Membrane distillation-crystallization of seawater reverse osmosis brines

This work aims at investigating the performance, in terms of water recovery and NaCl crystallization kinetics, of a membrane distillation-crystallization (MDC) bench-scale plant operated on brines discharged from a seawater reverse osmosis (RO) unit. Experimental tests carried out on artificial RO concentrates resulted in the production of 21 kg/m³ of NaCl crystals, exhibiting substantially a ordinary cubic shape with size between 20 and 200 μm; the final water recovery factor increased up to 90%. Analogous investigations carried out on RO brines from natural seawater were affected by the presence of dissolved organic matter, showing a 20% reduction of the amount of salt crystallized, and a 8% decrease of the transmembrane flux. Growth rate of sodium chloride crystals generated from natural RO brines varied between 0.8 × 10^-8 and 2.8 × 10^-8 m/s; these values were 15-23% lower than those measured for NaCl crystals grown form artificial concentrates. In general, the NaCl crystal size distribution was characterized by a narrow dispersion (coefficient of variation within 35-40%). MDC operations were stable over 100 h as a result of a careful control of supersaturation, polarization phenomena, nucleation process and hydrodynamics.