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Alkylation of Isobutane with 1-Butene on a Solid Acid Catalyst in Supercritical Reaction Media Gabriel M. Santana and Aydin Akgerman* Chemical Engineering Department, Texas A&M University, College Station, Texas 77843-3122
Alkylation of isobutane with n-butenes to form highly branched isoparaffins is an important process in gasoline manufacture. This reaction is currently catalyzed using either liquid hydrofluoric acid or sulfuric acid. Although these liquid acids rapidly catalyze the alkylation reaction, there are a number of drawbacks associated with their use. Solid acids have been studied extensively as possible catalysts for alkylation. However, with solid acid catalysts, the catalyst deactivates rapidly with time on stream. It is been hypothesized and shown that when the reaction is carried in supercritical media, the solid acid catalyst deactivation slows down considerably; however, high temperatures are needed to bring the reaction mixture to supercritical conditions, which results in the formation of undesired products. In this study, the effects of using different operation conditions for alkylation on a USY zeolite were investigated. It was observed that the deactivation rate was slower in supercritical-phase alkylation. Carbon dioxide was used as a diluent to lower the critical temperature of the reaction mixture in the ratio 27/9/1 for carbon dioxide/isobutene/1-butene. The use of diluent favored both alkylate selectivity and coke precursor removal. Introduction Alkylation of isobutane with butenes is a very important petrochemical process to produce high-octane and clean-burning gasolines. Alkylates are produced commercially using either sulfuric acid (H2SO4) or hydrofluoric acid (HF) as catalysts. The advantages and disadvantages of using H2SO4 or HF have been highly debated; comparisons have been made based on the alkylate quality, cost of separation, capital and operational costs, safety, and environmental aspects.1 Recently, the objective is to replace the existing alkylation technology by a new process which utilizes different catalysts which are nontoxic, noncorrosive, easy to handle, and environmentally friendly. In the past decade, a great deal of attention and resources has been paid to the development of solid acid catalysts, which have environmental and safety advantages over the liquid acids. The search for such a solid acid catalyst has also been accompanied by attempts to develop new process technologies, several of which have reached the pilot plant stage, but none has yet been commercialized. The common factor and limitation of all of these investigations is that a high catalytic activity on solid acids for long periods of time on stream has not been yet realized; these catalysts deactivate rapidly. In recent years, reactions in supercritical media have been used to extract heavy hydrocarbons or coke precursors in situ from porous catalysts.2-7 Supercritical fluids have a surface tension close to zero, enabling them to penetrate the porous structure of catalyst particles. It has been shown that supercritical fluids have the ability to dissolve compounds that are largely insoluble in the gas or liquid phase. It has also been observed that supercritical fluids can remove coke precursors from a zeolite, increasing the catalyst lifetime.8-11 Recent studies on the use of supercritical fluids in the alkylation * To whom correspondence should be addressed. Tel: 979845-3375. Fax: 979-845-6446. E-mail:
[email protected].
reaction of isobutane with butenes12,13 have shown that supercritical conditions prolong the life of the catalyst. However, the high temperatures used to reach supercritical conditions (>408 K) lead to undesirable side reactions such as oligomerization and cracking (poor selectivity). Thus, it is desirable to be able to perform the reaction at lower temperatures and still be at supercritical conditions. To accomplish this, a compound with low critical temperature can be used as a diluent, reducing the critical temperature of the reaction mixture. The use of diluents to lower the critical temperature of the supercritical phase for the alkylation reaction of isobutane with butenes has been reported only by Subramaniam’s group, who has studied the effects of CO2 as a diluent using a zeolite and sulfated zirconia as catalysts. They observed that the catalyst reaches a steady-state-type activity after a certain time on stream.10 Clark and Subramaniam10 studied alkylation in gas, liquid, and supercritical phases. We duplicated some of their data, and our results essentially verify their results. However, we carried out alkylation at different supercritical-phase reaction conditions and showed that at low temperatures extended C8 alkylate production activity can be obtained. We showed that the paraffins dominate the product when CO2 is used as a diluent. A constant CO2/isoparaffin/olefin ratio of 27/9/1 was used when CO2 was employed as the diluent. Sets of olefin weight hourly space velocities (WHSV) were 0.2, 0.3, 0.6, and 0.9 h-1. Here we report on our findings in lowtemperature supercritical-phase alkylation. Experimental Setup and Methods A schematic of the experimental assembly used is shown in Figure 1. Isobutane and 1-butene were transferred from the dip-tube cylinders to the syringe pump reservoirs through an activated alumina bed (to remove peroxides and other trace impurities) and a 15 µm filter. CO2 from the cylinder was passed through an activated carbon bed, a 15 µm filter, and a cooling ice bath and
10.1021/ie000501t CCC: $20.00 © 2001 American Chemical Society Published on Web 08/04/2001
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Figure 1. Schematic representation of the experimental setup for gas-, liquid-, and supercritical-phase alkylation reactions.
pumped directly to the reactor using a chromatographic pump (LDC/Milton Roy ConstaMetricIII). Isobutane and 1-butene were fed to the reactor directly by the syringe pumps (Isco LC-5000 and LC-2600, respectively). Air and N2 were used in the catalyst regeneration step. The isobutane/1-butene mixture and CO2 were preheated and allowed to mix in a tubing coil (1/8 in. o.d.) around the reactor, and the resulting mixture entered the reactor from the bottom (the reactor was operated in the up-flow mode). The reactor was a stainless steel tube equipped with a thermowell and pressure relief valves. The catalyst was packed in the middle of the reactor (isothermal zone) between beds of glass beads of the same size as the catalyst. The reactor heating system consisted of a circular aluminum block with an axially drilled hole to house the reactor. A heating tape (for alkylation reactions) or an electric heater (for catalyst regeneration) was used to heat the aluminum block. The catalyst used in this research was an ultrastable Y-type zeolite, provided by UOP (LZY-84), which was dry-pelletized, crushed, and sieved into a variety of particle sizes with average diameters ranging from 210 to 495 µm. A Si/Al ratio of 7.16 was calculated from the information given by the supplier. The catalyst was activated in situ under high-purity nitrogen by increasing the reactor temperature to 723 K at a rate of 5 K/min and holding the catalysts at this temperature for 4 h. The reactor was pressurized prior to the beginning of the reaction to allow for a clean start of the reaction. The reactor pressure was set by two backpressure regulators (BPRs) employed in series (by Tescom Corp., series 1700). The BPRs allowed good pressure control up to ∼415 bar ((0.5 bar) and were maintained at 333 ( 1 K by means of a heating tape. The effluent was analyzed online for the gas-phase components, and also a liquid sample was collected (at ∼0 °C) for offline analysis. The online gas chromatograph (GC) was a SRI
8610 unit equipped with a flame ionization detector (FID) and a Carbograph 2AP packed column by Alltech. This column was capable of resolving for methane, propane, and all C4 paraffin and olefin isomers. The chromatographic data obtained from the online GC were used to calculate 1-butene conversions. The liquid phase was a C5+ fraction and was collected in a cooled vial, which was removed and replaced periodically throughout the reaction. The liquid samples were analyzed by a HP G1800A GCD system, which had an electron ionization detector (EID) and a mass spectrometer (MS). In this GC a Petrocol DH capillary column from Supelco was used. This column was specially designed to separate hydrocarbon mixtures in the gasoline composition range. Compounds were considered identified when the probability of the fit between the mass spectra of the observed peak and the mass spectra of a compound in the standard libraries was above 90%. Details of the experimental setup can be found in Santana.14 The X-ray diffraction patterns for the new calcinated zeolite and for the regenerated one were determined. Comparison of these diffraction patterns did not show any noticeable differences. Furthermore, the alkylation reaction was also duplicated, and good agreement of 1-butene conversion and product composition was observed between the fresh and regenerated catalysts. A careful start-up procedure was employed in order to reach reaction conditions without any deactivation of the catalysts. The reactor and exit lines up to the BPRs were initially filled with liquid CO2, the reactor pressure was set, and the CO2 flow was verified with the gas flowmeter. The reactor was brought to reaction temperature under a CO2 environment. Then, the flow was switched to the bypass, and both the isobutane and 1-butene feeds were started. These pumps were set to supply the appropriate amounts to meet the desired 1-butene WHSV and 27/9/1 molar ratio. After the
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Figure 2. 1-Butene conversion for supercritical-phase reactions with CO2. Comparison of the results from Clark and Subramaniam10 (line) at olefin WHSV ) 0.25 h-1 and the results from this study (marker) at olefin WHSV ) 0.3 h-1.
steady-state composition was obtained, the flow was switched to the reactor, and this was taken as time zero for the reaction. Liquid samples were collected with time on stream (TOS) after the first drop of liquid was condensed. In parallel, the effluent was also analyzed periodically using the online GC. After about 26 h on stream, the reaction was stopped by diverting the feed to the bypass line and isolating the reactor. This allowed one to take a sample of the feed at the end of the experiment in order to verify that there was no change in the feed ratio. After this, the catalyst bed was ready for regeneration.
Figure 3. 1-Butene conversion for supercritical-phase reactions with CO2, at 353 K, 137.9 bar, and CO2/I/O ) 27/9/1, at different space velocities.
Results and Discussion Clark and Subramaniam10 studied the isobutane/1butene alkylation reaction in gas, liquid, near-critical without CO2, and supercritical with CO2 reactions on an HY (USY) zeolite at WHSV ) 0.25 h-1. Our data at WHSV of 0.3 h-1 are the closest to their data.14 In general, the data trends in two studies are similar, and very good agreement is observed for the supercritical and near-critical reactions. The 1-butene conversion data for supercritical reactions with CO2 were carried out at different temperatures, pressures, CO2/isobutane/ olefin (CO2/I/O) ratios, and olefin WHSVs. They employed ratios of CO2/I/O ) 86/8/1 at 323 K and 155 bar and CO2/I/O ) 43/8/1 at 368 K and 140 bar. In this study we used CO2/I/O ) 27/9/1 at 353 K and 137.9 bar. The three data trends present similar shapes, the reactions showed an initial rapid decrease in conversion, and the trends of data appear to overlap (Figure 2). As TOS increased, the curves split in three different curves, which reached three different steady-state conversions. The conversion curve for the reaction at 368 K reached a final steady-state conversion of about 0.21, the reaction at 353 K reached a conversion of about 0.12, and the reaction at 323 K reached a conversion of about 0.10. In this case, in contrast to the gas- and liquid-phase reactions, higher conversion was reached at higher temperatures. However, the factor played by the degree of dilution in these experiments affected the conversion histories, because addition of CO2 decreases the concentration of both isobutane and 1-butene in the catalyst pores. Figure 3 shows 1-butene conversion with TOS for the supercritical phase with CO2 reaction. The change in the composition of the liquid-phase product with TOS at a WHSV of 0.3 h-1 is shown in Figure 4. Product distributions at other space velocities are available
Figure 4. Change in liquid product composition with TOS. Supercritical-phase alkylation with CO2, at 353 K, 137.9 bar, CO2/ I/O ) 27/9/1, and WHSV ) 0.3 h-1.
elsewhere,14 but they have similar trends. It was observed that 1-butene isomerized to trans- and cis-2butene, and the degree of isomerization decreased with TOS. 1-Butene conversions were calculated by neglecting isomerization; all of the isomers are considered as 1-butene. This is based on the assumption that butenes are not secondary reaction products from cracking and/ or disproportionation reactions, under the temperatures and pressures used here. Initially, for olefin WHSV ) 0.2, 0.3, and 0.6 h-1, total conversion was observed. However, for olefin WHSV ) 0.9 h-1, the initial conversion was ∼0.94. For all of the olefin WHSVs used, the conversion decreased rapidly during the first 2 h of TOS. After this the rate of decrease of conversion slowed until about 3-5 h of TOS, when a nearly constant conversion was attained. The final conversions obtained were ∼0.17, ∼0.13, and ∼0.10 at 0.2, 0.3, and 0.6 h-1 olefin WHSV, respectively. For olefin WHSV ) 0.9 h-1, the conversion continued decreasing slowly and did not show to hold a nearly steady value. However, at the maximum TOS observed, the conversion reached a value of about 0.04. The conversion curves in Figure 3 show that rapid deactivation occurred at relatively short TOS. The combination of low temperature and improvement of transport properties such as diffusivity and solubility may contribute to rapid obtainment of pseudoequilibrium. The concentration of oligomers on the catalyst surface in the particles reaches a value which is maintained constant by the continuous removal of heavy molecular weight hydrocarbons by the supercritical
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mixture. This also allows one to maintain a constant concentration of strong and weak acid sites on the catalyst surface in the particles. The relatively low temperature would increase the chemisorption enthalpy of carbocations, as well as the adsorption energy of molecular species, which is also valid for the liquidphase reactions. The composition of the liquid product presented in Figure 4 shows an initial high concentration of C9+’s, which indicates that the supercritical reaction medium solubilizes and removes the high molecular weight species from the catalyst structure, preventing their accumulation on the catalyst surface. The paraffinic nature of the initial products also indicates that supercritical reaction media improved the hydride transfer activity. This is also verified by comparing the composition of the liquid product after the rapid decrease of conversion. For supercritical phase reactions with CO2, the production of saturated isooctanes and C9+’s continues relatively much longer than in the gas phase, liquid phase, and supercritical phase without CO2 reactions.14 However, as TOS increased, unsaturated products started to get produced, and after about 10 h of TOS, the liquid product consisted of isooctenes, C9+’s, and C9)+’s. We observed a narrow liquid product spectrum in the supercritical reaction using CO2 as the diluent, which may be attributed to a number of factors.14 Lower temperatures are expected to reduce cracking reactions, decreasing the formation of C5’s, C6’s, and C7’s. The addition of CO2 as the diluent reduced the concentration of 1-butene, and therefore the oligomerization potential, resulting in lower concentrations of oligomers and/or heavier species on the catalyst surface hindering the alkylation reactions. A lower concentration of deactivating species on the catalyst surface allows access to a larger number of acid sites. Strong acid sites are responsible for alkylation reactions, and their improved accessibility increases the product quality. The catalyst removed from the reactor was light brown when CO2 was used as the diluent, in contrast to the dark brown or blackish color observed in the gas phase, liquid phase, and supercritical phase without CO2 reactions. When the isooctane percentages in the liquid product from the different reactions were compared, the values observed for the supercritical reaction without CO2, liquid-phase, and gas-phase reactions showed higher initial concentrations than those for the supercritical phase with CO2 reaction. However, for the supercritical phase with CO2, the highest concentrations of isononanes and heavier saturated products (isononanes+), isononenes, and heavier unsaturated products (isononenes+) were observed. This indicates clearly the effect of using CO2 as the diluent, which increases the removal of heavy products from the catalyst particles and their pores. The effect of relatively low operating temperature is also realized by the presence of heavy products, which indicates significantly lower cracking activity when compared to higher temperature reactions. In addition, this low temperature also favors selectivity toward trimethylpentanes (TMP) over dimethylhexanes (DMH) and methylheptanes (MH), which are shown in Figure 5. This is in contrast to that observed for the gas and supercritical phases without CO2 reactions. It was also observed that the isooctenes produced by the supercritical reaction with CO2 increased in way similar to that
Figure 5. Composition of the isooctane fraction of the liquid product from the supercritical phase with CO2 reaction at WHSV ) 0.3 h-1.
in the gas-phase reaction but shifted by time caused by the increased volumetric flow (CO2 used as the diluent). Literature Cited (1) Albright, L. F. Alkylation Will Be a Key Process in the Reformulated Gasoline Era. Oil Gas J. 1990, 88 (46), Nov 12, 79. (2) Tiltscher, H.; Wolf, H.; Schelchshorn, J. A Mild and Effective Method for Reactivation or Maintenance of the Activity of Heterogeneous Catalysts. Angew. Chem., Int. Ed. Engl. 1981, 20, 892894. (3) Saim, S.; Subramaniam, B. Isomerization of 1-Hexane over Pt/γ-Al2O3 Catalyst: Reaction Mixture Density and Temperature Effects on Catalyst Effectiveness Factor, Coke Laydown, and Catalyst Micromeritics. J. Catal. 1991, 131, 445-456. (4) Yokota, K.; Fujimoto, K. Supercritical-Phase FischerTropsch Synthesis Reaction: 2. The Effective Diffusion of Reactant and Products in the Supercritical-Phase Reaction. Ind. Eng. Chem. Res. 1991, 30, 95-100. (5) Lang, X. S.; Akgerman, A.; Bukur, D. B. Steady-State Fischer-Tropsch Synthesis in Supercritical Propane. Ind. Eng. Chem. Res. 1995, 34, 72-77. (6) Bukur, D. B.; Lang, X. S.; Akgerman, A.; Feng, Z. T. Effect of Process Conditions on Olefin Selectivity During Conventional and Supercritical Fischer-Tropsch Synthesis. Ind. Eng. Chem. Res. 1997, 36, 2580-2587. (7) Fan, L.; Nakamura, I.; Ishida, S.; Fujimoto, K. SupercriticalPhase Alkylation Reaction on Solid Acid Catalysts: Mechanistic Study and Catalyst Development. Ind. Eng. Chem. Res. 1997, 36, 1458-1463. (8) Tiltscher, H.; Wolf, H.; Schelchshorn, J. Utilization of Supercritical Fluid Solvent-Effects in Heterogeneous Catalysis. Ber. Bunsen-Ges. Phys. Chem. 1984, 88, 897-900. (9) Manos, G.; Hofmann, H. Coke Removal from a Zeolite Catalyst by Supercritical Fluids. Chem. Eng. Technol. 1991, 14, 73. (10) Clark, M. C.; Subramaniam, B. Extended Alkylate Production Activity during Fixed-Bed Supercritical 1-Butane/Isobutane Alkylation on Solid Acid Catalysts Using Carbon Dioxide as a Diluent. Ind. Eng. Chem. Res. 1998, 37, 1243-1250. (11) Savage, P. E.; Gopalan, S.; Mizan, T. I.; Martino, C. J.; Brock, E. E. Reaction at Supercritical Conditions: Applications and Fundamentals. AIChE J. 1995, 41, 1723-1778. (12) Husain, A. Solid Catalyzed Supercritical IsoparaffinOlefin Alkylation Processes. U.S. Patent 5,304,698, 1994. (13) Nakamura, I.; Ishida, S.; Fujimoto, K. Supercritical Phase Isobutane/Olefin Alkylation on Solid Acids. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1995, 40, 512-516. (14) Santana, G. M. Study of the Supercritical Alkylation Reaction of Isobutane with 1-Butene on a Solid Acid Catalyst. Ph.D. Dissertation, Texas A&M University, College Station, TX, May 2000.
Received for review May 22, 2000 Revised manuscript received May 22, 2001 Accepted May 22, 2001 IE000501T
