The present invention relates to the fields of plasma generation and chemistry.
As discussed in co-owned U.S. patent application Ser. No. 12/201,229 (published as U.S. Patent Application Publication No. 2009/0057131, and referred to hereinafter as “the '131 Publication”), the entire disclosure of which is incorporated herein by reference, a conventional plasma torch may be used to generate energized chemical species and electrons in liquid media through injection of non-thermal plasma (NTP). Currently, the interactions between non-thermal plasma (NTP) and liquid media are mainly utilized in water treatment. Such interactions are usually accomplished by the direct discharge of water and water plasma using various methods. Other approaches involve generating a direct current/alternating current discharge through a water/water vapor interface, or through gas bubbles. These approaches, however, require high-voltage pulses, with a corresponding high power consumption, and are limited by their low operating volumes. The approach described in the '131 Publication provides a plasma torch of simple, compact construction and a scalable method for operating such torches to generate reactive chemical species. However, the device disclosed in the '131 Publication produces only a single plasma discharge which, while more energy-efficient than discharges produced by other methods, limits the average energy of the plasma-activated species (PAS).
In one aspect of the present invention, a microhollow cathode discharge (MHCD) apparatus is used to stimulate chemical reactions within a fluid media by injecting plasma-activated species (PAS) in a gas carrier (i.e., a gas plasma) into the fluid media. In an embodiment according to this aspect of the present invention, the MHCD apparatus includes an electrically-conductive housing having a gas inlet and a gas outlet. An electrode is embedded in the housing between the gas inlet and gas outlet. The electrode has a bore with an electrically-conductive surface and is otherwise electrically insulated from the housing except at a location near the plasma outlet. The insulation is arranged so as to create a gas channel between the insulation and the housing.
Gas plasma is generated in a plasma reactor that is electrically-isolated from the MCHD apparatus and provided at the gas inlet at a sufficient pressure to drive the plasma through the bore of the electrode and air channel. A DC voltage is applied across the electrode and housing such that the electrode acts as cathode and the housing acts as an anode. The gas plasma is thus accelerated through the bore of the electrode and ejected into the fluid media where the PAS interact with the fluid, creating energized chemical species.
In another aspect of the present invention, a multicavity coupled plasma discharge (MCPD) apparatus is used to eject a gas plasma into a fluid media at higher energies than may be achieved using an MHCD device. In one embodiment according to this aspect of the invention, the MCPD apparatus is provided with a nozzle assembly that includes an electrode, an electrode insulator around the electrode, an electrically-conductive conduit having a gas inlet, and an electrically-conductive cup having a gas outlet. A dielectric material between the conduit and cup electrically isolate them from each other. The electrode has a bore with an electrically-conductive surface, and is exposed near a gas outlet in the cup. Otherwise, the electrode is electrically isolated from the conduit by the electrode insulator and a gas channel defined between the electrode insulator and the conduit.
Gas plasma is generated in a plasma reactor that is electrically-isolated from the MCPD apparatus and provided at the gas inlet at a sufficient pressure to drive the plasma through the bore of the electrode and the air channel. A DC voltage is applied across the electrode and conduit such that the electrode acts as cathode and the conduit acts as an anode. The gas plasma is thus accelerated through the bore of the electrode and eject it into the fluid media. Further, filamentous electrical discharges occur between the cup and the conduit, which increases the average energy of the PAS ejected through the gas housing and produces packets of PAS at rates in the nanosecond regime.
For a better understanding of the present invention, reference is made to the following detailed description of the exemplary embodiments considered in conjunction with the accompanying drawings, in which:
In one aspect of the present invention, a type of plasma injection device known as a microhollow cathode discharge (MHCD) apparatus is used as part of a conventional plasma torch to provide a simple and convenient non-thermal plasma (NTP)-fluid interface system for the direct injection of plasma-activated species (PAS) into a fluid medium, thus improving the overall efficiency of the plasma-medium interaction and reducing the power needed to generate energized chemical species compared to methods in which the water is ejected through the MCHD. The NTP, also referred to herein as a “gas plasma”, generally consists of PAS in a carrier gas. The term “gas plasma”, however, need not be limited to NTPs. MHCD apparatuses, in general, are discussed in U.S. Pat. No. 6,433,480, the disclosure of which is incorporated herein by reference.
A NTP discharge 28 is generated separately in a plasma reactor (not shown) and enclosed in the gas-buffered housing 12. Any electrically-isolated cup-like structure within or outside a plasma reactor may perform as a gas-buffered housing 12, as long as it is electrically isolated from the plasma reactor and allows gas flow to exit through the gas outlet 22. The gas-buffered housing 12 may be integral to the plasma reactor. Arrow 30 indicates a gas plasma (also “gas plasma 30”) entering the housing 12 through the gas inlet 20. A plume 32 that contains PAS driven by the flow of gas plasma 30 is shown exiting the gas outlet 22. In concept, the source of the gas plasma is not critical to the invention, and a NTP may be used.
In operation, the gas outlet 22 may submerged into a liquid or the plume 32 may be ejected into the atmosphere or other gaseous media. Gas should be provided at the gas inlet 20 so as to maintain the pressure in the housing 12 equal to or higher than the overall pressure of the environment into which the plume 32 is ejected, so that the housing 12 is not flooded through the gas outlet 22 and the discharge plasma 28 in the housing 12 is sustained. The housing 12 expels gas as a mixture of inflow gas and PAS.
When the gas outlet 22 is submerged in a liquid, the PAS may then interact with the liquid on the surfaces of gas bubbles expelled from the gas outlet 22, or with micro-liquid droplets that exist within gas bubbles created by interaction between the plume and the liquid. A quasi-steady gas cavity will also form at the gas outlet 22, causing a tremendous increase in the area of the liquid-gas interface, which leads to a much higher efficiency of conversion of the chemical species in the liquid.
When ejected into a gaseous media, such as the atmosphere, the PAS may convert constituent gas-phase molecules into reactive species, such as peroxides. The PAS may also convert chemical species at the surfaces of microdroplets or aerosols.
In a series of experiments discussed in the aforementioned '131 Publication, a DC micro-discharge plasma was generated using an MHCD apparatus of the type shown in
The arrangement of the electrical circuit shown in
When air was used as the working gas, direct oxidation of water was achieved in an extremely efficient way without discharging the water itself through the gas outlet 22. The hydrogen peroxide (H2O2) production rate was at least three times better than the best existing plasma-solution interaction method known to the inventors (i.e., capillary discharge in water, as discussed in Nikiforov, A. Yu., and Leys, C., “Influence of capillary geometry and applied voltage on hydrogen peroxide and OH radical formation in AC underwater electrical discharges”, Plasma Sources Sci. Technol. 16 (2007) 273-280, the disclosure of which is incorporated herein by reference).
A combination of two NTP-liquid interface systems may be opposed to each other with one gas-buffered housing biased positively to serve as a virtual anode and the other biased negatively to serve as a virtual cathode. With a flow of gasses from both systems, a gas discharge may be sustained within a quasi-steady state gas cavity generated between the opposing gas outlets.
The following, non-limiting, experimental example may be useful to further illustrate application of the invention in an embodiment according to its first aspect.
PAS Generation: PAS were generated via a MHCD structure, similar to the MHCD apparatus 10 shown in
Introducing PAS into de-ionized water: The apparatus described above was set to create PAS continuously. As the apparatus was held stationary in a vertical position, a beaker containing 100 ml of de-ionized water was raised towards the gas outlet 22 of the gas-buffered housing 12 on a z-stage until the outer surface of the gas outlet 22 was about 2 cm below the surface of the de-ionized water. The flow of ambient air was controlled at a constant rate of about 30 ml/s and allowed to bubble out of the gas outlet 22. The PAS were introduced into the water continuously for about 15 minutes.
Measurement of the H2O2 concentration: The concentration of H2O2 in the treated water sample was evaluated using a HACH® hydrogen peroxide test kit (Model HYP-1; HACH Company, Loveland, Colo., USA). Ammonium molybdate solution was added to the treated de-ionized water sample, followed by the addition of HACH® Sulfite 1 reagent powder. After mixing, the color of the sample turned into a dark blue that was almost black. After 5 minutes, about 1 ml of the prepared sample was collected, and sodium thiosulfate titrant was added drop by drop until the color disappeared completely. Each drop of sodium thiosulfate titrant was counted as 1 mg/L of H2O2.
The H2O2 test showed that about 80 mg of H2O2/L of de-ionized water was produced during 15 minutes of direct introduction of PAS. The amount of H2O2 produced in similar tests would be dependent on air flow rate and electrical current.
pH test: The pH of the treated de-ionized water was tested using a standard pH paper test strip. No obvious color change was observed in tests made on the treated sample, indicating that there was no discernable deviation from the initial liquid pH of 7.
Ion current measurement outside of the gas outlet: The apparatus described above was held vertically, with ambient air as the working gas. Air flow and electrical current were maintained at about 30 ml/s and about 20 mA, respectively. An aluminum plate was connected to an ammeter to detect the ion current outside of the gas outlet 22. The distance between the surface of the aluminum plate and the outer surface of the gas outlet 22 was varied from 0.1 cm to 20 cm. The detected negative ion current was observed to decay with increased distance and ranged from 1 mA at a distance of 0.1 cm to 1 nA at 20 cm.
Application of Multicavity Coupled Plasma Discharges (MCPDS) to the Injection Of Plasma-Activated Species (PAS) into Fluid Media
In another aspect of the present invention, a multicavity coupled plasma discharge (MCPD) is used to inject PAS into fluid media at higher frequencies and higher average energies than may be achieved using a conventional plasma torch with an MHCD. A MCPD is a mode of plasma discharge in which a single primary discharge is used to initiate one or more secondary plasma discharges. In such embodiments of the present invention, multiple MCPDs are generated using a single direct current power source to initiate the primary discharge. The power needed to sustain the subsequent secondary discharges is drawn from the primary discharge by means of an active or a passive coupling scheme.
Referring to
In some embodiments of the invention, the cathode bore 60 has an effective inner diameter of about 1 mm. In some embodiments of the invention, the cathode 58 is between about 180 mm and about 200 mm in length. In some embodiments of the invention, the cathode 58 is made of copper. In some embodiments of the invention, the cathode insulator 52 has an outer diameter of about 3 mm. In some embodiments of the invention, the cathode insulator 52 is made of ceramic. In some embodiments of the invention, the extension tube 48 has an inner diameter between about 4 mm and about 5 mm. In some embodiments of the invention, the extension tube 48 is made of copper.
Continuing to refer to
Continuing to refer to
It may be seen that the cathode insulator 52 and cathode 58 extend into the chamber 80 defined by the insulating bodies 74, 76. The ceramic stop washer 88 is secured within the chamber 80. The cathode 58 passes through the ceramic stop washer 88 and is mechanically joined to the cathode contact cup 84 near a proximal end 92 of the cathode 58, such that the cathode bore 60 remains open. The spring 86 is positioned between the ceramic stop washer 88 and cathode contact cup 84 so as to provide a resilient mechanical connection between them. The arrangement of the ceramic stop washer 88, spring 86, and cathode contact cup 84 is such that the cathode 58 is suspended within the cavity 40 of the MCPD apparatus 34 and remains centered within the extension tube 48. The cathode contact cup 84 is also positioned near a proximal end 94 of the body 38, for reasons discussed further hereinbelow.
At the proximal end 94 of the body 38, the cavity 40 is closed by an electrically-insulating end cap 96 to which the high-voltage contact washer 90 is attached. A high-voltage connector 98 for connection to a high-voltage source (not shown) extends through the end cap 96 such that it is in electrical communication with the high-voltage contact washer 90. The cathode contact cup 84 is in contact with the high-voltage contact washer 90, and such contact is maintained through action of the spring 86. The end cap 96 is arranged such that it may be turned to move the high-voltage contact washer 90 and cathode contact cup 84 in a longitudinal direction, and thus adjust the position of the distal end 68 of the cathode 58 relative to the anode cup 66.
Reference numbers used throughout the remainder of the specification should be read with respect to
In some embodiments of the invention, the nozzle opening 102 has an effective diameter between about 0.8 mm and about 1 mm. In some embodiments of the invention, the nozzle opening 102 has a length between about 1.2 mm and about 1.4 mm. In some embodiments of the invention, the anode cup cavity 100 has an effective diameter between about 3 mm and about 4 mm. In some embodiments, the anode cup cavity 100 has a length between about 3 mm and about 4 mm. In some embodiments, the anode cup cavity 100 is made of brass.
The extension tube 48 and the anode cup 66 are separated by a gap (not shown) containing a dielectric material 106. In some embodiments, the dielectric material 106 is a liquid or solid material that also acts as a seal between the extension tube 48 and the anode cup 66. In other embodiments, the distal end 72 of the extension tube 48 is set back from the abutment 104 so as to provide fluid communication between the air channel 54 and the environment of the nozzle assembly 36 between the extension tube 48 and the anode cup 66. In such embodiments, a portion of the gas flowing through the air channel 54 may be diverted to flow between the anode cup 66 and extension tube 48 and into the environment. In such embodiments, the gas may serve as the dielectric material 106. It may be observed that, because of the arrangement of the nozzle assembly 36, the gas that serves as the dielectric material 106 may be that same gas that drives the primary discharge (i.e., PAS plume 108). Reference numbers not previously mentioned with regard to
The presence of a dielectric material 106 between the extension tube 48 and the anode cup 66 has the effect of electrically-decoupling the anode cup 66 from the extension tube 48. For comparison, in the MHCD apparatus 10 of
The secondary discharge may be a series of filamentary discharges or, in some cases, a continuous arc discharge. Depending on the distance between the anode cup 66 and the extension tube 48 and the dielectric properties of the dielectric material 106, the secondary discharge may be either an arc or a high-frequency filamentary discharge. The pulse frequency of the filamentary discharge between the anode cup 66 and extension tube 48 can be adjusted according to the dielectric properties of the dielectric material 106, the input current, or the spacing between the anode cup 66 and extension tube 48.
To aid in understanding the formation of secondary discharges,
Based on the foregoing discussions regarding
Further to the above discussion, experimentation has shown that the additional voltage drawn by secondary discharges in a MCPD torch results in about a three-fold increase in the frequency of filamentous discharges (i.e. about 1.5 to about 4.5 MHz) and a three-fold increase in average energy of the ejected electron bullets over the performance of a MHCD torch. Without being limited by theory, it appears that the ejection of electron bullets may be necessary to produce the chemical conversions observed in liquid media with an MHCD or MCPD torch. Thus, the ejection of electron bullets related to the secondary discharges of a MCPD torch would greatly enhance the rates of chemical conversion that can be achieved. Very low rates of conversion, if any, would be achieved in a micro-arc discharge regime.
As may be understood from the disclosures made herein, MCPDs make it possible to convert a DC-driven atmospheric pressure micro-flow discharge to a power modulator unit, thereby creating conditions for an additional source of energy (e.g., for a secondary plasma source), utilizing a single power supply. Beyond the embodiments disclosed in detail herein, this approach may be used to supply high-frequency voltages for other discharges using a single power supply, whether such discharges are located on a single device or on remote devices. One such application that has been demonstrated is the harnessing of the secondary discharge from an MCPD torch to power a MHCD torch. Air was used as the PAS carrier and dielectric material 106 in the MCPD torch. The anode cup 66 of the MCPD torch was connected to the cathode (i.e., embedded electrode 16 of
Other advantages include the provision of a high ratio of voltage amplification (e.g., a ratio of 1 to 20), and ultra-fast time compression (e.g., direct current pulse in the nanosecond regime). All of these advantages are achieved within a small-volume gap within the MCPD apparatus (i.e., within a volume of a few cubic centimeters). Further, in view of the disclosure of MCPD devices made herein, one having ordinary skill in the relevant arts would realize that MCPD devices generating multiple secondary plasma discharges may also be made.
It should be understood that the embodiments of the invention discussed herein are merely exemplary and that a person skilled in the relevant arts may make many variations and modifications without departing from the spirit and scope of the invention.
This application is a continuation-in-part of U.S. patent application Ser. No. 12/201,229, filed on Aug. 29, 2008, the disclosure of which is incorporated herein by reference, which claims benefit of U.S. Provisional Patent Application No. 60/969,326, filed Aug. 31, 2007, and U.S. Provisional Patent Application No. 61/128,675, filed May 23, 2008, and further directly claims benefit of U.S. Provisional Patent Application No. 61/128,675, filed May 23, 2008, the disclosure of which is incorporated herein by reference.
Number | Date | Country | |
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60969326 | Aug 2007 | US | |
61128675 | May 2008 | US |
Number | Date | Country | |
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Parent | 12201229 | Aug 2008 | US |
Child | 12471037 | US |