Asymmetric hollow fiber

Zeolite/polymer mixed-matrix membranes can be fabricated into dense film, asymmetric flat sheet, or asymmetric hollow fiber. Similar to commercial polymer membranes, mixed-matrix membranes need to have an asymmetric membrane geometry with a thin selective skin layer on a porous support layer to be commercially viable. The skin layer should be made from a zeohte/polymer mixed-matrix material to provide the membrane high selectivity, but the non-selective porous support layer can be made from the zeohte/polymer mixed-matrix material, a pure polymer membrane material, or an inorganic membrane material. 

H2S removal from gas streams 243 Asymmetric hollow-fiber modules coupled with concentrated alkaline solution... 

Yoshino, M., Nakamura, S., Kita, H., Okamoto, K.-i., Tanihara, N. and Kusuki, Y. (2003) Olefin/paraffin separation performance of asymmetric hollow fiber membrane of 6FDA/BPDA-DDBT copolyimide. Journal of Membrane Science, 212, 13-27. 

Visser, T., Koops, G.H. and Wessling, M. (2005) On the subtle balance between competitive sorption and plasticization effects in asymmetric hollow fiber gas-separation membranes. Journal of Membrane Science, 252, 265-277. 

Asymmetric hollow fibers provide an interesting support for enzyme immobilization, in this case the membrane structure allows the retention of the enzyme into the sponge layer of the fibers by crossflow filtration. The amount of biocatalyst loaded, its distribution and activity through the support and its lifetime are very important parameters to properly orientate the development of such systems. The specific effect that the support has upon the enzyme, however, greatly depend upon both the support and the enzyme involved in the immobilization as well as the method of immobilization used. 

FIGURE 10 Current asymmetric hollow-fiber formation process for gas separation membranes. 

Polymeric materials are still the most widely used membranes for gas separation, and for specific apphcations the separation technology is well established (see Section 4.6). Producing the membranes either as composites with a selective skin layer on flat sheets or as asymmetric hollow fibers are well-known techniques. Figure 4.5 shows an SEM picture of a typical composite polymeric membrane with a selective, thin skin layer of poly(dimethyl)siloxane (PDMS) on a support structure of polypropylene (PP). The polymeric membrane development today is clearly into more carefully tailored membranes for specific... 

As a self-supported cylinder, hollow fiber membrane is required to withstand high transmembrane pressure without collapsing. Modulus of elasticity is a crucial parameter for the calculation of the collapse pressure of a given fiber. With a much more porous overall structure, asymmetric hollow fibers specifically require a high modulus of elasticity to avoid collapse of the... 

Li, D.F. Chung, T.S. Wang, R. Liu, Y. Fabrication of fluoropolyimide polyethersulfone (PES) dual layer asymmetric hollow fiber membranes for gas separation. Journal of Membrane Science 2002, 198, 211-223. 

Hon-celluloslc Membranes. Despite an Intensive search for more favorable membrane polymers, cellulose acetate remained the best material for reverse osmosis until 1969 when the first B-9 permeator for brackish water desalination was Introduced by Du Font. Richter and Hoehn ( ) Invented aromatic polyamide asymmetric hollow-fiber... 

If an asymmetric hollow fiber with the skin on the outside is to be produced, the precipitant in the inner bore is replaced by an inert gas and the fiber is spun into the precipitation bath. Between the precipitation bath and the spinneret there is an air gap as indicated in Figure 1.36 (b) where the fiber may be drawn to obtain the desired dimensions before precipitation. Hollow fibers have, therefore, often significantly smaller diameters than the nozzle. 

Figure 3.11 is a photomicrograph of a typical asymmetric hollow fiber with the skin on the inside wall. Hollow fibers for UF have the skin on the inside whereas hollow fibers for RO are smaller and have the skin on the outside. This is because of the high pressure required in RO commercially available fibers cannot withstand internel pressures up to 400 psi and above. Thus, for RO, the feed stream is pressurized on the outside of the fiber, and the permeate flows from the lumen of the fiber (see Figure 3.12). Actually, considerable research effort has been spent in developing composite hollow fibers for RO which can withstand internal pressures up to 900 psi because, as we shall see, flowing the feed stream down the lumen of the fiber greetly reduces concentration polarization effects.

The spinning of asymmetric hollow fibers with the skin on the inside closely resembles the procedure used in casting flat-sheet membranes. Figure 3.1510 is a schematic diagram of a spinneret used to spin these fibers. The degassed and filtered polymer solution is forced under pressure into a coaxial tube spinneret. The liquid is extruded through an annular orifice and the hollow fiber (still liquid) is stabilized and precipitated by an internal coagulating fluid (usually water) which flows out the center tube. 

Asymmetric hollow fiber membranes can also be used as selective supports for enzymes. A biocatalyst suspension can in fact be forced through the unskinned surface of asymmetric membranes so that biocatalysts, either enzymes or whole cells, although still suspended, are effectively immobilized within the macroporous spongy part of the membranes.42-53 The enzymatic activity can thus be spread over a large surface, although substrates and products can only diffuse to and from the biocatalyst. 

A theoretical analysis of such enzyme membrane reactors was carried out by Rony46 and Waterland et al.47 In the Waterland model, which refers to asymmetric hollow fibers, each fiber is assumed to be at the nodal position in an equilateral triangular mesh, so that it is equidistant from six adjacent fibers. 

Figure 7.27 Asymmetric hollow fiber schematic radial cross section.47...

Figure 7.30 Axial section of an asymmetric hollow fiber. The substrate solution is fed to the lumen of the membrane under laminar regime.47...

Hydrolysis of high-molecular-weight protein in milk (trypsin and Asymmetric hollow fiber with gelified enzyme Production of baby food... 

Hydrolysis of cellulose to cellobiose Asymmetric hollow fiber Production of ethanol... 

In the simulation of the HMD process, the gas-permeation membrane used is assumed to be an asymmetric hollow-fiber membrane. For this type of membrane, gas permeation does not depend on the flow pattern on the permeate side as the porous supporting layer prevents mixing of the permeate fluxes (Pan, 1986). A schematic of the flow pattern in an asymmetric hollow-fiber membrane is shown in Figure 10.2. Hence, a simple cross-flow model is sufficient to describe the membrane behavior. 

Figure 10.2 Schematic of the flow pattern in an asymmetric hollow-fiber membrane.

The membrane model in the implemented ACM module was first validated with the experimental results of Pan (1986). In the experiments, the effect of varying stage cut (that is, the fraction of feed that permeates through the membrane) on mole fraction of H2 in the permeate was studied using asymmetric hollow fiber membranes in co- and counter-current configurations. Parameters of the membrane module can be found in Table lO.A.l stage cut varies from 0.34 to 0.55 in the counter-current configuration and from 0.35 to 0.6 in the... 

Pan, C.Y. (1986) Gas separation by high-flux, asymmetric hollow-fiber membrane. AIChE Journal, 32 (12), 2020-2027. 

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