Xylene , isomers

The 3D autocorrelation vector of the three xylene isomers in Figure 8-4 differ only with respect to the component relating to the two methyl groups. For o-xylene it is... 

Dimethyl derivatives of benzene are called xylenes There are three xylene isomers the ortho (o) meta (m) and para (p) substituted derivatives... 

Fnedel-Crafts acylation of the individual isomers of xylene with acetyl chlonde and alu minum chloride yields a single product different for each xylene isomer in high yield in each case Write the structures of the products of acetylation of o m and p xylene... 

Xylene isomerization Xylene isomers m-Xylenes [108-38-3] o-Xylenes [95-47-6] p-Xylenes [106-42-3] Xylenes... 

Ciyst lliz tion. Low temperature fractional crystallization was the first and for many years the only commercial technique for separating PX from mixed xylenes. As shown in Table 2, PX has a much higher freezing point than the other xylene isomers. Thus, upon cooling, a pure soHd phase of PX crystallizes first. Eventually, upon further cooling, a temperature is reached where soHd crystals of another isomer also form. This is called the eutectic point. PX crystals usually form at about —4° C and the PX-MX eutectic is reached at about —68° C. In commercial practice, PX crystallization is carried out at a temperature just above the eutectic point. At all temperatures above the eutectic point, PX is stiU soluble in the remaining Cg aromatics Hquid solution,... 

The xylene isomers are flammable Hquids and should be stored in approved closed containers with appropriate labels and away from heat and open flames. Limits for transportation by air are 5 L on passenger planes and 60 L on cargo planes. 

Xylene Isomeri tion. The objective of C-8-aromatics processing is the conversion of the usual four-component feedstream (ethylbenzene and the three xylenes) into an isomerically pure xylene. Although the bulk of current demand is for xylene isomer for polyester fiber manufacture, significant markets for the other isomers exist. The primary problem is separation of the 8—40% ethylbenzene that is present in the usual feedstocks, a task that is compHcated by the closeness of the boiling points of ethylbenzene and -xylene. In addition, the equiUbrium concentrations of the xylenes present in the isomer separation train raffinate have to be reestabUshed to maximize the yield of the desired isomer. 

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. 

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). 

The MTDP process, which is similar to the Tatoray process, produces an equilibrium composition of xylene isomers. A -xylene yield of 24% in the xylene product is formed at 42—48 wt % toluene conversion over the heterogeneous catalyst at 390—495°C, 4.2 MPa (600 psig), 1 2 Hquid hourly space velocity, and 4 H2/hydrocarbon molar feed ratio. A new ZSM-5 catalyst, which has higher activity and stability than the current catalyst, has been reported (93). 

The selective alkylation of toluene with methanol to produce -xylene as a predominant isomer can be achieved over shape-selective catalysts (99—101). With a modified ZSM-5 zeoHte catalyst, more than 99% -xylene in xylene isomers can be produced at 550°C. This -xylene concentration exceeds the equiHbrium concentration of 23% (99). The selective synthesis of -xylene using relatively low cost toluene is economically attractive however, this technology was not commercialized as of 1991. 

Transall lation. Two molecules of toluene are converted iato one molecule of benzene and one molecule of mixed xylene isomers ia a sequence called transalkylation or disproportionation. Economic feasibiUty of the process strongly depends on the relative prices of benzene, toluene, and xylene. Operation of a transalkylation unit is practical only when there is an excess of toluene and a strong demand for benzene. In recent years, xylene and benzene prices have generally been higher than toluene prices so transalkylation is presendy an attractive alternative to hydrodealkylation (see also Btx... 

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. 

Fig. 25. Schematic diagram of a system used to separate xylene isomers (69). PC = pressure control, TC = temperature control, and FC = flow control.

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. 

Orthoxylene (the highest boiling xylene isomer) is separated from the other xylenes and the heavier C, aromatics by fractionation. The meta and lighter xylenes are taken overhead in a xylenes splitter containing 160 trays. Orthoxylene is then separated from the C, aromatics in a 50-plate rerun column. Product purity from such a fractionation is typically 99-1- %. 

Although many of the aromatic compounds based on benzene have pleasant odors, they are usually toxic, and some are carcinogenic. Volatile aromatic hydrocarbons are highly flammable and burn with a luminous, sooty flame. The effects of molecular size (in simple arenes as well as in substituted aromatics) and of molecular symmetry (e.g., xylene isomers) are noticeable in physical properties [48, p. 212 49, p. 375 50, p. 41]. Since the hybrid bonds of benzene rings are as stable as the single bonds in alkanes, aromatic compounds can participate in chemical reactions without disrupting the ring structure. 

Properties There are three xylene isomers, commonly known as orr/zo-xylene, meta-xylene, and para-xylene. They are all colorless liquids. The orr/zo-isomer boils at 144°C, the meta- at 139.1°C, and the para-at 138.5°C. 

As an example of the selective removal of products, Foley et al. [36] anticipated a selective formation of dimethylamine over a catalyst coated with a carbon molecular sieve layer. Nishiyama et al. [37] demonstrated the concept of the selective removal of products. A silica-alumina catalyst coated with a silicalite membrane was used for disproportionation and alkylation of toluene to produce p-xylene. The product fraction of p-xylene in xylene isomers (para-selectivity) for the silicalite-coated catalyst largely exceeded the equilibrium value of about 22%. 

As illustrated in Figure 10.6, the high para-selectivity in the toluene disproportionation is caused by the selective removal of p-xylene from the silica-alumina particles, which leads to an apparent equilibrium shift between the xylene isomers. 

Since their development in 1974 ZSM-5 zeolites have had considerable commercial success. ZSM-5 has a 10-membered ring-pore aperture of 0.55 nm (hence the 5 in ZSM-5), which is an ideal dimension for carrying out selective transformations on small aromatic substrates. Being the feedstock for PET, / -xylene is the most useful of the xylene isomers. The Bronsted acid form of ZSM-5, H-ZSM-5, is used to produce p-xylene selectively through toluene alkylation with methanol, xylene isomerization and toluene disproportionation (Figure 4.4). This is an example of a product selective reaction in which the reactant (toluene) is small enough to enter the pore but some of the initial products formed (o and w-xylene) are too large to diffuse rapidly out of the pore. /7-Xylene can, however. 

All these substrates can be degraded nnder aerobic conditions and, althongh there appear to be important differences among the xylene isomers and mutant strains have been isolated that can degrade all three isomers (Di Lecce et al. 1997). Reviews have covered varions aspects of this problem ... 

Another possibility is to measure the interaction between liquid crystal26 and two xylene isomers, para- and meta-xylene, which is demonstrated in Figure 13. 

Isarom A catalytic process for isomerizing the xylene isomers, developed by Institut Frangais du Petrole. The catalyst is aluminum trifluoride. 

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. 

MGCC [Mitsubishi Gas-Chemical Company] Also called JGCC. A process for extracting m-xylene from mixed xylene isomers by making the fluoroboric acid complex. All the xylene isomers form such complexes, but that formed by the m-isomcr is much more stable than the others. Development started in 1962 by 1979, three plants were operating. 

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