An evaluation of membrane properties and process characteristics of a scaled-up pressure retarded osmosis (PRO) process

This work presents a systematic evaluation of the membrane and process characteristics of a scaled-up pressure retarded osmosis (PRO). In order to meet pre-defined membrane economic viability (≥5 W/m2), different operating conditions and design parameters are studied with respect to the increase of the process scale, including the initial flow rates of the draw and feed solution, operating pressure, membrane permeability-selectivity, structural parameter, and the efficiency of the high-pressure pump (HP), energy recovery device (ERD) and hydro-turbine (HT). The numerical results indicate that the performance of the scaled-up PRO process is significantly dependent on the dimensionless flow rate. Furthermore, with the increase of the specific membrane scale, the accumulated solute leakage becomes important. The membrane to achieve the optimal performance moves to the low permeability in order to mitigate the reverse solute permeation. Additionally, the counter-current flow scheme is capable to increase the process performance with a higher permeable and less selectable membrane compared to the co-current flow scheme. Finally, the inefficiencies of the process components move the optimal APD occurring at a higher dimensionless flow rate to reduce the energy losses in the pressurization and at a higher specific membrane scale to increase energy generation. Crown Copyright © 2015 Published by Elsevier B.V. All rights reserved.

A systematic evaluation and comparison between the co-current and the counter-current scaled-up PRO process is developed in this study. The significant operating conditions and design parameters of a scaled-up PRO process are investigated. It includes the hydraulic pressure applied on the draw solution, the initial flow rates of the draw and the feed solution, the permeability and selectivity of the membrane, structural parameter, and the inefficiencies of the process components such as HP, ERD and HT. On the basis of the results, some conclusions can be drawn: 1) dimensionless flow rate has an important role in the performance of the scaled-up PRO process in terms of both the SEE and the APD in the membrane module level. At a particular dimensionless flow rate, the process performance between the co-current and the counter-current flow scheme is not significantly different in high APD operations; 2) In the scaled-up PRO process, with the increase on the specific membrane scale, the detrimental effect of the RSP becomes significant. The accumulated solute leakage shifts the maximum SEE occurring at the lower membrane permeability in a larger scale PRO process. The ability to increase the SEE by enhancing the membrane permeability is better in the case of the counter-current flow PRO process.; 3) The machines' inefficiencies drive the maximum APD occurring at a higher dimensionless flow rate to reduce the energy losses in pumping and pressurization and a higher specific membrane scale to increase the salinity energy generation. The energy losses caused by the inefficiencies shrunk the salinity energy generation, especially at the small dimensionless flow in a small scale process.