Types of Separation Processes and Methods

Many chemical process materials and biological substances occur as mixtures of different components in the gas, liquid, or solid phase. In order to separate or remove one or more of the components from its original mixture, it must be contacted with another phase. The two phases are brought into more or less intimate contact with each other so that a solute or solutes can diffuse from one to the other. The two bulk phases are usually only somewhat miscible in each other. The two-phase pair can be gas-liquid, gas-solid, liquid-liquid, or liquid-solid. During the contact of the two phases the components of the original mixture redistribute themselves between the two phases. The phases are then separated by simple physical methods. By choosing the proper conditions and phases, one phase is enriched while the other is depleted in one or more components. 

Absorption. When the two contacting phases are a gas and a liquid, the unit operation is called absorption. A solute A or several solutes are absorbed from the gas phase into a liquid phase in absorption. This process involves molecular and turbulent diffusion or mass transfer of solute A through a stagnant nondiffusing gas B into a stagnant liquid C. An example is absorption of ammonia A from air B by the liquid water C. Usually, the exit ammonia-water solution is distilled to recover relatively pure ammonia. 

Another example is absorbing SO2 from the flue gases by absorption in alkaline solutions. In the hydrogenation of edible oils in the food industry, hydrogen gas is bubbled into oil and absorbed. The hydrogen in solution then reacts with the oil in the presence of a catalyst. The reverse of absorption is called stripping or desorption, and the same theories and basic principles hold. An example is the steam stripping of nonvolatile 

When the gas is pure air and the liquid is pure water, the process is called humidification. Dehumidification involves removal of water vapor from air. 

Distillation. In the distillation process, a volatile vapor phase and a liquid phase that vaporizes are involved. An example is distillation of an ethanol—water solution, where the vapor contains a concentration of ethanol greater than in the liquid. Another example is distillation of an ammonia-water solution to produce a vapor richer in ammonia. In the distillation of crude petroleum, various fractions, such as gasoline, kerosene, and heating oils, are distilled off. 

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