Molecular Thermodynamics for Scaling Prediction in Membrane Distillation

The scaling phenomenon is a major challenge in membrane distillation (MD) because MD is typically applied for treatment of hypersaline wastewater. Scaling not only decreases the vapor flux through the membrane but also reduces the lifetime of the membrane. The scaling-induced pore wetting caused by precipitation of the minerals at the membrane-water interface leads to water breakthrough in the membrane and eventually failure in the MD performance. Theoretically, the necessary conditions for a dissolve salt to precipitate can be found from its solubility constant and activity of the constituents in the brine. The composition and concentration of the dissolved solids can be used to determine the onset of precipitation in MD. A comprehensive thermodynamic model is, thereby, needed to estimate the electrolyte behavior and to predict the onset of precipitation at different brine compositions.

The MD has shown potential for treating hypersaline brine streams, but it is significantly impaired by the scaling of minerals. In this presentation, we will discuss the thermodynamic model that we have developed based on the eNRTL model (Song and Chen, I&ECR, 2009, 48(16), 7788-97) - a state-of-the-art electrolyte model - to investigate the fluid phase equilibria of brine at different compositions. We chose produced water (PW) resulted from hydraulic fracturing of the unconventional oil and gas as the model hypersaline brine for our thermodynamic modeling. The eNRTL model includes all the major ionic species that are present in PW, which could be incorporated into custom modeled simulations for more accurate prediction of the thermodynamic properties of the brine in the membrane-based separations. With fully-parameterized eNRTL model, we predicted salt precipitation in the treatment of PW samples with MD. Our thermodynamic modeling findings is in agreement with the experimental results from our scaling MD tests. This model is a tool that can help forecasting the precipitation of salts for any brine composition at different concentrations and temperatures. This approach could be further expanded to other desalination processes that are impaired by scaling of the dissolved electrolytes.