Effect of zinc chloride and PEG concentrations on the critical flux during tangential flow microfiltration of BSA precipitates

Open Access
Li, Zhao
Graduate Program:
Chemical Engineering
Master of Science
Document Type:
Master Thesis
Date of Defense:
July 06, 2017
Committee Members:
  • Andrew L. Zydney, Thesis Advisor
  • Themistoklis Matsoukas, Committee Member
  • Darrell Velegol, Committee Member
  • protein precipitation
  • critical flux
  • microfiltration
  • fouling
  • fouling×
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 a suspension of precipitated protein was a function of the effective particle size of the precipitate, which could be controlled by the precipitation conditions. Experimental studies were performed to evaluate 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). Data were obtained using 0.2 µm pore size hollow fiber polyethersulfone microfiltration membranes. The critical flux varied with shear rate to approximately the 1/3 power, consistent with predictions of the classical concentration polarization model. The critical flux increased significantly with increasing zinc chloride concentration, going from 60 L/m2/hr for a 2 mM ZnCl2 solution to 200 L/m2/hr for an 8 mM ZnCl2 solution. In contrast, the critical flux achieved a maximum value at an intermediate PEG concentration of around 6 weight percent at a fixed ZnCl2 concentration of 3.5 mM. 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 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.