(495e) Evaluating the Scaling Potential in Crossflow Membrane Distillation Modules

Membrane distillation (MD) can have a role to play in desalting highly saline brines that have considerable osmotic pressures where RO operation becomes more expensive and problematic. Using MD in this way would allow increasing levels of recovery and help eliminate/reduce the problem of concentrate disposal vexing inland desalination. For this promise to be realized, MD must show itself to be more resistant to scaling than RO and thus not limited by it in the way that RO is. The authors present an analysis of the scaling potential in crossflow MD in terms of saturation index profiles throughout the module as a function of module configuration. Modelling shows that the highest scaling potential is to be found at the high temperature end of the module both due to the high temperature and concentration polarization associated with high local fluxes. However, temperature effects are expected to be far more important than concentration due to the high values estimated for mass transfer coefficients in shell-fed cross-flow MD modules. In fact, concentration polarization estimated in cross-flow MD units is lower than in spiral wound modules for similar flux values.

Preliminary results on the calcium sulfate system show that despite evidence that calcium sulfate precipitates at elevated temperatures, no significant loss in water vapor permeation is discerned. Possible explanations for this resistance to scaling are presented and discussed.