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In chemical engineering and related fields, a unit operation is a basic step in a process. Unit operations involve bringing a physical change such as separation, crystallization, evaporation, filtration etc. 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. Arthur Dehon Little propounded the concept of "unit operations" to explain industrial chemistry processes in 1916. 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:
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.
See also 
- Distillation Design
- Separation process
- Transport phenomena
- Unit Operations of Chemical Engineering
- Process simulation
- "The MIT Connection"http://libraries.mit.edu/archives/exhibits/adlittle/mit-connection.html Retrieved March 6, 2010.
- The Encyclopedia of Earth. "Walker, William H. http://www.eoearth.org/article/Walker,_William_H. Accessed April 4, 2010.