Influence of Membrane Structure and Permeability On Power Production in Pressure Retarded Osmosis Processes

Wednesday, November 10, 2010: 1:45 PM
Grand Ballroom F (Marriott Downtown)
Zeta L.T. Yu-Peralta, Department of Civil and Environmental Engineering, University of California, Los Angeles, Los Angeles, CA and Eric M. V. Hoek, Dept. of Civil and Environmental Engineering, University of California, Los Angeles, Los Angeles, CA

The concept of pressure-retarded osmosis (PRO) was first introduced by Norman in 1974 [1], but later named as PRO [2] and extended by Loeb [2, 3] and other researchers [4-8] . The main obstacle that hinders the development of PRO and limits power production is the lack of appropriate membrane design¬—specifically, a highly permeable and selective barrier layer and a support layer that produces minimal internal concentration polarization (ICP). Internal concentration polarization results from hindered diffusion of feed and/or draw solutes through the porous support layer of a composite membrane. The extent of ICP is theoretically a function of the porous support layer thickness and (macrovoid) porosity and tortuosity. So far, the highest output reported is 3.5 W/m2 of membrane area from a thin film composite membrane tested in the laboratory using simple salt solutions and converting the measured flux into power using a simple thermodynamic model [9]. In order for PRO to be economically viable, a membrane that generates at least 5 W/m2 is required [10]. This goal of the work to be presented is to elucidate the inter-relationships between the five known PRO structure-performance parameters (barrier layer water permeability and salt permeability; support layer porosity, tortuosity, and thickness) and their influence on PRO power production. In the talk, we will map out various combinations of the five structure-performance parameters required to make PRO economical using the commercial CTA membrane properties as the baseline for comparison. Our preliminary results suggest that for a given porous support layer increasing the barrier layer water permeability directly increases power density; however, if salt permeability increase proportionately the gain in power production is completely lost. Hence, PRO membranes must become more permeable to water, while maintaining the current salt permeability. For a given barrier layer, support layer tortuosity appears the most important parameter; as tortuosity approaches unity, power production is minimally affected by changes in support layer porosity and thickness. These results provide great insight into the structural changes required to produce a viable PRO membrane.

References

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