Xylene isomer separation

Xylenes. The main appHcation of xylene isomers, primarily p- and 0-xylenes, is in the manufacture of plasticizers and polyester fibers and resins. Demands for xylene isomers and other aromatics such as benzene have steadily been increasing over the last two decades. The major source of xylenes is the catalytic reforming of naphtha and the pyrolysis of naphtha and gas oils. A significant amount of toluene and Cg aromatics, which have lower petrochemical value, is also produced by these processes. More valuable p- or 0-xylene isomers can be manufactured from these low value aromatics in a process complex consisting of transalkylation, eg, the Tatoray process and Mobil s toluene disproportionation (M lDP) and selective toluene disproportionation (MSTDP) processes isomerization, eg, the UOP Isomar process (88) and Mobil s high temperature isomerization (MHTI), low pressure isomerization (MLPI), and vapor-phase isomerization (MVPI) processes (89) and xylene isomer separation, eg, the UOP Parex process (90). 

Column crystalhzers of the end-fed type can be used for purification of many eutectic-type systems and for aqueous as well as organic systems (McKay loc. cit.). Column ciystaUizers have been used for xylene isomer separation, but recently other separation technologies including more efficient melt ciystaUization equipment have tended to supplant the Phillips style ciystaUizer. 

The same situation was evidenced in the case of xylene isomers separation. The evapomeation is more efficient than that of pervaporation [83],... 

The technology of SMB chromatography has been widely used in the petrochemical (xylene isomer separation) and food industries (glucose-fructose separation) in... 

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].

Table 15.4 Comparison of xylene isomers separation by zeolite membrane and polymeric...

All the above mentioned high perm-selectivity of zeolite membranes can be attributed to the selective sorption into the membranes. Satisfactory performance can be obtained by defective zeolite membranes. Xylene isomers separation by zeolite membranes compared with polymeric membranes are summarized in Table 15.4. As shown, zeolite membranes showed much higher isomer separation performances than that of polymeric membranes. Specially, Lai et al. [41] prepared b-oriented silicalite-1 zeolite membrane by a secondary growth method with a b-oriented seed layer and use of trimer-TPA as a template in the secondary growth step. The membrane offers p-xylene permeance of 34.3 x 10 kg/m. h with p- to o-xylene separation factor of up to 500. Recently, Yuan et al. [42] prepared siUcalite-1 zeolite membrane by a template-free secondary growth method. The synthesized membrane showed excellent performance for pervaporation separation of xylene isomers at low temperature (50°C). 

After the ion-exchange of MFI zeolite membranes from Na-type to H-type, the alkylation of toluene with methanol was carried out at temperatures of 450 and 500°C. The reactants, in a molar composition of 2 toluene 1 methanol, were fed with a syringe pump. The carrier gas used from the outer to the inner side of the cylindrical membranes was He. The composition was analyzed by gas chromatography using a xylene-isomer separation column (Ben-tone 34, Supelco). The pressure difference across the membrane was controlled at approximately 10 kPa. 

Daramola MO, Burger AJ, Pera-Titus M, Giroir-Eendler A, Miachon S, Lorenzen L, Dahnon JA. Nanocomposite MFI-ceramic hollow fibre membranes via pore-pugging synthesis Prospects for xylene isomer separation. J Membr Sci 2009 337 106-112. 

Deng, Z., Nicolas, C.-H., Guo, Y., Giroir-Fendler, A., and Pera-Titus, M. (2010) Synthesis and characterization of nanocomposite B-MFl-alumina hollow fibre membranes and application to xylene isomer separation. Micropor. Mesopor. Mater., 133, 18-26. 

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