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 the nucleation occurs; the growth rate, depending on the supersaturation degree, is determined by a combination of the nature of the growing crystal surface 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 distillation - a separation process based on hydrophobic microporous membranes - has been employed to reach the supersaturation in the crystallization of NaCl from aqueous solutions: volatile components diffuse through the membrane pores as vapor by applying a temperature difference across the membrane which creates a gradient of the equilibrium partial pressures; that is the driving force for the operation. The experiments have been carried out in a laboratory plant for studying the distribution of crystal dimensions, nucleation, and growth rates as a function of the retention time, slurry density, temperature, and supersaturation level of the solution. The kinetic data and product size distribution obtained have been compared with the ones by traditional crystallizers.