Air pollution control engineering

The phenomenon of air pollution includes a sequence of events: the generation of pollutants at and their release from a source; their transport and transformation in and removal from the atmosphere; and their effects on human beings, materials, and ecosystems. Generally, either economically infeasible or technically impossible to design processes for absolutely zero emissions of air pollutants, we seek to control the emissions to a level such that effects are either nonexistent or minimized.

Air pollutants are divided into two main groups: particulates and gasses. Because particulates consist of solids and/or liquid material, air pollutants, therefore, encompass all three basic forms of matter. Gaseous pollutants include gaseous forms of sulfur and nitrogen. Particulates may be subdivided into several groups. Atmospheric particulates consist of the solid or liquid material with diameters smaller than about 50 µm. Fine particulates are those with diameters smaller than 3 µm. The term “aerosol” is defined specifically as particulates with diameters smaller than about 30–50 µm (this does not refer to the large particulates from aerosol spray cans). Particulates with diameters larger than 50 µm settle relatively quickly and do not remain in the ambient air.

Theoretically, absorption of a pollutant in a gas phase into a contacting liquid phase occurs when the liquid contains less than the equilibrium concentration of the pollutant. In other words, the pollutant in the gas phase must have some solubility in the liquid phase. For absorption into the liquid phase to occur, the maximum concentration of the same pollutant in the liquid phase must be avoided initially. This is because the concentration difference across the phase boundary is the driving force for absorption to occur between the two phases. Additionally, absorption (mass transfer) from gas into liquid (or vice versa) is dependent on the physical properties of the gas–liquid matrix (e.g., diffusivity, viscosity, density) as well as the conditions of the scrubber system (e.g., temperature, pressure, gas and liquid mass flow rates). Absorption of a pollutant is enhanced by lower temperatures, greater liquid–gas contact surfaces, higher liquid–gas ratios, and the higher concentration of the pollutant in the gas phase (or, alternatively, the lower concentration of the pollutant in the liquid phase). In some instances, elevated pressures are used to give added driving force of the pollutant into the liquid stream as well. Wet scrubbers are often the technology of choice if high removal efficiencies of acid gasses are required.

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