Downstream Processes BIE/CEE 5930/6930 Spring 2008.
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Downstream Processes BIE/CEE 5930/6930 Spring 2008
Ultrafiltration and Microfiltration Microfiltration –0.1 to 10 μm filter sizes –Used to separate cells Ultrafiltration –MW range 2000 to 500,000 (2 to 500 kilo Daltons (kD)) –Used to concentrate or sieve proteins based on size –Anisotropic membranes A thin membrane with small pores supported by a thicker membrane with larger pores Low MW solutes pass through the filter and high MW solutes are retained Pressure driven process –Can result in concentration polarization and gel formation at membrane surface
Ultrafiltration and Microfiltration At steady state: –Rate of convective transport of solute towards membrane = rate of diffusive transport of solute in opposite direction Gel polarization equation
Cross flow or tangential flow filtration Pressure applied parallel to membrane instead of perpendicular to it Fluid flows parallel to membrane and prevents accumulation of solute at the surface Ultrafiltration and Microfiltration
Pressure drop given by: Modified Hagen-Poiseuille Equation for turbulent flow in a pipe Hagen-Poiseuille Equation for laminar flow in a pipe These expressions relate pressure drop due to linear flow of fluid through the pipe
Filtration flux (J) is a function of –Transmembrane pressure drop (ΔP M ) –Gel layer concentration (C G ) –Mass transfer coefficient (K) –Bulk solute concentration (C B ) If no solute is present, then Flux is a function of ΔP M only. If solute is present and R G is constant, flux still increases linearly with ΔP M If gel polarization occurs, R G is not constant and flux will no longer be a function only of ΔP M } From Gel polarization eq.
Ultrafiltration and Microfiltration ΔP M may be applied in two ways –Increasing inlet pressure (P i ) –Decreasing ΔP by increasing P o (Back- pressure) P i is constrained by pumps available or membrane properties If P i is constant, applying a little back pressure is good because we get higher ΔP M If too much back pressure is applied, –ΔP decreases and thereby velocity –Gel polarization starts to occur –Lower velocity decreases mass transfer rate If velocity is too high, we get high pressure drop (ΔP) and low ΔP M again resulting in low flux