TFC Membrane with a Cellulose Support: Relating Performance and Swelling Upon Exposure to NaCl

TFC Membrane with a Cellulose Support: Relating Performance and Swelling upon Exposure to NaCl

 

Inger Lise Alsvik¹, Katherine Zodrow², Menachem Elimelech², May-Britt Hägg¹

 

1) Department of Chemical Engineering, Norwegian University of Science and Technology, Trondheim, Norway

2) Department of Chemical and Environmental Engineering, Yale University, New Haven, USA

 

Pressure retarded osmosis (PRO) is an emerging, clean and renewable energy source. The Norwegian company Statkraft opened the world's first PRO pilot plant in 2009. Statkraft's ambition is to build a full-scale power plant in 2015. The two key components in a PRO power plant are the pressure exchanger and the membrane. The synthesis of new membranes specifically designed for the PRO process is essential for the development of this technology. Two types of membranes have been tested in PRO, asymetric cellulose acetate membranes and thin film composite (TFC) membranes. It would be desirable to develop a membrane combining the hydrophilic nature of cellulose acetate and the high permeability of the TFC membranes with the thin selective layer. However, coating of hydrophilic support membranes, like cellulose acetate, with polyamide by interfacial polymerization (IP) is difficult. This is most likely due to the unfavorable interaction between the hydrophilic support and the relatively hydrophobic polyamide. TFC membranes with a cellulose triacetate (CTA) support layer have been fabricated by the use of a modified interfacial polymerization (IP) method. In this method, a linking monomer is used to form a covalent bond between the CTA support and the polyamide (PA) active layer [1, 2]. In this project, the effect of three different linking monomers was investigated. The linking monomers were trimesoyl chloride (TMC), succinyl chloride and malonyl chloride. TMC is a trifunctional molecule, while succinyl and malonyl chloride are bifunctional. A titration was performed on the CTA support treated with the three different linking monomers. The number of charged groups was about twice as much on the support treated with TMC compared to the supports treated with succinyl chloride and malonyl chloride. An increased number of ionizable groups on the support would influence swelling/de-swelling of the polymer. Subsequently, this might influence membrane performance. The membranes were tested in reverse osmosis (RO) and in forward osmosis (FO) using three different draw solution concentrations (1, 2 and 5 M NaCl). The three-dimensional membrane morphology in different NaCl solutions was visualized using confocal laser scaning microscopy (CLSM). The morphology of the membrane with TMC as linking monomer was clearly influenced by the NaCl concentration in the solution. Interestingly, the number of macrovoids visible in CLSM increased at higher NaCl concentrations. The membranes made with succinyl and malonyl chloride as linking monomers were less influenced by the NaCl concentration. The membrane made using succinyl chloride as linking monomer had a decrease in numbers of macrovoids visible in CLSM with increasing NaCl concentration. The membrane made with malonyl chloride as linking monomer displayed little change in the number of macrovoids as a function of NaCl concentration. The changes in membrane morphology were compared with the results obtained in FO and RO.

 

1.             Nilsen, T.-N. and I.L. Alsvik, Thin Film Composites. PCT Patent Application NO2011/000162, May 31, 2011.

2.             Alsvik, I.L., T.N. Nilsen, and M.B. Hägg, Preparation of Thin Film Composite Polyamide membranes on a hydrophilic Cellulose Acetate support. To be published, 2012.