In chemical engineering and related fields, a unit operation is a basic step in a process. For example in milk processing, homogenization, pasteurization, chilling, and packaging are each unit operations which are connected to create the overall process. A process may have many unit operations to obtain the desired product. Historically, the different chemical industries were regarded as different industrial processes and with different principles. In 1923 William H. Walker, Warren K. Lewis and William H. McAdams wrote the book The Principles of Chemical Engineering and explained the variety of chemical industries have processes which follow the same physical laws. They summed-up these similar processes into unit operations. Each unit operation follows the same physical laws and may be used in all chemical industries. The unit operations form the fundamental principles of chemical engineering. Chemical engineering unit operations consist of five classes:
- Fluid flow processes, including fluids transportation, filtration, solids fluidization
- Heat transfer processes, including evaporation, condensation
- Mass transfer processes, including gas absorption, distillation, extraction, adsorption, drying
- Thermodynamic processes, including gas liquefaction, refrigeration
- Mechanical processes, including solids transportation, crushing and pulverization, screening and sieving
Chemical engineering unit operations also fall in the following categories:
- Combination (mixing)
- Separation (distillation)
- Reaction (chemical reaction)
Chemical engineering unit operations and chemical engineering unit processing form the main principles of all kinds of chemical industries and are the foundation of designs of chemical plants, factories, and equipment used.
