Q Aeration Absorption and Stripping

Aeration, absorption, and stripping are unit operations that rely on flow of masses between phases. When a difference in concentration exists between two points in a body of mass, a flow of mass occurs between the points. When the flow occurs between two phases of masses, a transfer of mass between the phases is said to occur. This transfer of mass between phases is called mass transfer. Examples of unit operations that embody the concept of mass transfer are distillation, absorption, dehumidification, liquid extraction, leaching, and crystallization.

Distillation is a unit operation that separates by vaporization liquid mixtures of miscible and volatile substances into individual components or groups of components. The separation of water and alcohol into the respective components of liquid air into nitrogen, oxygen, and argon; and the separation of crude petroleum into gasoline, oil, and kerosene are examples of the distillation unit operation.

Absorption is a unit operation that removes a solute mass or masses from a gas phase into a liquid phase. Aeration of water dissolves air into it; thus, aeration is absorption. Another example of absorption is the "washing" of ammonia from an ammonia-polluted air. In this operation, ammonia is removed from the air by its dissolution into the water.

The reverse flow of masses from the liquid phase into the gas phase is called stripping. In stripping, the solute molecule is removed from its solution with the liquid into the gas phase.

Dehumidification is the removal of a solute liquid vapor from a gas phase by the solute condensing into its liquid phase. The removal of water vapor in air by condensation on a cold surface is dehumidification. The reverse of dehumidification is humidification. In this unit operation, the flow of the solute is from the liquid phase evaporating into the gas phase. The end result of this movement is saturation of the gas. For example, during heavy rains, the atmosphere may become saturated with water vapor, the degree of this saturation being measured by the relative humidity. Liquid extraction is the removal of a solute component from a liquid mixture called the raffinate using a liquid solvent. In this operation, the solvent preferentially dissolves the solute molecule to be extracted.

Leaching is a unit operation where a solute molecule is removed from a solid using a fluid extractor. This is similar to liquid extraction, except that the solute to be removed comes from a solid rather than from a liquid as in the case of liquid extraction. Also, the fluid extractor may be a fluid or a gas. For example, pollutants can be leached out from solid wastes in a landfill as rain percolates down the heap.

Crystallization is a unit operation where solute mass flows toward a point of concentration forming crystals. The driving force for the transfer of mass from liquid into the solid phase is the affinity of the solute to form into a solid. An example of this operation is the making of ice from liquid water. The formation of snow from water vapor in the atmosphere is also a process of crystallization.

Of all the unit operations of mass transfer, this chapter will only discuss aeration, absorption, and stripping. As mentioned, aeration is a form of absorption. Because this operation plays a very important and significant role in water and waste-water treatment, however, we will give it a separate heading and call it specifically aeration.

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