Degasification

Degassing means the controlled removal of gases and other volatile substances such as solvents or moisture from liquids and solids. It usually happens as a step in designated degassing. The independent escape of gases other hand, is usually referred to generally as outgassing. If only moisture to be removed, it is called a drying.

Reasons for degassing

In casting resins, concrete, paint, metal and plastic melts and the like air and other gases contained herein may form bubbles, then stay composed as voids after curing or solidification. The viscosity of fluids can be increased by blowing, which is often undesirable.

Often intended by degassing a later outgassing of odorous or harmful substances avoided or unneeded solvents are removed.

The removal of dissolved or entrapped as bubbles prevents substances besides various other negative effects such as:

• Hydrogen embrittlement of steel
• Food spoilage by oxidation
• Hydrolysis of transformer oil from moisture
• Corrosion of steam boilers, piping, etc. by oxygen and carbon dioxide in the liquid
• Failure of the hydraulic systems with increased compressibility (for example, in vehicle brake )

Method for degassing

Vacuum degassing

The most common method for degassing is to be degassed material be exposed to a vacuum. It is similar in principle to the vacuum drying.

According to Henry's law the concentration of a gas is directly proportional to the partial pressure of the gas corresponding to the fluid in a liquid. Evacuating a few mbar causing a sharp drop in the partial pressure, and thus of the gas concentration in the liquid. Because of the temperature dependence of the Henry's constant can be further improved by increasing the temperature at a constant negative pressure, the Entgasungsqualität.

In the form of bubbles trapped gases (e.g., entrapped air) to be removed by the vacuum. According to the law of Boyle -Mariotte applies to the pressure and the volume

The bubbles so puffed up with pressure decrease. According to the Stokes equation for the rise velocity of the bubbles, the relationship

Wherein the bubble radius and the dynamic viscosity of the liquid. The increase in volume so the bubbles rise much more quickly to the surface, where they burst.

It is also clear that a reduction in viscosity, for example, by increasing the temperature can accelerate degassing.

It facilitates and accelerates the degassing when the gas has to travel the shortest possible route to the surface of the liquid. Is advantageous, if the medium is present as a thin layer (so-called Dünnschichtentgasung ).

Process technology, eg in Vergussanlagen degassing of fluids is often done in parallel to a mixing process in vacuum mixers. These often include cone-like internals, over which the material flows to Dünnschichtengasung. Following the same principle, but with continuous throughput, work Durchlaufentgaser.

Also plants for the compounding of thermoplastics, such as twin-screw extruder, equipped with degassing zones evacuated, there to remove low-molecular components such as monomers, oligomers, solvents, air, or of reaction or decomposition products from the polymer melt. For solvent-based plastics is the challenge here in the great amount of escaping gases.

There are also various methods for vacuum degassing molten steel in the secondary metallurgy.

In solids, complex processes occur. Here, between deposited on the surface ( adsorpierten ) particles and into the interior of the solid taken up ( absorbed ) or trapped there ( occluded ) particles must be distinguished. The former can be directly detached from the surface ( desorptieren ), the latter must first diffuse to the surface and then desorptieren, which is extremely time consuming. By increasing the temperature the rate of gas release increases exponentially.

Technically, the degassing process here in heated vacuum chambers or cabinets.

Degassing by ultrasound

Ultrasound is introduced into a liquid, such as a horn, so in her a high-frequency alternating pressure field builds up. By periodically created momentary vacuum voids are formed. This effect is known as cavitation. The cavities are formed primarily of gas inclusions which act as so-called cavitation nuclei, which are weak points in the liquid, where these tears.

The dissolved gas diffuses in the cavitation bubbles in and prevents them fully implode during the subsequent increase of pressure: the bubbles grow with each oscillation operation.

Standing waves formed by reflections, so the bubbles are pushed to their nodes, where they unite (coalescence ), and finally migrate through the buoyancy at the surface.

Also, molten metal can be degassed by this method. Complete but degassing can be achieved only in conjunction with other methods.

Thermal degassing

Because of the temperature dependence of the Henry constant can degassing also be achieved solely by increasing the temperature, such as at the bubbles in the cooking pot before reaching the actual boiling point is clear. The thermal degassing is particularly applied to free the feed water of steam boilers and other hot water system from the corrosion-promoting gases oxygen and carbon dioxide.

Oxygen removal by chemical bonding

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

Oxygen with inert gas elimination

Food can be (mostly nitrogen) stored in an inert atmosphere or washed by her, so that escape existing gases such as oxygen in the atmosphere. It is also possible to have bubbled such a sparingly soluble entrainer as fine bubbles through liquids or melts ( stripping).

Addition of Enlüftungsadditiven

So-called ventilation additives are chemicals. Merging several smaller air bubbles to a large and thus favor the ascent to the surface

Bubbles are stabilized by surfactants in liquid at the air-liquid interface, i.e., those surfactants cause a repulsive effect between the boundary surfaces. A deaerator is such that it is in the liquid rather poorly soluble, and therefore accumulates at the interface and displaces the surfactants. Thus, the repulsive effect is canceled and the bubbles can merge.

Vent are mainly used where a technical degassing is no longer possible, eg by air bubbles from paints or casting resins to remove after application. Removal of dissolved substances is with them but not possible.

The vent can also be improved by lowering the viscosity additives. Defoamers are however not primarily cause degassing, but the bursting of bubbles on the surface in order to prevent foam formation.