Experimental study of the potential of concentrated NaCl solutions for use in pressure-retarded osmosis process

Abstract

Pressure retarded osmosis is a process that enables useful work generation from the salinity difference of solutions. The literature most often considers using pressure retarded osmosis with natural sodium chloride (NaCl) solutions, such as seawater, dedicated for open systems. To explore the full potential of this process, however, optimized, highly concentrated solutions of various compounds can be used. The presented research is focused on evaluating the impact of increasing draw solution temperature and concentration on the permeate flow in the osmotic process. The permeate flow is directly related to achievable work in this process, therefore, it is important to find feed and draw solution parameters that maximize it. An experimental setup developed in this study provides full control over the process parameters. Furthermore, the performance characteristics of the membrane over process time were investigated, as it became evident during preliminary experiments that the membrane impact is significant. The studies were conducted without back-pressure, in a configuration typical of the forward osmosis process, with solution circulation on both sides of the membrane. The obtained results show a clear positive impact of both the temperature and concentration increase on the potential output of a pressure retarded osmosis system. The membrane behaviour study allowed for correct interpretation of the results, by establishing the dynamics of the membrane degradation process.