degasing

Degassing refers to the controlled removal of gases and other volatile substances such as solvents or moisture from liquids and solids . It usually takes place as a process step in degassing devices provided for this purpose. The independent escape of gases, on the other hand, is usually referred to as outgassing . If moisture is to be removed, one speaks of drying .

Reasons for degassing

Defect due to gas inclusion in a metallic workpiece ( "blow hole" )

Melting in casting resins , concrete , paints , metal and plastic, etc. Ä. Air and other gases contained can form bubbles , which then remain as cavities after hardening or solidification . The viscosity of liquids can also be increased by bubbles, which is often undesirable.

Often, a subsequent outgassing of odorous or harmful substances should be avoided by degassing or solvents that are no longer required should be removed.

The elimination of dissolved substances or substances trapped as bubbles also prevents various other negative effects such as:

Hydrogen embrittlement of steel
Food spoilage due to oxidation

Hydrolysis of transformer oil by moisture

Corrosion of steam boilers , pipelines, etc. due to oxygen and carbon dioxide in the liquid

Failure of hydraulic systems due to increased compressibility (e.g. in vehicle brakes )

Procedure for degassing

Vacuum degassing

mobile vacuum system for processing transformer oil

Vacuum laboratory oven for drying and degassing

The most common method of degassing is to subject the substance to be degassed to a vacuum . In principle, it is similar to vacuum drying .

According to Henry's law , the concentration of a gas in a liquid is directly proportional to the partial pressure of the corresponding gas above the liquid. The evacuation to a few mbar causes a sharp drop in the partial pressure and thus also the gas concentration in the liquid. Because of the temperature dependence of Henry's constant , the degassing quality can be further improved by increasing the temperature while maintaining the negative pressure.

Gases enclosed in the form of bubbles (e.g. air stirred in ) are also removed by the vacuum. According to Boyle-Mariotte's law , pressure and volume apply ${\ displaystyle p}$ ${\ displaystyle V}$

{\ displaystyle p \ cdot V = {\ text {const.}}}

The bubbles inflate when the pressure drops. According to Stokes' equation , the relationship applies to the rate of ascent of the bubbles ${\ displaystyle v}$

{\ displaystyle v \ sim {\ frac {r ^ {2}} {\ eta}}}

,

wherein the bubble radius and the dynamic viscosity of the liquid. Due to the increase in volume, the bubbles rise to the surface much faster and burst there. ${\ displaystyle r}$ ${\ displaystyle {\ eta}}$

It is also clear that a decrease in viscosity, e.g. B. by increasing the temperature, the degassing can accelerate.

It facilitates or accelerates degassing if the gas has to cover the shortest possible path to the surface of the liquid. It is therefore advantageous if the medium is present as a thin layer (so-called thin-layer degassing).

Process engineering z. B. in potting systems , the degassing of fluids often takes place in parallel to a mixing process in vacuum mixers . These often contain cone-like internals through which the material for thin-film degassing flows. Continuous degassers work according to the same principle, but with continuous throughput .

Systems for compounding thermoplastics , such as twin screw extruders , also have evacuated degassing zones in order to remove low molecular weight components such as monomers , oligomers , solvents, air or reaction or decomposition products from the polymer melt . With solvent-based plastics, the challenge here is the large amount of escaping gases.

There are also various methods of vacuum degassing of steel melts in secondary metallurgy .

More complex processes take place with solids. Here must be between deposited on the surface ( a d sorpierten ) particles and incorporated into the interior of the solid body ( a b sorbed ) or enclosed therein ( occluded ) particles can be distinguished. The former can be detached directly from the surface ( desorptieren ), the latter have to get used to the surface diffuse and then desorbed, which takes much more time. The rate of gas release increases exponentially as the temperature rises.

In terms of process technology, degassing takes place here in heated vacuum chambers or cabinets.

Degassing using ultrasound

If ultrasound is introduced into a liquid, e.g. B. via a sonotrode , a high-frequency alternating pressure field builds up in it. The short-term negative pressure that occurs periodically causes cavities to form. This effect is known as cavitation . The cavities arise primarily from gas inclusions , which act as so-called cavitation nuclei, i.e. weak points in the liquid at which it tears.

The dissolved gas diffuses into the cavitation bubbles and prevents them from completely imploding again when the pressure rises: The bubbles grow with each oscillation process.

If standing waves are formed by reflections , the bubbles are pushed to their nodes, where they combine ( coalescence ) and finally migrate to the surface due to the buoyancy.

Metal melts can also be degassed using this process. However, complete degassing can only be achieved in conjunction with other methods.

Thermal degassing

Due to the temperature dependence of Henry's constant, degassing can also be achieved simply by increasing the temperature, as is evident, for example, from the formation of bubbles in the saucepan before the actual boiling point is reached. Thermal degassing is used in particular to remove the corrosive gases oxygen and carbon dioxide from the feed water of steam boilers and other hot water systems.

Removal of oxygen through chemical bonding

Oxygen can be removed by chemical bonding with suitable reducing agents . In the food industry, oxygen absorbers in packaging or suitable enzymes are added directly to the food.

Elimination of oxygen with inert gas

Food can in an inert gas (usually atmospheric nitrogen are) stored or washed from her, so that existing gases such as oxygen leak into the atmosphere. It is also possible to let such a sparingly soluble entrainer bubble through liquids or melts as fine bubbles ( stripping ).

Addition of deaeration additives

Stabilization of bubbles by surfactants in aqueous solution. The hydrophilic red ends repel each other.

So-called ventilation additives are chemicals that allow several smaller air bubbles to merge into one large one and thus promote the ascent to the surface.

Air bubbles are stabilized in liquids by surfactants at the air-liquid interface, i.e. H. these surfactants have a repellent effect between the interfaces. A deaerator is designed in such a way that it is rather poorly soluble in the liquid and therefore collects at the interface and displaces the surfactants. This removes the repulsive effect and allows the bubbles to fuse.

Ventilators are mainly used where technical degassing is no longer possible, e.g. B. to remove air bubbles from paints or casting resins after application . However, they cannot be used to remove dissolved substances.

The ventilation can also be improved with viscosity-reducing additives. Defoamers , on the other hand, should not primarily cause degassing, but rather the bursting of the bubbles on the surface in order to prevent foam formation .

Web links

Animation of the ultrasound process

Individual evidence

↑ ^a ^b ^c Karl Jousten u. a .: Wutz manual vacuum technology . Ed .: Karl Jousten. 10th edition. Vieweg + Teubner, Wiesbaden 2010, ISBN 978-3-8348-0695-6 , chap. 6.1. Sorption phenomena and their meaning - terms and terminology, p. 202–204 ( limited preview in Google Book search).
↑ ^a ^b ^c ^d Frank Lechner: Degassing of polymer melts with co-rotating twin-screw extruders . In: Klemens Kohlgrüber (ed.): The co-rotating twin screw extruder . Hanser Verlag, Munich 2007, ISBN 978-3-446-41252-1 , p. 191–212 ( limited preview in Google Book search).
↑ ^a ^b ^c Gottfried Wilhelm Ehrenstein: fiber composite plastics . Materials - processing - properties. 2nd Edition. Hanser Verlag, Munich 2006, ISBN 978-3-446-22716-3 , chap. 5 processing, p. 189 ( limited preview in Google Book search).
↑ Uwe J. Möller, Jamil Nassar: Lubricants in operation . 2nd Edition. Springer, Berlin / Heidelberg / New York 2002, ISBN 3-540-41909-8 , chap. 3.15.3.2 Finely distributed air in the lubricating oil, p. 205–208 ( limited preview in Google Book search).
↑ ^a ^b ^c ^d Norbert Buchner: Packaging of food . Springer, Berlin / Heidelberg / New York 1999, ISBN 3-540-64920-4 , chap. 4.311 Degassing food before packaging, p. 188 f . ( limited preview in Google Book search).
↑ ^a ^b ^c Thomas Brock, Michael Groteklaes, Peter Mischke: Textbook of paint technology . Ed .: Ulrich Zorll. 2nd Edition. Vincentz Verlag, Hannover 2000, ISBN 978-3-87870-569-7 , chap. 2.4.2.1. Defoamers and deaerators, p. 169 f . ( limited preview in Google Book search).
↑ ^a ^b Bodo Müller: Additive compact . Vincentz Verlag, Hannover 2009, ISBN 978-3-86630-915-9 , chap. 3.3 Ventilation of powder coatings, p. 57 f . ( limited preview in Google Book search).
↑ ^a ^b Chr. Edelmann: gas delivery . In: Manfred von Ardenne u. a. (Ed.): Effects of physics and their applications . 3. Edition. Harri Deutsch Verlag, Frankfurt am Main 2005, ISBN 978-3-8171-1682-9 , pp. 320 f . ( limited preview in Google Book search).
↑ ^a ^b ^c H. Kutruff: Acoustic Cavitation . In: Manfred von Ardenne u. a. (Ed.): Effects of physics and their applications . 3. Edition. Harri Deutsch Verlag, Frankfurt am Main 2005, ISBN 978-3-8171-1682-9 , pp. 927–930 ( limited preview in Google Book search).
↑ ^a ^b ^c Ludwig Bergmann , Clemens Schaefer : Mechanics - Acoustics - Heat theory (= textbook of experimental physics . Volume 1 ). Walter de Gruyter, Berlin 1945, ISBN 978-3-11-151095-8 , IX. Cape. Acoustics, p. 434 ( limited preview in Google Book search).
↑ Andreas Freund: Experimental investigation and design of mini-structured evaporators of different designs . Logos Verlag, Berlin 2010, ISBN 978-3-8325-2664-1 , Appendix A: Degassing of the working medium, p. 137–139 ( limited preview in Google Book search).

[wutz-1] Karl Jousten u. a .: Wutz manual vacuum technology . Ed .: Karl Jousten. 10th edition. Vieweg + Teubner, Wiesbaden 2010, ISBN 978-3-8348-0695-6 , chap. 6.1. Sorption phenomena and their meaning - terms and terminology, p. 202–204 ( limited preview in Google Book search).

[doppelschnecke-2] Frank Lechner: Degassing of polymer melts with co-rotating twin-screw extruders . In: Klemens Kohlgrüber (ed.): The co-rotating twin screw extruder . Hanser Verlag, Munich 2007, ISBN 978-3-446-41252-1 , p. 191–212 ( limited preview in Google Book search).

[faser-3] Gottfried Wilhelm Ehrenstein: fiber composite plastics . Materials - processing - properties. 2nd Edition. Hanser Verlag, Munich 2006, ISBN 978-3-446-22716-3 , chap. 5 processing, p. 189 ( limited preview in Google Book search).

[schmier-4] Uwe J. Möller, Jamil Nassar: Lubricants in operation . 2nd Edition. Springer, Berlin / Heidelberg / New York 2002, ISBN 3-540-41909-8 , chap. 3.15.3.2 Finely distributed air in the lubricating oil, p. 205–208 ( limited preview in Google Book search).

[lebensmittel-5] Norbert Buchner: Packaging of food . Springer, Berlin / Heidelberg / New York 1999, ISBN 3-540-64920-4 , chap. 4.311 Degassing food before packaging, p. 188 f . ( limited preview in Google Book search).

[lack-6] Thomas Brock, Michael Groteklaes, Peter Mischke: Textbook of paint technology . Ed .: Ulrich Zorll. 2nd Edition. Vincentz Verlag, Hannover 2000, ISBN 978-3-87870-569-7 , chap. 2.4.2.1. Defoamers and deaerators, p. 169 f . ( limited preview in Google Book search).

[additive-7] Bodo Müller: Additive compact . Vincentz Verlag, Hannover 2009, ISBN 978-3-86630-915-9 , chap. 3.3 Ventilation of powder coatings, p. 57 f . ( limited preview in Google Book search).

[effekte_gas-8] Chr. Edelmann: gas delivery . In: Manfred von Ardenne u. a. (Ed.): Effects of physics and their applications . 3. Edition. Harri Deutsch Verlag, Frankfurt am Main 2005, ISBN 978-3-8171-1682-9 , pp. 320 f . ( limited preview in Google Book search).

[effekte_kav-9] H. Kutruff: Acoustic Cavitation . In: Manfred von Ardenne u. a. (Ed.): Effects of physics and their applications . 3. Edition. Harri Deutsch Verlag, Frankfurt am Main 2005, ISBN 978-3-8171-1682-9 , pp. 927–930 ( limited preview in Google Book search).

[berg-10] Ludwig Bergmann , Clemens Schaefer : Mechanics - Acoustics - Heat theory (= textbook of experimental physics . Volume 1 ). Walter de Gruyter, Berlin 1945, ISBN 978-3-11-151095-8 , IX. Cape. Acoustics, p. 434 ( limited preview in Google Book search).

[verdampfer-11] Andreas Freund: Experimental investigation and design of mini-structured evaporators of different designs . Logos Verlag, Berlin 2010, ISBN 978-3-8325-2664-1 , Appendix A: Degassing of the working medium, p. 137–139 ( limited preview in Google Book search).