Dual-Layer Hollow Fibers

Compared with single-layer asymmetric hollow-fiber membranes, the dual-layer ones spun fiom a simultaneous extmsion process are more attractive because of the following reasons  

Not only do they keep all the advantages of asymmetrie membranes, but they also would greatly save the cost of high-performance materials as mueh as 95% or even more, depending on the ratio of the inner to the outer layer thicknesses. 

By means of the inner supporting layer and eo-extmsion process, it is possible to deploy brittle but highly selective and permeable materials as the selective layer to form a composite membrane. 

The simultaneous eo-extrusion approach eliminates the secondary step of depositing a selective layer upon hoUow-fiber membranes, thus making the production of duallayer asymmetric hoUow-fiber membranes mueh more straightforward and cost effective compared to the preparation of other types of composite membranes. 

The study of dual-layer asyimnetric hollow-fiber membranes formed by the phase-inversion process started in the late 1980s. In 1987, Yanagimoto invented dual-layer asymmetric flat-sheet and hollow-fiber membranes to improve the antifouling properties of membranes for ultrafiltration and microfiltration (Yanagimoto, 1987, 1988). Since then, Kuzumoto and Nitta (1989) simultaneously extruded inner and outer dopes containing the same polymer but different solvents and additives to improve water permeability. Ekiner et al. (1992) disclosed the procedures for the fabrication of dual-layer hollow fibers for gas separation. Li et al. (2002) developed a delamination-fiee dual-layer asymmetric 

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SSZ-13 Sheath Ultem Core Ultem and Matrimid Silane coupling Dual layer hollow fiber Gas separations (e.g.. O2/N2) [75]... 

Figure 7.13 SEM images of a dual-layer hollow fiber with zeolite insert. Enlargement reveals good polymer adhesion to the zeolite particles (Adapted from Ref. [62]).

Husain, S. (2006). Mixed Matrix Dual Layer Hollow Fiber Membranes for Natural Gas Separation, in Chemical Biomolecular Engineering, PhD Dissertation, Georgia Institute of Technology. 

Liu, Y. Chung, T.S. Wang, R. Liu, D.F. Chng, M.L. Chemical cross-linking modification of polyimide/poly(ethersulfone) dual layer hollow fiber membranes for gas separation. Industrial Engineering Chemistry Research 2003, 42 (6), 1190-1195. 

Jiang, L.Y. Chung, T.S. Li, D.F. Cao, C. Kulprathipanja, A. Fabrication of matrimid/ polyethersulfone dual layer hollow fiber... 

PI materials have been used for the dehydration of water/alcohol mixtures by the pervaporation method.P84 co-PI hollow fibers and zeolite filled P84 CO-PI membranes " have been used for the pervaporation dehydration of isopropanol. In addition, P84 co-PI based dual-layer hollow fiber membranes serve for the dehydration of tetrafluoropropanol. ... 

K. Y. Wang, T.-S. Chung, and R. Rajagopalan. Dehydration of tetrafluoro-propanol (TFP) by pervaporation via novel PBI/BTDA-TDI/MDI co-poly-imide (P84) dual-layer hollow fiber membranes. J. Membr. Sci., 287(1) 60-66, January 2007. 

P. Wang, M.M. Teoh, T.-S. Chung, Morphological architecture of dual-layer hollow fiber for membrane distillation with higher desalination performance. Water Research 2011, 45, 5489-5500. 

Dual layer hollow fiber membranes have become increasingly attractive due to which they can be fabricated in a single step using the non-solvent induced phase inversion method. A good lamination between the two layers as well as a regular morphology are critical to get a functional hollow fiber membrane [80]. 

In addition, dual layer hollow fibers have been manufactured by a dry jet-wet spinning technique [81]. Two different polymers, i.e., PAI and poly(ether sul-fone), were used as the selective layer on the outer side and the porous support layer on the inner side, respectively. The dual layer substrates were subsequently modified crosslinking followed by multilayer polyelectrolyte depositions in order to get a nanofiltration skin on the outer layer. 

Setiawan L, Shi L, Krantz WB, Wang R. Explorations of delamination and irregular structure in poly(amide-iniide)-polyethersulfone dual layer hollow fiber membranes. J Membr Sci 2012 423 24 73-84. 

Setiawan L, Wang R, Tan S, Shi L, Fane AG. Fabrication of poly(amide-imide)-poly-ethersulfone dual layer hollow fiber membranes applied in forward osmosis by combined polyelectrolyte cross-Unking and depositions. Desalination 2013 312 99-106. 

Y. Wang, S.H. Goh, T.S. Chung, N. Peng, Polyamide-imide/polyetherimide dual-layer hollow fiber membranes for pervaporation dehydration of C1-C4 alcohols, J. Membr. Sci. 326 (1) (2009) 222-233. 

Figure 31.9 illustrates the dual-layer spinneret design in our lab (Li et al., 2002). The outer-layer dope, inner-layer dope, and bore fluid are delivered to the orifice by passing through three independent channels. The most difficult parts of spinneret design arc precision and channel alignment. To run the dual-layer hollow-fiber spinneret, two metering pumps were employed to deliver inner and outer dopes. 

Figure 31.10 Schematic diagram of the influence of shrinkage percentage on dual-layer hollow-fiber structure (Li et al., 2004a).

Figure 31.12 SEM morphology of different skins of the dual-layer hollow fibers (Jiang et al., 2004).

TABLE 31.2 Elemental Analysis of Interface of Dual-Layer Hollow Fibers Spun with an Air Gap of 1.5 cm at 25°C of Coagulation"... 

Effects of Posttreatment on Dual-Layer Hollow-Fiber Membranes... 

Figure 31.13 Visual estimation of the dense-selective layer thickness of dual-layer hollow-fiber membranes with different heat treatment methods (left) 15°C for 3 h and right) 150°C for 1 h. (Li et al., 2004b).

It is possible to produce dual-layer hollow fibers, for several applications, using a triple orifice spinnerets. The production of dual-layer fibers is more complex due to the presence of two dope solutions. One of the most critical issues is delamination of the outer layer. A modified spinneref, with an indented middle tube, has been proposed to induce interdiffusion between the two fiber layers and prevent delamination [49]. 

N. Widjojo, T.S. Chung, and W.B. Krantz, A morphological and structural study of Ultem/P84 copolyimide dual-layer hollow fiber membranes with delamination-free morphology. Journal of Membrane Science 294 (2007) 132-146. 

FIGURE 7.18 Distribution of nanosilica particles at the cross section of PVDF/nanosilica dual-layer hollow-fiber membranes. (Data from P. Sukitpaneenit and T.S. Chung, Ind. Eng. Chem. Res., 51, 978-993, 2012.)... 

F.Y. Li, Y. Li, T.S. Chung, H. Chen, Y.C. Jean, and S. Kawi. (2011). Development and positron aimihilation spectroscopy (PAS) characterization of polyamide imide (PAI) polyethersulfone (PES) based defect-free dual-layer hollow fiber membranes with an ultrathin dense-selective layer for gas separation, J. Memb. Sci. 378 541-550. 

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