Effect of salt concentration on permeation performance in hollow fiber type PRO membrane module

Abstract

The new sustainable power generation technique which can convert the salinity gradient energy to the hydroelectric energy is expected. This technique is called Pressure Retarded Osmosis (PRO). Clarification of the relationship between the performance of the PRO module and permeation characteristics is important. It's already known that increase of salt concentration can increase the permeation flux through the membrane. As the conventional researches, the effects of increase of fresh water concentration and concentration polarization have evaluated. In this research, relationship between the salt concentration and membrane module is focused. The effects of fresh water dissipation and flow state of salt water in hollow fiber membrane module as new factors are researched with experiment and numerical simulation. As result, in the case of low salt concentration, permeation is not caused sufficiently in module. On the other hand, in the case of high salt concentration, very low permeation flux area exists extensively, and the effects of concentration polarization and fresh water dissipation are relatively large. Therefore salt water flow rate and module shape should be changed for each salt concentration.Key words : Pressure Retarded Osmosis(PRO), Forward Osmosis(FO), Hollow fiber, Salt concentration, Concentration polarization, Hollow fiber membrane module IntroductionGlobal warming has been proceeding in the world, because a large amount of carbon dioxide is exhausted. Nuclear power generation is undesirable, because safety of radiation exposure is concerned. Then, reduction of carbon dioxide and improvement of safety for power generation are required. For these requirements, a lot of conversion power generation techniques with sustainable energy like the solar, wind, biomass, and ocean energy have been suggested. Particularly, Pressure Retarded Osmosis (PRO) suggested by S.Loeb is expected as one of these power generations with sustainable energy (Loeb and Norman, 1975). PRO is one of osmosis, solution of high osmotic pressure is pressurized. This pressure is lower than osmotic pressure difference between fresh water and salt water. In PRO, pressure of the fresh water is increased without the pump power. This pressurized mixed water can generate the electric power. However, Lee et al. mentioned that PRO is technically feasible, but not economically viable with currently available membrane (Lee et al., 1981). This means that the permeation flow rate is low and the required power to supply the salt water and fresh water is large. Thus the permeation flow rate from membrane module is important to generate power.Three principal problems to reduce the permeation flow rate have been researched. These problems are concentration polarization, membrane fouling, and increase of fresh water concentration. Concentration polarization cause salinity gradient in membrane and on membrane surface. This phenomenon leads to reduction of osmotic pressure difference through the membrane. Sagiv et al. had evaluated the effect ...