P-Xylene separation

Displacement-purge forms the basis for most simulated continuous countercurrent systems (see hereafter) such as the UOP Sorbex processes. UOP has licensed close to one hundred Sorbex units for its family of processes Parex to separate p-xylene from C3 aromatics, Molex tor /i-paraffin from branched and cyclic hydrocarbons, Olex for olefins from paraffin, Sarex for fruc tose from dextrose plus polysaccharides, Cymex forp- or m-cymene from cymene isomers, and Cresex for p- or m-cresol from cresol isomers. Toray Industries Aromax process is another for the production of p-xylene [Otani, Chem. Eng., 80(9), 106-107, (1973)]. Illinois Water Treatment [Making Wave.s in Liquid Processing, Illinois Water Treatment Company, IWT Adsep System, Rockford, IL, 6(1), (1984)] and Mitsubishi [Ishikawa, Tanabe, and Usui, U.S. Patent 4,182,633 (1980)] have also commercialized displacement-purge processes for the separation of fructose from dextrose. 

Aromax (2) Also known as Toray Aromax. A chromatographic process for separating p-xylene from its isomers. Similar to the Parex (1) process, it operates in the liquid phase at 200°C, 15 bar. Developed in 1971 by Toray Industries, Japan. 

Guo, G. and Long, Y. (2001) Process for separating p-xylene with hydrophobic silicic zeolite by selective adsorption. C.N. Patent 1,280,977. 

The free sulphonic acids [e.g. toluene-p-sulphonic acid, for preparation see, Expt 6.37, Note (1)], as opposed to their sodium salts, may sometimes be obtained directly if the sulphonation reaction is carried out with continuous removal of the water formed in the reaction, conveniently by using a Dean and Stark water separator. p-Xylene-2-sulphonic acid (Expt 6.40) is an example of a sulphonic acid whose solubility in water is such that it crystallises directly from the reaction medium and hence it may readily be isolated. 

Chevron (2) A process for separating p-xylene from its isomers by continuous crystallization, using liquid carbon dioxide in direct contact with the xylene as the refrigerant. Developed by the Chevron Research Company in 1966. 

Eluxyl A process for separating p-xylene from its isomers, using an adsorbent-solvent technique. The process is based on simulated countercurrent adsorption, where the selective adsorbent is held stationary in the adsorption column. The feed mixture to be separated is introduced at various levels in the middle of the column, as in the Sorbex process. The p-xylene product can be more than 99.9% pure. Developed by IFP and Chevron Chemical. A large pilot plant was built in Chevron s site at Pascagoula, MS, in 1994, and a commercial plant on the site was announced in 1996. By 2005, eight plants had been licensed by Axens, of which three were operating. 

Crystallization was for many years the only industrial technique employed to separate p-xylene from its isomers. 

In oil processing, separation of aromatic isomers Cg (ethylbenzene 7b= 136°C,p-xylene 7b= 138.3°C, m-xylene Ty, = 139.1°C, >-xylene T], = 144.4°C) is required. According to the literary data, the following isomers of hydrocarbons are separated p-xylene/m-xylene, p-xylene/o-xylene, -hexane/2,2-dimethylbutane, -hexane/3-methylpentane, and n-butane/f-butane [8,83,130-137]. Pervaporation method is the most effective for this purpose. To separate the isomers, membranes based on various polymers were used. Good separation for aU isomer mixtures was attained by the polyimide Kapton film (fip = 1.43-2.18) but parylene films and cellulose acetate also exhibited a relatively high separation factor (fip = 1.22-1.56 and /3p = 1.23-1.56, respectively). Temperatures >200°C were required to obtain a reasonable flux through the polyimide film and a pressure of about 20 atm was necessary to keep the feed stream liquid [8]. 

Fig. 4. ZSM-5 membrane performance in xylene isomer separation, p-xylene, o-xylene permeance and mixture separation factor (SF) are plotted versus temperature of permeation for typical c-oriented (A) and b-oriented (B) films [110].

This technique, of the Arosorb type (Sun Oil), is uneconomic for the treatment of complex cuts from which the paraffins and naphthene are to be eliminated. Hence it has not been generalized. However, to obtain special gasolines with very low aromatics contents, similar processes are sometimes employed. The main value of adsorption emerged through the possibility it offers of separating p-xylene from its isomers in aromatic CB cuts, with high yields (see Section 4,3.3.2). 

The crystallization point of p-xylene is markedly higher than that of the other Cg-aromatics it is therefore possible to separate p-xylene by crystallization. Since p-xylene can be better adsorbed than the accompanying Cg-aromatics, adsorption can also be used in its recovery. 

Crystallization is used to separate p-xylene from its C8 isomers. The melting point of p-xylene is 13 C which is significantly higher than the other xylenes or ethylbenzene. Therefore, p-xylene can be selectively crystallized by cooling a C8 mixture. 

Another method to separate p-xylene relies on the difference in shape of the isomers. The para isomer can be selectively adsorbed into pores of certain zeolites. It is then rinsed out of the zeolite bed with a desorbing solvent that... 

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