Coalescence

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Coalescence (from Latin coalescere , roughly " growing together") describes in an older, interdisciplinary scientific language derived from Latin, the growing together or merging of separately perceptible things or parts.

General

In this most general meaning, coalescence in biological and medical parlance stands for the growing together of organs in contact . In population genetics , one speaks of coalescence when both parents have a common ancestor, which is the case with closed populations after a determinable number of generations. In nuclear magnetic resonance spectroscopy , the elimination of intermolecular couplings in the observable spectrum from a certain temperature is referred to as coalescence. In historical linguistics , coalescence is the process of merging neighboring sounds into one sound.

Separation of dispersions

Schematic representation of the confluence of droplets of an emulsion to form a continuous phase

Coalescence has a specific technical meaning in meteorology , colloid chemistry and process engineering : There the term describes the separation of components of a dispersion through the merging of the finely divided colloids to form a continuous phase . It does this by the confluence of droplets in an aerosol or emulsion, or by agglomeration of particles in a suspension . Gas bubbles in a foam also close together in this way. The interfacial tension , the kinetic energy and the collision rate of the colloids have an influence . The number and size of the colloids increase the likelihood of coalescence, while the viscosity of their medium reduces them. For the formation of precipitation, the coalescence efficiency is higher when drops of different sizes collide. When solids agglomerate like liquid and gaseous substances, we speak of flocculation . In the case of sintering , a distinction is made between the desired coagulation of the solid particles that are intended to combine to form a workpiece and undesired coalescence.

Colloids of light liquids in water or of gases in liquids separate from their dispersion medium when they reach a certain volume due to static buoyancy . Solids with a greater density than the dispersion medium separate from it by sedimentation . A dispersion continually segregates itself through the tendency of its colloids to coalesce, depending to a certain extent on their pH values . The processes have not been clarified in detail , especially with regard to the effect of ions and dipoles .

Examples

Coalescence can be a natural process, such as the formation of rain in a cloud , or it can be accelerated or slowed down in a targeted manner by processes and additives : Foam forms on the surface of soapy water because soap reduces the surface tension of the water, so that the air bubbles dissolve connect slower. Coalescence can be undesirable, as in the case of colloid systems in detergents, foodstuffs or cosmetics, which contain dispersants , surfactants or emulsifiers to prevent them or must be shaken before use, or it can be brought about intentionally, as in the case of polyurethane dispersions as a coating or adhesive or with different separation processes .

For example, coalescence is decisive for the “drying” of dispersion coatings such as dispersion paints and varnishes. After the water has evaporated, the plastic particles flow together and form a film. A film forming below the minimum film formation temperature to allow the polymer, so-called coalescing (film-forming auxiliaries: water-soluble low volatility are solvents ), as for example, Texanol , butyl glycol or dipropylene glycol methyl ether . These swell the polymer particles and temporarily lower their glass transition temperature so that the particles can fuse with one another. After the coalescing agent has evaporated, the original glass transition temperature is restored.

Apparatus

A whole range of apparatus in process engineering make use of coalescence to separate colloids, especially the large group of emulsion splitting plants . There are also the following applications:

Coalescence separator

A coalescence stage is often built into a light liquid separator (oil separator). This stage consists of a container with an inlet and an outlet, which contains an insert made of lipophilic material with a large surface on which the colloids collect by adsorption . For example, this insert consists of V-shaped sheets that are arranged one above the other like roofs. The smallest oil droplets distributed in the water flow through holes in the kinked edge of the sheet metal to form large oil droplets with more buoyancy and can thus be separated due to the difference in density on the surface.

This means that the last 3% of light liquid can be almost completely separated off, while without an adsorption coalescence separator an efficiency of only 97% can be achieved.

Coalescence filter

The term coalescence filter is mainly used for apparatus for cleaning aerosols such as smoke . With a coalescence filter, air or a liquid is passed through a very permeable sand, fiber or wire mesh packing, whereby oil and dirt particles are very likely to hit the filter surface and get stuck due to their adhesive force ( stickiness ). Separated sticky substances like oil can catch non-sticky dirt particles. Examples are the old air filters of car and motorcycle engines with oiled wire mesh inlays (metal knitted fabric) or oil mist separators (demisters) in suction systems of cutting machines with cooling lubricants.

The separation efficiency is often unsatisfactory, which is why other filter principles (surface filters such as the typical cartridge filters with cleaning), pore filters, electrostatic filters (for smoke, etc.) are used or connected downstream for non-sticky fine dust .

literature

Web links

Wiktionary: coalescence  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. ↑ http://de.pons.com/ü Zusammenarbeit / latein- deutsch / coalescere , accessed on May 30, 2017.
  2. ^ Roche Lexicon Medicine, 5th ed., Urban & Fischer, Munich 2003, p. 1016. ISBN 3-437-15156-8
  3. ↑ Heike Bickeböller, Christine Fischer: Introduction to Genetic Epidemiology, Berlin, Springer 2007, p. 102. ISBN 978-3540335689
  4. ↑ Manfred Hesse, Herbert Meier, Bernd Zeeh: Spectroscopic methods in organic chemistry, 7th edition, Thieme, Stuttgart 2005, p. 103. ISBN 978-3135761077
  5. ↑ Helmut Glück (Ed.): Metzler Lexikon Sprache, Metzler, Stuttgart 2000, p. 323. ISBN 978-3476015198
  6. ↑ Bettina Giesen: Modeling and simulation of the synthesis of nanoparticles in the gas phase: Investigations on the interaction of coagulation and coalescence, Cuvillier, Göttingen 2006, pp. 15–19. ISBN 978-3865377265
  7. ↑ Entry on minimum film forming temperature. In: Römpp Online . Georg Thieme Verlag, accessed on September 27, 2014.
  8. ↑ Elmar Brügging: Reactions in light liquid separators , Kassel Univ. Press, Kassel 2014, pp. 33–35. ISBN 978-3862197385
  9. ↑ Klaus Görner, Kurt Hübner (Ed.): Gasreinigung und Luftreinhaltung, Springer VDI, Berlin 2013, p. G-67. ISBN 978-3642563096