Polymer membrane separation

Thiele, W. and Foerster, H.-J. (2006) Progress in electrochemical ozone generation and disinfection of ultra-pure water using new electrochemical cell with polymer membrane separators (in German). Proceedings of the Annual GDCh Meeting, Bayreuth 2006. 

Donnan equilibrium is a well-understood phenomenon which is observed at any interface that prevents diffusion of at least one (but not all) charged species between two phases a thin polymer membrane separating two liquid mixtures provides one example. Donnan behavior also occurs at other interfaces (9) and is especially important in ion exchange resins (10). 

In a membrane cell, a polymer membrane separates the anode and cathode compartments. This allows the passage of Na" " ions but blocks the CP and OH ions (Figure 9.24). The concentration of Na" (aq) and OH (aq) gradually rises in the cathode region and is continuously removed. 

The size of the permeant molecule, as well as the chemical affinity between the permeant and the polymer, is an important determinant of permeability. Polymers can act as molecular sieves, allowing some molecules to pass through rapidly while retarding the passage of others. This is the principle used industrially in polymer membrane separation of gas blends. 

Pressure or concentration difference serves as the driving force for isothermal diffusion through polymer films. Molecules at higher concentration sorb into the solid polymer and can move through the matrix of polymer chains with subsequent desorption from the film surface of different concentration. If the polymer membrane separating two solutions is permeable, the diffusion of molecules from the high concentration side to the low concentration side is called osmosis or dialysis. 

The S-Brane process for the desulfurization of naphtha fractions was developed for application in refineries. A polymer membrane separates a permeate enriched with sulfur compounds, which is added to the conventional hydrodesulfurization process. The low-sulfur retentate can be further processed with no additional desulfurization [85]. 

Microporous polymer membranes are the most commonly used separators in lithium-ion batteries. Majority of the microporous polymer membrane separators are based on semicrystalline polyolefin materials, such as polyethylene (PE), polypropylene (PP), PE-PP blends and high-density polyethylene (HDPE)-ultrahigh molecular polyethylene (UHMWPE). 

ILs— tetraethyl and tetrabutyl ammonium nitrate— were used as additives for protein refolding. Pure liquid tetra-alkyl ammonium nitrates were utilized to denature the protein. EAN is a colorless to slightly yellow-colored IL having no characteristic odor and works as an amphoteric solvent. EAN is a liquid electrolyte at room temperamre and involves dissociable protons thus, it is also called as protic IL [79-82], which can be used as medium electrolytes for fuel cells [83] and polymer membrane separators [84]. The properties and apphcations of EAN were recently reviewed in the literature [85], EAN is miscible with water to form mixtures at aity composition, and both the component ions favorably form hydrogen bonds with water [86]. 

First MMMs were studied in the 1980s and are more and more investigated in recent years. MMMs receive attention as a possibility to enhance the properties of pure polymer membranes. Separation properties with MMMs can be above the Robeson upper bond (Figure AX which is a plot of permeability versus selectivity for most industrial relevant gas mixtures. Porous inorganic fillers can counteract the trade-off between selectivity and permeability, which is typical for pure polymer membranes. 

Keywords Siloxane polymers, membrane separation, composite membranes, gas and vapors transport... 

Although microporous membranes are a topic of research interest, all current commercial gas separations are based on the fourth type of mechanism shown in Figure 36, namely diffusion through dense polymer films. Gas transport through dense polymer membranes is governed by equation 8 where is the flux of component /,andare the partial pressure of the component i on either side of the membrane, /is the membrane thickness, and is a constant called the membrane permeability, which is a measure of the membrane s ability to permeate gas. The ability of a membrane to separate two gases, i and is the ratio of their permeabilities,a, called the membrane selectivity (eq. 9). 

Reverse osmosis membrane separations are governed by the properties of the membrane used in the process. These properties depend on the chemical nature of the membrane material, which is almost always a polymer, as well as its physical stmcture. Properties for the ideal RO membrane include low cost, resistance to chemical and microbial attack, mechanical and stmctural stabiHty over long operating periods and wide temperature ranges, and the desired separation characteristics for each particular system. However, few membranes satisfy all these criteria and so compromises must be made to select the best RO membrane available for each appHcation. Excellent discussions of RO membrane materials, preparation methods, and stmctures are available (8,13,16-21). 

Most commercially available RO membranes fall into one of two categories asymmetric membranes containing one polymer, or thin-fHm composite membranes consisting of two or more polymer layers. Asymmetric RO membranes have a thin ( 100 nm) permselective skin layer supported on a more porous sublayer of the same polymer. The dense skin layer determines the fluxes and selectivities of these membranes whereas the porous sublayer serves only as a mechanical support for the skin layer and has Httle effect on the membrane separation properties. Asymmetric membranes are most commonly formed by a phase inversion (polymer precipitation) process (16). In this process, a polymer solution is precipitated into a polymer-rich soHd phase that forms the membrane and a polymer-poor Hquid phase that forms the membrane pores or void spaces. 

Factors affecting RO membrane separations and water flux include feed variables such as solute concentration, temperature, pH, and pretreatment requirements membrane variables such as polymer type, module geometry, and module arrangement and process variables such as feed flow rate, operating time and pressure, and water recovery. 

Fig. 1. Formation of an ultrafUtration membrane A, unprecipitated polymer solution B, polymer solution separating into two phases C, pore fingers with...

Practical appHcations have been reported for PVP/ceUulosics (108,119,120) and PVP/polysulfones (121,122) in membrane separation technology, eg, in the manufacture of dialysis membranes. Electrically conductive polymers of polyaruline are rendered more soluble and hence easier to process by complexation with PVP (123). Addition of small amounts of PVP to nylon 66 and 610 causes significant morphological changes, resulting in fewer but more regular spherulites (124). 

Polymer Membranes These are used in filtration applications for fine-particle separations such as microfiltration and ultrafiltration (clarification involving the removal of l- Im and smaller particles). The membranes are made from a variety of materials, the commonest being cellulose acetates and polyamides. Membrane filtration, discussed in Sec. 22, has been well covered by Porter (in Schweitzer, op. cit., sec. 2.1). 

Process Description Gas-separation membranes separate gases from other gases. Some gas filters, which remove hquids or sohds from gases, are microfiltration membranes. Gas membranes generally work because individual gases differ in their solubility and diffusivity through nonporous polymers. A few membranes operate by sieving, Knudsen flow, or chemical complexation. 

The statistical properties of polymer chains in a quenched random medium have been the subject of intensive investigations during the last decades, both theoretically [79-89] and experimentally [90-96], because diffusion in such media is of great relevance for chromatography, membrane separation, ultrafiltration, etc. 

As the main disadvantage of liquid membrane systems is the instability over a longer period of time, another approach would be to perform separation through a solid membrane [22]. Enantioselective polymer membranes typically consist of a nonse-lective porous support coated with a thin layer of an enantioselective polymer. This... 

In a simple version of a fuel cell, a fuel such as hydrogen gas is passed over a platinum electrode, oxygen is passed over the other, similar electrode, and the electrolyte is aqueous potassium hydroxide. A porous membrane separates the two electrode compartments. Many varieties of fuel cells are possible, and in some the electrolyte is a solid polymer membrane or a ceramic (see Section 14.22). Three of the most promising fuel cells are the alkali fuel cell, the phosphoric acid fuel cell, and the methanol fuel cell. 

PSB electrolytes are brought close together in the battery cells where they are separated by a polymer membrane that only allows Na ions to go through, producing about 1.5 V across the membrane. Cells are electrically connected in series and parallel to obtain the desired voltage and current levels. The net efficiency of this battery working at room temperature is about 75%. It has been verified in the laboratory and demonstrated at multi-kW scale in the UK [92]. 

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