Influence of membrane spacers on separation characteristics in ultrafiltration

Abstract:
The efficiency of membrane processes like ultrafiltration (UF), nanofiltration and reverse osmosis to a great extent depends on optimized flow conditions within the membrane module. Membrane spacers implemented in flat sheet membrane modules such as spiral wound modules, largely contribute to an efficient operation since mass transfer rates are substantially increased by the spacer induced flow, while fouling is significantly reduced. The increase in mass transfer is attributed to a consistent boundary layer renewal being a direct result of the hydrodynamics caused by the spacer: Flow towards the membrane surface, recirculation regions as well as moving vortices have been identified as the main mechanisms for mass transfer enhancement. In addition, high shear forces induced by the flow onto the membrane surface result in lower fouling propensity. Although significant work has been performed regarding the understanding of mass transfer enhancement mechanisms and spacer optimization, the influence of membrane spacers on separation characteristics such as molecular weight cut-off or selectivity of a membrane process is not yet well understood. Separation characteristics are not only determined by the membrane selected, but are largely influenced by the operating conditions such as flux and cross flow velocity. This contribution extends the current understanding of the influence of hydrodynamics, spacers and spacer geometry on the separation characteristic in ultrafiltration applications. Different net-spacer geometries and a new micro-structured membrane spacer are applied to filtration of well defined dextrans solutions. For a broad range of operating conditions the influence of these spacer geometries on filtration characteristics and mass transfer is investigated. Molecular weight cut-off measurements are performed and sieving curves are obtained using size-exclusion chromatography. Results from this study demonstrate that in addition to a high increase in mass transfer, significant changes in the sieving coefficient are observed, differing widley for the spacers under investigation. Further, it is shown that by choosing the right operating conditions and a suitable spacer geometry a higher selectivity can be obtained. Finally, this work correlates the influence of the different spacers on the separation characteristic to the spacer induced hydrodynamics by means of computational fluid dynamics calculations.

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