Hollow libers

The excellent niass-transfer properties conferred by the hollow-liber configuration led to numerous applications. Commercial applications have been established in the medical field in gas separations and pervaporation other applications are in various stages of dcselopmem. 

Hollow libers olTcr three primary advantages over llal-sheet or tubular membranes, First, hollow fibers exhibit higher productivity per unit volume second, they are self-supporting and third, high recovery in individual units can be tolerated,... 

When the operation of the hollow-liber membrane is to he reversed, and permeation from (he bore to ouier zone is required, circumferential dress and pressure drop along the liher capillary ihore) musi he considered in the design of the fiber unit. 

In preparation of permselective hollow-fiber membranes, morphology must be controlled to obtain desired mechanical and transport properties. Fiber lubrication is performed wilhottl a easting surface. Therefore, in the moving unsupported thread line, the nascent hollow -liber membrane musi establish mechanical integrity in a very short time. 

Fig 2 Composite hollow liher production scheme I PHI = polycihyleneimine TDI = toluene 2,4-diisocyanate). Anisotropic (porous skint polysullone hollow liber is rolled into bath A and is titled vertically no avoid droplet t onnationt into a heating lube. The libel is then passed through bath B and is annealed in a ventilated heating tube 11 tO C)... 

Although these composite fibers were developed lor reverse osmosis lltcir acceptance in the desalinatiun industry has been limited due to insufficient selectivity and oxidative stability. The concept, however, is extremely viable composite membrane fiat films made from interfacial polymerization have gained wide industry approval. Hollow libers using Ibis technique to give equivalent properties and life, yet lo be developed, should be market tested during the 1990s. 

Interpenetrated Mall Matrix. Ion-exchange hollow libers can be produced by polymerizing an ionic monomer within (he porous wall matrix of a hollow fiber. Requirements of such a fabrication are th the monomers should not dissolve or plastici/c die polymer from which the fibers arc made. f2i the heat generated during the polymerization and contraction prior to the formation of new interpenetrating polymer should be minimized and ( ) the polymerization should not occur within the lumen (and hence cause plugging of the fiber). One drawback of such fibers is brittleness. 

Ultrafiltration (UF) refers to the removal of high molecular weight colloids (10,000 MW) up to particles less than 0.05 pm in diameter [10]. Like MF, UF places a mechanical barrier into the flow stream to separate the solid and liquid phases. The most common UF is a cross-flow hollow fiber type whereby a UF module contains hundreds of hollow microfibers. Whether or not the medium to be filtered flows inside or outside, the microfibers depends on the characteristics of the waste stream. UF is different from MF not only because UF can filter very small particles and some colloids, but also because of the cross-flow dynamics inside the UF module that keeps the surface of the hollow libers clean. Because of the cross flow, UF modules require a reject stream as well as a permeate stream. In other words, 100% of the liquid that enters the UF module does not exit as permeate. Figure 19.1 shows the differences between the UF and microlilters with respect to flow path. 

A reference case for the C02-selective WGS membrane reactor was chosen with the C02/H2 selectivity of 40, the C02 permeability of 4000 Barrer, the inlet sweep-to-feed molar flow rate ratio of 1, the membrane thickness of 5jum, 52,500 hollow libers (a length of 61 cm, an inner diameter of 0.1 cm, and a porous support with a porosity of 50% and a thickness of 30jLon), both inlet feed and sweep temperatures of 140 °C, and the feed and sweep pressures of 3 and latm, respectively. With respect to this case, the effects of C02/H2 selectivity, C02 permeability, sweep-to-feed ratio, inlet feed temperature, inlet sweep temperature, and catalyst activity on the reactor behavior were then investigated. 

The selectivity of metal catalysts improves in some reactions with alloying for example the alumina-supported Pd-Cu catalyst hydrogenales butadiene to 1-butene with 99% selectivity, i.e. the isomerization is less than lOli. The explanation is that hydrogen adsorption decreased on the Cu-containing catalysts . Similarly, better selectivities were observed with a polymer anchored Pd, or a Pd-Co catalyst in the gas-phase hydrogenation of butadiene and cyclopentadiene in a hollow-liber reactor 2 in the liquid-phase hydrogenation of 1,5-hexadiene with Pd-Ag catalyst. ... 

Fig. 4 Calculated loss spectra of metal and dielectric-coated metal hollow-libers...

Spiral wound, hollow libers, plate and frame, and tubular (use for small flow, high value, and severe fouling applications). 

Like enzymes, whole cells are sometimes immobilized by attachment to a surface or by entrapment within a support. One motivation for this is similar to the motivation for using biomass recycle in a continuous process. The cells are grown under optimal conditions for cell growth but are used at conditions optimized for production of a secondary metabolite. A hollow-liber reactor, similar to those used for cross-flow filtration, can be used to entrap the cells while allowing input of the substrate and removal of products. Attachment of the cells to a nonreactive material such as alumina allows a great variety of reactor types including packed beds, fluidized and spouted beds, and air-lift reactors. Packed beds with a biofilm on the packing are commonly used for wastewater treatment. A semicommercial process for beer used an air-lift reactor to achieve reaction times of one day compared to five to seven days for the normal batch process. Unfortunately, the beer suffered from a mismatched flavor profile that was attributed to mass transfer limitations. 

The polymer concentration in the solution is adjusted depending on the viscosity. Higher solution viscosities are required for the production of hollow-liber membranes, as compared to flat sheet production, because the fiber fabrication is performed without a casting surface. 

G. C. Kapantaidakis, G. H. Koops, and M. Wessling. Preparation and characterization of gas separation hollow liber membranes based on poly-ethersulfone-polyimide miscible blends. Desalination, 145(l-3) 353-357, September 2002. 

Hollow liber (outer diameter <0.5 mm) and capillary (outer diameter >0.5 nun) ceramic membranes constitute the very last development in the field of inorganic membranes. As for polymer hollow fiber m branes, a large surface/volume ratio is expected for the modules as well as the possibility of keeping uppermost properties of ceramics, that is, high mechanical, chanical, andtanperature resistance. Production of ceramic hollow fibers or capillaries with an asynunetric or homogeneous structure (Figure 9.6a) can be achieved in... 

In order to integrate the heat exchangers in the membrane system and to eliminate the temperature difference in membrane module by feeding hot air, the high-temperature sealing problem associated with the fabrication of hollow liber modules and membrane systems has to be solved. 

Lu et al. [Ill] later reported on the development of solid-state electrochemical linear actuators with a polyaniline yarn-in-hollow fiber configuration using an ionic liquid electrolyte. These yarn-in-hollow fiber actuators, which were constructed using a triflic acid-doped polyaniline solid fiber inserted into a triflic acid-doped polyaniline hollow liber. A porous polyacrylonitrile insert separated the two electrodes, which contained the [BMIM] [Bp4] electrolyte (Figure 2.27). It was demonstrated... 

Pure polymers lack the degree of hardness required in the wall and therefore tend to collapse after the removal of the core. Accordingly, the presence of ceramic precursor in the polymer solution was essential not only to stabilize the compound jet and entrain the oil uniformly but also to form rigid walls necessary for maintenance of the tubular shape. It was also suggested that the sheath should be able to withstand the capillary forces during core extraction in order to maintain the hollow fiber morphology. Further, the use of immiscible solutions was necessary to form continuous hollow libers with a smooth interior surface. When a miscible solution was... 

Fig. 22 (a) TEM and (b) SEM images of as-spun hollow libers after the oily cotes had been extracted with octane. The walls of these tubes were made of a composite containing amorphous Ti02 and PVP. Adapted with permission from [180]. Copyright (2004) American Chemical Society... 

Hollow liber membranes are numerous small hollow fibers with semi-permeable walls, and are assembled within a cylindrical shell/jacket to function as a bioreactor. One of the clinical applications of hoUow fiber bioreactors is the hemodialyzer. These hollow fiber membranes are produced by solution-based processing method by solvent phase separation. This process has been used to produce filtration membranes in the past [11], and is now being used to produce tissue engineering scaffolds [12-14]. 

Butyl acetate H-USY zeolite + amorphous silica layer Hollow libers 75... 

Zhitomirsky I, Gal-Or L. Formation of hollow libers by electrophoretic deposition. Mater Utt 1999 38(l) 10-7. 

Tharakan, J.P., Chau, PC., 1986. Operation and pressure distribution of immobilized cell hollow liber bioreactors. Biotechnol. Bioeng. 28, 1064-1071. 

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