Carbon molecular sieve membranes selectivity

Carbon molecular sieve membranes Resistant to contaminants Intermediate hydrogen flux and selectivity Intermediate hydrogen flux and selectivity High water permeability Pilot-scale testing in low temperature WGS membrane reactor application Need demonstration of long-term stability and durability in practical applications... 

Carbon molecular sieve membranes. Molecular sieve carbons can be produced by controlled pyrolysis of selected polymers as mentioned in 3.2.7 Pyrolysis. Carbon molecular sieves with a mean pore diameter from 025 to 1 nm are known to have high separation selectivities for molecules differing by as little as 0.02 nm in critical dimensions. Besides the separation properties, these amorphous materials with more or less regular pore structures may also provide catalytic properties. Carbon molecular sieve membranes in sheet and hollow fiber (with a fiber outer diameter of 5 pm to 1 mm) forms can be derived from cellulose and its derivatives, certain acrylics, peach-tar mesophase or certain thermosetting polymers such as phenolic resins and oxidized polyacrylonitrile by pyrolysis in an inert atmosphere [Koresh and Soffer, 1983 Soffer et al., 1987 Murphy, 1988]. 

Hagg MB, Lie JA, and Lindbrathen A. Carbon molecular sieve membranes—a promising alternative for selected industrial appUcations. In Li NN, Drioli E, Ho WSW, and Lipscomb GG, eds. Annals of the New York Academy of Sciences vol. 984 Advanced Membrane Technology. New York The New York Academy of Sciences, 2003, pp. 329-345. 

Carbon molecular sieve membranes are used in gas separation technology, for example, to recover CO2 and H2O from natural gas, and other purification steps. A variety of polymeric precursors for carbon molecular sieve membranes are available, such as poly(imide), poly(acrylonitrile) phenolic resins, and poly(fiuduryl alcohol). PPE can be modified in various ways, which procedure is advantageous for tailoring the selectivity. ... 

Since the pioneering paper of Koresh and Soffer on carbon molecular sieve membranes in 1983 [300] much research has been carried out in the field of carbon-based gas-separation membranes. Selectivities and permeabilities far above the performance of the best polymers have been obtained for carbon molecular sieve membranes by many researchers. One example is a recent publica-... 

The carbon-based membranes show superior permeability-selectivity combination to polymeric ones and are categorized in three classes carbon membranes, carbon molecular sieve membranes and carbon nanotubes. ... 

Another kind of membrane material is represented by a group of nanoporous, hydrogen-selective carbon molecular sieve membranes, which exhibit excellent permeation characteristics and hydrogen permeabilities competitive with metallic membranes (Harale et al., 2007). Furthermore, they are unaffected by CO or hydrogen sulfide (H2S) contamination. Nevertheless, their hydrothermal stability is not guaranteed at r > 623 K. 

Carbon molecular sieve membranes, which are currently developed (2003) on different porous carriers in various geometries [2.15, 2.16], are an interesting alternative to the hollow fibre membranes. They promise higher permeability with higher selectivity at the same time. 

Barsema JN, Van Der Vegt NFA, Koops GH, Wessling M (2002) Carbon molecular sieve membranes prepared from porous fiber precursor. J Membr Sci 205 (1-2) 239-246 Soffer A, Azariah A, Amar A, Cohen H, Golub D, Saguee S, Tobias H (1997) Method of improving the selectivity of carbon membranes by chemical vapor deposition. US Patent... 

Nguyen C, Do DD, Haraya K, Wang K (2003) The structural characterization of carbon molecular sieve membrane (CMSM) via gas adsorption. J Membr Sci 220 (1-2) 177-182 Nguyen C, Do DD (1999) Adsorption of supercritical gases in porous media Determination of micropore size distribution. J Phys Chem B 103 (33) 6900-6908 Katsaros FK, Steriotis TA, Ramanos GE, Konstantakou M, Stubos AK, Kanellopoulos NK (2007) Preparation and characterization of gas selective microporous carbon membranes. Microporous Mesoporous Mater 99 (1-2) 181-189... 

Despite all the advantages, polymeric membranes eannot overcome the polymer upper-bound limit between permeability and selectivity. On the other hand, some inorganie membranes sueh as zeolite and carbon molecular sieve membranes oIFct mueh higher permeability and selectivity than polymeric membranes but are expensive and difficult for large-scale manufacture. Therefore, it is highly desirable to provide an alternate eost-effeclive membrane in a position above the trade-off curves between permeability and seleelivity. 

As an example of the selective removal of products, Foley et al. [36] anticipated a selective formation of dimethylamine over a catalyst coated with a carbon molecular sieve layer. Nishiyama et al. [37] demonstrated the concept of the selective removal of products. A silica-alumina catalyst coated with a silicalite membrane was used for disproportionation and alkylation of toluene to produce p-xylene. The product fraction of p-xylene in xylene isomers (para-selectivity) for the silicalite-coated catalyst largely exceeded the equilibrium value of about 22%. 

Purification with PSA and Polymeric Membranes. The PSA process is based on the selective adsorption of gaseous compounds on a fixed bed of solid adsorbent in a series of identical adsorption beds. The adsorbent is an active carbon or a carbon-molecular sieve. Each bed undergoes a... 

Successful separation of alkanes and alkenes has been documented when microporous membranes have been used [79,138]. The physiochemical properties, size, and shape of the molecules will play an important role for the separation, hence critical temperatures and gas molecule configurations should be carefully evaluated for the gases in mixture. On the basis of gas properties and process conditions, the separation may be performed according to selective surface flow or molecular sieving (refer to Section 4.2 on transport). The transport may also be enhanced by having a Ag compound in the membrane. The Ag ion will form a reversible complex with the alkene, and facilitated transport results. Selectivities in the range of 200-300 have been reported for separation of ethene-ethane and propene-propane [138]. Successful separation of alkanes and alkenes will be important for the petrochemical industry. Today the surplus hydrocarbons in the purge gas are usually flared. Membranes which should be suitable for this application are the carbon molecular sieves (see Section 4.3.2) and nanostructured materials (Section 4.3.3). 

Ethylene has been separated from ethane by a silver nitrate solution passing countercurrent in a hollow fiber poly-sulfone.165 This separation has also been performed with the silver nitrate solution between two sheets of a polysilox-ane.166 A hydrated silver ion-exchanged Nafion film separated 1,5-hexadiene from 1-hexene with separation factors of 50-80.167 Polyethylene, graft-polymerized with acrylic acid, then converted to its silver salt, favored isobutylene over isobutane by a factor of 10. Olefins, such as ethylene, can be separated from paraffins by electroinduced facilitated transport using a Nafion membrane containing copper ions and platinum.168 A carbon molecular sieve made by pyrolysis of a polyimide, followed by enlargement of the pores with water at 400 C selected propylene over propane with an a-valve greater than 100 at 35°C.169... 

Recently, carbon molecular sieves have been fabricated in the form of planar membranes and hollow tubes by the pyrolysis of polyacrylonitrile in suitable forms (12-16). Very high separation selectivities have been reported with these materials. Their pore sizes are in the range from 3 to 5.2A. Selectivities of greater than 100 1 are observed between molecules which differ by as little as 0.2A in their critical dimensions. Kinetics of adsorption on these materials have been determined (2.,ii,l ) -... 

On carbon films prepared fi om commercially available poly(imide) films with a thickness of 0.1 mm, selective permeation of hydrogen gas was recently found [105]. This permselectivity may help to develop applications of these carbon films in fuel cells for vehicles, since CO in H2 gas supplied from an on-board reformer has to be removed through a molecular sieving membrane. 

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