Alkylation Introduction Process chemistry and thermodynamics. to C 9 C 7

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1 4.3 Alkylation Introduction The alkylation reaction is the addition of an alkyl group to any hydrocarbon. In the petroleum industry, however, the term alkylation is used for the reaction of low molecular weight olefins with a light isoparaffin to form a liquid hydrocarbon. The alkylation process was commercialized during the second half of the 1930s to convert the light hydrocarbons in the Fluid Catalytic Cracking (FCC) off-gases into more useful, liquid products. During the Second World War, it experienced a tremendous growth due to the need for high-octane aviation fuel. From 1950 to 1970, the world capacity remained relatively flat due to the comparative cost of other gasoline blending components. The lead phase-down in many countries and additional environmental regulations after the 1970s increased the demand for alkylate as a blending stock for motor gasoline. In addition, the phase-out of MTBE (methyl tert-butyl ether) in some US states has further increased the need for high-octane clean streams. At the beginning of the millennium, 13% of the US gasoline marked was alkylate. In fact, alkylate is a high-octane blend-stock (RON, Research Octane Number, 93-98; MON, Motor Octane Number, 90-95) free of undesirable components such as sulphur, benzene and other aromatics. It is mostly made up of C 7 to C 9 highly branched paraffins and is produced primarily by reacting isobutane with light olefins in the presence of strong catalysts, such as hydrofluoric and sulphuric. Due to safety and corrosion problems caused by the use of liquid strong s, a number of companies have carried out research to commercialize a solid alkylation catalyst. In fact, though the process has been a reliable and safe producer of a prime highoctane gasoline for many decades, in recent years it has been the object of environmental concerns and of research and development efforts Process chemistry and thermodynamics The alkylation unit is traditionally fed with the FCC off-gases and is normally installed in refineries equipped with catalyting cracking units. The usual feedstocks are isobutane and light olefins, mostly C 3 and C 4 ; the olefins from cokers (if available) are also sent as feeds to alkylation. Table 1 shows the composition of two typical olefinic feedstocks from FCC; the alkylation process requires more isobutane from sources other than cracking. The process chemistry is extremely complex due to the large number of possible side reactions. The main product is a mixture of isoparaffins called alkylate. Table 1. Typical olefin feeds from FCC units (weight %) C 3 1-C 4 2-C 4 C 5 C 3 n-c 4 i-c 5 others C 4 cut Wide cut The symbol as superscript indicates that there is a double link. VOLUME II / REFINING AND PETROCHEMICALS 181

2 PROCESSES TO IMPROVE THE QUALITIES OF DISTILLATES Isoparaffins with tertiary carbon atoms react with the olefins. Among isoparaffins, isobutane has been commonly used, since isopentane is a valuable liquid hydrocarbon blended directly with commercial gasoline. However, gasoline reformulation has reduced the acceptable vapour pressure, and isopentane has become an interesting material for propylene alkylation (Detrick et al., 2004). Some typical reactions of the alkylation process include the following: C CH 2 CH isobutene isobutane C CH3 CH 2 CH 2,2,4-trimethylpentane The reaction proceeds via the carbocation mechanism. The initiation step (step 1) generates the carbocation (initially C 3 + or C 4 +, depending on feed composition) by protonation of the olefin. The catalytic solvents capable of transferring this proton to the olefin are strong s. The more stable tertiary butyl cation is then generated by transfer of a hydride ion (step 2). The direct formation of a cation from isobutane at roughly room temperature requires a solvent system with ity similar to or higher than H 2 (Marcilly, 2003). The most diffused commercial processes have traditionally used HF and H 2 (Table 2). The propagation reaction involves the tertiary butyl cation, which reacts with the olefin to form a larger cation (step 3), and then generates a new tertiary butyl cation and the alkylate product (step 4). This sequence is illustrated below using propylene and isobutene as an example reaction: CH 2 CH H CH CH 2 CH CH 2 1-butene CH isobutane CH HC CH 2 CH 2 CH 2 C CH 2 C 2,2-dimethylhexane CH CH 2 C CH CH 2 CH propylene isobutane CH 2 CH 2 C 2,2-dimethylpentane CH CH 2 C CH CH 2 C HC It should be recalled that 2,2,4-trimethylpentane (isoctane) is one of the two standard hydrocarbons for octane number definition, and that its ON (Octane Number) is 100. The butene isomerization to isobutene (in the alkylation feed) is an important reaction to produce high-octane hydrocarbons from feeds containing appreciable quantities of 1-butene. A number of alkylation units processing butenes have an upstream isomerization unit (Detrick et al., 2004). CH 2 CH 2 C An important side reaction of the process is the hydrogen transfer reaction, most pronounced in HF catalyzed processes fed with propylene (the symbol = as superscript indicates that there is a double link): C 3 C 3 i-c 4 i-c 8 (trimethylpentane) C 182 ENCYCLOPAEDIA OF HYDROCARBONS

3 ALKYLATION Table 2. Typical properties of fresh alkylation s (Marcilly, 2003) PROPERTY HF H 2 Molecular weight Boiling temperature ( C) Melting temperature ( C) Specific weight (d 15 4 ) Viscosity (cp) (0 C) 33 (15 C) Hammett ity (H o )* * Acidity of the industrial s during operation. The overall reaction is: C 3 H 6 2 H 10 C 3 H 8 propylene isobutane propane The octane number of trimethylpentane is sensibly higher than that of dimethylpentane normally obtained from propylene. However, in this reaction two molecules of isobutane are required to produce only one molecule of alkylate. A number of other side reactions may be involved in the process; the most common include the polymerization and cracking. The polymerization of olefins results in the production of low-octane, high boiling point components, which are undesirable. This reaction is minimized by using high isobutane/olefin ratios and choosing proper reaction conditions. The heavier polymerization products are known as Acid Soluble Oils (ASOs) or red oils and tend to deactivate the catalyst. ASOs are unsaturated compounds with more than about 10 carbon atoms per molecule that can react with H 2. Alkylation reactions are higly exothermic (on average kj/mol); the reaction equilibrium is, therefore, shifted to the alkylate formation at low temperature and high pressure. Moreover, lower temperatures minimize the formation of by-products due to polymerization and cracking reactions Process kinetics CH CH 2 The traditional alkylation reaction takes place in a medium in which the hydrocarbon drops are dispersed C trimethylpentane in a continuous phase. Being that olefins are more soluble in than the isoalkane, one may expect a high conversion to polymers; this, however, is not in agreement with industrial practice. An explanation could be that the carbocations formed by the interaction of the with the olefin, which initiate the reaction chain, are found to a larger extent at the interface between the two phases, with the carbonium ions oriented towards the hydrocarbon phase (Raseev, 2003). The isoalkane in the hydrocarbon phase can then interact with the carbonium ion. This opinion is not unanimous, but it allows the alkylation reaction to be treated as a homogeneous process where the reaction rate is proportional to the interfacial area. The rate will then increase with the degree of dispersion and, therefore, with the decrease in size of the hydrocarbon droplets. This is confirmed experimentally: in fact, the octane number and, generally, the alkylate quality increase by intensifying the mixing in the reactor (Li et al., 1970). With good mixing and the proper operating conditions, alkylation occurs almost instantaneously Catalysts and reaction conditions Strong s: HF and H 2 In order to favour the thermodynamics and to minimize the formation of by-products, the alkylation reaction is carried out at the lowest possible temperature. This is kinetically possible by using large amounts of strong catalysts; the world market has long been split between H 2 and HF (see again Table 2). The strength of the two compounds is similar when they carry traces of impurities (Marcilly, 2003). The catalysts must be used almost pure, since the alkylation reaction requires a strong ity in order to attain kinetics that are economically acceptable. In general, the HF alkylate has a higher octane number due to the hydrogen transfer reactions; however, the process economics should be analyzed while keeping in mind the higher isobutane consumption and the lower catalyst consumption when using HF. During operation, the is contaminated by and soluble organic matter, which decrease the total ity; in such conditions, the isobutane solubility is higher (e.g. 0.4% by weight in H 2 and 3.6% by weight in HF; Marcilly, 2003). The process temperature depends on the type. The oxidizing properties of H 2 suggest a temperature generally less than 12 C. However, the viscosity increases rapidly by lowering the temperature, which restricts the useful temperature range between about 2-12 C (5 C being a good VOLUME II / REFINING AND PETROCHEMICALS 183

4 PROCESSES TO IMPROVE THE QUALITIES OF DISTILLATES compromise). HF is not an oxidant and thus the useful temperature can be in the range of C (normally between C), which simplifies the reactor cooling systems. The reaction pressure is fixed at a level capable of keeping the reaction media in the liquid phase. In both cases, an excess of isobutane must be used to avoid olefin polymerization; the excess isobutane is recycled to the reactor after separation of the alkylate product. The reaction medium is composed of two phases: the phase (continuous phase) and the hydrocarbon phase (dispersed phase). The reacting hydrocarbons are those solubilized into the phase. The s physical characteristics at the process temperature impose a much more effective stirring in the case of H 2.In fact, one of the key differences between HF and H 2 alkylation is the handling of the catalyst. The catalyst activity decreases with time due to dilution, ASO formation and impurity build-up. The HF can be fractionated to remove and ASO. H 2 must be removed from the unit and regenerated by completely decomposing the to SO 2 -SO 3 and condensing them back to H 2. This regeneration process can be done at the site or, usually, outside of the refinery in remote locations. For the above-mentioned reasons, the H 2 consumption is normally much higher than HF consumption, in spite of the fact that HF can form an azeotrope with (the so-called CBM, Constant Boiling Mixture), responsible for losses. Tables 3 and 4 illustrate the influence that both the type of catalyst and the olefin bear on alkylate yield and quality (Joly, 2001). As already mentioned, the impurities of the feed greatly affect yield, alkylate formation and composition, especially in the case of H 2 catalyst. For HF catalyst, consumption is normally less than 1 kg/t since the catalyst is regenerated by simple distillation. Mitigation of the risk due to use: solid catalysts Although extensive experience shows that alkylation plants, regardless of catalyst choice, can be operated safely and with low process risks; the process catalysts have been subject to critical attention in the last decades. Hydrofluoric is very volatile (boiling point: 19.5 C) and produces dangerous mists in the event of an accidental release. Refiners with sulphuric alkylation units must ship large quantities of spent offsite for regeneration, thus creating potential transportation hazards. Both concentrated s are contained in carbon steel and become very corrosive when diluted with. The refining industry has developed a number of mitigation strategies to face these problems: curtain systems, rapid dump methods, remotely operated isolation systems, etc. At the same time, catalyst producers and process licensors have developed, and in some cases commercialized, solid-phase catalysts. Several pilot-size units are operating and a number of processes have been presented (Refining processes, 2002, 2004; Meyers, 2004; D Amico et al., 2006). Solid catalysts have long been investigated; these include exchanged Table 3. Yield and octane number of the product from H 2 alkylation process Type of feed Propylene Butenes Amylenes Yield (vol C 5 /vol olefin) consumption (vol/vol olefins) Catalyst consumption (kg/t C 5 ) MON RON Table 4. Yield and octane number of the product from HF alkylation process Type of feed C 3 1-C 4 2-C 4 C 3 C 4 C 5 Yield (vol C 5 /vol olefin) consumption (vol/vol olefins) MON RON The symbol as superscript indicates that there is a double link. 184 ENCYCLOPAEDIA OF HYDROCARBONS

5 ALKYLATION zeolites, ion-exchange resins such as Amberlyst, perfluoropolymers with sulphonic groups along its backbone (Nafion), super solids (chlorinated alumina, sulphated zirconia) and liquid supers immobilized on solids. Examples of solid catalysts promoted by strong s are: alumina (or zeolites)/bf 3, silica/cf 3 SO 3 H, silica/sbf 5. Most catalysts are proprietary and little information is normally given regarding their composition. Solid catalysts can improve safety and production costs, but tend to deactivate rapidly under alkylation conditions due to build up of coke and heavy compounds on the catalyst surface. Burning off the heavy hydrocarbons through high-temperature oxidation quickly destroys the catalyst activity. To solve the deactivation problem, some companies have developed new reactor types and new generation systems based on the desorption of the heavy hydrocarbons with the use of a hydrogen stream (Roeseler, 2004). Another approach proposes the use of supercritical fluids as the reaction media; as an example, supercritical CO 2 was found to be good at dissolving the heavy coke material on the catalysts surface (Subramaniam, 2001). Some companies have proposed special additives that reduce the tendency of HF to form mists. On-site sulphuric regeneration is available to eliminate the shipment of spent and regenerate ; although this technology has been available for half a century, only few refineries operate on-site regeneration Sulphuric alkylation processes Sulphuric alkylation was the first to be developed, during the decade preceding the Second World War. Essentially, the H 2 processes consist in a reaction section where an emulsion of hydrocarbons and is formed (and the reaction occurs), and in a settling section that separates and recycles the. A fractionation section separates the alkylate from the excess isobutane, which is recycled to the reactor. There are currently two major alkylation processes using H 2 as catalyst: Stratco effluent refrigeration alkylation, and ExxonMobil cascade autorefrigeration process; each uses different approaches for the design of the reaction and refrigeration sections. Stratco process The Stratco reactor is a horizontal pressure vessel containing an inner tube bundle, which acts as an exchanger to remove the heat of reaction, and a mixing impeller (Fig. 1). It operates at a pressure of about 3.5 to 5.0 bar, sufficient to keep the two phases in the liquid state. The and hydrocarbon feed come into contact and are vigorously stirred by the impeller blades. An emulsion is formed and the reaction takes place almost instantly; the contact time is very short and the side reactions are kept to a minimum. The high recycle rate of the emulsion allows an efficient control of the reaction temperature. The general scheme of the process is shown in Fig. 2. The dehydrated olefin feed is mixed with the recycle isobutane and cooled in the feed/effluent exchangers; is removed in the coalescer before entering the reactor. A portion of the emulsion in the contactor reactor is withdrawn from the discharge side of the impeller and sent to the settler, which separates the reacted hydrocarbon phase from the emulsion. The settled is returned to the suction side of the impeller. Acid is purged from the unit, usually on a continuous basis, and fresh is introduced so that the strength is kept high. The hydrocarbon phase, containing the alkylate product and the isobutene, is sent to the tube bundle in the reactor by reducing the pressure to about bar, across a back pressure control valve. At this pressure, the lighter components of the effluent stream are vaporized, reducing the temperature to below 0 C. Additional vaporization occurs in the tube bundle as the net effluent stream removes the heat of reaction (Graves, 2004). The stream from the tube bundle is sent to the suction trap/flash drum to separate the vapour and liquid phase. The liquid isobutene from the flash drum-side is directly Fig. 1. Stratco-type alkylation reactor. coolant to settler pressure relief hydrocarbon feed driver coolant drain VOLUME II / REFINING AND PETROCHEMICALS 185

6 PROCESSES TO IMPROVE THE QUALITIES OF DISTILLATES n-butane product compressor accumulator economizer propane product olefin feed isobutane feed Fig. 2. Simplified flow diagram of the H 2 Stratco process. spent fresh alkylate product recycled to the reactor, while the liquid from the suction trap-side is transferred to the effluent fractionation section after caustic washing or passing over a bauxite bed for the elimination of sulphates. Isobutane is recycled back to the reactor section. The vapour phase from the flash drum is compressed, cooled and condensed. Propane is eliminated in the depropanizer, whose bottoms are recycled to the reactor. ExxonMobil process The ExxonMobil cascade process uses the autorefrigeration system to remove the heat of reaction and to maintain the low reaction temperature (4-5 C) needed for alkylation. The reactor is a horizontal vessel containing a number of compartments with mixers in each stage to emulsify the hydrocarbon- mixture. The reaction is held at low pressure and the heat of reaction is eliminated by evaporating an isobutane stream directly fed into one end of the reactor. The is fed on the same end and moves together with the isobutane by overflowing from one compartment to the other. The olefin feed is split and added into each compartment. It is not necessary to maintain a high pressure in the reactor to prevent vaporization of light hydrocarbons: the pressure varies from about 1.5 bar in the first stage (richer in isobutane) to about 0.5 bar in the last stage. Usually, the reactor contains a settling zone at the end. A flow scheme of the process is shown in Fig. 3 (Lerner and Citarella, 1991). Olefin feed is mixed with recycle isobutane from the deisobutanizer, cooled and fed to the reactor. Water condensed at a lower temperature is removed in the coalescer. The vapours leaving the reactor are routed to the refrigerator section where they are compressed, condensed and sent to the economizer (an intermediate pressure flash drum), which reduces the power requirements of the refrigeration compressor. A slipstream of refrigerant (isobutane) is depropanized after being caustic and washed. Propane is separated overhead, while isobutane-rich bottoms are returned to the process. The reactor liquid product is sent to the settler, from where the settled is recycled back to the reactor. The hydrocarbon portion (containing alkylate, excess isobutane and n-butane) is caustic and washed to remove any components, and is sent to the deisobutanizer. Overhead from the tower is an isobutane-rich stream, which is recycled to the reactor, while the bottoms are sent to the debutanizer for the separation of the alkylate product from butane. Feed impurities and small amounts of polymerized olefins that form ASO tend to accumulate in the recycle. Therefore, a spent purge is taken from the process to remove these oils and fresh makeup is added to maintain sufficient strength. 186 ENCYCLOPAEDIA OF HYDROCARBONS

7 ALKYLATION compressor KO drum economizer fresh caustic caustic washing spent caustic washing waste depropanizer propane recycle refrigerant deisobutanizer butane coalescer waste reactor M M M M caustic washing washing debutanizer settler olefin feed plus isobutane recycle recycle fresh caustic waste alkylate make-up spent olefin feed spent caustic make-up isobutane Fig. 3. Flow diagram of the H 2 ExxonMobil process Fluoridic alkylation processes In 1994, there were 127 HF units and 92 H 2 units in the world refineries (Joly, 2001). In the Refining processes handbook (Refining processes, 2004), the declared licences for the HF units were 160, which almost doubled the declared H 2 licences. Of course not all the licensed plants were still working, but the data provide a rough idea of the diffusion of the processes. At normal design conditions, an HF alkylation unit requires a higher ratio of isobutane to olefin (I/O) than a H 2 unit. Both processes fractionate the isobutane from the reactor effluent stream and recycle it back to the reactor. Due to its higher I/O ratio, an HF alkylation unit is designed with a larger fractionation section. The low HF viscosity and better solubility of isobutane in the allow simpler reactors to be used: it is sufficient to inject the hydrocarbon feed into the phase to obtain a good emulsion. Therefore, the HF units do not have mechanical stirring devices. Water can be used to cool the reactor, given the higher reaction temperatures. The Conoco-Phillips and UOP (Universal Oil Products) technologies shared the market at the beginning of the third millennium. The Conoco-Phillips process The original Phillips process is characterized by its very simple reactor, similar to that shown in Fig. 4 (Gary and Handwerk, 1975). Essentially, it is composed of an cooler, a riser reactor and a settler. Acid circulation is by gravity differential and an circulation pump is not necessary. The residence time in the tubular reactor is in the order of half a minute. A basic flow scheme of the Phillips process is illustrated in Fig. 5. The more recent version converts VOLUME II / REFINING AND PETROCHEMICALS 187

8 PROCESSES TO IMPROVE THE QUALITIES OF DISTILLATES riser reactor hydrocarbons (20-27 C) hydrocarbon feed cooler hydrocarbon product settler accumulator cooling Fig. 4. Simplified scheme of the Phillips HF reactor. propylene, amylene, butenes and isobutane to motor fuel using ReVAP (Reduced Volatility Alkylation Process) alkylation. Both the olefin and isobutane feeds are dehydrated by passing them through a solid bed desiccant. Good dehydration is essential to minimize potential corrosion of process equipment, which results from addition of to hydrofluoric. The olefin and isobutane feeds are then mixed with hydrofluoric at a sufficient pressure to maintain all components in the liquid phase. The reaction mixture is allowed to settle into two liquid phases. The is withdrawn from the bottom of the settler and passed through a cooler to remove the reaction heat; it is then recycled and mixed with fresh feed. A slipstream of is withdrawn from the settler and fed by a pump to the rerun column to remove dissolved and polymerized hydrocarbons. The overhead product from the rerun column is mostly hydrofluoric, which is condensed and returned to the system. The bottom product from the rerun column is a mixture of ASO and HF- azeotrope. These components are separated in a settler (not shown in the flow diagram). The ASO is used for fuel and the HF- mixture is neutralized with lime or caustic. This rerun operation is added to maintain the activity of the hydrofluoric catalyst. The hydrocarbon phase removed from the top of the settler is a mixture of propane, isobutane, normal butane, and alkylate, along with small amounts of hydrofluoric. These components are separated by fractionation and the isobutane is recycled to the reactor. Propane and normal butane products are passed through caustic treaters to remove trace quantities of hydrofluoric. The design of the settler-cooler-reactor section is critical to good conversion in a hydrofluoric alkylation unit. The UOP process In the UOP HF-alkylation process, a pump provides the inlet pressure into the reactor nozzles, which allow the hydrocarbon phase to be dispersed in the continuous phase. The dried olefin and isobutene are fed at different reactor heights, while the is fed at the reactor bottom. The mixing between hydrocarbons and phases is improved by the pumping design. The reactor heat is removed by means of cooling. A simplified flow scheme of a typical C 4 HF alkylation unit is shown in Fig. 6. Similar schemes are available for the C 3 -C 4 alkylation units (Detrick et al., 2004). The combined feed enters the shell of the reactor-heat exchanger through several nozzles, which maintain an even temperature in the reactor. The effluent from the reactor flows to the settler and the is recycled to the reactor. The hydrocarbon phase (containing dissolved HF ) is charged to the iso-stripper. The alkylate and n-butane are recovered from the bottom and as a side stream, respectively. Isobutane is also dotained as a side-cut and recycled to the reactor. The overhead, mainly consisting of isobutane, propane and, is in part charged to the HF stripper. An HF alkylation unit fed with C 3 -C 4 olefins is normally equipped with a depropanizer, which may be also required with C 4 olefins, if the propane quantity is too high. A small slipstream of circulating HF is regenerated internally, to keep the purity at the requested level. The internal regeneration technique has virtually eliminated the need for an regeneration (Detrick et al., 2004). All effluent streams and process vents, sewer effluents and regeneration bottoms are treated either with KOH or alumina. The KOH is regenerated on a periodic basis by using lime. 188 ENCYCLOPAEDIA OF HYDROCARBONS

9 ALKYLATION reactor settler from depropanizer section recycle main fractionator recontactor standpipe reactor riser make-up cooling cooler tank rerun column steam eductor depropanizer feed mixer nozzle ASO stripping recycle isobutane olefin feed steam butane to KOH treater isobutane isobutane from depropanizer section total alkylate to storage Fig. 5. Typical scheme of the Phillips HF process. Risk mitigation in the HF alkylation plants In order to reduce aerosol formation in the event of an HF release, UOP proposes the Alkad process, to be used with HF alkylation technology. The Alkad process is a passive mitigation system that will reduce aerosol from any leak occurring in the unit. The alkylation reactions take place in the presence of a liquid additive, which form a long chain of associated HF molecules; in this form, HF loses its tendency to form aerosols when released to the atmosphere Processes with solid catalysts Since the 1990s, a number of companies have proposed alkylation processes based on solid catalysts, both using fixed-bed or riser reactors. At the moment, most solid catalyst processes are at the pilot stage and very few commercial units are installed. The Alkylene process The UOP Alkylene process was developed during the late 1990s, based on a liquid transport reactor (riser reactor) to promote fast and good contact between hydrocarbons and solid catalyst, with in situ regeneration capability. The liquid-phase operation minimizes abrasion problems. A novel catalyst (HAL- 100) was developed with declared good performance and long stability; it is easily regenerated without a high temperature carbon burn. A simplified flow diagram of the process is shown in Fig. 7 (Roeseler, 2004). Reactants and catalyst flow up the reactor riser at a rate of about one foot per second as the reaction occurs. The catalyst is quickly separated from the hydrocarbon at the top of the riser and falls by gravity into the reactivation zone. The catalyst flows slowly downward and it is contacted with a recycle of cold isobutane saturated with hydrogen. Heavy hydrocarbons are hydrogenated and desorbed from the catalyst. The reactivated catalyst flows down and back into the riser bottom. A catalyst slipstream is reactivated at a higher temperature in a separate vessel to completely remove the small quantities of the residual heavy hydrocarbons. Alkylate from the reactor is sent to a fractionation section similar to those of the liquid processes. Other processes The Topsoe process employs a catalyst system of the supported liquid phase type. The super VOLUME II / REFINING AND PETROCHEMICALS 189

10 PROCESSES TO IMPROVE THE QUALITIES OF DISTILLATES isostripper recycle cooling settler regenerator HF stripper recycle alumina treater olefin feed make-up reactor ASO and CBM to neutralization alkylate KOH treater n-butane KOH treater KOH treater to depropanizer Fig. 6. Typical scheme of a UOP HF-alkylation unit fed with C 4 cuts (Detrick et al., 2004). liquid is adsorbed on a porous solid support. This enables the use of simple fixed-bed reactors. However, through proper operation of the reactor system, the liquid nature of the catalyst can be used to achieve simple maintenance of catalytic activity. The AlkyClean process is licensed by ABB Global, Akzo Nobel and Fortum Oil and Gas, and uses a solid catalyst. The reactors are undergoing a mild liquid-phase regeneration using isobutane and hydrogen. The process does not produce any ASO Alkylene reactor /H 2 catalyst reactivation zone olefin feed products hot reactivation vessel isobutane recycle Fig. 7. Simplified flow diagram of the Alkylene process. H 2 light ends LPG alkylate or require post treatment of the reactor effluent or final product Hazards and corrosion problems Among refinery processes, alkylation plants are somewhat unique because they use strong aggressive s and contain large volumes of LPG. A key element of hazard management is directed at preventing the release in the first place. Extensive experience demonstrates that alkylation units can be operated safely, and with minimum process risk to employees or neighbouring communities (Scott, 1992). The plants are constructed according to standards and with materials that are specifically designed to make the plant as safe as possible. In case of liquid exposure, both s will cause serious burns. HF also has the property of penetrating the tissues and reacting with the calcium and magnesium in the body. In the presence of, HF forms an azeotrope that contains 36% HF and boils at 109 C. HF is volatile, and when spilled it forms stable aerosol clouds. Vapour and aerosol cause serious inhalation hazards (lung damage). Both concentrated s can be contained in carbon steel, but they become very corrosive when diluted 190 ENCYCLOPAEDIA OF HYDROCARBONS

11 ALKYLATION with. The aggressiveness of both s varies with its concentration, temperature, nature of contaminants and velocity relative to exposed surfaces. In H 2 alkylation, the hydrocarbons are emulsified in concentrate and reacted at low temperatures; the remains fairly concentrated, diluting to about 88%. Stainless steel (304 L or 316 L) should be preferred in areas where high velocity may be encountered, such as pumps, valves and bends (Schillmoller, 1998a). Austenitic stainless steels are resistant to anydrous HF, but are unreliable in dilute. Alloy 400 (Monel) containing Ni and Cu and minor amounts of Fe and Mn has been the standard material in alkylation plants for many years and has been used for heat exchangers, columns, reboilers and overhead condensers with concentrations between 85 and 95%. Alloy 400 is resistant to all concentrations of HF, including anhydrous, over a wide range of temperatures, in the absence of oxygen or sulphur dioxide contamination. HF contaminated with even small amounts of oxygen can cause severe pitting and cracking of steel and Monel (Schillmoller, 1998b). Special attention should be given to the welding, which can be severely corroded if not done properly. HF also attacks all materials containing silica (glass, china, etc.), asbestos and many plastics, except teflon. In addition to the usual refinery safety procedures, the operators of the area in the alkylation units must follow special ad hoc procedures Process and operating variables The main process variables that influence the yields and quality of the alkylate product are: a) quality of the feed; b) strength and composition; c) isobutene/olefin ratio; d) temperature; e) mixing and space velocity. In the case of the liquid catalysts, the properties of the two s can explain the differences between the related processes and the operating conditions. The higher viscosity and surface tension of sulphuric as opposed to hydrofluoric make it much more difficult to obtain a fine dispersion of the hydrocarbon in the phase. The higher solubility of isobutane in HF leads to a higher ratio of isobutane to olefins in the phase (especially at the interface) with respect to H 2 processes; the secondary reactions are then reduced and the quality of the alkylate is improved. Quality of the feed Impurities increase catalyst consumption. The hydrocarbon feed should be dried and desulphurized, especially in HF units. Diolefins cause heavy losses of sulphuric. The olefin type in the feed, particularly the ratio of butylene to propylene, affects the product quality and consumption: in propylene alkylation, the octane number can be five units lower and consumption almost double (Parkash, 2003). The olefin type also influences the heat of reaction, isobutane consumption and alkylate yield. Acid strength Acid composition at the equilibrium is an important parameter that influences alkylate quality. There is a minimum strength required by the process, which varies depending on the and olefin type and spent composition. At lower concentrations polymerization becomes predominant. Water lowers the catalytic activity three to five times faster than hydrocarbon diluents (Parkash, 2003). However, some is necessary to ionize the. The optimum and minimum contents for H 2 units are close to 99% and 90%, respectively. In HF processes, the best alkylate quality is produced with a content of about 2.8% (Joly, 2001). The impurities present in the feed can be absorbed or react with the catalyst, causing a decrease in strength and the need to increase the make-up. Isobutane/olefin ratio Isobutane/Olefin (I/O) ratio is the most important operating parameter: it controls alkylate yields and quality, as well as catalyst consumption. Polymerization occurs in the phase and is the most important reaction competing with the alkylation reaction. During polymerization, two or more olefin molecules react to form a polymer, which causes lower product octane and increased consumption. The solubility in the phase is much higher for olefins than for isobutane; therefore, a large excess of the latter must be maintained in the reaction zone to ensure that enough isobutane is absorbed at the interface. The usual I/O ratio ranges from 5 to 8 in H 2 units and from 10 to 15 in HF processes. Temperature The alkylation process is thermodynamically favoured by low temperatures. Reaction temperature has a greater effect in H 2 than in HF processes. Reducing the reaction temperature minimizes the polymerization rate relative to the alkylation rate, resulting in a higher octane number and lower VOLUME II / REFINING AND PETROCHEMICALS 191

12 PROCESSES TO IMPROVE THE QUALITIES OF DISTILLATES consumption. In the case of H 2, temperatures below 2-4 C are generally avoided because of the very high viscosities of the phase. Also, very low temperatures slow down settling rates and favour carryover. The temperature in the reactor depends on the olefin feed rate, which influences the reaction heat. Efficient removal of the heat from the reactor is essential for all catalytic systems. Mixing and space velocity Increasing mixing produces a better and finer dispersion of the hydrocarbon droplets in the emulsion, increasing the surface area, and thus the kinetics and product quality. In the case of liquid catalysts, the space velocity can be a measure of the olefin concentration in the phase and may be defined as follows: olefin space velocity = olefin in the reactor (m 3 /h)/ in the reactor (m 3 ) As the olefin space velocity increases, the octane number tends to decrease and the consumption increases. In general, the residence time of the reactants is not a limiting parameter because the alkylation reaction occurs almost instantaneously. References D AMICO V. et al. (2006) Consider new methods to dedottleneck clean alkylate production, «Hydrocarbon Processing», February, Detrick K.A. et al. (2004) UOP HF alkylation technology, in: Meyers R.A. (editor in chief) Handbook of petroleum refining processes, New York, McGraw-Hill, Chapter Gary J.H., Handwerk G.E. (1975) Petroleum refining. Technology and economics, New York, Marcel Dekker, 152. Graves D.C. (2004) Stratco effluent refrigerated H 2 alkylation process, in: Meyers R.A. (editor in chief) Handbook of petroleum refining processes, New York, McGraw-Hill, Chapter Joly J.F. (2001) Aliphatic alkylation, in: Leprince P. (edited by) Conversion processes, Paris, Technip, Lerner H., Citarella V.A. (1991) Improve alkylation efficiency, «Hydrocarbon Processing», November, 89. Li K.W. et al. (1970) Alkylation of isobutane with light olefins using sulfuric. Operating variables affecting physical phenomena only, «Industrial & Engineering Chemistry Process Design and Development», 9, Marcilly C. (2003) Catalyse o-basique. Application au raffinage et à la pétrochemie, Paris, Technip, 2 v.; v. I, Meyers R.A. (editor in chief) (2004) Handbook of petroleum refining processes, New York, McGraw-Hill. Parkash S. (2003) Refining processes handbook, Amsterdam, Elsevier, Raseev S. (2003) Thermal and catalytic process in petroleum refining, New York, Marcel Dekker, Refining processes handbook 2002 (2002) «Hydrocarbon Processing», November, Refining processes handbook 2004 (2004) «Hydrocarbon Processing», CD. Ritter S.K. (2001) Alkylate rising, «Chemical and Engineering News», 11, Roeseler C. (2004) UOP alkylene process for motor fuel alkylation, in: Meyers R.A. (editor in chief) Handbook of petroleum refining processes, New York, McGraw-Hill, Chapter Schillmoller C.M. (1998a) Select alloys that perform well in sulfuric, «Chemical Engineering Progress», 2, 38. Schillmoller C.M. (1998b) Select the right alloys for hydrofluoric service, «Chemical Engineering Progress», 11, Scott B. (1992) Identify alkylation hazards, «Hydrocarbon Processing», 10, 77. Subramaniam B. (2001) Enhancing the stability of porous catalysts with supercritical reaction media, «Applied Catalysis. A: General», 212, Carlo Giavarini Dipartimento di Ingegneria Chimica, dei Materiali, delle Materie Prime e Metallurgia Università degli Studi di Roma La Sapienza Roma, Italy 192 ENCYCLOPAEDIA OF HYDROCARBONS