| PATENT ASSIGNEE'S COUNTRY | USA |
| UPDATE | 08.00 |
| PATENT NUMBER | This data is not available for free |
| PATENT GRANT DATE | 01.08.00 |
| PATENT TITLE |
Production of alkyl aromatics by passing transalkylation effluent to alkylation zone |
| PATENT ABSTRACT |
A process for producing alkyl aromatics using a transalkylation reaction zone and an alkylation reaction zone is disclosed. The transalkylation reaction zone effluent passes to the alkylation reaction zone where aromatics in the transalkylation reaction zone effluent are alkylated to the desired alkyl aromatics. This process decreases the capital and operating costs of recycling the aromatics in the transalkylation reaction zone effluent. This process is well suited for solid transalkylation and alkylation catalysts. Ethylbenzene and cumene may be produced by this process. |
| PATENT INVENTORS | This data is not available for free |
| PATENT ASSIGNEE | This data is not available for free |
| PATENT FILE DATE | 28.07.98 |
| PATENT REFERENCES CITED | This data is not available for free |
| PATENT PARENT CASE TEXT | This data is not available for free |
| PATENT CLAIMS |
What is claimed is: 1. A process for producing an alkylated aromatic hydrocarbon, said process comprising: a) passing a first feed stream comprising an aromatic substrate hydrocarbon and a second feed stream comprising a first alkyl aromatic hydrocarbon having more than one alkyl group to a first reaction zone, transalkylating said aromatic substrate hydrocarbon with said first alkyl aromatic hydrocarbon in the presence of a first solid catalyst in said first reaction zone to produce a second alkyl aromatic hydrocarbon having at least one more alkyl group than said aromatic substrate hydrocarbon, and recovering from said first reaction zone a first effluent stream comprising said aromatic substrate hydrocarbon and said second alkyl aromatic hydrocarbon; b) passing a third feed stream comprising an olefinic hydrocarbon and at least an aliquot portion of said first effluent stream to a second reaction zone, alkylating said substrate aromatic hydrocarbon with said olefinic hydrocarbon in the presence of a second solid catalyst in said second reaction zone to produce said second alkyl aromatic hydrocarbon; and c) recovering said second alkyl aromatic hydrocarbon from said process. 2. The process of claim 1 further characterized in that said process comprises: d) withdrawing from said second reaction zone a second effluent stream comprising said second alkyl aromatic hydrocarbon and said first alkyl aromatic hydrocarbon; e) separating said second effluent stream into a product stream comprising said second alkyl aromatic hydrocarbon and a recycle stream comprising said first alkyl aromatic hydrocarbon; and f) providing at least a portion of said second feed stream in (a) of claim 1 from at least a portion of said recycle stream. 3. The process of claim 1 further characterized in that said process comprises: d) withdrawing from said second reaction zone a second effluent stream comprising said second alkyl aromatic hydrocarbon and said aromatic substrate hydrocarbon; e) separating said second effluent stream into a product stream comprising said second alkyl aromatic hydrocarbon and a recycle stream comprising said aromatic substrate hydrocarbon; and f) providing at least a portion of said first feed stream in (a) of claim 1 from at least a first portion of said recycle stream. 4. The process of claim 3 further characterized in that a second portion of said recycle stream is passed to said second reaction zone. 5. The process of claim 1 wherein said first solid catalyst comprises a zeolite selected from the group consisting of zeolite Y, zeolite beta, ZSM-5, PSH-3, MCM-22, MCM-36, MCM-49, and MCM-56. 6. The process of claim 1 wherein said second solid catalyst comprises a zeolite selected from the group consisting of zeolite Y, zeolite beta, ZSM-5, PSH-3, MCM-22, MCM-36, MCM-49, and MCM-56. 7. The process of claim 1 wherein said first solid catalyst comprises zeolite Y and said second solid catalyst comprises zeolite beta. 8. The process of claim 1 wherein said aromatic substrate hydrocarbon comprises benzene, said olefinic hydrocarbon comprises ethylene, and said second alkyl aromatic hydrocarbon comprises ethylbenzene. 9. The process of claim 1 wherein said aromatic substrate hydrocarbon comprises benzene, said olefinic hydrocarbon comprises propylene, and said second alkyl aromatic hydrocarbon comprises cumene. 10. The process of claim 1 wherein said first effluent stream comprises an aromatic selected from the group consisting of a trialkyl aromatic, a tetraalkyl aromatic, a pentaalkyl aromatic, a hexaalkyl aromatic, and a diphenylalkane. 11. The process of claim 1 further characterized in that said second reaction zone operates under at least partial liquid phase conditions. 12. The process of claim 1 further characterized in that said alkylating occurs at a temperature of 250 to 350.degree. F. 13. The process of claim 1 further characterized in that said alkylating occurs at a molar ratio of phenyl groups per alkyl group of less than 3:1. 14. The process of claim 1 wherein said second reaction zone comprises at least two beds of said second solid catalyst and further characterized in that said aliquot portion of said first effluent stream is passed to a first bed of said at least two beds, said third feed stream is passed to said first bed of said at least two beds, and a fourth feed stream comprising said olefinic hydrocarbon is passed to a second bed of said at least two beds. 15. The process of claim 14 further characterized in that said passing said third feed stream to said first bed comprises passing a first portion of a common stream comprising said olefinic hydrocarbon to said first bed and said passing said fourth feed stream to said second bed comprises passing a second portion of said common stream to said second bed. 16. The process of claim 1 wherein said first solid catalyst and said second solid catalyst have the same composition. 17. The process of claim 1 wherein said first reaction zone and said second reaction zone are contained in a common vessel. 18. The process of claim 1 further characterized in that said passing said first feed stream and said second feed stream to said first reaction zone comprises combining said first feed stream and said second feed stream to form a combined stream and passing said combined stream to said first reaction zone. 19. A process for the production of cumene, said process comprising: a) contacting an aromatic feed comprising benzene, a benzene recycle stream comprising benzene, and a diisopropylbenzene stream comprising diisopropylbenzene in a transalkylation reaction zone with a transalkylation catalyst at transalkylation conditions to produce a transalkylation zone effluent comprising benzene, cumene, and diisopropylbenzene; b) contacting said transalkylation zone effluent and an olefin feed comprising propylene and propane in an alkylation reaction zone with an alkylation catalyst at alkylation conditions to provide an alkylation zone effluent comprising propane, benzene, cumene, diisopropylbenzene, and heavier hydrocarbons; c) separating said alkylation zone effluent in a benzene separation zone into a benzene fraction comprising benzene and propane and a benzene bottoms stream comprising cumene, diisopropylbenzene, and said heavier hydrocarbons; d) separating said benzene fraction in a light ends column into said benzene recycle stream and a light ends stream comprising propane; e) separating said benzene bottoms stream into a product stream comprising cumene and a cumene bottoms stream comprising diisopropylbenzene and said heavier hydrocarbons; and f) separating said cumene bottoms stream into a heavies stream comprising said heavier hydrocarbons that is removed from the process and said diisopropylbenzene stream. 20. The process of claim 19 further characterized in that water is passed to said alkylation reaction zone and said light ends stream comprises water. 21. A process for the production of ethylbenzene, said process comprising: a) contacting an aromatic feed comprising benzene, a benzene recycle stream comprising benzene, and a diethylbenzene stream comprising diethylbenzene in a transalkylation reaction zone with a transalkylation catalyst at transalkylation conditions to produce a transalkylation zone effluent comprising benzene, ethylbenzene, and diethylbenzene; b) contacting said transalkylation zone effluent and an olefin feed comprising ethylene in an alkylation reaction zone with an alkylation catalyst at alkylation conditions to produce an alkylation zone effluent comprising benzene, ethylbenzene, diethylbenzene, and heavier hydrocarbons; c) separating said alkylation zone effluent in a benzene separation zone into said benzene recycle stream comprising benzene and a benzene bottoms stream comprising ethylbenzene, diethylbenzene, and said heavier hydrocarbons; d) separating said benzene bottoms stream into a product stream comprising ethylbenzene and an ethylbenzene bottoms stream comprising diethylbenzene and said heavier hydrocarbons; and e) separating said ethylbenzene bottoms stream into a heavies stream comprising said heavier hydrocarbons that is removed from the process and said diethylbenzene stream. |
| PATENT DESCRIPTION |
FIELD OF THE INVENTION This invention relates to a hydrocarbon conversion process. The invention more specifically relates to the production of alkylaromatic hydrocarbons by the reaction of an acyclic olefinic hydrocarbon with an aromatic feed hydrocarbon. BACKGROUND OF THE INVENTION The alkylation of aromatics with olefins to produce monoalkyl aromatics is a well developed art which is practiced commercially in large industrial units. One commercial application of this process is the alkylation of benzene with ethylene to produce ethylbenzene which is subsequently used to produce styrene. Another application is the alkylation of benzene with propylene to form cumene (isopropylbenzene) which is subsequently used in the production of phenol and acetone. Those skilled in the art are therefore familiar with the general design and operation of such alkylation processes. The performances of alkylation processes for producing monoalkyl aromatics are influenced by the stability and activity of the solid catalyst at the operating conditions of the process. For example, as the molar ratio of aromatic per olefin increases, currently available catalysts typically exhibit an improved selectivity to the monoalkyl aromatic. But even at a high molar ratio of aromatic per olefin, several polyalkyl aromatic by-products such as dialkyl aromatics and trialkyl aromatics accompany monoalkyl aromatic production. Although the formation of dialkyl and trialkyl aromatics might, at first glance, be viewed as by-products that represent a reduction in the efficient use of the olefin, in fact each can be readily transalkylated with the aromatic using a transalkylation catalyst to produce the monoalkyl aromatic. So-called combination processes combine an alkylation zone with a transalkylation zone in order to maximize monoalkyl aromatic production. One disadvantage of combination processes is that separate reaction zones for alkylation and for transalkylation duplicate costly equipment. Each reaction zone requires what amounts to its own reaction train, including separate and independent reaction vessels, heaters, heat exchangers, piping, and instrumentation. Another disadvantage of combination processes is the great expense associated with recovering and recycling unreacted aromatic from the effluents of the alkylation and transalkylation reaction zones. Alkylation reaction zones generally operate at a molar ratio of aromatic per alkylation agent that is at least 1:1 in order to ensure a high selectivity toward the monoalkyl aromatic. Transalkylation reaction zones generally operate at a molar ratio of aromatic per dialkyl aromatic that is much greater than the stoichiometric ratio of 1:1 in order to ensure a high conversion of the dialkyl aromatic to the monoalkyl aromatic. Consequently, the alkylation and transalkylation reaction zone effluents contain a significant quantity of unreacted aromatic, which must be removed in order to obtain the monoalkyl aromatic product and which must be recycled in order to ensure the efficient use of the aromatic. Prior art combination processes lessen the great expense incurred in removing and recycling the unreacted aromatic contained in the alkylation and transalkylation reaction zone effluents by routing each alkylation and transalkylation effluent stream to a single, common product recovery facility, in which the very same distillation columns remove unreacted aromatic from both effluent streams and recycle unreacted aromatic to both reaction zones. Incidentally, a no less important function of these distillation columns in the prior art is the removal of polyalkyl aromatics other than dialkyl and trialkyl aromatics and of other heavy alkylation and transalkylation by-products such as diphenylalkanes, which are collectively referred to herein as heavies. Although sharing common product equipment in this manner reduces the capital expense of a combination process, the energy requirements for vaporizing and condensing the aromatic from the effluent streams remains undiminished. Thus, the high utilities expenses of combination processes as well as the costly duplication of reaction zones has given impetus to research with a goal of minimizing energy requirements and of integrating the alkylation and transalkylation zones even further. SUMMARY OF THE INVENTION This invention is an economical and efficient combination process for producing an alkyl aromatic by alkylation and by transalkylation. In this invention, the transalkylation effluent stream passes to the alkylation reaction zone instead of to a product recovery facility as in the prior art. This invention places the transalkylation and alkylation reaction zones in series, with polyalkyl aromatics passing to the transalkylation reaction zone with the alkylation substrate, which is usually benzene, and the transalkylation effluent passing to the alkylation reaction zone. This invention decreases the formation of polyalkyl aromatics, especially dialkyl aromatics, in the alkylation reaction zone, because polyalkyl-aromatics that are present in the transalkylation effluent and which this invention passes to the alkylation zone tend to inhibit the production of polyalkyl aromatics in the alkylation reaction zone. Because the alkylation reaction zone makes less polyalkyl aromatics, the transalkylation reaction zone does not need to convert as many polyalkyl aromatics, and, therefore, the capital and operating costs associated with recycling the alkylation substrate to the transalkylation reaction zone in a combination alkylation-transalkylation process decrease. The costs associated with recycling the alkylation substrate can be further decreased by diverting to the transalkylation reaction zone some or all of the alkylation substrate that passes directly to the alkylation reaction zone in prior art processes. Because diverting this benzene to the transalkylation reaction zone increases conversion of polyalkyl aromatics in the transalkylation reaction zone, the alkylation reaction zone may be operated with less alkylation substrate being passed to the alkylation reaction zone. Thus, less excess alkylation substrate is present in the alkylation effluent stream and, therefore, less capital and utilities need to be spent to recover the desired alkylaromatic product from the excess alkylation substrate in the alkylation effluent stream. Consequently, this invention can be operated in a manner that vaporizes, condenses, and recycles a decreased quantity not only of polyalkyl aromatics but also of excess alkylation substrate. Additional cost savings are attainable with this invention by consolidating the alkylation and transalkylation reaction zones into a single reactor vessel, and by eliminating in whole or in part recycling of the alkylation effluent stream, if any, to the alkylation reaction zone. Thus, in summary, a combination process that uses this invention can operate with significantly lower capital and utility costs compared to a prior art combination process. Combination processes that will benefit most from this invention include those in which passing the transalkylation reactor effluent to the alkylation reaction zone does not have significant adverse effects on the production of monoalkyl aromatic in the alkylation zone or on the deactivation rate of the alkylation catalyst. For this reason, this invention is particularly applicable to combination processes that use beta zeolite as the alkylation catalyst, because at alkylation conditions beta zeolite produces nearly the equilibrium amount of monoalkyl aromatic and because, surprisingly, beta zeolite is not rapidly deactivated by polyalkyl aromatics in the alkylation feed. This invention is also particularly applicable to those combination processes that benefit from operation at a relatively high molar ratio of phenyl groups per alkyl group in the transalkylation reaction zone and a relatively low molar ratio of aromatic per alkyl group in the alkylation reaction zone. In a broad embodiment, this invention is a process for producing an alkylated aromatic hydrocarbon. Aromatic substrate hydrocarbons and first alkyl aromatic hydrocarbons having more than one alkyl group pass to a first reaction zone. In the first reaction zone, aromatic substrate hydrocarbons transalkylate with first alkyl aromatic hydrocarbons in the presence of a first solid catalyst to produce a second alkyl aromatic hydrocarbon having at least one more alkyl group than the aromatic substrate hydrocarbon. A first effluent stream comprising aromatic substrate hydrocarbons and second alkyl aromatic hydrocarbons is recovered from the first reaction zone. Olefinic hydrocarbons and at least an aliquot portion of the first effluent stream are passed to a second reaction zone. In the second reaction zone, substrate aromatic hydrocarbons are alkylated with olefinic hydrocarbons in the presence of a second solid catalyst to produce second alkyl aromatic hydrocarbons. The second alkyl aromatic hydrocarbons are recovered from the process. In a more limited embodiment, this invention is a process for the production of cumene. An aromatic feed comprising benzene, a benzene recycle stream comprising benzene, and a diisopropylbenzene stream comprising diisopropylbenzene are contacted in a transalkylation reaction zone with a transalkylation catalyst. The transalkylation reaction zone operates at transalkylation conditions and produces a transalkylation zone effluent comprising benzene, cumene, and diisopropylbenzene. The transalkylation effluent stream and an olefin feed comprising propylene and propane are contacted in an alkylation reaction zone with an alkylation catalyst. The alkylation reaction zone operates at alkylation conditions and provides an alkylation zone effluent comprising propane, benzene, cumene, diisopropylbenzene, and heavier hydrocarbons. A benzene separation zone separates the alkylation zone effluent into a benzene fraction comprising benzene and propane and a benzene bottoms stream comprising cumene, diisopropylbenzene, and the heavier hydrocarbons. A light ends column separates the benzene fraction into a light ends stream comprising propane and the benzene recycle stream. The benzene bottoms stream is separated into a product stream comprising cumene and a cumene bottoms comprising diisopropylbenzene and the heavier hydrocarbons. The cumene bottoms stream is separated into a heavies stream comprising the heavier hydrocarbons and the diisopropylbenzene stream. The heavies stream is removed from the process. In another more limited embodiment, this invention is a process for the production of ethylbenzene. An aromatic feed comprising benzene, a benzene recycle stream comprising benzene, and a diethylbenzene stream comprising diethylbenzene are contacted in a transalkylation reaction zone with a transalkylation catalyst at transalkylation conditions. A transalkylation zone effluent comprising benzene, ethylbenzene, and diethylbenzene is produced. The transalkylation effluent stream and an olefin feed comprising ethylene are contacted in an alkylation reaction zone with an alkylation catalyst at alkylation conditions. An alkylation zone effluent comprising benzene, ethylbenzene, diethylbenzene, and heavier hydrocarbons is produced. The alkylation zone effluent is separated in a benzene separation zone into the benzene recycle stream comprising benzene and a benzene bottoms stream comprising ethylbenzene, diethylbenzene, and the heavier hydrocarbons. The benzene bottoms stream is separated into a product stream comprising ethylbenzene and an ethylbenzene bottoms comprising diethylbenzene and the heavier hydrocarbons. The ethylbenzene bottoms stream is separated into a heavies stream comprising the heavier hydrocarbons that is removed from the process and the diethylbenzene stream. INFORMATION DISCLOSURE Prior art alkylation processes are well described in the literature. U.S. Pat. No. 4,051,191 describes catalysts, reaction conditions, and a separation method for the recovery of cumene that uses a rectification zone and a two-column fractionation train. U.S. Pat. Nos. 4,695,665 and 4,587,370 are particularly directed to the separation of products and the recovery of recycle streams from processes for the alkylation of aromatic hydrocarbons, and U.S. Pat. No. 4,695,665 discloses the use of a flash drum in combination with an effluent rectifier to recover unreacted feed components. U.S. Pat. No. 4,891,458 describes the use of beta zeolite for the alkylation aromatic hydrocarbons with alkenes to produce alkyl aromatics. U.S. Pat. No. 4,891,458 also discloses that transalkylation can occur in an alkylation reactor, and that additional monoalkyl aromatic hydrocarbons can be produced in an alkylation reactor by recycling polyalkyl aromatic hydrocarbons to the alkylation reactor to undergo transalkylation. U.S. Pat. No. 4,922,053 describes a process for alkylating benzene with ethylene in a multibed reactor wherein polyethylbenzenes are recycled to the first alkylation bed and also to one or more of the other alkylation beds in order to increase ethylbenzene yield. U.S. Pat. No. 5,030,786 discloses an alkylation process wherein the feed stream is dehydrated to enhance the performance of beta or Y zeolites in the alkylation process. U.S. Pat. No. 5,336,821 describes the use of beta zeolite for the alkylation of aromatic hydrocarbons in a process that is improved by an indirect heat exchanger to recover the heat of reaction. In one embodiment, the alkylation reactor effluent passes through the heat exchanger and is recycled to the alkylation reactor. Prior art transalkylation processes are well described in the literature. U.S. Pat. No. 4,083,886 describes a process for the transalkylation of the alkyl aromatic hydrocarbons that uses a zeolitic catalyst. U.S. Pat. No. 4,891,458 describes the use of beta zeolite for the transalkylation of aromatic hydrocarbons with polyalkyl aromatic hydrocarbons. European Patent Application EP 0 733 608 A1 discloses the use of an alumina silicate catalyst having an average crystal size of less than about 0.5 microns for the transalkylation of polyalkyl benzenes with benzene in a reaction zone with an alkylating agent such as ethylene. Combination processes that produce alkyl aromatic products by using an alkylation reaction zone and a transalkylation reaction zone are also well known. U.S. Pat. No. 4,008,290 describes a combination process in which the alkylation effluent and the transalkylation effluent are passed to a common separation zone, which separates the two effluents into product, by-product, and recycle streams including a recycle benzene stream. A portion of the alkylation effluent is recycled to the alkylation reaction zone in order to decrease the portion of the recycle benzene stream that is recycled to the alkylation reaction zone. U.S. Pat. No. 5,003,119 describes a combination process for producing monoalkyl aromatics in which the alkylation effluent passes to the transalkylation reaction zone, and the transalkylation effluent passes to a separation zone. U.S. Pat. No. 5,003,119 also describes passing dialkyl aromatics to the alkylation reaction zone. U.S. Pat. No. 5,177,285 discloses an alkylation process that is improved by maintaining the feed to the alkylation zone in a relatively wet condition and the feed to the transalkylation zone in a relatively dry condition. The process operates with a relatively pure ethylene feed as an alkylating agent with a large excess of benzene. U.S. Pat. No. 5,723,710 describes a surface-modified zeolite beta which exhibits stability and long catalyst life when used in alkylation and transalkylation of aromatic compounds. The teachings of U.S. Pat. No. 5,723,710 are incorporated herein by reference. BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 and 2 are schematic illustrations of embodiments of the invention. DETAILED DESCRIPTION OF THE INVENTION This invention is suitable generally for alkylation substrates and alkylation agents, and more specifically to aromatic alkylation substrates and olefinic alkylating agents. Benzene is the aromatic alkylation substrate of principal interest, but alkyl-substituted benzenes may be used. More than one aromatic alkylation substrate may be used. Monoolefins are the principal olefinic alkylating agent, but other diolefins, polyolefins, acetylenic hydrocarbons, and substituted hydrocarbons can be used. The olefinic alkylating agent preferably contains from 2 to 4 carbon atoms, but olefins having from 2 to 20 carbon atoms may be used. Ethylene and propylene are the preferred olefinic alkylating agents. More than one olefin may be used. In addition to alkylation substrates and alkylation agents, this invention may also be suitable for transalkylation agents. The transalkylation agent transalkylates with the alkylation substrate to produce the desired product, namely the product of alkylating the alkylation substrate with the alkylating agent. The transalkylation agent may be introduced to the present invention via a feed from a source that is external to the present invention, but more commonly the transalkylation agent is a by-product of alkylating the alkylation substrate with the alkylating agent in the process that uses the present invention. Where the transalkylation agent is such an alkylation by-product, a transalkylation agent from an external source may not be needed, and the transalkylation agent can be passed to the transalkylation zone by recovering the transalkylation agent from the alkylation effluent stream and passing a stream enriched in transalkylation agent and depleted in desired product to the transalkylation reaction zone. Alternately and less preferably, some of the alkylation effluent stream may be passed without separation to the transalkylation reaction zone. This invention is suitable specifically for aromatic transalkylation agents having more than one alkyl group, and dialkyl benzenes are the principal aromatic transalkylation agents for producing monoalkyl benzenes. As the number of alkyl groups on the desired aromatic product increases, the number of alkyl groups on the principal aromatic transalkylation agent increases. Generally, the alkylation substrate, alkylation agent, and transalkylation agent are hydrocarbons. As used herein, the term "hydrocarbon" means a compound that contains carbon and hydrogen and that may contain other atoms as well, such as halogens (e.g., fluorine, chlorine, and bromine), oxygen, sulfur, and nitrogen. These other atoms may be present, for example, in substituent groups that are substituted on the aromatic ring of an aromatic alkylation substrate or of an aromatic transalkylation agent. The desired alkyl aromatic product has at least one more alkyl group than the aromatic substrate. One of the widely practiced hydrocarbon conversion processes to which this invention is applicable is the production of cumene by alkylation of benzene with propylene and by transalkylation of benzene with polyisopropylbenzenes that are alkylation by-products. Therefore, the discussion herein of this invention refers mainly to cumene processes. For the sake of clarity, the discussion herein of cumene transalkylation precedes that of cumene alkylation because an essential element of this invention is passing some transalkylation effluent to the alkylation zone. It is not intended that this discussion limit the scope of this invention as set forth in the claims. In the transalkylation reaction zone, diisopropylbenzene and higher polyisopropylbenzenes transalkylate with benzene to produce cumene (isopropylbenzene). Generally, a catalyst promotes the transalkylation in the transalkylation reaction zone. The transalkylation catalyst for the present invention may be one of a class of aluminosilicate molecular sieves known as zeolites. The zeolitic molecular sieves suitable for use in the present invention are crystalline aluminosilicates which in the calcined form may be represented by the general formula: Me.sub.2 /n O: Al.sub.2 O.sub.3 :xSiO.sub.2 : yH.sub.2 O where Me is a cation, n is the valence of the cation, x has a value of from about 5 to 100, and y has a value of from about 2 to 10. Detailed descriptions of zeolites may be found in D. W. Breck, Zeolite Molecular Sieves, John Wiley and Sons, New York 1974, and in other standard references. Suitable zeolites for the transalkylation catalyst include zeolite Y, zeolite beta, ZSM-5, PSH-3, MCM-22, MCM-36, MCM-49, and MCM-56. In general, each of the zeolites described hereinafter as suitable for the alkylation catalyst is suitable for the transalkylation catalyst. A preferred zeolite Y for transalkylation is essentially free of residual non-H.sup.+ cations, by which it is meant that the non-H.sup.+ cation content of the zeolite Y is less than 200 wppm calculated as NH.sub.3 equivalents. A preferred zeolite for the transalkylation catalyst is zeolite beta as disclosed in U.S. Pats. No. 4,891,458 and 5,081,323, the teachings of which are incorporated herein by reference. Another preferred zeolite beta is the surface-modified zeolite beta that is described hereinafter and which is disclosed in U.S. Pat. No. No. 5,723,710, the teachings of which are incorporated herein by reference. The zeolite is generally present in an amount of at least 50 wt.-% of the catalyst and more preferably in an amount of at least 90 wt.-% of the catalyst. In most cases, the balance of the transalkylation catalyst other than the zeolite is a refractory inorganic oxide binder. The preferred inorganic oxide is alumina, with gamma-alumina, eta-alumina, and mixtures thereof being particularly preferred. Where the catalyst comprises a zeolite and an inorganic oxide, the zeolite content may be from 5 to 99 wt.-% of the catalyst, and the inorganic oxide may be from 1 to 95 wt.-% of the catalyst. Preferred transalkylation catalysts are zeolite Y with an alumina or silica binder and zeolite beta with an alumina or silica binder. The transalkylation reaction can be carried out in a broad range of operating conditions that result in a high conversion of diisopropylbenzene (DIPB) to cumene. DIPB conversion is limited by equilibrium governed mainly by the ratio of phenyl groups per alkyl group and is generally greater than 30% and preferably greater than 50%. Operating conditions generally include a temperature of from about 210.degree. F. (99.degree. C.) to about the critical temperature of the alkylation substrate, which is about 554.degree. F. (290.degree. C.) for benzene and may be 889.degree. F. (475.degree. C.) or even higher for heavier alkylation substrates. The transalkylation pressure would generally be from 1 to about 130 atmospheres, but set so that the reactants are in the liquid phase. Accordingly, the preferred pressure for the transalkylation reaction zone range is from 10 to about 50 atmospheres. A total liquid hourly space velocity (LHSV) of from 0.5 to 50 hr.sup.-1 is desirable, with 0.5 to 5 hr.sup.-1 being preferred. The water concentration is typically less than 200 wppm, and preferably less than 20 wppm, and more preferably less than 5 wppm. The molar ratio of phenyl groups per alkyl group, which is often referred to as the phenyl/alkyl ratio, is a key operating variable for transalkylation because the equilibrium conversion of polyalkyl aromatics is a function of the phenyl/alkyl ratio. The numerator of this ratio is the number of moles of phenyl groups passing through the transalkylation zone during a specified period of time. The number of moles of phenyl groups is the sum of all phenyl groups, regardless of the compound in which the phenyl group happens to be. In the context of cumene production, for example, one mole of benzene, one mole of cumene, one mole of diisopropylbenzene (DIPB), and one mole of triisopropylbenzene (TIPB) each contribute one mole of phenyl group to the sum of phenyl groups. The denominator of this ratio is the number of moles of alkyl groups passing through the transalkylation zone during the same specified period of time. The number of moles of alkyl groups is the sum of all alkyl and alkenyl groups, regardless of the compound in which the alkyl or alkenyl group happens to be. Thus, in the production of cumene (isopropylbenzene), the number of moles of propyl groups is the sum of all propyl and propenyl groups, regardless of the compound in which the propyl or propenyl group happens to be. For example, one mole of propylene and one mole of cumene each contribute one mole of propyl group to the sum of propyl groups, whereas one mole of DIPB contributes two moles of propyl groups and one mole of TIPB contributes three moles of propyl groups. For cumene production, the phenyl/propyl ratio is generally from 8:1 to 1:1, and preferably from 6:1 to 3:1. Where the transalkylation feed consists of only benzene and DIPB, the phenyl/propyl ratio may be computed from the molar ratio of benzene per DIPB, which is referred to as the benzene/DIPB ratio, by using the mathematical formula, phenyl/propyl ratio =1/2x (benzene/DIPB ratio +1). This mathematical formula is sufficiently accurate as an approximation of the phenyl/propyl ratio when the cumene content or the TIPB content of the transalkylation feed is less than 1.0 vol.-%. For ethylbenzene production, the phenyl/ethyl ratio is generally from 10:1 to 1:1, and preferably from 7:1 to 2:1. The transalkylation reaction zone may be operated and arranged in any manner that provides the desired operating conditions and the required contacting of reactants and catalyst. A single contacting stage in transalkylation is routinely used, in part because the transalkylation reactions are neither very exothermic nor very endothermic. The transalkylation effluent stream contains not only the desired monoalkyl aromatic product (cumene) but also unreacted transalkylation reactants as well as transalkylation by-products. Of the transalkylation reactants, benzene is usually the most abundant, because in transalkylation benzene is generally present in a stoichiometric excess to the polyisopropylbenzenes. Diisopropylbenzenes (DIPB) in the transalkylation feed also are generally present in the transalkylation effluent stream because the DIPB conversion in transalkylation is limited by equilibrium to less than 100 %. Higher polyalkylbenzenes such as triisopropylbenzenes (TIPB) and tetraisopropylbenzenes also may be present in the transalkylation effluent, either as an unreacted transalkylation reactant or as a transalkylation by-product from the reaction of a polyalkylbenzene with another polyalkylbenzene rather than with benzene. As mentioned previously, passing some of the transalkylation effluent stream to the alkylation reaction zone is an essential element of this invention. The portion of the transalkylation effluent stream that passes to the alkylation reaction zone is generally from about 5 to 100 % of the transalkylation effluent stream, and preferably from about 70 to 100%. The portion of the transalkylation effluent stream that passes to the alkylation reaction zone is preferably an aliquot portion of the transalkylation effluent stream. The alkylation reaction zone feed stream contains not only components that exit the transalkylation reaction zone but also additional components that are injected into the transalkylation effluent stream. For instance, the alkylating agent (propylene) enters the transalkylation effluent stream between the transalkylation and alkylation reaction zones. In addition, if the quantity of alkylation substrate (benzene) in the transalkylation effluent is insufficient for the alkylation reaction zone or if the temperature in the alkylation reaction zone requires adjustment, then additional fresh or recycle benzene may be combined with the transalkylation effluent stream and passed to the alkylation reaction zone. Of course, as mentioned previously, if additional benzene is required for alkylation, then it may be preferable, subject to transalkylation space velocity constraints, for that benzene to be first passed through the transalkylation reaction zone, where a higher ratio of benzene per diisopropylbenzene and a higher ratio of phenyl groups per propyl groups would tend to increase diisopropylbenzene conversion. Finally, water may be added to the transalkylation effluent stream, because some alkylation catalysts benefit from operating at a higher concentration of water than transalkylation catalysts. The alkylation reaction zone can operate over a broad range of operating conditions. The alkylation reaction zone is ordinarily operated to obtain an essentially complete conversion of the alkylating agent (propylene) to cumene, diisopropylbenzene, or heavier polyisopropylbenzenes. Propylene conversion is generally more than 99.5 % and preferably more than 99.9 %. Preferably, the operating conditions result in nearly equilibrium concentrations of cumene being produced in the alkylation reaction zone. The concentration of cumene is generally greater than 80 % of the equilibrium concentration, and preferably greater than 95 %. To help attain such a high cumene concentration, a stoichiometric excess of benzene over propylene is generally present in alkylation reaction zone feed. The molar ratio of benzene per propylene is generally from 20:1 to 1:1, and preferably from 5:1 to 1:1. The preferred molar ratio of phenyl groups per propyl group in the alkylation reaction zone is from 5:1 to 1:1. Temperatures usually range from about 210.degree. F. (99.degree. C.) to about the critical temperature of the alkylation substrate, which is about 554.degree. F. (290.degree. C.) for benzene and may be 889.degree. F. (475.degree. C.) or even higher for heavier alkylation substrates. Temperatures are preferably in the range of about 250 to 350.degree. F. (121 to 177.degree. C.) and more preferably in the range of about 270 to 350.degree. F. (132 to 177.degree. C.). Pressures can also vary within a wide range of from about 1 atmosphere to about 130 atmospheres. Because liquid phase conditions are generally preferred within the alkylation reaction zone, the pressure should be sufficient to maintain the benzene in a liquid phase and will typically fall in a range of from 10 to 50 atmospheres. The benzene liquid hourly space velocity (LHSV) is generally from about 0.5 to 50 hr.sup.-1, and preferably from about 1 to 10 hr.sup.-1. The water concentration in the alkylation reaction zone is generally greater than 50 wppm, but may be over 500 wppm, depending on the particular catalyst. The propylene concentration in the alkylation reaction zone is generally less than 12 wt.-% and preferably less than 6 wt.-% in order to minimize formation of alkylation by-products. This invention is typically suitable to the case where the stream containing the alkylating agent contains non-alkylating materials, and preferably with low concentrations of the non-alkylating materials. For example, in a cumene process the propylene-containing stream commonly contains some propane. This invention is particularly applicable where the propylene stream of a cumene process contains from 0 to about 50 wt.-% propane. A catalyst promotes the initial alkylation of the alkylation substrate in the alkylation reaction zone. A wide variety of catalysts can be used in the alkylation reaction zone. Suitable catalysts for use in the alkylation reaction zone will comprise any catalyst that is not deactivated rapidly as a consequence of including heavies in the alkylation reaction zone feed. In addition, the presence of heavies should not have a deleterious effect on the approach to equilibrium cumene concentrations in the alkylation reaction zone. If polyisopropylbenzenes are present in the alkylation reaction zone feed, those with fewer propyl groups are preferred, as are low concentrations of any polyisopropylbenzenes that are present. Preferably, the concentration of polyisopropylbenzenes is less than 5 wt.-% of the alkylation reaction zone feed. Also, the presence of cumene in the alkylation feed should not have a significant adverse effect on the production of cumene in the alkylation reaction zone. The preferred alkylation catalyst for use in this invention is a zeolitic catalyst. Suitable zeolites include zeolite Y, zeolite beta, ZSM-5, PSH-3, MCM-22, MCM-36, MCM-49, and MCM-56. Zeolite Y is described in U.S. Pat. No. 3,130,007. A preferred zeolite Y is essentially free of residual non-H.sup.+ cations, by which it is meant that the non-H.sup.+ cation content of the zeolite Y is less than 200 wppm calculated as NH.sub.3 equivalents. Thus, the number of H.sup.+ acid sites in the preferred zeolite Y is maximized. Zeolite beta is described in U.S. Pat. No. 3,308,069 and Re 28,341. The topology of zeolite beta and the three zeolite beta polytypes are described in the article by Higgins, et al., in Zeolites, Vol. 8, November 1988, starting at page 446; and in the letter by M. M. J. Treacy et al., in Nature, Vol. 332, Mar. 17, 1988, starting at page 249. Suitable zeolite betas include, but are not limited to, the naturally occurring mixture of the three polytypes, any one of the three polytypes, or any combination of the three polytypes. The use of zeolite beta in alkylation and transalkylation is disclosed in U.S. Pat. Nos. 4,891,458 and 5,081,323, and the use of pristine zeolite beta in alkylation is disclosed in European Patent EP 432,814 B1. Suitable zeolite betas include, but are not limited to, pristine zeolite beta in which the H.sup.+ ion has at least partially replaced the contained metal cation, as disclosed in European Patent EP 432,814 B1; and zeolite beta into which certain quantities of alkaline, alkaline-earth, or metallic cations have been introduced by ion exchange, as disclosed in U.S. Pat. No. 5,672,799. Various modifications of zeolite beta are also suitable for use in this invention. Suitable modified zeolite betas include, but are not limited to, zeolite beta which has been modified by steam treatment and ammonium ion treatment, as disclosed in U.S. Pat. No. 5,522,984; and zeolite beta in which the H.sup.+ ion has at least partially replaced the contained metal cation, with the zeolite beta being modified by isomorphous substitution of aluminum by boron, gallium, or iron, as disclosed in European Patent EP 432,814 B1. Suitable zeolites for use in this invention also include zeolites that are synthesized by modified preparation methods, such as, but not limited to, a preparation method comprising forming a reaction mixture comprising water, a source of silicon dioxide, a source of fluoride ions, a source of tetraethylammonium cations, and, optionally, a source of an oxide of a trivalent element, as disclosed in PCT International Publication Number WO 97/33830. ZSM-5 is described in U.S. Pat. No. 3,702,886 and Re 29,948. PSH-3 is disclosed in U.S. Pat. No. 4,439,409. MCM-22 is disclosed in U.S. Pat. Nos. 4,954,325 and 4,992,606, and its structure is described in the article in Science, Vol. 264, pp. 1910-1913 (Jun. 24, 1994). U.S. Pat. Nos. 5,077,445; 5,334,795; and 5,600,048 describe the use of MCM-22 to produce alkylaromatics. MCM-36 is disclosed in U.S. Pat. Nos. 5,250,277 and 5,292,698. The use of a catalyst comprising MCM-36 to produce alkylaromatics such as ethylbenzene is disclosed in U.S. Pat. Nos. 5,258,565 and 5,600,048. The synthesis of MCM-49 is described in U.S. Pat. No. 5,323,575, and the use of MCM-49 to produce alkylaromatics including the liquid phase production of ethylbenzene is described in U.S. Pat. Nos. 5,371,370; 5,493,065; and 5,600,048. MCM-56 is disclosed in U.S. Pat. No. 5,362,697. The use of MCM-56 to produce ethylbenzene and other alkylaromatics is disclosed in U.S. Pat. Nos. 5,453,554 and 5,600,048. A preferred zeolite beta for use in alkylation in this invention is disclosed in U.S. Pat. No. 5,723,710, the teachings of which are incorporated herein by reference. This preferred zeolite is a surface-modified zeolite beta which results from acid washing of a templated native zeolite beta. That is, the formation of the surface-modified zeolite beta starts with a templated beta where the template is, for example, a tetraalkylammonium salt, such as tetraethylammonium salt. It is critical to acid wash a templated zeolite beta in order to protect the internal sites of the zeolite and to prevent dealumination. The templated zeolite beta is treated with a strong acid at a pH between about 0 up to about 2, although a pH under 1 is preferred. Acids which may be used include nitric acid, sulfuric acid, phosphoric acid, and so forth. For example, a weak, 0.01 molar nitric acid may be used in conjunction with ammonium nitrate to perform the acid wash, although substantially higher concentrations, up to about 20 weight percent nitric acid, are preferred. Nitric acid is a preferred acid since it is a non-complexing acid and therefore does not encourage dealumination. Treatment of the templated zeolite beta with strong acid may be effected over the temperature range between about 20.degree. C. (68.degree. F.) up to about 125.degree. C. (257.degree. F.). It is important that acid washing be done under conditions not so severe as to effect dealumination. The time over which acid washing is conducted in preparing the preferred zeolite is quite temperature dependent. It is critical in the formation of the surface-modified zeolite beta that there be no significant bulk dealumination of the zeolite. Thus, as a general statement it can be said that acid washing should be done for a time insufficient to effect dealumination. For example, using 0.01 molar nitric acid and circa 40% ammonium nitrate at 70.degree. C. (158.degree. F.), contact times of 2-3 hours are found adequate to modify the environment of surface aluminum without causing significant bulk dealumination. Using circa 15% nitric acid with ammonium nitrate to treat a circa 25 weight percent slurry at 85.degree. C. (185.degree. F.), a 90-minute treatment is effective. The dependent variables in acid washing include acid concentration, slurry concentration, time and temperature, and suitable conditions at which surface-modified zeolite beta can be prepared without significant bulk dealumination are readily determined by the skilled artisan. Next the template is removed by calcination at temperatures in the range of 550-700.degree. C. (1022-1292.degree. F.). Calcination conditions are well known in the art and need not be elaborated upon here. It also needs to be mentioned that powdered zeolite itself is not usually used as the alkylation catalyst. Therefore, in the more usual case after the templated zeolite beta is acid washed it is mixed with a conventional binder, extruded, and the extrudate is ultimately calcined. But the critical portion of the preparation of the preferred zeolite is the acid wash of the templated beta according to the foregoing description. Acid washing a calcined (i.e., non-templated) zeolite beta does not produce the surface-modified material of the preferred zeolite. It has been found that after treatment as described above the surface aluminum atoms are chemically modified. It has been hypothesized that the modification is in the form of replacement of strong acid sites at the catalyst surface by weaker acid sites. What has been definitely observed is that the surface aluminums of the preferred modified zeolite beta have 2 p binding energies as measured by x-ray photoelectron spectroscopy of at least 74.8 electron volts. As mentioned previously, the zeolite will usually be used in combination with a refractory inorganic oxide binder. Preferred binders are alumina and silica. Preferred alkylation catalysts include zeolite Y with an alumina or silica binder, and zeolite beta or the previously-described surface-modified zeolite beta with an alumina or silica binder. The zeolite will usually be present in an amount of at least 50 wt.-% of the catalyst, and preferably in an amount of at least 70 wt.-% of the catalyst. This process is useful for any arrangement of transalkylation reaction zone and alkylation reaction zone wherein the effluent of the former is passed to the latter. However, it has been found that a beta zeolite or a Y type zeolite contained in an alumina binder will perform very well when used in both the alkylation reaction zone and the transalkylation reaction zone. Therefore, both reaction zones may use the same catalyst. Accordingly, in a preferred embodiment of this invention for cumene production, beta zeolite is used as the catalyst in both the alkylation and transalkylation zones. There is no requirement, however, that the alkylation reaction zone and the transalkylation reaction zone use the same catalyst. Accordingly, a preferred embodiment for ethylbenzene production uses Y zeolite as the transalkylation catalyst and beta zeolite as the alkylation catalyst. This combination of Y zeolite for transalkylation and beta zeolite for alkylation is particularly preferred for producing ethylbenzene when the a molar ratio of phenyl groups per alkyl group in the alkylation zone of less than 3:1, because it is believed that this embodiment compensates somewhat for the low yield of ethylbenzene that would otherwise occur during alkylation at low phenyl/alkyl ratios. In this preferred ethylbenzene embodiment, the transalkylation catalyst generally operates at a temperature of from about 356 to about 464.degree. F. (180 to 240.degree. C.), and the alkylation catalyst generally operates at a temperature of from about 329 to about 500.degree. F. (165 to 260.degree. C.). There is also no requirement that the transalkylation reaction zone and the alkylation reaction zone be in separate vessels. However, it is believed that an alkylation reaction zone and a transalkylation reaction zone will require less capital expense and be less mechanically complex when both are in a single, common reaction vessel. Therefore, in the preferred embodiment of this invention, both reaction zones are in the same reaction vessel. The alkylation reaction zone may be operated and arranged in any manner that provides the desired operating temperatures and number of contacting stages. Multiple contacting stages in the alkylation reaction zone are routinely used to provide cooling by the staged addition of reactants and/or cooled reactor effluent quench to multiple beds of alkylation catalyst. The multiple injection of the reactants or quench serves to cool the stages between alkylation catalyst beds, to provide temperature control, and to reduce the concentration of alkylating agent (e.g., propylene or ethylene). Ordinarily, the alkylation catalyst is arranged in multiple beds to permit series flow of the alkylation substrate (e.g., benzene) and parallel flow interbed injection of the alkylating agent (e.g., propylene or ethylene). Thus, in the usual situation, benzene is provided in a molar excess to the olefin, all of the benzene that is provided to the multiple beds is introduced into the first bed, and the effluent of each of the beds flows to the next bed in the series. In this way, unreacted benzene from each bed is made available to react in the next bed in the series. However, where the alkylation catalyst is arranged in multiple beds, the alkylation substrate can also be injected between beds of alkylation catalyst. Multiple beds of alkylation catalyst means two or more beds of alkylation catalyst. In theory, there is no upper limit as to the number of alkylation catalyst beds, but in practice the maximum number of beds of alkylation catalyst is determined by a number of factors, including the ability to distribute flow in each bed uniformly, the ability to inject alkylating agent and/or alkylation substrate to each bed evenly, the exothermic temperature rise in each bed, and the clearance and/or access required to assemble and maintain the mechanical equipment associated with each bed. A typical alkylation unit usually has 4 or 6 alkylation catalyst beds, but it is also possible to have as many as 10, 20, or more alkylation catalyst beds. In one embodiment of this invention, the portion of the transalkylation effluent stream that passes to a multibed alkylation reaction zone is itself divided into sub-portions, preferably aliquot sub-portions. One sub-portion of the transalkylation effluent stream passes to the first bed in the series, and a second sub-portion is passed to another bed in the series. In the case where there are only two alkylation catalyst beds, between about 1 and about 99% of the portion of the transalkylation effluent stream that passes to the alkylation reaction zone passes to the first bed in the series, and the remainder, i.e., between about 1 and about 99% of the transalkylation effluent stream that passes to the alkylation reaction zone, passes to the second alkylation bed. Where there are three or more alkylation catalyst beds, between about 1 and about 99 % of the portion of the transalkylation effluent stream that passes to the alkylation reaction zone passes to the first bed in the series, and for any other alkylation bed in the series, between about 0 and about 99% of the portion of the transalkylation effluent stream that passes to the alkylation reaction zone passes to that other bed in the series. Thus, where a portion of the transalkylation effluent passes stage-wise to an alkylation reaction zone having four alkylation catalyst beds, numerous possibilities for the distribution of the portion of the transalkylation effluent exist. One possibility is 25% to each of the four beds. A second possible distribution is 10% to the first bed, 40% to the second bed, 0% to the third bed, and 50% to the fourth bed. A third possible distribution is 30% to the first bed, 10% to the second bed, 60% to the third bed, and 0% to the fourth bed. Because the transalkylation effluent is generally at a temperature that is less than the effluent temperature of each alkylation bed, the transalkylation effluent that is injected between the alkylation beds can provide not only multiple injection of reactants but also quench for the alkylation catalyst beds. Where the alkylation reaction zone contains separate alkylation catalyst beds, the separate beds may be arranged in a single vessel or in multiple vessels. In practicing this invention with multiple alkylation catalyst beds, a common vessel may contain the transalkylation reaction zone and one or more alkylation reaction catalyst beds. However, for very large units separate vessels for the transalkylation catalyst bed and for one or more alkylation catalyst beds may be more advantageous. The alkylation effluent generally is a mixture of the desired monoalkyl aromatic product with a wide variety of undesired by-products. For example, in the alkylation of benzene with ethylene to produce ethylbenzene, the alkylation reaction zone can also produce diethylbenzene and triethylbenzene in addition to other ethylene condensation by-products. Similarly, in the alkylation of benzene with propylene to produce cumene, the reaction zone can produce diisopropylbenzenes, triisopropylbenzenes, tetraisopropylbenzenes, heavier polyisopropylbenzenes, and other heavy alkylated aromatics such as diphenylpropane in addition to other propylene condensation by-products. Transalkylation also yields additional alkyl aromatic by-products, which in this invention can be alkylated in the alkylation reaction zone to produce still other by-products. Generally, the alkylation effluent stream also contains a substantial amount of unreacted aromatic substrate (benzene), because aromatic substrate is ordinarily present in a stoichiometric excess in both transalkylation and alkylation. Therefore, a number of separation stages are needed to separate the desired aromatic product from the by-products and the unreacted aromatic substrate. A number of combinations of columns and separators can be used to recover the desired alkyl aromatic product and to produce recycle streams of aromatic substrate and polyalkylated aromatics for transalkylation. Typically, a first column separates light paraffins that entered the process with the alkylating agent and that passed through the alkylation reaction zone without reacting. A second column separates the aromatic substrate from the remaining heavier components of the alkylation effluent stream. Alternatively, the order of these first two columns may be reversed, in which case a low alkane (e.g., propane) content of the alkylating agent stream (e.g., propylene-containing stream) is preferred. One or more additional separation columns separate the desired aromatic product from by-product streams that contain lighter or heavier by-products. Heavy by-products that are not suitable for transalkylation are usually rejected from the process. The operation of this invention and of an arrangement of separation zones to recover product and to produce recycle streams can be better understood by referring to FIG. 1. FIG. 1 schematically illustrates the major equipment used in performing the process of this invention for the production of cumene. In the process, fresh benzene feed flows through a line 28 and mixes with a stream in a line 84 that contains recycle benzene as well as recycle diisopropylbenzenes and triisopropylbenzenes. The mixture flows through a line 86 and enters a bed 12 of zeolitic transalkylation catalyst. A vessel 10 houses the transalkylation bed 12, an alkylation bed 14, and an additional alkylation bed 16. After contact with the catalyst in bed 12, the transalkylation zone effluent from bed 12 flows through the vessel 10, receives propylene via lines 32, 34, and 42, and enters bed 14 of zeolitic alkylation catalyst. Effluent from bed 14 receives propylene via lines 32, 34, and 38, and enters bed 16 which contains zeolitic alkylation catalyst. Alkylation bed 16 effluent flows from vessel 10 and flows via lines 30 and 50 to a vessel 20, which contains another series of alkylation beds 18, 22, 24, and 26 which all contain zeolitic alkylation catalyst. Alkylation bed 16 effluent enters bed 18, bed 18 effluent enters bed 22, bed 22 effluent enters bed 24, and bed 24 effluent enters bed 26. Incidentally, water (not shown) may be injected into any, some, or all of the alkylation beds. Lines 32 and 36 supply propylene for the alkylation beds in the vessel 20. The propylene for bed 18 flows through lines 44, 46, 48, and 50 and enters vessel 20 upstream of bed 18. Similarly, propylene for bed 22 flows through lines 44, 46, and 52; propylene for bed 24 flows through lines 44 and 54; and propylene for bed 26 flows through line 56. Alkylation zone effluent from alkylation bed 26 flows from vessel 20 via a line 58. The line 58 from vessel 20 supplies alkylation zone effluent to a benzene column 40. Although not shown in FIG. 1, line 58 may optionally supply alkylation zone effluent to each alkylation bed 14, 16, 18, 22, 24, and 26 by recycling to a point upstream of each bed. From benzene column 40 are withdrawn a fraction containing benzene and light hydrocarbons in a line 62 and a bottom stream containing higher boiling hydrocarbons in a line 64. Line 62 passes the fraction to a depropanizer 60. Although not shown in the FIG. 1, a portion of the fraction in the line 62 may optionally be recycled to a point upstream of alkylation bed 14 in vessel 10 to provide additional benzene with a relatively high water concentration for the alkylation reaction zone. Depropanizer 60 recovers in a line 68 a purified benzene stream having a relatively low water concentration. A line 66 passes an overhead stream containing propane and water from the depropanizer 60. The line 64 carries the bottom stream of the benzene column 40 to a cumene column 70. A line 72 recovers a cumene product overhead, and a line 74 transfers a bottoms stream to a heavies column 80 for the recovery of diisopropylbenzene and triisopropylbenzene via a line 82, a lighter boiling material via a line 78, and heavy aromatics via a line 76. A purified stream of diisopropylbenzene and triisopropylbenzene returns as recycle via the line 82, combines with benzene in the line 68, and the combination is carried by the lines 84 and 86 to the transalkylation catalyst bed 12 in the vessel 10. As described previously, the effluent from the transalkylation bed 12 flows through vessel 10 to the top of alkylation catalyst bed 14. |
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