GB997838A - Recovering polymer from dilute polymer dispersions - Google Patents

Abstract

Solvent is separated by evaporation from a dilute solution, suspension or slurry of a thermoplastic material, such as a polyolefine, polystyrene or polyvinyl chloride, by passing the solution through the tubes of a multitubular heat-exchanger, the shell of which is supplied with heating-medium at such a temperature that solvent is evaporated, the flow of solution through the tubes being stabilized (i) by recycling to the feed-line some of the concentrate withdrawn from the heat exchanger, or (ii) by submitting the solution to preliminary partial flash vaporization, and then introducing the mixture of vapours and solution into the tubes of the heat-exchanger. Capillary tubes may be provided at the entrance of the heating-tubes in order to assist in stabilizing the flow. In an example, a solution of polypropylene in xylol, containing 8,6% polymer by weight, is preheated to 227 DEG C., a pressure of 108 p.s.i.g. being maintained on the solution by means of a motorized needle-valve operated by a pressure indicator controller. After flashing to a pressure of 21 p.s.i.g., the vapour-liquid mixture is forced into the tubes of a vertical heat-exchanger. Each tube has a ( 1/8 inch x 1 inch) capillary at the inlet. The mixture of vapour and liquid is discharged into a receiver held at 4 p.s.i.g., and the vapour is withdrawn to a condenser. The liquid, containing about 84% polymer, is pumped through a second heat-exchanger maintained at about 250 DEG C.; and molten polypropylene is discharged, together with xylene vapour, into a receiver at a pressure of 0,2 mm. Hg. The molten polymer is extruded into a water bath; and the solid strands so obtained are cut into pellets. Dichlorobenzene vapour may be used as heating-medium in the heat-exchangers.