Fabrication of wet phase inversion capillary membrane, dimension and diffusion effects

Abstract

A protocol already exists for fabrication of a capillary membrane having an internal ultrafiltration skin supported by a finger-like pore structure in the external capillary wall (Jacobs and Leukes, 1996; Jacobs and Sanderson, 1997). These membranes have been produced at the Institute of Polymer Science, University of Stellenbosch, South Africa. Two major applications emerged from the development of these internally skinned membranes. One application was in the production of potable water by Ultra-filtration (UF) from sources containing coloured water. A second application was in the immobilization of a white rot fungus in a ."gradostat" membrane bioreactor. Here a nutrient gradient through the membrane wall and fungal mat can be established and manipulated in order to stimulate continuous production of secondary metabolites (extra-cellular enzymes). These enzymes are useful in the degradation of polycyclic aromatic compounds, notably PCB species in contaminated water and soils (Jacobs and Sanderson, 1997). Two objectives emerged from experiences with the above applications. The first objective was to improve membrane performance in UF applications. In this case a reduction was sought in trans-membrane pressure differential required to attain a desired flux without sacrificing rejection. The pressure required for a given desired flux across a membrane depends on the resistance of the membrane skin layer and of its supporting sub-layer which together comprises the capillary wall and defmes its overall structure. If any of these resistances could be reduced, the overall resistance to transport of water would be reduced. Then it would be possible to operate the membrane at lower trans-membrane pressure differences. On the other hand, operation with higher pressure would also increase flux but require a thicker capillary wall to resist this pressure. In the attempt to optimise these properties of the capillary membrane, capillary membranes produced in the study reported here were tested to find the relationship of flux performance with the structures that resulted from varying key parameters affecting structure and integrity. The objective in the case of immobilizing fungi in membrane bioreactor applications was to attain thicker walls thus providing better support for the fungal mass. The internally skinned capillary membrane has finger-like microvoids that start next to the UF skin layer and extend across the capillary membrane wall and open at the external membrane periphery, giving an ideal structure for retaining the fungal biomass. The idea of a membrane with this type of morphology to immobilize white rot fungi was to anchor the growing fungus within these microvoids which imitate the natural environment in which these organisms live, that is, in the fibrous structure of decaying wood. The requirement to inoculate the microvoids with fungal spores (reproductive cells), implies that they need to be accessible from the outside, requiring a membrane wall that is externally unskinned. In the formation ofthe capillary membrane the processes of formation of the porous UP skin and the finger-like microvoids are mainly governed by diffusion of solvent out of a polymer dope (gel phase) and of non-solvent into the dope phase. Such exchanges are of primary importance between the bore fluid (containing non-solvent) and dope (containing solvent) or between the external spinning bath (high in solvent content) and dope. Diffusion effects also occur between the nascent pore voids and the precipitating polymer matrix. There are also expected to be some convection effects due to shear between the bore fluid and the moving dope gel phase and due to shrinkage ofthe gel phase. The variables selected for experimentation m the study reported here were: the dope extrusion rate (DER); dope composition (viscosity effects); bore fluid flow rate (BFF); bore fluid composition and wall thickness and diameter effects (determined largely by spinneret dimensions). Each of these has an expected effect on membrane structure and its resulting performance. Most were varied over narrow ranges indicated in the literature and by experience to be effective and critical. In addition, the effects of altering the walI thickness were investigated by using two different spinneret sizes. The external spinning bath composition (solvent content) was reported in the literature to be a particularly important parameter in the formation of externally unskinned membranes. Maintaining a high content of solvent in the external spinning bath could prevent skin formation. Too high a solvent content could, however, prevent phase transition and lead to later precipitation ofa dense skin on contact with the non-solvent in the later (humidification and rinsing) steps in the fmishing of the capillary membrane product. The external bath composition was therefore varied so as to find the bath composition that would match the cloud point for the polymer dope employed. As expected, the thickness of the membranes increased with DER increase. However, it was found that there is a critical wall thickness where an external skin layer is formed as a result of increasing the DER. A certain volumetric ratio ofDER to BFF (1,5:1 for this study) was therefore maintained in order to produce externally unskinned membranes. This shows that although the final membrane structure is detennined by the casting dope formulation, the fabrication protocol plays an equally important role in controlling structural properties and perfonnance. There was no significant change with the membrane thickness as a result of changing BFF but the voids became longer and more in number as the BFF was increased. Too high solvent content (99% NMP in this study) resulted in an external skin layer being formed. According to Smolders et.al. (1992), when the solvent content in the external spinning bath is too high, the polymer at the surface of the newly fonned membrane slowly dissolves in the external spinning bath re-forming a dope-like solution. When the newly formed membrane passes through the humidifier, the dope-like solution solidifies to form an external skin. At the same instance, too low solvent (93% for this study) resulted in external skin being fonned. Externally unskinned membranes were formed at 94 and 96% NMP bath composition. The use of a small spinneret resulted in very thin walled externally unskinned membranes.