Ozone generator
An ozone generator , also called an ozonizer , is a technical aid for producing ozone from atmospheric oxygen or water. Due to the instability of the ozone , it is necessary that the ozone is produced directly at the point of use. It is therefore important from a practical point of view that the ozone generator is optimized as a production technology in such a way that the sufficient amount of gas that is required for ozone treatment can be produced economically and as insusceptible as possible to interference.
Ozone as an important oxidizing agent in many areas of application
Ozone (O 3 ), a gas under standard conditions , is a strong oxidizing agent that develops its oxidizing effect even at room temperature . Due to its strong oxidizing effect, the gas is unstable even at room temperature.
The effect of the ozone arises from the atomic oxygen produced during the decomposition of the molecule , which is itself highly reactive and has an oxidative effect. As so-called “ active oxygen ”, ozone is, as it were , a carrier of this reactive atomic oxygen. The disintegration reaction is as follows:
Due to this decomposition mechanism with a high oxidative effect, ozone is of considerable importance in many applications. The compound described for the first time by Christian Friedrich Schönbein in 1839 was used as a disinfectant in drinking water treatment as early as 1906 . The ozone is also used in sewage treatment plants. In the 1970s, the gas was used for the first time in the USA to disinfect the sewage treatment plant drain.
It is precisely this disinfecting effect on organic compounds , for example bacteria or viruses , that makes ozone treatment an important process. In contrast to chlorine as an alternative oxidizing and disinfecting agent, ozone treatment is above all largely environmentally friendly. The compounds decomposed by the ozone are biodegradable and even the unused ozone after the decomposition reaction decomposes automatically, with only oxygen remaining as a decomposition product. In addition to this application, ozone is also used, for example, in medical ozone therapy , for cleaning room air or in bleaching processes for paper.
Cost-intensive and low ozone yield
As early as 1857, Werner von Siemens developed a process to produce ozone with the help of silent electrical discharge (also called “corona discharge”, comparable to a lightning strike ). To do this, he used an aluminum cylinder and a glass cylinder as electrodes for his ozone generator . The glass cylinder encloses the aluminum cylinder and acts as a dielectric . It also has a coating of conductive material on its outside . When high voltage is applied to the ozone generator, an electric field is created through which oxygen is passed. Ozone is formed. The amount of ozone formed depends on the gas pressure and the voltage level. The higher the voltage, the more ozone is created, the higher the gas pressure, the less ozone is formed. Modern devices of this type of process use several such tube systems one behind the other and continuously cool or dry the process gas for reasons of optimization. This also increases the ozone yield in the ozone generator. If pure oxygen is used, the yield is significantly higher than if air is used. In addition, nitrogen oxides are produced as a by-product when air is used in the ozone generator. On an industrial scale, the voltage is up to 20 kV, and the cooling process also requires a lot of energy . Such a process with this ozone generator is therefore very costly, if the complex technology is also taken into account.
As an alternative to an ozone generator that uses high voltage to generate ozone, it is also possible to use an ozone generator that uses a light source to generate ultraviolet light . This approach is based on the processes in the stratosphere that provide for the earth's ozone layer. Such an ozone generator with UV light uses ambient air to generate ozone without producing nitrogen oxides as a by-product . This makes the ozone generator fundamentally cheaper, as there is no need to dry the air or concentrate the oxygen. It can also be used in humid ambient air without any problems. However, a disadvantage is that this ozone generator produces lower yields of ozone. In addition, it must be ensured that the air flow flowing through is permanently exposed to UV light in order to form ozone. As a result, such an ozone generator can only be used to a limited extent if the air flow or a water flow used flows very quickly through the ozone generator.
Both processes are cost-intensive overall, are quite prone to failure and tend to produce lower yields of ozone.
Plasma and electrolysis
Two other ways of producing ozone in an ozone generator are the use of cold plasma and electrolysis .
The cold plasma is generated in an ozone generator by means of a dielectric barrier discharge. If pure oxygen gas is now passed through this ozone generator, the oxygen molecules are split into atomic oxygen, which can then recombine to form ozone. The amount of ozone produced in this way is comparable to the production with corona discharge, but significantly larger than with the production of ozone in an ozone generator with UV light. An ozone generator of this type is very expensive, however.
An ozone generator that uses electrolysis to generate ozone splits water molecules into H 2 , O 2 and O 3 with the help of an applied voltage . The hydrogen gas produced is removed after the fission process, only oxygen and ozone remain. In contrast to the corona discharge, no nitrogen oxides are generated as a by-product in the ozone generator. In addition, the ozone yield is significantly higher in relation to the energy used. A high surge voltage is usually required for the electrolysis of ozone from water. This can be avoided if a suitable catalyst is selected for the ozone generator, for example lead dioxide or boron-doped diamond. With regard to drinking water treatment , however, the use of lead dioxide in the ozone generator is to be rated negatively. In this case, other catalysts have to be used.
More modern developments
On the basis of an electrochemical cell, ozone generators have been developed in recent years in which oxygen and ozone are obtained from water at the anode . This gas mixture can then be discharged directly, but is usually dissolved directly in the electrolyzed water and used there. Depending on the electrode materials used, increasing amounts of hydrogen are formed at the cathode in relation to ozone. When generating ozone from water, ozone can in the best case be formed according to the following sum equation, whereby in the purely theoretical case three molecules of hydrogen are formed for one ozone molecule:
According to the Fraunhofer Institute for Interfacial Engineering and Biotechnology (IGB), ideally a mixture of approx. 18% ozone and 82% oxygen is formed. The equation can therefore be written approximately as follows:
12 H 2 O → O 3 + 4.5 O 2 + 12 H 2
Hydrogen peroxide is also formed depending on the electrode configuration and current density. This can be done on both the cathode and the anode:
Cathode: 2 H 3 O + + 2 e - -> H 2 O 2 + 2 H 2
Anode: 2 OH - -> H 2 O 2 + 2 e -
Hydrogen peroxide has the advantage that it can also be used in combination with the ozone for water treatment and disinfection. Hydrogen peroxide , in turn, is an oxidizing agent. The use of both oxidizing agents therefore increases efficiency. The advantage of the electrochemical cells lies in their compactness and easy handling. In addition, only hydrogen is generated as a real undesirable by-product in this process. The energy costs related to the generation of ozone are cheaper than with the corona process. In addition to purifying water, this ozone generator can also be used, for example, in room air treatment or in decolorization processes. A use of this type of ozone generator for drinking water treatment usually fails due to the presence of dissolved salts, as z. B. calcium carbonate is deposited on the electrodes, whereby the performance of the cells decreases very quickly. Also today (as of 2018) electrochemical processes are not yet suitable for the economic generation of larger amounts of ozone in the range of 100 g / h and are usually only used in the pharmaceutical sector to prevent the recontamination of ultrapure water.
Individual evidence
- ↑ EH Riesenfeld: The ozone, its formation and use. Ed .: The natural sciences. No. 15, 1927, 777-784 .
- ↑ Langlais B. and Reckhow DAB, Deborah R .: Ozone in Water Treatment - Application and Engineering . Ed .: Fl .: Lewis Publishers. 1991.
- ↑ Paraskeva P. and Graham NJD: Ozonation of Municipal Wastewater Effluents . Ed .: Water Environment Research. 2002, p. 569-580 .
- ↑ a b c d OCEGE - Environmentally friendly disinfection with electrolytic ozone generator - Fraunhofer IGB. Retrieved February 20, 2018 .
- ↑ R.Dehmlow: Oxygen-Ozone Therapies. Methods and practical application. Ed .: Elsevier Urban & Fischer. Munich 2008.
- ↑ Dohan, JM; WJ Masschelein: Photochemical Generation of Ozone: Present State-of-the-Art, Ozone Sci. Closely. 9 (4) . No. 315-334,1987 .
- ↑ Gujral, SS; Nand, P .; Vashist, N .: Ozone Therapy: A Milestone in the Treatment of Ailments. In: Indo Global Journal of Pharmaceutical Sciences . No. 167-173 , 2013.
- ↑ a b c Becker, Michael Friedrich: Electrolytic ozone generation - conception and scientific evaluation of a complete system for surface disinfection in the beverage sector .
- ↑ Foller, Peter C .; Tobias, Charles W .: The Anodic Evolution of Ozone. In: Journal of the Electrochemical Society . 1982, p. 506-515 .