GB932576A - Drying apparatus and method - Google Patents

Abstract

932,576. Drying systems and apparatus. PILLISBURY CO. April 18, 1961 [April 29, 1960], No. 13922/61. Class 34 (2). Apparatus and method for the low temperature drying of moisture containing material comprises subjecting the material to a zone of reduced pressure, lowering the temperature of the material by evaporation of moisture therein under the effects of the reduced pressure, distributing a refrigerated liquid condensing agent around the periphery of the zone and cascading the condensing agent over surfaces in close proximity to the material to be dried, adsorbing moisture vapour freed from the material in the cascading condensing agent, withdrawing the agent and adsorbed moisture from the zone of reduced pressure, separating the adsorbed moisture from the agent, recirculating the agent for redistribution to the reduced pressure zone and recooling the recirculated agent just prior to introduction to the zone. Material to be dried is frozen either prior to insertion on trays into a chamber 10 or by the action of vaporizing the water content therein under vacuum conditions within the chamber, which conditions are achieved by a pump (not shown) and a conduit 15. The trays are positioned on hollow shelves 12 through which a heating medium passes from an inlet 13 to vaporize any water remaining on the material, the water vapour so produced being removed by a refrigerated adsorbtion liquid which is introduced into the chamber via a cooler unit 20 and conduit 19, through a distributer tube 17 having apertures 18 for cascading down condensing panels 16 attached to the walls of the chamber. The water vapour is immediately condensed by solution in the adsorbtion liquid when a water miscible condensing agent is used or is condensed as ice crystals when a water immiscible liquid is used. The mixture of liquid and condensed vapour is drawn off through a sump 23 and pumped to a separated means which varies according to the adsorbtion liquid utilised. With a water miscible liquid e.g. a brine of lithium bromide or lithium chloride, the mixture is heated as it passes through a heat exchanger 27 in countercurrent flow with reconcentrated brine being returned from an evaporator 30 for reintroduction to the drying chamber through the conduit 19 and cooler unit 20. The temperature of the liquid is further raised in the evaporator by a heating coil 29 so as to evaporate and expel water vapour therefrom and so reconcentrate the brine. The flow of adsorbtion liquid into the chamber is controlled by a valve 34 operated by a motor 35 actuated by a unit 36 responsive to the level of the mixture in an outlet conduit 24 below the sump 23. In a modifiation, Fig. 2 (not shown), a refrigerated water immiscible liquid condensing liquid having a density less than water or ice e.g. polydimethyl siloxane fluid is utilized so that the water vapour produced in the drying chamber is condensed as ice crystals and is entrained in the cascading fluid. The fluid and crystals enter a vertical settling tank wherein the heavier crystals sink to the bottom are there heated by a coil and so discharged and the reconcentrated fluid is separated and recirculated through the cooler unit to the chamber. In another modification, Fig. 3 (not shown), a refrigerated water immiscible liquid is utilized having a density between those of ice and water, the ice and fluid being separated in a first vertical separator tank from which both the ice free fluid is withdrawn for recirculation from the base and also the ice crystals are withdrawn from the top of the tank together with a portion of the liquid to a second tank where the crystals arc melted, the fluid rising to the top of the tank for separation and recirculation. In a further modification, Fig. 4 (not shown), a refrigerated water immiscible liquid is utilized having a density greater than those of either ice and water. The liquid and crystals pass from the chamber into a vertical separator tank wherein the heavier liquid sinks to the bottom of the tank and is withdrawn therefrom for recirculation, the ice crystals being melted at the top of the tank to faciliate discharge.