Molecular thermodynamics for scaling prediction in filtration of hypersaline brine: Case of membrane distillation

Abstract:

Membrane distillation (MD) process for water treatment is significantly impaired by scaling of dissolved minerals. The type and content of minerals – generally measured as total dissolved solids (TDS) – in hypersaline brine not only reduce the MD flux but also controls the scaling behavior on the membrane surface. The scaling-induced pore blockage can further reduce the water flux and eventually leads to membrane wetting. The scaling problem is even more pronounced in the treatment of produced water (PW) as it contains 3-7 times higher TDS concentrations, compared to the seawater. Theoretically, the necessary conditions for a salt to precipitate can be traced from its solubility product constant and activity of the constituents within the solution. Therefore, a comprehensive thermodynamic model is necessary to represent the electrolyte behavior and to predict the onset of precipitation of different salts in a complex solution like PW. We used eNRTL model, a state-of-the-art electrolyte model, to address the PW fluid phase equilibria. The eNRTL was successfully used for the modeling of high salinity real PW samples. We developed an Aspen Custom Modeler (ACM) model and incorporated our eNRTL model for predicting the performance of vacuum membrane distillation (VMD). We evaluated the scaling potential of 18 different single salts that were likely to precipitate in the typical temperature range of VMD process on the MD membranes, as displayed in figure below.

With fully parameterized eNRTL model, we predicted salt precipitations for two different PW samples and compared the results against the experimental results. Scaling layer on the membrane surface was characterized by electron microscopy imaging followed by elemental analysis via EDX and by XRD analysis. Results suggest that our thermodynamic model is well capable of predicting the scalants on the membrane surface. Furthermore, we predicted the precipitation of salts with tendency to scale once the PW is concentrated, at different temperatures suitable for MD operation.