ZA200107013B

ZA200107013B - Method and apparatus for producing ozone.

Info

Publication number: ZA200107013B
Application number: ZA200107013A
Authority: ZA
Inventors: Barend Visser
Original assignee: Univ Potchefstroom
Priority date: 2001-08-24
Filing date: 2001-08-24
Publication date: 2002-11-24

Description

METHOD AND APPARATUS FOR PRODUCING OZONE

TECHNICAL FIELD

This invention relates to a method and apparatus for producing ozone. ~~ BACKGROUND ART

A known method for producing ozone includes the steps of passing oxygen at 1 atmosphere and 25°C through concentric metallised glass tubes to which low- : frequency power at 50-500 Hz and 10-20 kV is applied. Due to the relatively slow change in potential (5kV per millisecond), a corona or silent electric - discharge is maintained between the electrodes. A disadvantage of this method is that energy is lost in the form of heat, and a relatively low yield ratio of ozone is achieved.

OBJECT OF THE INVENTION

: It is accordingly an object of the present invention to provide a method and apparatus for producing ozone with which the aforesaid disadvantage may be overcome or to provide a useful alternative to the known method.

SUMMARY OF THE INVENTION

According to the invention there is provided a method of producing ozone comprising the steps of: - passing an oxygen-containing fluid through an electrode region; - generating intermittent bursts of corona discharge in the electrode region by energizing an electrode in the region with intermittent voltage pulses; - characterized in that the voltage pulses cause an electric field in the electrode region to change at a rate faster than 3kV/mm/10ns.

In this specification, the terms “oxygen-containing fluid” and “fluid comprising oxygen” are used to denote a fluid comprising molecular oxygen, that is O,.

Each voltage pulse may have a pulse width shorter than 100ns.

The electric field may typically change at a rate in the order of 10kV/mm/10ns.

Further according to the invention there is provided apparatus for producing ozone comprising: - a housing defining a passage for an oxygen-containing fluid; - first and second electrodes disposed adjacent the passage; and : - voltage pulse generating means connected to at least one of the electrodes for generating voltage pulses between the electrodes; and - the voltage pulses causing an electric field between the electrodes to change at a rate faster then 3kV/mm/10ns.

Each voltage pulse has a pulse may have a pulse width shorter than 100ns.

The electric field may typically change at a rate in the order of 10kV/mm/10ns

The voltage pulse generating means may comprise a self-oscillating circuit.

The self-oscillating circuit may comprise a field effect transistor (FET) which may be connected to at least one of the electrodes and a switch circuit for the FET, the switch circuit may comprise charge storage means and switching means connected to a gate of the FET, the switching means being operative to deposit charge from the charge storage means onto the gate, thereby to improve a rise time of a voltage applied to said at least one of the electrodes.

The charge storage means may comprises a capacitor and the switch means may comprise a SIDAC.

The electrodes may be connected to a secondary winding of a transformer, a primary winding of the transformer may be connected in a drain-source circuitof the FET.

The passage may extend between an inlet to the housing an outlet therefrom.

In one embodiment the first electrode may be an annular electrode disposed in the housing and the passage may extend through a clearance defined between the first electrode and the housing.

The housing may be a metal housing serving as the second electrode and an insulating layer for the first electrode may be disposed between the first electrode and the housing.

In another embodiment the housing may be made of an electricity insulating material, the first electrode may be disposed circumferentially on the outside of the housing and the second electrode may be provided inside the housing spaced from an inner wall of the housing, to define the passage between the second electrode and the inner wall.

According to another aspect of the invention there is provided apparatus for producing ozone comprising: - a housing defining a passage for an oxygen-containing fluid, - first and second electrodes disposed adjacent the passage; and - voltage pulse generating means; - the voltage pulse generating means comprising a field effect transistor (FET)as switched voltage output device connected to at least one of the electrodes; and a switch circuit for the FET comprising charge storage means and switching means connected to a gate of the FET, the switching means being operative to deposit charge from the charge storage means onto the gate, thereby to improve a rise time of a voltage pulse applied to said at least one of the electrodes.

4(a)

According to yet another aspect of the invention there is provided a method of driving a field effect transistor (FET) having charge storage means and switching means connected to a gate of the FET comprising the step of applying from the charge storage means to the gate a charge pulse driving a voltage on the gate beyond a maximum voltage rating of the FET.

The voltage may be driven two to four times beyond the maximum voltage rating ~ ofthe FET.

According to still another aspect of the invention a circuit for driving a field effect transistor (FET) having a gate comprises charge storage means and switching : means connected to the gate of the FET and which are adapted to apply from the charge storage means to the gate a charge pulse which drives a voltage on the gate beyond a maximum voltage rating of the FET.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described further, by way of example only, with reference to the accompanying drawings wherein: figure 1 is an exploded perspective view of apparatus according to a first embodiment of the invention for producing ozone; figure 2 is an exploded perspective view of a closure and electrode assembly of the apparatus of figure 1;

RX figure 3 is a perspective view of the closure and electrode assembly of figure 2 when assembled; figure 4 is a schematic representation of an electronic circuit used to oo 5 generate a train of voltage pulses that is applied to the electrode assembly of figures 2 and 3. figures 5(a);(b);(c); and (d) are voltage waveforms against a first time scale at points a, b, ¢, and d in figure 4; figures 6(a);(b);(c); and (d) are the same wave forms against a larger time scale; figure 7 is a cross-sectional view on fine VII in figure 3; figure 8 is a partially broken away perspective view of apparatus according : to a second embodiment of the invention for producing ozone; and : figure 9 is a cross-sectional side view of a central portion of the apparatus of figure 8.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

~ Referring to figure 1, apparatus according to a first embodiment of the invention for producing ozone, is generally designated by reference numeral 10.

The apparatus 10 includes a tubular anodised aluminium housing 12 having an open end 14 and a closed end 16, and a separate closure 18 for closing the open end. The apparatus 10 further includes an electrode assembly 20 mountable on the closure 18 and pulse generating means in the form of an electronic circuit 30 (shown in figure 4) for energising the electrode assembly 20. "Aninlet 22 to the housing is provided in the closed end 16 and an outlet 24 is defined in the closure 18. A passage 21 (shown in figure 7) extends from the inlet 22 to the outlet 24.

As best shown in figures 1 and 7, the electrode assembly 20 comprises an insulating disc or base 20.1 of an ozone and corona resistant material, such as : glass, alumina etc and an annular electrode 20.2 mounted on the face of the base 20.1 facing away from the closure 18. The base 20.1 is provided with a plurality of spaced peripheral notches 20.3, the purpose of which will be described hereinafter.

The closure 18 is provided with an annular ridge formation 18.1. As best shown in figure 7, when the electrode assembly 20 is mounted on shoulder ~ formations on the closure 18, the ridge formation 18.1 is disposed in close proximity, but with a clearance 23 of approximately 0.3mm from the base 20.1,

The aforementioned passage 21 extends from the inlet 22 along the tubular

: © WO 00/50338 PCT/ZA00/00031— a housing 12, through the notches 20.3 in the base, through the clearance 23 between the ridge formation 18.1 and the base 20.1, and out via the outlet 24.

As will be described hereinafter, a rapidly changing electric field is established h) in the passage 21 in the region of the ridge formation 18.1 causing a corona discharge and oxygen flowing along the passage 21 in use, therefore passes through the field. The effect of the electric field is that instantaneous ionisation of oxygen is achieved by the corona discharge to produce ozone from the oxygen, without substantial energy loss in the form of heat generated.

The applicant has found that the ozone yield ratio is dependant on the rise time t, the fall time t, and width w, of the pulses 50 (shown in figure 5(d)) in the train ) 52 of pulses (shown in figure 6(d)) applied to the electrode assembly 20. Itis believed that the shorter the rise and fall times and/or the pulse width, the more efficient the apparatus becomes.

A self-oscillating circuit 30 for energizing the electrode assembly 20 is shown in figure 4. Voltage waveforms as measured at points a, b, c_ and d are shown in figures 5(a), (b), (c) and (d) respectively and also in figures 6(a), (b), (c) and (d) respectively.

The circuit 30 comprises a capacitor 34 in parallel with a SIDAC 36 and inductor

37. The SIDAC is connected to the gate 39 of a field effect transistor (FET) such as a MOSFET 38 of the type IRF 740, for example. The SIDAC 36 conducts current when a voltage exceeding a certain threshold (100V for example) is applied across it. A primary winding of a transformer 43 is connected in the drain-source circuit 45 of the MOSFET 38. The secondary winding of the transformer is connected to the electrode assembly 20 as shown in figure 4.

A DC voltage of about 150V is applied at point 41 of the circuit. Initially the : 10 potential difference across the SIDAC 36 is insufficient to cause the SIDAC 36 to switch on and hence the capacitor 34 is charged up. When the voltage over the SIDAC 36 exceeds the aforementioned threshold voltage of the SIDAC 36, it switches on, resulting in a closed circuit from the capacitor 34 to the gate 39 of the MOSFET 38, partially discharging the capacitor 34 and hence charging the gate 39. The result is that a charge will now be shared between the capacitor 34 and the gate 39, so that some voltage, preferably sufficiently above the gate threshold voltage (typically 6V) relative to ground, is applied to the gate. The current that discharges from the capacitor 34 through to SIDAC 36 is applied to the gate 39 of the MOSFET 38 slightly prior to the onset of current flow in the drain-source circuit 45. As a result of the current from the capacitor, the voltage on the gate exceeds the aforementioned threshold voltage by a sufficient amount. The resulting signals at points a, b, c and d are shown in figures 5(a) to (d) respectively and in figures 6(a) to (d), respectively.

Using this method, the gate voltage may for short intervals 51 be driven approximately two to four times 53 beyond the maximum voltage rating of some

MOSFET's without destroying the device.

As will be seen from figures 5(d) and 6(d) each of the pulses 50 in the train 52 of voltage pulses applied to the electrode assembly has a 30% - 70% slope or rise time t.and a fall time t,of better than 2kV/100ns, preferably in the order of 3kV/10ns. Furthermore, the width of the puises w, as they pass through the average value 54 is shorter than 100ns, preferably shorter than 30ns.

The peak value of the voltage applied to the electrode assembly is in the order of 3kV and with the clearance between the electrode 20.2 and the ridge 18 A in the order of 0.3mm, the maximum electric field strength E is bigger than 3kV/mm, preferably in the order of 10kV/mm.

Referring to figures 7 and 8, apparatus according to a second embodiment of the invention for producing ozone, is generally designated by reference numeral 100.

The basic working of the apparatus 100 is similar to that of apparatus 10, but the construction of apparatus 100 differs in that the housing 102 is manufactured from an insulating material. The apparatus 100 includes a first electrode 104, which comprises a conductive annulus extending around the housing 102 and a second electrode 106 disposed inside the housing 102. - 5

The second electrode 106 is provided with an annular ridge formation 106.1 disposed in close proximity to the inner wall of the housing 102, in the region of the first electrode 104. The first electrode 104 is connected to the self-oscillating circuit and the second electrode 106 is earthed. A corona discharge is therefore 10 established between the ridge formation 106.1 and the inner wall of the housing : 102, causing the production of ozone as hereinbefore described. it will be appreciated that there are many variations in detail on the method and apparatus according to the invention without departing from the scope and spirit of the appended claims.

Claims

1. A method of producing ozone comprising the steps of: - passing an oxygen-containing fluid through an electrode region; - generating intermittent bursts of corona discharge in the electrode region by energizing an electrode in the region with intermittent voltage pulses; : - characterized in that the voltage pulses cause an electric field in the electrode region to change at a rate faster than 3kV/mm/10ns.

2. A method as claimed in claim 1 wherein each voltage pulse has a pulse width shorter than 100ns.

3. A method as claimed in claim 1 or claim 2 wherein the electric field changes at a rate in the order of 10kV/mm/10ns.

4. Apparatus for producing ozone comprising: - a housing defining a passage for an oxygen-containing fluid; - first and second electrodes disposed adjacent the passage; and - voltage pulse generating means connected to at least one of the electrodes for generating voltage pulses between the electrodes; and - the voltage pulses causing an electric field between the electrodes to change at a rate faster then 3kV/mm/10ns.

5. Apparatus as claimed in claim 4 wherein each voltage pulse has a pulse width shorter than 100ns.

6. Apparatus as claimed in claim 4 or claim 5 wherein the electric field changes at a rate in the order of 10kV/mm/10ns

7. Apparatus as claimed in any one of claims 4 to 6 wherein the voltage pulse generating means comprises a self-oscillating circuit. ;

8. Apparatus as claimed in claim 7 wherein the self-oscillating circuit comprises a field effect transistor (FET) which is connected to the at least one of the electrodes and a switch circuit for the FET, the switch circuit comprising charge storage means and switching means connected to a gate of the FET, the switching means being operative to deposit charge from the charge storage means onto the gate, thereby to : improve a rise time of a voltage applied to said at least one of the electrodes.

9. Apparatus as claimed in claim 8 wherein the charge storage means comprises a capacitor and the switch means comprises a SIDAC. >

10. Apparatus as claimed in claim 8 or claim 9 wherein the electrodes are connected to a secondary winding of a transformer, a primary winding of the transformer being connected in a drain-source circuit of the FET.

11. Apparatus as claimed in any one of claims 4 to 10 wherein the passage extends between an inlet to the housing an outlet therefrom.

12. Apparatus as claimed in any one of claims 4 to 11 wherein the first electrode is an annular electrode disposed in the housing and wherein the passage extends through a clearance defined between the first electrode and the housing.

13. Apparatus as claimed in claim 12 wherein the housing is a metal housing serving as the second electrode and wherein an insulating layer for the first electrode is disposed between the first electrode and the housing.

14. An apparatus as claimed in any one of claims 4 to 11 wherein the housing is of an electricity insulating material, wherein the first electrode is disposed circumferentially on the outside of the housing and wherein the second electrode is provided spaced from an inner wall of the housing, to define the passage between the second electrode and the inner wall.

15. Apparatus for producing ozone comprising: - a housing defining a passage for an oxygen-containing fluid; - first and second electrodes disposed adjacent the passage; and - voltage pulse generating means; - the voltage pulse generating means comprising a field effect transistor (FET) as switched voltage output device connected to at least one of the electrodes; and a switch circuit for the FET comprising charge storage means and switching means connected to a gate of the FET, the switching means being operative to deposit charge from the charge storage means onto the gate, thereby to improve a rise time of a voltage pulse applied to said at least one of the electrodes.

16. A method of driving a field effect transistor (FET) having charge storage means and switching means connected to a gate of the FET comprising the step of applying from the charge storage means to the gate a charge pulse driving a voltage on the gate beyond a maximum voltage rating of the FET.

17. A method as claimed in claim 16 wherein the voltage is driven two to four times beyond the maximum voltage rating of the FET.

18. A circuit for driving a field effect transistor (FET) having a gate, the circuit comprising charge storage means and switching means connected to the gate of the FET and which are adapted to apply from the charge storage means to the gate a charge pulse which drives a voltage on the gate beyond a maximum voltage rating of the FET.

14(a)

19. A method of producing ozone substantially as herein described with reference to the accompanying diagrams.

20. Apparatus for producing ozone substantially as herein described with reference to the accompanying diagrams.

21. A method of driving a field effect transistor substantially as herein described with reference to the accompanying diagrams.

22. A circuit for driving a field effect transistor substantially as herein described with reference to the accompanying diagrams. ORIGINAL FILED: 237° AUGUST 2001. AMENDED COPY: DATED T Js HE21% DAY OF NOVEMBER 2002 ”% - - 7 rd fo nn am om — DM NE PATENT ATTORNEY FOR APPLICANT