The influence of temperature on ion transport in nanofiltration membranes

Abstract

Nanofiltration (NF) is a pressure-driven membrane-based solution treatment technology, similar to reverse osmosis (RO). The niche set of applications for NF includes those requiring selectivity between monovalent ions and multivalent ions. Incoming streams to be treated can occur over a wide range of temperature, usually between 20-100°C. The advantage of operating NF at higher temperatures is increased water recovery, while the key drawback is diminished salt retention. Although the change of these performance metrics is widely reported in literature, few sources explain the causative mechanisms. Temperature-variation of the mobilities of both solute and solvent species, the solute diffusivity and solvent viscosity are well-studied and are used to explain changes in transport through the membrane in most literature. However, NF membranes are defined by several structural and charge-based properties, which are likely to be affected by temperature. This thesis elucidates the effect of individual membrane properties and mobilities on NF permeate quality, and finally compares sets of parameters on the extent to which they explain the change in NF selectivity with temperature change. Modeling results show that neither membrane parameter changes nor mobilities can alone explain selectivity changes with temperature. With increasing pressure, however, the net effect of membrane parameters increasingly overshadows that of the mobilities. As mentioned previously, NF is used particularly for monovalent-multivalent ion separations. The study of 'fractionation,' involving separation of sodium-chloride from sodium-sulfate is one such application, and is often conducted between 20-50°C. Previous studies do not indicate whether selectivity between the charged species improves or deteriorates at higher temperature. In this thesis, a selectivity metric, M, is introduced and an analytical framework is established to explain its variation with temperature variation, as well as other membrane and operating parameters. The conclusion is that selectivity decreases at higher temperature, which can be mitigated by design-focus on enhanced charge acquirement by the membrane at elevated temperature. The final segment of this work introduces a common modeling framework for pressure-driven (NF and RO) and osmotically driven membrane processes (forward osmosis, FO) to identify both similarities and dissimilarities in salt transport mechanisms. RO and FO membranes are traditionally considered non-porous membranes, while NF membranes are known to possess nanometer-sized pores. However, experimental and spectroscopy results in the past decade report detection of pores in RO and FO membranes. The modeling results in this thesis show that all three membranes can be modeled as porous, the key distinction being that the pressurized modes allow salt-water transport coupling.