Melt crystallization by controlled evaporative cooling. The caprolactam-water system in batch operation

An adiabatic evaporative cooling crystallization process was developed, in which cooling is achieved by evaporating a cooling agent under a reduced pressure. In this way, the actual cooling is achieved in a condenser, without a solid phase being present. It is shown that the optimum operating strategy can be determined from the phase diagram. Caprolactam was crystallized from caprolactam-water mixtures by evaporating water. For this system the optimum process conditions are at 5 wt% water, a temperature of 53°C, and a pressure of 4.2 kPa. The temperature is controlled adequately by controlling the vapor mass flow rate instead of the pressure, which leads to an excellent and easy control system and therefore a reproducible batch crystallization process. The permeability is high and it is shown that it is possible, after the first heterogeneous nucleation event, to suppress or even avoid nucleation during the outgrowth of these nuclei. The permeability increases with the increasing crystal mass content. Therefore, a certain target permeability is attained most quickly at the highest allowable cooling rate. (C) 2000 Elsevier Science Ltd. All rights reserved.; An adiabatic evaporative cooling crystallization process was developed, in which cooling is achieved by evaporating a cooling agent under a reduced pressure. In this way, the actual cooling is achieved in a condenser, without a solid phase being present. It is shown that the optimum operating strategy can be determined from the phase diagram. Caprolactam was crystallized from caprolactam-water mixtures by evaporating water. For this system the optimum process conditions are at 5 wt% water, a temperature of 53°C, and a pressure of 4.2 kPa. The temperature is controlled adequately by controlling the vapor mass flow rate instead of the pressure, which leads to an excellent and easy control system and therefore a reproducible batch crystallization process. The permeability is high and it is shown that it is possible, after the first heterogeneous nucleation event, to suppress or even avoid nucleation during the outgrowth of these nuclei. The permeability increases with the increasing crystal mass content. Therefore, a certain target permeability is attained most quickly at the highest allowable cooling rate.