The performance of helical screw-thread inserts in tubular membranes

A detailed study of the performance of helical screw-thread inserts in tubu lar membranes has been carried out at the University of Oxford. The design combines a predominantly helical flow, in which Dean vortices are generated , with an approximately axial how, which modifies the Dean vortices into a continuous 'corkscrew vortex'. This promotes good mixing of the feed fluid and minimises concentration polarisation effects. Computational fluid dynam ics (CFD) modelling of the flow has been carried out. Comparisons of the pe rformance of plain membranes with membranes fitted with various inserts are reported. The behaviour of the screw thread inserts was modelled using the commercial CFD code 'Fluent'. Good agreement was found between the CFD pre dictions and experimental measurements. Laminar flow conditions appear to b e maintained at high cross-flow rates, suggesting that the helical insert g eometry could improve the filtration of shear-sensitive fluids such as bloo d. Measurements using bovine blood and suspensions of marine algae supporte d this conclusion. Testing of tubular membranes fitted with one-start and t hree-start helical inserts has been completed. The inserts were tested unde r microfiltration conditions using yeast solutions, under ultrafiltration c onditions using reconstituted powdered milk solutions, and under nano filtr ation conditions using synthetic dyes. Two pumped loops were built for thes e experiments. The helical inserts produced much higher fluxes at low cross -flow rates than membranes without inserts (up to a factor of 6 higher), an d also performed better than a plain cylindrical insert which had a similar pressure drop versus flow characteristic. Thus helical inserts improve the filtration of shear-sensitive fluids at low cross-flows; they also produce high Auras with low cross-flow rates, which is beneficial if high value pr oducts are being separated. By suitably modifying the geometry of the inser t, lower fluxes and pressure drops can be obtained: this offers the opportu nity of tailoring the insert performance to the requirements of a particula r process. (C) 2001 Elsevier Science B.V. All rights reserved.