Improvements in process and apparatus for purification of crystals

摘要

In a process for crystal purification in which a slurry of liquid and crystallized solids is moved through a liquid removal zone, a reflux zone, and a solids melting zone, part of the melt of the solids melting zone being removed from the system and another part being forced in a direction countercurrent to the direction of movement of solids in the reflux zone, the material in the zones is subjected to an intermittent back pressure simultaneously with the movement of the solids. A liquid feed mixture of two or more components is forced by a pump 3 into a chiller A, where one of the components is crystallized, and then into a column 4, having a filtration zone B where liquid passes through a cylindrical filter 14 and leaves by a pipe 16. The crystal mass passes through a reflux zone C towards a heater 17 which melts the crystals. Part of the melt is withdrawn through a pipe 18 and the remainder is forced back through reflux zone C to effect crystal purification, the resulting liquid being drawn off through outlet 16 with the filtrate. A piston 21 in a cylinder 19 applies a back-pressure to the material in the column at a frequency of 15 to 200 pulsations per minute. A thermocouple 29 is connected to a temperature recorder-controller TRC which operates a valve 30 in pipe 18 and is adjusted so that the rate of withdrawal of melt in pipe 18 is varied to maintain a desired rate of advance of crystals through reflux zone C. The withdrawal of filtrate in pipe 16 may be controlled by a solenoid valve 32, so that filtrate is removed only on the compression stroke of piston 21. The withdrawal of melt by pipe 18 also may be controlled so that melt withdrawal ceases during the compression stroke of the piston 21. In a modification the cylinder 20 is spaced from the column 4 by a conduit, and in another modification the pulsation is produced by a diaphragm in the side of the column which is actuated by a piston acting on an intermediate fluid. In examples, para-xylene is separated from a mixture of xylenes and ethyl benzene. Lists are given of compounds which may be separated from mixtures containing them. The process also may be used to remove water, in the form of ice crystals, from fruit and vegetable juices and other liquid food products.