Evapoporometry determination of pore-size distribution and pore fouling of hollow fiber membranes

Abstract

Hollow fiber (HF) membranes are used in many applications for which characterization of the pore-size distribution (PSD) is necessary. Current techniques for determining the PSD require relatively expensive dedicated equipment. Moreover, most techniques are not applicable for characterization of fouled membranes. Evapoporometry (EP) is a novel characterization technique based on vapor-pressure depression that can detect the full spectrum of pore sizes in ultrafiltration (UF) membranes. In prior studies EP was used to determine the PSD of only flat sheet membranes using just isopropyl alcohol (IPA) as the volatile wetting test liquid. Here it is extended to HF membranes using both IPA and water as the test liquids. The average pore diameter of polyvinylidene fluoride (PVDF) HF membranes using IPA and water as the test liquids was 30.2±1.4nm and 27.4±2.0nm, respectively. The average pore diameter using IPA and water for polyacrylonitrile (PAN) HF membranes was 11.8±1.0nm and 12.1±2.8nm, respectively. Liquid displacement porometry (LDP) gave a markedly smaller average pore diameter of 20.7±3.8nm for the PVDF membranes compared to evaluating the pore diameter with EP owing to compaction under displacement pressures as high as 6MPa. LDP gave a substantially larger average pore diameter of 26.5±1.0nm for the PAN membranes compared to evaluating the pore diameter with EP owing to its inability to characterize pores smaller than 14nm. EP characterization of the PSD before and after fouling the PVDF membrane with humic acid and bentonite showed a 35% reduction in the effective average pore diameter owing to internal pore fouling.