Polyethylbenzenes

Polyethylbenzenes (diethylbenzene, triethylbenzene, etc.) are also formed as unwanted byproducts through reversible reactions in series with respect to ethylbenzene but parallel with respect to ethylene. For example,... 

These polyethylbenzenes are recycled to the reactor to inhibit formation of fresh polyethylbenzenes. ... 

For example, ia the iadustriaHy important alkylation of benzene with ethylene to ethylbenzene, polyethylbenzenes are also produced. The overall formation of polysubstituted products is minimized by recycling the higher ethylation products for the ethylation of fresh benzene (14). By adding the calculated equiUbrium amount of polyethylbenzene to the benzene feed, a high conversion of ethylene to monoethylbenzene can be achieved (15) (see also... 

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. 

Fig. 1. Mobil-Badger vapor-phase ethylbenzene process where PEB = polyethylbenzene.

In order that the amount of side reaction should be reduced and to minimise the production of polyethylbenzenes, the molar ratios of feedstock and products are approximately as indicated in the following equation (Figure 16.4). 

After passing through the reaction chamber the products are cooled and the aluminium chloride, which is in the form of a complex with the hydrocarbons, settles out. The ethylbenzene, benzene and polyethylbenzenes are separated by fractional distillation, the ethylbenzene having a purity of over 99%. The polyethylbenzenes are dealkylated by heating at 200°C in the presence of aluminium chloride and these products together with the unchanged benzene are recycled. 

The vapor-phase Badger process (Eigure 10-2), which has been commercialized since 1980, can accept dilute ethylene streams such as those produced from ECC off gas. A zeolite type heterogeneous catalyst is used in a fixed bed process. The reaction conditions are 420°C and 200-300 psi. Over 98% yield is obtained at 90% conversion." Polyethylbenzene (polyalkylated) and unreacted benzene are recycled and join the fresh feed to the reactor. The reactor effluent is fed to the benzene fractionation system to recover unreacted benzene. The bottoms... 

Figure 10-2. The Badger process for producing ethylbenzene (1) reactor, (2) fractionator (for recovery of unreacted benzene), (3) EB fractionator, (4) polyethylbenzene recovery column.

The Friedel-Crafts reaction has one major drawback. It doesn t stop at the mono-substitution stage. That is, the catalyst works so well, that the benzene will pick up two, three, or more ethylene molecules, forming diethylbenzene, triethylbenzene, or higher polyethylbenzenes. See Figure 8 2.) The problem is that chemically its easier to alkylate EB than it is benzene. One way to control the problem is to carry out the reaction in the presence of a large excess of benzene. When an ethylene molecule is in the neighborhood of one..EB molecule and 20 benzene molecules, chances are that the ethylene will hook up with benzene, even though it prefers EB. 

The effluent stream leaving the reactor is cooled and then treated with caustic (sodium hydroxide) and water to remove the catalyst. The cleaned up stream then contains about 35% unreacted benzene, 50% EB,.15% polyethylbenzene (PEB), and a small amount of miscellaneous heavy materials. 

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 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. 

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. 

It has been shown that single ring aromatic alkylation reactions such as benzene to ethylbenzene take place primarily within the 12- ring (12-MR) system, and that the 10-ring (10-MR) system contributes little to the ethylbenzene reaction. A key feature of MCM-22 is its ability to operate stably at low benzene-to-ethylene ratios with minimal production of polyethylbenzenes (PEBs) or ethylene oligomers. The excellent ethylbenzene selectivity of the MCM-22 catalyst is likely due to confinement effects within this 12-MR pore system and to the very facile desorption... 

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. 

It is favored by an increase in pressure and a decrease in temperature. However the reaction go far to the right, below 60(y C It is accompanied by side conversions, especially of consecutive alkylations, leading to the formation of polyethylbenzenes. Dealkylation, dismutation and Isomerization reactions also occur. 

The composition of the reactor effluent depends on the molar ratio of the-benzene to the ethyl groups in the feed. It has been demonstrated that the optimal conditions are satisfied with values of this parameter between 2 and 2.5. In this case, the alkylate obtained has the following composition ethylbenzene 41 to 43 weight per cent, benzene 38 to 40, diethylbenzenes 12 to 14, triethyl nzenes 2 to 3, heavier polyethylbenzenes 1.5 to 2, miscellaneous 1.5 to 2. Any decrease in the ratio of benzene to ethyl groups has the effect of reducing the formation of the higher molecular weight components. 

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. 

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. 

In the 1960s, Bl Paso employed pbo ]Aoric add deposited on kieselguhr around 325 C and about 6.10 Pa absolute. This catalyst is capable of converting the polyethylbenzenes, but their recycle results in a drop in catalyst activity due to the cracking reactions and the formation of coke deposits. 

The alkylation reaction can be performed under two rather different conditions. If, for example, in a well-stirred laboratory semi-batch reactor a given amount of ethylene is added rapidly to the benzene under given reaction conditions such that the ethylene is completely absorbed, it will be found that the product formed immediately after all ethylene is added is relatively rich in higher polyethylbenzenes and relatively poor in the desired monoethylbenzene. However, if the same amount of ethylene is added relatively slowly to another batch of benzene, the reaction product will be relatively rich in monoethylbenzene and poor in the undesired polyethylbenzenes. 

The rapid initial absorption and reaction of ethylene forms polyethylbenzenes unselectively, and the slow liquid phase reaction of the polyethylbenzenes with unreacted benzene results in an approach towards thermodynamic equilibrium. That is, although the amounts of higher polyethylbenzene is negligible under thermodynamic control, they can be considerably under kinetic control. 

---