Permeation of organic vapors

P Meares. Transient permeation of organic vapors through polymer membranes. J Appl Polym Sci 9 917-932, 1965. 

Curry, J.G.,"Transient Permeation of Organic Vapors Through Elastomeric Membranes", Ph.D. Dissertation, University of Alabama, Tuscaloosa, 1972. 

Rogers, C. E., V. Stannett and M. Szwarc The sorption, diffusion, and permeation of organic vapors in polyethylene. J. Polymer Sci. 45, 61 (I960). 

Meares, P, Transient Permeation of Organic Vapors Through Polymers Membranes, J. Appl. Polym. Sci., 9, 917, 1965. 

Spiral-wound modules are much more commonly used in low pressure or vacuum gas separation appHcations, such as the production of oxygen-enriched air, or the separation of organic vapors from air. In these appHcations, the feed gas is at close to ambient pressure, and a vacuum is drawn on the permeate side of the membrane. Parasitic pressure drops on the permeate side of the membrane and the difficulty in making high performance hollow-fine fiber membranes from the mbbery polymers used to make these membranes both work against hollow-fine fiber modules for this appHcation. 

For numerous technical appHcations the pressure ratio does not exceed 10 or 20. An example is the separation of organic vapors, where a typical pressure ratio is about 10. Figure 7.12 shows a plot of permeate concentration versus selectivity at a pressure ratio of 10. 

The advantage of these mbbery membranes besides high flux and acceptable selectivities is the preferential permeability of organic vapors. The preferred permeation of the condensable organic vapors is desirable in order to avoid condensation on the membrane surface. 

Today vapor-permeation processes are widely used in the dehydration of organic solvents, or in the removal of methanol from other organic components, or in the removal of VOCs from gas streams. In the literature the term Vapor permeation is often related to the removal of organic vapors ( VOCs ) from air or gas streams only. In these applications the more-permeable component is brought close to saturation by cooling, compression, or both pretreatment steps. Thus there is no real reason for such a narrow definition and the means by which the vapor has been produced has no influence either on the nature of the membrane or the mechanism of the separation process. 

Of the two developing membrane processes Usted in Table 3, gas separation and pervaporation, gas separation is the more developed. At least 20 companies worldwide offer industrial membrane-based gas separation systems for a variety of applications. In gas separation, a mixed gas feed at an elevated pressure is passed across the surface of a membrane that is selectively permeable to one component of the feed. The membrane separation process produces a permeate enriched in the more permeable species and a residue enriched in the less permeable species. Important, well-developed applications are the separation of hydrogen from nitrogen, argon, and methane in ammonia plants the production of nitrogen from air the separation of carbon dioxide from methane in natimal gas operations and the separation and recovery of organic vapors from air streams. Gas separation is an area of considerable current research interest the munber of applications is expected to increase rapidly over the next few years. 

Pervaporation is a relatively new process with elements in common with reverse osmosis and gas separation. In pervaporation, a liquid mixture contacts one side of a membrane, and the permeate is removed as a vapor from the other. Currendy, the only industrial application of pervaporation is the dehydration of organic solvents, in particular, the dehydration of 90—95% ethanol solutions, a difficult separation problem because an ethanol—water azeotrope forms at 95% ethanol. However, pervaporation processes are also being developed for the removal of dissolved organics from water and the separation of organic solvent mixtures. These applications are likely to become commercial after the year 2000. 

A crystalline or semicrystalline state in polymers can be induced by thermal changes from a melt or from a glass, by strain, by organic vapors, or by Hquid solvents (40). Polymer crystallization can also be induced by compressed (or supercritical) gases, such as CO2 (41). The plasticization of a polymer by CO2 can increase the polymer segmental motions so that crystallization is kinetically possible. Because the amount of gas (or fluid) sorbed into the polymer is a dkect function of the pressure, the rate and extent of crystallization may be controUed by controlling the supercritical fluid pressure. As a result of this abiHty to induce crystallization, a history effect may be introduced into polymers. This can be an important consideration for polymer processing and gas permeation membranes. 

It should be recognized that all plastic materials over a time period allow a certain amount of water vapor, organic gas, or liquid to permeate the thickness of the material. It is only a matter of degree of permeation between various materials used as barriers against vapors and gases. It has been found that the permeability coefficient is a function of the solubility coefficient and diffusion coefficient. The process of permeation is explained as the solution of the vapor into the incoming surface of the barrier, followed by diffusion through the barrier thickness, and evaporation on the exit side. 

Membranes can also be used to alter the vapor-liquid equilibrium behavior and allow separation of azeotropes. The liquid mixture is fed to one side of the membrane, and the permeate is held under conditions to maintain it in the vapor phase. Most separations use hydrophyllic membranes that preferentially pass water rather than organic material. Thus, pervaporation is commonly used for the dehydration of organic components. 

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