Finite element analysis of forward osmosis process using NaCl solutions

A steady-state finite element (FEM) 2D model was developed to account for forward osmosis (FO) desalination using NaCl solutions. The model involves mass and momentum transport equations, which were solved for boundary conditions developed in the present study. The resulted solved model fits well the existing water flux data sets representing the main FO configurations. Membrane orientation and other factors affecting water flux were discussed quantitatively in detail. It was found that a major reduction in water flux results mainly from resistance due to the draw concentration polarization (CP) layer that in some cases may be higher than the concentration drop through the membrane skin which is proportional to the driving force. The concentration of draw solution must therefore be optimized with other parameters in order to increase the overall effective driving force for an actual FO system. The driving force reduction induces increased resistivity to water flux along the membrane due to the concentration/dilution effects of feed/draw salt concentrations, respectively. Shorter membranes, narrower channels and thinner membrane support layers increase water flux through the membrane. The present model is an efficient quantitative tool for optimizing a given FO system.