Gaseous pollutants adsorption

Transport from the atmosphere to land and water Dry deposition of particulate and gaseous pollutants Precipitation scavenging of particulate and gaseous pollutants Adsorption of gases onto particles and subsequent diy and wet deposition Transport within the atmosphere Turbulent dispersion and convection Atmospheric transformation Diffusion to the stratosphere Photochemical degradation Oxidation by free radicals and ozone Gas-to-particle conversion... 

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. 

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. 

If enclosure is not possible, the sources of emission are selectively aspirated. Gaseous pollutants and vapors are collected and passed to central or decentralized disposal equipment [e.g., combustion plants (thermal or catalytic) or adsorption units], particulate impurities being retained in filters... 

Filtration Process for Air Quality Management. - Purification of gaseous pollutants is associated with adsorption, absorption and incineration. A new... 

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. 

The RFGC technique, a flow perturbation method developed in 1980, is used to measure the following directly from experimental data rate constants for adsorption, desorption, and chemical reaction of gaseous pollutants with the solid surface of the objects, di sion coefficients of the pollutants which are diffused into the pores of the solid, deposition velocities and reaction probabilities for the action of air pollutants on the same surfaces, adsorption energies, local monolayer capacities, local adsorption isotherms, and probability density... 

From the above-calculated parameters, through the following Eqs. 19 and 20, the overall deposition velocity (Vd), which is equivalent to an overall mass transfer coefficient of the gaseous pollutant to the solid surface, corrected for the activated adsorption/desorption and surface reaction, and the reaction probability 7 of the pollutant with the surface under study are found ... 

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... 

In recent years considerable emphasis has been placed on energy conservation and environmental pollution control. As discussed earlier, the former problem is likely to be solved, at least in part, by the introduction or wider use of more efficient fuels such as biomass or nuclear fuel, coupled with more controlled application of what is at present waste heat from chemical reactions. This is now more than ever possible because of the improved and improving design of heat pumps, boilers, heat exchangers, etc., the use of which is more nearly optimized by computer control. Emissions of gaseous pollutants have been decreasing in recent years by improved methods of condensation, adsorption. 

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