Ind. Eng. Chem. Res. 2003, 42, 4283-4289
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Alkylation of Isobutane with C3-C5 Olefins To Produce High-Quality Gasolines: Physicochemical Sequence of Events Lyle F. Albright* School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47907
Alkylation of isobutane with C3-C5 olefins currently produces alkylates that are the highestquality and cleanest-burning gasolines. Four processes are widely used; two employ sulfuric acid as the catalyst, whereas the other two employ liquid HF. Processes that employ solid catalysts have yet to be commercialized. This paper comprises a review of recent findings concerning the fundamentals (chemistry of alkylation plus side reactions, mass- and heat-transfer steps in the acid/hydrocarbon dispersions, characteristics of dispersion, etc.), i.e., the physicochemical events. A sequence of over 30 chemical, physical, and transfer steps is outlined and explained, for probably the first time. Better understanding of the fundamentals strongly suggests that much improved processes can be developed. Introduction Essentially the same alkylation processes have been used for over 30 years in petroleum refineries to produce the gasolines having the highest quality. In the past 15 years, a much better understanding has been obtained of all of the physicochemical phenomena that occur: the chemistry and especially undesired side reactions and physical and transfer steps in the liquid-liquid dispersions in the reactor, decanters, and hydrocarbon washers. The alkylate product obtained with sulfuric acid as a catalyst is produced at, or at least close to, the interfaces in acid/hydrocarbon dispersions.1,2 The alkylate with systems using HF is probably also produced at or near the interfaces. Larger interfacial areas result in increased rates of alkylation plus higherquality alkylates when C4 olefins and sulfuric acid are used.3 Dispersions obtained with HF tend to have much larger interfacial areas as compared to those containing sulfuric acid, but the former produce more undesired byproducts.4 Much effort has been made in the past 20-30 years to develop alkylation processes using solid catalysts, which would minimize safety problems. Although claims have been made that specific processes are ready to be commercialized, operational problems still seem to be unsolved. In this work, commercial processes and those using solid catalysts are discussed and compared. For processes using sulfuric acid as the catalyst, recent physicochemical information suggests methods for reducing operating costs, producing higher-quality alkylates, and lowering feedstock requirements. Chemistry of Alkylation The chemistry reported here is divided into the following categories: production of alkylate via four mechanisms, roles of isoalkyl sulfates and fluorides as intermediates, and undesired side reactions. Production of Alkylates. The desired alkylates are mixtures of mainly C5-C16 isoparaffins produced by four chemical mechanisms.4-7 Mechanism 1 is the only true alkylate sequence. Overall, 1 mol of isobutane reacts * E-mail:
[email protected].
with a single mole of olefin; a rather large number of chemical steps actually occur that are generally not the chain mechanism often indicated in the literature. With propylene, C4 olefins, and C5 olefins, the products are C7 isoparaffins (generally dimethylpentanes), C8 isoparaffins [generally trimethylpentanes (TMPs) or dimethylhexanes (DMHs)], and C9 isoparaffins (often either trimethylhexanes or dimethylheptanes), respectively. Mechanism 2 produces the entire range of C5 through about C16 isoparaffins. According to this route, C10-C20 cations are first formed by polymerization-type reactions followed by fragmentation steps to produce C4-C16 cations; H- transfer then produces the isoparaffins. Mechanism 3 is when C10-C16 cations are produced and then converted to C10-C16 isoparaffins as a result of H- transfer. Isobutane and olefins react on essentially a 1:1 molar basis when C4 olefins are employed, and mechanisms 1-3 are grouped together. Mechanism 4, also referred to as the self-alkylation of isobutane, is overall the reaction of 2 mol of isobutane with 1 mol of olefin to form alkylate plus light nparaffins or isoparaffins. With propylene, propane is formed. With C4 olefins, n-butane or isobutane is produced; the formation of isobutane is not easily detected because of the large excess of isobutane in the reaction mixture. With C5 olefins, n-pentane and increased amounts of isopentane are produced. These n-paraffins are undesired in the gasoline pool. Mechanism 4 is generally of considerable importance when HF is used as the catalyst, but it is of no or little importance with sulfuric acid when propylene and C4 olefins are used. Because of mechanism 4, often 6-8% more isobutane is consumed to produce the same amount of alkylate when HF is the catalyst.8 Questions still exist concerning the chemistry of alkylations using C5 olefins in regard to the relative importance of the four alkylation mechanisms. The differences in compositions of alkylates produced with different C5 olefins have not yet been well explained. Problem areas are next discussed, employing data for alkylations using sulfuric acid and C5 olefins.7 (1) The relative importance of mechanism 1 for various C5 olefins is as follows: 2-pentenes > 1-pentene . 2-methylbutene-2 (2MB2). The much different results between 2-pentenes and 1-pentene indicate that isomer-
10.1021/ie0303294 CCC: $25.00 © 2003 American Chemical Society Published on Web 08/12/2003
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ization of 1-pentene to 2-pentenes is incomplete; yet 1-butene isomerizes rapidly and almost completely to 2-butenes before alkylation, as discussed later. The production of n-pentane for alkylations with n-pentenes indicates that at least a small fraction of the n-pentenes form alkylates via mechanism 4. Mechanism 2 is obviously of major importance, especially when 2MB2 is used. (2) All C5 olefins tested produced isopentane in relatively large amounts; the differences in amounts are small but in the following order: 2MB2 > 2-pentenes > 1-pentene. Yet if mechanism 4 were of major importance, much larger amounts of isopentane would be produced with 2MB2. No detectable n-pentane was produced when 2MB2 was used. (3) Using the compositions of alkylates reported by Albright and Kranz,7 carbon and hydrogen balance calculations indicate that a considerably greater number of moles of isobutane than of C5 olefins react; molar ratios of reactants were about 1.25 or more. Relatively little C11 and heavier isoparaffins were produced. Mechanism 2 explains the results for 2MB2 better than mechanism 4; fragmentation of the heavy cations apparently resulted in considerable i-C5+’s that then went on to form isopentane. Less information is known for alkylations using both HF and C5 olefins, except that even higher amounts of isopentane are produced. Mechanism 4 is probably of major importance.9 Some HF users deliberately promote mechanism 4 reactions with propylene and 1-butene to obtain higher-quality alkylates that contain more TMPs. TMPs are a family of four C8 isomers that have the highest quality, with research octane numbers (RONs) varying from 100 to 109.6.10 Dimethylhexanes (DMHs), also C8 isomers, are less branched and have much lower RON values, varying from 55.5 to 76.3. Dimethylpentanes, produced in large amounts from propylene, and trimethylhexanes, often produced in large amounts with C5 olefins, have RON values that average about 88 and 90, respectively. Roles of Isoalkyl Sulfates and Isoakyl Fluorides. Isoalkyl sulfates produce both alkylate and byproducts. Isoalkyl fluorides likely have similar roles. The importance of these compounds was not realized by many investigators until perhaps 30 years ago; yet since then, other investigators have essentially ignored them. At one time, isobutane and the olefin were thought to react as a chain reaction by a two-step reaction sequence.11 Yet when feed mixtures of isobutane and olefin are put into contact with with sulfuric acid, the olefin initially reacts more rapidly12sso obviously, a simple chain reaction is, at most, only partly true. Instead, isoalkyl sulfates are rapidly produced from n-olefins. Also C10 and heavier cations are produced, especially from isoolefins,5,13,14 as intermediates in both mechanisms 2 and 3. Both isoalkyl acid sulfates, soluble in sulfuric acid, and di-isoalkyl sulfates, soluble in the liquid hydrocarbon layer, can be produced when normal olefins and sulfuric acid contact each other.13 Temperatures employed can vary from at least -20 to 30 °C. Higher yields of sulfate esters are produced at lower temperatures; hence byproducts, including conjunct polymers, are then formed to only a minor extent. The molar ratio of sulfuric acid to olefin determines whether mono-isoalkyl or di-isoalkyl sulfates are formed in the laboratory.
Isobutane is nonreactive in these experiments when limited amounts of sulfuric acid are employed. For commercial alkylation processes using sulfuric acid,4,15,16 some di-isoalkyl sulfates are always dissolved in the hydrocarbon product streams, which are mixtures of mainly unreacted isobutane and alkylate. Monoisoalkyl acid sulfates are, however, in the acid phase leaving the reactor. Olefins investigated included propylene, which produces isopropyl sulfates; 1-butene and 2-butenes, which produce sec-butyl (i.e. 2-butyl) sulfates; and 1-pentene and 2-pentenes, which form both 2-pentyl and 3-pentyl sulfates. Mono-isoalkyl acid sulfates are obviously formed first. They then react with more olefins, producing di-isoalkyl sulfates. These reactions occur at or at least close to the interfaces of the hydrocarbon/acid dispersions. Di-sec-butyl sulfate (produced using n-butenes) has been recovered by evaporation of isobutane from liquid mixtures of isobutane and di-sec-butyl sulfate. An oily liquid was recovered that is quite stable at room temperature. Attempts have been made with isobutylene to produce sulfates, which would presumably be tert-butyl sulfates.13 There is no evidence that such sulfates have ever been produced at operating conditions similar to those used in alkylation units. During alkylation, the initial reactions with isobutylene result in the rapid formation of oligomers (heavy olefins) and C12-C20 cations. The formation of sulfates explains how n-olefins react during the initial stages of alkylation. It also explains why different n-olefins require drastically different operating conditions for alkylation. Isopropyl sulfates are much more stable than sec-butyl sulfates and especially sec-pentyl sulfates. Higher temperatures, stronger acids, and/or longer residence times are needed when propylene is employed as compared to n-butenes and n-pentenes. Ethyl sulfates (and ethyl fluoride) are so stable that sulfuric acid (and HF) are unsuitable for alkylations with ethylene. Fluorides have been produced at high yields in the laboratory when liquid n-olefins were put into contact with liquid HF. During commercial alkylations, a much lower fraction of the olefins react to form fluorides than during processes using sulfuric acid, as explained later. Both sulfates and fluorides react in several ways, as discussed next. Formation of Alkylates from Sulfates and Fluorides. Albright, Spalding et al.,6,13 and Albright et al.14,17 investigated the production of both sec-butyl acid sulfate and di-sec-butyl sulfate and subsequent production of alkylates. With an excess of isobutane and with sufficient sulfuric acid, alkylates with RONs of 99-100 can be produced at temperatures of about -20 to 0 °C. The following reactions occur: First, di-sec-butyl acid sulfate (when used) is converted to sec-butyl acid sulfate, which dissolves in the acid phase. Second, this latter sulfate decomposes to release sulfuric acid and mainly 2-butenes (which are thermodynamically the more stable nbutenes). Third, the 2-butenes produced react with t-C4H9+’s to form almost exclusively TMP+’s (few heavier cations are produced because the internal ratio of isobutane/olefin is very high). Fourth, H-’s convert the TMP+’s to TMPs. These H-’s are transferred mainly from conjunct polymers; of course, H-’s are supplied to the conjunct polymers from isobutane. The alkylates produced sometimes contain 85-90% TMPs. Very high quality alkylates are also produced when sec-butyl fluoride reacts with isobutane. Modifying processes
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using HF to produce high levels of isoalkyl fluorides might significantly minimize the possibilities of forming aerosol clouds of HF; such clouds are currently a major safety concern. Isomerization of n-Olefins. When sulfuric acid is used as the catalyst, most of the 1-butene is isomerized before alkylation to 2-butenes. Less isomerization occurs with HF.11 It has been proposed that 1-butene (and also 2-butenes) is converted to sec-butyl cations in the presence of the strong acids and that these cations then decompose forming mainly 2-butenes, the more stable isomers. Yet why is isomerization much less pronounced with HF? sec-Butyl cations are formed in the presence of HF, as such cations are the precursors to n-butane, formed via mechanism 4. Hence, sec-butyl cations are probably not the major intermediates for isomerization of n-butenes. The following explanations are postulated instead: (1) sec-Butyl acid sulfates are the main precursors for the isomerization, and they are produced in high yields. sec-Butyl fluoride is, however, formed (with HF) to a much lesser extent. (2) The kinetics of alkylation, as will be discussed in more detail later, is much faster when HF is the catalyst rather than sulfuric acid. Because the kinetics of alkylation is relatively slow with sulfuric acid, 1-butene might have longer to form sulfate esters and, hence, isomerize to 2-butenes. Production of Conjunct Polymers and Pseudo-Alkylate. When sulfuric acid containing dissolved sec-butyl acid sulfate is allowed to warm from -20 °C to about 15-20 °C, the sulfate rather rapidly decomposes,18 forming conjunct polymers dissolved in the acid and a pseudoalkylate (a mixture of C4-C16 isoparaffins having a low RON, often in low 80’s). The pseudo-alkylate collects as a liquid layer on top of the acid. The composition of the pseudo-alkylate indicates that its H/C ratio is about 2.25. The conjunct polymers have a H/C ratio of about 1.75.19 The olefins used to produce the isoalkyl acid sulfates obviously have a H/C ratio of 2.0. Material balances of the carbon and hydrogen atoms indicate that the weight of the pseudo-alkylate is at most slightly greater than that of the conjunct polymers. Experimental data confirm this conclusion. Experimental information indicates that iso-olefins (isobutylene and isopentenes) are more apt than nolefins to form conjunct polymers.18,20 The intermediates formed in this case are mainly C10 and heavier cations. Conjunct polymers are produced mainly from olefins and to a much lesser extent from isobutane. During alkylation, three factors that substantially increase the production of conjunct polymers in sulfuric acid are as follows: smaller internal ratios of isobutane to olefins, which also promote mechanism 2; increased operating temperatures; and sulfuric acids with higher acidities, such as 96-99.5%. At lower acidities, the catalytic activity decreases, so that the stability of the sulfate esters increases, in which case, a significant fraction of the sulfates fails to react in the reactor to form alkylates. Hence, higher fractions of isoalkyl acid sulfates are present in the used acids. A portion of the acid sulfates decomposes in both the reactor and the decanter, producing conjunct polymers and pseudo-alkylate. This pseudo-alkylate mixes with the regular alkylate, reducing its quality. More decomposition probably occurs in the decanter than in the
reactor as the sulfates have less contact with isobutane in the former. Hydrocarbon layers have formed on top of used acid in storage tanks.21-23 Such layers have, on occasion, resulted in serious accidents (fires and/or explosions). Some, if not many, refinery personnel believe that these layers form because of incomplete separation of acid/ hydrocarbon dispersions in the decanters. In such cases, the hydrocarbon layers would be true alkylates with a high RON. An alternate explanation, which might often be more correct, is that acid sulfates decompose and the layers are mainly pseudo-alkylates having low RONs. Conjunct polymers contain numerous CdC double bonds and some conjugated dienes.18,19 These double bonds react with sulfuric acid (or HF) to form conjunct polymer sulfates (or conjunct polymer fluorides), which are good surfactants and accumulate at the interfaces. They are also good sources of H-’s since they contain numerous tertiary C-H bonds, and experimental evidence indicates that most H-’s that react with isoalkyl cations are supplied by the conjunct polymers. Replenishment of the H-’s in the polymers occurs from isobutane to form t-C4H9+’s. The optimal concentration of conjunct polymers in sulfuric acid is often about 5-7%. As the amounts increase to these values, both the quality and yields of the alkylate increase.24 Also the formation of conjunct polymers decreases. As the amounts increase above 5-7%, the catalytic activity of the acid decreases significantly. Hence, smaller fractions of the isoalkyl sulfates react to form alkylates. Eventuall,y the catalytic activity becomes sufficiently low that used acids need to be discarded and then regenerated. The viscosity of the acid phase increases, thereby affecting the character of the acid/hydrocarbon dispersion.25 Larger interfacial areas are formed in the dispersions as the amounts of conjunct polymers in the acid phase increase. Considerably less information about the formation of conjunct polymers has been reported for HF units. Isoalkyl fluorides and heavy isoalkyl cations are likely precursors. Because of the higher temperatures employed in reactors using HF, possibly even higher fractions of the olefins and heavier cations react to form conjunct polymers and pseudo-alkylate. In one reactor of Phillips Petroleum Co., the HF phase contained 8-12% conjunct polymers and 82-85% HF.26 The unaccounted material is thought to be mainly HF that reacted to form conjunct polymers fluorides. Degradation of Alkylates. Sulfuric acid acts as an oxidizing agent for both isoparaffins and conjunct polymers, even at temperatures as low as 10 °C. It is an especially good oxidizing agent for all four TMPs and other isoparaffins having tertiary C-H bonds.2,27 2, 3, 4-TMP, which has three tertiary C-H bonds, is the most easily oxidized TMP. During oxidation, SO2 and water are produced as the sulfuric acid reacts; a mixture of C4 to at least C16 isoparaffins is also formed. Both the quality and quantity of the alkylate are reduced, with experimental results indicating reductions up to perhaps 1% on occasion. In addition to isobutane, isopentane and other isoparaffins containing tertiary C-H bonds can be alkylated. TMPs are included, and extensive reactions occur when TMPs and olefins come into contact with each other. Intermediate cations such as C12-C20 cations form; reactions of mechanisms 2 and 3 then produce alkylates of much reduced quality. Such contact between
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TMPs and olefins occurs in current reactors, especially those using sulfuric acid. In such reactors, the dispersions are recirculated numerous times, and residence times are generally in the 15-30 min range. Such oxidation and undesired alkylation reactions can be reduced by first, increasing the kinetics of alkylation; second, removing the acid/hydrocarbon dispersions from the reactor as soon as alkylation is complete; and third, separating the dispersions rapidly. Oxidation of Conjunct Polymers. Oxidation reactions also occur in used sulfuric acid.28 Some hydrogen atoms on the conjunct polymer are oxidized. The H/C ratio of the polymers decreases as SO2 and water are formed. The rates of these reactions increase with higher acid strengths and higher temperatures. Tank cars filled with used acids have, on occasion and especially during the summertime, experienced considerable sulfur dioxide evolution, with the result that the conjunct polymer has sometimes been converted to a solid, carbon-rich deposit in the bottom of the tank car. Physical Phenomena during Alkylation The following physical factors all have important effects in the overall alkylation process: the characteristics of the liquid-liquid dispersion; the transfer of reactants, intermediates, and products in the dispersion; the kinetics of producing the dispersion; and the separation of the dispersion. Acid/Hydrocarbon Dispersions. In current commercial alkylation processes, the dispersions are acidcontinuous with dispersed hydrocarbon droplets. Sulfuric acid/hydrocarbon dispersions generally have volumetric ratios ranging from about 45:55 to 60:40. Hydrocarbon-continuous dispersions tend to occur at lower ratios. Albright and Eckert29 have reported, however, a method of starting up an agitated vessel to obtain acidcontinuous dispersions at ratios as low as 20:80. Purdue investigations29 performed using a small cylindrical glass vessel equipped with a mechanical agitator suggest that a period of several seconds is often required to obtain equilibrium conditions in sulfuric acid/hydrocarbon dispersions. On the basis of experimental work performed at equilibrium conditions, the following generalizations can be made: (1) Interfacial areas of acid-continuous dispersions are much higher than those of hydrocarbon-continuous dispersion for identical agitator speeds.25 Mosby and Albright30 found that alkylates produced in acidcontinuous dispersions using C4 olefins had higher qualities, often by 2-3 RON units. (2) Increased agitation in acid-continuous dispersions using C4 olefins results in higher-quality alkylates.3 Using 2-butenes, the RON values of the alkylates increased from 86.2 to 98.7 as the agitator speed increased from 1000 to 4000 rpm. Increased agitation obviously results in larger interfacial areas (with increased numbers of droplets having much smaller diameters). (3) Increased levels of agitation also result in higher rates of alkylation. Li et al.3 found in one case that the optimal residence times in the reactor were about 5 min. In industrial alkylation units, residence times are in the 20-30 min range with presumably lower levels of agitation. (4) The acid/hydrocarbon volumetric ratio has a large effect on the interfacial area between the two phases.25 In one test, the interfacial area in the mixing vessel
increased by a factor of about 2 as the ratio increased from 50:50 to 70:30. Obviously, the average size of the droplets decreases considerably at the higher ratio, as the number of droplets increases. (5) The temperature of the dispersion affects the interfacial areas in the range from -3.5 to 19 °C.25 In one test, the lowest area occurred at 15 °C. Temperature has large effects on the liquid viscosities and especially those of sulfuric acid. Temperature also affects the interfacial surface tensions and, to a relatively small extent, the densities of the two liquids. (6) Larger interfacial areas result as the amounts of conjunct polymers in the acid increase;25 in one test, the area increased by a factor of 3. The conjunct polymers obviously act as surfactants. They also increase the viscosity of the acid phase. Coalescence and Fragmentation of Droplets. The start-up period in which hydrocarbon droplets are formed has been investigated only to a limited extent. At equilibrium conditions, the rates of coalescence and fragmentation become equal. During gravity separations, as employed in industrial units, coalescence obviously predominates. Albright and Eckert29 found in a laboratory unit that acid-continuous dispersions thought to have interfacial areas similar to those in industrial units often separated in 2-20 min. Dispersions being separated sometimes develop stable froths that separate even more slowly. Hydrocarbon droplets move upward rather slowly to the surface through the acid phase, which has a rather high viscosity, often 40-70 cP. Some commercial separators (decanters) often provide 45 min for separation. Gravity separation of hydrocarbon-continuous dispersions, however, tends to be much quicker. In these cases, the acid droplets sink relatively rapidly through the low-viscosity hydrocarbons. In alkylation reactors, droplets in the dispersions repeatedly coalesce and later fragment as agitation continues, as droplets move closer to the agitator and then away, and as the hydrocarbon feedstocks are added to the reactor. Droplets of different compositions and sizes are obviously present in the reactor. Assuming that the isobutane and olefin feeds are premixed, the following hydrocarbons are present in different droplets: first, premixed feed; second, the product mixture of unreacted isobutane and alkylate; and third, hydrocarbons with a wide range of intermediate compositions. In cascade reactors using sulfuric acid as the catalyst, part of the isobutane is evaporated from at least some droplets located presumably at the upper levels of the dispersion. Such vaporization removes the heats of reaction and maintains the desired temperatures. It also reduces the isobutane concentrations in the droplets. In some cascade reactors, relatively pure isobutane and olefins are both added as feed. Hence, droplets of relatively pure isobutane and pure olefin would initially form. As a result, hydrocarbon droplets having a wide range of compositions often occur. In alkylation units using sulfuric acid, the product hydrocarbon phase from the alkylation reactors always contains a rather small amount of di-isoalkyl sulfates. These sulfates need to be removed from the hydrocarbon phase before entering the distillation unit; otherwise, they would decompose in the distillation unit, thus releasing sulfuric acid. To remove the sulfates, the hydrocarbon mixture is usually sent to an agitated unit where the liquid hydrocarbons are put into contact with
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either sulfuric acid or dilute caustic. A liquid-liquid dispersion is formed, and the sulfates transfer to the liquid-liquid interface, where they react. When caustic is used, the sulfates are converted to salts and are discarded. When sulfuric acid is used, they are converted in iso-alkyl acid sulfates, which can then be used to produce alkylate. These extraction (or wash) units need to provide adequate interfacial areas and contact times to remove all of the sulfates. Heat Transfer. The overall alkylation reactions are highly exothermic. The olefin reactions to form sulfates or fluorides plus heavier olefins and cations contribute greatly to the overall exothermicites. Agitation or pumping energy is also added to all commercial reactors to produce the desired dispersions. Two of the four commercial reactors provide heat-transfer surfaces to maintain essentially constant temperatures.15,16 In the reactor designed by Phillips Petroleum Co., a temperature rise occurs, as this reactor provides little or no heat transfer. In cascade units, partial vaporization of the light hydrocarbons and especially isobutane is employed to maintain rather uniform temperatures. Such a technique, however, increases the range of hydrocarbon compositions in the dispersed droplets. STRATCO, Inc., has twice modified the heat-transfer coils in its reactors. Several years ago, it provided coils with larger heat-transfer areas.31 Recently,32 STRATCO developed a technique to obtain more uniform flow of coolant (a cold mixture of unreacted isobutane and alkylate) through each of the numerous coils. Hence, the overall heat-transfer coefficients are increased. The increased heat transfer could be used to advantage in at least three ways: to increase production capacity, to operate at lower temperatures, and/or to operate at higher levels of agitation so that larger interfacial areas in the dispersions could be obtained. Sequence of Physicochemical Steps As already indicated, many steps, or events, occur in the reactor system during alkylation. At least 30 physicochemical events have a significant effect on the overall process for alkylations using sulfuric acid as the catalyst. Thirty-two events are outlined below that occur in cascade reactors using sulfuric acid, plus also in the accompanying decanter and wash system for hydrocarbons. (1) In the dispersed droplets in the acid/hydrocarbon dispersion, isobutane (event 1) and olefin (event 2) transfer to the interface between the two immiscible liquids. The kinetics of transfers obviously depend on the size and shape of the droplets, the flow patterns in the droplets, the temperature, and the specific hydrocarbon being transferred. Essentially no chemical reactions occur in the interior of the droplets. (2) At, or at least near, the outer surface of the droplet, the following reactions occur. (a) tert-Butyl cations form in mainly two ways: (i) Isobutane is converted to a tert-butyl cation (event 3) when a hydride ion (H-) is transferred from isobutane to one of the many cations known to occur in conjunct polymers. (ii) Isobutylene reacts with a proton from the acid (event 4). (b) A rather high fraction of n-olefins react with sulfuric acid, first forming isoalkyl acid sulfates (event 5) and then di-isolkyl sulfates (event 6). These reactions are reversible (events 7 and 8). Alternatively the
sulfates, when present as sec-butyl or sec-pentyl sulfates, can produce an isomerized n-olefin (event 9). For example, 1-butene or 1-pentene can be isomerized to 2-butenes or 2-pentenes, respectively. Structural isomerization is generally of no importance. (c) Many iso-olefins react to form C10-C20 cations (event 10). As a rule, a much smaller faction of the n-olefins react to form the heavy cations (event 11). All C3-C5 olefins form heavy cations to at least a limited extent. (d) Alkylates are produced by mechanisms 1-4 (events 12-15). The chemistry of all of these mechanisms involves several chemical steps. The operating conditions and the specific olefins employed have a major effect on the relative importance of the four mechanisms. With sulfuric acid, mechanism 4 (event 15) is of no importance, except to a limited extent with C5 olefins. (3) The alkylate formed and present at the acid/ hydrocarbon interface experiences the following events: (a) Most of alkylate transfers into the hydrocarbon droplets (event 16). (b) A rather small fraction reacts by a series of decomposition steps involving sulfuric acid (event 17). TMPs are particularly susceptible to such attack, resulting in the production of a mixture of C4-C16 isoparaffins. (c) Numerous isoparaffins in the alkylate contain tertiary C-H bonds from which a hydride ion can be removed. In essence, these isoparaffins can be alkylated with C3-C5 olefins (event 18). All TMPs can degrade in this way. (4) The exothermic heats of reaction transfer to the acid phase (event 19). (5) Isoalkyl sulfates produced at the interfaces experience the following events: (a) Most isoalkyl acid sulfates transfer into the acid phase (event 20). With sufficient time, some transfer back to the interface (event 21) and can be converted to alkylate or di-isoalkyl sulfate. (b) Most di-isoalkyl sulfates transfer into the hydrocarbon phase (event 22). With sufficient time, some transfer back to the interface (event 23). At the interface, these sulfates can be converted to alkylate or isoalkyl acid sulfate. (c) Some isoalkyl acid sulfates dissolved in the acid phase decompose in the reactor, forming conjunct polymers and pseudo-alkylate (event 24). Some decompositions also occur in the decanter (event 25) employed to separate the dispersions. Some decompositions also occur in storage tanks for the used acid. 6) Heavy cations, such as C10-C20+’s, react to form conjunct polymers and pseudo-alkylate (event 26). Isobutylene and isopentenes are most effective in producing these heavy cations. (7) The following physical steps (or events) occur in the acid/hydrocarbon dispersions: (a) Hydrocarbon droplets are produced when hydrocarbon feeds are introduced or jetted into the acid phase (event 27). Different alkylation processes employ quite different methods of introducing the feeds. (b) Numerous hydrocarbon droplets fragment (event 28) and later coalesce (event 29), often numerous times. As already discussed, such a sequence provides some benefits and some detrimental effects in current reactors. Currently, a wide range of droplet sizes and compositions result, which produces alkylates with a wide range of compositions.
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(c) In cascade reactors, part of the hydrocarbon liquid is vaporized (event 30). Such vaporization might promote partial separation of the hydrocarbon droplets from the acid phase. (d) Partial separation of the dispersion (event 31) followed by redispersion (event 32) has been detected in different locations in a given stage of a cascade reactor.33 In a STRATCO reactor, a higher acid/ hydrocarbon ratio was detected in the bottom portion of a unit several years ago. When partial separation of the phases occurs in the reactor, there is an undesired decrease of the interfacial areas. Obviously, not all of these events are of the same importance in the overall alkylation process, but all affect the production of both alkylate and byproducts in at least some cases. Use of Solid Catalysts Numerous investigations have been made using porous solid catalysts for the alkylation of isobutane with light olefins. Typically, the catalysts deactivate rapidly, often within hours or even minutes.34 Furthermore, only small amounts of C9 and heavier isoparaffins are detected in the alkylate. Material balances of carbon and hydrogen atoms made using the composition of the alkylate product indicate that the alkylate yields are low; hence, yields of the byproducts are high. These results can be explained as follows: (1) Light feed hydrocarbons diffuse into the catalyst pores relatively slowly,35,36 but the heavier isoparaffin diffuse much more slowly. Probably some heavy product molecules never escape from the pores. 2) Isoparaffins and especially heavier ones are strongly adsorbed on the catalyst surfaces. When higher temperatures are employed to facilitate desorption, decomposition reactions are promoted. Deactivating species plus conjunct polymers are then produced. It is recommended that a catalyst under consideration should first be tested for diffusion of hydrocarbons in the pores and for adsorption and desorption. Conclusions Both chemical and physical steps are of major importance for alkylations of isobutane regardless of the catalyst used. For alkylations in a cascade reactor using sulfuric acid as the catalyst, 32 events (or steps) have been identified. Improved processes using sulfuric acid will result when the following developments are realized: (1) Hydrocarbon droplets are smaller and have more uniform sizes and compositions. (2) Reactors are developed that provide increased interfacial areas between the acid and hydrocarbon phases, more uniform residence times of both phases, and also better controlled operating conditions. (3) Improved methods of adding the olefin feeds are developed. Different olefins have significantly different optimal operating conditions. Consideration of the sequence of events (or steps) identified earlier will likely provide guidance in developing an improved process. The number of events differs somewhat with different reactors and catalysts. Current alkylation processes using HF apparently meet relatively well the first two criteria reported above. Processes using sulfuric acid fail significantly however, although improvements can likely be developed. Select
alkylation plants using sulfuric acid have demonstrated major improvements by using multiple reactors for different olefin feeds. Further improvements appear possible. Literature Cited (1) Kramer, G. M. Alkylation Studies. In Industrial and Laboratory Alkylations; Albright, L. F., Goldsby, A. R., Eds.; ACS Symposium Series 55, American Chemical Society: Washington, DC, 1977; Chapter 1, p 1. (2) Doshi, B.; Albright, L. F. Degradation and Isomerization Reactions Occurring During the Alkylation of Isobutane and Light Olefins. Ind. Eng. Chem. Process Des. Dev. 1976, 15, 53. (3) Li, K. W.; Eckert, R. E.; Albright, L. F. Alkylation of Isobutane with Light Olefins Using Sulfuric Acid: Operating Variables Affecting Physical Phenomena Only. Ind. Eng. Chem. Process Des. Dev. 1970, 9, 434. (4) Albright, L. F.; am Ende, D. J. AlkylationsHomogeneous. In Encyclopedia of Catalysis; Horvath, I. T., Ed.; John Wiley and Sons: New York, 2003; Vol. 1, pp 191-210. (5) Albright, L. F. Updating Alkylate Gasoline Technology. CHEMTECH 1998, 26 (6), 40. (6) Albright, L. F.; Spalding, M. A.; Faunce, J.; Eckert, R. E. Alkylation of Isobutane with C4 Olefins. III. Two-Step Process Using Sulfuric Acid as Catalyst. Ind. Eng. Chem. Res. 1988, 27, 391. (7) Albright, L. F.; Kranz, K. Alkylation of Isobutane with Pentenes Using Sulfuric Acid as a Catalyst: Chemistry and Reaction Mechanism. Ind. Eng. Chem. Res. 1992, 31, 475. (8) Albright, L. F. Alkylation of Isobutane with C3-C5 Olefins: Comparison of Feedstock Consumption, Acid Usage, and Alkylate Quality for Different Processes. Ind. Eng. Chem. Res. 2002, 41, 5627. (9) Abbott, R. G.; Randolph, B. B. Control of Synthetic Isopentane Production During Alkylation of Amylenes. U.S. Patent 5,382,747, Jan 17, 1995. (10) Albright, L. F.; Eckert, R. E. New Equations Help Rapidly Determine Alkylate Octane Numbers. Oil Gas J. Jan. 18, 1999; pp. 51-54. (11) Iverson, J. O.; Schmerling, L. Adv. Pet. Chem. Refin. 1958, 1, 337. (12) Shlegeris, R. J.; Albright, L. F. Alkylation of Isobutane with Various Olefins in the Presence of Sulfuric Acid. Ind. Eng. Chem. Process Des. Dev. 1969, 8, 92. (13) Albright, L. F.; Spalding, M. A.; Nowinski, J. A.; Ybarra, R. M.; Eckert, R. E. Alkylation of Isobutane with C4 Olefins. I. First-Step Reactions Using Sulfuric Acid Catalyst. Ind. Eng. Chem. Res. 1988, 27, 381. (14) Albright, L. F.; Doshi, B. M.; Ferman, M. A.; Ewo, A. TwoStep Alkylation of Isobutane with C4 Olefins: Reactions of C4 Olefins with Sulfuric Acid. In Industrial and Laboratory Alkylations, ACS Symposium Series 55; Albright, L. F., Goldsby, A. R., Eds.; American Chemical Society: Washington, DC, 1977; Chapter 6, p 96. (15) Albright, L. F. Improving Alkylate Gasoline Technology. CHEMTECH 1998, 29 (7), 46. (16) Albright, L. F. AlkylationsIndustrial. In Encyclopedia of Catalysis; Horvath, I. T., Ed.; John Wiley and Sons: New York, 2003; Vol. 1, pp 191-210. (17) Albright, L. F.; Doshi, B. M.; Ferman, M. A.; Ewo, A. TwoStep Alkylation of Isobutane with C4 Olefins: Reaction of Isobutane with Initial Reaction Products. In Industrial and Laboratory Alkylations; ACS Symposium Series 55, Albright, L. F., Goldsby, A. R., Eds.; American Chemical Society: Washington, DC, 1977; Chapter 7, pp 109-127. (18) Albright, L. F.; Spalding, M. A.; Kopser, C. G.; Eckert, R. E. Alkylation of Isobutane with C4 Olefins. II. Production and Characterization of Conjunct Polymers. Ind. Eng. Chem. Res. 1988, 27, 386. (19) Miron, S.; Lee, R. J. Molecular Structure of Conjunct Polymers. J. Chem. Eng. Data 1963, 8, 150. (20) Li, K. W.; Eckert, R. E.; Albright, L. F. Alkylation of Isobutane with Light Olefins using Sulfuric Acid: Operating Variables Affecting Both Chemical and Physical Phenomena. Ind. Eng. Chem. Des. Dev. 1970, 9, 441.
Ind. Eng. Chem. Res., Vol. 42, No. 19, 2003 4289 (21) Albright, L. F.; Lang, E. G. Sulfuric Acid Recovery from Alkylation Units and Other Processes. In Encyclopedia of Chemical Processing and Design; McKetta, J. J., Ed.; Marcel Dekker: New York, 1996; Vol. 55, pp 456-468. (22) Kalas, D. Acid Storage Tank Blows at Martinez, CA, Refinery. Contra Costa Times Oct 10, 1993. (23) Findings and Recommendations of Motiva Refinery Sulfuric Acid Tank Farm Disaster; U.S. Chemical Safety and Hazards Investigation Board: Washington, DC, Aug 28, 2002. (24) Albright, L. F.; Houle, L.; Smutka, A. M.; Eckert, R. E. Alkylation of Isobutane with Butenes: Effect of Sulfuric Acid Compositions. Ind. Eng. Chem. Process Des. Dev. 1972, 11, 446. (25) am Ende, D. J.; Eckert, R. E.; Albright, L. F. Interfacial Area of Dispersions of Sulfuric Acid and Hydrocarbons. Ind. Eng. Chem. Res. 1995, 34, 4343. (26) Eastman, A. D.; Randolph, R. B.; Moore, W. P.; Heald, R. L. Online Monitoring of HF. Hydrocarbon Process. 2001, 80 (8), 95-100. (27) am Ende, D. J.; Albright, L. F. Degradation and Isomerization of Isoparaffins While in Contact with Sulfuric Acid in Alkylation Units: Chemistry and Reaction Kinetics. Ind. Eng. Chem. Res. 1994, 33, 840. (28) Sung, S.; Szechy, G.; Albright, L. F. Decomposition of Spent Alkylation Sulfuric Acid to Produce Sulfur Dioxide and Water. Ind. Eng. Chem. Res. 1993, 32, 2490. (29) Albright, L. F.; Eckert, R. E. Formation and Separation of Sulfuric Acid/n-Heptane Dispersions: Application to Alkylation. Ind. Eng. Chem. Res. 2001, 40, 4032.
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Received for review April 16, 2003 Revised manuscript received June 30, 2003 Accepted July 1, 2003 IE0303294
