(691c) Network-Based Finite Element Analysis for Studying the Effect of Support Structure On Internal Concentration Polarization During Forward Osmosis

Forward osmosis is one of the most promising technologies for water treatment with low energy requirement and energy generation (pressure retarded osmosis).  A typical forward osmosis membrane is composed of a very thin active layer and a thick porous support layer.  In contrast to the reverse osmosis, the forward osmosis process is mainly driven by the osmotic pressure difference between the draw solution and the feed solution.  Previous studies found that severe flux decline could be caused due to the loss of effective driving force, which is extremely sensitive to the concentration profile developed within the porous support layer, namely the internal concentration polarization.

However, most of classical models accounting for the internal concentration polarization during forward osmosis are based on a linear approximation of the support structure while the complex geometry is neglected or lumped into the macroscopic phenomenological coefficients.  Little work has been done to systematically study the underlying mechanisms accounting for the effects of the porous substructures on the coupled transport phenomena.

In the current work, the support structure was approximated by a three-dimensional network with stochastic disconnectivity, which was mathematically characterized by a blockage probability function.  Then, a variety of membrane morphologies were simulated by varying the blockage probability in different coordinate directions.  Both the bulk flow and the solute flow in the networks were numerically evaluated by using the finite element analysis, and therefore visualized to provide deeper insights into the interplay between the elaborate porous structures and the osmotically-driven convective diffusion.  The presented modeling results also quantitatively revealed the profound correlations between the macroscopic transport parameters and the characteristics of the support substructures, thereby offering a very useful tool to optimize the support structure for controlling the internal concentration polarization during forward osmosis.