Solvent-resistant nanofiltration membranes: Separation studies and modeling

The primary focus of the research is to extend the principles of Nanofiltration (NF) to non-aqueous systems using solvent-resistant NF membranes. Several different levels of interaction are introduced when organic solvents are used with polymeric membranes and thus quantification of polymer-solvent interactions is critical. Pure solvent permeation studies were conducted to understand the mechanism of solvent transport through polymeric membranes. Different membrane materials (hydrophilic and hydrophobic) as well as different solvents (polar and non-polar) were used for the study. For example, hexane flux at 13 bar through a hydrophobic silicone based NF membrane was ∼0.6 × 10−4 cm3/cm2. s. and that through a hydrophilic aromatic polyamide based NF membrane was ∼6 × 10−4 cm3/cm2. s. A simple model based on a solution-diffusion approach which uses solvent physical properties (molar volume, viscosity) and membrane properties (surface energy, etc.) is used for correlating the pure solvent permeation through hydrophobic polymeric membranes.; Solute transport studies were performed using organic dyes and triglycerides in polar and non-polar solvents. For example, the rejection of Sudan IV (384 MW organic dye) in n-hexane medium is about 25% at 15 bar and that in methanol is about −10% at about 20 bar for a hydrophobic (PDMS-based) membrane. However, for a hydrophilic polyamide based NF membrane, the direction of separation is reversed (86% in methanol and 43% in n-hexane). From our experimental data with two types of membranes it is clear that coupling of the solute and solvent fluxes cannot be neglected. Two traditional transport theories (Spiegler-Kedem and Surface Force-Pore Flow model) that consider coupling were evaluated with literature and our experimental solute permeation data. A model based on a fundamental chemical potential gradient approach has been proposed for explaining solute separation. The model uses solute, solvent and membrane physical properties and uses the Flory-Huggins and UNIFAC theories as activity coefficient models. This model has been used to obtain a correlation for the diffusion coefficients of solutes in hexane through a hydrophobic membrane. This correlation along with convective coupling can be used to predict separation behavior for different solutes and at different temperatures.