In chemistry and chemical engineering, a separation process is used to transform a mixture of substances into two or more distinct products. The separated products could differ in chemical properties or some physical property, such as size, or crystal modification or other separation into different components.

Barring a few exceptions, almost every element or compound is found naturally in an impure state such as a mixture of two or more substances. Many times the need to separate it into its individual components arises. Separation applications in the field of chemical engineering are very important. A good example is that of crude oil. Crude oil is a mixture of various hydrocarbons and is valuable in this natural form. Demand is greater, however, for the purified various hydrocarbons such as natural gases, gasoline, diesel, jet fuel, lubricating oils, asphalt,etc.

Separation processes can essentially be termed as mass transfer processes. The classification can be based on the means of separation, mechanical or chemical. The choice of separation depends on the pros and cons of each. Mechanical separations are usually favored if possible due to the lower cost of the operations as compared to chemical separations. Systems that can not be separated by purely mechanical means (e.g. crude oil), chemical separation is the remaining solution. The mixture at hand could exist as a combination of any two or more states: solid-solid, solid-liquid, solid-gas, liquid-liquid, liquid-gas, gas-gas, solid-liquid-gas mixture, etc.

Depending on the raw mixture, various processes can be employed to separate the mixtures. Many times two or more of these processes have to be used in combination to obtain the desired separation. In addition to chemical processes, mechanical processes can also be applied where possible. In the example of crude oil, one upstream distillation operation will feed its two or more product streams into multiple downstream distillation operations to further separate the crude, and so on until final products are purified.

[edit] Various types of separation processes

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• Adsorption.
• Centrifugation and cyclonic separation, separates based on density differences.
• Chromatography separates dissolved substances by different interaction with (i.e., travel through) a material.
• Crystallization.
• Decantation.
• Demister (vapor), removes liquid droplets from gas streams.
• Distillation, used for mixtures of liquids with different boiling points.
• Drying, removes liquid from a solid by vaporisation.
• Electrophoresis, separates organic molecules based on their different interaction with a gel under an electric potential (i.e., different travel).
• Elutriation.
• Evaporation.
• Extraction.
  • Leaching.
  • Liquid-liquid extraction.
  • Solid phase extraction.
• Flotation.
  • Dissolved air flotation, removes suspended solids non-selectively from slurry by bubbles that are generated by air coming out of solution.
  • Froth flotation, recovers valuable, hydrophobic solids by attachment to air bubbles generated by mechanical agitation of an air-slurry mixture, which float, and are recovered.
  • Deinking, separating hydrophobic ink particles from hydrophilic paper pulp in paper recycling.
• Flocculation, separates a solid from a liquid in a colloid, by use of a flocculant, which promotes the solid clumping into flocs.
• Filtration, Mesh, bag and paper filters are used to remove large particulates suspended in fluids (e.g., fly ash) while membrane processes including microfiltration, ultrafiltration, nanofiltration, reverse osmosis, dialysis (biochemistry) utilising synthetic membranes, separates micrometre-sized or smaller species.
• Fractional distillation
• Fractional freezing
• Oil-water separation, gravimetrically separates suspended oil droplets from waste water in oil refineries, petrochemical and chemical plants, natural gas processing plants and similar industries.
• Magnetic separation.
• Precipitation.
• Recrystallization.
• Sedimentation, separates using density differences.
  • Gravity separation.
• Sieving.
• Stripping.
• Sublimation.
• Vapor-liquid separation, separates by gravity, based on the Souders-Brown equation.
• Winnowing.
• Zone refining.

[edit] See also

• Analytical chemistry
• Chemical engineering
• Unit operations