Carbon molecular sieve Subject

The main focus of this volume is on imderstanding the transport of molecules in microporous solids such as zeolites and carbon molecular sieves, and the kinetics of adsorption/desorption. This subject is of both practical and theoretical interest, since the performance of zeohte-based catalysts and adsorbents is strongly influenced by resistances to mass transfer and intracrystalline diffusion. However, at an even more basic level, the performance of microporous catalysts and adsorbents depends on favorable adsorption equilibria for the relevant species, so a general imderstanding of the fundamentals of adsorption equilibrium is a necessary prerequisite for understanding kinetic behavior. This chapter is intended to provide a concise summary of the general principles of adsorption equiHbriiun and of the main features of sorption kinetics in microporous solids, which generally depend on a combination of both equilibriiun and kinetic properties. 

Micropore diffusion in zeolites and carbon molecular sieves has been widely studied. The subject has been reviewed by Barrer, Karger et al., ... 

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). 

It has been observed that membranes of carbon molecular sieves can exceed the upper bound of conventional polymeric membranes. The carbon molecular sieve membranes are produced by carbonization of aromatic polymers (e.g., polyimides), yielding pore dimensions in the range of O2 and N2 molecular dimensions. Polyimide/poly(vinyl pyrrolidone) blends subjected to carbonization conditions also yielded carbon molecular sieve membranes that exceeded the upper bound limit for conventional polymeric membranes [195, 196]. Specific values were O2 permeability of 560-810 barrers with a O2/N2 separation factors of 10-7 well above the upper bound. 

The physical-chemical properties of a synthetic gallophosphate molecular sieve, the 30-A supercage cloverite , have been assessed [18]. Instead of attempting to list the burgeoning number of fullerene publications, attention is drawn to the formation and characterization of fullerene-like nanocrystals of tungsten disulfide [19,20]. Preparation, characterization and utilization of carbon nanotubes have been the subject of a number of reports from several laboratories [21-27]. 

Experience in air separation plant operations and other cryogenic processing plants has shown that local freeze-out of impurities such as carbon dioxide can occur at concentrations well below the solubility limit. For this reason, the carbon dioxide content of the feed gas subject to the minimum operating temperature is usually kept below 50 ppm. The amine process and the molecular sieve adsorption process are the most widely used methods for carbon dioxide removal. The amine process involves adsorption of the impurity by a lean aqueous organic amine solution. With sufficient amine recirculation rate, the carbon dioxide in the treated gas can be reduced to less than 25 ppm. Oxygen is removed by a catalytic reaction with hydrogen to form water. 

To obtain carbon membranes with molecular sieving properties, pore diameters in the range of a few angstroms are required. The associated pyrolysis procedures and post treatments are more involved. This subject will be treated later under section 3.2.10 Molecular Sieving Membranes. 

There are many methods for the desulfurization of nature gas, which can be classified into dry desulfurization, wet desulfurization, and catalytic adsorption. In the dry desulfurization, some solid sorbents, such as iron oxide, zinc oxide, activated carbon (AC), zeolites, and molecular sieves, are used. In wet desulfurization method, liquid-phase chemical/physical solvent absorption systems are usually used for scrubbing H2S amine-based processes are subject to equipment corrosion, foaming, amine-solution degradation, and evaporation, and require extensive wastewater treatment. As a result, this sulfur removal technology is complex and capital intensive,44 although the processes are still employed widely in the industry. The desulfurization of coal gasification gas will be reviewed in detail in Section 5.5. In the catalytic-adsorption method, the sulfur compounds are transformed into H2S by catalytic HDS or into elemental sulfur or SOx by selective catalytic oxidation (SCO), and then, the reformed H2S and SOx are removed by the subsequent adsorption. 

Tlie BET equation, however, is subject to various limitations when applied to microporous carbons. Thus, constrictions in the microporous network may cause molecular sieve effects and molecular shape selectivity. Diffusion effects may also occur when using N2 at 77 K as the adsorbate since at such low temperatures the kinetic energy may be insufficient to penetrate all the micropores. For this reason adsorption of CO2 at higher temperatures (273 K) is also used. CO2 and N2 iso erms are complementary. Thus, whereas from the CO2 isothenn micropores of up to approximately 10 m width can be measured, the Na can be used to test larger pores. Despite these limitations the BET surlace area is the parameter most commonly used to characterize the specific surlace area of carbon adsorbents. 

The adsorption and the isolation technique is an efficient method in environmental problems, the toxic materials in sorts of industrial waste-water and exhaust ffimes can be absorbed by using various adsorbents, so that the ffimes and the liquid are up to standard of environmental protection. The key problem of the adsorption and the isolation technique lies in the adsorbents the commonly used adsorbents are activated carbon, silica gel, acid terra alba and zeolite molecular sieve, etc. [20-22]. But, not only the adsorption characterization of these materials but also the operating characterization and the reproducing ability of these materials are all very weak. So searching for a high quality adsorption material has become a subject concerned by experts all over the world. 

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. ... 

Purification adsorption of nitrogen trifluoride is NOT recommended. Purification process using dry media such as activated charcoal (carbon) or molecular sieve may be subject to rapid exotherms from sudden exposure of the media to large quantities of nitrogen trifluoride. 

Coke furnace wastewater containing dissolved organic carbon (DOC) of 69 ppm and two peaks at the molecular weight of 400 and 10,000 Da was subjected to the filtration by the activated carbon membrane. Figure 9.20 displays (permeate concentration in chemical oxygen demand (COD)/feed concentration in COD=C/Co) versus effluent volume. Each curve indicates the typical breakthrough cirrve but interestingly the curve never exceeds (C/Co) ratio of 0.68, which means 32% of the COD is removed by the sieve mechanism. Hence, the COD removal by the... 

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