Chapter 14 - Concentration Polarization in Silica Membranes

Abstract

Concentration polarization is one of the most important problems in most of the membrane separation processes. This phenomenon has generally been investigated in liquid membrane separation processes and in most cases of gas membrane separation processes it has been neglected. However, in the recent years, by modifying membranes in gas separation processes, concentration polarization should be considered seriously. A mathematical model was represented for concentration polarization to clear the effects of different parameters theoretically. Theoretical and experimental studies confirmed that the higher permeation rates result in more considerable concentration polarization effect, even at low selectivities. Also, the effects of feed flow rate, pressure, and composition on the concentration polarization in membrane gas separation processes were investigated. Concentration polarization in the silica or silica-derived membranes was studied at two different categories: gas separation and desalination processes. In gas separation by silica-based membranes, the effects of concentration polarization have generally been neglected, but in the recent years some of the researchers have studied separation of gas mixtures and found that the reduction of permeate flux and selectivity compared to those for single gases can be related to the concentration polarization and pore blockage. In desalination processes, salt concentration near the surface of micro and mesoporous silica membranes results in a considerable decrease in the membrane performance at pervaporation and nanofiltration processes. In both cases, increasing of temperature has been reported as an experienced technique to reduce concentration polarization effect on membrane performance.