Impact of membrane imprinting on fluid flow in spacer-filled ultrafiltration cassettes: Computational fluid dynamics analysis

Tangential flow filtration (TFF) cassettes for protein ultrafiltration use woven spacers to define the feed flow path and enhance mass transfer. Compression of the cassette can imprint the spacer into the membrane, reducing the effective feed-channel height and altering the local channel geometry, an effect that has been ignored in previous studies. In this work, membrane imprinting in a commercial Pellicon® 3 cassette with 2/1 twill-weave spacer was quantified using optical profilometry and incorporated into a computational fluid dynamics (CFD) model. The spacer created an impression in the membrane that was 95 μm deep, resulting in a 325 μm channel height for the 515 μm spacer thickness. The calculated pressure drop was in excellent agreement with experiments, accurately predicting the more than 5-fold increase in pressure drop with a 4-fold increase in feed flow rate. CFD simulations showed that membrane compression substantially alters the local flow field creating recirculation zones downstream of the spacer filaments at average feed fluxes above 70 L/m²/h. The CFD model was also used to examine the effect of spacer flow attack angle. Rotating the spacer by 90° caused more than a 50% increase in pressure drop, with the average shear stress on the top and bottom membranes differing by as much as 25% depending on the angle of attack. These results highlight the importance of membrane imprinting and the detailed spacer/cassette geometry on the flow in screened cassettes used for protein ultrafiltration.