Ind. Eng. Chem. Res. 2009, 48, 1409–1413
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Present and Future Alkylation Processes in Refineries Lyle F. Albright Forney Hall of Chemical Engineering, Purdue UniVersity, West Lafayette, Indiana 47907
Isobutane has been alkylated with light (C3-C5) olefins for many years to produce high-quality, clean-burning gasolines. Processes that will significantly reduce operating costs and also produce higher-quality gasolines are proposed. These recommendations are based on recent improved understanding of process fundamentals, including the complicated chemical steps and the physical steps between and near the two or more phases in the reactor. Processes using sulfuric acid can be modified to reduce acid consumption significantly. Several solid catalysts have recently been proposed. All exhibit rapid catalyst deactivation for reasons explained in this article. Introduction Alkylation of isobutane with C3-C5 olefins results in the production of the highest-quality hydrocarbons for the gasoline pool.1 Currently, approximately 13-15% of this pool2 is produced by alkylation to form products with research octane numbers (RONs) of about 93-97. The motor octane number (MON) is about 2-3 points smaller. Alkylation products (or alkylates) were first produced in the late 1930s and early 1940s as a cooperative effort of several American companies to produce higher-quality gasolines for the warplanes of that time. These alkylates helped Allied planes perform better than enemy planes during World War II. Two catalysts used in the 1940s and since to produce alkylate are sulfuric acid and HF. In all cases, dispersions are formed in which the hydrocarbon phase is dispersed as droplets in the liquid catalyst phase. After World War II, alkylate was blended in automotive gasolines. Fifty years ago, about 75% of the alkylate was produced using sulfuric acid. During the next 30 years, the importance of HF grew until about 50% of the alkylate was produced by HF. Then, two incidents occurred, one in 1986 and the second in 1987, that clarified the dangers of HF.1 First, tests indicated that liquid HF can form aerosol clouds containing lethal levels of HF that can drift downwind for at least several miles. Second, an accident at a Texas refinery released considerable amounts of gaseous HF. As a result, over 1000 individuals in the neighborhood were evacuated. There were no fatalities, but many people were hospitalized for observation. Since then, the relative importance of alkylations with sulfuric acid has increased. Much effort has since been made to develop methods to minimize HF danger and also to formulate safer catalysts such as solid catalysts. It was recently announced3 that a small unit using a solid catalyst to produce alkylate will be built. All alkylation processes involve complicated chemistry4 that results in the formation of numerous cations plus hydride ions (used to convert the cations mainly to isoparaffins). Side reactions are also of importance. The overall chemistry varies to a significant degree for different catalysts. In addition, numerous transfer steps occur between the hydrocarbon and catalyst phases.5 In this article, the pros and cons of sulfuric acid, HF, and solid catalysts are compared. Recommendations are made that could result in improved alkylation processes. Chemistry of Isobutane Alkylations Four chemical sequences produce C5-C16 mixtures of hydrocarbons (mainly isoparaffins). They have been designated as mechanisms 1-4.1,4,5
Mechanism 1. This mechanism is the only one for true alkylation. When isobutane is alkylated with C3-C5 olefins, C7-C9 isoparaffins, respectively, are produced. The simplified chemistry with a C4 olefin is shown as two reaction steps as follows t-C4H9+ + C4 olefin f i-C8H17+ i-C8H17+ + i-C4H10 f i-C8H18 + t-C4H9+ The i-C8H18 product is generally a trimethylpentane (TMP) or less likely an undesired dimethylhexane (DMH). DMHs are produced in large amounts from 1-butene when HF is the catalyst. t-C4H9+’s (tertiary butyl cations) are produced in three ways, one of which involves transfer of a proton (H+) from the acid to isobutylene. There are also two methods to form t-C4H9+’s from isobutane. H- (hydride ion) can be transferred from isobutane to the hydrocarbon cation, such as shown in the second reaction step above, or H- can be transferred from isobutane to the conjunct polymers (CPs) dissolved in the acid phase, in which case a tertiary C-H bond is temporarily formed. Eventually, the H- transfers as in the second reaction step above. These transfers occur at or at least close to the interface between the two liquid phases. Both are generally of major importance. Alkylates produced with sulfuric acid as the catalyst and with 1-pentene as the olefin have a different composition than those produced using 2-pentenes.6 The former alkylates contain appreciably less C9 isoparaffins (hence, mechanism 1 is of much less importance). Alkylates produced from 1-pentene, however, contain more TMPs. With sulfuric acid, the alkylates produced from 1-butene and 2-butenes give alkylates with almost identical compositions. Cyclopentene produces mainly conjunct polymers (CPs). Mechanism 2. This mechanism produces a mixture of C4-C16 isoparaffins and is the main mechanism for the production of lighter plus heavier isoparaffins. Some isobutane is also produced. Initially, C10-C20 cations are formed as two to five olefin molecules add to t-C4H9+. These heavy cations fragment, forming both C4-C16 cations and olefins. Transfer of H- and H+ ions results in C4-C16 isoparaffins. Mechanism 2 is of considerably greater importance with branched olefins, namely, isobutylene and isopentenes, as compared to normal olefins. For this mechanism, the ratio of TMPs to DMHs tends to be relatively low. Mechanism 3. This mechanism first produces C10-C16 cations as in mechanism 2. Hydride transfer then results in the production of heavy isoparaffins.
10.1021/ie801495p CCC: $40.75 2009 American Chemical Society Published on Web 01/08/2009
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Mechanism 4. This mechanism is of considerable importance when HF is the catalyst. Both propane and n-butane are produced in significant amounts when propylene and n-butene, respectively, are employed with HF as the catalyst. The overall chemistry is often shown with propylene as follows4
Less information has been reported for side reactions with HF as the catalyst, but apparently the levels of such reactions are similar.
2i-C4H10 + C3H6 f TMP + C3H8
Sulfuric acid and HF have been employed as catalysts for over 60 years. Five processes have been widely used for many years. Sulfuric acid processes were designed by Stratco (which was purchased by the DuPont Co. several years ago), ExxonMobil, and M.W. Kellogg Co.1 No new Kellogg units have been built in the past 35 years, but a significant number of their plants are still operating. The reactors of ExxonMobil and Kellogg are similar in design. Phillips Petroleum (now ConocoPhillips) and UOP designed units using HF as the catalyst. Their reactors are of a different design. UOP announced in December 2007 that they had purchased the Conoco-Phillips alkylation technology and hence could broaden their alkylation offerings. Safety Concerns. HF is known to be a much more dangerous catalyst, especially since 1986. First tests indicating that toxic aerosol clouds can be formed and second the loss of considerable gaseous HF from a refinery clarified dangers. In addition, shortly thereafter, two operators were killed as a result of a HF leak at a refinery in Corpus Christi, TX.1 Several methods to reduce dangers have now been reported.1 Conoco-Phillips and UOP have developed and used additives for HF in at least some of their units to reduce the dangers. For this purpose, ConocoPhillips recommends sulfolane, whereas UOP has recommended pyridine or picolene. Claims have been made of reducing the amount of HF in the aerosol clouds by 60-90%. Details on how far the toxic clouds might travel when additives are used have not been reported. In the alkylation units, the additives need to be recovered and recycled. The amounts of additives employed are unknown. Water sprays around HF units have also been used in several plants. Their effectiveness in reducing HF leaks has not been reported. In addition, following an accident, it is uncertain whether the sprays would still be operable. Facilities for rapid transfer of liquid HF to an alternative storage tank are also provided in some units. These safety procedures substantially increase operating costs. Sulfuric acid is a dangerous chemical, but the dangers are relatively localized. One problem is the formation of a pseudoalkylate layer on top of the used acid in storage tanks.10,13,14 It is now known that some sec-alkyl sulfates produced in the alkylation reactor are carried to sulfuric acid storage tanks. Here, they decompose to form both conjunct polymers and pseudoalkylate. Some companies maintain a nitrogen blanket on their storage tanks and also drain the pseudo-alkylate at regular intervals. Advantages of HF Processes. HF processes have the following advantages as compared to sulfuric acid processes:1 Temperatures employed in HF reactors are significantly higher, usually 30-40 °C, as compared to 5-10 °C in sulfuric acid units. In HF processes, the costs to remove the heats of reaction plus energy supplied by agitation are much lower. Cooling water can be used, whereas refrigeration is required for sulfuric acid units. Much more rapid kinetics occur in HF-type units. The Conoco-Phillips reactor is simply a vertical tube that has a residence time estimated to be about 10 s.1 For sulfuric acidtype processes, residence times are in range of about 20-30 min. The size and costs of the reactors for HF are much lower. Distillation is employed in the refinery to regenerate used HF. Operating costs are relatively low. Used sulfuric acid is usually shipped, however, to a nearby plant where sulfuric acid
Unfortunately, in addition to TMPs, appreciable amounts of light ends, DMHs, and heavier paraffins are also produced, resulting in a rather low-quality alkylate. Consumption of isobutane on a relative basis is almost double that for the remaining alkylate.7 Calculations indicate that 15-25% of the propylene is often converted to propane and 6-10% of the n-butenes to n-butane. With HF as the catalyst, increased amounts of both n-pentane and isopentane are produced when either n-C5 or i-C5 olefins are used.8 With sulfuric acid, much smaller amounts of n-pentenes are converted to n-pentane.6,7 Normal paraffins produced by mechanism 4 are unsuitable gasolines. Side Reactions. It has long been known that conjunct polymers (CPs) are produced during alkylation. When present in relatively low amounts in the acid, they have beneficial effects. First, they serve as a reservoir for hydride ions, and second, they act as surfactants to produce desired and larger interfacial areas between the two liquid phases. Unfortunately, however, CPs reduce the catalytic activity of the liquid acids. Eventually, the catalyst needs to be replaced. Many years ago, Miron and Lee9 reported that the chemical structure of CPs was about -(C1H1.75)20-40. Albright and co-workers4,10 much later found that CPs and pseudo-alkylate are produced simultaneously as indicated by the following simplified reaction scheme C4 olefins f i-C16-i-C20 cations f CPs + pseudo-alkylate (C4-C16 isoparaffins) The pseudo-alkylate contains the same isoparaffins as regular alkylates, but it has a significantly lower octane number. Relatively high concentrations of DMHs are formed. The heavy cations formed as intermediates are produced by reactions similar to those of mechanism 2. The weight of the CPs is essentially equal to that of the pseudo-alkylate. Branched olefins produce more CPs and pseudo-alkylate than normal olefins. Often, 2-3% of the olefins are converted to these byproduct when both sulfuric acid and HF are used as catalysts. Propylene and pentenes also react to form these byproducts. CPs are also formed if the following impurities are present in the feedstocks: conjugated dienes (such as butadiene), acetylenic compounds, sulfur-containing hydrocarbons, and cyclopentene. Increased water concentrations in the catalyst likely promote CP production. When sulfuric acid is used as the catalyst, TMPs and other highly branched isoparaffins decompose in the liquid-liquid dispersions.11-13 This acid acts as an oxidizing agent, forming heavy cations, SO2, and water. The heavy cations react via mechanism-2-type reactions to form a mixture of C4-C16 isoparaffins. Of the four TMPs, 2,3,4-TMP, having three tertiary CsH bonds, is most reactive. Some of the TMP+ ions initially formed isomerize to other TMP+ ions (and then forms other TMP molecules). It is thought that such decompositions also occur with HF as the catalyst. The above results indicate the importance of rapidly separating the dispersions containing sulfuric acid once the alkylate product is formed. Yet, many alkylation processes using sulfuric acids provide decanters having residence times of up to 30 min.
Current Alkylation Processes
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is produced. The costs of regenerating this acid have often been as high as 25-30% of the total operating costs.1,14 Potential ways to reduce these costs are reported later in this article. Costs of obtaining large interfacial areas in the dispersion are considerably less when HF is the catalyst because it has a much lower viscosity than sulfuric acid. Advantages of Processes Using Sulfuric Acid. Considerably less DMHs are generally produced, and hence, alkylate qualities are often higher when sulfuric acid is the catalyst. With HF, 1-butene always produces appreciable DMHs.1,4,5 Less isobutane is consumed with sulfuric acid as the catalyst, as propane and n-butane are not produced by mechanism 4.4 Savings, of course, depend on the olefins used, but are estimated to be as high as 5-10%.8 With HF, rather large amounts of isopentanes are also produced via mechanism 4.8 The isobutane/olefin ratio fed to the reactors using sulfuric acid are often 7-9 as compared to 12-15 for HF.1,14 Hence, distillation costs to recover and recycle the unreacted isobutane are considerably less in sulfuric acid units. Alkylation Units Being Built In its June 2007 and October 2008 issues, Hydrocarbon Processing3 reported that 36 alkylation units either are being enlarged or are grassroot units. In the U.S., seven units are being enlarged. Seven units are being constructed in Asia. The remaining plants are located in Europe, Africa, South America, Canada, and Mexico. Most of the new capacity will employ units designed by Stratco, Phillips-Conoco, and UOP. A unit in Romania, designed by South Korean Engineering and Construction (SKEC), will use sulfuric acid as the catalyst. The type of reactor being employed is unknown. For several other units, the design was not reported. One unit to be built in Lithuania that was designed by Exelus will employ a solid catalyst. Solid Catalysts for Alkylation Processes Solid catalysts have been investigated for over 50 years. To obtain the large surface areas needed to catalyze the alkylation reactions, porous solids are apparently always used. Larger surface areas are generally obtained with some combination of more and smaller pores. At least three types of catalyst beds have been tested: packed beds, fluidized beds, and moving beds. To date, relatively rapid deactivation of the catalyst has always been a problem. For the Exelus process, which uses a packed bed, the catalyst is reactivated about every 12 h.17,18 Two reactors are required: one is on stream while the other is being regenerated. For the alkylene process of UOP, Roeseler et al.19 reported that tests indicated over 9000 cycles in over 9 months. Calculations indicate that reactivations were made approximately every 0.7-0.8 h. Hydrogen gas is employed for reactivations. The UOP catalyst has platinum deposited on its surface; hence, it acts as the catalyst both for hydrogenating the “deactivating species” and also for alkylation. UOP has reported that, in its units, the catalyst is partially deactivated in a moving-bed reactor and then completely activated in another reactor. Oxygen or air has also been employed for the reactivation of other catalysts. Numerous transfer stops obviously occur in the pores of the solid catalysts. Both isobutane and the feed olefins diffuse inward in the pores, and product molecules diffuse outward. Sherwood et al.20 outlined several problems that occur in the porous catalyst. The rates of transfer or diffusion vary for different molecules because of differences in size, shape, and
molecular weight. In the pores, numerous reaction steps occur. Conditions in the pores are conductive to forming, in addition to alkylate, both conjunct polymers and pseudo-alkylate. Diffusion of conjunct polymer in particular would be slow and probably nonexistent. The conjunct polymers would likely complex or bond with the inner surfaces of the solid catalyst. Certain companies17-19 that market processes using solid catalysts claim that conjunct polymers are not produced in their processes. They have not, however, reported any known experimental evidence to support their claims. Instead, they claim that coke, presumably mainly carbon, is formed; however, experimental evidence is lacking. The discussion of Sherwood et al.20 supports conclusions that catalyst deactivation is affected by the following variables: First, the porosity and size of the pores affect deactivation. Small pores would likely deactivate much more quickly than big pores. Second, the size of the solid catalyst particles has an impact. Small particles probably deactivate more rapidly than large particles. Third, deactivation is affected by the dimensions of the packed bed and the reaction zone in the catalyst bed that produces alkylate, which moves in the bed while it is on stream. Larger beds would have more pronounced hot spots in the middle of the bed due to poorer heat transfer. Fourth, higher temperatures almost certainly result in more rapid deactivation and in reduced yields of the desired alkylate product. Fifth, the ratio of isobutane to olefins has an effect on deactivation. Ratios as high as 10-15 have occasionally been recommended. Sherwood et al.20 reported that the composition of the reactants in the phase outside the pores is generally very different from that in the pores. Sixth, the composition of olefins has an impact because branched olefins almost certainly cause more rapid deactivation. It is recommended that refineries considering alkylation units employing solid catalyst should seek details concerning the following factors: (1) time required before catalyst needs to be regenerated; (2) qualities and yields of alkylate as a function of time while the catalyst is being used and hence deactivated; (3) catalyst information including composition of the catalyst plus possible precious metals, range of pore size and particle size, and expected longevity of the catalyst; (4) operating conditions for alkylation with different olefins; (5) predicted quality and yield of alkylate obtained for various operating conditions and for different olefins; and (6) recommended operating conditions for reactivation of the catalyst. For example, does reactivation affect the longevity of the catalyst such as loss of precious metals or size of particles? What special safety precautions are needed during regeneration? Both hydrogen and oxygen have safety concerns. Possible Improved Processes Using Sulfuric Acid Several approaches on how processes using sulfuric acid can be improved need investigation. First, reduced acid consumption seems possible. In the past several years, acid consumption has been reduced in several refineries from about 0.5-0.6 lb/gal to as little as 0.3 lb/gal. Stratco21,22 has patented techniques to feed different olefins to different reactors. The optimum operating conditions differ for different olefins. In addition, techniques to reduce impurities in the feedstream help. In the alkylation reactor, both conjunct polymers and water are produced during alkylation, and they build up in the acid phase. The question that is yet to be completely answered is which of the two is the more deleterious. As already mentioned, conjunct polymers at small to moderate levels have beneficial effects. Several refineries specify feed acids having 99.5%
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acidity. Other refineries employing feed acids have as little as 98.5% acidity. Albright et al.23 found that good-quality alkylates were produced with sulfuric acid having acidities down to 85% when the water content of the acid was maintained at
