Separation (process engineering)

from Wikipedia, the free encyclopedia

A separation process uses the different physical and chemical properties of mixed substances to separate them from one another. The separation of substance mixtures is one of the most important basic procedural operations. Most raw materials and products of chemical reactions are mixtures of substances. These must be separated for further processing.

The waste water treatment and sorting of recyclable materials from light and composite packaging ( yellow sack ) are also among the separation process. In metallurgy, the separation of metals in the form of melt is summarized under the technical term refining .


Separation processes are of great economic importance as they are responsible for 40–90% of the capital employed and the operating costs of industrial production. The separation processes used include washing, extraction, pressing, drying, clarification, evaporation, crystallization and filtration. Several separation processes are often carried out one after the other. Separation processes have different functions:

  • Purification of raw materials and products as well as separation of by-products
  • Recycling of solvents and unreacted reactants ( raw materials)
  • Removal of contaminants from wastewater and air


Heterogeneous mixtures (e.g. of liquid and solid ) can be separated by mechanical separation processes such as filtration or centrifugation. Homogeneous mixtures can be separated by molecular separation processes. Molecular separation processes are either equilibrium-based or rate-controlled. In equilibrium-based separation processes, two immiscible phases of different composition are formed; an example of this is distillation (in distillation the vapor has a different composition than the liquid). Speed-controlled processes are based on different transport speeds of compounds through a medium, examples of which are adsorption, ion exchange or crystallization.

A homogeneous mixture can be separated into two phases by an “ energy separating agent ”, a “ mass separating agent ”, a barrier or external fields. Energy separation agents create a second phase (which differs from the first phase in composition and is immiscible with it); they include the techniques most commonly used in industry. For example, the addition of heat (the release agent) to a liquid (first phase) results in the formation of vapor (second phase). Mass release agents are other chemicals or substances. They absorb or dissolve one of the products selectively, they are either in liquid form (for sorption, extractive distillation or extraction) or as a solid (adsorption or ion exchange). The use of a barrier that retains only one of the two compounds but not the other ( semipermeable membranes ) is less common in their application; external fields are only used in special applications.

Thermal separation processes

Thermal separation processes are all separation processes based on the establishment of a thermodynamic phase equilibrium . There are essentially three different separation processes.

Separation due to the boiling point

  1. In rectification and distillation , an equilibrium is established between a liquid phase and a vapor phase (or gas phase). The separation takes place here by repeated evaporation and condensation. This method works well for components with widely different boiling points , but fails for azeotropic systems . Methods used to separate azeotropic mixtures are for example
  2. During drying , a distribution equilibrium is established between a liquid and a solid and the gas phase. Drying is one of the most common thermal separation processes.
  3. During stripping , substances (e.g. moisture) are transferred from a liquid phase into the gas phase by desorption processes . For this purpose, the liquid phase is brought into contact with a gas using the countercurrent principle (for drying with a dry gas that can absorb water vapor).
  4. Burning with the aim of changing substance properties .

Separation due to freezing point

  1. When freezing out , a substance is deposited from a solution by cooling. This process is used for components with very different freezing points and in some cases for separating azeotropically boiling systems. In process engineering, freezing out is called crystallization because of the transition to a solid . A distinction is made between static (discontinuous) and dynamic (continuous) crystallization.

Separation by sublimation

  1. In the case of sublimation , a solid changes directly from the solid state of aggregation to the gaseous state of aggregation. Then the sublimated substance is deposited on a cooled surface in the solid state of aggregation. If the sublimation vapor pressure of the contamination differs sufficiently from the valuable material to be recovered, this seldom used thermal separation process is effective. The sublimation method is mostly used on a laboratory scale. One example of a scaling up in technically and economically relevant dimensions is freeze-drying .

Separation due to solubility

The solubility is a material property which sometimes is connected to other separation effects.

Mechanical separation processes

The mechanical separation processes use differences in the mechanical properties of the components. These can also be visual or optical reflective features.

  • Picking through targeted removal
  • Sorting by separating the total
  • Separation by blast of air z. B. of coffee beans according to the roast color with "stink beans" with high oil content

Separation due to surface wettability

  • Flotation is a separation process in which fine-grain solid mixtures in an aqueous slurry (suspension) are separated with the help of air bubbles due to the different surface wettability of the particles.

Separation due to density

Differences in density of substance mixtures can be used with the following methods.

Separation due to particle size

Separation due to particle inertia

When using movements, the inertia of the particles can be used for separation.

Separation due to magnetizability

The magnetisability is used for the magnetic material properties .

Separation due to electrical mobility

Separation as a result of chemical reactions

The course of reactions changes the physical properties , which in turn can be used for separation.

The zone melting process for obtaining high-purity single crystals or metals makes use of the fact that impurities in the melt have a more energetically favorable chemical environment (lower chemical potential ) than in the solid body and therefore migrate from the solid body into the melt.


Individual evidence

  1. ^ A b c André B. de Haan, Hans Bosch: Industrial separation processes: fundamentals . De Gruyter, Berlin 2013, ISBN 978-3-11-030669-9 .