Membranes slow gases

Carbon Dioxide-Methane Much of the natural gas produced in the world is coproduced with an acid gas, most commonly CO9 and/or H9S. While there are many successful processes for separating the gases, membrane separation is a commercially successfufcompetitor, especially for small instaUations. The economics work best for feeds with very high or veiy low CH4 content. Methane is a slow gas CO9, H9S, and H9O are fast gases. 

Product Losses Account must be taken of the product loss, the slow gas in the permeate (such as CH4), or the fast gas in the residue (such as H2). Figure 22-79 illustrates the issue for a membrane used to purify natural gas from 93 percent to pipeline quality, 98 percent. In the upper figure, the gas is run through a permeator bank operating as a single stage. For the membrane and module chosen, the permeate contains 63 percent CH4. By dividing the same membrane area into... 

Palladium membranes have been prepared by electroless deposition on porous stainless steel substrates.60 The performance of these membranes as hydrogen diffusion electrodes was evaluated using a three-electrode cell in alkaline solution. The technical feasibility of these membranes as gas diffusion electrodes has been proven however, the activity of hydrogen oxidation was high at the beginning and significantly decreased with time. This behavior was attributed to the slow entry of hydrogen at the H2/Pd interface. 

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... 

Figure 8. Design parameters for single-compartment parallel plate reactor (membrane chlor-alkali cell) with slow gas evolution in two dimensions.

Gas separation by membranes will always have to compete with other separation processes such as cryogenics, absorption and adsorption systems. Membranes usually are less competitive in very large scale operations where the fast gas is less than about 20% of the feed gas, unless the slow gas is the desired product. Membranes also are not usually the method of choice when extremely pure product gas is required. Membranes do, however, have distinct advantages in small to medium scale operations, in situations where gas is available at pressure, in situations where high recovery is paramount, and in applications where simplicity and minimal maintenance are of prime importance (such as in remote locations). Membranes are very well suited for applications in which the non-permeate is the product of interest, since it is obtained at pressure. Examples are acid gas removal from natural gas and gas dehydration. 

Decreasing the feed gas flow rate below a critical value decreases the separation efficiency due to a boundary layer effect. The concentration of the fast gas is depleted in the feed gas adjacent to the membrane surface, reducing its partial pressure and therefore its rate of permeation. Since the rate of permeation of the slow gas is not affected (or is increased) this reduces permeate purity. The critical flow rate is determined by the degree of mixing at the membrane surface, and this is a function of gas velocity gas properties such as viscosity, density, and diffusivity and module design. 

Plasticization and Other Time Effects Most data from the literature, including those presented above are taken from experiments where one gas at a time is tested, with Ot calculated as a ratio of the two permeabihties. If either gas permeates because of a high-sorption coefficient rather than a high diffusivity, there may be an increase in the permeabihty of all gases in contact with the membrane. Thus, the Ot actually found in a real separation may be much lower than that calculated by the simple ratio of permeabilities. The data in the hterature do not rehably include the plasticization effect. If present, it results in the sometimes slow relaxation of polymer structure giving a rise in permeabihty and a dramatic dechne in selectivity. 

Some bead materials possess porous structure and, therefore, have very high surface to volume ratio. The examples include silica-gel, controlled pore glass, and zeolite beads. These inorganic materials are made use of to design gas sensors. Indicators are usually adsorbed on the surface and the beads are then dispersed in a permeation-selective membrane (usually silicone rubbers). Such sensors possess high sensitivity to oxygen and a fast response in the gas phase but can be rather slow in the aqueous phase since the gas contained in the pores needs to be exchanged. Porous polymeric materials are rarer and have not been used so far in optical nanosensors. 

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