PROCESS FOR THE CRYSTALLISATION OF DIFFICULTLY SOLUBLE MATERIALS AND CRYSTALLISATION APPARATUS FOR CARRYING OUT SUCH PROCESS

Materials dissolved up to satn. level in a solvent wherein they are difficult to dissolve in normal pressure and temp. conditions, esp. terephthalic acid in water, acetic acid, methanol or a mixt. of two of these, are crystallised by removal of heat from the soln. by passing over the soln. a dried carrier gas which becomes charged with solvent vapour. The gas is then cooled, solvent is removed by condensation, absorption or adsorption, the gas is reheated and fed back into the soln. The gas is initially hotter than the soln. and is introduced below the liq. surface. The vapour pressure difference between the soln. at its entry point and in the container is 0-100 (3-60)bar. The appts. is also claimed. Suitable carrier gases include air, N, H, CO2 and inert gases. Applicable to the basic crystallising unit types with mixed or classified suspensions and mixed or classified prod. removal. The units are of moderate height, undesirably small crystals are not produced, and contamination by by-prods. is reduced.