Potential for scaling by sparingly soluble salts in crossflow DCMD

Membrane distillation (MD) whether of the DCMD (direct contact membrane distillation) or VMD (vacuum membrane distillation) variety can have a role to play in desalting highly saline waters that have considerable osmotic pressures where reverse osmosis (RO) operation becomes more expensive and problematic. Using MD in this way would allow increased recovery and help reduce the problem of concentrate disposal vexing inland desalination. To realize this promise, MD must show itself to be more resistant to scaling than RO and thus not limited by it in the way that RO is. An analysis of the scaling potential in hollow fiber membrane-based crossflow DCMD is presented in terms of the saturation index profiles throughout the hollow fiber membrane module as a function of the location in the module for the sparingly soluble salt, CaSO₄, in various forms (such as, gypsum and anhydrite). Modeling shows that the highest scaling potential is to be found at the high temperature end of the module both due to the high brine temperature and concentration polarization associated with high local fluxes. Concentration effects are far more important than temperature, although concentration polarization estimated in crossflow hollow fiber DCMD units is lower than that in spiral wound modules in RO for similar flux values. Experimental results at high saturation indices on the calcium sulfate system ranging between 1.13 and 1.93 show that despite evidence that calcium sulfate precipitates at elevated temperatures, no significant loss in water vapor permeation is observed. Possible explanations for this resistance to scaling are presented and discussed.