CHEMICALS FROM XYLENES

Xylenes (dimethylbenzenes) are an aromatic mixture composed of three isomers (o-, m-, and p-xylene). They are normally obtained from catalytic reforming and cracking units with other C₆, C₇, and C₈ aromatics. Separating the aromatic mixture from the reformate is done by extraction-distillation and isomerization processes . para-Xylene is the most important of the three isomers for producing terephthalic acid to manufacture polyesters. m-Xylene is the least used of the three isomers, but the equilibrium mixture obtained from catalytic reformers has a higher ratio of the meta isomer. Table 1.1 shows the thermodynamic composition of C8 aromatics at three temperatures.

Table 1.1:Thermodynamic equilibrium composition of C₈ aromatics at three temperatures

m-Xylene is usually isomerized to the more valuable p-xylene. As mentioned earlier, xylene chemistry is primarily related to the methyl substituents, which are amenable to oxidation. Approximately 65% of the isolated xylenes are used to make chemicals. The rest are either used as solvents or blended with gasolines. The 1998 U.S. production of mixed xylenes for chemical use was approximately 9.5 million pounds. P Xylene alone was about 7.7 million pounds that year.