Membrane crystallizers

Well-controlled crystallization is the best method for preparing materials that are extremely pure. Furthermore, it can also be highly advantageous by operating the crystallization so that the crystals pick up impurities from water. Crystal growth generally starts at solute concentrations at which t he nucleation occurs; the growth rate, depending on the supersaturation deg ree, is determined by a combination of the nature of the growing crystal su rface and the diffusional rate. The purpose of this work is to introduce an innovative methodology, the membrane crystallization, to produce crystals from solutions. For the present application, direct contact membrane distil lation-a separation process based on hydrophobic microporous membranes-has been employed to reach the supersaturation in the crystallization of NaCl f rom aqueous solutions: volatile components diffuse through the membrane por es as vapor by applying a temperature difference across the membrane which creates a gradient of the equilibrium partial pressures; that is the drivin g force for the operation. The experiments have been carried out in a labor atory plant for studying the distribution of crystal dimensions, nucleation , and growth rates as a function of the retention time, slurry density, tem perature, and supersaturation level of the solution. The kinetic data and p roduct size distribution obtained have been compared with the ones by tradi tional crystallizers.