Membrane distillation of concentrated brines—Role of water activities in the evaluation of driving force

Membrane distillation crystallisation (MDC) can be used to recover crystalline products from solutions. MDC of a concentrated solution of magnesium sulphate of 375 g/l was investigated. It was found that the MDC of epsomite was achievable at a feed temperature of only 33 °C and a distillate temperature of approximately 17 °C. The orthorhombic crystals were formed at a growth rate of 1.6 × 10−8 m s−1. The transmembrane flux was then measured for the membrane distillation (MD) of two concentrated solutions of mixed electrolytes, MgSO4 and NaCl with concentrations of 225 and 225 g/l, respectively, for one mix; and 275 and 137.5 g/l, respectively, for the other mix. For the mixtures of salts, only sodium chloride was precipitated while magnesium sulphate remained in solution but increased in concentration. The geochemical equilibrium speciation program, PHRQPITZ, was used to determine solution activities. These results were verified by experimental vapour pressure values determined by dynamic vapour–liquid equilibrium (VLE) experiments to calculate the vapour pressures of the solution (and hence driving force) at each stage during the distillation. The accountability of water activities showed how the trend of decreasing flux fitted with that of the driving force for the distillation process. When operated in batch concentration mode the flux and driving force showed a constant initial rise (due to stabilisation of temperature and concentration profiles and fluid dynamics inside the module) after which it plateaus off before rapidly declining. Preliminary computer modelling has demonstrated how the understanding of membrane distillation processes can be improved by being able to predict driving forces.