Hydrocarbon processing xylene isomer 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. 

A brief list of some important separations which are carried out with zeolite adsorbents is given in Table 1.3. A much more comprehensive list has been given by Collins and reprinted by Breck. The large majority of these applications are purification processes in which the zeolite is used as a selective adsorbent to remove an undesirable impurity such as water, HjS, or radioactive Kr. Important examples of true separation processes include the linear/branched chain hydrocarbon separation, carried out with zeolite A and the separation of xylene isomers which utilizes various cationic forms of the X and Y zeolites. Some of these processes are considered in greater detail in Chapters 10-12. 

Distillations are perfect candidates for Raman process monitoring since many potential fluorescent or interfering species are reduced or removed. The Institut Francais du Petrole holds two relevant patents discussing the use of Raman spectroscopy to control the separation of aromatics by adsorption on simulated moving beds and the separation of isomers, particularly aromatic hydrocarbons with 8-10 carbon atoms such as para-xylene and orffio-xylene.67 68... 

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]. 

In the separation of mixtures of aromatic hydrocarbon isomers, the para-isomer is generally preferentially absorbed and can be separated on the basis of kinetic effects. The most well-known example is the recovery of p-xylene (Parex process). 

Molecular sieving by controlling access of molecules to the internal surfaces and by restricting molecular dilfusivity is particularly important in processes that require the separation of branched from linear alkanes or in resolution of the different isomers of xylene relevant dimensions of some important hydrocarbons of these types are given in Table 7.2. Small-pore zeolites such as Na-A are particularly important for the separation of n-alkanes and n-alkanols from their branched isomers, whereas medium-pore zeolites such as ZSM-5 show adsorption of p-xylene but very slow (or no) adsorption of o-xylene. Molecular sieving is also important in restricting the size of molecules that leave the pores of zeolites after catalytic reaction within them. This product diffusivity selectivity is described in detail for specific examples in the next chapter, but the intra-zeolitic isomerisation of xylenes in ZSM-5 to give predominantly p-xylene product is an excellent example. 

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