There is renewed interest in the possibility of using precipitation for initial capture of high value therapeutic proteins as part of an integrated continuous downstream process. These precipitates can be continuously washed using tangential flow filtration, with long term operation achieved by operating the membrane modules below the critical filtrate flux for fouling. Our hypothesis was that the critical flux for the precipitated protein would be a function of the properties of the precipitate as determined by the precipitation conditions. We evaluated the critical flux using a flux-stepping procedure for model protein precipitates (bovine serum albumin) generated using a combination of a crosslinking agent (zinc chloride) and an excluded volume precipitant (polyethylene glycol [PEG]). The critical flux varied with shear rate to approximately the 1/3 power, consistent with predictions of the classical polarization model. The critical flux increased significantly with increasing zinc chloride concentration, going from 60 L/m2/h for a 2 mM ZnCl2 solution to 200 L/m2/h for an 8 mM ZnCl2 solution. In contrast, the critical flux achieved a maximum value at an intermediate PEG concentration. Independent measurements of the effective size and viscosity of the protein precipitates were used to obtain additional understanding of the effects of ZnCl2 and PEG on the precipitation and the critical flux. These results provide important insights into the development of effective tangential flow filtration systems for processing large quantities of precipitated protein as would be required for large scale continuous protein purification by precipitation.
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