THE ULTRAFILTRATION OF VISCOUS FLUIDS

While it is well known that high viscosities in the concentration boun dary layer have a detrimental effect on mass transfer in ultrafiltrati on, less attention has been paid to the influence of the bulk fluid vi scosity. This paper shows that the increase in the bulk fluid viscosit y that occurs as a fluid is concentrated can cause the value of the ma ss transfer coefficient to either increase or decrease depending upon the flow regime. Under laminar conditions an increase in the bulk visc osity increases the shear stress at the membrane surface which can lea d to an improvement in mass transfer. Under turbulent conditions the d ecrease in turbulence that accompanies an increase in bulk viscosity c auses a fall in the value of the turbulent mass transfer coefficient. These two phenomena have been demonstrated from the shape of flux-conc entration plots for the polysaccharide gums xanthan and pectin.