POLYSULFONE HOLLOW FIBER ULTRAFILTRATION MEMBRANES

Abstract

A thorough study on the technology for producing polyethersulfone (PES) hollow fiber (HF) membranes was carried out. Effects of spinning compositions and spinning cond1t10ns on membrane physical dimensions, nascent fiber velocity (NFV) and membrane morphology/performance were investigated. A large proportion of this work was on PES at a polymer concentration of 20 wt %; and the important variables studied included internal water coagulant flow rate (WFR), length of air gap (LAG), extrusion pressure (BP), additive polyvinylpyrrolidone (PVP) concentrations in the spinning solution, and other secondary additives. Even though the number of variables and the range of variables involved are limited, several facts and directions of technological significance have emerged from this work. The influence of these variables on membrane physical dimensions, such as fiber outside diameter (OD), fiber inside diameter (ID), fiber wall thickness and OD/ID ratio were studied. Relationships between fiber dimensions and spinnerette dimension were found in this work, i.e. OD was always smaller than spinnerette diameter; while ID could be larger or smaller than spinnerette ID. Furthermore, with the given spinnerette dimensions, more than two fold variations in fiber OD and ID could be possible, depending on experimental conditions. These results provide a useful basis for the design of spinnerettes to meet the specification of HF membrane production. Effects of WFR, LAG, EP, PVP concentration, and secondary additives on membrane morphology/performance were characterized by membrane flux PR and its ability to separate polyethylene glycol (PEG) solutes of varied molecular weight. Increase in PEG separation indicates a decrease in membrane pore size in the bore side skin layer of the membrane; while increase in PR indicates a decrease in the thickness of the skin layer, an increase in the number of pores in the skin layer of the membrane bore surface, and an increase in the asymmetricity of the pore structure of the membrane. Thus, PR and PEG separation data offer quantitative measurements of membrane morphology/performance. Experimental results show that high performance HF membranes can be produced by a proper combination of WFR, LAG, EP and inclusion of selected additives. HF membranes prepared in this work yielded reasonably solute separation and high flux at low operating pressure of 20 psi, and these membranes could be applied at operating pressures varying from a few psi up to 100 psi. Results reported in this thesis offer a firm experimental basis for making hollow fiber membranes for a wide variety of reverse osmosis/ultrafiltration applications.