Transalkylation of polyethylbenzenes

The earlie industrial developments of vapor phase alkylation processes involved the use of alumina base catalyst systems. They include the Koppers technique industrialized in the Second World War, which operates around 310°C and between 6 and 6. 10 Pa absolute, but does not allow the transalkylation of polyethylbenzenes. 

Liquid phase transalkylation of polyethylbenzenes with benzene yields ethylbenzene (43). For REX catalyst, optimum conditions of 232°, 800 psig, LHSV = 2, and a CeHs/CCgHslgCsHi molar ratio of 9... 

In the Mohil-Badger vapor-phase process, fresh and recycled benzene are vaporized and preheated to the desired temperature and fed to a multistage fixed-bed reactor. Ethylene is distributed to the individual stages. Alkylation takes place in tile vapor phase. Separately, file polyethylbenzene stream from the distillation section is mixed with benzene, vaporized and heated, and fed to the transalkylator, where polyethylbenzenes react with benzene to form additional ethylbenzene. The combined reactor effluent is distilled in the benzene column. Benzene is condensed in the overhead for recycle to the reactors. The bottoms from the benzene column are distilled in the ethylbenzene column to recover the ethylbenzene product in the overhead. The bottoms stream from the ethylbenzene column is further distilled in the polyefitylbenzene column to remove a small quantity of residue. The overhead polyethylbenzene stream is recycled to the reactor section for transalkylation to ethylbenzene. 

Description Ethylene reacts with benzene in either a totally liquid-filled or mixed-phase alkylation reactor (1) containing multiple fixed-beds of ExxonMobil s proprietary catalyst, forming EB and very small quantities of polyethylbenzenes (PEB). In the transalkylation reactor (2), PEB is converted to EB by reaction with benzene over ExxonMobil s transalkylation catalyst. PEB and benzene recovered from the crude EB enter the transalkylation reactor. 

A small amount of polyethylbenzene (PEB) is recovered in the overhead of the PEB column (5) and recycled back to the transalkylation reactor (2) where it is combined with benzene over a second proprietary zeolite catalyst to produce additional EB product. A small amount of flux oil is recovered from the bottom of the PEB column (5) and is usually burned as fuel. 

This type of catalyst does not simultaneously allow alkylation of the benzene and transalkylation of the polyethylbenzenes. Hence distinct units must be employed, operating in different conditions ... 

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. 

The ethylation of naphthalene was not shape-selective even over HM. However, the transethylation of naphthalene with polyethylbenzenes over HY gave high selectivity for 2,6-DEN with high yield.78 Tetraethylbenzene was the best transalkylating agent among di-, tri-, and tetraethylbenzenes to form... 

Alkylation of benzene with ethylene gives ethylbenzene,283,284,308,309 which is the major source of styrene produced by catalytic dehydrogenation. High benzene ethylene ratios are applied in all industrial processes to minimize polyethylation. Polyethylbenzenes formed are recycled and transalkylated with benzene. Yields better than 98% are usually attained. Reactants free of sulfur impurities and water must be used. 

In the EBMax process, benzene is fed to the bottom of the liquid-filled multibed reactor. Ethylene is co-fed with the benzene and also between the catalyst beds. Polyethylbenzenes, which are almost exclusively diethylbenzenes, undergo transalkylation with benzene in a second reactor. Mobil-Badger offers both liquid phase and vapor phase transalkylation processes. The vapor phase process removes benzene feed coboilers such as cyclohexane and methylcyclopentane as well as propyl and butylbenzenes. Because the EBMax process produces very low levels of propyl and butylbenzenes, for most applications, the more energy efficient liquid phase process is preferred. Worldwide, there are currently ten licensed EBMax units with a cumulative ethylbenzene production capacity of five million metric tons per year. 

The polyethylbenzenes can be recycled to the alkylation reactor. Aluminum chloride has the advantage of activating their transalkylation, which can therefore take place at the same time as the main reaction. 

Since only a small amount of aluminum chloride is used in this homogeneous alkylation process, more care has to be taken to control the method and rate of addition of ethylene to the benzene. The alkylation reaction vessel is designed to accommodate simultaneously both the very rapid ethylene-benzene reaction and the relatively slow polyethylbenzene transalkylation reactions. By careful design of the reactor and control of operating conditions, the formation of higher polyethylbenzenes can be minimized. 

The transalkylation and isomerization reactions can be satisfactorily explained by the Streitwieser mechanism( ). This mechanism proposes a 1,1-diphenylethane-type intermediate. For example, para-diethylbenzene. (Figure 3) Such an intermolecular mechanism is consistent with the experimental data and does not require the assumption of a sequence of intramolecular 1,2 shifts. The decay of the polyethylbenzenes towards equilibrium is consecutive and not concurrent. The catalyst seems to be associated with the most basic center and when it reaches steady-state, the catalyst transfers to the next most basic one. There is also a concurrent intramolecular isomerization such as 1,2,4 triethylbenzene going to 1,3,5 triethylbenzene. There is hence a movement towards isomer equilibrium as well as product equilibrium. 

Two important premises made were first, that the alkylation was an irreversible reaction with transalkylation a separate and independent reaction, and second, that the reaction rate of molecular ethylene in the liquid was not rate controlling as long as the concentrations of the higher polyethylbenzenes were low. This allowed the kinetic constants for the transalkylation model to be evaluated from transalkylation data when the combined alkylation-... 

Modem alkylation processes make use of solid catalysts based on zeolites. According to different technologies, the reaction can be performed in vapour or liquid phase. The selection of a suitable chemical reactor for ethylbenzene is discussed in the Example 8.3. A conceptual flowsheet is depicted in Fig. 7.31 for a vapour-phase process (Mobil-Badger), one of the most widely used. The reactor works at 390-440 C and 0.6-3 MPa. Besides the main product ethylbenzene (EB), polyethylbenzenes (PEB) are formed, their amount depending on the reaction conditions. Large excess of benzene, over 6 1, is needed to shift the equilibrium to the desired product. The reaction mixture is sent to the separation section. Final yield can increase over 99% by converting PEB s to EB in a separate transalkylation reactor. 

An early liquid phase process used an aluminum trichloride catalyst at 85°- 95°C at pressures just above atmospheric. A low ethylene/benzene ratio was used to limit the formation of diethylbenzene and other polyethylbenzenes. By-products could, however, be recycled with benzene and were recovered as ethylbenzene by transalkylation. Ethylbenzene selectivity was about 94% based on benzene and higher on et lene. The catalyst that formed in solution was thought to be HAlCLt-w-C 5C2H5, which gradually deactivated and was replenished as required. Other acid catalysts such as boron trifluoride can be used in the hquid phase process, which is still widely used in older plants. 

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