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2.2. organic materials This section discusses adsorbents that are based on organic material, whether synthetic or naturally occurring. A wide variety of organic materials have been used as for “sorption,” besides activated carbon or charcoal. Some might function as solid “absorbents” rather than adsorbents. Among these are cellulose (the most abundant biopolymer in nature), chitin (the second most abundant biopolymer in nature), collagen, wool, starch-polyacrylamide gels (which absorb many times their own weight of water at ambient temperature, but release most of it by gentle heating), polysaccharrides derived from corn, and miscellaneous forms of biomass (e.g., residue from crop harvests). Some of these may have niches, but none would be considered a general purpose adsorbent. Here we will focus on commercial adsorbents. activated carbon Although the choices among zeolites are immense, activated carbons are even more diverse. For example, the base materials that comprise activated carbons include: wood, coal, peat, coconut shells, saran, recycled tires, and others. The final adsorbents all look, to the casual observer, pretty much the same, i.e., black granules or pellets, but appearances can be deceptive. Activation produces a distribution of internal pores, and affects the carbon surface (e.g., graphitic versus oxidized), generally to enhance its adsorptive capacity. Thus, by varying activation conditions, differences of the internal surfaces can be induced, even for materials that appear to be identical. Another feature that varies, depending on the nature of the base material, is ash content, which is of course inorganic. Typical values are between 2 and 25%, but the average is about 7%. Alkali ash near or at the surface can be removed by acid washing or other minerals may be deposited by impregnation. The microscopic structure (pore size distribution and surface area), surface qualities, and chemical composition all strongly affect adsorption characteristics, and they therefore affect the performance parameters (capacity, selectivity, regenerability, kinetics, compatibility, and cost). In fact, there are so many variables that it is surprising that firms are able to maintain specifications of specific products, through tight quality control. Manufacturers include Waterlink - Barnebey - Sutcliffe, Calgon, Ceca Division of Elf Atochem, Kansai Coke & Chemicals, Norit, Showa Denko, and Westvaco. Effective surface areas generally range from 300 to 1,500 m2/g, depending on the base material, activation method, density, etc., although some made from petroleum coke exceed 3,000 m2/g. Surface areas are treated about the same as engine horsepower or displacement are for automobiles: those with the largest tend to imply that they are the best. Assuming that area correlates with capacity, and that all other factors are equal, that assertion might be valid. Otherwise, it is prudent to make an objective assessment of all the performance criteria. Common forms are beads 1 to 3 mm dia., granules, extrudates (pellets) 2 to 4 mm dia., and powder. Some typical applications are: water and wastewater treatment to remove hazardous organic compounds or those that impart odor or taste, cleanup of off-gases containing volatile organic compounds (especially solvents which might be recovered, and odoriferous chemicals which are merely trapped), upgrading methane from substandard natural gas wells, food decolorization, and pharmaceutical purification. Impregnated activated carbons are widely used in gas masks and to remove other specific contaminants in gas or water. Impregnants include sulfuric acid (for ammonia or mercury), iron oxide (for hydrogen sulfide or mercaptans), zinc oxide (for hydrogen cyanide), and a combination of heavy metal salts (for phosgene, arsine, and nerve gases). Pretreatment for gas-phase applications is often performed as the last step of manufacture, due to the large quantities employed because it would be impractical to do on-site. requires heating to about 200°C. Adding to the diversity is a relatively new type of product called a “carbon molecular sieve,” analogous to the zeolite molecular sieve mentioned before. While micropores in zeolites tend to have rounded apertures, the carbon-based counterparts are more slit-like, as in the space between layers of graphite. To date, only one type of commercial separation employs this material: separation of nitrogen from air. This pressure swing process exploits the difference between sizes of oxygen (3.43D) and nitrogen (3.68D), and can achieve 99.9% nitrogen purity. 8PDF Image | ADSORBENT SELECTION
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