Deaeration

During degassing, gas bubbles, mostly air bubbles, are removed from a liquid, for example bubbles from a gel.  However, a degassing process is also necessary to remove gases in raw liquids, which would subsequently outgas in the finished product and lead to gas bubbles there.

Degassing or outgassing is understood to mean the release of gases present in liquid or solid materials. The processes used for this are, for example, pressure relief or temperature increase. The degassing process often occurs in nature. In industry, for instance, water degassing is used everywhere that product quality and stability are guaranteed by the lowest oxygen values. This is the case, for example, in the chemical and pharmaceutical industry, as well as in the drinks industry.

A well-known and everyday form of degassing of a liquid is demonstrated when a bottle of carbonated mineral water is opened. Mixing processes often make subsequent degassing of the product necessary, as air and gas inclusions (or pockets) are often formed during the processing of liquid to highly viscose substances. These inclusions usually have a negative effect on the product. They can, for example, lead to fats and oils becoming rancid, to oxidisation, to discolouring and changes to the smell and taste of the products.  Moreover, they can cause the surface of coated materials to become porous and perforated. In contrast, degassed products are chemically more stable and have a longer shelf life.

Outgassing is most strongly triggered by the heating of liquids in conjunction with a drop in pressure. It is dependent on the solubility of the gases, which normally decreases with dropping partial pressure and rising temperatures according to Henry’s law. In addition, gases can also be released by natural means or by the formation of pores during the outgassing of foam materials.

Applications

Degassing is mainly used in the production of oxygen-sensitive products and in products on which the presence of bubbles would have a negative influence.  Both requirements apply to cosmetic products, as oxygen reduces the shelf life and the gas bubbles would accelerate coagulation of the dispersed phase. In some applications, such as the production of battery masses/compounds, the presence of oxygen would render the finished product unusable.

There are a series of production processes in which liquids have to be degassed. Oil and lubricants, for example, must be degassed before pumping to minimise the pump wear that is caused by cavitation. Lacquers must be degassed before hardening or before use. Liquid foodstuffs, such as beverages and sauces, are degassed in order to increase their shelf life, as degassing inhibits the growth of aerobic microbes. A further example is shampoo, which should not contain any air and is prone to foaming due to its own tensides and can increase the volume to such an extent that filling is impossible. The Agitation  process is closely connected to the efficiency of the degassing process and should also be considered from this point of view.
Information on some applications in which venting is used can be found here:

• Foam baths
• Shower gels
• Shampoo

Details

During degassing, it is of particular importance that a targeted expansion of the air bubbles under vacuum is used, in order to enable the gas bubbles to rise in the product. Care should be taken here that the vessel is large enough to hold the expanded product mass without allowing the inside of the lid to be wetted or sucked in by the vacuum pump. The agitator may not be used to reduce the size of the air bubbles in this process; it must, however, produce sufficient mixing in the vessel, so that the air bubbles reach the surface more quickly, where they can burst and merge into the gas phase. Without an agitator, this process would be extremely time-consuming and would depend entirely on the rate of ascent of the gas bubbles.
Degassing of liquids is often made more difficult by the fact that the gas bubbles to be removed have been stabilised by the use of surface-active substances. These excipients, which have actually been employed to stabilise the produced emulsion or dispersion, surround the gas bubbles and render them so robust that their degassing represents a special challenge for process technology. This can be solved by conveying-optimised agitators in the vessel. The degassing process can be significantly shortened by means of large annular surfaces, which convey the product to the surface and there effectively support the bursting of the air bubbles, even in the foaming state.

We will be pleased to advise you on the subject of degassing.