MINIMIZING FINE CRYSTALS IN CONTINUOUS CRYSTALLIZATION-BY-COOLING PROCESS

摘要

1443632 Continuous crystallisation HERCULES Inc 4 Sept 1974 [17 Sept 1973] 38600/ 74 Heading BIG A method for the continuous crystallisation of a crystallisable solute from a solution thereof, wherein the production of fine crystals is minimised, comprises establishing and maintaming the hydrostatic pressure or head of the circulation stream at the point of introduction of the feed stream at least equal to and preferably greater than the difference in the vapour pressure of the solvent in the incoming feed stream and the vapour pressure of the solvent in the circulation stream arriving at that point. It is advantageous to carry out the crystallisation in two or more stages, for example as shown in Fig. I, wherein the first crystallisation stage comprises a crystalliser vessel 14 containing a crystalliser body 16 at a pressure greater than atmospheric pressure. The feed stream of hot solution at a temperature substantially in excess of the atmospheric pressure boiling point temperature of the solvent therein is produced by way of the feed stream conduit 34 into the first stage return conduit 32. The feed stream promptly cools adiabatically to the preventing temperature of the circulation stream flowing through the return conduit 32, Because of the hydrostatic head or pressure H at the point of feed stream introduction into the circulation stream, no flashing of the solvent in the feed stream occurs. Any nucleation that occurs in the mixed stream that results from the joining of cooling of the feed stream. When the mixed stream enters the crystalliser body 16, evaporation of solvent takes place and heat is thereby removed from the body 16, the extent of evaporation is controlled by regulating the release of solvent vapour through the vapour discharge conduit 18, by means of control valve 20 to condenser 22. The temperature of the crystalliser body 16 and the circulation stream flowing through the pump 30 and return conduit 32 is maintained at the temperature at which the vapour pressure of the solvent in the circulation stream is at least equal to and preferably greater than the vapour pressure of the solvent in the incoming feed stream 34 less the hydrostatic pressure at the bottom of the hydrostatic leg 38. Part of the slurry withdrawn from the crystalliser body 16 through conduit 26 is conducted by way of conduit 40 into the second crystalliser stage vessel 44, in this stage the crystalliser body 46 is maintained under subatmospheric pressure. By regulating the vacuum conduit valve 57 the extent of evaporation of solvent from, and thus the temperature of, the crystalliser body 46 are controlled. The temperature of the crystalliser body 46 and the circulation stream flowing through the pump 60 and the return conduit 62 are maintained at a value at which the vapour pressure of the solvent in the circulation stream arriving at the point of entry of the first stage discharge stream is at least equal to and preferably greater than the vapour pressure of the solvent in the first stage discharge stream in the conduit 40 less the hydrostatic pressure at the bottom of the hydrostatic leg 68. A typical application is in the crystallisation of dimethyl terephthalate from methanol.