GB853815A - Apparatus for separating solids and liquids - Google Patents

Abstract

Apparatus for separating floatable polyolefins from mixture with a liquid consists of a confined skimming tank adapted to hold the mixture, means for introducing the mixture, means for withdrawing the liquid, inclined auger means having a lower terminus openly communicating with the top section of the skimming p tank, and means for maintaining a liquid level above the lower terminus of the auger means, the walls of the top section of the skimming tank forming an apex which is truncated by the lower terminus of the auger means (see Group I). The mixture may be that obtained by spraying a solution of polymer into water whereby the polymers are precipitated and dispersed in the water and removed from the solution as described in Specification 835,301. Preferred polymerization methods with which the apparatus may be associated are described in Specifications 790,195 and 804,641. In a method described with reference to Fig. 1 (not shown) of the present specification, a mono-olefin, e.g. ethylene, is passed to a polymerization zone together with a chromium oxide catalyst containing hexavalent chromium associated with silica-alumina dispersed in a solvent. Additional solvent, e.g. cyclohexane, may also be introduced. The temperature range is 230-300 DEG F., cooling water being applied through a heat exchanger. Polymerization zone effluent passes at 280 DEG F. to a monomer removal zone, a catalyst removal zone and a solvent "adjustment" zone (where solvent is removed or added), in turn. The solution is here cooled to 240 DEG F. and its concentration adjusted to 4,5% polymer. Thereafter the solution is dispersed in water at lower temperature (e.g. 100 DEG F.). The resulting mixture has a temperature in the range 100-130 DEG F., preferably 120 DEG F., and is pumped past a pressure-reducing valve which maintains a pressure of 35-45 p.s.i.g. The mixture passes to a stripping zone where it is agitated at 170 DEG F. at atmospheric pressure, or at 140 DEG F. at 7 p.s.i.a. Steam is supplied. Steam and solvent pass overhead and are separated. Polymer solids-water mixture passes to the apparatus first referred to (Figs. 2 and 3, not shown). Withdrawn water may be returned to the stripping zone. The process may be used for obtaining polymers or copolymers of propylene or butylene or copolymers of mono-olefins and diolefins such as butadiene or isoprene. Other catalysts mentioned are triethyl aluminium plus titanium tetrachloride, and mixtures of ethylaluminium halides with titanium tetrachloride. Another group of catalysts consists of a halide of a Group IV metal, e.g. titanium tetrachloride, silicon tetrabromide, zirconium tetrachloride, tin tetrabromide, with one or more free metals selected from the group sodium, potassium, lithium, rubidium, zinc, cadmium, and aluminium. Examples of solvents are: propane, isobutane, n-pentane, isopentane, iso-octane; those paraffins having from 5 to 12 carbon atoms per molecule are preferred. Other solvents mentioned are cyclohexane and methylcyclohexane. Details are given of a specific example of a solid polyethylene preparation.