suppressing the reverse reaction and allowing higher MTBE conversions (DeGarmo et al., 1992). Kinetics of MTBE synthesis on Amberlyst 15 catalyst were studied first by Ancillotti et al. (1977, 1978), and
* To whom correspondence may be addressed. 0888-588519512634-0730$09.00/0
subsequently by Gicquel and Torck (19831, Subramaniam and Bhatia (1987), and Al-Jarallah et al. (1988). Reaction rate expressions were developed, as is customary, in terms of component concentrations. However, due t o the nonideality of the liquid mixtures owing to their disparate polarities, component activities are preferred over component concentrations, or mole fractions, in the rate expressions. Rate equation in terms of activities was first proposed by Rehfinger and Hoffmann (1990a,b) for MTBE synthesis, and recently by Fite et al. (1994) for ETBE synthesis. The UNIFAC method was utilized to predict the activity coefficients in the reaction mixture. Its use has been substantiated by the agreement between the predictions and the experimental data for the case of thermodynamic equilibrium of MTBE (Colombo et al., 1983). All of the above cited studies for MTBE were conducted either in laboratory batch reactors or in a CSTR, so that initial or differential reaction rates could be directly calculated. However, the applicability of the proposed rate expressions has not been examined in detail in integral reactors under conditions corresponding to those in industrial packed-bed reactors. Thus, an integral reactor with a catalyst bed packed with unground catalyst was selected in the present investigation so as to determine the efficacy of existing rate expressions and evaluate any diffusional limitations for the purpose of reactor design and analysis at high conversions approaching those in industrial practice. It should be pointed out that a reaction rate cannot be directly obtained from an integral reactor performance analysis even in the absence of any diffusional retardation. Rather, a proposed reaction mechanism and the corresponding rate expression may be tested by integrating it along the catalyst bed and comparing with the experimental reactor outlet conversion. In addition, to distinguish kinetic effects from thermal effects, it was decided to minimize the temperature gradients along the catalyst bed and to ensure isothermal operation by diluting catalyst with inert particles and by circulating constant temperature water around the external surface of the reactor bed. Another objective of the present study was to explore the influence of intraparticle diffusion on the observed isobutylene conversion under conditions equivalent to those in industrial reactors. This influence is usually intentionally eliminated in intrinsic kinetics studies by reducing the catalyst particle size. For instance, Sub-
0 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34,No. 3,1995 731
Rod uct Collect ion
II
CV: CheckValve LSV Liquid Smpling Valve
NV: Needle Valve R V Mief Valve F? Pressure Gauge T: Thermocouple
Figure 1. Schematic diagram of the upflow packed-bed (integral) reactor system for etherification reaction.
ramaniam and Bhatia (1987) selected a n average particle size of 0.4 mm for MTBE kinetics experiments in a batch reactor. Fite et al. (1994) crushed the catalyst (of average diameter about 0.74 mm) to a size less than 0.1 mm for ETBE kinetics experiments in a differential reactor. Although a detailed analysis of the intraparticle diffusion of methanol during MTBE synthesis was performed by Rehfinger and Hoffmann (1990b), it was done primarily under conditions when isobutylene is in a large excess. Intraparticle diffusion effects in MTBE synthesis have not been examined systematically over a wide range of conditions, particularly when the alcohol, in comparison to the olefin, is slightly in excess of the stoichiometric ratio, as in the industrial process. On the other hand, when isobutylene is in large excess, its concentration within the catalyst particle is relatively uniform, whereas the methanol concentration decreases toward the particle center due to the intraparticle diffusion limitation (Berg and Harris, 1993). Catalyst effectiveness factors larger than unity are then obtained as a result of the negative order of reaction in methanol activity. However, if isobutylene and methanol are present in a ratio closer t o the reaction stoichiometry, both isobutylene and methanol concentrations decrease toward the particle center, with a partial compensating effect and increased likelihood of normal behavior. Since the MTBE synthesis is equilibrium limited and, in fact, the equilibrium constant is required in the complete rate expression, a thermodynamic analysis was also performed. Thus, the present study includes an analysis of (1)the MTBE equilibrium constant, (2) intrinsic MTBE synthesis kinetics in an integral reactor, and (3) the influence of intraparticle diffusion limitations under conditions close to those in commercial processes.
Experimentation Apparatus. A schematic diagram of the upflow packed-bed (integral) reactor system is presented in Figure 1. Reservoirs of methanol and isobutylene were each placed on a top loading balance (Denver Instru-
ment DI-5K and Mettler BD1201, accuracy f 0 . 1 g) for continuous gravimetric mass flow rate measurements. Liquid isobutylene, because of its low normal boiling point of 266.9 K, was batchwise transferred from the cylinder to the isobutylene reservoir under pressure. In order to do so, a small pressure drop was needed between the cylinder outlet and the reservoir inlet, which was realized by cracking open the vent valve on the reservoir. The suction tube and the pump head for isobutylene pump (Gilson 305-1OOSC) were cooled with ice to avoid bubble formation. Methanol (Gilson 305lOSC) and isobutylene were independently pumped into a preheater tube packed with 3 mm glass beads where they were mixed and heated by circulating water in its jacket to the selected reaction temperature. The mixture was then introduced into a 3/8 in. stainless steel tubular packed-bed reactor with an effective length of 12 in., where the MTBE synthesis reaction took place. A back pressure regulator (UpchurchP738) and a needle valve were used to hold the reactor at a pressure of 1.72 MPa (250 psig) in order to maintain isobutylene in the liquid phase. Two type-K thermocouples(Omega KTSS116G)were placed inside the reactor, one in the middle of the catalyst bed and the other located 1in. into the catalyst bed inlet, and were used in conjunction with a digital thermometer (Omega MDSS116-KC1). The reactor was maintained a t a constant temperature by circulating water from a bath (Fisher Isotemp 9101, stability f O . O 1 K), at a selected temperature, through a jacket enclosing the reactor. Materials. Amberlyst 15 ion exchange resin (obtained from Sigma Chemical Co.) was used as the catalyst. Properties of the Amberlyst 15 catalyst are provided in Table 1. Depending upon the reaction temperature, a n amount varying between 1 and 4 g of dry catalyst was uniformly diluted with inert silicon carbide grains (obtained from McMaster-Carr) and then carefully packed into the tubular reactor. Placed between two 10-pm stainless steel frits, the reactor ends were filled with glass wool t o prevent any catalyst loss from the reactor bed. Before use, the catalyst was
732 Ind. Eng. Chem. Res., Vol. 34, No. 3, 1995 Table 1. Physical Properties of Amberlyst 15 Catalyst (Kunin et al., 1962) average particle diameter, mm surface area, m2/g porosity, % average pore diameter, nm apparent density, g/cm3 packing density, g/cm3 ion exchange capacity, equivlg swelling ratio
0.74 42.5 32 28.8 1.01 0.61 4.3 10-3
.-E .-
L I"
0.10
1
i
0.5a
Rehfinger and Hoffmann (1990b). L
washed first with an aqueous solution of 6 vol % nitric acid, and then with ethanol, and finally dried overnight in a vacuum oven at 378 K. The conditioned catalyst was kept in a sealed vial until use. To ascertain the influence of the catalyst particle size, a sample of Amberlyst 15 catalyst (of average diameter 0.74 mm) was also ground and sieved to obtain a granulometric fraction with the size 0.125-0.25 mm (average diameter 0.1875 mm). Methanol was obtained from Fisher (99.9%) with a water content of about 0.02 wt %. Isobutylene was obtained from Matheson (purity > 99%). MTBE was purchased from Aldrich Chemical (purity 99.8%). All chemicals were used without further treatment. No solvents or diluents were present in any of the experiments. Procedure. After the reactor was purged with nitrogen, pure methanol was pumped into the reactor until a pressure of about 0.20 MPa (50 psig) was reached. Then, liquid isobutylene at 273 K was pumped into the reactor to attain a final pressure of about 1.72 MPa (250 psig). At this point, the feed rates were set to the desired values, and the reactor was heated to the desired temperature. The reactor operation was continued until a steady state was achieved. In order to assure that all MTBE produced and unreacted methanol were condensed, the product was trapped in an icecooled collector. Samples of the condensed product were taken periodically and analyzed in a gas chromatograph (Perkin Elmer Autosystem) equipped with an Alltech PORAPAK R column (6 ft x 1/8 in.) at an oven temperature of 170 "C, and the results were used t o monitor the approach to the steady state. The steady state product composition was then used to compute the fractional isobutylene conversion from the following relationship:
where O M ~ OisHthe molar ratio of methanol t o isobutylene in the feed and XMeOH and XMTBE are the mole fractions of methanol and MTBE in the product. Equilibrium experiments of MTBE decomposition were also carried out in the aforementioned upflow pack-bed reactor at a pressure of 1.72 MPa (250 psig) using a stop-flow procedure. Because of the difficulty in handling the low-boiling-point isobutylene, MTBE decomposition was found to be experimentally more convenient than MTBE synthesis for determining the equilibrium constant as a function of temperature. First, a prerun was performed at a sufficiently high space velocity such that the established steady state conversion would be sufficiently removed from the equilibrium conversion of MTBE. Next, the feed was completely stopped and the reactor was operated batchwise €or an adequate period of time, depending upon
Time, min
Figure 2. Typical observation of MTBE conversion change during a thermodynamic equilibrium stop-flow experiment. T = 353 K.
the temperature, to reach the equilibrium state. Then, the feed was restored and the product stream was periodically sampled until the MTBE conversion dropped back to the original steady state as shown in Figure 2. The peak value of observed MTBE decomposition conversion was taken as the equilibrium conversion and was used to calculate the equilibrium constant at the selected temperature. This procedure was found to be preferable to the conventional usage of batch reactors for the determination of equilibrium constant because of the tendency of the volatile isobutylene to escape during sampling from a batch reactor.
Results and Discussion Preliminary experiments indicated that, without catalyst dilution with silicon carbide particles, there were significant temperature gradients along the packed catalyst bed. For instance, with 4 g of catalyst charge, a WHSV of around 8 h-l, and an external circulating water temperature of 313 K, temperature variations of up to 6 K between the bed inlet and center were typically observed due to the exothermicity of the reaction. The temperature variations were even greater at the higher reaction temperatures, thus making kinetics measurements from such a reactor difficult to interpret. This problem was addressed by diluting the catalyst with inert particles to reduce the reaction heat released per unit of heat exchange area and promote heat transfer. Thus, sufficient amounts of silicon carbide grains (size 0.25-0.45 mm) were used to reduce the temperature gradients in the bed. The dilution weight ratio varied from 5 to 30. This reduced the temperature variation t o less than 1 K under all the conditions investigated. A separate run at the highest temperature explored (353 K) with the reactor loaded only with silicon carbide particles indicated no MTBE formation, thus confirming their inertness. Experiments were also conducted to investigate the intraparticle transport effects on reaction rate. This was done by performing experiments under operating conditions that were identical except for the catalyst particle size. The two average particle sizes used were respectively 0.1875 and 0.74 mm. It was determined that the intraparticle diffusion effect was not significant, as attested to by isobutylene conversion, up to a temperature of 333 K for conversions below 85%. At temperatures above 333 K, higher reaction rates were always obtained with the smaller particle size. This is similar to what was observed by Fite et al. (1994) with feed composition close to stoichiometry, but is opposite
Ind. Eng. Chem. Res., Vol. 34, No. 3, 1995 733 of the abnormal observation that larger particle size resulted in higher reaction rates (Rehfinger and Hoffmann, 1990a; Fite et al., 1994). This difference in behavior is due t o the different feed compositions used. While feed composition ratio close to the stoichiometric ratio was used in this study to simulate industrial conditions, olefin was present in large excess in feed as compared t o alcohol when the above mentioned abnormal behavior was observed. A detailed discussion of this phenomenon is given in a later section of this paper. Thus, experiments at temperatures up t o 333 K were conducted with the original Amberlyst 15 catalyst of average size 0.74 mm, and those at temperatures above 333 K were conducted with both the catalyst sizes: 0.1875 and 0.74 mm. The molar ratio of methanol to isobutylene in the feed was maintained in the range of 1.0-1.12, roughly equivalent to that used in industry (Brockwell et al., 1991). In addition, any external transport limitations were tested by varying the velocity of the liquid mixture in the reactor. This was accomplished in two different ways, i.e., by changing the catalyst loading in the reactor while maintaining a constant WHSV, or by changing the reactor diameter while maintaining constant flow rate and catalyst loading. Thus, at the temperature of 333 K, experiments were conducted with 1 and 4 g of catalyst but at the same space velocity of WHSV = 8 h-l. At 353 K, experiments were carried out in two different reactors of diameter 3/16 in. and 3/8 in. The obtained isobutylene conversions between comparative runs were very similar thus indicating the absence of external mass transport effect. MTBE Reaction Equilibrium. Several correlations for the equilibrium constant, K,versus temperature for MTBE synthesis reaction in liquid phase, MeOH IB MTBE, have been proposed in the literature. The expression given by Colombo et al. (1983) is
+
In K = -10.0982
+ 4254.05T1 - 0.2667 In T
(2)
while that by Rehfinger and Hoffmann (1990a) is
+
In K = 357.094 - 1492.77T' - 77.4002 In T 0.507563T - 9.12739 x 1 0 - 4 p 1.10649 x 10-61"7 - 6.27996 x 10-loT' 3)
+
and the expression given by Izquierdo et al. (1992 is
-
+
In K = 1144 - 14634T' 233 In T 1.066T 1.077 x 1 0 - 3 p 5.306 x 10-71"7 4)
+
The alternate thermodynamic approaches utilized in the above cited sources have been recently carefully evaluated by Jensen and Datta (1995) for the liquid phase ETBE synthesis. The method of directly using the standard state thermodynamic properties for the liquid phase, if available, was recommended. This seems rather obvious because the least uncertainties would then be introduced into the thermodynamic properties. Equations 2-4 are compared in Figure 3 along with the data obtained over the temperature range of 343-373 K by Al-Jarallah et al. (1988) and over 313-353 K by Izquierdo et al. (1992). A significant difference is observed among the above expressions. In particular, eq 4 does not match the experimental values by the same authors. This greatly undermines the confidence of directly using any of these without further experimental scrutiny.
/
~
~
"
l
"
"
1
'
"
'
1
"
'
'
1
100 Y i C
m v) c
I
s
E, e .=I
10
'C
E
o o
1
2.7
2.8
2.9
Izquierdo et a1 (1 992)
3.1
3
lOOOrr,
AI-Jarallah a( 81 (1988)
3.2
3.3
K"
Figure 3. Comparison of present experimental data for equilibrium constant with proposed expression and literature correlations and data for the MTBE synthesis reaction. Table 2. Thermochemical Properties of Liquid Phase Methanol, Isobutylene, and MTBE (TRC, 1992): C,J= a bT CF d F J/(mol-K),T i n K
+
+
methanol -238.91 L ~ G ;kJ/mol ,~, -166.64 a , J4mol.K) 7.696b b, J/(mol.K2) 0.1617b c, J4m0l.K~) 2.058 x 6, Jl(m01.K~) 2.874 x 10-76 Cp,l,J/(mol.K) 9 4 9
AH;,,, kJ/mol
+
MTBE isobutylene" -37.7 -315.13c -119.87d 60.672 53.41e 35.44 0.7335e 0.8020 -3.124 x 10-3 -1.625 x 2.152 x 5.045 x 10-6 142.lf 196.2f
a Obtained from Jensen and Datta (1995). Obtained by fitting the Gallant (1970) data (over 273-373 K) to an exponential relation that was then expanded into a series and truncated to third order. Adjusted from the TRC value of -313.6 kJ/mol. Adjusted from the TRC value of -120.1 kJ/mol. e Estimated by the Rowlinson-Bondi equation (Reid et al., 1987). f Average heat capacity, integrated over 298-353 K.
The experimental results obtained in this study for equilibrium constants are also plotted in Figure 3 and are compared with the literature correlations and data. Theoretical calculations were performed using the thermodynamic properties given in Table 2. The standard enthalpy and Gibbs free energy of formation for liquid phase MTBE were slightly adjusted respectively to -315.13 kJ/mol (from the TRC value of -313.6 kJ/mol) and -119.87 kJ/mol (from the TRC value of -120.1 kJ/ mol) t o obtain the best fit between the calculations and the experiments. The equation obtained with the hence and AG;,, of MTBE is adjusted values of AH;,,
+ +
+
In K = -13.482 4388.7T1 1.2353 In T 0.013849T 2.5923 x 1 0 - 5 p - 3.1881 x 10-'1"7 (5) if the cubic relations of heat capacity are used. Alternatively
In K = 22.370
+ 3195.1T1 - 4.8232 In T
(6)
if the average heat capacities over a temperature range from 298 t o 353 K are utilized. Actually, eqs 5 and 6 are roughly equivalent in data fitting accuracy over the temperature range investigated. As shown in Figure 3, eq 5 agrees closely not only with the present experimental results, but also with the Rehfinger and Hoffmann (1990a) correlation, eq 3, as well as the experimental data obtained by Al-Jarallah et al. (1988) and Izquierdo et al. (1993). A n exception is the discrepancy
734 Ind. Eng. Chem. Res., Vol. 34, No. 3, 1995
-
sis and rate expressions may be derived to correlate experimental data (Subramaniam and Bhatia, 1987). However, it seems more appropriate to adopt the Langmuir-Hinshelwood-Hougen-Watson (LHHW) approach for a heterogeneous catalytic mechanism assuming that the rate-determining step is the surface reaction between adsorbed (protonated) methanol and adsorbed (protonated) isobutylene. This approach was adopted by Rehfinger and Hoffmann (1990a) for MTBE synthesis and by Fite et al. (1994) for ETBE synthesis. The LHHW mechanism was also adopted in this study although the other mechanisms were examined as well. Based on the LHHW approach and the dual site ratedetermining step described above, the rate expression takes the following form in terms of species activities:
303 K --t 313 K 323 K 333 K 343 K
rn
CI
I
0.0
1
" " " " " " " " " " " " " " " '
0
50
100
150 200 250
300
WHSV, h-' Figure 4. Isobutylene conversion as a function of WHSV and temperature for O M ~ O= H 1.05 (nominal) and a!, = 0.74 mm (average).
at the highest temperature, 373 K. It is possible that the experimental results at this temperature are affected by side reactions. The presence of side reactions, mainly the formation of isobutylene dimers, have been experimentally observed by Rehfinger and Hoffmann (1990a,c)when intraparticle diffusion affected the overall reaction rate at conditions such as at high temperatures and at low alcohol t o olefin ratios. In short, both eq 3 and eq 5 are recommended for determining equilibrium constant and for use in the reaction rate expression. The simpler expression, eq 6, is also adequate. Intrinsic Kinetics of MTBE Synthesis. Experimental isobutylene conversions obtained from the integral packed-bed reactor are shown in Figure 4, as a function of weight hourly space velocity over a wide range (WHSV = 3-300 h-l) at different reaction temperatures (303-353 K). The molar ratio of methanol to isobutylene in the feed was maintained at the nominal value of 1.05, but actual ratios in different experiments varied from 1.0 to 1.12. In general, isobutylene conversion increased as temperature increased and/or space velocity decreased. However, at very low space velocities (as shown in the upper-left corner of Figure 4), the conversion dropped as temperature increased at a given WHSV, apparently as a result of increasing thermodynamic limitations at the higher temperatures, characteristic of exothermic reactions such as MTBE synthesis. Similar results were also obtained in batch reactors (Al-Jarallah et al., 1988). Since the packed-bed reactor used is integral, experimental evaluation of a rate expression involves substitution of the appropriate rate expression into the integrated form of reactor mass balance equation
(8)
in the absence of any "inert" solvent. If it is further assumed that the polar methanol molecules are preferentially adsorbed on the ion exchange resin catalyst as compared to the relatively nonpolar isobutylene and MTBE and that the fraction of unoccupied sites is small, eq 8 reduces to (Rehfinger and Hoffmann, 1990a)
where k k r K I d K M e O H , and K is given by eq 5. Substituting eq 9 into eq 7 yields aMeOHL
dX= RHS (10)
~MTBE uMeO#IB
-
7
where uj = q y j (jrefers to MeOH, IB, and MTBE) is the activity of speciesj. The activity coefficients, yj, are estimated by the UNIFAC method (Fredenslund et al., 1975; Skjold-Jorgensen et al., 1979; Gmehling et al., 1982; Macedo et al., 1983; Tiegs et al., 1987). The mole fractions, x j , are related to fractional isobutylene conversion, X , in terms of molar ratio of methanol to isobutylene in the feed, OMeOH, by
v
XMTBE
=
+0
A
MeOH
-
(11)
and (7)
to test for linearity between ( W/FIB,~) and the right hand side integral. The experimental data used for intrinsic kinetics evaluation were limited to the runs from which the obtained isobutylene conversions were significantly lower than the corresponding equilibrium conversions, and temperatures were limited t o below 333 K. This was done since intraparticle diffusion began to limit the overall reaction rate at the higher temperatures and conversions, as discussed later. In the absence of diffusional limitations, MTBE reaction taking place in the liquid phase over acidic resins may be described as pseudo-homogeneous cataly-
X
= i h x , where
AX = XlB/N (i = 0, 1, ...,iV) (12)
X I Bis the isobutylene conversion at the reactor outlet; N is the number of intervals for numerically evaluating the integral in eq 10. Therefore, if eq 9 is appropriate, a plot of RHS in eq 10 versus (WIFIB,~)at each temperature should give a straight line passing through the origin with the slope being equal to the pseudo-rate constant, k, at the temperature. The results are shown in Figure 5. Satisfactory linear relationship is obtained at each temperature. The rate constants at 303,313, 323, and 333 K are 0.01182, 0.03471, 0.1024, and 0.2448 mol/ (h-g) (dry catalyst), respectively. Further, a replicate
Ind. Eng. Chem. Res., Vol. 34, No. 3, 1995 736 Table 3. Activation Energy in the Literature for the Liquid Phase MTBE Synthesis over Amberlyst 15 Catalyst reactor type temperature range, K activation energy, kJ/mol reference batch reactor batch reactor batch reactor batch reactor CSTR packed-bed, integral reactor
71.2 82.0 76.7 87.9 92.4 85.4
333-353 323-368 313-328 343-373 323-363 303-333
0.8
0.6 0.4 0.2 0.0
0
5
1 0 1 5 2 0 2 5 3 0 3 5
Ancillotti et al. (1977) Gicquel and Torck (1983) Subramanian and Bhatia (1987) Al-Jarallah et al. (1988) Rehfinger and Hoffmann (1990a) present work
pseudo-rate constant obtained by Rehfinger and Hoffmann (1990a1, as shown in Figure 6. It may, hence, be concluded that the simplified rate expression given by eq 9 accurately predicts the performance of integral packed-bed reactors under the range of compositions investigated and under the conditions of isothermality and absence of diffusional retardation. Conversely, it may be concluded that the integral reactor used can yield reliable kinetics under the experimental conditions investigated. The final recommended rate expression for intrinsic kinetics of liquid phase MTBE synthesis under the range of compositions investigated, thus, is (Rehfinger and Hoffmann, 1990a)
W/Fiai, g.h/mol Figure 5. Integral analysis of the kinetics experiments using eq 10 at temperatures from 303 to 333 K for d, = 0.74 mm.
a
+. v)
s C
-
Present experiment Rehfinger & Hoffmann (1990a)
-.%* ,----.---._
I
o . o l ' ' ' ' ' ' ' ' ' ' i ' " ' " ' ' ' i ' ' " " -. ''~ 2.7
\,
2.8
2.9
3
3.1
3.2
3.3
, ,
-
,I 3.4
lOOOTT, K-' Figure 6. Arrhenius plot of the temperature dependence of the MTBE reaction pseudo-rate constant obtained from the present packed-bed reactor as compared with that determined by Rehfinger and Hoffmann (1990a) in a CSTR.
run (shown as a crossed square in Figure 5) of the first kinetics experiment (point "f" in Figure 5 ) after all runs were completed indicates no discernable catalyst deactivation. The dependence of the pseudo-rate constant, k, on temperature is assumed to be described by the Arrhenius relation k = A0 exp(-EIRT). Thus, a plot of logarithmic rate constant against reciprocal temperature, Figure 6, yielded the effective activation energy, E = 85.4 kJ/mol, and A0 = 6.3 x 10l2mol4h-g). It should be noted, however, that since k = krKI$KMeoH, the effective activation energy E = Er MGJB - MG,MeOH, where E, is the true activation energy and M G j is the enthalpy of adsorption of speciesj (IB or MeOH) on Amberlyst 15 catalyst. The calculated effective activation energy value is compared in Table 3 with those determined in batch reactors and continuous stirred tank reactors by other investigators for the liquid phase MTBE synthesis over Amberlyst 15 catalyst. The activation energy determined in the present study is about 4% larger than the mean value of the literature data. In addition, the calculated rate constant over a temperature range of 303-333 K agrees well with the
+
with A0 = 6.3 x 10l2 mol4h-g) and E = 85.4 kJ/mol. Equilibrium constant, K, is given in eq 5, and activity coefficients are computed by the UNIFAC method. It may be mentioned that from the reaction stoichiometry, - ~ M ~ O H= -rIB = " E , although eq 13 is expressed in terms of the rate of disappearance of isobutylene. The rate expression, eq 13, shows that the rate of reaction is strongly inhibited by methanol. A mechanistic explanation for this is that, as assumed in deriving eq 9, the rate-limiting step is the surface reaction between adsorbed methanol and adsorbed isobutylene. Due t o the large polarity difference, however, methanol would have a much higher affinity for being adsorbed than isobutylene and, consequently, largely displaces isobutylene from the catalyst surface, thus inhibiting the overall reaction rate. Intraparticle Diffusional Limitations. As already indicated, intraparticle diffusion begins to limit the reaction rate for an average catalyst particle size of 0.74 mm at temperatures above 333 K and at conversions approaching equilibrium. A direct evidence of diffusional limitation at the higher temperatures is that higher isobutylene conversions were obtained with smaller particle size. As can be seen in Figure 7, the predictions from eq 13 a t 353 K are much higher than the experimental conversions obtained with normal catalyst particles ( d , = 0.74 mm), but agree with the experimental conversions with ground particles (d, = 0.125-0.25 mm, average 0.1875 mm), evidently as a result of the eliminated intraparticle diffusion resistance. Calculations indicate that the generalized Thiele modulus (Bischoff, 1965) based on methanol mass balance, i.e.,
,
R* dCkOH
CMeOH,e
for this reaction increases dramatically when the reaction gets close to the corresponding equilibrium conver-
736 Ind. Eng. Chem. Res., Vol. 34, No. 3, 1995 X"
The parameter, #,, in eq 14 is the Thiele modulus based on the h i s length parameter for a first-order reaction:
Equilibrium
.-0
$
0.6
0.2
0.0 0
1
4
2 3 W/Fi,,I, g.h/mol
5
Figure 7. Influence of catalyst particle size on isobutylene conversion at 5" = 353 K, O M ~ O=H1.05, and varying space time.
P ui
-333 K .......__ 353 K
TI
'Q, .-b)
E
I,,
6
a
=N
9
.... .... ..-.._.._._____........-----
2
,
E
a
,m
v
*,/;I
4 1
0
which also takes into account the Amberlyst 15 catalyst swelling. D e f f p e 0in ~ eq 15 is the effective diffusion coefficient of methanol in the multicomponent system (Berg and Harris, 1993). Use of the dimensionless methanol concentration in the definition of generalized Thiele modulus does not necessarily contradict the rate expression in terms of activities. Actually, a rate expression like eq 13 can also be expressed in terms of methanol mole fraction through an invariant transformation (Berg and Harris, 1993) or an eigenvector transformation (Sundmacher and Hoffmann, 1992) since the activity coefficients are also implicitly related to methanol mole fraction through the UNIFAC method. A generalized approach t o incorporate the intraparticle diffusion influence is to calculate the catalyst effectiveness factor by (Lee, 1985):
,
_______________-----_ _ - I
-
0
0.6 0.8 Isobutylene Conversion, Xi, 0.4
0.2
1
where the dimensionless methanol concentration at the particle center, CLeOH,c, may be approximated by the theoretical solution corresponding to a first-order reaction:
Figure 8. Generalized Thiele modulus versus isobutylene conversion at O M ~ O=H 1.05 and d, = 0.74 mm. e
X' C-
0
Equilibrium
0.8
2Q,
4 i
which has been modified to account for the reversibility of the MTBE synthesis reaction. Concentrations of isobutylene and MTBE in the catalyst particle are related t o that of methanol as follows (Lee, 1985):
> C
8
cj
Experimental, d = 0.74 mm Calculated from eq 13
n
0.0 0
5
10 W/F,,,,
15 20 , g.h/mol
25
30
Figure 9. Illustration of intraparticle diffusional limitations at high isobutylene conversions for T = 333 K and d, = 0.74 mm.
sion, Figure 8. The increasing diffusional limitation with isobutylene conversion qualitatively likely explains the increasing discrepancy between experimental data and calculated values by using eq 13, particularly at isobutylene conversion above 80% at the temperature of 333 K as shown in Figure 9. Attempts failed t o account for this discrepancy by retaining the vacant site term and more adsorption terms in the denominator of eq 8 or by using alternate reaction mechanisms and rate expressions. The solid line with dots in Figure 8 indicates the threshold below which the reverse reaction rate is less than 5% of the forward reaction rate. It can be seen that the generalized Thiele modulus is usually higher than 1.5 at conversions above this threshold.
= 'j,s
+
(")( v
~
MeOH beffj e ~ )('MeOH ~
- CMeOH,s)
(18)
Although isobutylene, being the limiting reactant, would be the obvious choice, methanol was selected in the effectiveness factor calculation as the key reactant since this was also done by Rehfinger and Hoffmann (1990b) and by Berg and Harris (1993). However, since -rMeoH = -rIB and (-rMeOH)&s = (-rIB)&s, the Catalyst effectiveness factor calculated for methanol is equal to that for isobutylene. Thus, the catalyst effectiveness factor calculated from eq 16 for methanol may be used in the integral reactor mass balance equation for isobutylene. Numerical integration indicates that eq 16 can be approximated by the corresponding first-order expression, eq 19, with an error of less than 5% if #g is larger than 1.5: (19) Equations 16 and 19 reach the same asymptotic solution, q = l/#g, in the limit of strong intraparticle resistance (#g 2 3). On the other hand, provided that the reaction is normal, for which the criterion is that the reaction rate gradient at the particle external surface, R , is positive (Kubota and Yamanaka, 1969)
Ind. Eng. Chem. Res., Vol. 34, No. 3, 1995 737 10.0
a C C Q E
-1
IB Conversion -
I
.-0 n% 3 E n al > .c. v)
V
al
!s al
[r
-2 1 '
0
"
"
'
"
1 ' 4
0.5
"
"
1
I
'
"
1.5
"
2
w
1.o
c
0
0.1
2.7 2.8 2.9
\
-
e
-mR s
3
0.10
\ \
3.1
3.2 3.3 3.4
lOOO/T, K"
Initial Molar Ratio of MeOH/IB,OhneoH Figure 10. MTBE forward dimensionless reaction rate gradient a t the particle external surface as a function of initial molar MeOWIB ratio at 333 K and different isobutylene conversion levels.
3
Figure 12. Arrhenius plot of effective diffusion coefficient of methanol in Amberlyst 15 for liquid phase MTBE synthesis.
It is of interest to examine whether the MTBE synthesis reaction behaves as a normal reaction under the conditions where Rehfinger and Hofhann (1990a,b) observed effectiveness factors higher than unity. In most of the Rehfinger and Hoffmann's experiments, isobutylene was present in large excess in comparison to methanol, and isobutylene conversions were up t o 15%. It is clear from Figure 10 that the value of S2 would likely be negative and the reaction would not behave as a normal reaction. This could give rise to catalyst effectiveness factors greater than unity, a result that was observed experimentally by Rehfinger and Hoffmann (1990b), and simulated by Berg and Harris (1993). The thus calculated catalyst effectiveness factor was utilized t o relate the observed reaction rate to the intrinsic reaction rate that is evaluated at the particle external surface conditions:
1 0.1
1
10
Generalized Thiele Modulus,9, Figure 11. Comparison of approximate methods with numerical integration for catalyst effectiveness factor calculations. T = 333 K, O M ~ O=H1.05, and d, = 0.74 mm.
(22) This relationship was then used in the integral reactor performance equation for data analysis:
and that qjg is less than 1.5, the effectiveness factor can also be estimated from eq 19 with qjg replaced with a modified Thiele modulus, qjm, defined by the following expression (Haynes, 1986):
It is known (Haynes, 1986) that kinetic models exhibiting abnormal behavior, such as dual site LHHW expressions of eq 8, behave as normal reactions over a wide range of parameter values. The values of S2 for the MTBE synthesis forward reaction were computed as a function of initial molar ratio of methanol to isobutylene and isobutylene conversion in Figure 10 at a temperature of 333 K. It can be seen that the criterion as represented by eq 20 for normal reactions is satisfied so long as the methanol to isobutylene ratio in the feed is stoichiometric or over, which was always true in the present investigation. Therefore, the Haynes approximation, eq 21, can be used. An example is given in Figure 11 at a temperature of 333 K for O M ~ O=H1.05 and d , = 0.74 mm. The use of Haynes approximation greatly improves the accuracy of calculations of the effectiveness factor for #g < 1.5 over simply using eqs 14 and 19.
(23) The value of the effective diffision coefficient of methanol was adjusted to obtain the best fit between experimental and theoretical conversions at each temperature. Next, an Arrhenius relation was used t o account for the temperature dependence of the effective diffusion coefficient, as shown in Figure 12. The activation energy of the methanol effective diffusion coefficient thus obtained is 35.4 kJ/mol with an apparent effective diffusion coefficient of 2.3 x m2/s at 333 K. The activation energy of methanol diffusion coefficient found in the present study is within the range of 25-42 kJ/ mol for liquid diffusion in macroreticular resins (Gicquel and Torck, 19831, but is somewhat lower than the 45 kJ/mol for isobutylene diffusion estimated from the reported diffusion coefficients for liquid phase isobutylene hydration over Amberlyst 15 catalyst (Leung et al., 1986). The effective diffision coefficient at 333 K found in the present study is slightly lower than the value of m2/s obtained by Rehfinger and Hoffmann 3.5 x (1990b). Figure 13 compares the experimental conversions with the calculated values from eq 23. At each temperature, the agreement is fairly good considering
738 Ind. Eng. Chem. Res., Vol. 34, No. 3, 1995 1.o ............................
: Equilibrium
0.8
0.8 1
I r - I
y,
0.6
.
0 C .-0 1.o
0.4 0.2
0.0 20 l
40 "
'
I
80
60 '
~
~
l
~
'
~
.............................................................
5
0
100 J
'
,,,,
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I
,,,,
10
,
) , ,
15
,id)j
,
, ) , ,
20
25
30
~
-0 . 1 0.8
0.6 0.4
0.2 0
20
40
60
80
100
o.oy* ' 0
1.o
1.o
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2 0.0
0.0 0
20
40
60 80 W/F,,,,, g.h/mol
100
'.
*
"
1
I
" "
2
" "
3
" 5
4
1
5
l l . . I l . . . . I , . , I I I , , , l
0
1
2
3
4
5
W/F,,,,, g.h/mol
Figure 13. Comparison of theoretical conversions (curves) using eq 23 with experimental data. (0, 0.1875 mm; 0 , 0.74 mm) at (a) 303, (b) 313, (c) 323, (d) 333, (e) 343, and (0 353 K.
the fact that an approximate solution for effectiveness factor was utilized to account for the intraparticle diffusion effects. In particular, the influence of catalyst particle size on isobutylene conversion was adequately simulated as shown in Figure 13f at the temperature of 353 K. In addition, the agreement at high conversions is significantly improved as shown in Figure 13d at the temperature of 333 K as compared to Figure 9. The effect of catalyst particle size on isobutylene conversion was next theoretically investigated using the developed model incorporating intraparticle diffusional influence. The calculations at a temperatures of 333 K for three conversion levels with an initial molar MeOW IB ratio of 1.05 are plotted in Figure 14. The particle size does not seem to affect the reaction as long as it is smaller than 1.0 mm at 10% isobutylene conversion. This value decreases to 0.4 mm at 80% conversion. The average diameter of the commercial Amberlyst 15 catalyst is about 0.74 mm. Therefore, in integral reactors, intraparticle diffusion limitation may be negligible during the first part of the catalyst bed length, but could become significant in the later part of the bed where the isobutylene conversion is high. Conclusions
Liquid phase MTBE synthesis was studied in an upflow isothermal packed-bed integral reactor. Applicability of the simplified intrinsic reaction rate ex-
10
I
'
'
"'"'I
'
'
" " "
1
k
1 10% IB Conversion - - - - - 50%
......... 80%
0.011 0.01
I
'
'
" ' c , , '
'
' " ' , * , '
'
0.1 1 Particle Diameter, dp, mm
'
'"'Y
10
Figure 14. Influence of catalyst particle size on calculated space times at T = 333 K, O M ~ O=H1.05, and isobutylene conversions of lo%, 50%, and 80%.
pression, eq 13, was confirmed by means of integral analysis over the range of conditions investigated. The expression is based on the LHHW mechanism where the surface reaction between adsorbed methanol and adsorbed isobutylene is the rate-determining step. Thermodynamic equilibrium experiments were also performed and an expression, eq 5, is proposed for the equilibrium constant. Intraparticle diffusional limitation was found to be significant in two cases: (1)at the higher temperatures (above 333 K) and (2) at high
Ind. Eng. Chem. Res., Vol. 34, No. 3, 1995 739 isobutylene conversion (above 80% even at the lower temperatures). A model incorporating the intraparticle diffusion influence predicts isobutylene conversions in good agreement with the experimental data in the integral packed-bed reactor under conditions corresponding to those used in industry. A methanol effective diffusion coefficient of 2.3 x m2/sat 333 K was obtained with an activation energy of 35.4 kJ/mol. An analysis of the resultant MTBE synthesis reaction rate expression indicates that the reaction behaves as a normal reaction if the feed molar methanollisobutylene ratio is over the stoichiometric ratio. However, if isobutylene is present in large excess as compared to methanol, the reaction could behave as an abnormal reaction, which could give rise to isothermal catalyst effectiveness factors larger than unity.
Acknowledgment The funding provided by the National Renewable Energy Laboratories for this study is gratefully acknowledged, as are the critical comments of Drs. Bahman Rejai and Marvin Klotz.
Nomenclature aj = activity of speciesj , = xjyj = parameter defined in eq 13, moI4h-g) C, = concentration of speciesj, mol/m3 Cj* = dimensionless concentration, = Cj/Cj,, CP,1= liquid phase heat capacity, J/(mol*K) D e ~= j effective diffusion coefficient of speciesj , m2/s d, = catalyst particle diameter, m E = effective activation energy, kJ/mol E,= true activation energy, kJ/mol FIB,^ = reactor inlet molar flow rate of isobutylene, mom k = reaction pseudo-rate constant, = krKI$KMeOH, mol4h.g) K = equilibrium constant Kj = adsorption equilibrium constant of speciesj k, = true reaction rate constant, mol4h.g) N = number of intervals for numerical integration, eq 12 R = gas constant, 8.314 J(mo1.K) R* = dimensionless reaction rate, = -rIB/(-rIB), = -rMeod
A0
(-rMeOH)s
-rj = reaction rate of disappearance of speciesj , moV(h-g) s = catalyst swelling ratio, volume increase divided by dry
volume T = temperature, K W = catalyst weight, g X = isobutylene conversion at a point within the reactor XIB= isobutylene conversion at reactor outlet xj = mole fraction of speciesj AGk, = standard free energy of formation, liquid phase, kJ/mol AH;,, = standard enthalpy of formation, liquid phase, kJ1 mol AHG= ~ enthalpy of adsorption of speciesj on Amberlyst 15, kJ/mol Greek Letters & = generalized Thiele modulus defined by eq 14 & = Thiele modulus defined by eq 21 q& = Thiele modulus defined by eq 15 yj = activity coefficient of speciesj 7 = catalyst effectiveness factor OM~OH = molar ratio of methanol to isobutylene in feed vj = stoichiometric coefficient of speciesj ea = apparent density of catalyst, kg/m3 P = reaction rate gradient at external surface, defined by eq 20
Subscripts c = evaluated at particle center conditions e = evaluated at equilibrium conditions IB = isobutylene MeOH = methanol MTBE = methyl tert-butyl ether obs = observed s = evaluated at particle external surface conditions Abbreviations ETBE= ethyl tert-butyl ether IB= isobutylene MeOH= methanol MTBE= methyl tert-butyl ether RHS = right hand side of eq 10 WHSV = weight hourly space velocity (total mass flow rate, g/h)/(reactor catalyst charge, g), h-l
Literature Cited Al-Jarallah, A. M.; Mohammed, A. B.; Lee, A. K. K. Kinetics of Methyl Tertiary Butyl Ether Synthesis Catalyzed by Ion Exchange Resin. Can. J. Chem. Eng. 1988,66, 802-806. Ancillotti, F.; Mauri, M. M.; Pescarollo, E. Ion Exchange Resin Catalyzed Addition of Alcohols to Olefins. J. Catal. 1977,46, 49-57. Ancillotti, F.; Mauri, M. M.; Pescarollo, E.; Romagnoni, L. Mechanisms in the reaction between olefins and alcohols catalyzed by ion exchange resins. J. Mol. Catal. 1978,4,37-48. Berg, D. A.;Harris, T. J. Characterization of Multicomponent Diffusion Effects in MTBE Synthesis. Znd. Eng. Chem. Res. 1993,32,2147-2158. Bischoff, K. B. Effectiveness Factors for General Reaction Rate Forms. AIChE J. 1965,11,351-355. Bitar, L. S.;Hazbun, E. A.; Piel, W. J. MTBE Production and Economics. Hydrocarbon Process. 1984,Oct, 63-66. Brockwell, H. L.; Sarathy, P. R.; Trotta, R. Synthesize Ethers. Hydrocarbon Process. 1991,Sept, 133-141. Colombo, F.; Con, L.; Dalloro, L.; Delogu, P. Equilibrium Constant for the Methyl-tert-butyl Ether Liquid Phase Synthesis by Use of UNIFAC. Znd. Eng. Chem. Fundam. 1983,22,219-223. DeGarmo, J. L; Parulekar, V. N.; Pinjala, V. Consider Reactive Distillation. Chem. Eng. Prog. 1992,3,43-50. Fite, C.;Iborra, M.; Tejero, J.; Izquierdo, J . F.; Cunill, F. Kinetics of the Liquid-Phase Synthesis of Ethyl tert-Butyl Ether (ETBE). Znd. Eng. Chem. Res. 1994,33,581-591. Fredenslund, A.; Jones, R. L.; Prausnitz, J. M. Group-Contribution Estimation of Activity Coefficients in Nonideal Liquid Mixtures. MChE J. 1975,21,1086-1098. Gallant, R. Physical Properties of Hydrocarbons; Gulf Publishing: Houston, 1968;Vol. 1,pp 61-72. Gicquel, A.; Torck, B. Synthesis of Methyl Tertiary Butyl Ether Catalyzed by Ion-Exchanged Resin: Influence of Methanol Concentration and Temperature. J . Catal. 1983,83,9-18. Gmehling, J.; Rasmussen, P.; Fredenslund, A. Vapor-Liquid Equilibria by UNIFAC Group Contribution. Revision and Extension 2.Znd. Eng. Chem. Process Des. Deu. 1982,21,118127. Haynes, H. W., Jr. An Explicit Approximation for the Effectiveness Factor in Porous Heterogeneous Catalysts. Chem. Eng. Sci. 1986,41,412-415. Izquierdo, J. F; Cunill, F. C.; Vila, M.; Tejero, J.; Iborra, M. Equilibrium Constants for Methyl tert-Butyl Ether LiquidPhase Synthesis. J . Chem. Eng. Data 1992,37,339-343. Jensen, K. L.; 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. Kubota, H.; Yamanaka, Y. Remarks on Approximate Estimation of Catalyst Effectiveness Factor. J. Chem. Eng. Jpn. 1969,2, 238-240. Kunin, R.; Meitzner, E.; Oline, J. A,; Fisher, S. A.; Frisch, N. Characterization of Amberlyst 15.Znd. Eng. Chem. Prod. Res. Deu. 1962,1 , 140-144. Ladisch, M. R. Catalysts for Etherification Processes. Presented at the National Conference on Octane Quality and Reformulated Gasoline, San Antonio, TX, March 1990.
740 Ind. Eng. Chem. Res., Vol. 34,No. 3, 1995 Lee, H. H. Heterogeneous Reactor Design; Buttenvorth Publishers: Boston, 1985; pp 117-126. Leung, P.; Zorrilla, C.; Recasens, F.; Smith, J. M. Hydration of Isobutene in Liquid-full and Trickle-bed Reactors. AIChE J . 1986,32, 1839-1847. Macedo, E. A.; Weidlich, U.; Gmehling, J.; Rasmussen, P. VaporLiquid Equilibria by UNIFAC Group Contribution. Revision and Extension 3. Ind. Eng. Chem. Process Des. Dev. 1983,22,676678. Marceglia, G.; Oriani, G. MTBE as Alternative Motor-Fuel. Chem. Econ. Eng. Rev. 1982, 14 (41, 39-45. Rehfinger, A,; Hoffmann, U. Kinetics of Methyl Tertiary Butyl Ether Liquid Phase Synthesis Catalyzed by Ion Exchange Resin-I. Intrinsic Rate Expression in Liquid Phase Activities. Chem. Eng. Sci. 1990a, 45, 1605-1617. Rehfinger, A,; Hoffmann, U. Kinetics of Methyl Tertiary Butyl Ether Liquid Phase Synthesis Catalyzed by Ion Exchange Resin-11. Macropore Diffusion of Methanol as Rate-Controlling Step. Chem. Eng. Sci. 1990b, 45, 1619-1626. Rehfinger, A,; Hoffmann, U. Formation of Di-isobutene, Main ByProduct of Methyl Tertiary Butyl Ether Synthesis Catalyzed by Ion Exchange Resin. Chem. Eng. Technol. 199Oc, 13, 150156. Reid, R. C.; Prausnitz, J. M.; Poling, B. E. The Properties of Gases and Liquids, 4th ed.; McGraw-Hill: New York, 1987; pp 136143. Reisch, M. Top 50 Chemicals Production Rose Modestly Last Year. Chem. Eng. News 1994, April 11, 12-15.
Skjold-Jorgensen, S.; Klobe, B.; Gmehling, J.; Rasmussen, P. Vapor-Liquid Equilibria by UNIFAC Group Contribution. Revision and Extension. Ind. Eng. Chem. Process Des. Deu. 1979, 18, 714-722. Subramaniam, C.; Bhatia, S.Liquid Phase Synthesis of Methyl tert-Butyl Ether Catalyzed by Ion-Exchanged Resin. Can. J. Chem. Eng. 1987, 65, 613-620. Sundmacher, K.; Hoffmann, U. Importance of Irreversible Thermodynamics for Liquid Phase Ion Exchange Catalysis: Experimental Verification for MTBE Synthesis. Chem. Eng. Sei. 1992, 47,2733-2738. Tiegs, D.; Gmehling, J.; Rasmussen, P.; Fredenslund, A. VaporLiquid Equilibria by UNIFAC Group Contribution. Revision and Extension 4. Ind. Eng. Chem. Res. 1987,26, 159-161. TRC Thermodynamic Tables, Hydrocarbons and Nonhydrocarbons; Thermodynamic Research Center, Texas A&M University: College Station, 1992;Vol. 5,5000 (alcohol);6040 (MTBE).
Received for review July 5, 1994 Revised manuscript received October 24, 1994 Accepted November 10, 1994@ IE940411M
Abstract published in Advance ACS Abstracts, February 1, 1995. @
