Studies On Separation Performance Of Nanofiltration Membranes In Organic Solvents

Nanofiltration (NF) is a relatively new membrane process lying between ultrafiltration (UF) and reverse osmosis (RO) and has been extensively applied in water treatment. One of its major advantages is low operating pressure and thus is an energy-saving process compared to RO. In recent years, research and application of NF to organic systems has been extensively explored. However, membrane instability in organic solvents is a major drawback and has limiting its applications to organic systems. Development of solvent resistance membranes and membrane stability are an actively pursued frontier research area in the world.In this thesis, the membrane stability was investigated by employing scanning electronic microscopy (SEM) and atomic force microscopy (AFM) and measuring membrane performance. The effect of operating conditions, organic solute, solvents, and membrane materials was investigated.First, the stability of a polyamide NF membrane (Desal-DK) in aqueous phase and organic solvents was observed. SEM and AFM examination revealed that the membrane structure changed to some extent after pre-treatment, especially for pure organic solvent pretreatedmembranes. Comparing with non-pretreated membrane, the roughness for pure organic solvent pretreated membranes was increased. While for water-organic solvent pretreated membranes, lower roughness was observed.The effect of membrane pre-treatment on membrane performance (flux and rejection) was investigated. Significant variation in rejection and flux in methanol was observed with the pretreated membranes. This is most likely one of the reasons that significant differences of membrane fluxes and rejections were observed for the same type of the membrane in organic solvent in the literature. The results demonstrated that the interactions between membrane and solvent, solute and membrane and membrane and solute play an important role in separation efficiency.The influence of operating conditions (applied pressure, temperature and concentration), solute charge, solvent polarity and membrane material was examined. The results showed that the permeation flux was significantly affected at low applied pressure and high temperature while significant variation in solute rejection was observed at high pressure and low temperature. The investigation of membrane flux decline in organic solvents demonstrated that the membrane concentration polarization was likely the major cause resulting in the increased rejection with the increase in the solute concentration. No charge effect on the rejection was observed in organic solvents and solution-diffusion and size exclusion are the major...