OPERATION CONDITIONS IN AN OZONIZER CONTAINING A WIRE PACKING

Transcription

1 OPERATION CONDITIONS IN AN OZONIZER CONTAINING A WIRE PACKING Janusz Ozonek a, U]\V]WRI-DKRáNRZVNL b, Iwo Pollo c,a Satiko Okazaki d a) Department of Chemical Technology, Lublin Technical University b) Department of Electrical Machines, Lublin Technical University c) Private University of Environmental Protection, Radom, Poland d) Sophia University, Kioi-cho 7-l,Chiyoda-ku, Tokyo , Japan Abstract. Investigations into an ozone generation process in an ozonizer containing a copper wire packing in the discharge zone are presented. Electrical characteristics of such discharges have been analyzed and energy dependences that are characteristic for the ozone synthesis process in an ozonizer with a conducting packing have been determined. 1. INTRODUCTION An increasing number of ozone applications, especially within the environment protection technologies (water treatment, sewage treatment, preventing the emission of noxious substances into the atmosphere) causes a significant growth of interest in the possibility of designing an ozone synthesis apparatus that could ensure an increase of an ozone production yield and an energy efficiency enhancement of within the ozone synthesis process. Among a number of propositions elaborated in various research centers attempts to apply so called "mixed" discharges seem to be dominating and these are for example surface or silent discharges as well as electrical discharges coupled with other methods of creating advantageous conditions of ozone synthesis like the application of ultraviolet radiation. Some research teams have attempted to introduce a solid packing to the discharge zone that performs a role of a catalyst. Both nonconducting [1] and conducting kinds of a packing [2] have been tried and a one first applied by S. Okazaki in 1988 presently meets a growing interest. The presented work is based on experiments on the course of ozone synthesis in the mentioned kind of an apparatus [3,4]. Particular attention has been paid to the obtained ozone concentrations, and energy efficiency of the process. Properties of the experimental apparatus with a wire packing designed by the authors have been analyzed in the course of the experiments. 2. EXPERIMENTAL SET-UP A diagram of the experimental setup is shown in the Fig. 1. The electrical feeding system has been adapted to feed an ozonizer with a voltage of up to 15 kv and frequency of 50 Hz. The electrical measurement system is composed of a kilovoltmeter of the C-96 type, a measuring probe Tetronix P6015A, an oscilloscope Tetronix model TDS3012B, and a meter to measure a current intensity. The analytical system has been set to a continuous measurement of ozone concentration by means of a photometric method (at λ= nm) with the application of an ozone analizer BMT 964.

2 R[\JHQ 'U\LQJ FROXPQ )ORZ FRQWURO F 2 220V AC 3RZHUVRXUFH 0HDVXULQJDQG FRQWUROV\VWHP P$ N9 & Fig. 1. The experimental setup 1- oscilloscope, 2- voltage divider, 3- ozonizer, 4- ozone concentration measurement Tests performed to assess a conducting packing effect on the ozone synthesis process efficiency have been done with the application of an ozonizer whose diagram is presented in Fig.2. An essential element of the ozone generator is a packing made of a copper wire coil of the 1.2 mm diameter placed in the discharge zone. The tests have been performed with an ozonizer of the discharge gap size of d=6.0 mm. In the course of the performed tests feeding voltage of the ozonizer and a current passing through the discharge gap have been measured. Active power supplied to the discharge zone has been determined from the surface area of an adequate Lissajous figure whose area is proportional to the active power absorbed by the ozonizer. The gas flow has been varied within the range of Ndm 3 /h. Measurements have been carried out at the temperature of the electrode liquid coolant of 18 o C. R[\JHQ H[WHUQDOFRROHG HOHFWURGH R]RQHR[\JHQ +9 FRSSHUZLUH ZDWHU ZDWHU Fig. 2. An ozonizer with a wire packing

3 3. RESULTS AND DISCUSSION The knowledge of electrical characteristics of an ozonizer makes possible to evaluate it as an electrical energy receiver that converts electrical energy to the energy of ozone synthesis. A feeding voltage on the value of a current passing through the discharge zone as a function of a feeding voltage is presented in the Fig. 3. The measured total volume of the ozonizer with a wire packing is given there as well. The value of a total volume calculated for an ozonizer without a packing and with a discharge gap of d=6.0 mm is C c =36.87pF. The presence of a packing brings about an increase of the ozonizer volume by almost 5 times and according to the Manley formula at constant values of an ozonizer feeding voltage and of a dielectric layer volume active power supplied to the discharge zone is bigger. 2,50 I U [ma] 2,00 1,50 1,00 C c =0.1794*10-9 F 0,50 0,00 0,00 2,00 4,00 6,00 8,00 10,00 12,00 U [kv] Fig. 3. Feeding voltage vs. current passing through an ozonizer with a wire packing depending on the discharge gap size (f=50hz, substrate gas - oxygen.) An example oscillogram obtained in the course of taking measurements for a discharge gap of 6.0 mm and a feeding voltage of U= 9 kv is presented in the Fig. 4. It follows from an analysis of a dynamic charge-voltage characteristics that the forming discharges are of a mixed character i.e. they exhibit properties of a silent discharge as well as of a corona discharge. The obtained Lissajous curves are of a different character than those obtained in the case of ac corona discharges [5] and of classical silent discharges which proves a different operation mode of the presented apparatus. Q [nc] 2,5 µc U [kv] 2 kv Fig. 4. Lissajous curve for an ozonizer with a conducting packing substrate gas - oxygen, F v =90Ndm 3 /h, f= 50Hz Fig, 5 presents preliminary results illustrating changes in ozone concentration as a function of

4 energy density. F 2 >J2 ( Y >:KO@ Fig. 5. Ozone concentration vs. energy density for an ozonizer with a wire packing The introduction of a conducting packing to a discharge gap because of the developed surface should also influence the course of physical and chemical phenomena within the ozone synthesis process. Electrical characteristics, both static and dynamic ones, of ozonizers with and without a packing don not differ much, experimental data however indicate an active role a packing performs in the process [6, 7, 8]. In ozone generators containing a metal packing their discharge gap size does not influence the obtained ozone concentration so much as it is in the case of no-packing ozonizers. In traditional ozonizers that operate with a small discharge gap its size is limited by technical conditions. A conducting packing causes a formation of a strongly asymmetric electrical field and advantageous conditions for the development of streamers not only in the vicinity of the inner electrode but over the whole discharge area. Corona discharges initiate a process of a low-temperature plasma formation and an initial gas ionization while the very ozone synthesis process proceeds in a silent discharge. In tubular ozonizers of the Siemens type small discharge gaps of the mm order are applied for to obtain high ozone concentrations at comparatively low energy consumption [9]. A discharge zone volume is of no decisive effect on the obtained ozone concentrations as it is in the case of typical ozonizers where along with the gap size growth at constant electric feeding conditions ozone concentration dramatically decreases. In such a setup hydrodynamic conditions are strongly disturbed and the course of a heat exchange between gas in the gap and the cooled electrode is more advantageous. 4. CONCLUSIONS The obtained results of testing an ozonizer with a conducting packing in its discharge zone indicate that the application of such packing can intensify the ozone synthesis process. The following factors contribute to the mentioned effect: a character of electrical discharges that form in a discharge gap; a change in hydrodynamic conditions (resulting from a linear velocity increase and disturbed gas flow) that brings about acceleration of heat transfer to the cooled electrode by means of a forced convection; a greater thermal conduction in a packing than of in a substrate gas which makes a heat transfer to the cooled electrode easier. The obtained results show how essential is the effect of a conducting packing placed in a discharge

5 gap on the ozone synthesis process course and its energy efficiency and that it can make a good prospective design solution for new types of industrial ozonizers. ACKNOWLEDGENTS The presented research work has been sponsored by the national Committee for Scientific Research in Poland within the project No 7T09B09321 REFERENCES [1] Schmidt-Szaáowski K., Kowalczyk K., Jodzis S.; Application of silica as a catalyst for ozone synthesis; Polish J. Appl. Chem., 1993, 37, [2] Okazaki S., Sigimitsu H., Niwa H., Kogoma M., Moriwaki T., Inomata T., Ozone formation from the reaction of O 2 -activated N 2 molecules and a new type of ozone generator with fine wire electrode, Ozone Sci. Eng 1988, 10, [3] Okazaki S. Ando A., Kogoma M.: A new type ozonizer with fine metal wire electrode, Proceedings of the III International Scientific Symposium: Ozone Synthesis, Properties and Applications, Wydawnictwo Lubelskie, Lublin 2000, [4] Okazaki S. Ando A., Jahoákowski K., Ozonek J., Pollo I.: Effect of conducting filling on energy efficiency of ozone synthesis, Proceedings of the III International Scientific Symposium: Ozone Synthesis, Properties and Applications, Wydawnictwo Lubelskie, Lublin 2000, [5] Rz czy ska M., Pollo I., Szyma ski W., Ozone synthesis in all-metal generator for semi-corona discharges with a polyester dielectric, Plasma Chemistry VI, Wydawnictwo Politechniki Lubelskiej, Lublin 1997, [6] Samoilovich V.G., Gibalov V.I., Kozlov K.V.: Fizicaskaja chimia bariernogo razriada. Izd. Mosk. Univ., Moskva [7] Quederni A., Limvorapitiuk Q., Bes R., Mora J.C., Ozone decomposition on glass and silica, Ozone Sci. Eng., 1996, 18, [8] Schmidt-SzaáowskiK., Borucka A., Catalytic activity of silica in ozone formation in electrical discharges, Plasma Chem. Plasma Proces., 1990, 10, 443 [9] Tabata Y., Kuzumoto M., Development of a very narrow discharge gap ozone generator with 1 kg/hour production capacity, Proc. of 13th Ozone World Congress, Kyoto (Japan) 1997, vol. 2,