Micro-structured spacers for intensified membrane process performance

The efficiency of membrane processes like ultrafiltration or reverse osmosis depends on optimized flow conditions within the membrane module. Spacers implemented in flat sheet membrane modules largely contribute to efficient operation since mass transfer rates are substantially increased by the spacer'induced flow while fouling is reduced. The use of spacers is not free of drawbacks and although generally beneficial to mass transport, it is associated with higher energy dissipation in the flow channel and can lead to additional operating problems such as intensified bio'fouling. Further, the application of membrane spacers to the treatment of fluids with high solid loads is prohibited, since the spacer filaments imply a high risk of channel blockage. Within this thesis, a new type of micro'structured membrane spacer design resembling structured packings or static mixers is introduced that reduces current shortcomings of net spacers. The new spacer type has been analyzed regarding its potential to enhance mass transfer and to reduce fouling. It was further investigated if the new spacer design can open up new applications for flat'sheet membranes systems incorporating spacers, where severe fouling and clogging problems so far eliminate the benefit of spacers. Evaluation of the spacer induced hydrodynamic showed that several of the current problems associated with the application of membrane spacers can be mitigated by using the new spacer design. Application of the new spacer results in an increase in local as well as overall shear force on the membrane surface. In addition, the shear pattern imposed by the micro'structured spacer displays no continuous region of low shear stress, which combined indicates a reduced risk of particle precipitation and channel blockage. Intensified mass transport is obtained by application of the new micro' structured membrane spacer compared to standard net spacers, leading to a higher overall performance in terms of trans'membrane flux. An economic comparison of micro'structured and net spacer at similar membrane packing density revealed that significant gains in flux are obtained; under the selected process conditions a 50% increase in flux at similar cross flow power consumption was observed. Further, higher process selectivity was obtained, which is most relevant in fractionation processes. Due to the reduced fouling propensity by application of the new micro'structured spacers and the absence of continuous fouling regions, the new spacer geometry seems suited for applications that display high fouling propensity and impose a higher risk of channel blockage. The structured spacers have therefore been applied to submerged membrane systems, where application of spacers so far has been prohibited. Air sparging could be significantly reduced, while at the same time trans'membrane flux could be increased two'fold without loss in process performance.