Hollow fiber membrane structure

Mansourizadeh, A., Ismail, A.F. 2011. A developed asymmetric RVDF hollow fiber membrane structure for COj absorption. Int. J. Greenh. Gas Control 5 374—380. 

BOE hollow-fiber membrane structure allows the reduction of the pressure drop along the hollow-fiber bore that leads to both greater salt rejection due to greater dilution effects and greater permeate water flow. 

Hollow fiber membranes are primarily homogeneous. In use, their lower permeability is compensated for by large surface per unit volume of vessel. Fibers are 25-250 pm outside dia, wall thickness 5-50 pm. The cross section of a vessel for reverse osmosis may have 20-35 million fibers/sqft and a surface of 5500-9000 sqft/cuft of vessel. Recently developed hollow fibers for gas permeation processes have anisotropic structures. 

Hollow-fiber membranes form a tubular structure which is usually arrayed as a parallel fiber bundle within a cylindrical container. The cells are trapped on the shell side of the hollow fibers while aerated nutrient medium is rapidly recirculated through the fibers. This type of membrane support can provide extra protection against contamination. However, the major disadvantages of membrane... 

Another method of producing composite hollow fibers, described by Kusuki etal. at Ube [108] and Kopp et al. at Memtec [109], is to spin double-layered fibers with a double spinneret of the type shown in Figure 3.37. This system allows different spinning solutions to be used for the outer and inner surface of the fibers and gives more precise control of the final structure. Often, two different polymers are incorporated into the same fiber. The result is a hollow fiber composite membrane equivalent to the flat sheet membrane shown in Figure 3.26. A reason for the popularity of composite hollow fiber membranes is that different polymers can be used to form the mechanically strong support and the selective layer. This can reduce the amount of selective polymer required. The tailor-made polymers developed for gas separation applications can cost as much as... 

US 5- 10/g. Single-layer hollow fiber membranes contain 10-20 g of polymer per square meter of membrane, for a material cost alone of US 50-200/m2. Thus, using a composite structure consisting of a relatively inexpensive core polymer material coated with a thin layer of the expensive selective polymer reduces the overall membrane material cost significantly. 

J.J. Kim, T.S. lang, Y.D. Kwon, U.Y. Kim and S.S. Kim, Structural Study of Micro-porous Polypropylene Hollow Fiber Membranes Made by the Melt-Spinning and Cold-Stretching Method, J. Membr. Sci. 93, 209 (1994). 

An important point to consider about hollow fiber membranes is their morphology. Hollow fiber membranes can be either symmetric or asymmetric.16 Symmetric membranes have continuous pore structure throughout. Asymmetric membranes have a dense upper layer or skin layer that is then supported with a sublayer that is significantly more porous. Figure 6.2 shows SEM images of... 

Salts rejected by the membrane stay in the concentrating stream but are continuously disposed from the membrane module by fresh feed to maintain the separation. Continuous removal of the permeate product enables the production of freshwater. RO membrane-building materials are usually polymers, such as cellulose acetates, polyamides or polyimides. The membranes are semipermeable, made of thin 30-200 nanometer thick layers adhering to a thicker porous support layer. Several types exist, such as symmetric, asymmetric, and thin-film composite membranes, depending on the membrane structure. They are usually built as envelopes made of pairs of long sheets separated by spacers, and are spirally wound around the product tube. In some cases, tubular, capillary, and even hollow-fiber membranes are used. 

T. Kobayashi, M. Todoki, M. Fujii, T. Takeyama and H. Tanzawa, Permeability and structure of PMMA stereocomplex hollow fiber membrane for hemodialysis, in E. Drioli and M. Nakagaki (Eds.), Proc. Eur.-fpn Cong. Membr. Membr. Processes, Plenum, New York, NY, 1986, pp. 507-513. 

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... 

Major polymer applications aerospace, electronics (mostly films and coatings), photosensitive materials for positive imaging, solar cells, hollow fiber membranes, composites. unclear power plants, space shuttle, microprocessor chip carriers, structural adhesives... 

Membranes. Seven membranes were evaluated in this work. These were flat-sheet and hollow-fiber membranes originally developed for RO applications. All have the anisotropic, skinned structure depicted in Figure 2. 

A further requirement of a CCRO membrane is that it should have an open, microporous sublayer structure. Such membranes allow effective diffusion of ethanol into the membrane from a recirculation solution supplied on the permeate side of the membrane. In our survey of various flat-sheet and hollow-fiber membranes, a monomer-derived polyamide composite membrane designated 3N8 was identified which satisfied this requirement. Other membranes tested either exhibited small or no measurable flux increases with permeate-side recirculation and are thus not suited to CCRO applications. 

At present, hollow-fiber membranes used for gas and liquid separations are mostly prepared from amorphous polymers by means of a phase inversion technique. In this technique, a polymer is dissolved in a suitable solvent or solvent mixtures and spim into a coagulation bath, where solvent exchange occurs between the extruded fiber and coagulant, yielding the asymmetric membrane structure. 

With the increase of the concentration of PPESK in the casting solution, the viscosity strongly increases and becomes shear-rate dependent. Then the morphology of the hollow fiber membranes changes from a finger-like structure to sponge-like structure. 

Hollow fiber membranes made from poly(imide)/sulfonated PES, with a phthalide group, exhibit a high selectivity in the vapor permeation of mixtures of methanol and methyl-terf-butyl ether (MTBE) as high as 12,000. The structures of the polymers used are shown in Figure 7.11. 

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). 

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