Unit operation

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An ore extraction process broken into its constituent unit operations (Quincy Mine, Hancock, MI ca. 1900)

In chemical engineering and related fields, a unit operation is a basic step in a process. Unit operations involve a physical change or chemical transformation such as separation, crystallization, evaporation, filtration, polymerization, isomerization, and other reactions. 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 require many unit operations to obtain the desired product from the starting materials, or feedstocks.

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.[1] 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.[2] They summed-up these similar processes into unit operations. Each unit operation follows the same physical laws and may be used in all relevant chemical industries. The unit operations form the fundamental principles of chemical engineering.

Chemical engineering unit operations consist of five classes:

  1. Fluid flow processes, including fluids transportation, filtration, and solids fluidization.
  2. Heat transfer processes, including evaporation, condensation, and heat exchange.
  3. Mass transfer processes, including gas absorption, distillation, extraction, adsorption, and drying.
  4. Thermodynamic processes, including gas liquefaction, and refrigeration.
  5. Mechanical processes, including solids transportation, crushing and pulverization, and screening and sieving.

Chemical engineering unit operations also fall in the following categories which involve elements from more than one class:

Furthermore, there are some unit operations which combine even these categories, such as reactive distillation and stirred tank reactors.

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[edit]


  1. ^ "Arther Dehon Little". Scatter Acorns That Oaks May Grow. MIT Libraries. Retrieved 13 November 2013. 
  2. ^ "Arthur D. Little, William H. Walker, and Warren K. Lewis". Chemistry in history. Chemical Heritage Foundation. Retrieved 13 November 2013.