Membranes for Organic Vapor Separation

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. 

The selectivities of various organic vapors over nitrogen of a rubbery thin-film composite membrane are shown in Fig. 1.1. 

The length of the bar shows the highest average selectivity obtained by single gas measurements at ambient temperatures. The selectivity that can be achieved in a technical process is dependent on the membrane structure, module configuration and process parameters. The temperature and the partial pressure of the organic compound have a direct impact on the membrane selectivity. 

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Lai ZP, Bonilla G, Diaz I, Nery JG, Sujaoti K, Amat MA, Kokkoli E, Terasaki O, Thompson RW, Tsapatsis M, and Vlachos DG. Microstructural optimization of a zeolite membrane for organic vapor separation. Science 2003 300 456-460. 

The recovery of organic vapors from waste gas streams using polymeric membranes is a well established process (7). Typically, composite membranes are used for this process. These membranes consist of a diin, selective rubbery layer coated onto a microporous support material. The selectivities of these membranes for organic vapors over nitrogen are typically about 10-100. Currently, commercial vapor separation membrane applications include small systems (10-100 scfin) to recover fluorinated hydrocarbons (Freons) and other high-value solvent vapors from process vent streams to large systems (100-1,000 scfin) for recovery of hydrocarbon vapors in the petrochemical industry (7). 

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. 

Yet another unique class of inorganic membrane materials called pillared clay and carbon composite membranes has been studied for gas separation [Zhu et al., 1994]. The permeation rates of benzene, chlorobenzene and 1,3-dichlorobenzene vapors through these membranes can be different by orders of magnitude as indicated earlier. This may open the door for these types of membranes for separating organic mixtures. 

FIGURE 9.3 Dependence of Henry s solubility coefhcient on Van der Waals volume of penetrant molecules for the systems of natural mbber/hydrocarbons. (From Semenova, S.I., Membranes in Russian), 13, 37, 2002 Baker, R.W. and Wijmans, J.G., Membrane separation of organic vapors from gas streams. In Paul D, Yampolskii Yu, Eds., Polymeric Gas Separation Membranes. CRC Press, 1994 353-397 Crank, J. and Park, G., Ed., Diffusion in Polymers. London, Academic Press, 1968.)... 

In these mixtures, the vapor or organic compound can either adsorb preferentially on the zeolite pores or undergo capillary condensation in pores of small diameter, therefore blocking the membrane for the other components in the mixture (i.e., permanent gas). The separation selectivity toward the blocking molecule decreases with temperature due to the decrease in adsorption and capillary condensation. 

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