Method for the generation of ozone

Abstract: There is provided a highly efficient and compact ozone generating apparatus in which a very short air gap (5) of about 0.2 mm is formed at high accuracy. Non-discharge portions are dispersed and disposed to cover an entire discharge space, or a spacer (61) is provided to form the non-discharge portion. Further, an elastic body is mounted on a back face of an electrode, thereby enhancing an air gap accuracy of the discharge space.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an ozone generating method, and more particularly to an ozone generating method which can generate high concentration ozone at high efficiency. Description of the Related Art Fig. 1A is a sectional view showing a conventional ozone generating apparatus of a so-called Otto-Plate type which is disclosed in, for example, Ozonizer Handbook, Ozonizer Ad Hoc Committee in the Institute of Electrical Engineers of Japan, (Corona Publishing Co., Ltd., 1960) p.249, and Fig. 1B is a front view of a left half of the ozone generating apparatus. In the drawings, reference numeral 1 means a power source, 2 is grounded metallic electrodes, and 3 is high-voltage electrodes opposed to the grounded electrodes 2 and connected to the power source 1, and high voltage is applied to the high-voltage electrodes 3. Further, reference numeral 4 means dielectrics (glass plates) mounted on surfaces of the grounded electrodes 2 and the high-voltage electrodes 3, 5 is a discharge space in which discharge is generated, and 6 is electrical insulating (dielectric) spacers to form the discharge spaces 5. Reference numerals 7 and 8 mean arrows respectively showing a gas supply port and gas exhaust ports, and 9 is an exhaust pipe to exhaust an ozonized gas. Fig. 2A is a sectional view showing another ozone generating apparatus of a so-called Lowther Plate type disclosed in, for example, S. D. Razumovskii et al., Ozone and its reactions with organic compounds, ELSEVIER, (1984), and Fig. 2B is a sectional view taken along line B-B of Fig. 2A. In the drawings, the same reference numerals are used for component parts having the same functions as those in Figs. 1A and 1B, and descriptions thereof are omitted. Reference numeral 41 means ceramic layers applied onto the grounded electrodes 2 and 3, having the same function as that of the glass plates 4. A description will now be given of the operation. In the conventional ozone generating apparatus, a.gas exhausting hole is provided in the grounded electrode 2, the high-voltage electrode 3, and the dielectric plate 4 at their intermediate portions. In the above publication disclosing the Otto-Plate type of apparatus, no description is given of the spacer 6. However, in order to ensure an interval (an air gap length) between the dielectrics 4, 4, the electrical insulating spacer is mounted around the discharge space 5 in reality in such a way that the spacer does not interfere with a gas inflow. An oxygen-containing raw gas is introduced in a direction of the arrow 7 from an entire circumference of a peripheral portion of the ozone generating apparatus. Then, oxygen is partially turned into ozone when the gas passes through the discharge space 5 in which the discharge is caused by high voltage supplied from the power source 1. As a result, the ozone-containing gas is taken out as an ozonized gas in a direction of the arrow 8 through the gas exhausting pipe 9 mounted at the intermediate portion. In the discharge spaces 5, heat is generated due to the discharge. Consequently, if the gas passing through the discharge space 5 is not effectively cooled, a gas temperature in the discharge space 5 is increased, and an amount of ozone generation is reduced. Hence, the grounded electrodes 2 and the high-voltage electrodes 3 are cooled by electrical insulating liquid such as insulating oil, thereby reducing a rise of the gas temperature. The ozone generating apparatus in Figs. 2A and 2B has the same basic structure as that of the ozone generating apparatus shown in Figs. 1A and 1B. In these two ozone generating apparatus the gas supply port and the gas exhaust port are separately mounted, and the gas flows in a direction shown in the drawings. Further, in the ozone generating apparatus shown in Figs. 1A and 1B, the electrical insulating spacers 6 (made of, for example, silicone) are illustrated. The spacers 6 can ensure the interval (the air gap length) between the electrodes 2 and 3, and are used as sealing members to prevent gas leakage from the discharge spaces. A description will now be given of a characteristic of the conventional ozone generating apparatus with reference to Figs. 3 to 6. In the drawings, reference numeral Q N means a flow rate of the raw gas (converted according to Standard Temperature and Pressure [STP]), W is discharge power, C 03 is an ozone concentration (converted according to STP) at the gas exhausting port of a discharge portion, T w is a temperature of cooling water, d is a discharge gap length, S is a discharge area between the electrodes 2 and 3, and η is ozone yield. W/Q N means discharge power consumption per gas molecule, and serves as an important parameter of an ozone generation characteristic. W/S means discharge power (power density) per unit area of the discharge space between the electrodes 2 and 3, and serves as a parameter reflecting the gas temperature. The ozone yield η means the amount of ozone generation per unit discharge power, and can be expressed as η = C 03 /(W/Q N ). In view of performances (about a compact size and an efficiency) of the ozone generating apparatus, η and W/S are preferably set to larger values, and C O3 is also preferably set to a larger value. Fig. 3 is a diagram showing a relationship between power consumption per molecule W/Q N and the ozone concentration C O3 when the power density W/S and the discharge gap length d are kept constant, and the temperature of cooling water is varied. As set forth above, the power consumption per molecule W/Q N serves as a basic parameter related to ozone generation, and the ozone yield η is more reduced as the power consumption W/Q N is more increased (in the drawing, the straight lines can be described when the ozone yield η is constant, and the ozone yield η becomes larger in the upper line). Further, the temperature T w of cooling water does not have a great effect when the power consumption W/Q N is small. However, when the power consumption W/Q N becomes larger, the ozone concentration C O3 (and the ozone yield η) becomes larger as the temperature T w of cooling water becomes lower. That is, for higher concentration ozone, it is important to set a low temperature of cooling water and keep a low gas temperature. Fig. 4 shows a relationship between the power consumption W/Q N and the ozone concentration C 03 when the temperature T w of cooling water and the discharge gap length d are kept constant, and the power density W/S is varied. It can be understood that an increase in the power density W/S results in the same effect as that obtained by an increase in the temperature T w of cooling water in Fig. 3. This is because the increase in the power density W/S and the increase in the temperature T w of cooling water can have the same effect on an increase in the gas temperature in the discharge space 5. Fig. 5 shows the ozone concentration C O3 with respect to the power consumption W/Q N when the temperature T w of cooling water and the power density W/S are kept constant, and the discharge gap length d is varied in the range from 0,8 to 1,6 mm. An increase in the discharge gap length d can provide an effect which is very similar to the effect obtained by the increase in the temperature T w of cooling water. Here, when an average gas temperature  av in the discharge space is defined as the following expression (1), another average gas temperature in the discharge space of the ozone generating apparatus in which the only single side of the electrodes is cooled can be represented by the expression (2). Further, the expression (3) can be held when both sides of the electrodes are cooled. where x denotes a distance in an air gap direction, d is the discharge gap length, (x) is the gas temperature in case of the distance x, ka is coefficient of heat transfer of the gas, and T w is the temperature of cooling water. From the expressions (1) to (3), though different coefficients are required according to methods of cooling the electrode, it can be seen that the average gas temperature  av is proportional to the discharge power density W/S and the air gap length d. That is, when the air gap length d is set to a small value, the average gas temperature  av can be limited to a lower value even if constant power is supplied, resulting in the high concentration ozone as in the case of d = 0,8 mm in Fig. 5. However, when the set air gap length d is extremely short, and a plurality of ozone generating units are provided to have a multi-stage structure, a larger variation is generated in the air gap length d of the discharge space of the ozone generating units. Therefore, another variation is caused in the flow rates Q N of gases passing through the discharge spaces, thereby causing still another variation in the discharge power W supplied into the discharge spaces. As a result, the equivalent power consumption W/Q N is increased so that an ozone generating efficiency is reduced as shown in Figs. 3 to 5. Further, as shown in Fig. 6, it is known that an excessively small air gap length d lowers an ozone exciting efficiency itself. Fig. 6 is a diagram illustrated in Czech, J. Phys., B38, (1988), Fig. 7, p.648, in which the transverse axis defines the air gap length, and the ordinate axis defines the ozone generating efficiency. Further, the symbols O and + respectively represent the results of two cases, that is, one case where air is used as the raw gas and the other case where oxygen is used as the raw gas. The article teaches that the optimal air gap length for the ozone generation is in an approximate range of 0,8 to 1,1 mm (see the first line on page 645). In particular, it is emphasized that a narrow air gap of 0,6 mm or less reduces the ozone exciting efficiency. Hence, in the conventional ozone generating apparatus, the air gap length is set in the range of 0,8 to 1,5 mm, and a thermal problem is avoided by operation with the power density W/S in a low range. That is, the apparatus is designed to have a large form and have a large discharge area, thereby improving the ozone generating efficiency. US-A-4 213 838 discloses an ozone generating apparatus with two mutually opposed electrodes, each coated with a dielectric material, wherein the electrodes are spaced apart at a distance of between 0,01 and 0,25 inches. US-A-4 049 707 discloses an ozone generating apparatus with two opposed electrodes, wherein one electrode is coated with a dielectric material. This document does not disclose specific pressure conditions. JP-A-55075905 also discloses an ozone generating apparatus. It further discloses the use of a gap length between 2,25 mm and 5.00 mm on a value of p·d of 2,34·10 4 - 3,12·10 4 Pa·cm (175,5-234,0 Torr·cm). The conventional ozone generating apparatus is provided as set forth above. As a result, there are problems in that, for example, a low gas temperature in the discharge space should be held, and for this purpose, the power density W/S should be reduced by providing the large ozone generating apparatus and the large discharge area S. SUMMARY OF THE INVENTION In order to overcome the problems as described above, it is the object of the present invention to provide an ozone generating in which a, very short air gap length of about 0,6 mm or less is used at high efficiency, a high cooling performance of a gas can be provided, a simple structure and a compact shape can be realized, and high concentration ozone can be obtained. It is also the object of the present invention to provide a large-capacity ozone generating method having a good ozone generating efficiency. According to the present invention, there is provided an ozone generating method in which a discharge gap length is set to a value of 0,6 mm or less, and the product pd of the discharge gap length d and a gas pressure p is set to a value of 1,6·10 4 Pa·cm (120 Torr·cm) or more. As stated above, in the ozone generating method according to the present invention, the discharge gap length d is set to the value of 0,6 mm or less, and the product pd of the air gap length d of the discharge space and the gas pressure p is set to the value of 1,6·10 4 Pa·cm (120 Torr·cm) or more. Consequently, it is possible to reduce production of nitrogen oxide NO x even when the discharge gap length is short, and realize highly efficient ozone generation event when a raw gas having a high nitrogen partial pressure is employed. The invention will more fully appear from the following detailed description when the same is read in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are for purpose of illustration only and are not intended as a definition of the limits of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1A and 1B are a sectional view and a front view showing a conventional ozone generating apparatus; Fig. 2A and 2B are a sectional view and a front view showing another conventional ozone generating apparatus; Fig. 3 is a graph diagram showing one example of an ozone generation characteristic in the conventional ozone generating apparatus; Fig. 4 is a graph diagram showing another example of the ozone generation characteristic in the conventional ozone generating apparatus; Fig. 5 is a graph diagram showing still another example of the ozone generation characteristic in the conventional ozone generating apparatus; Fig. 6 is a graph diagram showing one example of an ozone generating efficiency with respect to a discharge gap length; Fig. 7 is a sectional view showing a first embodiment for use in the present invention; Fig. 8 is a plan view showing a dielectric electrode in the first embodiment; Fig. 9 is a plan view showing a stress buffer Plate in the first embodiment; Fig. 10 is a graph diagram showing ozone generating characteristics in case air having an oxygen to nitrogen ratio of 1:4 is used as a raw gas; Fig. 11 is a graph diagram showing the result of Fig. 10 as the product pd of the discharge gap length d and the gas pressure p; Fig. 12 is a sectional view showing a second embodiment for use in the present invention; Fig. 13 is a sectional view showing a third embodiment for use in the present invention; and Figs. 14A and 14B are waveform diagrams showing waveforms of voltage applied to electrodes in the third embodiment in Fig. 11. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1 One preferred embodiment of the invention will now be described referring to the accompanying drawings. Fig. 7 is a sectional view showing a first embodiment for use in the present invention, in which the same reference numerals are used for component parts identical with those in the prior art shown in Figs. 1A and 1B, and descriptions thereof are omitted. In Fig. 7, reference numeral 11 means power supply plates connected to a power source 1 through a fuse 12, and 31 is conduction layers (electrodes) in electrical contact with the power supply plates 11. The conduction layer 31 corresponds to a high-voltage electrode 3 in the prior art shown in Figs. 1A and 1B. Reference numeral 4 means a dielectric including an alumina ceramics plate. Fig. 8 shows a relationship between a size of the ceramics plate 4 and a size of the conduction layer 31. In Fig. 8, reference numeral 32 means a hole (a gas supply mechanism) provided in the ceramics plate 4 at its intermediate portion to serve as a gas passage. The conduction layer 31 includes a silver metallized layer mounted on a single side of the ceramics plate 4 to have a thickness of 40 microns. The power supply plate 11 and the conduction layer 31 become at the same potential if the power supply plate 11 is even partially in contact with the conduction layer 31. Therefore, even when any gap is made between the power supply plate 11 and the ceramics plate 4, both of them are kept at the same potential. Accordingly, no electric field is applied to the gap so that no void discharge is caused. In addition, the conduction layer 31 is formed so as not to cover an entire surface of the ceramics plate 4, and an outer periphery and an inner periphery of the ceramics plate 4 have areas on which no conduction layer 31 is formed. It is thereby possible to prevent creeping discharge toward the grounded electrode 2 through the outer periphery or the inner periphery of the ceramics plate 4 (see Fig. 7). Though a distance required to the creeping discharge between the conduction layer 31 and the grounded electrode 2 depends upon applied voltage, it is typically sufficient to set the distance to 2 mm or more. Reference numeral 61 means a metallic spacer interposed between the grounded electrode 2 and the ceramics plate 4. The ceramics plate 4 and the grounded electrode 2 are provided to define, through the spacer 61, a discharge space 5 in which discharge is generated. An oxygen-containing gas is supplied through a gas supply port (a gas supply mechanism) 7 into the discharge space 5. Consequently, in the discharge space 5, the oxygen-containing gas is partially ozonized by the discharge caused between the conduction layer 31 and the grounded electrode 2. An employed gas may include a mixed gas of nitrogen and oxygen, or air. In this case, as the gas contains the least amount of moisture possible and has the highest oxygen concentration possible, a higher efficiency can be obtained, The ceramics plate 4 and the grounded electrode 2 are in surface contact with each other through the metallic spacer 61. Heat generated in the ceramics plate 4 passes through the spacer 61, and can be effectively absorbed by the cooled grounded electrode 2. In the first embodiment shown in Fig. 7, two ozone generating units are opposed to each other. A stress buffer plate (an elastic body) 100 made of ethylene propylene rubber (hereinafter abbreviated EP rubber) having ozone resistance is inserted between the two ozone generating units. Further, an unillustrated pressure mechanism is used to press the upper grounded electrode 2 from a direction of the arrow A, thereby assembling an apparatus. That is, the spacer 61 is interposed between the grounded electrode 2 and the ceramics plate 4, and a back of the ceramics plate 4 is pressed by drag of the stress buffer plate 100 serving as the elastic body, thereby keeping a constant air gap length of the discharge gap 5. In other words, the stress buffer plate 100 absorbs force generated in, for example, the ceramics plate 4 due to mechanical stress or thermal stress. Therefore, the stress buffer plate 100 can prevent degradation of an air gap length accuracy and breakage of the ceramics plate 4, both of which are mainly caused due to distortion of the ceramics plate 4. In a method in the prior art shown in Figs. 2A and 2B, a spacer 6 made of silicone is inserted into a gap defined by electrodes 2 and 3 or ceramics layers (dielectrics) 41 at an outer peripheral portion causing no discharge. In the method, however, the air gap length may be varied due to thermal distortion of the electrodes 2 and 3. In the embodiment, the stress buffer plate 100 as shown in Fig. 9 is employed in order to avoid the problem. In Fig. 9, reference numeral 100 means the stress buffer plate made of EP rubber, 101 is an opening (a gas supply mechanism) serving as a gas passage, and 102 is a partial surface of the EP rubber, which is coated with fluoroplastic to avoid corrosion of the EP rubber due to ozone. The stress buffer plate 100 has substantially the same size as that of the discharge space 5, and is mounted on a back face of the conduction layer 31 as shown in Fig. 7. It is thereby possible to uniformly dispose the stress buffer plates 100 so as to cover an entire surface of the discharge space 5 from the outside thereof. Further, it is possible to ensure the air gap length of the spacer 61, and keep a high air gap length accuracy of the discharge space 5. That is, in the structure in the embodiment, the air gap length can be left constant even when a thickness of the ceramics plate 4 is varied due to, for example, thermal expansion, and a thickness of the stress buffer plate 100 is varied to buffer the variation. Accordingly, the structure in the embodiment is effective particularly when a very short air gap of about several hundreds microns is required. Further, as shown in Fig. 7, the air gap is held between the high-voltage conduction layers 31 at the same potential to be surrounded by the conduction layers 31. As a result, no electric field is applied to the stress buffer plate 100, and no creeping discharge is caused. For the purposes of, for example, avoiding degradation of the stress buffer plate 100 due to the creeping discharge, as shown in Fig. 7 the stress buffer plate 100 preferably has an area equal to or less than that of the conduction layer 31 applied onto the surface of the ceramics plate 4, and the stress buffer plate 100 is preferably surrounded by the conduction layers 31 at the same potential. Alternatively, the stress buffer plate 100 may be entirely made of fluoroplastic. A description will now be given of the operation. In Fig. 7, the gas is sucked in a direction of the arrow 7 from a peripheral portion of the grounded electrode 2 to pass through the discharge space 5, thereafter flowing in a direction of the arrow 81 and passing through a gas exhaust pipe (a gas supply mechanism) 9 to be exhausted in the direction of the arrow 8. Operating gas pressure is set to two atmospheres (atm). Further, it came clear that, when the raw gas contained nitrogen, it was possible to obtain an ozone generating characteristic entirely different from the above characteristic. In Fig. 10, air having an oxygen-nitrogen ratio of 1:4 was employed as the raw gas, and the ozone generating characteristic was observed while varying the discharge gap length d and the gas pressure p. Though it was also confirmed that an increase in the power consumption W/Q N reduced the ozone concentration in a high power consumption W/Q N area, the area is not shown in the drawing. From the drawing, it can be seen that, as the discharge gap length d became larger, or as the gas pressure p became higher, higher concentration ozone could be obtained. Fig. 11 shows the result of Fig. 10 as the product pd of the discharge gap length d and the gas pressure p. From the drawing, it can be understood that, as the product pd is more increased, higher concentration ozone, can be obtained. It is necessary to set the product pd to 1,6·10 4 Pa·cm (120 Torr·cm) or more in order to obtain a predetermined high concentration ozone. The result has a tendency opposite to that of the experimental result using the oxygen as the raw gas. Further, it came clear that a characteristic identical with that in Fig. 11 could be obtained even when a nitrogen rate was substantially equal to an oxygen rate. When the nitrogen rate was more increased, that is, a nitrogen rich gas was used as the raw gas, the ozone was destroyed by nitrogen oxide (NO x ) generated by the discharge, resulting in a phenomenon in which high concentration ozone could not be obtained. Further, as the electronic energy becomes higher, that is, the product pd becomes smaller, the nitrogen oxide (NO x ) can be more easily produced. Thus, the high concentration ozone can not be obtained in an area of the small product pd. Such a physical model is well known in an area having the discharge gap length d of 0,8 mm or more. However, in the above experiment, it could initially be confirmed that the same phenomenon occurred even in case of the discharge gap length of 0,6 mm or less. Embodiment 2 In the embodiment 1, the electrode is grounded to form the grounded electrode 2, and high voltage is applied to the conduction layer 31 from the power source 1 to establish a high-voltage electric field between the grounded electrode 2 and the conduction layer 31. However, as shown in Fig. 12, high voltage may be applied to grounded electrodes 2 from the power source 1, and conduction layers 31 may be grounded through a fuse 12 to serve as grounded electrodes, thereby establishing the high-voltage electric field between both the electrodes. Embodiment 3 Alternatively, as shown in Fig. 13, a middle point grounded power source 11 may be used, and high voltage having anti-phases may be respectively applied to electrodes 2 and conduction layers 31 from the middle point grounded power source 11 so as to establish a high-voltage electric field. It is thereby possible to reduce a potential difference between ground potentials of both the electrodes to a value half potential difference obtained when one electrode is grounded like the electrode 2 in the embodiment 1. That is, when the one electrode is grounded, and a peak value of an electrode potential on the high voltage side is defined as v, a peak value of the potential of both the electrodes with the middle point grounded power source 1' can be expressed as v/2 as shown in Figs. 14A and 14B. It is thereby possible to halve an insulating distance required between the electrode and a ground of the ozone generating apparatus. Further, it is possible to provide an ozone generating apparatus having a compact form. As set forth above, in the ozone generating apparatus according to the present invention, the gas pressure in the discharge space is set to one atmosphere or more, and the discharge gap length of the discharge space is set to 0,4 mm or less. Consequently, highly efficient ozone generation can be realized in the high ozone concentration area. As a result, it is possible to provide a highly efficient and high concentration ozone generating apparatus. In the ozone generating apparatus, when the nitrogen rate is higher than the oxygen rate, and the discharge gap length set to 0,6 mm or less, the product pd of the gas pressure p and the discharge gap length d is set to a value of 1,6·10 4 Pa·cm (120 Torr·cm) or more. In this case, even when the nitrogen-rich raw gas is employed, it is possible to reduce production of the nitrogen oxide, and generate high concentration ozone at high efficiency. As a result, it is possible to provide a highly efficient and high concentration ozone generating method.

Claims: An ozone generating method, wherein an oxygen an nitrogen containing gas is supplied between two electrodes (2, 31) which are mutually opposed to generate discharge in discharge space (5) in response to high voltage applied therebetween and between which at least one dielectric (4) is interposed, the discharge gap length d is set equal or less than 0,6 mm and the gas being supplied between the electrodes (2, 31) so as to generate ozone by the discharge characterized in that a gas is supplied, whose concentration of nitrogen is higher than that of oxygen and whose gas pressure p of the discharge space (S) is set such, that the product p·d of the gas pressure p and the discharge gap length d is 1.6·10 4 Pa·cm (120 Torr·cm) or more.

Inventors: KUZUMOTO MASAKI, MUKAI MASAHIRO, OCHI JUNJI, OZAWA TATEKI, TABATA YOUICHIRO, YAGI SHIGENORI, YOSHIZAWA KENJI, KUZUMOTO, MASAKI, MUKAI, MASAHIRO, OCHI, JUNJI, OZAWA, TATEKI, TABATA, YOUICHIRO, YAGI, SHIGENORI, YOSHIZAWA, KENJI, KUZUMOTO, MASAKI, C/O MITSUBISHI DENKI K.K., MUKAI, MASAHIRO, C/O MITSUBISHI DENKI K.K., OCHI, JUNJI, C/O MITSUBISHI DENKI K.K., OZAWA, TATEKI, C/O MITSUBISHI DENKI K.K., TABATA, YOUICHIRO, C/O MITSUBISHI DENKI K.K., YAGI, SHIGENORI, C/O MITSUBISHI DENKI K.K., YOSHIZAWA, KENJI, C/O MITSUBISHI DENKI K.K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1995
Main classification: C01B13
IPC codes: H01T23, C01B13