(A) study on the cellulose acetate hollow fiber membranes made by dry-wet spinning

Abstract

Hollow fiber membranes for reverse osmosis were manufactured by dry-wet spinning of cellulose acetate solution. The dope used was 27\% by weight of cellulose acetate (E-400-25) in 40:60 by weight of formamide-acetone mixture; the spinning was through a single hole needlein-orifice hollow filament spinnerette. In order to coagulate the inner wall of the hollow fiber, water-formamide or water-acetone mixture was used as an internal coagulant. The effect of the coagulation bath temperature, the air-drying time, the internal coagulant composition, the annealing temperature, the stretch ratio, and the operating pressure on the hollow fiber membrane properties were studied. The hollow fiber membranes in this study were of the asymmetric type with a structure comprised of a dense selective outer surface (thickness about 1-5 $\mu{m}$) and a porous gelled substructure. And the fiber had outside and inside diameters of 450 and 188 $\mu{m}$, respectively. At an operating pressure of 23.5 atm the permeate flux was 0.4 gal.ft.$^{-2}$ day$^{-1}$, with about 96\% salt rejection, when charging with a 5000 ppm NaCl solution. In this study, the optimum spinning conditions are as follows : Coagulation bath temperature ; below $15\,^\circ\!C$ Internal coagulant composition ; 100\% water Air-drying time ; 1-2 sec. Annealing temperature ; 70-$75\,^\circ\!C$ for 5 min. in water bath.