Adsorbents for gas separation

Burchell, T.D., Judkins, R.R. and Rogers, M.R., A carbon fiber based monolithic adsorbent for gas separation. In Proc. 23rd Biennial Con/, on Carbon, American Carbon Society, 1997, pp. 158 159. 

Deng, S.G. Synthesis and Properties of Nanos-tructured Adsorbents for Gas Separation and Environmental Applications, Ph.D. dissertation, University of Cincinnati, Cincinnati, OH, 1996. 

Most commercial adsorbents for gas-phase appHcations are employed in the form of pellets, beads, or other granular shapes, typically about 1.5 to 3.2 mm in diameter. Most commonly, these adsorbents are packed into fixed beds through which the gaseous feed mixtures are passed. Normally, the process is conducted in a cycHc manner. When the capacity of the bed is exhausted, the feed flow is stopped to terminate the loading step of the process, the bed is treated to remove the adsorbed molecules in a separate regeneration step, and the cycle is then repeated. 

FIG. 16-55 Adsorbent wheels for gas separation a) horizontal with fixed beds (h) vertical monolith. Reprinted with permission ofUOF.)... 

Amorphous adsorbents, 1 587-589 for gas separation, 1 631 properties and applications, l 587t Amorphous aluminum hydroxide, 23 76 Amorphous carbohydrates, material science of, 11 530-536 Amorphous carbon, 4 735 Amorphous cellulose, 5 372-373 Amorphous films, in OLEDs, 22 215 Amorphous germanium (a-Ge), 22 128 Amorphous glassy polymers, localized deformation mechanisms in, 20 350-351... 

Crystal lattice packing, 12 249-250 Crystal lattice vibrations, 14 236 Crystalline adsorbents, 1 586, 589. See also Molecular sieves Zeolites for gas separation, 1 631 properties and applications, l 588t Crystalline alkali silicates, atomic structure of, 22 454-455 Crystalline cellulose, 5 373-379 Crystalline epoxy resins, 10 373-374 Crystalline flake graphite, 12 793 manufacture and processing of, 12 781-784... 

C.A. (1993) Adsorbent-filled membranes for gas separation. Part 1. Improvement of the gas separation properties of polymeric membranes by incorporation... 

It was once taken for granted that adsorbents were required in the form of fine powders, porous granules or extrudates. However, other physical forms such as porous fibres are now available for special applications such as membranes for gas separation or water treatment Carbon cloth provides an interesting example of a highly active form of fibrous carbon. 

Two novel hybrid concepts for gas separation using adsorption technology have emerged in recent years. They include (a) adsorbent membranes, and (b) simultaneous adsorption and reaction. 

J.M. Duval, B. Folkers, M.H.V Mulder, G. Desgrandchamps, and C.A. Smolders, Adsorbent filled membranes for gas separation. Pan 1. Improvement of the gas separation properties of polymeric membranes by incorporation of microporous adsorbents, J. Membrane Sci. 50 189 (1983). 

An adsorbent is often tailor-made to suit a separation need or a process can be designed to best fit the properties of an adsorbent. Special adsorbents are also available for specific applications (e.g., removal of mercury vapor, drying of reactive fluids, resistance to acids, etc). More recently, adsorbents have been produced that use reversible chemisorption as the mechanism for gas separation. Creation of new adsorbents and modification of existing adsorbents continue to be an active area of research and development. 

Recently activated carbons such as activated carbon fibers (ACFs) and superhigh surface area carbons have been developed. New activated carbons have more uniform micropore size distribution and greater surface area than traditional activated carbons. The carbon membranes for gas separation have been also developed lately[8]. The activation of the polyamide film leads to self-supported activated carbon film whose surface area is larger than 1100 m /g [9]. Thus various kinds of carbon adsorbents have been developed to find new applications. Scientific studies on activated carbon have been increasing according to development of these new carbon adsorbents with a special relevance to energy and environmental demands. In particular, controll of an adsorptive ability of activated carbon is requisite for new application. Consequently, basic principles for control of the micropore filling mechanism of activated carbons are shown here. 

Molecular engineering of an activated carbon surface provides a very interesting opportunity to beneficially alter the carbon s core adsorptive characteristics for gas separation. Polar groups can be introduced to the surface of a weakly polar carbon by judicious surface oxidation. Thus, a hydrophobic carbon can be converted to a hydrophilic adsorbent. Figure 22.9(a) shows the water vapor adsorption isotherms at 297 K (specific amount of water adsorbed tj as a function of relative water vapor pressure x) on the original Ceca carbon (Type V... 

The interaction of carbon nanotnbes with their environment, and in particular with gases or liquids adsorbed either on their internal or external surfaces, is attracting increasing attention dne to the possible infinence of such adsorption on the CNT electronic properties (application to chemical sensors) and to the possibility of nsing these materials for efficient gas storage or for gas separation [11]. 

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