Unit operation

(chemical engineering) The basic physical operations of chemical engineering in a chemical process plant, that is, distillation, fluid transport, heat and mass transfer, evaporation, extraction, drying, crystallization, filtration, mixing, size separation, crushing and grinding, and conveying.

A structure of logic used for synthesizing and analyzing processing schemes in the chemical and allied industries, in which the basic underlying concept is that all processing schemes can be composed from and decomposed into a series of individual, or unit, steps. If a step involves a chemical change, it is called a unit process; if physical change, a unit operation. These unit operations cut across widely different processing applications, including the manufacture of chemicals, fuels, pharmaceuticals, pulp and paper, processed foods, and primary metals. The unit operations approach serves as a very powerful form of morphological analysis, which systematizes process design, and greatly reduces both the number of concepts that must be taught and the number of possibilities that should be considered in synthesizing a particular process.

Most unit operations are based mechanistically upon the fundamental transport processes of mass transfer, heat transfer, and fluid flow (momentum transfer). Unit operations based on fluid mechanics include fluid transport (such as pumping), mixing/agitation, filtration, clarification, thickening or sedimentation, classification, and centrifugation. Operations based on heat transfer include heat exchange, condensation, evaporation, furnaces or kilns, drying, cooling towers, and freezing or thawing. Operations that are based on mass transfer include distillation, solvent extraction, leaching, absorption or desorption, adsorption, ion exchange, humidification or dehumidification, gaseous diffusion, crystallization, and thermal diffusion. Operations that are based on mechanical principles include screening, solids handling, size reduction, flotation, magnetic separation, and electrostatic precipitation. The study of transport phenomena provides a unifying and powerful basis for an understanding of the different unit operations. See also Absorption; Adsorption; Centrifugation; Chemical engineering; Clarification; Cooling tower; Crystallization; Dehumidifier; Diffusion; Distillation; Drying; Electrostatic precipitator; Evaporation; Filtration; Flotation; Heat exchanger; Humidification; Ion exchange; Kiln; Leaching; Magnetic separation methods; Mechanical separation techniques; Mixing; Pump; Pumping machinery; Sedimentation (industry); Solids pump; Solvent extraction; Transport processes; Unit processes.

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. 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:

1. Fluid flow processes, including fluids transportation, filtration, solids fluidization
2. Heat transfer processes, including evaporation, condensation
3. Mass transfer processes, including gas absorption, distillation, extraction, adsorption, drying
4. Thermodynamic processes, including gas liquefaction, refrigeration
5. 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.

See also

• Distillation Design
• Separation process
• Transport phenomena
• Unit Operations of Chemical Engineering
• Process simulation

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