Direct contact cooling techniques in melt suspension crystallization and their effect on the product purity

Experiments have been carried out to investigate the use of direct contact cooling techniques for the separation of different organic mixtures by melt suspension crystallization. Air as a gas coolant, water and sodium chlorid e solution as liquid coolants were used at atmospheric pressure to cool dow n the melt and generate crystals. The purity of the product was described w ith the effective distribution coefficient k(eff). In the system dodecanol- decanol, a gas coolant lead to higher product purities (k(eff) between 0.2 and 0.6) than a liquid coolant (k(eff) about 0.8). For the systems caprolac tam/water (with coolant air) and p-xylene/o-xylene (with a 20 mass% sodium chloride solution as coolant) higher purities than in the system dodecanol/ decanol could be achieved (k(eff) in the range of 0.2 and lower). Liquid co olants required additional separation steps because of the miscibility betw een coolant and melt. The influence of the solid-liquid separation (vacuum filtration, pressure filtration, centrifugation) on the product purity was investigated. Although pure crystals may be the result from the crystalliza tion step, a pure product could not be received by filtration or centrifuga tion. The experiments showed that solid-liquid separation and the system pr operties itself and not process parameters like superficial velocity of the coolant or residence time of the melt influenced the product purity. The y ield could be estimated from an energy balance. (C) 2001 Elsevier Science L td. All rights reserved.