Membrane distillation crystallization of concentrated salts-flux and crystal formation

Membrane distillation crystallization (MDC) can be used to process highly concentrated aqueous solutions. In MDC, membrane distillation (MD) is used to recover water and to generate the desired supersaturation in the crystallizer where product crystals can be precipitated. This paper discusses factors influencing flux at close to saturation and the formation of salt crystals. The flux behaviour was investigated using two aqueous salt solutions of sodium sulfate (Na2SO4) and sodium chloride (NaCl) which have different solubility-temperature coefficients. For both salts it was observed that MD was operable at high concentrations at feed temperatures of 50 and 60 °C with fluxes up to 20 L m−2 h−1. It was found that both concentration and temperature polarization influenced the performance of MD. When operated in batch concentration mode without the crystallizer the flux gradually declined due to vapour pressure suppression and concentration polarization, up to a critical degree of saturation. The flux data could be predicted by heat and mass transfer modeling or by a simple empirical relationship involving the overall vapour pressure driving force and the degree of saturation. Beyond the critical degree of saturation, rapid flux decline was observed due to crystal deposition and scale formation on the membrane which reduced the membrane permeability. The Na2SO4 solution was able to operate at slightly higher degrees of saturation, which may be because the negative solubility-temperature coefficient favours solubility in the polarization layer. In MDC the temperature and saturation level both in the MD and the crystallizer are critical operating parameters. Anhydrous sodium sulfate crystals can be produced by means of MDC with a relatively narrow crystal size distribution and average size of 60-80 μm.