WO2009069117A2

WO2009069117A2 - Corona discharge generating device and method

Info

Publication number: WO2009069117A2
Application number: PCT/IL2008/001509
Authority: WO
Inventors: Yakov E. Krasik
Original assignee: Aquapure Technologies Ltd.
Priority date: 2007-11-29
Filing date: 2008-11-17
Publication date: 2009-06-04
Also published as: WO2009069117A3

Abstract

A corona discharge generating device includes a housing having a bottom wall and a side wall made of conductive material adapted to be electrically connected to a negative high-voltage pulse generator. A ferrofluid layer is provided inside the housing, on the bottom wall. An insulating sheet is provided inside the housing, over the ferrofluid layer, for supporting a layer of electrolyte liquid. A discharge electrode is placed at a predetermined distance over the layer of electrolyte liquid, and adapted to be connected at a positive high-voltage. A magnet is provided for inducing a magnetic field on the ferrofluid layer, so as to form magnetically induced cones on the surface of the ferrofluid layer, in order to enhance formation of Taylor cones on the surface of the electrolyte liquid, when the negative high-voltage pulse is applied on the housing.

Description

CORONA DISCHARGE GENERATING DEVICE AND METHOD

FIELD OF THE INVENTION

[001] The present invention relates to corona discharge. More specifically the present invention relates to a corona discharge generating device and method.

BACKGROUND OF THE INVENTION

[002] Corona discharge has many commercial and industrial applications. One of the many known applications of corona discharge is the production of ozone using free- radicals and ions generated in corona reaction. Among the many industrial applications for Ozone is the treating of liquids. Various oxidation processes for treating liquids are used in many industries, such as, for example, the petrochemical industry, refineries, the pharmaceutical industry, the chemical industry, the textile industry, the Pulp & Paper industry, the water disinfection & treatment industry, the pesticide industry, and many more. Oxidation processes are used in various applications such as, for example, the following: Pretreatment for biological wastewater treatment; Wastewater polishing; Ground water remediation to treat MTBE (methyl tertiary butyl ether), Dioxane, Trichloro ethylene, perchloro ethylene, and other organo-halogens; Landfill leachate treatment; Cooling tower water reuse; Storm water run-off treatment; Shipboard wastewater treatment. [003] It is known that a crucial factor in the ignition of corona discharge is attributed to the electrode properties.

[004] For example, it is necessary to use an electrode with a particular geometry to ensure that the electric field E_p is greater than the critical electrical field E_s which is necessary for the ignition of corona discharge. [005] In order to achieve a high value of the electric field E_p using a rather moderate voltage amplitude V, a standard corona discharge electrode configuration may use either thin wires having diameters of several tens of microns or knife-type electrodes. In the both cases a significant electric field enhancement is achieved due to the small area of the emitting surface that increases surface charge density. [006] The abovementioned corona electrodes have a major drawback, which is short life- time due to erosion occurring on the electrode emitting surface during the corona discharge process. Intense electron emission from the corona electrode leads to the formation of relatively dense plasma in the vicinity of the emitting surface. Plasma ions are accelerated towards the electrodes and erode them. Another disadvantage of these electrodes is the necessity to place the electrode at a certain distance from the grounded electrode in order to avoid possible electrical breakdown. In the case of generation of ozone and radicals for water purification or sterilization of various types of surfaces, this distance reduces significantly the total amount of ozone and radicals capable of reaching their target due to the finite mobility and short life-time of ozone and radicals. [007] Taylor showed (G. I. Taylor, Proc. Royal Society (London) A 280, 383 1964) that application of high-voltage to the surface of liquid can lead to the formation of cone- shaped structures having an angle a of approximately 49.3°. These cone-shaped structures, usually referred to as "Taylor cones", are formed as a result of the balance between electrostatic and surface-tension forces and due to liquid polarization properties. For high enough electrical stress, electrical discharges can occur at the apex of the cones. [008] Taylor cones were used for corona discharge ignition and ozone production as described, for instance by J. A. Robertson, M. A. Bergougnou, W. L. Cairns, G. S. P. Castle and I. I. Inculet, "A new type of ozone generator using Taylor cones on water surface" (IEEE Trans, on Indust. Appl., vol. 34, no. 6, November/December 1998, pp. 1218-1224). [009] Robertson et al. showed that an application of an electric field of 2- 10⁴ V/cm and a pulse duration of 20 ms may lead to the formation of Taylor cones and intense corona discharge and ozone generation. The method proposed by Robertson et al. includes applying relatively low-amplitude electric fields at long time duration to the metal electrode while the grounded electrode was the water surface itself. The drawback of the proposed setup is that such a setup is not suitable for achieving a reproducible and reliable formation of Taylor cones that would result in a moderate rate of ozone production. [0010] It is desirable to introduce a novel corona discharge generating device which substantially prolongs the life-time of the electrode used and allows for producing ozone and radicals in abundance. SUMMARY OF THE INVENTION

[0011] There is thus provided, according to embodiments of the present invention, a corona discharge generating device comprising: [0012] a housing having a bottom wall and a side wall made of conductive material adapted to be electrically connected to a negative high- voltage pulse generator;

[0013] a ferrofluid layer provided inside the housing, on the bottom wall;

[0014] an insulating sheet provided inside the housing, over the ferrofluid layer, for supporting a layer of electrolyte liquid; [0015] a discharge electrode placed at a predetermined distance over the layer of electrolyte liquid, and adapted to be connected at a positive high- voltage; and

[0016] a magnet for inducing a magnetic field on the ferrofluid layer, so as to form magnetically induced cones on the surface of the ferrofluid layer, in order to enhance formation of Taylor cones on the surface of the electrolyte liquid, when the negative high- voltage pulse is applied on the housing.

[0017] Furthermore, in accordance with some preferred embodiments of the present invention, the electrolyte liquid comprises water.

[0018] Furthermore, in accordance with some preferred embodiments of the present invention, a screening electrode is provided on an edge of the wall of the housing so as to prevent electrical discharge between the discharge electrode and the wall of the housing.

[0019] Furthermore, in accordance with some preferred embodiments of the present invention, the device is further provided with at least one port for allowing water to flow into and out of the housing, over the insulating sheet.

[0020] Furthermore, in accordance with some preferred embodiments of the present invention, said at least one port comprises an inlet port and an outlet port.

[0021] Furthermore, in accordance with some preferred embodiments of the present invention, the magnet comprises a permanent magnet.

[0022] Furthermore, in accordance with some preferred embodiments of the present invention, the discharge electrode comprises a metal plate. [0023] Furthermore, in accordance with some preferred embodiments of the present invention, the metal plate is flat. [0024] Furthermore, in accordance with some preferred embodiments of the present invention, there is provided a method for generating corona discharge comprising: [0025] providing a corona discharge device that includes a housing having a bottom wall and a side wall made of conductive material, a ferrofluid layer provided inside the housing, on the bottom wall, an insulating sheet provided inside the housing, over the ferrofluid layer, for supporting a layer of electrolyte liquid, a discharge electrode placed at a predetermined distance over the layer of electrolyte liquid, and adapted to be connected at a positive high- voltage; [0026] providing a magnet for inducing a magnetic field on the ferrofluid layer; [0027] electrically connecting the housing to a negative high- voltage pulse generator; [0028] depositing a layer of electrolyte liquid on the insulating sheet; [0029] connecting the discharge electrode to a positive high- voltage; and [0030] applying the negative high- voltage pulse on the housing. [0031] Furthermore, in accordance with some preferred embodiments of the present invention, the electrolyte liquid comprises water.

[0032] Furthermore, in accordance with some preferred embodiments of the present invention, the method comprises allowing the water to flow through the device. [0033] Furthermore, in accordance with some preferred embodiments of the present invention, the method comprises providing a screening electrode on an edge of the wall of the housing so as to prevent electrical discharge between the discharge electrode and the wall of the housing.

[0034] Furthermore, in accordance with some preferred embodiments of the present invention, the negative high-voltage pulse comprises a negative high-voltage pulse in the range of 10 to 100 nanoseconds duration.

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with objects, features, and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanying drawings in which: [0036] Fig. 1 is a cross-sectional schematic illustration of a corona discharge generating device, in accordance with embodiments of the present invention.

[0037] Fig. 2 illustrates a corona discharge generating device, used for water treatment, according to embodiments of the present invention. [0038] Fig. 3 illustrates a method for generating corona discharge, according to embodiments of the present invention.

[0039] It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

[0040] In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, and components have not been described in detail so as not to obscure the present invention. [0041] A corona discharge configuration according to embodiments of the present invention may improve significantly the reliability and reproducibility of corona discharge induced at apexes of the Taylor cones formed on the surface of an electrolyte liquid, such as, for example, water. When using water as the electrolyte liquid, such a corona discharge generating device configuration may be used, for example, as an ozone generator for the purpose of treating the water. [0042] A cross-sectional schematic illustration of a corona discharge generating device design according to embodiments of the invention is shown in Fig. 1.

[0043] A corona discharge generating device (10) according to embodiments of the invention, may include a metal housing (12) made metal, preferably a non-corrosive metal, such as, for example, stainless steel. The metal housing is connected to a negative high voltage pulse source (14). As depicted at Fig. 1, the edge of the metal housing walls has a rounded shape in the form of a tube or a solid cylindrical edge. The rounded shape serves as a screening electrode (16) to prevent inadvertent corona discharge at the edges of the metal housing. The corona discharge generating device further includes a layer of ferrofluid (18). Ferrofluids are colloidal mixtures composed of nanoscale ferromagnetic or ferromagnetic particles suspended in a carrier fluid, usually an organic solvent or water, e.g. Vitreloy liquid-metal alloy, available from Liquidmetal Technologies Lie, Rancho Santa Margarita, CA, USA, placed within the metal housing. An insulating sheet (20) is positioned above the liquid metal in order to separate this liquid and the water layer which is placed above it and thus, avoid decontamination of the water layer by metal liquid microparticles. The insulating sheet may be, for example, a plastic film such as MYLER® polyester film or KAPTON® polyimide film, available from DuPont®. [0044] A water layer (22) is deposited over the insulating sheet. The corona discharge generating device, according to embodiments of the present invention, uses the water needed to be treated as the negatively-charged electrode (cathode) in the generation of ozone in the corona discharge process. The water may be stationary or flow over the insulating sheet above the metal liquid layer. [0045] The corona discharge generating device further includes a positive biased electrode (24), such as, for example, a flat metal plate made, for instance, of stainless steel, positioned above the water surface substantially parallel to the surface. The distance between the positive biased electrode and the surface of the water may be in the order of a few millimeters. When a sufficiently high voltage capable of causing an electric field of between 10⁴ and 10⁶ Volt/cm is applied to the water via the metal housing, Taylor cones are formed on the surface of the water. At the apices of the Taylor cones corona discharge occurs that generates ozone directly on the surface of the water.

[0046] A magnet (26), such as, for example, a permanent magnet, is positioned externally adjacent to the metal housing underneath the metal liquid layer. The magnetic field of the magnet can be in the range of several kGs which is typical value for commercially available permanent magnets (e.g. Neodymium, Samarium cobalt magnets available from Magnetic Component Engineering Inc., Torrance, CA, USA).

[0047] When a high-voltage negative pulse is applied on the housing the ferromagnetic particles within the metal liquid, influenced by the magnetic field of the adjacent magnet, induce cones on the surface of the metal liquid, which enhance the formation of Taylor cones on the surface of the water layer above, thus enhancing the corona discharge between the apexes of the Taylor cones on the surface of the water layer and the flat electrode.

[0048] Fig. 2 illustrates a corona discharge generating device, used for water treatment, according to embodiments of the present invention: [0049] Water is allowed to flow through housing (12) of the water treatment device (10), using at least one port fludicially connected to a water supply. In the embodiment shown in Fig., 2 water is being fed in through inlet (30) and out through outlet (32). The inlet and outlet are preferably made of insulating materials, and a long enough portions of insulating piping are used on both ends so as to prevent shock hazards. [0050] Fig. 3 illustrates a method for generating corona discharge, according to embodiments of the present invention. The method includes:

[0051] Providing a corona discharge generating device, according to embodiments is provided (40), which includes a housing having a bottom wall and a side wall made of conductive material adapted to be electrically connected to a negative high- voltage pulse generator; a metal liquid layer provided inside the housing, on the bottom wall; an insulating sheet provided inside the housing, over the metal liquid layer, for supporting a layer of electrolyte liquid; a discharge electrode placed at a predetermined distance over the layer of electrolyte liquid, and adapted to be connected at a positive high-voltage. A magnet is provided (42) for inducing a magnetic field on the metal liquid. The housing is electrically connected to a negative high-voltage pulse generator (44). A layer of electrolyte liquid is deposited on the insulating sheet (46). The discharge electrode is connected to a positive high-voltage (48) and the negative high-voltage pulse is applied on the housing (50). [0052] Generation of ozone, using a corona discharge generating device, according to embodiments of the present invention, may include applying a high- voltage pulse of a some tens of nanoseconds duration (typically within a range of 10 to 100 nanoseconds) through the housing, to the water, while applying DC positive biased voltage to the metal electrode placed above the water level at a typical distance of a few millimeters from the water surface. Using a high-voltage pulse of nanosecond time duration may significantly increase the amplitude of the electric field as compared to applying the high- voltage pulse to a metal electrode while the water to be treated is grounded. The use of a high- voltage pulse of nanosecond time duration is suitable for applying it to water whose resistivity is of several kΩ. Thus almost all the energy may be dissipated in corona discharge and not through the high-resistant water resulting in an efficient process of ozone generation. [0053] Decoupling of the water with an end customer can be facilitated by providing for a long path of water to the customer using dielectric tubes. Indeed, the tap water specific resistance can be estimated as around 10³Ω-cm. Thus, several meters length of a water path with a cross-sectional area of a few cm² results in several MΩ resistance which will decrease current losses to a negligible level, of a few mA. [0054] According to embodiments of the present invention, the polarity of the biased voltage may be opposite to the polarity of the high-voltage pulse applied to the water. Thus, application of a DC biased voltage to the metal electrode placed above the water level allows decreasing the amplitude of the high-voltage pulse applied to the water and reducing the time delay in the appearance of Tailor's cones. [0055] Li order to decrease the amplitude of the both biased voltage and the high- voltage pulse applied to the water, to reduce the time delay in the appearance of the Taylor cones, and to achieve active control over the quantity and space distribution of the Taylor cones, a magnet is provided at the bottom of the metal housing (which serves as the water reservoir). Application of a ferrofluid together with a magnetic field enables developing variable structures of the liquid surface with sharp cone-type geometries whose sharpness and quantity depends on the strength of the magnetic field. Thus, using a metal liquid layer with induced cone structures may dictate the structure of the Taylor cones at the surface of the water. The decoupling of the metal liquid from the water may be achieved using a thin dielectric sheet. [0056] The corona discharge generating device, according to embodiments of the present invention facilitates generating corona discharge using controllable shape of electrode made of water employing the Taylor cones effect. Thus, the life time of the corona discharge generating device is practically infinite. In a corona discharge generating device according to embodiments of the present invention the generation of ozone and radicals occurs in the vicinity of the water surface in contract to other commonly used corona discharge devices. [0057] While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents will now occur to those of ordinary skill in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.

Claims

CLAIMS[0058] What is claimed is:

1. A corona discharge generating device comprising: a housing having a bottom wall and a side wall made of conductive material adapted to be electrically connected to a negative high- voltage pulse generator; a ferrofluid layer provided inside the housing, on the bottom wall; an insulating sheet provided inside the housing, over the ferrofluid layer, for supporting a layer of electrolyte liquid; a discharge electrode placed at a predetermined distance over the layer of electrolyte liquid, and adapted to be connected at a positive high-voltage; a magnet for inducing a magnetic field on the ferrofluid layer, so as to form magnetically induced cones on the surface of the ferrofluid layer, in order to enhance formation of Taylor cones on the surface of the electrolyte liquid, when the negative high- voltage pulse is applied on the housing.

2. The device as claimed in claim 1, wherein the electrolyte liquid comprises water.

3. The device as claimed in claim 1, wherein a screening electrode is provided on an edge of the wall of the housing so as to prevent electrical discharge between the discharge electrode and the wall of the housing.

4. The device as claimed in claim 1, further provided with at least one port for allowing water to flow into and out of the housing, over the insulating sheet.

5. The device as claimed in claim 4, wherein said at least one port comprises an inlet port and an outlet port.

6. The device as claimed in claim 1, wherein the magnet comprises a permanent magnet.

7. The device as claimed in claim 1, wherein the discharge electrode comprises a metal plate.

8. The device as claimed in claim 7, wherein the metal plate is flat.

9. A method for generating corona discharge comprising: providing a corona discharge device that includes a housing having a bottom wall and a side wall made of conductive material, a ferrofluid layer provided inside the housing, on the bottom wall, an insulating sheet provided inside the housing, over the ferrofluid layer, for supporting a layer of electrolyte liquid, a discharge electrode placed at a predetermined distance over the layer of electrolyte liquid, and adapted to be connected at a positive high- voltage,; providing a magnet for inducing a magnetic field on the ferrofluid layer; electrically connecting the housing to a negative high- voltage pulse generator; depositing a layer of electrolyte liquid on the insulating sheet; connecting the discharge electrode to a positive high- voltage; applying the negative high- voltage pulse on the housing.

10. The method as claimed in claim 9, wherein the electrolyte liquid comprises water.

11. The method as claimed in claim 9, comprising allowing the water to flow through the device.

12. The method as claimed in claim 9, comprising providing a screening electrode on an edge of the wall of the housing so as to prevent electrical discharge between the discharge electrode and the wall of the housing.

13. The method as claimed in claim 9, wherein the magnet comprises a permanent magnet.

14. The method as claimed in claim 9, wherein the discharge electrode comprises a metal plate.

15. The method as claimed in claim 9, wherein the negative high- voltage pulse comprises a negative high- voltage pulse in the range of 10 to 100 nanoseconds duration.