Permeability/permeation

Plastic film Film thickness Permeation Permeability... 

Gas Permeation. Permeability of oxygen and nitrogen was evaluated for supplied air by a gas chromatographic method (Yanaco GTR-20). 

A comparison of moisture vapor permeation through various polymers can be seen in Table 3.5. Notice that PCTFE is only second to FEP and both are among the most resistant plastics to water vapor permeation. Permeability data can be found in Appendixes I through IV. 

Permeability coefficient Permeation Permenorm 5000 H2 Permenorm 3601 K2 Permenorm Z PermeOx Permethrin... 

As an excellent barrier resin, PTEE is widely used in the chemical industry. However, it is a poor barrier for fluorocarbon oils because similarity in the chemical composition of a barrier and a permeant increases permeation. Most Hquids and gases (other than fluorocarbons) do not permeate highly crystalline PTFE. Permeabilities at 30deg C (in mol/(m-s-Pa) X 10 ) are as follows CO2, 0.93 N2, 0.18 He, 2.47 anhydrous HCl, <0.01 (89). 

Membranes made by interfacial polymerization have a dense, highly cross-linked interfacial polymer layer formed on the surface of the support membrane at the interface of the two solutions. A less cross-linked, more permeable hydrogel layer forms under this surface layer and fills the pores of the support membrane. Because the dense cross-linked polymer layer can only form at the interface, it is extremely thin, on the order of 0.1 p.m or less, and the permeation flux is high. Because the polymer is highly cross-linked, its selectivity is also high. The first reverse osmosis membranes made this way were 5—10 times less salt-permeable than the best membranes with comparable water fluxes made by other techniques. 

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). 

Membrane-retained components are collectively called concentrate or retentate. Materials permeating the membrane are called filtrate, ultrafiltrate, or permeate. It is the objective of ultrafiltration to recover or concentrate particular species in the retentate (eg, latex concentration, pigment recovery, protein recovery from cheese and casein wheys, and concentration of proteins for biopharmaceuticals) or to produce a purified permeate (eg, sewage treatment, production of sterile water or antibiotics, etc). Diafiltration is a specific ultrafiltration process in which the retentate is further purified or the permeable sohds are extracted further by the addition of water or, in the case of proteins, buffer to the retentate. 

Diafiltration is an ultrafiltration process where water or an aqueous buffer is added to the concentrate and permeate is removed (50). The two steps may be sequential or simultaneous. Diafiltration improves the degree of separation between retained and permeable species. 

Permeation in the vinyUdene chloride copolymer and the polyolefins is not affected by humidity the permeability and diffusion coefficient in the ethylene—vinyl alcohol copolymer can be as much as 1000 times greater with high humidity (14—17). 

The permeability varies with temperature according to equation 12 where is a constant, E is the activation energy for permeation, E. is the gas constant, and Tis the absolute temperature. 

Reverse Osmosis. A reverse osmosis (RO) process has been developed to remove alcohol from distilled spirits without affecting the sensory properties (14). It consists of passing barrel-strength whiskey through a permeable membrane at high pressure, causing the alcohol to permeate the membrane and concentrating the flavor components in the retentate. 

Foamed or Cellular CeUular plastics such as polyurethane and polystyrene do not hold up or perform well in the ciyogenic temperature range because of permeation of the cell strnc tnre by water vapor, which in turn increases the heat-transfer rate. CeUular glass holds up better and is less permeable. 

The more permeable component is called the. st ga.s, so it is the one enriched in the permeate stream. Permeability through polymers is the product of solubihty and diffusivity. The diffusivity of a gas in a membrane is inversely proportional to its kinetic diameter, a value determined from zeolite cage exclusion data (see Table 22-23 after Breck, Zeolite Molecular Sieves, Wiley, NY, 1974, p. 636). 

Plasticization and Other Time Effects Most data from the literature, including those presented above are taken from experiments where one gas at a time is tested, with Ot calculated as a ratio of the two permeabihties. If either gas permeates because of a high-sorption coefficient rather than a high diffusivity, there may be an increase in the permeabihty of all gases in contact with the membrane. Thus, the Ot actually found in a real separation may be much lower than that calculated by the simple ratio of permeabilities. The data in the hterature do not rehably include the plasticization effect. If present, it results in the sometimes slow relaxation of polymer structure giving a rise in permeabihty and a dramatic dechne in selectivity. 

An important characteristic of pervaporation that distinguishes it from distillation is that it is a rate process, not an equilibrium process. The more permeable component may be the less-volatile component. Perv oration has its greatest iitihty in the resolution of azeotropes, as an acqiinct to distillation. Selecting a membrane permeable to the minor corTiponent is important, since the membrane area required is roughly proportional to the mass of permeate. Thus pervaporation devices for the purification of the ethanol-water azeotrope (95 percent ethanol) are always based on a hydrophihc membrane. 

Membrane Pervaporation Since 1987, membrane pei vapora-tion has become widely accepted in the CPI as an effective means of separation and recovery of liquid-phase process streams. It is most commonly used to dehydrate hquid hydrocarbons to yield a high-purity ethanol, isopropanol, and ethylene glycol product. The method basically consists of a selec tively-permeable membrane layer separating a liquid feed stream and a gas phase permeate stream as shown in Fig. 25-19. The permeation rate and selectivity is governed bv the physicochemical composition of the membrane. Pei vaporation differs From reverse osmosis systems in that the permeate rate is not a function of osmotic pressure, since the permeate is maintained at saturation pressure (Ref. 24). 

All organic coatings show varying degrees of solubility and permeability for components of the corrosive medium, which can be described as permeation and ionic conductivity (see Sections 5.2.1 and 5.2.2). An absolute separation of protected object and medium is not possible because of these properties. Certain requirements have to be met for corrosion protection, which must also take account of electrochemical factors [1] (see Section 5.2). 

Of particular interest in the usage of polymers is the permeability of a gas, vapour or liquid through a film. Permeation is a three-part process and involves solution of small molecules in polymer, migration or diffusion through the polymer according to the concentration gradient, and emergence of the small particle at the outer surface. Hence permeability is the product of solubility and diffusion and it is possible to write, where the solubility obeys Henry s law,... 

Low air and gas permeability. The amorphous and highly saturated nature of the polymer chain prevents permeation of air and gases. 

Permeability. The low density of plastics is an advantage in many situations but the relatively loose packing of the molecules means that gases and liquids can permeate through the plastic. This can be important in many applications such as packaging or fuel tanks. It is not possible to generalise about the performance of plastics relative to each other or in respect to the performance of a specific plastic in contact with different liquids and gases. 

The main reason for producing multi-layer co-extruded films is to get materials with better barrier properties - particularly in regard to gas permeation. The following Table shows the effects which can be achieved. Data on permeability of plastics are also given in Figs 1.13 and 1.14. 

Permeation rates are dependent on the ehemieal makeup of the eontamination. This ineludes the size of the eontaminant (how large or small the moleeule or partiele is) and on the pore size of the proteetive material (for instanee, impermeable rubber suits, tyveks, or eotton eoveralls). Chemieal eharaeteristies (i.e., polarity, vapor pressure, pH) of both the eontaminant and the proteetive material also determine permeability. Keep in mind that gases, vapors, and low-viseosity liquids tend to permeate more readily than high-viseosity liquids or solids [2],... 

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