In 1857, von Siemens developed the first industrial ozone generator, which was based on corona discharges. Two
concentrical glass tubes were used; the outer tube was covered externally by a layer of tin, and the inner tube was
covered internally by a layer of tin. Air was circulated through the annular space between the tubes. This technology was
later improved by the addition of circulating cooling fluids along the discharge air or oxygen gap, resulting in lower
generation temperatures and less thermal destruction of the ozone.
The generation of ozone involves the intermediate formation of atomic oxygen radicals which can react with molecular oxygen. All processes that can dissociate molecular oxygen into oxygen radicals are potential ozone generation reactions. Energy sources that make this action possible are electrons or photon quantum energy. Electrons can be used from high-voltage sources in the silent corona discharge, from nuclear sources, and from electrolytic processes. Suitable photon quantum energy includes UV light of wavelengths lower than 200 nm and γ-rays.
In nature, ozone generation occurs when oxygen molecules reacts in the presence of electrical discharges, e.g., lightning, and by action of high energy electromagnetic radiation. Some electrical equipment inadvertently generates levels of ozone that can be easily smelled; this is especially true if there is a spark or a very high voltage.
Ozone Generation by Corona Discharge
Corona discharge in a dry process gas containing oxygen is presently the most widely used method of ozone generation for water treatment. A classical production line is composed of the following units: gas source (compressors or liquefied gas), dust filters, gas dryers, ozone generators, contacting units, and off gas destruction.
It is of utmost importance that a dry process gas is applied to the corona discharge. Limiting nitric acid formation is also important in order to protect the generators and to increase the efficiency of the generation process. In normal operation of properly designed systems, a maximum of 3 to 5 g nitric acid is obtained per kilogram ozone produced with air. If increased amounts of water vapor are present, larger quantities of nitrogen oxides are formed when spark discharges occur. Also, hydroxyl radicals are formed that combine with oxygen radicals and also ozone. Both reactions reduce the ozone generation efficiency. Consequently, the dryness of the process gas is of relevant importance to obtain a yield of ozone. Moreover, with air, nitrogen oxides can form nitric acid, which can cause corrosion. The presence of organic impurities in the feed gas should be avoided, including impurities arising from engine exhaust, and leakages in cooling systems.
The formation of ozone through electrical discharge in a process gas is based on the non homogeneous corona discharge in air or oxygen. There are numerous distributed micro discharges by which the ozone is effectively generated. It appears that each individual micro discharge lasts only several nanoseconds, lasting about 2.5 to 3 times longer in air than in oxygen. The current density ranges between 100 and 1000 A.cm-2. By using oxygen or enriching the process air in oxygen, the generating capacity of a given ozone generator can be increased by a factor ranging form 1.7 to 2.5 versus the production capacity with air, depending on the design parameters (for example, gas discharge gap and current frequency). The nominal design capacity at which operation can be performed on a permanent basis must be considered to be at least 20 to 30 percent. The yield obtained when using an oxygen-enriched process gas is increased with a smaller gas space and an increased electrical current frequency. Since all variations result in energy loss in the form of heat, cooling of the process gas is very important. The most efficient form of cooling is the “both-side” cooling system, which is a system that has cooling on both the high-voltage side and on the ground side. However, in case of accidental breakage of the dielectric, the cooling liquid (for example, water) enters the discharge gap and causes short- circuiting of the entire system. Therefore, cooling only the ground side is the safer design.
The plasma created by the high voltage field between the electrodes (see schematic of an ozone system and photos of an actual system) in an ozone generator is called a corona and is similar to the effect observed in lightning and static discharges. Ozone is formed by the following reactions:
|A||1/2 O2 = O||Heat of Reaction A = +59.1 Kcal|
|B||O + O2 = O3||Heat of Reaction B = -24.6 Kcal|
|AB||3/2 O2 = O3||Heat of Reaction AB = +34.5 Kcal|