Advances in solvent-resistant nanofiltration membranes: experimental observations and applications

Nanofiltration (NF) and reverse osmosis (RO) are well-established membrane technologies for applications involving aqueous streams. The principles of NF transport (diffusion, convection, and Donnan exclusion) are effectively used to develop novel membrane materials and applications in aqueous medium. Use of NF in a non-aqueous medium holds strong potential for the food, refining, and pharmaceutical industries because of the low energy costs involved with such membrane processes. Further understanding and development of solvent-resistant NF membranes provides opportunities for various hybrid processing ranging from reactor-membrane to distillation-membrane combinations. This paper provides a comprehensive overview of literature results and our own work in the area of non-aqueous systems. For solvent-based systems, potential membrane swelling and solvent-solute coupling needs to be considered for membrane design and transport theories. A simplified transport theory for pure solvents has been developed using solvent (molar volume, viscosity) and membrane properties (membrane surface energy). This model and has been verified with literature data for both hydrophilic and hydrophobic membranes. Membrane characterization and preconditioning aspects need to be given serious consideration for evaluating membrane performance. In addition to permeability and separation results, some novel applications of NF in non-aqueous solvents are included in this paper.