Membrane separation 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. 

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. 

During the 1970s, considerable research and developmental work was devoted to membranes. Many potential applications were identified, but commercialization was slow. In 1977, Monsanto demonstrated its first full scale membrane separator at Texas City, Texas, in a hydrogen/carbon monoxide ratio adjustment application (Burmaster and Carter, 1983). In 1979, Monsanto commercialized its hollow fiber membrane module as the Prism separator. From 1979 to 1982 Prism separators were evaluated in several refinery hydrogen purification applications (Bollinger et al., 1982). The success of these pilot tests established the commercial viability of gas separation with membranes. The first large scale commercial CO2 membrane separation project was the installation of two membrane separation facilities at the Sacroc tertiary oil recovery project in West Texas in 1983. Up to 80 MMscfd of gas has been processed in these facilities (Parro, 1984). 

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. 

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... 

Thus, the maximnm value of the selectivity is determined by the ratio of the flow of smaller molecules through dense polymer material and through the pores to the flow of larger molecules to the pores only. A further increase of the specific energy input of the plasma treatment y > Ym) leads to a slow decrease of the membrane selectivity due to a slow decrease of polymer material permeability with respect to smaller molectrles. The initial porosity of the polymer membranes essentially limits the maximtrm value of their selectivity to gas separation. When the lestosil membrane is especially prepared without pores, its selectivity with respect to He-CILt separation exceeds 1000 after microwave plasma treatment (Sections 9.9.2-9.9.S Arbatsky et al., 1988,1990). 

The mixed-conducting membrane eliminates the cryogenic air separation plant and it forms a safety barrier between the natural gas and air. The membrane becomes more productive in a configuration like this, because the slow oxygen desorption at the permeate side is enhanced by the chemical reaction. 

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