Process membrane gas separations

Innovative process designs are being developed to reduce the size of the membrane unit and the energy needed to separate, condense and inject the carbon dioxide. It seems possible to reduce the energy consumption of the membrane process to about 20-25% of the power plant output. If this work is successful and these membrane plants are built, this application will dwarf all other gas-separation membrane processes. 

An example of a multistage gas separation membrane process is given in Figure 7.1 for natural gas treating [Spillman, 1989]. It is generally accepted that for a gas separation membrane to be cost effective, a separation factor of at least 5-10 (sometimes even higher) will be required. 

Figure 7.1 A multistage gas separation membrane process for natural gas treating [Spillman, 1989]...

R. Spillman, Economics of gas separation membrane processes, in R.D. Noble and S.A. Stern, (Eds.), Membrane Separation Technology. Elsevier, Amsterdam, 1995, Chap. 13, pp. 589-667. 

Figure VIII - 24. Three-stage gas separation membrane"process [7].

Figure 10.19 Single-stage to multistage gas separation membrane processes for treating natural gas, (Reproduced from [317] with permission.)...

L. Zhao, A parametric study of CO2/N2 gas separation membrane processes for post-combustion capture, J. Membr. Sci., 2008,325,284-294. 

Process Description Gas-separation membranes separate gases from other gases. Some gas filters, which remove hquids or sohds from gases, are microfiltration membranes. Gas membranes generally work because individual gases differ in their solubility and diffusivity through nonporous polymers. A few membranes operate by sieving, Knudsen flow, or chemical complexation. 

Vapor Feed A variant on pei vaporation is to use vapor, rather than liquid, as a feed. While the resulting process could be classified along with gas-separation membranes, it is customarily regarded as pei vaporation. 

In gas separation with membranes, a gas mixture at an elevated pressure is passed across the surface of a membrane that is selectively permeable to one component of the mixture. The basic process is illustrated in Figure 16.4. Major current applications of gas separation membranes include the separation of hydrogen from nitrogen, argon and methane in ammonia plants the production of nitrogen from ah and the separation of carbon dioxide from methane in natural gas operations. Membrane gas separation is an area of considerable research interest and the number of applications is expanding rapidly. 

Chlorine Processing Beyond the Millennium - the Use of Gas-separation Membranes... 

The great advantage of the absorption process over the synthesis of a by-product was its direct recovery of chlorine. Such a process or one that uses chlorine in another on-site process with steady demand is the ideal. More vigorous liquefaction is one approach to reducing the amount of chlorine value to be disposed of, and it has usually been chosen as the substitute for absorption. In this chapter, we discuss the use of gas-separation membranes as an alternative. 

Reverse osmosis, pervaporation and polymeric gas separation membranes have a dense polymer layer with no visible pores, in which the separation occurs. These membranes show different transport rates for molecules as small as 2-5 A in diameter. The fluxes of permeants through these membranes are also much lower than through the microporous membranes. Transport is best described by the solution-diffusion model. The spaces between the polymer chains in these membranes are less than 5 A in diameter and so are within the normal range of thermal motion of the polymer chains that make up the membrane matrix. Molecules permeate the membrane through free volume elements between the polymer chains that are transient on the timescale of the diffusion processes occurring. 

W.J. Koros and I. Pinnau, Membrane Formation for Gas Separation Processes, in Polymeric Gas Separation Membranes, D.R. Paul and Y.P. Yampol skii (eds), CRC Press, Boca Raton, FL, pp. 209-272 (1994). 

Most gas separation processes require that the selective membrane layer be extremely thin to achieve economical fluxes. Typical membrane thicknesses are less than 0.5 xm and often less than 0.1 xm. Early gas separation membranes [22] were adapted from the cellulose acetate membranes produced for reverse osmosis by the Loeb-Sourirajan phase separation process. These membranes are produced by precipitation in water the water must be removed before the membranes can be used to separate gases. However, the capillary forces generated as the liquid evaporates cause collapse of the finely microporous substrate of the cellulose acetate membrane, destroying its usefulness. This problem has been overcome by a solvent exchange process in which the water is first exchanged for an alcohol, then for hexane. The surface tension forces generated as liquid hexane is evaporated are much reduced, and a dry membrane is produced. Membranes produced by this method have been widely used by Grace (now GMS, a division of Kvaemer) and Separex (now a division of UOP) to separate carbon dioxide from methane in natural gas. 

Kaldis, S.P., Skodras, G. and Sakellaropoulos, G.P. (2004) Energy and capital cost analysis of C02 capture in coal IGCC processes via gas separation membranes. Fuel Processing Technology,... 

FIGURE 10 Current asymmetric hollow-fiber formation process for gas separation membranes. 

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