Transalkylations benzene

Coke formation on H-mordenite catalyst during the benzene transalkylation with C9 aromatics... 

This work deals with the study of the coke formation on H-mordenite during the benzene transalkylation with C9 aromatics, under several reaction conditions, in order to evaluate the condition which results in the lowest catalyst deactivation for industrial purposes. It was found that coke was produced in all samples but it was maintained around 4% (weight) without damage to activity and selectivity to toluene and xylenes. The coke was hydrogenated and could be easily removed. The soluble coke was mostly constituted by aliphatic hydrocarbons, while the insoluble coke was amorphous. These results were explained by the mordenite structure as well as by the presence of hydrogen. The best condition to perform the reaction depends much more on the selectivity to toluene and xylenes rather than on coke production. 

The transalkylation of toluene with trimethylbenzenes (TMB) is one of the most important reactions of conversion of methylaromatics aiming at the xylene production. However, with the recent market reduction of benzene as a consequence of environmental restrictions, benzene transalkylation with C9 aromatics emerges as a potentially important reaction for commercial applications and its investigation has attracted increasing attention. 

Transalkylation is also catalyzed by acids, but requires more severe conditions than isomerization. As shown below, the methyl migration is intermolecular and ultimately produces a mixture of aromatic compounds ranging from benzene to hexamethylbenzene. The overall equiHbrium constants for all possible methylbenzenes have been deterrnined experimentally and calculated theoretically (Fig. 2 and Table 3). 

Xylenes Produetion Via Toluene Transalkylation and Disproportionation. The toluene that is produced from processes such as catalytic reforming can be converted into xylenes via transalkylation and disproportionation. Toluene disproportionation is defined as the reaction of 2 mol of toluene to produce 1 mol of xylene and 1 mol of benzene. Toluene transalkylation is defined as the reaction of toluene with or higher aromatics to produce xylenes ... 

Additioaal uses for higher olefias iaclude the productioa of epoxides for subsequeat coaversioa iato surface-active ageats, alkylatioa of benzene to produce drag-flow reducers, alkylation of phenol to produce antioxidants, oligomeriza tion to produce synthetic waxes (qv), and the production of linear mercaptans for use in agricultural chemicals and polymer stabilizers. Aluminum alkyls can be produced from a-olefias either by direct hydroalumination or by transalkylation. In addition, a number of heavy olefin streams and olefin or paraffin streams have been sulfated or sulfonated and used in the leather (qv) iadustry. 

Fig. 3. Unocal—Lummus—UOP ethylbenzene process AR = alkylation reactor TR = transalkylation reactor BC = benzene column ...

Fig. 5. UOP cumene process Rx = reactor R = rectifier BC = benzene column CC = cumene column T = transalkylation.

Propylene feed, fresh benzene feed, and recycle benzene are charged to the upflow reactor, which operates at 3—4 MPa (400—600 psig) and at 200—260°C. The SPA catalyst provides an essentially complete conversion of propylene [115-07-1] on a one-pass basis. A typical reactor effluent yield contains 94.8 wt % cumene and 3.1 wt % diisopropylbenzene [25321-09-9] (DIPB). The remaining 2.1% is primarily heavy aromatics. This high yield of cumene is achieved without transalkylation of DIPB and is unique to the SPA catalyst process. 

The cumene product is 99.9 wt % pure, and the heavy aromatics, which have a research octane number (RON) of 109, can either be used as high octane gasoline-blending components or combiaed with additional benzene and sent to a transalkylation section of the plant where DIPB is converted to cumene. The overall yields of cumene for this process are typically 97—98 wt % with transalkylation and 94—96 wt % without transalkylation. 

Zeolite Catalysts. Uaocal has iatroduced a fixed-bed fiquid-phase reactor system based oa a Y-type zeofite catalyst (62). The selectivity to cumene is geaeraHy betweea 70 and 90 wt %. The remaining components are primarily polyisopropylbenzenes, which are transalkylated to cumene ia a separate reactioa zoae to give an overall yield of cumene of about 99 wt %. The distillation requirements iavolve the separation of propane for LPG use, the recycle of excess benzene to the reaction zones, the separation of polyisopropylbenzene for transalkylation to cumene, and the production of a purified cumene product. 

Xylenes. The main appHcation of xylene isomers, primarily p- and 0-xylenes, is in the manufacture of plasticizers and polyester fibers and resins. Demands for xylene isomers and other aromatics such as benzene have steadily been increasing over the last two decades. The major source of xylenes is the catalytic reforming of naphtha and the pyrolysis of naphtha and gas oils. A significant amount of toluene and Cg aromatics, which have lower petrochemical value, is also produced by these processes. More valuable p- or 0-xylene isomers can be manufactured from these low value aromatics in a process complex consisting of transalkylation, eg, the Tatoray process and Mobil s toluene disproportionation (M lDP) and selective toluene disproportionation (MSTDP) processes isomerization, eg, the UOP Isomar process (88) and Mobil s high temperature isomerization (MHTI), low pressure isomerization (MLPI), and vapor-phase isomerization (MVPI) processes (89) and xylene isomer separation, eg, the UOP Parex process (90). 

The Tatoray process, which was developed by Toray Industries, Inc., and is available for Hcense through UOP, can be appHed to the production of xylenes and benzene from feedstock that consists typically of toluene [108-88-3] either alone or blended with aromatics (particularly trimethylbenzenes and ethyl-toluenes). The main reactions are transalkylation (or disproportionation) of toluene to xylene and benzene or of toluene and trimethylbenzenes to xylenes in the vapor phase over a highly selective fixed-bed catalyst in a hydrogen atmosphere at 350—500°C and 1—5 MPa (10—50 atm). Ethyl groups are... 

The Xylene Plus process of ARGO Technology, Inc. (95,96) and the FINA T2BX process (97) also use a fixed-bed catalyst in the vapor phase for transalkylation of toluene to produce xylenes and benzene. The Mobil low temperature disproportionation (LTD) process employs a zeoHte catalyst for transalkylation of toluene in the Hquid phase at 260—315°C in the absence of hydrogen (98). 

The transalkylation reaction is essentiaHyisothermal and is reversible. A high ratio of benzene to polyethylbenzene favors the transalkylation reaction to the right and retards the disproportionation reaction to the left. Although alkylation and transalkylation can be carried out in the same reactor, as has been practiced in some processes, higher ethylbenzene yield and purity are achieved with a separate alkylator and transalkylator, operating under different conditions optimized for the respective reactions. 

Alkylator Feed heater Transalkylator Benzene column... 

Transall lation. Two molecules of toluene are converted iato one molecule of benzene and one molecule of mixed xylene isomers ia a sequence called transalkylation or disproportionation. Economic feasibiUty of the process strongly depends on the relative prices of benzene, toluene, and xylene. Operation of a transalkylation unit is practical only when there is an excess of toluene and a strong demand for benzene. In recent years, xylene and benzene prices have generally been higher than toluene prices so transalkylation is presendy an attractive alternative to hydrodealkylation (see also Btx... 

Ethylbenzene can also be produced by catalytic alkylation of benzene with ethylene. Benzene is alkylated with ethylene in a fixed bed alkylator. An excess of benzene is used to suppress the formation of di- and triethyl- benzenes. The excess benzene is removed from the alkylate by fractionation and recycled to the alkylator. The ethylbenzene is separated from the polyalkylated benzenes which are in turn fed to a separate reactor. Here benzene is added to convert the polyalkylated benzenes to monoethylbenzene by transalkylation. 

In the UOP process (Figure 10-5), fresh propylene feed is combined with fresh and recycled benzene, then passed through heat exchangers and a steam preheater before being charged to the reactor.The effluent is separated, and excess benzene recycled. Cumene is finally clay treated and fractionated. The bottom product is mainly diisopropyl benzene, which is reacted with benzene in a transalkylation section ... 

Figure 10-5. A flow diagram of the UOP cumene process " (1) reactor, (2,3) two-stage flash system, (4) depropanizer, (5) benzene column, (6) clay treatment, (7) fractionator, (8) transalkylation section.

The synergism of a dual-catalyst system comprising of Pt/ZSM-12 and H-Beta aiming to improve the benzene product purity during transalkylation of aromatics has been studied. Catalyst compositions of the dual-catalyst system were optimized at various reaction temperatures in terms of benzene product purity and premium product yields. Accordingly, a notable improvement in benzene purity at 683 K that meets the industrial specification was achieved using the cascade dual-bed catalyst. 

The catalytic performances obtained during transalkylation of toluene and 1,2,4-trimethylbenzene at 50 50 wt/wt composition over a single catalyst Pt/Z12 and a dualbed catalyst Pt/Z 121 HB are shown in Table 1. As expected, the presence of Pt tends to catalyze hydrogenation of coke precursors and aromatic species to yield undesirable naphthenes (N6 and N7) side products, such as cyclohexane (CH), methylcyclopentane (MCP), methylcyclohexane (MCH), and dimethylcyclopentane (DMCP), which deteriorates the benzene product purity. The product purity of benzene separated in typical benzene distillation towers, commonly termed as simulated benzene purity , can be estimated from the compositions of reactor effluent, such that [3] ... 

Figure 1. Effects of H-Beta ratio (y) in the dual-bed catalyst Pt/Z12(x) HB(y) on (a) benzene purity and (b) product yields during transalkylation reaction (see text) at 623 K.

A dual-bed catalyst system has been developed to tackle the key problems in benzene product impurity during heavy aromatics transalkylation processing over metal-supported zeolite catalysts. It was found that by introducing zeolite H-Beta as a complementary component to the conventional single-bed Pt/ZSM-12 catalyst, the cascaded dual-bed catalyst shows synergistic effect not only in catalytic stability but also in adjustments of benzene product purity and product yields and hence should represent a versatile catalyst system for heavy aromatics transalkylation. 

Tatoray [Transalkylation aromatics Toray] A process for transalkylating toluene, and/or trimethylbenzenes, into a mixture of benzene and xylenes. Operated in the vapor phase, with hydrogen, in a fixed bed containing a zeolite catalyst. Developed jointly by Toray Industries and UOP and now licensed by UOP. First operated commercially in Japan in 1969 as of 1992, 23 units were operating and 6 more were in design and construction. 

Karge and Ladebeck (90) studied the alkylation of benzene with olefins over aluminum-deficient, beryllium exchanged mordenite and found a considerable extension of the lifetime of the catalyst, as compared to H-mordenite. The authors were able to carry out quite efficiently the alkylation reaction as well as the transalkylation of ethylbenzene at relatively low temperatures. 

Production of p-xylene via p-xylene removal, i.e., by crystallization or adsorption, and re-equilibration of the para-depleted stream requires recycle operation. Ethylbenzene in the feed must therefore be converted to lower or higher boiling products during the xylene isomerization step, otherwise it would build up in the recycle stream. With dual-functional catalysts, ethylbenzene is converted partly to xylenes and is partly hydrocracked. With mono-functional acid ZSM-5, ethylbenzene is converted at low temperature via transalkylation, and at higher temperature via transalkylation and dealkylation. In both cases, benzene of nitration grade purity is produced as a valuable by-product. 

Figure 6. Transalkylation of an ethylbenzene-xylene feed over HZSM-4. TMB = trimethylbenzene, DMEB = dimethylethyl-benzene, DEB = diethylbenzene, and ETol = ethyltoluene. Feed 16% EB, 62% m-xylene, 22% o-xylene. Temperature ...

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