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Abstract
Emerging membrane technologies such as forward osmosis (FO) and membrane distillation (MD) can provide alternative engineering approaches to current water-treatment membrane technologies, but without the high-pressure requirements. Currently, greater industrial implementation of these technologies is hindered by limitations with low flux, flow polarisation issues, design optimisation and issues with membrane deformation. An experimental and numerical assessment of a plate-and-frame (PF) FO module, revealed significant occlusion of the draw-channel under applied transmembrane-pressure (TMP), at points up to 70% while under an applied TMP of 1.45bar. Subsequently, 3D computational fluid dynamics (CFD) simulations were performed and validated against pressure loss data under TMP, to reveal the impact of flow indicators known to affect concentration polarisation (CP), such as Reynolds number, velocity profiles and shear strain. The pressure-loss method was then applied to a range of commercially available modules, found to occlude a cross-sectional area from 12-16% for the spiral would (SW) types and 49% 1.45bar for the PF module. CP models were then developed in conjunction with flux data to establish the degree of CP occurring in the modules. The CP data was then related to a CFD characterisation to establish detailed relationships on the impact of TMP on CP effects. Finally, a solar vacuum-membrane distillation (solar-VMD) system was developed and assessed experimentally to apply the lessons learned from the FO investigation in another emerging membrane technology. Lab-scale experiments were used to develop and validate a CFD model, using predictive hydrodynamic factors such as Reynolds number and shear strain, to mitigate temperature polarisation (TP) using turbulence promoters. A parametric analysis of the CFD data revealed the flux improvements and TP mitigation available through the addition of a baffle, combined with an economic analysis for real world use (demonstrating a viable decentralised drinking and hot-water supply). Flux performance of the MD system was found at >8LMH in solar conditions of ~800W/m2, with a payback period of 2.06 years. Overall, this thesis provides a detailed assessment of the impacts of applied TMP in FO processes, as well as potential design optimisation pathways by furthering the knowledge of CFD analysis in emerging membrane technologies.
