Hollow fibres for baromembrane separation of liquids

Abstract

Hollow fibres for reverse osmosis are spun from solutions of polymers in mixtures of a solvent with a swelling agent; the solvent must have a higher vapor pressure than the swelling agent and must evaporate in the preforming stage to prevent formation of a layer of highly concentrated solution on the outside of the liquid fibre which decomposes into phases according to a spinodal mechanism on entering the spinning bath. The layers of the solution near the center of the liquid fibre decompose into phases according to a nucleation mechanism. After hydrothermal treatment (annealing), a hollow fibre with a wall of asymmetric structure is formed, and the surface layer only has diffusion permeability while the layer nearer the center of the fibre has phase permeability. In order to increase the permeability of the ultrafiltration fibre and fibre matrix for reverse osmosis, the precipitating agent must primarily diffuse inside the jet of spinning solution during spinning.