Abstract
High-performance polybenzimidazole (PBI) hollow-fiber membranes (HFMs) were fabricated through a continuous dry-jet wet spinning process at SRI International. By adjusting the spinning air gap from 4″ (10.2 cm) to 0.5″ (1.3 cm), the HFM pore sizes were enlarged dramatically without any significant change of the fiber dimensional size and barrier layer thickness. When fabricated with an air gap of 2.5″ (6.4 cm) and a surface modified by NaClO solution, the PBI HFM performance was comparable to that of a commercial reverse osmosis (RO) HFM product from Toyobo in terms of salt (NaCl) rejection and water permeability. The PBI RO HFM was positively surface charged in acidic conditions (pH < 7), which enhanced salt rejection via the Donnan effect. With an air gap of 1.5″ (3.8 cm), the PBI HFM rejected MgSO4 and Na2SO4 above 95%, a result that compares favorably with that achieved by nanofiltration. In addition, the PBI HFM has a defect-free structure with an ultra-thin barrier layer and porous sublayer. We believe PBI HFMs are ideal for water purification and can be readily commercialized.
Highlights
Polybenzimidazole (PBI) is an attractive candidate for hollow-fiber (HF) membrane separation because of its extremely high continuous operating temperature [1], robust mechanical stability [2], and outstanding chemical resistance [3,4,5]
Spiral-wound modules based on flat-sheet membranes have dominated the market for large desalination plants, with Toyobo providing hollow fibers (HFs) modules derived from cellulose triacetate (CTA) [6]
The PBI solution was supplied by PBI Performance Product and blended with dimethylacetamide (DMAc), polyvinylpyrrolidone (PVP), and isopropyl alcohol (IPA) to prepare dope solution for HF spinning
Summary
Polybenzimidazole (PBI) is an attractive candidate for hollow-fiber (HF) membrane separation because of its extremely high continuous operating temperature (as high as 250 ◦ C) [1], robust mechanical stability [2], and outstanding chemical resistance [3,4,5]. As described in the literature, the pore size of dry-jet wet spun HFs can be influenced by air gap [16,17,18,19,20], water-soluble additives [21], non-solvent concentrations in the spinning solution [22], bore solution composition [16,23,24], coagulation bath composition [24], and thermal/chemical post treatment [23,25]. Because there is a threshold for the fiber orientation, the curved shape of the average pore size as a function of air gap could be completely different: when the spinning air gap is increased, membrane pore size may narrow and gas or water permeability may decline [17,18,26] These effects are reported to vary, with some investigators obtaining opposite results [19] or noting that pore size may first decrease and increase [20]. The resultant fiber products were characterized and evaluated in separation tests to indicate products with good potential for commercialization
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