Adsorption, gaseous pollutant removal

Although the continuous-countercurrent type of operation has found limited application in the removal of gaseous pollutants from process streams (Tor example, the removal of carbon dioxide and sulfur compounds such as hydrogen sulfide and carbonyl sulfide), by far the most common type of operation presently in use is the fixed-bed adsorber. The relatively high cost of continuously transporting solid particles as required in steady-state operations makes fixed-bed adsorption an attractive, economical alternative. If intermittent or batch operation is practical, a simple one-bed system, cycling alternately between the adsorption and regeneration phases, 1 suffice. 

One way to control gaseous pollutants like SO2 and SO3 is to remove the gases from fuel exhaust systems by absorption into a liquid solution or by adsorption onto a solid material. Absorption involves dissolving the gas in a liquid while adsorption is a surface phenomenon. In each case, a subsequent chemical reaction can occur to further trap the pollutant. Lime and limestone are two solid materials that effectively attract sulfur dioxide gas to their surfaces. The ensuing chemical reaction converts the gaseous pollutant to a solid nontoxic substance that can be collected and disposed or used in another industry. 

NOXIOUS GAS REMOVAL. Gaseous pollutants can be removed from air streams either by absorption, adsorption, condensation, or incineration. A list of typical gaseous pollutants that can be treated with these four methods is given in Table 9. Generally, condensation is not utilized as a method for removing a solvent vapor from air or other carrier gas unless the concentration of the solvent in the gas is high and the solvent is worth recovery. Since condensation cannot remove all of the solvent, it can only be used to reduce the solvent concentration in the carrier gas. 

The use of dry adsorbents like activated carbon and molecular sieves has received considerable attention in removing final traces of objectional gaseous pollutants. Adsorption is generally carried out in large, horizontal fixed beds often equipped with blowers, condensers, separators, and controls. A typical installation usually consists of two beds one is onstream while the other is being regenerated. 

The removal of gaseous pollutants from dryer exhaust may be accomplished by several possible processes. Among these are absorption, adsorption, condensation, and incineration [11-16]. The choice of a given process is usually determined by physical and chemical characteristics of the dried product and by economic and environmental considerations. Table 53.3 summarizes some of the basic characteristics of the gaseous emission control equipment. 

Devices used to reduce the number of particulates emitted include settling chambers, baghouses, cyclones, wet scrubbers, and electrostatic precipitators. Electrostatic precipitators impart the particles with an electric charge to aid in their removal. They are often used in power plants, at least in part because of the readily available electric power to rim them. Water-soluble gaseous pollutants can be removed by wet scrubbers. Other options for gaseous pollutants are adsorption on activated carbon or incineration of combustible pollutants. Because sulfur is contained in the coal used as fuel, coal-fired power plants produce sulfur oxides, particularly troublesome pollutants. The main options for reducing... 

A countercurrent moving-bed adsorption column is used to remove benzene from a gaseous emission. Activated carbon is employed as the adsorbent. The flowrate of the gas is 1.2 kg/s and it contains 0.027 wt/wt% of benzene. It is desired to recover 99% of this pollutant. The activated carbon entering the column has 2 X 10 wt/wt% of benzene. Over the operating range, the adsorption isotherm (Yaws et al., 1995) is linearized to... 

However, activated carbons are the most extensively applied industrial adsorbents for the removal of pollutants from gaseous and aqueous and nonaqueous streams, because of their exceptionally powerful adsorption properties and their readily modifiable surface chemistry [217,218], Carbon is the primarily applied adsorbent in the case of liquid-solid adsorption systems. 

Due to its high vapor pressure at the operating temperature of the electrolysis, mercury, whose circulating tonnage represents 700 to 2400 kg/t per day of chlorine production capacity, pollutes the different gaseous streams produced (chlorine, hydrogen). Similarly, it contaminates the different liquids produced by the operation (spent brine, caustic soda, etc.). This results in substantial losses, which must be limited for economic as well as environmental reasons. Whereas small. amounts of mercury in the chlorine (0.1 to 0.2 g/t) are not detrimental to its subsequent uses, the same cannot be said of caustic soda, especially for food applications, in which it is removed by filtration (up to 15 ppb), for hydrogen, from which it is removed (up to 3 to 5 ppb) by absorption in sodium hypochlorite, adsorption on activated charcoal etc, and aqueous wastes, from which it is removed (up to 5 to 10 ppb) by precipitation, adsorption, reduction or extraction. The spent brine, which normally contains 1 to 10 ppm mercury and occasionally 1000 ppm, is usually recycled and therefore does not require treatment... 

This chapter provides a comprehensive summary of surface science involved in the application of activated carbon for air cleaning from sulfur containing species such as hydrogen sulfide, sulfur dioxide, and mercaptans. Moreover, the removal of organic sulfur-containing compounds from both gaseous and liquid fuel is addressed. The emphasis is placed on the role of activated carbon surfaces, either unmodified or modified in the processes of adsorption and catalytic oxidation of these pollutants. 

Activated carbon (AC) has been most effective adsorbent for the removal of a wide range of contaminants from aqueous or gaseous environment. It is a widely used adsorbent in the treatment of wastewaters due to its exceptionally high surface areas which range from 500 to 1,500 m g, well-developed internal microporosity stmcture [1]. While the effectiveness of ACs to act as adsorbents for a wide range of pollutant materials is well noted and more research on AC modification are presented due to the need to enable ACs to develop affinity for special contaminants removal from wastewater [2], It is, therefore, essential to imderstand the various important factors that influence the adsorption capacity of AC due to their... 

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