Influence of Sublayer Structure On Concentration Polarization and Membrane Performance In FO Processes

Influence of sublayer structure on concentration polarization and membrane performance in FO processes

Jincai Su¹, Rui Chin Ong¹, Bradley J. Helmer², Tai-Shung Chung^1,*

¹Department of Chemical & Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117576

²Eastman Chemical Company, P.O. Box 1972, Kingsport, Tennessee 37662

*Corresponding author, Email: chencts@nus.edu.sg, Fax: (65)-6779 1936

 

Key words: Forward osmosis; Hollow fiber membrane; Internal concentration polarization; Sublayer structure

  Abstracts

Forward osmosis (FO) has been considered as a promising technology for water-related applications. Intensive studies have been conducted in the last several years on the development of high performance FO membranes and novel draw solutions. A serious problem of the FO process is concentration polarization (CP). Since most FO membranes are asymmetric, CP may occur at the surface of the membrane active layer (external concentration polarization, ECP) or within the membrane sublayer (internal concentration polarization, ICP). The influence of ECP can be reduced to some extent through enhancing the turbulence of draw solutions and using draw solutions with lower viscosity. However, these approaches are not effective for suppressing ICP. Running the feed or the draw solution against the porous sublayer would result in the concentration of the feed or the dilution of the draw solution. Both cases would inevitably reduce the effective driving force, thus influencing the water flux.

In order to find out the relationship between ICP and the membrane sublayer, we have prepared hollow fiber membranes with different sublayer structures and evaluated their FO performance with DI water feed. It is observed that varying the sublayer structure does not apparently affect the FO performance with running the draw solution against the selective layer but has significant influence on the performance if running the draw solution against the sublayer. Theoretical studies are carried to examine how different structural parameters, i.e. sublayer porosity, tortuosity and thickness, affect the FO performance. Desired FO membrane should have no sublayer. Since phase inversed membranes always have a sublayer, the preferable sublayer should have high porosity, low tortuosity and small thickness. Based on experimental observation and theoretical modeling, desired sublayer structure are proposed for FO membranes.