Permeation rates

Barnes and co-workers have studied mixed-monolayer systems [278,281,283,284] and found some striking nonidealities. Mixed films of octadecanol and cholesterol, for example, show little evaporation resistance if only 10% cholesterol is present [278] apparently due to an uneven granular microstructure in films with cholesterol [284]. Another study of cellulose decanoate films showed no correlation between holes in the monolayer and permeation rate [285]. Polymerized surfactants make relatively poor water evaporation retarders when compared to octadecanol [286]. There are problems in obtaining reproducible values for r [287] due to impurities in the monolayer material or in the spreading solvent. 

Acrylonitrile—methyl acrylate—iadene terpolymers, by themselves, or ia blends with acrylonitrile—methyl acrylate copolymers, exhibit even lower oxygen and water permeation rates than the iadene-free copolymers (110,111). Terpolymers of acrylonitrile with iadene and isobutjlenealso exhibit excellent barrier properties (112), and permeation of gas and water vapor through acrylonitrile—styrene—isobutjleneterpolymers is also low (113,114). 

The permeabUity of ceUular polymers to gases and vapors depends on the fraction of open ceUs as weU as the polymer-phase composition and state. The presence of open ceUs in a foam allows gases and vapors to permeate the ceU stmcture by diffusion and convection dow, yielding very large permeation rates. In closed-ceUed foams the permeation of gases or vapors is governed by composition of the polymer phase, gas composition, density, and ceUular stmcture of the foam (194,199,215,218,219). 

Poly(vinyl chloride). To be converted into film, poly(viayl chloride) [9002-86-22] (PVC) must be modified with heat stabilizers and plasticizers, which increase costs. Plasticized PVC film is highly transparent and soft, with a very high gas-permeation rate. Water-vapor transmission rate is relatively low. At present, PVC film is produced by blown-film extmsion, although casting and calendering are employed for heavier gauges (see Vinyl POLYAffiRS). 

Corrosion attack on the polymer is influenced by permeation rate, as weU as internal stresses or fatigue, that distorts or fractures the resin glass fiber... 

Ethanol removed by the vapor stream can be recovered by condensation, vapor recompression, or scmbbiag. Ia the first two methods, the coaceatratioa of the recovered ethanol depends on the relative humidity of the sweep stream and the ratio of sweep and permeation rates. In scmbbiag, the rate of water deflvery to the Hquid-gas coatactor affects the ethanol concentration ia the recovered stream. 

Electrically assisted transdermal dmg deflvery, ie, electrotransport or iontophoresis, involves the three key transport processes of passive diffusion, electromigration, and electro osmosis. In passive diffusion, which plays a relatively small role in the transport of ionic compounds, the permeation rate of a compound is deterrnined by its diffusion coefficient and the concentration gradient. Electromigration is the transport of electrically charged ions in an electrical field, that is, the movement of anions and cations toward the anode and cathode, respectively. Electro osmosis is the volume flow of solvent through an electrically charged membrane or tissue in the presence of an appHed electrical field. As the solvent moves, it carries dissolved solutes. 

Euture electrotransport therapeutic systems will differ substantially from those just described. They will draw on advances in microelectronics and transdermal system technology to provide transdermal therapy for compounds with low passive permeation rates, patterned or pulsed dmg deHvery,... 

Permeation rate Standard cubic feet/miniite kmol/s... 

Equation (22-106) gives a permeate concentration as a function of the feed concentration at a stage cut, 0 = 0, To calculate permeate composition as a function of 0, the equation may be used iteratively if the permeate is unmixed, such as would apply in a test cell. The calculation for real devices must take into account the fact that the driving force is variable due to changes on both sides of the membrane, as partial pressure is a point function, nowhere constant. Using the same caveat, permeation rates may be calciilated component by component using Eq. (22-98) and permeance values. For any real device, both concentration and permeation require iterative calculations dependent on module geometiy. 

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). 

The vapor vent valves are connected to the tank vapor control valve, and ultimately to the carbon canister by tubing that is resistant to swelling in the presence of fuel vapors. The tubing material must also have a low HC permeation rate, so that the evaporative emissions are not increased due to release of HC molecules. The tank vapor control valve connects the carbon canister to two fuel tank vapor sources the vapor vent valve lines and a refueling vent tube. 

Temperature inereases generally inerease the eontaminant permeation rate [2], However, temperatures between 40° and 90° F usually do not have a signifieant effeet. 

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],... 

Many fluorine containing polymers are being evaluated for possible use as permeable selective membranes. The fluorine containing polymers have been shown to increase permeation rates without decreasing in the selec-... 

It is not possible at present to provide an equation, or set of equations, that allows the prediction from fu st principles of the membrane permeation rate and solute rejection for a given real separation. Research attempting such prediction for model systems is underway, but the physical properties of real systems, both the membrane and the solute, are too complex for such analysis. An analogous situation exists for conventional filtration processes. The general... 

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