The Influence of Flow Rate on the Ozone Decomposition in the Teflon Pipe

Ozone decomposition in the teflon pipe and its influence on the preciseness of experimental measurements has been studied. The ozone was produced by DC corona discharge fed by carbon dioxide. The ozone concentration has been measured by UV spectrometry method directly in the reactor and indirectly in the cell made of glass connected with the outlet of the reactor. It was found out that the difference strongly depends on the gas floe rate and at low flow rates can be even ore than 50%. Introduction In the experiments where ozone concentration is measured the results can be influenced by measurement technique. As the particular details of the experiments are often not specified in the paper the comparison of results produced by individual experiments can be problematic. In general there are two basic methods for ozone concentration measurement. The direct measurement (DM) is technique when the O3 concentration is measured directly in the discharge gap. The indirect measurement (IM) is technique when it is measured in the cell which inlet is connected with the outlet of the ozone generator tube by pipe. In our experiments we have compared these two methods. In the paper [1] the negative corona discharge in carbon dioxide and its mixtures with oxygen were studied. Apart from other aims of the experiment, the ozone concentration at wide range of gas flow rates (3-300 cm/min) was compared. The ozone concentration was measured by IM and the dimensions of pipe were not specified in the paper. In such experiment the consideration of different ozone decomposition due to different flow rates would be useful. On the other hand there are possibilities for using the IM technique without influence upon results. In paper [2] the influence of IM is insignificant because of the same gas flow rate in individual measurements. So, the results of all the measurements contained the same error and results were not distorted. In another paper [3] the IM had insignificant effect because of very high gas flow rates. There are also other papers in which the authors used the DM technique. We have published such type of paper recently [4]. Despite the fact that the gas flow rate was low, the reliability of measurements is high. It is evident that in some cases the IM technique is reliable solution for ozone concentration measurement (high gas flow rates; short not reactive pipes). But in some experiments it is useful to prefer DM technique despite the fact that it is technically demanding. The aim of this paper is to compare these two methods and partially illustrate which one to choose for which experiment. Experimental apparatus, results and discussion In our experiments the ozone was generated by negative corona discharge fed by pure carbon dioxide. The electrode system in the reactor was composed of coaxial wire and cylinder. The diameters of the electrodes were 1.6 cm and 125 μm. The active length of reactor was 10 cm. Both the electrodes were made of stainless steel. For DM method the reactor was placed directly into the UV spectrometer used for ozone concentration measurement (see Fig. 1.). For IM the reactor was connected with the cell in the spectrometer by 1 m long Teflon pipe (see Fig. 2.). The ozone concentration is always higher in case of DM. This effect is most likely caused by the decomposition of ozone by both heterogeneous (on the walls) and homogeneous (in the volume) processes in the Teflon pipe connecting the reactor and the glass cell placed in the spectrometer used for ozone 7 WDS'08 Proceedings of Contributed Papers, Part II, 7–10, 2008. ISBN 978-80-7378-066-1 © MATFYZPRESS