GB787084A - Isomerization of xylene - Google Patents

Abstract

<PICT:0787084/IV (b)/1> p-Xylene is produced by the isomerization of a mixture of xylene isomers consisting predominantly of o- and m-xylene and containing less than an equilibrium amount of p-xylene in which the mixture is fractionally distilled to separate an overhead fraction rich in m-xylene and a bottoms fraction rich in o-xylene, and in which the overhead fraction is contacted with a silica-alumina catalyst at 700-1000 DEG F. and at a space velocity of 0.5-5.0 v/v/hour and the bottoms fraction is separately contacted with a silica-alumina catalyst at a temperature of 100-150 DEG F. higher than that employed in treating the overhead fraction and at approximately the same space velocity. The catalysts are preferably periodically regenerated by contacting with a free oxygen-containing gas and passing steam through the catalyst for a period of 2-20 minutes immediately subsequent to the regeneration step. In an example, a feed consisting of ethylbenzene 5.8 weight per cent, para-xylene 18.6, meta-xylene 38.4, ortho-xylene 26.0, and C7 and C9 hydrocarbons and paraffins 11.2, is passed through a line 1 into a crystallization zone 2 where a part of the p-xylene is crystallized at a temperature of -75 DEG to -120 DEG F., the crystals being separated by a centrifugal filter 4. The mother liquor is removed by line 6 and a fractionated in zone 7 into a fraction containing chiefly m-xylene which is removed through line 8 and is isomerized in zone 14 at a temperature of 700-1000 DEG F. while the higher boiling fraction is removed through line 13 and isomerized at from 100-150 DEG F. higher than the temperature used for the overhead fraction. The isomerized mixture is distilled in zone 19 from which ethylbenzene and lighter components are taken off by line 24. A fraction consisting chiefly of m- and p-xylene is removed by line 25 and a fraction consisting predominantly of o-xylene by line 26.