Hollow fiber membrane phase inversion fabrication

The MIEC hollow fiber membranes are usually fabricated by a combined phase inversion and sintering process. The immersion induced phase inversion technique has been widely employed to prepare polymeric hollow fiber membranes. Due to the excellent binding capability of the polymers, a large amount of fine inorganic powder can be mixed inside the polymer solution to form a uniform mixture from which the ceramic hollow fiber precusors can be extruded at room temperature. Subsequent sintering would convert these precursor fibers into the ceramic hollow fiber membrane products. Figure... 

The formation of a porous structure results from phase separation (or phase inversion) mechanisms that are not limited to electrospraying. It is the process that controls membrane formation, as the solvent exchanges with a nonsolvent, polymer solution solidifies and polymeric device forms. The phase separation is fully investigated in fabrication of flat or hollow fiber membranes or in situ forming drug delivery systems. - Usually, quick evaporation of the solvent produces particles with porous or golf ball-shaped surfaces (Figure 22.26). 

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]. 

Chung, T.S.N. 2008. Fabrication of hollow-fiber membranes by phase inversion. Advanced Membrane Technology and Applications, 821-839. 

In the fabrication of FIFMMRs, the porous hollow fiber membranes with asymmetric structures are first prepared through a phase-inversion/ sintering technique, which has been described in Chapter 2 of this book and in many recent publications [31]. The microstructure of the hollow fibers can be tailored as expected for different applications by modulation of the suspension composition and the spinning parameters [32, 33]. Aran et al. [5,21] developed porous AI2O3 FIFMMRs with various geometrical parameters for gas-liquid-solid (G-L-S) reactions. The Pd-AhOa catalyst... 

FABRICATION OF HOLLOW-FIBER MEMBRANES BY PHASE INVERSION... 

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... 

Despite the fact that there has been extensive study on the preparation and characterizations of flat-sheet PVDF membranes [16-20] fabricated by NIPS, limited studies have been devoted to the fabrication and characterization of PVDF hollow-fiber membranes [21-24]. It is well accepted that the phase inversion process of hollow-fiber membranes is much more complex than that of flat-sheet membranes, and the controlling factors for the former during membrane formation are distinctly different from those for the latter. One essential difference is the dope formulation that affects the dope viscosity. Usually, a polymer dope possessing a viscosity of... 

It is well known that polymer concentration plays an essential role in membrane formation through the phase inversion process. The effect of polymer concentration on the morphology of PVDF hollow-fiber membranes has been demonstrated in many studies [21,34,37-42]. Generally, a concept of critical polymer concentration is often employed as a guideline for a proper selection of polymer content in dope solutions [4,5]. The critical polymer concentration can be determined from the correlation of viscosity versus polymer concentration at a specific shear rate and temperature. An example of the critical polymer concentration of PVDF/NMP dope solutions is illustrated in Figure 7.5. Depending on the specific application, PVDF dopes with a polymer content below the critical concentration are usually adopted to fabricate microporous PVDF hollow fibers for water-related applications such as MF [43], UF [40], and MD [8,10]. On the other hand, dopes possessing a polymer concentration above the critical value are chosen to fabricate PVDF hollow fibers with a relatively dense selective skin for gas separation and pervaporation [12,13,39]. 

T.S. Chung. (2008). Fabrication of hollow fiber membranes by phase inversion. In Advanced Membrane Technology and Applications, N. Li et al. (Eds.), John Wiley Sons, Inc., Hoboken, NJ, pp. 821-841. 

Apart from single-layer membranes, the use of the viscous fingering-induced phase inversion process allows a single-step formation of multilayer hollow-fiber membranes with flexible control of thickness and morphology of each layer. To date, dual- and triple-layer membranes have been fabricated, and the same methodology can be transferred toward membranes made of more layers, although the subsequent sintering and application of such manbranes may need further considerations. 

The viscous fingering-induced phase inversion process integrates microstructuring and continuous fabrication of ceramic hollow-fiber membranes in one step, whereas... 

Ceramic hollow-fiber support can be fabricated by a phase inversion-based extrusion/sintering technique [3], which allows a more flexible control over the membrane macro/microstructures by adjusting fabricating parameters such as air-gap, extrusion rate, internal coagulant composition, and the amount of nonsolvent additive in the spinning suspensions [4]. Such unique structural diversity delivers... 

Technology development on the fabrication of asymmetric membranes with an ultrathin dense layer has received much attention due to the fact that the thinner the dense layer is, the higher is the productivity. The fabrication of a hollow fiber with a desirable pore-size distribution and performance is not a trivial process as many factors influence fiber morphology during the phase inversion. 

It is difficult and time-consuming to fabricate membrane modules from the hollow fibers due to the poor mechanical strength. Moreover, the membrane modules are also damageable for both transportation and utilization. It would be better if the phase inversion technique can be modified to prepare monolith-structured membranes. 

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