Cellulose acetate hollow fiber membranes

Figure 12 Diffractogram of dried cellulose acetate hollow fiber membrane.

The general process for preparation of the precursors consists of four steps, i.e. dope formation, casting/spinning, dehydration and post-treatment. There are many parameters that will affect the precursor properties during the preparation process. An example for the optimization of spinning condition was reported by He et al., who reported that the optimal conditions for spinning cellulose acetate hollow fiber membranes was found to be as follows bore fluid, water+ NMP (85%) air gap 25 mm bore flow rate, 40% of dope flow rate (2.2 mL min ) and temperature of quench bath, 50... 

Chou, W.L., Yu, D.G. and Yang, M.C. 2005. The preparation and characterization of silverloading cellulose acetate hollow fiber membrane for water treatment Pohm. Adv. 

Phenol Activated carbon-filled cellulose acetate hollow-fiber membranes Pseudomonas putida ATCC 17484 Zhu et al., 2000... 

Zhu, G., Chung, T. S. and Loh, K. C. 2000. Activated carbon-filled cellulose acetate hollow-fiber membrane for cell immobilization and phenol degradation. Journal of Applied Polymer Science, 76,695-707. 

HoUow fibers are widely used for filtration, utilizing the semipermeable nature of their capillary walls. In the medical industry, hollow fiber bioreactors are often made from cellulose and synthetic polymers. Cellulose acetate and cuprammonium rayon are the widely used ceUulose-based hollow fibers, while synthetic hollow fibers are often made from polysulfone, polyamide, and polyacrylonitrile. Modifications can be made to these materials to improve their functions by using polymers based on phospholipid, a substance found in the human cell membrane. 2-methaCTyloyloxyethyl phosphoryl-choline (MPC) is a methacrylate monomer with a phospholipid polar group. When MPC-based copolymers are used as additives for polysulfone, protein adsorption and platelet adhesion can be effectively reduced, thereby improving blood compatibility. Cellulose acetate hollow fiber membranes can also be modified with MPC-based copolymers by means of blending or surface coating to obtain improved permeability. 

Membrane systems to remove acid gas from the natural gas emerged more than three decades ago. Today about half a dozen venders provide membrane systems for natural gas upgrading. Amongst them are UOP (Separex and Grace), who uses cellulose acetate spiral wound module and Natco (Cynara) who uses cellulose acetate hollow fiber membrane modules. Ube, Air Liquide and Air Products offer polyimide hollow fiber membrane modules, and MTR produces perfluoropolymer membranes in spiral wound modules. 

The hollow-fiber membrane bioreactor took the simple cylindrical geometry housing [dimension 13 mm inner diameter (ID) x 22 mm outer diameter (OD) x 40 mm L see Fig. 14.2]. Cellulose acetate hollow-fiber membranes [200 p,m ID, wall thickness of 14 p,m and molecular weight cutoff (MWCO) of 10 kDa] derived from hemodialysers used to construct the HFMBs. The hollow-fiber membranes were fixed in the bioreactor by using molded silicon rubber. The effective length of the fiber in the reactor was 30 mm with approximately 200 fibers in each bioreactor. The distance between adjacent fibers was approximately 400 p-m, of the order of the distance between natural blood capillaries in human bone. The volume external to the hollow fibers in each HFMB was approximately 3.5 mL, and this volume was available for the collagen gel together with the microcarriers with adherent cells. 

Preparation of Hollow Fiber Membrane. CTA (Cellulose Tri-Acetate) hollow fiber membranes were prepared by aplnning a dope solution of CTA followed by soaking and anealing. 

Hughes immobilized AgN03 solutions in cellulose acetate hollow fibers to prepare immobilized liquid membranes for ethylene and propylene transport. 

Separex s,41-43 Grace Membrane Systems cellulose ester,60 Envirogenics GASEP (trademark of Envirogenics Company)64 cellulose triacetate spiral-wound membranes, and Dow cellulose acetate hollow fibers are used to produce a salable product from sour gas streams. 

Several classes of polymeric materials are found to perform adequately for blood processing, including cellulose and cellulose esters, polyamides, polysulfone, and some acrylic and polycarbonate copolymers. However, commercial cellulose, used for the first membranes in the late 1940 s, remains the principal material in which hemodialysis membranes are made. Membranes are obtained by casting or spinning a dope mixture of cellulose dissolved in cuprammonium solution or by deacetylating cellulose acetate hollow fibers [121]. However, polycarbonate-polyether (PC-PE) block copolymers, in which the ratio between hydrophobic PC and hydrophilic PE blocks can be varied to modulate the mechanical properties as well as the diffusivity and permeability of the membrane, compete with cellulose in the hemodialysis market. 

Research and development of the hollow-fiber-type module using a cellulose acetate membrane was conducted by Monsanto, Toyobo, and others, in addition to Dow Chemical. Toyobo announced an RO module for one-pass desalination of seawater that used the cellulose triacetate hollow-fiber membrane module in 1979 (Orofino, 1970 Ukai et al., 1980). 

W.L. Chou, M.C. Yang, Effect of take-up speed on physical properties and permeation performance of cellulose acetate hollow fibers. Journal of Membrane Science, 250 (2005) 259-267. 

Currently, approximately one billion gal/day of water are desalted by reverse osmosis. Half of this capacity is installed in the United States, Europe, and Japan, principally to produce ultrapure industrial water. The remainder is installed in the Middle East and other desert regions to produce municipal drinking water from brackish groundwater or seawater. In recent years, the interfacial composite membrane has displaced the anisotropic cellulose acetate membrane in most applications. Interfacial composite membranes are supplied in spiral-wound module form the market share of hollow fiber membranes is now less than... 

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. 

In this context, only two polymers have ever been used on a large scale in asymmetric membranes cellulose acetate and Permasep B-9/B-10 aramids. The former polymer predates the era of reverse osmosis membranes. The latter polymer has been used in hollow fiber membranes for 15 years. Attempts to bring other new polymers into asymmetric membrane production have been few (PBIL, PBI, polypiperazineamides), generally without particular success. 

Meiny different supports have been used to prepare ILMs Including cellulose acetate reverse osmosis membranes (1 6, 25, 29, ), micro-porous polypropylene ultrafiltration membranes (31-3 T7 polyvinyl chloride filters (35), and hollow fiber cellulose acetate reverse osmosis membranes T36). Way et al. ( ) discuss the chemical and physical properties that must be considered when an ILM support Is selected. 

Low-density polyethylene and polypropylene in the form of flat-sheet and hollow-fiber membranes are used in plasmapheresis and as oxygenators in the heart-lung machine. Other materials commonly used in plasmapheresis are cellulose acetate, polycarbonate, and polysulfone [129]. 

Semipermeable membranes and hollow fibers are produced from cellulose acetate. Dry-jet wet-spinning techniques are described to provide asymmetric and homogeneous hollow fiber membranes. Manipulation of spinning conditions leads to morphologies that permit higher rejection and higher fluxes. The excellent balance of the hydrophobic-hydrophilic characteristics for cellulose acetate makes this polymer useful for reverse osmosis [89-93]. Cellulose acetate membranes and hollow fiber membranes are commercially available for hemopurification. [94], for ultrafiltration [95], and for other commercial separation processes. 

Fujii et al. [13] studied morphological structures of the cross section of various hollow fibers and fiat sheet membranes by high-resolution field emission scanning electron microscopy. Figure 6.8 shows a cross-sectional structure of a flat sheet cellulose acetate RO membrane. The layer near the top surface is composed of a densely packed monolayer of polymeric spheres, which is supported by a layer formed with completely packed spheres. The contours of the spheres in the top layer can be observed. The middle layer is also composed of loosely packed and partly fused spheres, which are larger than the spheres in the surface layer. In the middle layer, there are many microvoids, the sizes of which are the same as the spheres. The layer near the bottom is denser than the middle layer, and the spheres are deformed and fused. Interstitial void spaces between the spheres, which may be called microvoids, are clearly observed. This structure seems common for the flat sheet as well as the hollow fiber membranes. For example. Fig. 6.9 shows a cross section of a hollow fiber made of PMMA B-2 (a copolymer containing methyl methacrylate and a small amount of sulfonate groups). The inside surface layer is composed of the dense structure of compactly packed fine polymeric particles. The particle structure of the middle layer... 

Chun Xiu Liu and Renbi Bai (2006). Adsorptive removal of copper ions with highly porous chi-tosan/cellulose acetate blend hollow fiber membranes. Journal of Membrane Science 284(1 2), 313-322. 

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