Hollow fiber PSF fine porous supports with ultrahigh permeance for composite membrane fabrication: Novel inert bore liquid (IBL) spinning technique

Abstract

The development of high-performance thin-film composite (TFC) hollow fiber membranes coated on top of porous supports is a hot topic both in industry and in science. However, it has been documented in recent literature that a porous support can hinder mass transfer through a composite membrane. Taking this into account, ultra-high permeable polysulfone (PSF) hollow fiber supports with an outer porous skin layer suitable for the fabrication of composite membranes were developed in this work. To address this ambitious challenge, the concept of spinning with an inert bore liquid (IBL) using wet air gap conditions was developed and successfully implemented. While the external non-solvent (water) ensures the formation of a thin porous skin layer, the inert bore liquid makes it possible to implement two important conditions for the formation of macropores on the lumen side of the hollow fiber: i) the coagulant enters only from the outer side of the dope solution layer, ii) there is no noticeable transport of the dope solution components into the inert bore liquid and vice versa. The following alkanes were studied as the IBL: pentane, hexane, heptane, and iso-octane. For example, using hexane and heptane as the bore liquids, PSF hollow fiber supports with high values of CO2 gas permeance, namely 20400 and 36700 GPU, respectively, were achieved. These values are an order of magnitude larger than the permeance of similar PSF supports reported in the literature. The presence of macropores on the lumen side of these hollow fibers was demonstrated by scanning electron microscopy. Composite membranes with a selective layer of polydecylmethylsiloxane (PDecMS) were coated and the separation of an eight-component mixture of C1-C4 hydrocarbons in a hollow fiber module was studied. Using a modified resistance model, it was shown that the absence of a skin layer on the lumen side of the hollow fiber reduced the support resistance and increased the contribution of the PDecMS selective layer to the total resistance of the composite membrane (for N2 and CO2: from 11.3% to 21.2 % and from 13 % to 20.5 %, respectively).