1888
Ind. Eng. Chem. Res. 1993,32, 1888-1894
Catalyst-InducedYield Enhancement in a Tubular Reactor? M. R. Ladisch,' R. L. Hendrickson, M. A. Brewer, and P. J. Westgate Laboratory of Renewable Resources Engineering, Purdue University, 1295 Potter Center, West Lafayette, Indiana 47907-1295 A macroreticular cation-exchange resin with an acid capacity of 5.5 mequiv/g and a higher acid group density was evaluated for MTBE formation from isobutylene and methanol using a 10-ft-long nonisothermal tubular reactor system with the methanol/isobutylene feed at close to stoichiometric ratio. A side-by-side comparison to a standard sulfonated catalyst (4.7 mequiv/g) shows this catalyst has a higher activity and increases maximum conversion by 2-5% and liquid hourly space yields by 10-25 % . Selectivities for both catalysts were close to 1. Concentration-based equilibrium constants (K,)for the enhanced catalyst were 870-2500 at temperatures ranging from 343 to 313 K compared to 300-850 for the standard sulfonated catalyst over the same temperature range. The catalyst with the higher acid group density enhances the maximum conversion of MTBE and increases the rate of reaction relative to the sulfonated catalyst currently in wide use in the industry.
Introduction Methyl tert-butyl ether (MTBE) is both an octane blending and a clean air component in gasolines. MTBE was the fastest growing chemical in the 19808, with continued growth leading to a predicted demand of 388 OOO barreldday in the winter and 164 000 barrels/day in the summer as year-round, ground-level ozone requirements of the Clean Air Act amendments go into effect in 1995 (Alnsworth, 1991). A widely used and thoroughly studied catalyst for MTBE production is Amberlyst 15 (Amberlyst is a registered trademark of the Rohm and Haas Co., Philadelphia, PA). It is a sulfonated, strong cation-exchange resin and has excellent properties with respect to MTBE formation from methanol and isobutylene at temperatures below 90 OC. Operating conditions have been defined where the selectivity is close to one and conversions above 90% are attained. Work by Ancilloti et al. (1977,1978) established a zero-order dependence of rate on methanol at concentrations above 4 mol/L, which was later verified by Lee et al. (1981) and other researchers. Amberlyst 15 is also an effective catalyst for formation of ethers from other higher alcohols. Studies in stirred reactors show that the reactivity of the primary alcohols with isobutylene in the presence of Amberlyst 15 is 1-butanol > 1-propanol > ethanol > methanol (Ancilloti et al., 1977,1978). Ethyl tert-butyl ether (ETBE),formed from ethanol and isobutylene, is of particular interest since the ethanol can be derived from renewable resources (Tau and Davis, 1989; Ladisch and Svarczkopf, 1991). This paper focuses on the MTBE reaction. Several types of industrial reactors are used for MTBE production. These include tubular reactors, adiabatic reactors with recycle (i.e., a large-diameter tubular reactor), and catalytic distillation configurations (Wentzheimer, 1980; Childs, 1981; Makovec, 1982; Smith, 1982; Jones, 1985). For tubular and recycle reactors, reaction temperatures generallyrange from 40 to 100OC, at high enough back pressures (150-200 psig) to maintain the reactants in a liquid state. Liquid hourly space velocities (LHSV) are typically in the range 1-5 volumes of fluid passed through the reactor per unit time per volume of catalyst bed. + Basis of the AIChE oral presentation 57 g in the Symposium on Industrial Reaction Engineering: Design, Modeling, Troubleshooting; November 3, 1992, Miami Beach, FL. * To whom correspondence should be addressed. E-mail (Bitnet)address:
[email protected] 0888-5885/93/2632-1888$04.00l0
The increasing demand for MTBE could, in part, be met by higher productivity per given reactor volume. Improving the productivity poses a significant challenge since the conversions and selectivities with existing catalysts are already above 90%. The activity coefficients for the reactants, particularly methanol, however, are not equal to unity, and the concentration equilibrium constant (K,) may become catalyst dependent. We report results from a catalyst, Amberlyst 35,which shows both improved activity and the ability to induce an enhanced conversion of methanol and isobutylene (IB) to MTBE in a tubular reactor.
Materials and Methods Catalysts. The catalysts used in this work were Amberlyst 35 and Amberlyst 15 (Amberlyst is a registered trademark of the Rohm and Haas Co., Philadelphia PA). Both are strong acid macroporous cation-exchange resins having an -SOs-H+ functional structure with porosities in the range 35-40 % . Amberlyst 35 has a higher acid content (5.5. mequiv/g) than Amberlyst 15 (4.7 mequiv/g). Feedstocks. The feedstocks used in these studies were supplied as premixed, certified cylinders from Matheson (Joliet, IL) containing methanol, isobutylene, isobutane, and n-butane as the major components and 0.4% n-pentane and 0.4% n-propane as minor components. The isobutylene concentrations ranged from 0.35 % to 18% (weight basis). The methanol compositions were adjusted in the range 0.7-11 % to give the desired methanoUIB mole ratios which ranged from 1.03 to 3.77. The balance of the feed mixture consisted principally of isobutane and n-butane in approximately equal amounts with compositions in the range 71-86% alkanes. Analytical Methods. The liquid effluent was analyzed for methyl tert-butyl ether (MTBE), tert-butyl alcohol (tBA), and methanol using a 2 mm i.d. X 4 m long GC column, packed with 80-100 mesh Poropak Q with 5 % carbowax coating, and placed in a Varian Model 3700 gas chromatograph. A 5-pL sample of liquid containing a 2-propanol internal standard was withdrawn from a capped bottle using a 25-pL Pressure-Loc syringe (Precision Sampling, Baton Rouge, LA) and injected into the GC. The Pressure-Loc syringe was necessary since the sample had a pressure of 30-40 psig at room temperature. The presence of 70% butanes made it difficult to directly quantitate isobutylene, although special efforts were made in some cases to improve resolution and thereby detect 1993 American Chemical Society
Ind. Eng. Chem. Res., Vol. 32, No. 9,1993 1889 the small amounts of unreacted isobutylene. The carrier gas was nitrogen at a flow rate of 30 mL1min. Resolution of the peaks was obtained with a temperature program in which a column temperature of 50 "C was held for 4 min and then increased at 12 "C per min until 170 "C was attained. The 170 "C temperature was held for 13 min. Diisobutylene,which elutes late in the chromatogramafter MTBE, was not formed in significant quantities at the conditions of this work, as would be expected from Ancilloti et al. (1978). Dimethyl ether (DME), which can be formed from methanol, was analyzed using a 50-80 mesh mixture of 80% Poropak N with 20% Poropak Q packed in a 2 mm i.d. X 2.4 m long column operated at a nitrogen or helium carrier gas flow rate of 30 mL/min. A sample volume of 100 pL of vapor was withdrawn from the head space in the bottle at 4 "C and 14 psi with a l-mL Pressure-Loc syringe and injected into the GC column. The temperature program consisted of an initial temperature of 40 "C for 12 min followed by a temperature increase of 4 "C per minute until 170 "C was reached. The temperature was then held for 13 min. Details of the GC analysis are described elsewhere (Lin et al., 1986). The vapor pressure of DME was estimated from the correlation: In P = 13.41313 - 2675.8/(T + 273) where P = pressure in psi and T = temperature in "C. Parameters in this equation were estimated using data from Perry and Chilton (1973). Predicted values of P deviated from reported data by less than 2 % . All of the detectable DME was present in the vapor due to its high relative volatility, with calculated vapor pressures of 42.6 psi at 4 "C and 27.5 psi at -8 "C, respectively. This was confirmed by analysis of both the liquid and vapor phases of a sample containing a known amount of DME. Consequently, sampling and analysis of the vapor in the head space give an accurate value of the total DME present. (1) Sampling of Reactor Effluent. The reactor effluent was at 150 psig, with the pressure maintained by an adjustable outlet check valve. One set of runs was carried out with a feed tank blanketed with helium at 300 psig. In this case, the outlet check valve was adjusted to give a 300 psig back pressure. There was no discernible difference in reactor operation at 150 vs 300 psig for an unreacted feed stream. Therefore, the lower pressure was used in all other runs. After start up, the reactor was run until a consistent temperature profile was achieved. A sample valve was then opened while the effluent valve was simultaneously closed, and all of the product stream was diverted to a sample collection system. The reaction product stream is cooled upon leaving the reactor effluent line through a check valve, due to sudden expansion of the effluent. The cold effluent, which was mostly liquid, then entered the sampling system consisting of vapor traps which were submerged in a dry icelacetone bath at -70 "C. All of the sample was condensed in this trap. This was confirmed by connecting a second and third vapor trap, each containing butanol and submerged in a dry icelacetone bath. There were no measurable weight gains in these two traps, and subsequent GC analysis of the liquid butanol did not detect the volatile compounds DME, propane, or butane. The sample bottle was a serum type vial (Supelco, Bellefonte,PA) which was sealed with a septum and metal seal after the sample was added. Prior to sample addition,
the bottle, septum, and seal were weighed to obtain a tare weight. The bottle was then chilled in the dry icelacetone bath, and the cold sample at -70 "C was poured from the dry icelacetone vapor trap into the chilled bottle. The septum and cap were immediately placed in the bottle opening, the cap was sealed by crimping with a hand-held device (Supelco, Bellefonte, PA), and the bottle was again weighed. A small aluminum disk on the top of the cap was then removed, and the internal standard 2-propanol, was added (700 pL) using a syringe. The bottle, sample, and internal standard were again weighed to obtain a total sample weight, and the weight of the internal standard was determined by difference. The sample bottle and its contents were kept at -20 "C in a freezer until analysis. Immediately prior to liquid analysis, the bottle and its contents were brought to room temperature. Samplesfor vapor analysiswere equilibrated in a refrigerator to 4 OC prior to their injection. Standard mixtures for calibrating the GC were obtained directly from standard Matheson calibration mixture cylinders and handled in an identical manner. (2) Pilot Scale Reactors. The pilot scale reactors used in this work were fabricated from stainless steel and were jacketed for purposes of circulatingcooling or heating fluid around the reactor. The reactor dimensions were 1.93 cm i.d. X 152 cm long with an empty volume of 445 mL. A thermowell which had an outside diameter of 0.3175 cm and displaced 12 mL of bed volume traversed the length of the reactor. It was placed in the center of the reactor to facilitate measurement of temperature as a function of distance from the inlet. The maximum bed volume, with the thermowell in place, was 433 mL. The runs described in this work were carried out by placing two reactors in series. These reactors were positioned vertically, with effluent passing down through the reactors. A heat exchanger placed between the first and second reactors controlled the temperature of the influent to the second reactor. Water was passed through the jackets in an up flow direction at a rate of 1.8 Llmin. The inlet temperature of the water entering the bottom of the second reactor was controlled by a circulating water bath. When water was circulating through the jacket of both reactors, nonisothermal operation was achieved. Adiabatic operation was simulated in some runs by preheating the lead reactor to the initial reaction temperature by circulating water through the jacket. After the preheating, the water was drained from the jacket and the jacket inlet and outlet were sealed, leaving an empty space between the reactor wall and the outer wall of the jacket. This system had an overall heat-transfer coefficient of 0.2-0.4 cal/(cm2h "C) as determined from material and energybalances for the reactor influent and effluent. These simulated operating conditions were as close to adiabatic as could be achieved on this scale. The experimental results showed 90% (or more) of the reaction occurred in the lead reactors, with most of the heat also generated in this reactor. Consequently,the temperature of the second reactor was controlled at simulated adiabatic conditions by circulating water through the jacket to make up the small amounts of heat loss which would otherwise occur. A schematic representation of this configuration is shown in Figure 1. (3) Methanol Balance. The methanol balance gave an accountingfor the methanol fed to the reactor compared to that recovered in the effluent as unreacted alcohol, MTBE and DME. Since the DME concentration was less than 120 ppm, its contribution was negligible and therefore ignored for purposes of the methanol balance:
1890 Ind. Eng. Chem. Res.,Vol. 32, No. 9, 1993 Simulated Adiabatic Reactor
Table I. Side-by-Side Comparison of Amberlyst 36 and Amberlvst 16 at Inlet Temoerature of 55 "C' catalyst run
no. of measurements inlet temp ("0
AT ('C) f c ('C) beddepth(ft) LHSV (l/h) at 25 "C i fl IB in feed (wt %) MeOHiIB mole ratio IB conv (%) i LT (%) fractional MTBE selectivityi o methanol balance ( % I i r (5%) LHSY Wh)
Amberlyst 35 8 55 29 2.91 8.67 5.6 4 0.12 17.80 1.055 95.5i 1.86 0.99 i 0.01 100.5 4 1.63 1.157
*
Amberlyst 15 9 55 17 9.33 5.2 0.02 17.80 1.055 92.8 1.14 0.95 98.5i 0.72 1.046
*
a LHSV = liquid hourly apace velocity for liquid density at 25 'C (77OF): (vol of influent/h)/reactorbed vol. LHSY = liquid hourly space yield for MTBE density at 25 'C (77"Fb LHSV X (IB% /1W) x (IB conv %/100) x (MTBE selectivity) X (MW MTBE/MW IB) x (influent density/MTBE density). Influent density = 0.59 g/mL, estimated at 25 "C hy method of Yen and Woods (1966). MTBE density = 0.75 g/mL. MTBE selectivity = MTBE formed/IB converted. Note that if MTBE and tBA are added together the selectivity is 1.00.
Figure 1. Schematic diagram of pilot d e reactor system. Thermowell is not shown.
% methanol
where M-E and MM~OH denote moles of MTBE and methanol, respectively. The methanol balance gives a useful measure of overall experimental error. A value significantly higher than 1009% indicates a leak in the pumping, reactor, or sampling system. If a leak occurs, the lower boiling components, particularly the propane, butanes, and isobutylene will preferentially flash and thus concentrate the methanol and MTBE components in the product stream, which is monitored by our GC method. A constant flow across the reactor is assumed, and with the measured concentrations it is used to calculate the moles of methanol and MTBE in the product. If there is a leak, the condition of constant flow rate is not satisfied and the balance deviates from closure. A methanol balance which is significantly lower than 100% may indicate analytical difficulties or could reflect differences in the amount of methanol held up by the catalyst bed during a period when readion conditions are changed. The methanol balance also reflects sampleto-sample variations in the analytical procedures. Thedatareportedinthisworkrepresentanalyses having a methanol balance within i 3 % of closure. During each run, the effluent was sampled 2-3 times at lMO-min intervals. Eachsample was analyzedin triplicate. If more than one of the replicate analyses gave an unsatisfactory methanol balance, the results for that particular sample were not used. A run is defined as a single set of reactor operating conditions (flow rate, temperature, feed composition, and/or catalyst) carried out on a given day. A new run is defined by a change in conditions or the repetition of the same conditions on a subsequent day. Isobutylene (IB) conversion was calculated by dividing the moles of MTBE and tert-butyl alcohol (tBA) formed by the moles of IB in the feed. The fractional MTBE selectivityis based on the moles of MTBE formed divided by the sum of the moles of MTBE and tBA. The DIB formed corresponded to levels of less than 40 ppm, and
in most cases none was detected. Consequently, the effect of DIB on the conversion and selectivity calculations is negligible. The results are presented as average values of replicate analyses for given samples from a particular run or, where applicable, runs at the same starting conditions. The corresponding standard deviations are also given. Standard deviations are reported as well when a small but measurable variation of inlet temperature or LHSV occurred.
Results Side-by-SideComparison. A side-by-sidecomparison of Amherlyst 35 and Amberlyst 15 at nonisothermal operating conditions showed that the catalyst with the higher acid group density (Amberlyst 35)gave the higher conversions at 55 and 60 "C (Tables I and 11,respectively). The isobutylene concentration in the feed and the methanol/IB mol ratio were 17.8% (by weight) and 1.055, respectively, witha typical feed tank composition as given in Table I11 (feed 1). The bed depth varied within 8.339.33 feet due to settling under flow conditions. The variation in bed depth does not change the observation that the higher acidity catalyst is the more active catalyst since the two cases of shorter beddepths coincide with the Amberlyst 35 catalyst. The higher liquid hourly space yield of the Amberlyst 35 probably reflects a higher volumetric density of acid groups per unit volume of reactor for the Amberlyst 35 relative to Amberlyst 15. While higher acidity also results in higher DME concentrations, the DME level is only 50 ppm for Amberlyst 35 at an inlet temperature of 60 "C and a temperature rise of 32 "C, which is slightly higher than the 20 ppm for Amberlyst 15 at similar conditions (Table 11). Selectivities in all cases are close to 1 for both catalysts. Comparison of the two catalysts at a liquid hourly space velocity (LHSV), which is believed to be within the typical range of industrial practice, further confirms that the catalyst with the higher acidity gives both higher conversion and higher yield at 50 and 60 "C (Table IV). Even at70°C, withatemperature riseof 30 "C, DMEformation continues to be quite small at 128ppm. The IB conversion values of 86.9-87.5% for Amberlyst 15 in Table IV are
Ind. Eng. Chem. Res., Vol. 32, No. 9, 1993 1891 Table 11. Side-by-Side ComDarison of Amberlyst 35 and Amberlyst 15 at 60 "C. ~~~~
catalyst run no. measurements inlet temp ("C) AT ("C) f u bed depth (ft) LHSV (l/h) at 25 "C f u IB in feed, (wt %) MeOH/IB mole ratio IB conv ( % ) f u (%) fractional MTBE selectivity methanol balance (%) f u (%) DME ppm, total mass LHSY (l/h)
Amberlyst 35 3 60 28.8 f 0.5 9.33 5.2 f 0.05 17.80 1.055 94.7 f 1.02 0.98 99.7 a 0.44 na 1.06
Amberlyst 15 6 60 25 9.33 4.6 17.80 1.055 89.5 f 0.87 0.98 97.1 f 0.54 20 0.89
Amberlyst 35 8 60 32 8.33 2.6 17.80 1.055 96.2 2.00 0.98 100.3 f 1.96 50 0.53
Amberlyst 15
7 60 30 9.33 2.3 17.80 1.055 86.9 f 0.98 0.99 97.9 f 1.26 na 0.43 * AT ("C) f u = maximum temperature rise; u given for seta of runs where variation in AT was measurable. DME ppm, total mass, denotes result of DME analysis based on head space analysis, with liquid (20-25mL) and vapor in closed 120-mL bottle, at 4 "C and 14 psig pressure. Other abbreviations are defined in Table I. Table 111. TyDical Feed Tank ComDositions ~~
feed 1 compd methanol isobutylene isobutane n-butane n-pentane propane water MTBE methanol/IB mole ratio a
mol wt 32.04 56.11 58.12 58.12 72.15 44.11 18 88.15 -
feed '2 mol % 17.743 16.826 31.95 32.70 0.301 0.483
wt%
10.70 17.77 34.95 35.77 0.409 0.401
