ARE PORE-SIZE DISTRIBUTIONS IN MICROFILTRATION MEMBRANES MEASURABLE BY 2-PHASE FLOW POROSIMETRY

The issue of evaluating equivalent pore diameter distributions in memb rane microfilters from gas-liquid (g-l) porosimetry data has been crit ically examined. Experiments performed with one isotropic and one comp osite anisotropic membrane in both possible orientations revealed cons picous dependence of the obtained (g-1) porosimetry peaks on imposed p ressure ramp rates, rho. Interference of this kinetic effect can be el iminated from the measured data by extrapolation to rho = 0. The ramp rate effect is most likely caused by tortuous pore length distribution , and relatively long times required for liquid expulsion. For two exp eriments, the observed effects of rho could be reconciled with predict ions of the Schlesinger-Bechhold theory [Bechold et al., Kolloid Z., 5 5 (1931) 172-198]. The data obtained with the thin top layer of the co mposite membrane facing intruding air directly did deviate somewhat fr om the theory. Pores characterized by (g-l) porosimetry are likely of the ''throat type'', and their size distribution is considerably more narrow than that obtained for the ''node-type'' pores by SEM-image ana lysis [Zeman and Denault, J. Membrane Sci., 71 (1992) 221-231]. A sing le bivariate distribution function was constructed for these two disti nct pore populations. Flow-weighted or number fraction distributions c an be calculated from the extrapolated porosimetry data. For narrow ra nges of ''throat'' diameters, these distributions are fairly similar.