A study of extrusion shear and forced convection residence time in the spinning of polysulfone hollow fiber membranes for gas separation

Polysulfone hollow fiber membranes for gas separation were spun using a for ced convection technique. Experiments were designed to decouple the effect of extrusion shear from forced convection residence time in the dry gap all owing both factors to be investigated. The main objective was to study the pure influence of shear and its capacity to increase membrane selectivity. The results suggested that extrusion sheer influences phase inversion dynam ics. Increasing shear decreased active layer thickness and increased pressu re-normalized flux. This was discussed in terms of thermodynamic instabilit y and polymer precipitation/coalescence speed. Increasing shear was found t o increase selectivity to levels greater than the intrinsic value for the a morphous membrane polymer. This may be as a result of induced molecular ori entation in the active layer. However, a critical shear rate existed beyond which selectivity deteriorated. This was attributed to the development of surface pores as the active layer thins. Membranes spun at intermediate forced convection residence times exhibited the highest selectivities. Skin formation must be complete, but excessive r esidence time allows deleterious non-solvent encroachment from the lumen. T he results indicate that if enhanced selectivity and high flux are to be ac hieved, membranes should be spun at a high shear rate and an optimized resi dence time in order to minimize surface defects, increase the critical shea r rate, decrease active layer thickness and heighten molecular orientation. (C) 1999 Elsevier Science B.V, All rights reserved.