Carbon molecular sieve industrial applications

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). 

Hagg MB, Lie JA, Lindbrathen A. Carbon molecular sieve membranes—A promising alternative for selected industrial applications. In Li NN, Drioli E, Ho WSW, 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. 

Potential Industrial Applications for Carbon Molecular Sieve Membranes... 

Carbon molecular sieves find today wide industrial applications in gas mixture separations, gas purification processes, and as catalysts and catalyst supports. 

The commercial separation of air into N2 and O2, an industrially very important process, is achieved by either cryogenic distillation or pressure swing adsorption (PSA). The use of pillared clays forms an interesting alternative for the carbon molecular sieves and zeolites currently applied as adsorbents in PSA techniques. Both the capacity and the selectivity towards air components are very important features in gas adsorption applications. 

The subject area of activated carbon was by now a significant technology in several industries where the applications of carbons, prepared by thermal (as well as chemical activation) were of fundamental importance. This handbook provided a chapter (summary) of aspects which must be considered in discussions of the use of activated carbon. The chapter contains an Introduction, Production Methods, Precursors, Physical (Thermal) Activation, Chemical Activation, Combined Activations, Carbon Molecular Sieves, Activated Carbon Fibers and Cloths, Pelletized Activated Carbons, Washed, Treated and Impregnated Activated Carbon, as well as a section covering industrial production and applications. As such, this chapter is a substantial reference document and will remain so for some considerable time (Rodriguez-Reinoso, 2002). 

Although carbon molecular sieves (CMSs) and zeolites offer very attractive permeation properties and the selectivites are significantly higher than that of polymeric materials, processing challenges and high costs are still hindering their industrial applications. 

To use adsorption as a unitary operation in industrial, pollution abatement, or energy production applications, in most cases, a reactor where a dynamic adsorption process will take place is packed with a concrete adsorbent. The adsorbents generally used for these applications are active carbons, zeolites and related materials, silica, mesoporous molecular sieves, alumina, titanium dioxide, magnesium oxide, clays, and pillared clays. 

Zeolite-based materials are extremely versatile uses include detergent manufacture, ion-exchange resins (ie, water softeners), catalytic applications in the petroleum industry, separation processes (ie, molecular sieves), and as an adsorbent for water, carbon dioxide, mercaptans, and hydrogen sulfide. 

The development of ultrafine porous structure in active carbons (i.e., the preparation of molecular sieve carbons) has been the subject matter of a large number of investigations because these materials find applications in industrial separation processes. These materials have some distinct advantages over the zeoUte sieves. ... 

CMS are amorphous materials. Their pore structure below 5 A can not be studied by X-ray diffraction, in contrast to most mineral molecular sieves. Transmission electron microscopy has also not been found suitable for determining such small pore dimensions. The most effective method for characterization is the analysis of adsorption isotherms of small probe molecules with different critical dimensions, viz. O2, N2, CO2, CH4. These adsorption isotherms are useful in determining the pore size distribution, surface area, pore volumes and separation capacity of CMS. In addition, these isotherms give information on the potential industrial applications of these materials, e.g. for the separation of nitrogen from air or of carbon dioxide and methane from flue gases. 

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