MELT CRYSTALLIZATION WITH DIRECT-CONTACT COOLING TECHNIQUES

Melt crystallization using direct contact cooling techniques has been examined for the separation of an n-decanol-n-dodecanol mixture. Addit ionally, the heat transfer of melt crystallization by direct contact o f coolants has been investigated. Three coolants, i.e. air (as a gas), water (as a liquid) and butane (as a liquefied gas) were employed. Vo lumetric heat transfer coefficients obtained by direct contact cooling , in the melt crystallizer are in the range between 0.1 and 100 kW/(m( 3)K) and increased roughly with the 1.5 to 1.8 power of the linear sup erficial velocity of coolant. The volumetric production rate was round to be in the range between 0.01 and 0.2 kg/(m(3)s) for the coolant ai r, 0.05 to 1.4 kg/(m(3)s) for the coolant water and butane. The linear growth rate of crystals was proportional to the second power of subco oling. The quality of the crystals produced by this direct contact coo ling technique was good, and the crystals are significantly larger tha n those obtained using scraped-surface crystallizers. A comparison bet ween the effective distribution coefficient obtained before and after wiping has shown that the core of crystals is very pure and the impuri ty is concentrated in the outer layer of crystals. The described melt crystallization also shows the advantage of continuous operating, high thermal economy and no encrustation in spite of high mean temperature difference.