Applications of Gas Separating Membranes

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. 

The largest current application of gas-separation membranes is separation of nitrogen (N2) from air. Capillary modules formed into bore-side feed modules are used almost exclusively in this application [10, 11]. The feed air is compressed to 6-10 bar and pumped through the membrane capillaries. Oxygen (02) permeates the membrane preferentially, leaving an oxygen-depleted, nitrogen-rich residue stream. The first membranes used for this application were based on poly(4-methyl-l-pentene) and ethyl cellulose, and had 02/N2 selectivities of about 4. Because of the modest... 

The preceding discussion of gas separation membrane types illustrates the large number of options available. A correspondingly large number of potential opportunities for gas separation membranes exist, but economics ultimately must dictate which membrane approach, if any, should be used in each application. Moreover, the key requirements of durability, productivity, and separation efficiency must be balanced against cost in all cases. The current spectrum of applications of gas separation membranes... 

APPLICATIONS. The major applications of gas-separation membranes are to make products that are enriched in one or more components but are not of very high purity. Products of equal or greater purity can usually be obtained by liquefaction and distillation at low temperature, but the membrane processes have the advantage of operation at or near room temperature. 

Another industrial application of gas-separation membranes is the removal of carbon dioxide from natural gas. The CO2/CH4, selectivity is about 20 to 30 for polycarbonate, polysulfone, and cellulose acetate membranes at 35°C and 40 atm. A selectivity of over 60 can be obtained with Kapton , but this polymer is much less permeable than the others. Increasing the temperature raises the permeability of most polymers but generally causes a. slight decrease in selectivity. The operating temperature is chosen to be somewhat above the dew point of the residue gas. There is considerable COj absorbed in the membranes at high CO2 partial pressures, and the plasticization effect of CO2 increases the effective diffusion coefficients for all gases and makes the selectivity less than that based on pure-gas data. Methods of allowing for such nonlinear effects have been presented. ... 

The previous chapter outlined the phenomena and theory associated with gas-separation membranes. The fundamentals of mass transfer and the process design equations that model membranes were also addressed. In this chapter, our attention turns to the industrial application of gas-separation membranes, specifically separations with polymeric membranes. 

Each successful application of gas-separation membranes is the result of a whole series of successfiil technical and commercial activities. Steps included in this series include the following ... 

In gas separation two completely different types of membranes can be used in this process (alchou in different regimes of application) a dense membrane where transport takes place via diffusion, and a porous membrane where Knudsen flow occurs. A commercial application of gas separation membranes occurs in hydrogen recovery, the separation of air (oxygen/nitrogen) and of methane and carbon dioxide provide other examples. Pervaporation and vapour permeation make use of a dense separating layer. 

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