AN INVESTIGATION OF CONCENTRATION POLARIZATION PHENOMENA IN MEMBRANE FILTRATION OF COLLOIDAL SILICA SUSPENSIONS BY NMR MICROIMAGING

Nuclear magnetic resonance (NMR) micro-imaging has been used to invest igate concentration polarization phenomena in membrane filtration of c olloidal silica suspensions using a single tubular microfiltration mem brane, with the feedstock fed to the inner lumen of the membrane and t he filtrate removed from the (outer) shell side. H-1 NMR images, in wh ich the signal intensity is weighted by the longitudinal relaxation ti me (T-1) of the solvent (water) protons, clearly exhibit details of th e formation and dissipation of the silica particle concentration polar ization layers at the surface of the membrane in response to changes i n trans-membrane pressure difference and feedstock crossflow rate. The images were used to map the spatial distribution of the silica polari zation layer as a function of time, distance from the filter inlet, an d applied trans-membrane pressure difference. In each case the polariz ation layer was observed to be highly asymmetric, being much thicker a t the bottom of the module than at the top. The performance of the fil ter was compared for different orientations of the filter module. The permeate flux rate was shown to be highly dependent on the orientation of the filter axis with respect to the vertical, This is consistent w ith the fact that the observed asymmetry in the layer is caused by flo w of the polarization layer over the surface of the membrane due to gr avitational effects. Phase sensitive NMR flow imaging was used to map the ID distribution of the feedstock crossflow on the lumen side of th e membrane as well as measuring the axial flow profile within the conc entration polarization layer itself. The axial component of flow of th e polarization layers is driven, not by gravity, but by the shear indu ced by the feedstock crossflow. The flow profile of the polarization l ayer presented in this paper therefore provides direct experimental ev idence for fluidity and motion of concentration polarization layers, a n assumption which has been invoked for the development of some theore tical models but which has not previously been confirmed experimentall y. (C) 1998 Elsevier Science B.V.