(305b) Studies On Scaling Of Membranes In Desalination By 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 and CaCO3, individually or mixed together. 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 show that even when the precipitation rate is fast in the mixed CaSO4-CaCO3 system at elevated temperatures or high concentrations, no significant loss in water vapor permeation is observed. Possible explanations for this resistance to scaling are presented and discussed in terms of crossflow and the nature of the coating on the surface.