|Because of its relatively short half-life, ozone is always generated on-site by an ozone generator. The two main principles of ozone generation are UV-light and corona-discharge. Ozone generation by corona-discharge is most common nowadays and has most advantages. Advantages of the corona-discharge method are greater sustainability of the unit, higher ozone production and higher cost affectivity. |
UV-light can be feasible where production of small amounts of ozone is desired (e.g. laboratories) [3,67]. This chapter will concentrate only on the first mentioned principle.
An ozone production unit with corona-discharge consists of the following parts: oxygen source, dust filters, gas dryers, ozone generators, contacting units and torch destruction .
In the ozone generator, the corona-discharge element is present, which provides a capacitive load. In here ozone is produced from oxygen as a direct result of electrical discharge. This corona-discharge ruptures the stable oxygen molecule and forms two oxygen radicals. These radicals can combine with oxygen molecules to form ozone. To control and maintain the electrical discharge, a di-electric is present, carried out in ceramic or glass. The excessive heat of the electrodes is often cooled by cooling water, or by air (figure 1) .
Figure 1: outline corona-discharge generator
For the production of ozone, ambient air can be used (supplied by a compressor) or pure oxygen (supplied by an oxygen generator, or sometimes by oxygen bottles). To condition this air, air dryers and dust filters are used.
3O2 ⇌ 2O3
Figure 2 illustrates the relation between cooling water temperature and the yield of ozone generation. This figure shows that an increasing cooling water temperature results in a decreasing ozone production [1,5]. To limit the decomposition of ozone, the temperature in the discharge gap should not be higher than 25 °C. The general advise is that cooling water may increase 5 °C to 20 °C maximally. It is important that the temperature of the inlet air is not too high.
Figure 2: influence of water cooling on ozone generation efficiency
Humidity inlet air
Figure 3: influence humidity inlet air on efficiency of ozone production
To prevent these side-reactions, inlet air first passes a drying chamber before ozone is generated. For drying, an aluminum compound can be used, comparable with silica gel. In an ozone generator two or more drying chambers are used alternately. When a drying chamber is used for a certain period of time, humid air is led to the other drying chamber, while the first is regenerated.
Figure 4: influence of hydrocarbons on the generation yield of ozone
Produced amount of ozone versus oxygen concentration of inlet air
Figure 7: influence of oxygen concentration on ozone production at different electrical current