Is vacuum a liquid?
Flavor protection for thirst quenchers - vacuum degassing removes disruptive components from liquids
Anyone who has ever left a fresh glass of tap water at room temperature or heated a pot with water is familiar with the phenomenon: As the water temperature rises, gas bubbles form on the glass wall or on the bottom of the pot. They mainly contain carbon dioxide and oxygen. The gases get into the water from the air, most of them already in the waterworks. There, the water is aerated again and again during treatment. These two air gases in particular dissolve in the liquid. The British chemist William Henry recognized at the beginning of the 19th century that solubility depends on the partial pressure of the gas and the temperature of the liquid. It increases with temperature, but decreases with decreasing pressure.
Gases affect shelf life and taste
Even small amounts of gas can quickly become disruptive factors in the production of juice, wine, beer and soft drinks. Dissolved oxygen, for example, promotes germ growth and destroys vitamin C and other antioxidant substances. The smell, taste and shelf life of the drinks suffer from this. In addition, it can lead to unsavory discoloration. Dissolved carbon dioxide allows fibers and pieces of fruit to float, which then deposit on the surface. Free gas bubbles also increase foam formation, especially with drinks that have been heated beforehand. In this case, you often have to drastically reduce the filling speed.
The unwanted gases can be elegantly removed from the processed liquids with a vacuum. In the case of spray degassing, for example, the water is fed through spray nozzles into a vacuum container in which the pressure is less than 300 millibars. The gas bubbles expand and burst in the negative pressure of the degassing unit. The released gas is then sucked off by a vacuum pump.
With and without stripping gas
The so-called membrane degassing is mainly used for the production of juice, wine and beer: In a column, the beverage liquid flows around a hollow fiber membrane that only allows gases to pass through. The dissolved gases penetrate the membrane and are sucked off by means of a vacuum pump. In order to achieve the lowest possible residual gas content, a so-called stripping gas can also be used on the gas side. Mostly nitrogen is used for this. In any case, a vacuum is applied to the gas side. The pressure gradient "pulls" the dissolved gases out of the liquid.
For the production of still mineral water, in which a particularly low residual content of oxygen and carbon dioxide is sought, degassing is carried out by connecting several columns in series. There are similar processes in the chemical industry, where undesirable reactions of liquid components have to be avoided.
District heating networks not only need water that has been cleaned of solids and salts, but also degassed and de-aerated water. This is primarily an economic question, because circulating water containing gas increases operating costs: Dissolved carbon dioxide turns the water into an acid that, in particular, causes metal parts to corrode more quickly. To prevent this, ion exchangers are used, which in turn need a constant supply of chemicals for regeneration. If the water in the pipe network is degassed, this measure is superfluous. The procedure also prevents the undesirable effects of oxygen. This also promotes corrosion and also the growth of germs. The latter can lead to the formation of so-called biofilms, which, for example, clog filters or narrow the cross-section of valves and lines.
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