Theoretical Studies for Membrane-based Desalting Systems founded on Specific Energy and Nondimensionalization Analysis

Abstract

Membrane-based desalting systems such as seawater reverse osmosis (SWRO) have contributed significantly to relieving global water stress. In recent days, emerging technologies such as forward osmosis (FO) and pressure-retarded osmosis (PRO) are being researched vigorously to aid the mitigation of the water shortage and the energy problem, respectively. To facilitate the advances of each technology, various types of studies are undergoing. Even with the different objectives of each technology, every membrane-based desalting system harnesses osmotic pressure and hydraulic pressure to run each process. Those pressures are collectively called ‘driving pressures’ of membrane-based desalting systems. The driving pressures are worth further investigating considering their critical roles in the systems. For such an in-depth study, the essence of the driving pressures should be understood: every pressure is a kind of specific energy. Namely, the overall flow of membrane-based desalting systems needs to be investigated with a view of that energy is driving the given systems. In general, however, the driving pressures are barely recognized as the subjects to energy. Even in some studies, the driving pressures as specific energy are used to estimate the energy efficiency of membrane-based desalting systems such as PRO rather than to address the operational mechanisms. In this context, the objective of this dissertation is to broadly investigate the effects of the driving pressures on the performance of membrane-based desalting systems. For effective analysis, new dimensionless indices and conventional dimensionless numbers are utilized for each study. In a distant view, this dissertation is divided into two other parts that consist of three separate chapters. In the first chapter named ‘Performance assessment of PRO-hybridized processes with a dimensionless index’, overall efficiencies of PRO-hybridized processes are investigated with a new dimensionless index comprised of the specific energy terms of each process. In this chapter, with the new dimensionless index, the performances of SWRO-PRO and SWRO-membrane distillation-PRO are impartially compared. This first part, comprised of one chapter, represents the macroscopic analysis with respect to membrane-based desalting systems. On the other hand, the second part consists of two chapters and represents the microscopic analysis concerning membrane-based desalting systems. In the second chapter named ‘Theoretical analysis of a relation between the driving pressures in membrane-based desalting systems’, a relation of the driving pressures in membrane-based desalting systems is theoretically derived. Also, the effects of those driving pressures on the systems are elaborated founded on the redefinition of a conventional model. In the last chapter named ‘Influences of membrane parameters on the determination of convection-diffusion impacts’, changes in motional characteristics of systems are analyzed by connecting relevant membrane parameters to a transport model redefined in a prior chapter. Thus, the contents of the third chapter are closely related to those of the second chapter. This dissertation provides new methodologies for observing membrane-based desalting systems via driving pressures. Thus, it is expected that future studies can be inspired by this dissertation.