Unit operation is the basic principle of chemical engineering. Before 1923 the different chemical industries were regarded as different industrial processings and with different principles.

In 1923 Willian.H.Walker , Warren.K.Lewis and Willian.H.McAdams wrote a book The Principle of Chemical Engineering and thought the variety chemical industries have the processings follow the same physical laws. They sum up the same processing into unit operations. Each unit operation follows the same physical law and may use in all chemical industries. The unit operations form the principle of chemical engineering.

Chemical engineering unit operation consists of five classes:

1. Fluid flow process, including Fluids transportation , Filtration, Solids fluidization etc.
2. Heat transfer process, including Evaporation, Condensation etc.
3. Mass transfer process, including Gas absorption , Distillation, Extraction, Adsorption, Drying etc.
4. Thermodynamic process, including Gas liquefaction , Refrigeration etc.
5. Machanical process, including Solides transportation , Crushing and pulverization , Screening and sieving etc.

Chemical engineering unit operations and chemical engineering Unit processings form the main principle of all kind of chemical industries and are the foundation of designs of chemical plants and factories.