Fundamentals of hollow fiber formation for gas separation

Abstract

The high demands on high performance membranes for energy, water and life science usages provide the impetus for membrane scientists to search for a comprehensive understanding of membrane formation from molecular level to design membranes with desirable configuration and separation performance. This dissertation is to reveal the integrated science bridging polymer fundamentals such as polymer cluster size, shear and elongational viscosities, molecular orientation, stress relaxation to membrane microstructure and separation performance for gas separation.

In the first part of the work, the evolution of macrovoids and the integrated methods to completely remove the macrovoids have been examined. The origins of macrovoid have received great attention and heavy debates during the last five decades, but no convincing and agreeable comprehension has been achieved. It has been discovered that there should be critical values of polymer concentration, air-gap distance and take-up speed, only above all of which the macrovoid-free hollow fibers can be successfully produced from a one-polymer and one-solvent system. This observation has been confirmed for fibers spun from different materials such as polysulfone, P84 and cellulose acetate, and may be universally applicable for other polymers.

Torlon® polyimide-amide was employed as the membrane material in the rest of the work because it has very good thermal stability and high inherent selectivities for various gas pairs. The formation of defect-free as-spun hollow fiber membranes with an ultra-thin dense-selective layer is an extremely challenging task because of the complexity of phase inversion process during the hollow fiber fabrication and the trade-off between the formation of an ultra-thin dense-selective layer and the generation of defects. The second part of this dissertation studies the effects of spinneret dimension and hollow fiber dimension on hollow fiber formation for O2/N2 separation and it has been discovered: (1) As the spinneret dimension increases, a higher elongation draw ratio is required to produce defect-free hollow fiber membranes; (2) The bigger the spinneret dimension, the higher the O2/N2 selectivity; (3) The bigger the spinneret dimension, the higher the O2 permeance. The main rationale is that less shear stress is induced in a bigger spinneret if similar spinning conditions are used. Such less shear stress would result in less polymer chain orientation which allows the smaller gas molecules O2 to permeate through the membrane more preferentially.

The rheological properties of the Tolron® polymer solution and its role in the formation of macrovoid-free, defect-free and ultra-thin hollow fibers for gas separation have been studied as well. The balanced viscoelastic properties of dope solutions with reasonable values in both shear and elongational viscosities as well as the existence of hydrogen bonding, have been found to be crucial for the formation of a defect-free or ultra-thin dense layer. The rheological properties of the dope solutions vary significantly as a function of the spinning temperature and they can be adjusted by adding solvent or nonsolvent additives. The optimum rheological properties to fabricate Torlon® 4000T-MV hollow fibers appear at about 48ºC, and the resultant fibers have an O2/N2 selectivity of 8.37 and a dense layer thickness of 781 Å. Adding a reasonable amount of water into the Torlon® 4000T-MV solution could not only suppress the macrovoid formation but also reduce the dense-layer thickness to 488 Å. By comparison, the best Torlon® 4000TF fibers were spun at 24ºC with an O2/N2 selectivity of 8.96 and a dense-layer of 1116 Å.