Zeolite isomer separations

Isomar [Isomerization of aromatics] A catalytic process for isomerizing xylene isomers and ethylbenzene into equilibrium isomer ratios. Usually combined with an isomer separation process such as Parex (1). The catalyst is a zeolite-containing alumina catalyst with platinum. Developed by UOP and widely licensed by them. It was first commercialized in 1967 by 1992, 32 plants had been commissioned and 8 others were in design or construction. See also Isolene II. 

IsoSiv [Isomer separation by molecular sieves] A process for separating linear hydrocarbons from naphtha and kerosene petroleum fractions. It operates in the vapor phase and uses a modified 5A zeolite molecular sieve, which selectively adsorbs linear hydrocarbons, excluding branched ones. Developed by Union Carbide Corporation and widely licensed, now by UOP. The first plant was operated in Texas in 1961. By 1990, more than 30 units had been licensed worldwide. See also Total Isomerization. 

It may be noted that a large scalep-xylene process also involves an isomer separation (Parcx process [65] using an X-type zeolite) followed by isomerization and recycle. 

Ethyltoluene is manufactured by aluminum chloride-catalyzed alkylation similar to that used for ethylbenzene production. All three isomers are formed. A typical analysis of the reactor effluent is shown in Table 9. After the unconverted toluene and light by-products are removed, the mixture of ethyltoluene isomers and polyethyltoluenes is fractionated to recover the meta and para isomers (bp 161.3 and 162.0°C, respectively) as the overhead product, which typically contains 0.2% or less ortho isomer (bp 165.1°C). This isomer separation is difficult but essential because ethyltoluene undergoes ring closure to form indan and indene in the subsequent dehydrogenation process. These compounds are even more difficult to remove from vinyltoluene, and their presence in the monomer results in inferior polymers. The o-ethyltoluene and polyethyltoluenes are recovered and recycled to the reactor for isomerization and transalkylation to produce more ethyltoluenes. Fina uses a zeolite-catalyzed vapor-phase alkylation process to produce ethyltoluenes. 

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. 

Supported membrane concepts that were studied in the past 10 years include 5-p.m-thick dense Pd on supported y-alumina (Pacheco Tanaka et al., 2006), 10-p m-thick dense Lao.sSro.sCoOa.g for O2 purification (van der Haar, 2001), 60-nm thin amorphous silica for small gas molecule separation and water pervaporation (de Vos and Vetweij, 1998), >1 xm-thick MFI zeolite membranes for parajortho-xyicnc isomer separation (Caro et al., 2000), 0.4-2-p.m thick mesoporous y-alumina (Yu et al., 2006), and 4 p,m mesoporous C0AI2O4 for nanofiltration of liquids (Condom et al., 2006). 

Separation of isomers is an application where zeolite membranes could be specifically interesting because of their well-defined pores that lead to molecular sieving effects. An application that is often considered is the xylene isomerization and related reactions. 

Molecular sieving effect of the membrane has been evidenced using a mixture of two isomers (i.e. no Knudsen separation can be anticipated), n-hexane and 2-2 dimethylbutane (respective kinetic diameters 0.43 and 0.62 nm). Figure 10 shows the permeate contains almost only the linear species, due to the sieving effect of the zeolite membrane (pore size ca 0.55 nm). This last result also underlines that the present zeolite membrane is almost defect-fi ee. 

Z. (1996) Chemical liquid deposition zeolites with controlled pore-opening size and shape-selective separation of isomers. Ind. Eng. Chem. Res., 35, 430. 

Guo, G. and Long, Y. (2001) Static equilibrium studies on separation of dichlorobenzene isomers on binder-free hydrophobic adsorlaent of MFI type zeolite. Sep. Purif. Technol,... 

Zinnen, H.A. and Issa, K.C. (1992) Separation of dichlorophenol isomers with zeolite adsorbents. U.S. Patent 5,118,876. 

Barder, T. (1991) Separation of 2,7 diisopropylnaphthalene from a feed mixture comprising various diisopropylnaphthalene isomers with a zeolite adsorbent. U.S. Patent 5,012,039. 

Zinnen, H.A. and Fergin, R.L. (1990) Rejective separation of para-xylene from xylene isomers and ethylbenzene with zeolites. U.S. Patent 4,940,830. 

Namba, S., Kanai, Y., Shoji, H., and Yashima, T. (1984) Separation of p-isomers from disubstituted benzenes by means of shape-selective adsorption on mordenite and ZSM-5 zeolites. 

Zeolite membranes have also been employed for organic-organic separations where selectivity is based on adsorption and diffusion differences of non-aqueous mixtures. NaX and NaY zeolite were used in the separation of methanol from MTBE and benzene (800 < a< 10000) exploiting the more polar nature of methanol which is attracted to the electrostatic poles of the high A1 content zeolites [38]. Other separations include (i) separation of n-hexane from 2,2-DMB using ZSM5, (ii) benzene from p-xylene using MOR/FER and (iii) xylene isomers [34]. 

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. 

Because of the large demand for p-xylene, another method is now being used by Amoco to increase the percentage of the para isomer in mixed xylenes. They are heated at 300°C with an acidic zeolite catalyst, which equilibrates the three xylenes to an o,m,p ratio of 10 72 18%. The para isomer is separated by fractional crystallization, whereas the o,m mixture is reisomerized with the catalyst to produce more para product. Theoretically, all the xylenes could be transformed into the desired para isomer. The zeolite catalyst has the following structure. 

A third possibility of separating the para isomer has been used. This isomer can be selectively adsorbed on zeolites, then desorbed after the ortho and meta isomers have passed through. 

In the physical separation process, a molecular sieve adsorbent is used as in the Union Carbide Olefins Siv process (88—90). Linear butenes are selectively adsorbed, and the isobutylene effluent is distilled to obtain a polymer-grade product. The adsorbent is a synthetic zeolite, Type 5A in the calcium cation exchanged form (91). UOP also offers an adsorption process, the Sorbutene process (92). The UOP process utilizes a liquid B—B stream, and uses a proprietary rotary valve containing multiple ports, which direct the flow of liquid to various sections of the adsorber (93,94). The cis- and trans-isomers are... 

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