A basic step in a process or operation carried out in a chemical plant. A unit operation involves the study of physical, chemical, or biochemical changes that occur during the processing of materials. The design of equipment and systems is based on these operations and includes mixing, reaction, and separation. Unit operations may be classified as:

1. 1 Fluid flow processes including filtration and fluidization.

2. 2 Heat transfer including evaporation and condensation.

3. 3 Thermodynamic processes including heat pumps.

4. 4 Mechanical processes such as solids and particle handling.

5. 5 Mass transfer including distillation, drying, solvent extraction, and adsorption.

Arthur D. Little is credited with coining the concept of unit operation.