The isolation and study of rheological and mass transfer parameters in the spinning of advanced hollow fibre membranes for gas separation
The main objective of this investigation was to study in detail the fundamental parameters of extrusion shear and forced convection residence time in the dry gap in dry/wet spinning of hollow fibre gas separation membranes. To achieve this, studies were undertaken in which extrusion shear and forced convection residence time were, for the first time de-coupled and studied in isolation. Much of the previous work in the literature has concentrated on the effects of precipitation conditions i. e. phase inversion on membrane formation. This study adopts a rheological perspective and closely considers the mass transfer and skin formation processes in the dry gap. In the first phase of work two spinning programmes were carried out; one where extrusion shear was varied while forced convection residence time was kept constant and the second where forced convection residence time was varied at constant extrusion shear. In both campaigns the dry gap chamber height was altered as appropriate. The studies utilised a sophisticated multi-component polymer dope designed to produce asymmetric gas separation membranes from forced convection dry/wet phase inversion. The forced convection system utilised here was of a unique design that produced aggressive mass transfer conditions. The resulting fibres were studied using a combination of gas permeation testing, structural modelling studies, mechanical studies and electron microscopy. These studies showed the previously undiscovered effect of extrusion shear on membrane active layer thickness and the subtle effects of forced convection residence time. The fibres produced displayed above recognised intrinsic selectivities for the gas separations studied and this was attributed to shear enhanced molecular orientation in the membrane active layer. The membranes from this phase were however mechanically weak. The second phase of work utilised a homologous higher polymer content dope to spin tougher membranes - ones that would withstand an industrial environment. Again, the experimental campaigns were designed to isolate shear and forced convection residence time so both could be studied independently. The spinning rig had to be modified to accommodate the increase in viscosity of the high polymer content spinning dope. The second phase "industrial" membranes were again studied using gas permeation studies, structural modelling, mechanical studies and electron microscopy. Due to the unique nature of the forced convection process in the spin line, the resulting fibres while displaying increased mechanical strength and intrinsic selectivity where not productive enough to be utilised in an industrial environment. Their skins were too thick and hence membrane permeability was prohibitively low. To solve the strength vs. gas separation properties conflict, a third phase of research To solve the strength vs. gas separation properties conflict, a third phase of research was carried out to strengthen membranes spun from the lower concentration dope by introducing a sub micron filler: Vapour Grown Carbon Fibre (VGCF). It was hoped that these mechanically reinforced composite hollow fibres would be both robust and exhibit attractive gas separation properties. These unique membranes displayed the productivity of the previous low polymer content fibres and indeed increased mechanical strength, but were unfortunately less selective due to surface defects. The fourth phase of work involved developing two more models to closely study the mass transfer occurring in the forced convection process and the skin formation mechanism during dry/wet spinning. The first model developed described the different areas of mass transfer in the forced convection chamber. This model explained the unique nature of the forced convection process in this spin line. The second model related the formation of the active layer to residence time in the dry gap. Further work should be undertaken to study the effect of extrusion shear on polymer solubility and phase inversion in order to produce high strength super selective membranes in the future.