Thermodynamic analysis of a stand-alone reverse osmosis desalination system powered by pressure retarded osmosis

In this study, a methodology is developed to assess the feasibility of a reverse osmosis (RO) desalination system powered by a stand-alone salinity driven pressure retarded osmosis (PRO) technology. First, the proposed hybrid RO–PRO system is analysed as a thermodynamic cycle and its feasibility is mathematically interpreted using a feasible condition (FC) number, several dimensionless operational variables and a number of constraints to represent the objective of zero brine discharge. Then, a study of the stand-alone feasibility of a hybrid seawater RO– PRO system is carried out. The results show that lower RO water recovery and higher dimensionless flow rate improve the stand-alone feasibility of the system. A subsystem, a look inside the PRO, is developed to study the applied pressure and the required membrane area to achieve the operations with optimum FC numbers. It is found that the optimum applied hydraulic pressure is inversely proportional to the dimensionless flow rate in the feasible range of stand-alone operations and more area of membrane is required by a larger FC number. Finally, a case study of a selected operation is presented based on its energy performance, and two influencing factors, the inefficiency of the components and the salinity concentration of the feed water. Crown Copyright © 2014 Published by Elsevier B.V. All rights reserved.

In this study, a hybrid RO–PRO system is investigated to find a possible solution of the stand-alone salinity power driven desalination process. First, a thermodynamic analysis of the saline streams using P–Q and P–π plot is presented (Fig. 2). Based on the analysis, a methodology to determine the stand-alone feasibility of the hybrid system is developed mathematically including the feasible constant that represents the availability of the stand-alone RO–PRO, zero brine discharge constraint and other variables restricted in practice. Secondly, an investigation into the PRO subsystem is developed to study its required operations. Finally, a case study of the possible operation is validated and two influencing factors, the efficiency of the components and the salinity of the feed water, are discussed. Based on the results obtained, the following conclusions can be drawn: 1) The stand-alone feasibility of the hybrid RO–PRO can be determined by checking the value of the FC number. 2) The dimensionless water permeation Y P is proportional to the dimensionless flow rate ϕ in order to increase the FC number at certain RO water recovery Y. 3) Lower RO water recovery and higher ratio of the PRO feed volumetric flow rate to the combined PRO feed and draw flow rates improve the stand-alone feasibility of the hybrid system and the feasible range of the dimensionless water permeation. 4) For the same water recovery of the RO system, higher applied hydraulic pressure, but lower membrane area, is required to achieve the optimum FC numbers at the lower dimensionless water permeation rates. 5) With the decrease Fig. 11. The energy performance of the case study with respect to the efficiency of the pumps and ERD.