Giáo trình Tiếng Anh trong kỹ thuật hóa học: Phần 2

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Giáo trình Tiếng Anh trong kỹ thuật hóa học: Phần 2 UNIT 23 ADSORPTION Adsorption is a separation process in which certain components of a fluid phase are transferred to the surface of a solid adsorbent. Usually the small particles of adsorbent are held in a fixed bed, and fluid is passed continuously through the bed until the solid is nearly saturated and the desired separation can no longer be achieved. The flow is then switched to a second bed until the saturated adsorbent can be replaced or regenerated. Ion exhange is another process that is usually carried out in this semibatch fashion in a fixed bed. Water that is to be softened or deionized is passed over beads of ion-exchange resin in a column until the resin becomes nearly saturated. The removal of trace impurities by reaction with solids can also be carried out in fixed beds, and the removal of H2S from synthesis gas with ZnO pellets is a well-known example. For all these processes, the performance depends on solid-fluid equilibria and on mass-transfer rates. In this chapter the emphasis is on adsorption, but the general methods of analysis and design are applicable to other fixed-bed processes. ADSORBENTS AND ADSORPTION PROCESSES. Most adsorbents are highly porous materials, and adsorption takes place primarily on the walls of the pores or at specific sites inside the particle. Because the pores are generally very small, the internal surface area is orders of magnitude greater than the external area and may be as large as 2000 m 2/g- Separation occurs because differences in molecular weight, shape, or polarity cause some molecules to be held more strongly on the surface than others or because the pores are too small to admit the larger molecules. In many cases, the adsorbing components (or adsorbate) is held strongly enough to permit complete removal of that component from the fluid with very little adsorption of other components. Regeneration of the adsorbent can then be carried out to obtain the adsorbate in concentrated or nearly pure form. Applications of vapor-phase adsorption include the recovery of organic solvents used in paints, printing inks, and solutions for film casting or fabric coating. The solvent-laden air may first be sent to a water-cooled or refrigerated condenser to collect some of the solvent, but it is generally impractical to cool the gas far below ambient temperature in an attempt to eliminate solvent losses. The air with a small amount of solvent is passed through a bed of carbon adsorbent particles, which can reduce the solvent concentration to less than 1 ppm. The concentration may be set by government emission standards rather than by the economics of solvent recovery. Adsorption on carbon is also used to remove pollutants such as H2S, CS2 and other odorous compounds from air circulating in ventilation systems, and canisters of carbon are placed in most new automobiles to prevent gasoline vapors from being vented to the air. Drying of gases is often carried out by adsorbing the water on silica gel, alumina, or other inorganic porous solids. The zeolites, or molecular sieves, which are natural or synthetic aluminosilicates with a very regular, fine pore structure, are especially effective in preparing gases with low dew points ( — 75°C). Adsorption on molecular sieves can also be used to separate oxygen and nitrogen, to prepare pure hydrogen for synthesis gas, and to separate normal paraffins from branched paraffins and aromatics. 90 Adsorption from the liquid phase is used to remove organic components from aqueous wastes, colored impurities from sugar solutions and vegetable oils, and water from organic liquids. Adsorption can also be used to recover reaction products that are not easily separated by distillation or crystallization. Some of the same types of solids are used for both vapor- phase and liquid-phase adsorption, though often adsorbents with larger pores are preferred for use with liquids. Exercises 23.1 Translate into Vietnamese Adsorption, adsorbent, switched, regenerate, regeneration, achieve, exchange, soften, deionize, bead, column, trace, emphasis, applicable, fixed bed, ink, printing ink, paint, casting, fabric, laden, refrigerate, impractical, attempt, concentration, emission, circulating, ventilation, canister, vent, porous, zeolite, sieve, aluminosilicate, regular, pore, effective, dew point, crystallization. 23.2 Translate the following sentences paying attention to the words in bold type 1. Unvulcanized rubber is both plastic and elastic. 2. Rubber either in the form of latex or solid sheet, when stored for considerable periods develops increased hardness. 3. The generation of oxygenated groups in secondary reactions is consistent with our present knowledge of polyisoprene biosynthesis. 4. It has been suggested that the carbonyl groups (in storage hardening) are incorporated at intervals along the “hydrocarbon chain”. 5. As a result of Goodyear’s work, along with improvements by Hancock, rubber goods of wide utility and durability could be made. 6. Modern industry could no longer function properly without reinforced rubber. 7. Polystyrene has been used commercially long before the high styrene resins. 23.3 Find (in the list given below) synonyms to the following words. Translate these words in to Vietnamese to be able to can behaviour certain compressed complete definite contracted 91 enough to detect to find out different form ...