Hollow fiber membrane spinning dope

For polymeric hollow fiber membranes, spinning parameters are crucial factors that must be controlled during the preparation of membranes. These parameters include the amount and type of polymers, solvents, additives mixed into the spinning dope solution, the dope and bore fluid rate, the kind of bore fluid, the fiber take-up velocity, the air-gap distance (unless wet spinning is used), and the coagulant bath temperature and the kind of coagulant bath. 

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... 

Hollow fiber membranes with a positively charged nanofiltration selective layer have been fabricated by using asymmetric microporous hollow fibers made from a Torlon PAI type as the porous substrate followed by a post-treatment with poly(ethyleneimine) [79]. The membrane structure and the surface properties can be tailored by adjusting the polymer dope composition, spinning conditions, and the posttreatment parameters. 

It has been previously shown that the type of add used to dope polyaniline flat sheet membranes affects both their permeability and selectivity [48]. The selective nature of these hollow fiber membranes can be tailored during the fiber-spinning process by doping the polyaniline hollow fiber with the desired acid. The godet baths were filled with a 1 M addic solution of varying acid strength and size, and the... 

Asymmetric hollow fiber membranes of polysulfone, polyethersulfone, and polyphenylsulfone can be prepared by phase inversion spinning solvent/nonsolvent dopes, i.e., N-methylpyrrolidone/formamide. These asymmetric hollow fiber membranes possess a microscopically observable skin supported by a porous open cellular network. The walls of the open cells of the matrix are composed of arrays of interconnected spherical micelles. With increasing proximity to the outer surface, the packing density of the spherical micelles increase with a concomitant decline in interstitial porosity. At the outer surface layers, the packing of the micelles becomes... 

Rahbari-Sisakht et al. [54] investigated the effect of novel surface modifying macromolecules (nSMMs) on the morphology and performance of PSf hollow-fiber membrane for CO2 absorption. The performance of surface-modifled membrane in contactor application for CO2 absorption through distilled water as absorbent was studied. The results show that surface-modified membrane had higher performance compared to plain polysulfone membranes. With the membrane prepared from SMM in the spinning dope, a maximum CO2 flux of 5.8 X 10 mol/m s was achieved at 300 ml/min of absorbent flow rate, which was almost 76% more than the other membrane. In a long-term stability study, the initial flux reduction was found to be about 18% after 50 h of operation of the surface-modified membrane. 

Research effort at Albany International Research Co. has developed unit processes necessary for pilot scale production of several species of reverse osmosis hollow fiber composite membranes. These processes include spin-dope preparation, a proprietary apparatus for dry-jet wet-spinning of microporous polysul-fone hollow fibers, coating of these fibers with a variety of permselective materials, bundle winding using multifilament yarns and module assembly. Modules of the membrane identified as Quantro II are in field trial against brackish and seawater feeds. Brackish water rejections of 94+% at a flux of 5-7 gfd at 400 psi have been measured. Seawater rejections of 99+% at 1-2 gfd at 1000 psi have been measured. Membrane use requires sealing of some portion of the fiber bundle for installation in a pressure shell. Much effort has been devoted to identification of potting materials which exhibit satisfactory adhesion to the fiber while... 

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. 

Manufacture of gas-separation membrane modules is largely a machine-assisted, labor-intensive operation. Polymer dopes are typically prepared batchwise with sufficient hold time to insure uniformity. The membrane performance is largely controlled by the polymer precipitation step and very dependent upon phase behavior and precipitation kinetics. Thus, it is essential that processing conditions be maintained as uniformly and as constant as possible if product quality and uniformity is to be preserved. For this reason, membrane-fihn formation and hollow-fiber spinning processes are usually operated continuously or for extended run times. Since the intermediate film or fiber must eventually be converted into discrete items, the continuous process is typically interrupted by collection of the membrane formed on spools or fiber skeins where it may be inventoried briefly before batch processing into the final assembly resumes. 

A suitable polymer material for preparation of carbon membranes should not cause pore holes or any defects after the carbonization. Up to now, various precursor materials such as polyimide, polyacrylonitrile (PAN), poly(phthalazinone ether sulfone ketone) and poly(phenylene oxide) have been used for the fabrication of carbon molecular sieve membranes. Likewise, aromatic polyimide and its derivatives have been extensively used as precursor for carbon membranes due to their rigid structure and high carbon yields. The membrane morphology of polyimide could be well maintained during the high temperature carbonization process. A commercially available and cheap polymeric material is cellulose acetate (CA, MW 100 000, DS = 2.45) this was also used as the precursor material for preparation of carbon membranes by He et al They reported that cellulose acetate can be easily dissolved in many solvents to form the dope solution for spinning the hollow fibers, and the hollow fiber carbon membranes prepared showed good separation performances. 

Figure 31.12 displays the desirable skin morphologies at various locations of dual-layer membranes. Briefly, both the inner and outer skins of the inner layer are porous with intimate adhesion between them. When using two different materials, one can control the interlayer diffusion as a function of spinning conditions. Table 31.2 summarizes the elemental analysis data of the interfacial layers of hollow fibers spun from PES as the inner layer and Matrimid polyimide as the outer layer at different spinneret temperatures (Jiang et al., 2004). After being extruded from the spinneret, the two dopes should make contact with each other under normal conditions. When the spinneret temperature is low (i.e., 25°C), their viscosities are high, the diffusion rate of the polymer molecules between the two layers will not be fast therefore, no sulfur can be found in the outer layer, as shown in this table. When the spinneret temperature is increased to 60°C, the interlayer diffusion between the two polymers apparently occurs, as evidenced in Table 31.2, in which it is... 

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