Selective crystallization of paraxylene in a water-alkanol mixture

Claim 2

The process according to claim 2 wherein the liquid refrigerant consists of a methanol-water mixture containing from about 60 to about 70 percent v methanol and from about 0.03 to about 0.15 percent w sodium dodecylbenzene sulfonate.

Claim 3

The process according to claim 3, wherein the suspension after leaving the cooling zone is filtered or centrifuged, whereupon the crystal mass is washed, at a temperature below the melting point of paraxylene, with a liquid that is substantially immiscible with paraxylene which liquid wets the crystals preferentially and has an interfacial tension of less than about 5 dyne/cm. relative to the mother liquor.

Claim 4

The process according to claim 4, wherein the washing liquid has substantially the same composition as the liquid refrigerant and the washing is carried out in at least two stages, each time at a higher temperature but always at a temperature below the melting point of paraxylene.

Claim 5

In a process for separating paraxylene from a homogeneous, normally liquid mixture of paraxylene and closely boiling homologs, wherein the liqUid mixture is chilled to below the crystallization temperature of paraxylene to crystallize substantially only paraxylene by directly contacting the liquid mixture in a cooling zone, in countercurrent, with a liquid refrigerant that is substantially immiscible with the liquid mixture or with the resulting mother liquor, the improvement which comprises: a. conducting said contacting in a single cooling zone having a plurality of mixing stages; b. employing a liquid refrigerant which preferentially wets the crystals of paraxylene and which consists of i. alkanol-water mixtures containing from about 60 to about 70 percent by volume alkanol, and ii. from about 0.01 to about 3 percent by weight, based on the weight of liquid refrigerant, of a surface active agent selected from the group consisting of fatty acid soaps, petroleum sulfonates, alkali metal alkylbenzene sulfonates, sulfated alcohols, alkylbenzene sulfonates, long chain alcohol ethoxylates, fatty acid ethoxylates, and fatty acid sulfonates; c. crystallizing paraxylene at the refrigerant-liquor mixture interface; d. forming a suspension of paraxylene crystals dispersed in the liquid refrigerant, which suspension contains only small amounts of entrained mother liquor, by passage of the crystals across the refrigerant-mother liquor interface; e. removing from the cooling zone, as separate streams, a paraxylene-depleted mother liquor and the suspension of paraxylene crystals dispersed in liquid refrigerant; and f. recovering paraxylene crystals having a purity in excess of 99 percent from the crystal-refrigerant suspension.