Separation membranes stability

Ionomer membranes are used in fuel cells in order to separate the anode and cathode compartment and to allow the transport of protons from the anode to the cathode. The typical membrane is Nation , which consists of a perfluorinated backbone and side chains terminated by sulfonic groups. In the oxidizing environment of fuel cells, Nation , as well as other membranes, is attacked by reactive oxygen radicals, which reduce the membrane stability. Direct ESR was used recently in our laboratory to detect and identify oxygen radicals as well as radical intermediates formed in perfluorinated membranes upon exposure to oxygen radicals [73,74]. The three methods used to produce oxygen radicals in the laboratory and the corresponding main reactions are shown below. 

It was expected that sulphate removal from sodium bromide solutions would be very similar to sulphate removal from sodium chloride. Experimentation was carried out to determine sulphate rejection, membrane permeability and membrane stability in concentrated sodium bromide. The experimental work determined that nanofiltration is a useful process for separating these materials. 

Propranolol has two separate and distinct effects. The first is a consequence of the drug s (3-blocking properties and the subsequent removal of adrenergic influences on the heart. The second is associated with its direct myocardial effects (membrane stabilization). The latter action, especially at high clinically employed doses, may account for its effectiveness against arrhythmias in which enhanced (3-receptor stimulation does not play a significant role in the genesis of the rhythm disturbance. 

Membrane instability results in partial mixing of feed and stripping phases, which deteriorates the selectivity. In addition, raffinate and product are contaminated by the extractant, leading also to extractant losses. Economy of separation and hence industrial application of LM for separation of cephalosporins are strongly dependent on membrane stabilization. 

Much attention has been paid to the synthesis of fluorine-containing condensation polymers because of their unique properties (43) and different classes of polymers including polyethers, polyesters, polycarbonates, polyamides, polyurethanes, polyimides, polybenzimidazoles, and epoxy prepolymers containing pendent or backbone-incorporated bis-trifluoromethyl groups have been developed. These polymers exhibit promise as film formers, gas separation membranes, seals, soluble polymers, coatings, adhesives, and in other high temperature applications (103,104). Such polymers show increased solubility, glass-transition temperature, flame resistance, thermal stability, oxidation and environmental stability, decreased color, crystallinity, dielectric constant, and water absorption. 

The first major application of microfiltration membranes was for biological testing of water. This remains an important laboratory application in microbiology and biotechnology. For these applications the early cellulose acetate/cellulose nitrate phase separation membranes made by vapor-phase precipitation with water are still widely used. In the early 1960s and 1970s, a number of other membrane materials with improved mechanical properties and chemical stability were developed. These include polyacrylonitrile-poly(vinyl chloride) copolymers, poly(vinylidene fluoride), polysulfone, cellulose triacetate, and various nylons. Most cartridge filters use these membranes. More recently poly(tetrafluo-roethylene) membranes have come into use. 

Current polymeric materials are inadequate to fully meet all requirements for the various different types of membranes (cf. Section 2.2) or to exploit the new opportunities for application of membranes. Mixed-matrix membranes, comprising inorganic materials (e.g., metal oxide, zeolite, metal or carbon particles) embedded in an organic polymer matrix, have been developed to improve the performance by synergistic combinations of the properties of both components. Such improvement is either with respect to separation performance (higher selectivity or permeability) or with respect to membrane stability (mechanical, thermal or chemical). 

Yang, X.J., Fane, A.G. and Soldenhoff, K. (2003) Comparison of liquid membrane processes for metal separations Permeability, stability, and selectivity. Industrialsl Engineering Chemistry Research, 42, 392. 

Segmented polyimide-polydimethylsiloxane copolymers have been successfully synthesized both in laboratory and industrial quantities to produce multiphase siloxane-modified polyimides. The siloxane detracts somewhat from the otherwise excellent thermo oxidative stability of the polyimide, but it does produce a number of important properties. These include multiphase behavior, improved adhesion to many substrates, improvements in fire resistance and enhanced gas and liquid separation membranes, where one wishes not only to maximize the contribution of the siloxane to permeability, but also to utilize the imide to re-... 

Industrial separation membranes and ion-exchange resins can be made from chitin, especially for water purification. Chitin is also used industrially as an additive to thicken and stabilize foods and pharmaceuticals. Since it can be shaped into fibres, the textile industry has used chitin, especially for socks, as it is claimed that chitin fabrics are naturally antibacterial and antiodour (www.solstitch.net). Chitin also acts as a binder in dyes, fabrics and adhesives. Some processes to size and strengthen paper employ chitin. 

In theory CVI membranes are very promising. Especially membrane stability is expected to be very good, because the separative layer is located inside the support, where it is protected against mechanical damage and chemical attack. In addition, measured permselectivities of CVI-silica membranes are very high for H2 N2 values as high as 3000 have been measured [36,37],... 

The chemical stability of hydrogen separation membranes is a critical issue, because they will operate at elevated temperatures and pressures in atmospheres containing CO, CO2, and HjS, among other constituents. Figure 6.4 shows the hydrogen... 

How well economically a gas separation membrane system performs is largely determined by three parameters. The first parameter is its permselectivity or selectivity toward the gases to be separated. Permselectivity affects the percentage recovery of the valuable gas in the feed. For the most part, it is a process economics issue. The second is the permeate flux or permeability which is related to productivity and determines the membrane area required. The third parameter is related to the membrane stability or service life which has a strong impact on the replacement and maintenance costs of the system. 

Silicalite-1 membranes, supported on porous alumina ceramic discs, have been prepared by two different routes. In the first the zeolite membrane has been formed by in situ hydrothermal synthesis. Secondly a layer has been formed by controlled filtration of zeolite colloids. To optimise membrane stability, conditions have been established in which penetration of zeolite into the support sublayer occurs. The pore structure of these membranes has been characterised by a combination of SEM and Hg-porosimetry. The permeabilities of several gases have been measured together with gas mbeture separation behaviour. 

Chemical surface modification methods of gas-separation membranes include treatment with fluorine, chlorine, bromine, or ozone. These treatments result in an increase in membrane selectivity with a decrease in flux. Cross-linking of polymers is often applied to improve the chemical stability and selectivity of membranes for reverse osmosis, pervaporation, and gas-separation applications (41). Mosqueda-Jimenez and co-workers studied the addition of surface modifying macromolecules, and the use of the additive... 

During the last decade there has been intensified activity in research and development of ceramic membranes for gas separation applications. In several studies it is said that the market for these membranes will expand very rapidly in the near future [1-3], This market growth will be due to advantages such as high permeation and membrane stability as compared with other membrane separation technologies. 

---