Pressure retarded osmosis efficiency for different hollow fibre membrane module flow configurations

Abstract The principle of pressure retarded osmosis is to utilize parts of the free energy of mixing of solutions with different salinities for production of mechanical energy. The mixing process takes place over a semi-permeable membrane as water is transported from the less concentrated salt solution to the more concentrated salt solution due to difference in chemical potential of water. The PRO process will be operated with elevated pressure on the side of the membrane having the highest salt concentration, corresponding to approximately half the difference in osmotic pressures for the two solutions. The net increase in volume on the pressurised side can be utilized to run a turbine. This paper has focused on modelling the power production and pressure drop in hollow fibre modules with different flow configurations, i.e. co-current and counter current longitudinal flows and elements with longitudinal flow inside the fibres and radial flow in the shell. The overall PRO performance is similar for the different flow configurations. The radial flow configuration has a superior pressure drop in the shell side compared to the longitudinal flow configurations, but the total membrane area for a given element size will be larger for the longitudinal flow configurations.