Polymeric membranes membrane drying

Interfacial polymerization membranes are less appHcable to gas separation because of the water swollen hydrogel that fills the pores of the support membrane. In reverse osmosis, this layer is highly water swollen and offers Httle resistance to water flow, but when the membrane is dried and used in gas separations the gel becomes a rigid glass with very low gas permeabiUty. This glassy polymer fills the membrane pores and, as a result, defect-free interfacial composite membranes usually have low gas fluxes, although their selectivities can be good. 

Figure 13 Typical swelling and deswelling rates of cross-linked poly(acryloyl pyrroli-dine-co-styrene) between 27°C and 37°C. AS15 ( ) AS20 (A). The numbers indicate the content of styrene in the feed composition in moles during polymerization. Membrane thickness is 0.5 mm in the dried state. (From Ref. 34.)...

Relationship Between Nodular and Rejecting Layers. Nodular formation was conceived by Maler and Scheuerman (14) and was shown to exist in the skin structure of anisotropic cellulose acetate membranes by Schultz and Asunmaa ( ), who ion etched the skin to discover an assembly of close-packed, 188 A in diameter spheres. Resting (15) has identified this kind of micellar structure in dry cellulose ester reverse osmosis membranes, and Panar, et al. (16) has identified their existence in the polyamide derivatives. Our work has shown that nodules exist in most polymeric membranes cast into a nonsolvent bath, where gelation at the interface is caused by initial depletion of solvent, as shown in Case B, which follows restricted Inward contraction of the interfacial zone. This leads to a dispersed phase of micelles within a continuous phase (designated as "polymer-poor phase") composed of a mixture of solvents, coagulant, and a dissolved fraction of the polymer. The formation of such a skin is delineated in the scheme shown in Figure 11. 

Phospholipid vesicles form spontaneously when distilled water is swirled with dried phospholipids. This method of preparation results in a highly polydisperse array of multicompartment vesicles of various shapes. Extrusion through polymeric membranes decreases both the size and polydispersity of the vesicles. Ultrasonic agitation is the most widely used method for converting the lipid dispersion into single-compartment vesicles of small size. 

Enzyme micro-encapsulation is another alternative for sensor development, although in most cases preparation of the microcapsules may require extremely well-controlled conditions. Two procedures have usually been applied to microcapsule preparation, namely interfacial polymerization and liquid drying [80]. Polyamide, collodion (cellulose nitrate), ethylcellulose, cellulose acetate butyrate or silicone polymers have been employed for preparation of permanent micro capsules. One advantage of this method is the double specificity attributed to the presence of both the enzyme and the semipermeable membrane. It also allows the simultaneous immobilization of many enzymes in a single step, and the contact area between the substrate and the catalyst is large. However, the need for high protein concentration and the restriction to low molecular weight substrates are the important limitations to this approach. 

For many years polymeric membranes have been utilized widely for material separation without detailed characterization of the pore size and the pore size distribution. Most of the commercially available membranes are prepared by either a dry or a wet phase-inversion process. These membranes are formed by the phase separation of multicomponent polymer-solvent systems, the underlying principle being phase separation of the polymer solution. 

A membrane designated "Solrox" made by Sumitomo Chemical Company is closely related to the above plasma polymerized composite membranes. A 1980 report by T. Sano described the Sumitomo process (31). A support film was cast from a polyacrylonitrile copolymer containing at least 40 mole percent acrylonitrile. The support film was dried and exposed to a helium or hydrogen plasma to form a tight cross-linked surface skin on the porous polyacrylonitrile support film. Data in a U.S. Patent issued in 1979 to Sano et al showed that the unmodified support film had a water flux of 87 gfd (145 L/ sq m/hr) at 142 psi (10 kg/sq cm). After the plasma treatment a reverse osmosis test using 0.55 percent NaCl at 710 psi (4895 kPa) showed 10.5 gfd (17.5 L/sq m/hr) flux at 98.3 percent salt rejection (32). This membrane appears to fall between a conventional asymmetric membrane and a composite membrane. If the surface skin is only cross-linked, one might call it a modified asymmetric membrane. However, if the surface skin is substantially modified chemically to make it distinct from the bulk of the membrane it could be considered as a composite type. 

The method has already been employed for polymeric membranes by several authors (14-16). Although there are some limitations for using this technique, for example, cylindrical pores are assumed and the membranes have to be dried without damaging the pore structure before the measurements can start, results were obtained for UF membranes made from different polymeric materials (Cellulose Acetate (CA), Poly-2,6-dimethy1-1,4-Phenylene Oxide (PPO) and some other non cellulosic materials). 

Figure 5.8 Change in electronic conductivity of composite membranes (dry state) with polymerization time of pyrrole. A ferric ion form cation exchange membrane, NEOSEPTA CM-1, was immersed in an aqueous 0.745 N pyrrole solution for different periods, washed with pure water and then completely dried.

Gas permeation (GP) was described in detail in Examples 9.2 and 9.3. Since the early 1980s, applications of GP with dense polymeric membranes have increased dramatically. Applications include (1) separation of hydrogen from methane (2) adjustment of the H2/CO ratio in synthesis gas (3) oxygen enrichment of air (4) nitrogen enrichment of air (5) removal of C02 (6) drying of natural gas and air (7) removal of helium and (8) removal of organic solvents from air (Seader and Henley, 2006). 

Micro fuel cell designs without polymeric membranes can overcome some PEM-related issues such as fuel crossover, anode dry-out or cathode flooding. In these membraneless laminar flow-based fuel cells (LF-EC) two or more liquid streams merge into a single microfluidic channel. The stream flows over the anode and the cathode electrodes placed on opposing side walls within the channel. The reaction of fuel and oxidant takes place at the electrodes while the two liquid streams and their liquid-liquid interface provide the necessary ionic transport [122,123]. 

Because a vacuum is applied for the removal of the solutes on the membrane downstream face, this side of the membrane is ideally dry in comparison to the more swollen (if polymeric membranes are employed) and hence more flexible membrane upstream face resulting from the solute uptake. This anisotropy of the membrane in the direction of the diffusion of the solute always exists for polymeric membranes and results in a non-uniform diffusivity of solute within the membrane. In other words, the diffusion coefficient of solute i in the membrane, will be position-dependent and not constant across the membrane. 

The dry and wet processes are two main manufacturing methods to prepare microporous polymeric membranes. Both methods are conducted through an extruder and a stretching process to increase the porosity and improve the tensile strength. Generally, separators made by dry process exhibit distinct slit-pore and straight microstructures, whereas those made by wet process show intercormected spherical or elliptical pores. Both methods use cheap polyolefin materials, so the microporous polymeric membranes are not expensive. 

The fastest growing desalination process is a membrane separation process called reverse osmosis (RO). The most remarkable advantage of RO is that it consumes little energy since no phase change is involved in the process. RO employs hydraulic pressure to overcome the osmotic pressure of the salt solution, causing water-selective permeation from the saline side of a membrane to the freshwater side as the membrane barrier rejects salts [1-4], Polymeric membranes are usually fabricated from materials such as cellulose acetate (CA), cellulose triacetate (CTA), and polyamide (PA) by the dry-wet phase inversion technique or by coating aromatic PA via interfacial polymerization (IFP) [5]. 

Membranes made by poly(vinyl chloride) (PVC) were used for PV separation of benzene-C YH mixture. Polymer was dissolved in THF to give 4 and 8 wt% solution at room temperature. PVC exhibits a strong affinity for benzene only. The casted films were dried under a nitrogen atmosphere for 3 days and then completely dried under reduced pressure. Thus, the membrane made by PVC was more permeable to benzene than to CYH. Increasing the concentration of benzene resulted in increasing flux as well as decreasing selectivity. Flux depended on the amount of polymer in the casting solution. Thickness of the polymeric membranes affected the permeation rate (Yildirin et al. 2001). 

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