Membranes for gas permeation

The Pore Radius and Tortuosity of a Porous Membrane for Gas Permeation... 

The main emphasis in this chapter is on the use of membranes for separations in liquid systems. As discussed by Koros and Chern(30) and Kesting and Fritzsche(31), gas mixtures may also be separated by membranes and both porous and non-porous membranes may be used. In the former case, Knudsen flow can result in separation, though the effect is relatively small. Much better separation is achieved with non-porous polymer membranes where the transport mechanism is based on sorption and diffusion. As for reverse osmosis and pervaporation, the transport equations for gas permeation through dense polymer membranes are based on Fick s Law, material transport being a function of the partial pressure difference across the membrane. 

Hollow fiber membranes are primarily homogeneous. In use, their lower permeability is compensated for by large surface per unit volume of vessel. Fibers are 25-250 pm outside dia, wall thickness 5-50 pm. The cross section of a vessel for reverse osmosis may have 20-35 million fibers/sqft and a surface of 5500-9000 sqft/cuft of vessel. Recently developed hollow fibers for gas permeation processes have anisotropic structures. 

The values of permeability coefficients for He, O2, N2, CO2, and CH4 in a variety of dense (isotropic) polymer membranes and the overall selectivities (ideal separation factors) of these membranes to the gas pairs He/N2,02/N2, and CO2/CH4 at 35°C have been tabulated in numerous reviews (Koros and Heliums, 1989 Koros, Fleming, and Jordan et al., 1988 Koros, Coleman, and Walker, 1992). Moreover, several useful predictive methods exist to allow estimation of gas permeation through polymers, based on their structural repeat units. The values of the permeability coefficients for a given gas in different polymers can vary by several orders of magnitude, depending on the nature of the gas. Thevalues oftheoverall selectivities vary by much less. Particularly noteworthy is the fact that the selectivity decreases with increasing permeability. This is the well-known inverse selectivity/permeability relationship of polymer membranes, which complicates the development of effective membranes for gas separations. 

Normally when one of the two performance indicators of a porous ceramic membrane for gas separation (i.e., separation factor and permeability) is high, the other is low. It is, therefore, necessary to m e a compromise that offers the most economic benefit Often it is desirable to slightly sacrifice the separation factor for a substantial increase in the permeation flux. This has been found to be feasible with a 5% doping of silica in an alumina membrane [GaBui et al., 1992]. 

During the last decade there has been intensified activity in research and development of ceramic membranes for gas separation applications. In several studies it is said that the market for these membranes will expand very rapidly in the near future [1-3], This market growth will be due to advantages such as high permeation and membrane stability as compared with other membrane separation technologies. 

Cellulosic Membranes. The first asymmetric membrane for gas separation appeared in 1970 (Table II), and It was not surprising that this membrane was a modified CA membrane of the Loeb-Sourirajan type (17). Gelled CA membranes for water desalination must be stored wet In order to maintain their permeation performance. However, In gas permeation, wet, plasticized membranes tend to lose their properties with time due to plastic creep of the soft material under pressure and due to slow drying during which the microporous sublayer may collapse and thus increase the thickness of the dense skin-layer. Gantzel and Merten (17) dried CA membranes with an acetyl-content of 39.4% by quick-freezing and vacuum sublimation at... 

Facilitated or carrier-mediated transport is a coupled transport process that combines a (chemical) coupling reaction with a diffusion process. The solute has first to react with the carrier to fonn a solute-carrier complex, which then diffuses through the membrane to finally release the solute at the permeate side. The overall process can be considered as a passive transport since the solute molecule is transported from a high to a low chemical potential. In the case of polymeric membranes the carrier can be chemically or physically bound to the solid matrix (Jixed carrier system), whereby the solute hops from one site to the other. Mobile carrier molecules have been incorporated in liquid membranes, which consist of a solid polymer matrix (support) and a liquid phase containing the carrier [2, 8], see Fig. 7.1. The state of the art of supported liquid membranes for gas separations will be discussed in detail in this chapter. 

More recently, Teramoto et al. [24-25] referred to the use of a novel facihtated transport membrane for gas separation in which a carrier was supphed to the feed side (high-pressure side) and it was forced to permeate through a membrane to the permeate side (low-pressure side), and then the permeated carrier solution was recirculated to the feed side. Since the membrane was always wet with the carrier solution, the membrane became very stable with no open or unfilled pores present which usually caused membrane unselectively in traditional SLM. This new type of membrane has been named a bulk flow liquid membrane (BFLM). The membrane resulted to be stable over a discontinuous one-month testing period. 

The permeabilities of oxygen through the Vycor were constant with the pressure difference and decreased with the square root of the temperature (figure not included). So the main mechanism for gas permeation is Knudsen diffusion. However, the permeabilities of oxygen through the V20s-coated catalytic membrane were deviated from Knudsen diffusion. As the partial oxidation proceeded, the... 

Single membrane units can be evaluated based on their geometry and operation conditions. Zolandz and Fleming [4] provide a good description for gas permeation systems and models for design purposes. Sender [5] discusses the use of cascades (or staging) for various series and/or parallel sets of membrane modules. 

G. R. Gavalas, A review of Zeolite Membrane Preparation and Permeation Properties, in Y. Yampolskii, 1. Pinnau, B.D. Freeman (Eds), Materials Science of Membranes for Gas and Vapor Separation, John Wiley Sons Ltd, Chichester, UK, in press... 

Asymmetric phase-inversion membranes like the membranes employed in reverse osmosis are difficult to prepare as gas permeation is much more sensitive to micropores than RO due to the much higher diffusion coefficients of gases. For the same reason, the composite membrane differs from RO composite membranes in gas permeation, the top layer of the asymmetric support structure is responsible for the separation while it is the sole duty of the coating to plug the micropores. Consequently, the material of the coating chosen (silicone) has a high permeability but a low selectivity while the membrane material (poly-sulfone) has a high selectivity (and a much lower permeability). 

Lee LL, Tsai DS. Synthesis and permeation properties of silicon-carbon based inorganic membranes for gas separation. Ind Eng Chem Res. 2001 40 612-6. 

Table 7.5 Mixed gas-permeation properties of PTMSP and PMP. Feed 2% butane in methane, feed pressure 10 bar, permeate pressure atmospheric, temperature 25 °C. From I. Pinnau et al. In Polymer Membranes for Gas and Vapor Separation, ACS Symposium Series 733 (1999), 56-67.

---