Optimizing the operation of direct-flow hollow fibre filtration

Thesis

In current water industry, direct-flow hollow fibre filtration is increasingly used as an intermediate or complete water purification technique. Compare to classical crossflow filtration, direct-flow filtration avoids high pressure drop that incurs high energy cost in the feed channel and permits the use of smaller diameter fibres which increases the filtration area per unit volume. Such filtration is achieved by efficiently capping one end of the crossflow device and it is usually operated in dead-end mode with regular intermittent backwash. Direct-flow filtration therefore forms the dominant method for producing potable water from ground or surface water. This thesis focuses on the operational considerations and design aspects of the direct-flow filtration membrane modules, specifically those of the pressure driven inside feed format. From a comprehensive literature survey of direct-flow filtration it was established that there was a general lack of information on direct-flow filtration behaviours. In particular there were no tools to simulate the hydrodynamic efficiencies within these devices. This study therefore set out to develop various direct-flow filtration models which one can also apply in various industrial processes. With a dimensionless mathematical model for the fluid flow in an axisymmetric direct-flow pipe, simple investigations on direct-flow filtration was achieved. Then two realistic hollow fibre models which included finite wall thicknesses and fouling effects were developed. These models were used to investigate various influences upon the axial dependence of the transmembrane pressure and the cumulative filtration volume under non-fouling and fouling conditions. The current fouling model is limited to chemically inert, incompressible, suspended solids. Geometrical parameters including fibre radius, fibre wall thickness, fibre length, module void fraction and imposed average flux were varied over a broad range values encompassing the values applied in today’s water industry. Various scenarios of membrane fouling and cleaning process in which fibres were subject to identical hydrodynamic conditions have been investigated. The study has proposed two important metrics and a strategy for optimizing them to promote the best filtration performance: the module productivity parameter, and the filtration uniformity factor. Overall, optimized design ranges and operating regimes of the direct-flow hollow fibre filtration devices were summarised, and the modelling results accord with commercial products as used in industrial practice that has evolved over the last 20 years and show guidance for future designs.

Authors: Xu, Q

• DOI: 10.5287/ora-ppkdwddd0
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English