Mass transfer enhancements in ultrafiltration membrane applications using new micro-engineered membrane spacers

Abstract:
The efficiency of membrane processes like ultrafiltration (UF), nanofiltration, reverse osmosis or electrodialysis to a great extent depends on optimized flow conditions within the membrane module. Mass transport to the membranes as well as fouling propensity is determined by mixing conditions within the feed compartment of the membrane module. Eddy mixing induced by spacers placed into the feed compartment greatly influences fouling rate and enhances mass transport by reducing concentration polarization. In this work a new micro-engineered spacer design is investigated, which promises further advances in mass transport and higher energy efficiency of the overall process. The new spacer displays a constant cross sectional area in the main direction of flow resulting in low feed side pressure losses, while at the same time high mixing rates at the membrane surface are ensured by the specific shape of the spacer filaments (cf. Fig 1). Prototypes of the membrane spacers are manufactured using rapid prototyping technology and are implemented in a flat sheet membrane test cell applying 200 kDA UF membranes for experimental investigation. Membrane flux as well as pressure loss within the feed channel is measured within a relevant range of cross flow velocities, i.e. Re number range, and compared to filtration without any membrane spacer or application of commercially available net type membrane spacers. In addition molecular weight cut-off (MWCO) of well-defined dextran solutions depending on cross flow velocity is determined using HPLC measurements to analyze MWCO shift caused by application of the new membrane spacer. Implementation of the new spacer type shows significant mass transfer enhancements; permeate flux increases between 3 to 4 fold, depending on TMP and cross flow velocity, at similar energy requirements compared to mass transport in an undisturbed flow field. In addition strong MWCO shifts are observed to significantly lower molecular weights when applying the new membrane spacers.

Keywords:
mass transport, spacer

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