P XYLENES AND ETHYLBENZENE

The ultimate in xylene separation is claimed, however, by Hetzner (10), who first distills the mixture to remove o-xylene by taking m-p-xylene and ethylbenzene overhead in a column having about 35 to 60 theoretical plates. It is reported that concentrates containing up to 97% o-xylene have been produced by this process. The m-xylene, p-xylene, and ethylbenzene mixture is selectively sulfonated to remove m-xylene. In this operation, 2 moles of Sulfuric acid (96 to 98%) are added per mole of m-xylene in the mixture to be treated. After separation, the aqueous layer is hydrolyzed at 250° to 300° F. to recover a concentrate containing 90% or more m-xylene. The hydrocarbon layer is cooled to produce p-xylene crystals, which are separated by filtration or centrifugation. The 85 to 90% p-xylene concentrate is reprocessed to recover a final product containing 96% p-xylene. The mother liquor from the p-xylene crystallization contains impure ethylbenzene and is rejected from the system. 

The alkenylation reaction proceeds at 100° or below. Yields of 80-91 of mcmobutenylated alkylbenzene were obtained from toluene, o- and p-xylene, and ethylbenzene using sodium deposited cxi calcium oxide (23). 

If there is a CH2 or a CH group in a-position to an aromatic system, it is attacked preferentially by oxygen with formation of a substituted benzyl hydroperoxide, 317 examples being tetralin, ind ne, fluorene, cumene, p-xylene, and ethylbenzene. Temperatures required for autoxidation of such compounds are lower than for alkanes. 

Discuss the possible methods of separating mixtures of o-Xylene. m-Xylene, p-Xylene, and ethylbenzene in light of the following physical properties. 

The IR technique also enables us to determine the adsorption and desorption of binary mixtures, provided the IR spectra of the two components were sufficiently different so that the spectra of the mixtures could be reliably decomposed. This was, e.g., the case for the pairs benzene/ethylbenzene, benzene/p-xylene, and ethylbenzene/p-xylene (compare, e.g.. Figs. 3 and 16). 

Xylene isomers, and particularly p-xylene, are important petrochemical intermediates. p-Xylene is almost exclusively used as raw material in the production of polyester fibers, films, and solid-state packaging resins. The Cg aromatic cut obtained from naphtha reforming and steam cracking contains a mixture of xylenes (50-60 wt.% of m-xylene and 20-25 wt.% of o- and p-xylene) and ethylbenzene (15-30 wt.%). In the search for advanced energy-efficient chemical separation techniques, p-xylene production via... 

Diaz, C. Tojo, J. Isobaric vapor-liquid equilibria of the binary systems cyclohexane with o-xylene, m-xylene, p-xylene, and ethylbenzene at 101.325 kPa/. Chem. Eng. Data 2002,47, 1154-1158... 

Yan, T.Y., Separation of p-xylene and ethylbenzene from C8-aromatics using medium-pore zeolites, Ind. Eng. Chem. Res., 28(5), 572-577 (1989). 

Figure 12 shows the ultraviolet spectra of o-, m-, and p-xylenes and ethylbenzene. This mixture can be adequately analyzed by an ultraviolet method if extreme care is used (the analysis can be done more easily and more accurately by infrared or gas-liquid chromatography, however). 

It is convenient to divide the petrochemical industry into two general sectors (/) olefins and (2) aromatics and their respective derivatives. Olefins ate straight- or branched-chain unsaturated hydrocarbons, the most important being ethylene (qv), [74-85-1] propjiene (qv) [115-07-17, and butadiene (qv) [106-99-0J. Aromatics are cycHc unsaturated hydrocarbons, the most important being benzene (qv) [71-43-2] toluene (qv) [108-88-3] p- s.y en.e [106-42-3] and (9-xylene [95-47-5] (see Xylenes and ethylbenzene) There are two other large-volume petrochemicals that do not fall easily into either of these two categories ammonia (qv) [7664-41-7] and methanol (qv) [67-56-1]. These two products ate derived primarily from methane [74-82-8] (natural gas) (see Hydrocarbons, c -c ). 

An aromatic hydrocarbon feedstock consisting mainly of m-xylene is to be isomerised catalytically in a process for the production of / -xylene. The product from the reactor consists of a mixture of p-xylene, m-xylene, o-xylene and ethylbenzene. As part of a preliminary assessment of the process, calculate the composition of this mixture if equilibrium were established over the catalyst at 730 K. 

In a laboratory scale dehydrocyclization reaction using a dual-function catalyst, Davis (8) reported that the aromatic products were o-xylene, /w-xylene and ethylbenzene in approximately equal amounts (ca. 20-30% each) and p-xylene (ca, 15%). The formation of these was assumed to be from a direct 1,6-ring closure, as sketched in the following two diagrams ... 

In early measurements the solubility of [C2CiIm][PFg] in aromatic hydrocarbons (benzene, toluene, ethylbenzene, o-xylene, m-xylene, and p-xylene) and that of [C4QIm][PFg] in the same aromatic hydrocarbons, and in n-alkanes (pentane, hexane, heptane, and octane), and in cyclohydrocarbons (cyclopentane and cyclohexane) has been presented [96]. 

With toluene, in a Pyrex vessel, the products were ethylbenzene, m-and p-xylenes, and an unidentified product. In a quartz vessel o-xylene was also found and the unidentified product decreased, suggesting that the unidentified product was a methyl cycloheptatriene capable of undergoing photoisomerization to give xylenes. 

The distillate (overhead) from Unit H, containing mostly ethylbenzene [100-41-4],p-xylene, and w-xylene, and some 0-xylene becomes the feed for the />-xylene separation process (UnitJ). 

The activation energy of over-all oxidation catalyzed by 0.02M cobalt acetate and 0.04M NaBr is very small—8.3 kcal./mole for ethylbenzene, 8.7 for p-xylene, and 14.9 for n-dodecane. 

When naphtha or naphthenic gasoline fractions are catalytically reformed, they usually yield a Cx aromatics stream that is comprised of mixed xylenes and ethylbenzene. It is possible to separate the ethylbenzene and o-xylene by fractionation. It is uneconomic to separate the m- and p-xylenes in this manner because of the closeness of their boiling points. To accomplish the separation, a Werner-type complex for selective absoiption of p-xylene from the feed mixture may be used. Or, because of the widely different freezing points of the two xylene isomers, a process of fractional crystallization may be used. To boost the p-xylene yield, die filtrate from the crystallization step can be catalytically isomerized. 

XYLENES AND ETHYLBENZENE. Xylenes and ethylbenzene (EB) are Cs aromatic isomers having the molecular formula CsHi... Tile xylenes consist of three isomers methyl groups on the benzene ring. The molecular structures are shown below. 

It is obviously impossible to separate m-x ene and p-xylene by distillation, ance their boiling points differ by only (L8 C However, this separation method is feasible to isolate o-xylene and ethylbenzene, although supeifiactionations are required. Further more, the melting point of p-xylene is much higher than that of the other Cg compounds. This is why crystallization was initially employed to recover p-xylene from the mixture, which may be previously rid of ethylbenzene and o-xylene. 

As a rule, if the amount of o-xylene and ethylbenzene present in the feedstock rises, the temperature of formation of the m, p-xylenes eutectic decreases, as well as its p-xyleoe content Recover) is accordingly improved. 

With monoalkylbenzenes and m-xylene no evidence has been obtained for formation of an intermediate radical cation, the complex decomposing to a benzyl radical (path i) either by a direct HAT or via a concerted electron-proton transfer mechanism. The latter seems more likely on the basis of the observation of the order of reactivity ethylbenzene > isopropylbenzene > toluene (statistically corrected) for reaction i, in line with the stereoelectronic effects observed for the deprotonation of alkylbenzene radical cations [153]. With more readily oxidizable substrates such as p-xylene and methylbenzenes with >3 methyl groups, an analogous complex is formed this, however, is characterized by a lifetime of < 20 ns and decomposes into the intermediate radical cation (experimentally observed) from which the benzyl radical is formed by deprotonation (paths ii and Hi). 

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