Introduction to Ozone Generation Techniques – Corona, UV and Electrochemistry
Siemens developed the first ozone generator, which was based on corona discharges, in 1957. Today, ozone is produced by several different methods, both commercially and in the laboratory.
Ozone generation involves the intermediate formation of atomic oxygen radicals that can react with molecular oxygen. All processes that can dissociate molecular oxygen into oxygen radicals have the potential to generate ozone. The energy sources that make this action possible are electrons or photon energy. Electrons can be used from high voltage sources in corona discharge, from nuclear sources and from electrolytic processes. Suitable photon quantum energy includes UV light of wavelengths less than 200 nm and γ-rays.
In nature, ozone generation occurs when oxygen molecules react in the presence of electrical discharges, for example lightning, and by the 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 very high voltage.
Corona Discharge Ozone Generation
Corona discharge in an oxygen-containing dry process gas is currently the most widely used method of ozone generation for water treatment. Corona or plasma is created in an ozone generator by applying a high voltage between two electrodes. Ozone is formed by the following reactions:
A 1/2 O2 = O Heat of reaction A= +59.1 Kcal
BO + O2 = O3 Heat of reaction B = -24.6 Kcal
AB 3/2 O2 = O3 Heat of reaction AB= +34.5 Kcal
The global reaction (AB) that produces ozone requires energy and is an endothermic reaction that obtains energy from the electrical discharge. A basic ozone generation system consists of the following: source gas (compressed air or oxygen), gas dryers, and ozone generators.
It is of utmost importance that a dry process gas is applied to the corona discharge. Limiting the formation of nitric acid is also important to protect generators and increase the efficiency of the generation process. In normal operation of properly designed systems, a maximum of 3 to 5 g of nitric acid is obtained per kilogram of ozone produced with the air. If greater amounts of water vapor are present, greater amounts of nitrogen oxides are formed when spark discharges occur. In addition, hydroxyl radicals are formed that combine with oxygen radicals and also with ozone. Both reactions reduce the efficiency of ozone generation. Consequently, the dryness of the process gas is important for good ozone yield. Also, with air, nitrogen oxides can form nitric acid, which can cause corrosion.
Ozone formation by electrical discharge in a process gas is based on corona discharge in air or oxygen. In an ozone generator there are numerous distributed electrical micro-discharges (arc or plasma) by which ozone is effectively generated. It appears that each individual microdischarge lasts only several nanoseconds. The current density ranges between 100 and 1000 Amps/cm2. By using oxygen or enriching the process air with oxygen, the generating capacity of a given ozone generator can be increased by a factor ranging from 1.7 to 2.5 compared to air alone. Whether air or oxygen feed energy is lost as heat, cooling of the process gas is very important. In smaller systems this is often reduced by using ambient air to cool one or both electrodes. In larger systems, cooling is usually done with water, usually at the ground electrode.
Other methods of ozone generation include:
Photochemical ozone generation
The formation of ozone from oxygen exposed to ultraviolet light at 140-190 nm was first reported by Lenard in 1900 and fully evaluated by Goldstein in 1903. It was soon recognized that the active wavelengths for technical generation lay ahead. below 200nm. In view of current mercury-based UV lamp technologies, the 254nm wavelength is transmitted together with the 185nm wavelength, so ozone destruction occurs simultaneously with ozone generation. Furthermore, the relative emission intensity is 5 to 10 times higher at 254nm compared to the 185nm wavelength. Therefore, only a small amount of ozone can be produced.
Attempts to achieve a suitable photostationary state of ozone formation with mercury lamps have failed. The main reason for this failure is that thermal decomposition is concomitant with ozone formation. Except for small-scale uses or synergistic effects, UV photochemical generation of ozone has not found widespread use.
Electrolytic ozone generation
Electrolytic ozone generation is of historical importance because synthetic ozone was first discovered by Schönbein in 1840 through the electrolysis of sulfuric acid. The simplicity of the equipment may make this process attractive to small-scale users or users in remote areas.
Many potential advantages are associated with electrolytic generation, including the use of low-voltage direct current, no feed gas preparation, reduced equipment size, possible generation of ozone in high concentrations, and generation in water, which eliminates contact of ozone with water. processes. Problems and drawbacks of the method include: corrosion and erosion of the electrodes, thermal overload due to anodic overvoltage and high current densities, need for special electrolytes or low conductivity water, and with the in situ generation process, scaling and deposits are form on the electrodes, and the production of free chlorine is inherent in the process when chloride ions are present in the water or electrolyte used.
Radiochemical ozone generation
High-energy oxygen irradiation by radioactive rays can promote the formation of ozone. Even with the favorable thermodynamic performance of the process and the interesting use of waste fission isotopes, the chemoclear ozone generation process has not yet become a major application in water or wastewater treatment due to the complicated process requirements. .