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In chemistry and chemical engineering, a separation process is used to transform a mixture of substances into two or more compositionally-distinct products.

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.

• Adsorption
• Centrifugation and Cyclones - density differences
• Chromatography involves the separation of different dissolved substances as they travel through a material. The dissolved substances are separated based on their interaction with the stationary phase.
• Crystallization
• Decantation
• Demister (Vapor) - removing liquid droplets from gas streams
• Dissolved air flotation - suspended solids are encouraged to float to the top of a fluid by rising air bubbles.
• Distillation - used for mixtures of liquids with different boiling points, or for a solid dissolved in a liquid.
• Drying - removing liquid from a solid by vaporising it
• Electrophoresis Organic molecules, such as protein are placed in a gel. A voltage is applied and the molecules move through the gel because they are charged. The gel restricts the motion so that different proteins will make different amounts of progress in any given time.
• Elutriation
• Evaporation
• Extraction
  • Leaching
  • Liquid-liquid extraction
  • Solid phase extraction
• Flocculation - density differences utilization a flocculant such as soap or detergent
• Fractional freezing
• Filtration. Mesh, bag and paper filters are used to remove large particulates suspended in fluids, eg. fly ash, while membrane processes including microfiltration, ultrafiltration, nanofiltration, reverse osmosis, dialysis (biochemistry) utilising synthetic membranes can separate micrometre-sized or smaller species.
• Oil-water separation - gravimetric separator used to remove suspended oil droplets from wastewaters in oil refineries, petrochemical and chemical plants, natural gas processing plants and similar industries.
• Precipitation
• Recrystallization
• Sedimentation - density differences
  • Gravity separation
• Sieving
• Stripping
• Sublimation
• Vapor-liquid separation - designed by using the Souders-Brown equation.
• Winnowing
• Zone refining

• Analytical chemistry
• Chemical engineering
• Unit operations