Membrane separation fast gases

Hydrogen Hydrogen recovery was the first large commercial membrane gas separation. Polysulfone fiber membranes became available in 1980 at a time when H9 needs were rising, and these novel membranes qiiickly came to dominate the market. Applications include recovery of H9 from ammonia purge gas, and extraction of H9 from petroleum crackiug streams. Hydrogen once diverted to low-quahty fuel use is now recovered to become ammonia, or is used to desulfurize fuel, etc. H9 is the fast gas. 

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. 

State of the Art A desirable gas membrane has high separating power (ot) and high permeability to the fast gas, in addition to critical requirements discussed below. The search for an ideal membrane produced copious data on many polymers, neatly summarized by Robeson [J. Membrane ScL, 62, 165 (1991)]. Plotting log permeability versus log selectivity (ot), an upper bound is found (see Fig. 22-73) which all the many hundreds of data points fit. The data were taken between 20-50°C, generally at 25 or 35°C. 

Partial Pressure Pinch An example of the hmitations of the partial pressure pinch is the dehumidification of air by membrane. While O9 is the fast gas in air separation, in this apphcation H9O is faster still. Special dehydration membranes exhibit a = 20,000. As gas passes down the membrane, the pai-dal pressure of H9O drops rapidly in the feed. Since the H9O in the permeate is diluted only by the O9 and N9 permeating simultaneously, p oo rises rapidly in the permeate. Soon there is no driving force. The commercial solution is to take some of the diy air product and introduce it into the permeate side as a countercurrent sweep gas, to dilute the permeate and lower the H9O partial pressure. It is in effect the introduction of a leak into the membrane, but it is a controlled leak and it is introduced at the optimum position. 

The more permeable component is called the fast gas, so it is the one enriched in the permeate stream. Permeabihly through polymers is the product of solubility and diffusivity. The diffusivity of a gas in a membrane is inversely proportional to its kinetic diameter, a value determined from zeolite cage exclusion data (see Table 20-26 after Breck, Zeolite Molecular Sieves, Wiley New York, 1974, p. 636). Tables 20-27, 20-28, and 20-29 provide units conversion factors useful for calculations related to gas-separation membrane systems. 

The problem with use of polymeric membranes in this application is plasticization, leading to much lower selectivities with gas mixtures than the simple ratio of pure-gas permeabilities would suggest. For this type of separation, a Robeson plot based on the ratio of pure-gas permeabilities has no predictive value. Although membranes with pure-gas propylene/propane selectivities of 20 or more have been reported [43, 44], only a handful of membranes have been able to achieve selectivities of 5 to 10 under realistic operating conditions, and these membranes have low permeances of 10 gpu or less for the fast component (propylene). This may be one of the few gas-separation applications where ceramic or carbon membranes have an industrial future. 

Membrane separation is a relatively new and fast-growing field in supramolecular chemistry. It is not only an important process in biological systems, but becomes a large-scale industrial activity. For industrial applications, many synthetic membranes have been developed. Important conventional membrane technologies are microfiltration, ultrafiltration, electro- and hemodialysis, reverse osmosis, and gas separations. The main advantages are the high separation factors that can be achieved under mild conditions and the low energy requirements. 

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. 

Vu et al. [122] incorporated CMS materials into polymers to form MMM films for selective gas separations. The CMS, formed by pyrolysis of a PI precursor and exhibiting an intrinsic CO2/CH4 selectivity of 200, was dispersed into a polymer matrix. Pure-gas permeation tests of such MMMs revealed the CO2/CH4 selectivity was enhanced by as much as 40%-45% relative to that of the pure polymer. The effective permeabilities of fast-gas penetrants (e.g., O2 and CO2) through these MMMs are also improved relative to the intrinsic permeabilities of the unmodified polymer matrices. For a CO2/H2 gas mixture, the CO2 will be the fastest permeating component, and H2 will be retained on the feed side to avoid repressurization, in which case the polymer matrix dictates the minimum membrane performance. Properly selected molecular sieves can only improve membrane performance in the absence of defects. The polymer matrix must be chosen so that comparable permeation occurs in the two phases (to avoid starving the sieves) and so the permeating molecules are directed toward (not around) the dispersed sieve particulates. 

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