IMPROVED AROMATIC ALOUILATION PROCESS
Description This invention relates to a process for removing impurities from an alkylation process, and also relates to an improved alkylation process resulting therefrom. In an alkylation process of aromatics, alkylated aromatics are prepared by alkylating an aromatic compound with an alkylating agent. The alkylation process is typically carried out in the presence of an acid which may be in the form of either a liquid or a solid. Examples of such acids include A1C13, BF3 / and zeolites. Zeolites are preferred in many cases because they eliminate problems associated with disposal and recovery. The particular alkylated aromatic product that is often desired is a mono-alkylated aromatic compound such as ethylbenzene or eumeno (isopropyl benzene). The poly-alkylated aromatic compounds can be formed in the process of manufacturing the mono-alkylated product, and must be either removed or converted. Advantageously, trans-alkylation is employed to convert the poly-alkylated aromatic to the desired mono-alkylated aromatic compound. For example, in a process scheme for producing ethylbenzene, the undesirable diethylbenzene produced in the alkylation step is converted to ethylbenzene in a trans-alkylation step. In this way, a trans-alkylation step is often an integrated part of a high-throughput alkylation process. The feed stream of poly-alkylated aromatics to the trans-alkylation reactor may contain impurities such as aromatic or aliphatic olefins, aromatic or aliphatic diolefins, styrene, oxygenated organic compounds, sulfur-containing compounds, nitrogen-containing compounds such as collidine, compounds oligomers such as polystyrene, and combinations thereof. Although trans-alkylation processes in the vapor phase are typically resistant to the presence of such impurities, trans-alkylation processes in the liquid phase are highly susceptible to contamination, deactivation, catalyst plugging and similar situations by virtue of contact with any of these trans-alkylation feed contaminants. Many other factors favor the trans-alkylation units in liquid phase in a global alkylation process scheme, and therefore a method and an apparatus for effectively removing such contamination would be desirable. Many methods and materials have been proposed for the removal of contaminants from hydrocarbon streams. U.S. Patent No. 2,778,863 discloses a multi-step clay treatment process for aromatic-containing streams to overcome the problems of clay failure caused by diolefins in other clay treatment processes. Clays such as bentonite or synthetic alumina and / or silica-containing materials are disclosed in U.S. Patent No. 3,835,037, for use in a low temperature process for oligomerization / polymerization of olefinic color-forming impurities in a stream of aromatics such as a fraction of naphtha. A process using a silica-alumina disintegration catalyst in the form of slurry to make contact with and polymerize olefins and diolefins in a stream of steam-cracked (cracked) naphtha is proposed in U.S. Patent No. 3,400,169 . Proponents of the process disclosed in U.S. Patent No. 4,795,550 tested the aforementioned hydrocarbon purification processes and proposed the use of a liquid phase process with a solid medium comprising an aluminosilicate zeolite, such as faujasite, and a refractory oxide for removing olefin reactive impurities with bromine from streams containing aromatics. WO 99/38936 discloses a process where a stream of aromatics is pre-treated to remove diolefins before contact with an acidic active catalyst material that removes the ono-olefinic hydrocarbon contaminants reactive with bromine. Hydrocarbon separation processes that use the selective sorption properties of certain zeolite materials, including specially treated zeolite materials, have been proposed in U.S. Patent Nos. 3,888,939 and 4,309,281. The removal of nitrogen-containing compounds from a hydrocarbon stream using a selective adsorbent, such as ZSM-5, having an average pore size of less than 5.5 Angstroms, is disclosed in U.S. Patent No. 5,744,686. U.S. Patent No. 5,330,946 discloses a bentonite clay-based catalyst, suitable for removing olefins from aromatic streams, manufactured by adhering a plurality of smaller acid-activated bentonite clay particles using a strong mineral acid as a binder. The use of spent catalysts for purification of aromatic streams by saturation of diolefins and removal of CCR at a sufficiently low temperature to reduce olefin polymerization reactions is proposed in U.S. Patent No. 4,501,652. It would be desirable to have a simple, one-step process suitable for removing and / or converting most or all of the various different types of organic and inorganic contaminants that may be present in an alkylation / trans-alkylation process unit such that the Valuable liquid phase trans-alkylation catalyst material is not deactivated and / or plugged by these contaminants, thereby reducing downtime and capital costs, while improving yields and material costs. According to the invention, an alkylation process is provided comprising the steps of: (a) contacting at least one aromatic compound capable of being alkylated with at least one alkylating agent in the presence of a catalyst to provide an alkylation product comprising at least one mono-alkylated aromatic compound and at least one poly-alkylated aromatic compound; (b) contacting at least a portion of the alkylation product with a purification medium in a liquid phase pre-reaction step to remove impurities to form a purified stream comprising at least one poly-alkylated aromatic compound; and (c) contacting the purified stream with at least one aromatic compound capable of being alkylated under liquid phase conditions in a trans-alkylation section in the presence of a catalyst to convert at least a portion of said at least one aromatic poly compound -alkylated in a mono-alkylated aromatic compound. The purification medium is preferably a molecular sieve catalyst selected from the group consisting of MCM-22, MCM-36, MCM-49, MCM-56, MCM-58, beta zeolite, faujasite, mordenite, and combinations thereof, although MCM-22, MCM-36, MCM-49, and MCM-56 are preferred. The purification medium can purify the alkylation stream, before trans-alkylation, by a combination of sorption and catalytic conversion. The accompanying drawing is a simplified flow diagram of a process for producing ethylbenzene according to an embodiment of the invention. In the improved alkylation process of the invention, at least one aromatic compound capable of being alkylated is contacted with at least one alkylating agent under sufficient reaction conditions and in the presence of a catalyst to provide an alkylated product comprising at least one mono-alkylated aromatic compound and at least one poly-alkylated aromatic compound. Then, at least a portion of the product is contacted with a purification medium in a liquid phase pre-reaction step to remove impurities and form a purified stream comprising at least one poly-alkylated aromatic compound. The purified stream and at least one aromatic compound capable of being alkylated are then contacted under liquid phase conditions in a trans-alkylation section in the presence of a catalyst to convert at least a portion of said at least one poly aromatic compound. rented in a mono-alkylated aromatic compound. The term "aromatic", with reference to the compounds capable of being alkylated that are useful herein, should be understood in accordance with its recognized scope in the art, including mono- and poly-nuclear substituted and unsubstituted compounds. Compounds of an aromatic character possessing a hetero-atom (e.g., N or S) are also useful, provided that they do not act as catalyst poisons under the selected reaction conditions. Substituted aromatic compounds that can be alkylated in the present have at least one hydrogen atom linked directly to the aromatic nucleus. The aromatic rings can be substituted with one or more alkyl, aryl, alkaryl, alkoxy, aryloxy, cycloalkyl, halide and / or other groups, which do not interfere with the alkylation reaction. Suitable aromatic hydrocarbons include benzene, naphthalene, anthracene, naphthacene, perylene, coronenne and phenanthrene. Generally, alkyl groups that may be present as substituents on the aromatic compound contain from 1 to 22 carbon atoms and usually from 1 to 8 carbon atoms, and more usually from 1 to 4 carbon atoms. Suitable substituted alkyl aromatic compounds include toluene, xylene, isopropylbenzene, normal propylbenzene, alpha-methylnaphthalene, ethylbenzene, eumeno, mesitylene, durene, p-cyano, butylbenzene, pseudo-cumene, o-diethylbenzene, m-diethylbenzene, p-diethylbenzene, isoamylbenzene, iso-exylbenzene, penta-ethylbenzene, penta-ethylbenzene; 1, 2, 3, 4-tetraethylbenzene; 1,2,3,5-tetramethylbenzene; 1, 2, 4-triethylbenzene; 1, 2, 3-trimethylbenzene; m-butyltoluene; p-butyltoluene; 3, 5-diethyl toluene; o-ethyl toluene; p-ethyl toluene; m-propyl toluene; 4-ethyl-m-xylene; dimethylnaphthalenes; Ethylnaphthalene; 2, 3-dimethylanthracene; 9-ethylanthracene; 2-methylanthracene; o-methylanthracene; 9, 10-dimethyl-phenanthrene; and 3-methyl-phenanthrene. Higher molecular weight alkyl aromatic hydrocarbons may also be used as starting materials, and include aromatic hydrocarbons such as are produced by alkylation of aromatic hydrocarbons with olefin oligomers. Such products are frequently referred to in the art as alkylate and include hexylbenzene, nonylbenzene, dodecylbenzene, pentadecylbenzene, hexyl toluene, nonyl toluene, dodecyl toluene, pentadecyl toluene, etc. Very often, alkylate is obtained as a high boiling fraction in which the alkyl group attached to the aromatic nucleus varies in size from C6 to C12. The alkylating agents that are useful in the process of this invention generally include any organic compound having at least one alkylation group available, capable of reaction with the aromatic compound capable of being alkylated. Preferably, the alkylation group possesses from 1 to 5 carbon atoms. Examples of suitable alkylating agents are olefins such as ethylene, propylene, butenes and pentenes; alcohols (including monoalcohols, di-alcohols, tri-alcohols, etc.), such as methanol, ethanol, propanoles, butanols and pentanols; aldehydes such as formaldehyde, acetaldehyde, propionaldehyde, butyraldehyde, and n-valeraldehyde; and alkyl halides such as methyl chloride, ethylene chloride, propyl chlorides, butyl chlorides and pentyl chlorides. Mixtures of light olefins are especially useful as alkylating agents in the alkylation process of this invention. Accordingly, mixtures of ethylene, propylene, butenes and / or pentenes which are major constituents of a variety of refinery streams, for example fuel gas, stripped plant gas containing ethylene, propylene, etc., gas stripped from naphtha disintegrator. containing light olefins, propane / propylene fluidized catalytic disintegrator (FCC) refinery streams, etc., are useful alkylating agents herein. For example, a typical stream of light olefins of FCC has the following composition:% by weight% molar Ethane 3.3 5.1 Ethylene 0.7 1.2 Propane 14.5 15.3 Propylene 42.5 46.8 Isobutane 12.9 10.3 n-Butane 3.3 2.6 Butenes 22.1 18.32 Pentanes 0.7 0.4 Preferably, the aromatic compound capable of being alkylated is benzene, the alkylating agent is ethylene or propylene, and the desired mono-alkylated reaction product is ethylbenzene or eumeno, respectively. The alkylation catalyst used in the process of the invention is a molecular sieve that is selective for the production of mono-alkylated species, such as ethylbenzene and eumeno. Suitable molecular sieves include MCM-22 (described in U.S. Patent No. 4,954,325), PSH-3 (described in U.S. Patent No. 4,439,409), SSZ-25 (described in U.S. Pat. No. 4,826,667), MCM-49 (described in U.S. Patent No. 5,236,575), MCM-56 (described in U.S. Patent No. 5,362,697), and beta zeolite (described in U.S. Pat. United No. 3,308,069). The alkylation step of this invention is conveniently conducted under conditions including a temperature of 0 to 500 'C, and preferably 50 to 250 * C, a pressure of 0.2 to 250 atmospheres, and preferably 5 to 100 atmospheres, a molar ratio of aromatic compound capable of being alkylated to alkylating agent from 0.1: 1 to 50: 1, and preferably 0.5: 1 to 10: 1, and a space velocity by weight (WHSV) of feed of 0.1 to 500 hr "1, preferably 0.5 to 100 hr" 1. When benzene is alkylated with ethylene to produce ethylbenzene, the alkylation reaction is preferably carried out in the liquid phase. Suitable liquid phase conditions include a temperature between 300 and 600 'F (150 and 316 * C), preferably between 400 and 500 * F (205 and 260 * C), a gauge pressure of up to 3,000 psi (20,875 kPa), preferably between 400 and 800 psi (2,860 and 5,600 kPa), a velocity space between 0.1 and 20, preferably between 1 and 6, based on the ethylene feed, and a ratio of benzene to ethylene in the alkylation reactor from 1: 1 to 30: 1 molar, preferably from 1: 1 to 10: 1 molar. When benzene is alkylated with propylene to produce eumeno, the reaction is preferably carried out under liquid phase conditions, including a temperature of up to 250 ° C, for example up to 150 ° C, for example 10 to 125 'C; a pressure of 250 atmospheres or less, for example from 1 to 30 atmospheres; and a space velocity hour by weight of aromatic hydrocarbon from 5 to 250 hr "1, preferably from 5 to 50 hr" 1. In addition to the desired mono-alkylated aromatic compound, the alkylation product stream will contain poly-alkylated species that are separated and fed to the transalkylation section for reaction with additional, alkylating, aromatic compound, such as benzene. However, the alkylation product stream may also contain impurities such as, for example, olefins, diolefins, styrene, oxygenated organic compounds, sulfur-containing compounds, nitrogen-containing compounds, oligomeric compounds, and combinations thereof. These impurities can originate from external feed currents or can be produced in either liquid or vapor phase alkylation reactors, or can come from both of these sources. These impurities or contaminants can deactivate or plug the trans-alkylation catalyst, and in the process of the present invention, these impurities are removed by adsorption and reaction in a treatment step carried out in a "pre-reactor", which contains a means of purification. The removal of these impurities prolongs the cycle length of the trans-alkylation reactor, preventing poisoning and potential plugging of the valuable trans-alkylation catalyst. The operating conditions of the pre-reactor are such that the feed is in the liquid phase and at a sufficient temperature to react the olefins, diolefins, and styrene and other highly reactive molecules to form heavy alkyl aromatics. In embodiments of the invention, the aromatic stream to be purified, ie containing some or all of the impurities referred to above, is brought into contact with the purification medium in a suitable pre-reaction zone, such as, for example, in a flow reactor containing a fixed bed comprising the composition of the purification medium, under effective conditions of liquid phase to effect the removal of the impurities by reaction and / or adsorption. In the case of oxygenates and sulfur compounds as well as in the case of heavier oligomeric compounds, such as polystyrene, in addition to converting some of these molecules into heavier, less reactive molecules, the purification medium also acts as a sorbent bed . The conditions employed in the purification step include a temperature of 100 to 600 * F (38 to 315 * C), and preferably 150 to 500 'F (65 to 260' C), a space velocity hour by weight (WHSV) from 0.1 to 200 hr "1, and preferably 0.5 to 100 hr" 1, and a pressure from ambient to 400 psig (2,860 kPa). The purification medium can be a molecular sieve catalyst, such as beta, MCM-22, MCM-36, MCM-49, MCM-56, MCM-58, faujasite, or mordenite. Especially preferred are MCM-22, MCM-36, MCM-49, and MCM-56. MCM-22, MCM-36, MCM-49, and MCM-56 are especially effective in removing both olefins and styrenes from the heavy reforming streams and UDEX extract, making them react to produce heavy alkyl aromatics. The liquid phase operating conditions using MCM-22, MCM-36, MCM-49, and MCM-56 that are preferred to obtain these results are 10 to 40 WHSV, 270 to 410'F (130 to 210 * C), and 100 to 300 psig (790 to 2,170 kPa). MCM-22, MCM-36, MCM-49, and MCM-56 can also tenaciously adsorb nitrogen species such as collidine under the liquid phase conditions contemplated. Finally, alkylation studies have shown that olefins have little propensity to oligomerize on MCM-22, MCM-36, MCM-49, and MCM-56 under the liquid phase conditions contemplated. These three attributes of the molecular sieve purification medium of the invention: (1) high reactivity for alkylation, (2) strong retention of poisons such as basic nitrogen compounds, and (3) low reactivity for oligomerization, make MCM-22, MCM-36, MCM-49, or MCM-56 particularly preferred as a component of purification medium for the improved alkylation process of the invention. In embodiments of the invention where the purification medium is a molecular sieve catalyst, it may be desired to incorporate the purification medium with another material resistant to the temperatures and other conditions employed in the purification step. Such materials include active and inactive materials and naturally occurring or synthetic zeolites as well as inorganic materials such as clays, silica and / or metal oxides, such as alumina. The latter may be of natural occurrence or be in the form of gelatinous precipitates or gels, including mixtures of silica and metal oxides. The use of a material in conjunction with the new crystal, ie combined with it or present during the synthesis of the new crystal, which is active, tends to change the conversion and / or selectivity of the catalyst in certain organic conversion processes. Inactive materials suitably serve as diluents to control the magnitude of the conversion in a given process, so that products can be obtained in an economical and orderly manner without employing other means to control the rate of reaction. These materials can be incorporated into naturally occurring clays, for example bentonite and kaolin, to improve the crushing resistance of the catalyst under commercial operating conditions. The materials, ie clays, oxides, etc., function as binders for the catalyst. It is desirable to provide a catalyst having good crush resistance because in commercial use it is desirable to prevent the catalyst from disintegrating into powder-like materials. These clay and / or oxide binders have been used normally in order to improve the crushing resistance of the catalyst only; however, in the present context of the invention, active clay binders and the like can be used to improve the purification properties of the purification medium. Alternatively, binders may be selected such that they do not participate in the removal of impurities, ie they are passive in the process of the invention. Naturally occurring clays that can be formed in composite materials with the new crystal include the family of montmorillonite and kaolin, which families include the sub-bentonites, and the kaolins commonly known as clays Dixie, McNamee, Georgia and Florida or others in the which the main mineral constituent is haloisite, kaolinite, dickite, nacrite, or anauxite. Such clays can be used in the raw state, such as obtained by mining, or initially subjected to calcination, acid treatment or chemical modification. Useful binders for forming composite materials with the molecular sieve catalyst also include porous, high surface area oxides, such as silica, alumina, zirconia, titania or others, including porous, high surface area inorganic oxides. In addition to the above materials, the molecular sieve catalyst that serves as the purification medium can be formed into a composite material with a porous matrix material, such as silica-alumina, silica-magnesia, silica-zirconia, silica-toria, silica -berry, silica-titania, as well as ternary compositions such as silica-alumina-toria, silica-alumina-zirconia, silica-alumina-magnesia, and silica-magnesia-zirconia. The relative proportions of finely divided purification medium and inorganic oxide matrix vary widely, the content of the purification medium varying from 1 to 100% by weight and, more usually, particularly when the composite material is prepared in the form of beads , in the range of 2 to 90% by weight of the composite material. Optionally, the molecular sieve purification means can be tableted or pearled or otherwise produced in a shape configured so that no binder is present. The molecular sieve purification medium can also contain a metal function such that the unsaturated compounds are converted to saturated compounds in the presence of a hydrogen co-feed. For example, a hydrogenation component such as tungsten, vanadium, molybdenum, rhenium, nickel, cobalt, chromium, manganese, or a noble metal such as platinum or palladium, can be used where a hydrogenation-dehydrogenation function is to be carried out. . Such a component can be in the composition of the purification medium as a co-crystallization, exchanged in the composition insofar as an element of group IIIA, for example aluminum, is in the structure, impregnated therein or physically mixed with it. intimate way with it. Such a component can be impregnated in or on it, as for example, in the case of platinum, by treating the silicate with a solution containing a platinum-containing ion. Thus, platinum compounds suitable for this purpose include chloro-platinic acid, platinum chloride, and various compounds containing the platinum-amine complex. The improved alkylation process described herein, specifically the pre-reaction step carried out in the presence of a purification medium, can be carried out as a batch, semi-continuous or continuous operation using a bed catalyst system. fixed or moving. In embodiments of the invention, two pre-reactors can be located in parallel, so that they can be operated in an oscillating mode. The location of the pre-reactor can be directly upstream of the trans-alkylator or in the distillation section used to separate the mono-alkylated product and the aromatic compound capable of being alkylated, without reacting, from the alkylation effluent. This last arrangement is employed in the ethylbenzene process illustrated in the accompanying drawing. Referring to the drawing, ethylene and recycle benzene flow through line 11 to an alkylation reactor 12 and the alkylation effluent (including unreacted benzene, ethylbenzene and polyethylated benzenes) is fed from the reactor via line 13 to a column of benzene 14. Unreacted benzene is removed from the alkylation effluent in column 14 and fed, together with fresh benzene, through the recycle line 15 to the feed line 11. The residue in column 14 then passes to a ethylbenzene column 16 from which the desired ethylbenzene product is removed as a head. The residue in column 16 then passes to the polyethylbenzene column 17, from which polyethylbenzenes are removed as a head and fed to a trans-alkylator 18. The effluent of the trans-alkylator 18 is fed to line 13 for combination with the alkylation effluent and passage to the benzene column 14. According to the invention, the process shown in the drawing includes a pre-reactor 19 which is located downstream of the ethylbenzene column 16 and upstream of the polyethylbenzene column. The pre-reactor, as shown in the drawing, is designed to be exceeded when the catalyst is worn or if polymer formation causes excess pressure drop. The location of the pre-reactor in the distillation section of the improved alkylation process of the invention can be changed, depending on the impurity to be removed. If the major impurities to be removed are reactive olefins such as styrene, the pre-reactor may be located upstream of the ethylbenzene column 16 with co-feed of hydrogen to convert the unsaturated molecules to their saturated version. For example, styrene can be converted to ethylbenzene. In order to prevent plugging of the catalyst bed, the pre-reactor bed can optionally be "graded" by structuring the bed so that larger catalyst particles are placed at the bed inlet. In this way, the interstitial volume between the particles is larger at the inlet, for example the upper part of the bed, thereby allowing the accumulation of a larger amount of contaminant residue on the catalyst before the bed begins to constrict the flow. This will have the effect of prolonging the life of the bed. In the process of the invention, the purified stream is contacted under liquid phase conditions in a trans-alkylation section in the presence of a catalyst to convert at least a portion of the at least one poly-alkylated aromatic compound to an aromatic compound mono-alkylated It is generally known to improve the yield of mono-alkylated product by producing additional mono-alkylated product by trans-alkylation. The polyalkylated products can be recycled to the alkylation reactor to undergo transalkylation or can be reacted with additional aromatic feed in a separate reactor. It may be preferred to physically mix the queues of the mono-alkylated product distillation with a stoichiometric excess of the aromatic feed, and to react the mixture in a separate reactor over a suitable trans-alkylation catalyst. The trans-alkylation catalyst may be a catalyst comprising a zeolite such as MCM-49, MCM-22, MCM-56, PSH-3, SSZ-25, zeolite X, Y zeolite, beta zeolite, or mordenite. Such trans-alkylation reactions on zeolite beta are disclosed in U.S. Patent No. 4,891,458; and, in addition, such trans-alkylations using acid-deallated mordenite are disclosed in U.S. Patent No. 5,243,116. The effluent from the trans-alkylation reactor is physically mixed with the effluent from the alkylation reactor and the combined distilled stream. Inhalation of the poly-alkylated product stream can be taken to remove heavy non-reactive materials from the loop, or the stream of poly-alkylated product can be distilled to remove heavy materials prior to trans-alkylation. The pre-reactor of the invention is of particular value where the alkylation step is effected in the vapor phase using "dirty" feedstocks, such as diluted ethylene from gas released from FCC. It is feasible that polyethylbenzene (PEB) of such alkylation units is contaminated with impurities, such as those mentioned above, which can cause deactivation and / or clogging of the liquid phase trans-alkylation reactor. The process of the invention allows a reconfiguration of older alkylation process units with a liquid phase transalkylator, at a considerably lower capital cost. The use of liquid phase trans-alkylator instead of a vapor phase trans-alkylator will also produce a considerably higher product purity, specifically xylene impurities in the case of ethylbenzene production. The expansion of capacity is achieved by incorporating a liquid phase trans-alquilador in facilities that did not previously have trans-alkylation capacity, and makes it possible to eliminate bottlenecks in the alkylation unit. The present invention can obtain an improvement in increments in the overall performance and efficiency in the use of the feedstock. The present invention can also be used in units where, for any reason, the polyethylbenzene stream has a high level of olefins and styrene or other impurities that can deactivate transalkylation catalysts.