Rectification, and Absorption

All three unit operations use a veiy simple separation principle that consists of three steps  

In distillation and rectification the second phase (vapor) is generated by supply of heat. In absorption, however, an external liquid (solvent, absorbent) is added to the system at hand. 

Mersmann et al., Thermal Separation Technology Principles, Methods, Process Design, VDI-Buch, DOI 10.1007/978-3-642-12525-6 5, 

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However in many heat and mass transfer processes in fluids, condensing or boiling at a solid surface play a decisive role. In thermal power plants water at high pressure is vaporized in the boiler and the steam produced is expanded in a turbine, and then liquified again in a condenser. In compression or absorption plants and heat pumps, boilers and condensers are important pieces of equipment in the plant. In the separation of mixtures, the different composition of vapours in equilibrium with their liquids is used. Boiling and condensing are, therefore, characteristic for many separation processes in chemical engineering. As examples of these types of processes, the evaporation, condensation, distillation, rectification and absorption of a fluid should all be mentioned. 

In the tliermodynamic description of distillation, rectification, and absorption processes the concentrations of the mixture are advantageously expressed by molar fractions x (liquid phase) and y (gas or vapor phase) since molar fractions enable a more straightforward formulation of phase equilibrium. In turn, the amounts of the phases (G or L) have to be expressed in moles, too. 

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