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polymers Polymeric adsorbents tend to be opaque spherical beads, but the color depends strongly on the product. Most commonly they are white or tan, but some are brown, orange, or black. The first materials were originally the inert particles that would otherwise have been further treated to make macroporous or macroreticular ion exchange resins. As such, they were typically polystyrene/divinyl- benzene copolymers having a spherical shape and high pore volume. Some are still that sort of byproduct, but most are manufactured separately, with high performance in adsorption as their purpose. Internally, the polymer beads contain “microbeads” that are joined together at a few points each, creating a macropore structure. Each microbead is usually comprised of a gel, but may be made porous. In addition, some polymeric adsorbents are activated via pyrolysis, in much the same way as carbon (yielding the black materials alluded to above), yet the particles retain their strength and spherical shape. Manufacturers include Bayer, Dow Chemical, Hayes Separations, Mitsubishi, Purolite, and Rohm and Haas. Instead of being limited to styrene/divinylbenzene, polymeric adsorbents are also made from polymethacrylate, divinylbenzene/ethylvinylbenzene, or vinylpyridine and are sometimes sulfonated or chloromethylated, much as are ion exchange resins. As a result, some are sufficiently hydrophilic to be used as a desiccant, while others are quite hydrophobic. The effective surface area is usually smaller than for activated carbon, e.g., 5 to 800 m2/g. The corresponding pore diameters range from about 20 to 2,000 D, or from 3 to 2000 D if activated. The available forms are fairly limited: beads of 0.3 to 1 mm dia., usually in a relatively narrow range. Obtaining even smaller particles would not be a problem, since they are even used for gas chromatography, but larger particles are not yet commercially available. A minor drawback of these materials is that they tend to shrink and swell upon cyclic use. For gas-phase applications they may require conditioning prior to use, e.g., washing with water and/or another solvent followed by drying. The range of applications is somewhat restricted, since the cost of most polymeric adsorbents is typically about 10× that of others that are available. In some instances other adsorbents simply cannot perform, so polymeric materials are the only choice. In other cases they compensate for the cost differential by yielding much better performance, especially for high value-added uses. Current applications include: recovery and purification of antibiotics and vitamins, decolorization, decaffeination, hemoperfusion, separation of halogenated light organics from water, and treatment of certain industrial wastes such as aqueous phenolics and VOC recovery from off-gases. 3. Adsorption Characteristics This section describes the scientific and quantitative characteristics of adsorbents for specific applications. The properties discussed here are only those relevant as a basis for adsorbent selection. Others, which may be only indirectly relevant, are glossed over. In fact, the material presented here is just an overview, since to understand their impact requires fairly deep understanding of the field of adsorption. 3.1 Adsorption Equilibrium and Heats of Adsorption The concept of adsorption equilibrium is involved deeply in the measurement and correlation of adsorption capacity, selectivity, and regenerability data. Generally, equilibrium is the constraint that limits each of these vital factors for every adsorption application. Earlier in this article, the term adsorption capacity was used freely as the amount of adsorbate taken up by the adsorbent, per unit mass (or volume) of the adsorbent. Thefollowinggeneraldefinitionexpressestherelationshipwithasanarbitraryfunctionofpartial pressure or concentration and temperature. (1) In the next section we will see specific functions that are commonly used to represent data. From here on, we assume that proper pretreatment has been done. Even so, there may be other conditions that affect equilibrium, especially the presence of other compounds that may compete for space in the adsorbent, or may adsorb irreversibly and reduce the effectiveness of the adsorbent. 9PDF Image | ADSORBENT SELECTION
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