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Speaking of isotherms, there are only a handful of shapes (some are shown in Figure 1), but there are dozens of empirical forms, some of which are discussed later in this article. Beyond those, there exist many other means to express adsorption capacity: isosteres, isobars, and various indices, such as surface area, pore size distribution, Iodine Number and Molasses Number. The latter two are used exclusively to describe activated carbons. Isosteres and isobars, along with isotherms, are discussed more fully later in this article. Surface area is a relative term, usually measured by inferring the monolayer coverage within an adsorbent of a substance with known density and molecular dimensions (e.g., nitrogen near its normal boiling point). Values typically correlate with capacity, and for various adsorbents are in the range of, say, 5 to 3,000 m2/g. Related to the weight of a penny (2.5 g), the range is from a 12 ft x12 ft square to nearly two football fields! Some specific ranges are listed for common adsorbents in the next section. Surface area alone is not a proper basis for choosing an adsorbent. Figure 1. Classic Isotherm Types. The pore size distribution is a related property that indicates the fraction of the space within a particle occupied by micropores (dp20D), mesopores (20Ddp500D), and macropores (dp500D). Pore dimensions intuitively correlate with both capacity and kinetics, but the exact dependence is subtle. Figure 2 shows the pore size distributions of some common adsorbents. The Iodine Number is a rough measure of capacity for small molecules and correlates with surface area. The Molasses Number was developed for decolorization of cane sugar, and relates to adsorption of large molecules from a liquid. Sometimes people selling adsorbents will use terms like these, possibly not realizing that we do not use them everyday, as they do. It never hurts to ask, “What does that term mean to my application?” 3PDF Image | ADSORBENT SELECTION
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