Xylene Isomerization

Arene isomerization takes place in disproportionation and transalkylation processes of methylbenzenes, also intended for manufacturing para- and ortho-xylenes, used for production of terephthalic acid and phthalic anhydride, oligopolyesters, fibers, varnishes, plasticizers and other products.

 Due to the peculiarities of xylene thermodynamic equilibrium (minor change of xylene equilibrium concentration with the temperature), corrosion aggressiveness and non-regenerability of catalytic systems with high acidity, catalysts based on aluminum chloride or boron fluoride didn't get extensive industrial application.

The most widespread xylene isomerization catalysts became the ones of two types: based on amorphous or crystalline aluminosilicates, and also similar to them heterogeneous catalysts containing platinum.

Depending on xylene mixture composition and their separation method, aluminosilicate catalysts, operating at atmospheric pressure and in the temperature range from 450 to 550 °C, are used. Over these catalysts ethylbenzene is exposed generally to disproportionation that determines short cycle length of catalyst operation. The recommended content of ethylbenzene in isomerization feed shouldn't exceed 6–13%. Whilst P-and O-xylene yield makes about 93 wt. %.

The most large-scale process of arene isomerization is isomerization of xylenes, produced in processes of pyrolysis and catalytic reforming, toluene disproportionation and C9 aromatic hydrocarbon transalkylation. Produced in these processes para - and ortho-xylenes make a significant proportion of the total world C 8 arene output. Major quantity of these xylene isomers is used in the petrochemical industry.

Industrial processes of xylene isomerization are represented by three main types:

1）Isomerization in liquid phase (at temperature below 100 °C);

2）Isomerization in gas phase under atmospheric pressure;

3）Isomerization in gas phase under hydrogen pressure.

Now for xylene isomerization ,Application of these catalysts allows increasing “once-through” ethyl benzene conversion up to 65–75%, and also converting considerable part of non-aromatic hydrocarbons.