Separation of xylenes

Since 1971 mainly adsorptive separation processes are used to obtain high purity -xylene (55,84—86). A typical commercial process for the separation of -xylene from other Cg aromatics produces about 99.8% purity -xylene at greater than 95% recovery. 

Extraction of C-8 Aromatics. The Japan Gas Chemical Co. developed an extraction process for the separation of -xylene [106-42-3] from its isomers using HF—BF as an extraction solvent and isomerization catalyst (235). The highly reactive solvent imposes its own restrictions but this approach is claimed to be economically superior to mote conventional separation processes (see Xylenes and ethylbenzene). 

Exploiting the relative basicity of the xylene isomers, commercial units employ superacids, typically HE—BF, as the acid complexing agent for the separation of / -xylene (feedstock for isophthalic acid) (15). Amoco produces high purity / -xylene at its Texas City facility using the HE—BF process (see Btx processing). Similar processes can be used for the separation of high purity mesitylene and isodurene from their and C q isomers, respectively. 

One of the early column crystallizers was that iatroduced for the separation of xylene isomers (see Xylene and Ethylbenzene). In this unit, shown schematically ia Eigure 25, -xylene crystals are formed ia a scraped-surface chiller above the column and fed to the column. The crystals move downward counter-currenfly to impure Hquid ia the upper portion of the column and melted -xylene ia the lower part of the column. Impure Hquor is withdrawn from an appropriate poiat near the top of the column of crystals while pure product, xylene, is removed from the bottom of the column. The pulse unit drives melt up the column as reflux and iato a product receiver. 

Molecular sieving Separation on n- and isoparafins using 5A zeolite Separation of xylenes using zeolite... 

A similar situation is reported for the separation of xylene isomers [83],... 

M. (2001) Separation of xylene isomer vapors with oriented MFI membranes made by seeded growth. Ind. Eng Chem. Res., 40, 544-552. 

O Brien-Abraham,)., Kanezashi, M., and Un, Y.S. (2008) Effects of adsorption-induced microstrucmral changes on separation of xylene isomers through MFI-type zeolite membranes./. Memhr. Sci., 320, 505-513. 

The use of various zeolites for the separation of xylene isomers has received a considerable amount of attention. Numerous patents have been issued for such separations however, equilibrium selectivity factors tend to be relatively low (9). We found virtually no p-xylene selectivity for a dealuminized H-mordenite under equilibrium conditions. H-ZSM-5, however, did exhibit a distinct preference for p-xylene sorption even at equilibrium (Table V). 

Separation of xylenes separation of lower-molecular-weight aromatic hydrocarbon isomers, especially at the lower area of the nematic region. 

Analytical Properties Separation of xylene and ethyltoluene isomers Reference 16... 

Chromatography Vapour or liquid Solid adsorbent or liquid adsorbent on a solid support Separation of xylene isomers and ethylbenzene... 

Figure 3.6. Separation of xylene isomers on a polar column, DB-Wax. Courtesy of J W Scientific.

Figure 5. Separation of xylenes on 10-/im LiChrosorb RP-8 column (50 x 4.0 mm i.d.) (a) without CD, (b) with 3.1 O 1 M a-CD, and (c) with 2.7 x 10"J M fi-CD. Solvent composition 20 vol% ethanol in water flow rate 3.6 mL/min temperature 25 C. (Reprinted with permission from ref. 28. Copyright 1986 Elsevier Science Publishers.)...

As shown in Table 3 for the straight channels of MFI, p-xylene, of which the kinetic diameter is actually larger than the pore aperture diameter, can be accomodated by deforming the pore from circular to ellipsoidal. This phenomenon should be taken into account when studying the separation of xylenes with MFI-frameworks in a membrane configuration. 

The separation of xylene isomers on MFl zeolite membranes can be considered one example of intracrystaUine size exclusion and competitive adsorption (strongly dependent on coverage). The difference in their kinetic diameters ( 0.58 nm for p-xylene and 0.68 nm for o- and m-xylene) indicates the possibility of an effective separation using MFl membranes (see Table 10.1). The kinetic diameter of p-xylene is close to one of the MFl channels ( 0.55 nm) whereas o- and m-xylene might be excluded. Therefore, MFl zeolite channels and crystal grain boundaries determinate the permeation characteristics [27]. 

The separation of -hexane/2,2-dimethylbutane, (DMB), another separation where size exclusion takes place, has been accomplished by Handers et al. [156], using pervaporation and vapor permeation. Both, the n-Cg and DMB fluxes are higher in the former case due to the higher driving force in pervaporation, leading to a lower selectivity compared to vapor permeation. The separation of xylene isomers on MH membranes has been described in Section 10.4.2.1 as a separation where size exclusion takes place. The results of the separation of these isomers using pervaporation with FER membranes [20] and MFl [21] were not successful, and very low fluxes of 10 and 10 mol/m s, accompanied by separation factors not greater than 16, respectively, were obtained. The best results for the pervaporation of xylenes were obtained by Yuan et al. [129] who prepared a template free sihcahte-1 membrane, the separation factor for a 50/50 wt% mixmre of p-xylene/o-xylene at 50°C was 60, and the flux of p-xylene was 13.7 x 10 kg/m h. 

Yuan WH, Lin YS, and Yang WS. Molecular sieving MFI-type zeolite membranes for pervaporation separation of xylene isomers. JAm Chem Soc 2004 126 4776 777. 

Separation of xylene isomers by freezing Chemical methods of separating m-xylene One-stage preparation of TNX Two-stage preparation of TNX Nitration via DNX Nitration via MNX Three stage preparation of TNX Mononitraiion Nitration of MNX to DNX Nitration of dinitroxylene to trinitroxylene Moftonitration of xylene (I G. Leverkusen method)... 

W.H. Yuan, Y.S. Lin, and W.S. Yang, Molecular Sieving MFI-type Zeolite Membranes for Pervaporation Separation of Xylene Isomers. J. Am. Chem. Soc., 2004, 126, All6-4777. 

The best approach to improving separations is to work toward reactions that achieve 100% yields at 100% conversions. Frequently, this will require more selective catalysts. The previous chapter contained an example moving in this direction. Toluene was disproportionated to benzene and xylenes using a silica-modified zeolite catalyst.23 After removal of benzene and unchanged toluene by distillation, the xylene remaining was a 99% para-isomer. It was clean enough to put directly into the process of oxidation to terephthalic acid. This avoided the usual separation of xylenes by crystallization or by a molecular sieve. There are times when an equilibrium can be shifted by removal of a product or by-product continuously to give 100% conversion. The familiar esterification with azeotropic removal of water or removal of water with a molecular sieve is an example. 

Separation of organic/organic mixtures Extraction of water from an aqueous solution of ethanol Extraction of water from solutions of ethanol, acetic acid Separation of benzene/ -hexane mixtures Separation of xylene isomers... 

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