Abstract
The filtration performance of soft colloid suspensions suffers from the agglomeration of the colloids on the membrane surface as filter cakes. Backflushing of fluid through the membrane and cross-flow flushing across the membrane are widely used methods to temporally remove the filter cake and restore the flux through the membrane. However, the phenomena occurring during the recovery of the filtration performance are not yet fully described. In this study, we filtrate poly(N-isopropylacrylamide) microgels and analyze the filter cake in terms of its composition and its dynamic mobility during removal using on-line laser scanning confocal microscopy. First, we observe uniform cake build-up that displays highly ordered and amorphous regions in the cake layer. Second, backflushing removes the cake in coherent pieces and their sizes depend on the previous cake build-up. And third, cross-flow flushing along the cake induces a pattern of longitudinal ridges on the cake surface, which depends on the cross-flow velocity and accelerates cake removal. These observations give insight into soft colloid filter cake arrangement and reveal the cake’s unique behaviour exposed to shear-stress.
Highlights
Approaches for soft-matter filtration are limited by underlying models
Assuming the feed overflowing the particles with a diameter of 2 μm as characteristic length at the filter cake surface with an average cross-flow velocity of u > 0.1 cm/s, we calculated Peclet numbers Pe > 15000, which shows that the fluid transport is advection-dominated
We present a unique visualization of colloidal filter cakes during different process conditions in a cross-flow microfiltration module
Summary
Approaches for soft-matter filtration are limited by underlying models. Even though these models have increased in complexity and accuracy in recent years, further experimental verification is needed to cover multi-scale phenomena such as compression, deformation, and rearrangement of the filter c ake[24,25,26]. The wavelength of the pattern is larger than the flow depth, such that different flow velocities induce diverse transport rates of the particles These velocity gradients create instabilities, vortexes in the bedload, and a sinusoidal longitudinal pattern[32]. The average density of this layer is smaller compared to the sediment In such flows, mechanical instabilities can generate longitudinal patterns, as reported by Forterre and P ouliquen[33,34] and explained by the concept of granular temperature. Cross-flow flushing unexpectedly creates a longitudinal pattern on the cake surface, similar to the ones obtained in the geological context of granular beds. We analyze the behaviour of this pattern regarding the average cross-flow velocity, its stability, and its influence on cake removal
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