“Breakthrough” osmosis and unusually high power densities in Pressure-Retarded Osmosis in non-ideally semi-permeable supported membranes

Abstract

Osmosis is the movement of solvent across a membrane induced by a solute-concentration gradient. It is very important for cell biology. Recently, it has started finding technological applications in the emerging processes of Forward Osmosis and Pressure-Retarded Osmosis. They use ultrathin and dense membranes supported mechanically by much thicker porous layers. Until now, these processes have been modelled by assuming the membrane to be ideally-semipermeable. We show theoretically that allowing for even minor deviations from ideal semipermeability to solvent can give rise to a previously overlooked mode of “breakthrough” osmosis. Here the rate of osmosis is very large (compared to the conventional mode) and practically unaffected by the so-called Internal Concentration Polarization. In Pressure-Retarded Osmosis, the power densities can easily exceed the conventional mode by one order of magnitude. Much more robust support layers can be used, which is an important technical advantage (reduced membrane damage) in Pressure-Retarded Osmosis.

On the application of the Spiegler-Kedem model to forward osmosis

“…Following the S-K equations given in [8], these can be simplified for FO systems by to omitting the pressure term because there should be no overall pressure drop across the barrier layer. The resulting equations are:…”

“…Today it is generally accepted that if PRO is to be commercially viable then it will be necessary to use resources with a higher salinity than seawater, for example brine from a reverse osmosis desalination plant [6,7], but the power output will still be modest. However in a recent purely theoretical paper a thought provoking theoretical analysis by Yaroshchuk [8] suggested that under some conditions and with a membrane displaying 'leakiness' , a 'breakthrough' mode might occur even to the extent that with the appropriate membranes reverse solute diffusion would be eliminated. Indeed there was the prediction that with these appropriate membranes there would be co-current flow of both solvent and solute against a concentration gradient.…”

“…Previous exploration of the implications of the effect of minor deviations from ideal semi-permeability using the Spiegler-Kedem (S-K) model have confined themselves to a single value of the reflection coefficient [8]. In this article a range of values are used.…”

“…It has been suggested that the total solute flux can change sign, and that the "dramatic change in the behaviour is ultimately caused by the change in the direction of solute flow through the membrane" [8]. This specific possibility has been refuted elsewhere where it was also shown (contrary to statements in [8]) that the use of thick support layers is undesirable.…”

Solar powered desalination – Technology, energy and future outlook

“…Additionally, the development of novel membranes for PRO has been a subject of interest for researchers in recent years[140][141][142][143][144][145]. More research into PRO and particularly the development of efficient membranes is needed before it can be considered as a viable source of energy recovery for PVRO systems.…”