Patent Application
20110048251
The invention relates to a method of enhancing gas-phase reaction in a plasma. The invention further relates to a system for enhanced gas-phase reaction in a plasma.
A plasma is an ionized gas. Active species of ions, electrons, high-energy neutrals, radicals as well as ultra-violet emission in plasmas can be used for various things. Plasma generation or a generated plasma may also advantageously be utilized for various purposes or applications like ozone generation, hydrogen production, exhaust gas cleaning, pollution control, odor removal, fuel conversion, sterilization, oxidation, etc.
Ozone can be formed by recombination in triplets of single oxygen atoms which are split from the diatomic oxygen in a plasma. Fuel conversion is the chemical or physical transformation of a naturally occurring or already modified fuel to improve the quality of the fuel. A fuel conversion process may result in one or more upgraded fuel products which may be solid, liquid, or gaseous, and may generate chemicals or raw materials for chemical manufacture. For example, coal generally requires size reduction, washing, and removal of inert species. Natural gas may need removal of H2S and CO2 with separation of gas liquids and C2 compounds. However, “gas-to-liquid” fuel conversion (liquefaction) is also of significant importance. Here CH4, which is the major component of natural gas, is converted to liquid fuels in one or more than one step chemical reactions.
A variety of plasmas exists, including direct current plasmas, capacitively coupled plasmas, pulsed plasmas, magnetron plasmas, electron cyclotron resonance plasmas, inductively coupled plasmas, helicon plasmas, helical resonator plasmas, microwave plasmas, and plasma jets (see e.g. A Bogaerts et al. Spectrochimica Pt.B 57 (2002) 609-658.). Many of them are operated at low pressures, suffering from the drawbacks that they require expensive vacuum systems. Furthermore, methods are only well-developed for batch or semi-batch treatments. To overcome these drawbacks an atmospheric pressure plasma surface modification system can be used that not only avoids the need for vacuum equipment but also permits both the surface modification of large objects and production line continuous surface modification (see e.g. C Tendero et al. Spectrochimica Pt.B 61 (2006) 2-30.).
A prior art plasma application system is shown in
Other types or variations of plasma sources include dielectric barrier discharges (DBDs) with a single dielectric barrier located substantially in the middle between the two electrodes or with a single dielectric barrier covering only one of the electrodes. Such plasma sources are typically also referred to as volume discharge (VD) sources where a micro-discharge can take place in thin channels generally randomly distributed over the electrode- and/or dielectric-surface. Other DBD plasma sources include so-called surface discharge (SD) plasma sources typically comprising a number of surface electrodes on a dielectric layer and a counter-electrode on the reverse side of the dielectric layer. Such SD plasma sources may include a so-called SPCP (Surface-discharge-induced Plasma Chemical Processing) discharge element or CDSD (Coplanar Diffuse Surface Discharge) element. In a SPCP, electrodes are attached on the dielectric(s) and in a CDSD the electrodes are embedded in the dielectric(s).
Other types of plasma sources are e.g. so-called plasma torches such as arc plasma torches, cold plasma torches (see e.g. H Mortensen et al. Jpn. J. Appl. Phys. 45(10B) (2006) 8506-8511.), atmospheric pressure plasma jet (APPJ), pencil like torches, barrier torches, and microwave torches (see e.g. C Tendero et al. Spectrochimica Pt.B 61 (2006) 2-30.). Yet another type of plasma source is the so-called gliding arc (see for example A Fridman et al. J. Phys. D Appl. Phys. 38 (2005) R1-R24).
Additional types of plasma sources are low pressure plasmas, corona discharge (see e.g. A Bogaerts et al. Spectrochimica Pt.B 57 (2002) 609-658) and microplasmas (see e.g. V Karanassios Specrochimica Acta Pt.B 59 (2003) 909-928). See e.g. A Bogaerts et al. Spectrochimica Pt.B 57 (2002) 609-658, U Kogelschatz Plasma Chem. Plasma Proc. 23(1) (2003) 1-46, C Tendero et al. Spectrochimica Pt.B 61 (2006) 2-30 and A Fridman et al. J. Phys. D Appl. Phys. 38 (2005) R1-R24) for further details of plasmas and atmospheric pressure plasmas.
The three articles ‘Ozone generation by hollow-needle to plate electrical in an ultrasound field’, J. Phys. D: Appl. Phys. 37 (2004) 1214-1220,‘Ultrasound and airflow induced thermal instability suppression of DC corona discharge: an experimental study’, Plasma Sources Sci. Technol. 15 (2006) 52-58, and ‘Ultrasonic resonator with electrical discharge cell for ozone generation’, Ultrasonics 46 (2007) 227-234, by Stanislav Pekárek, Rudolf Bálek et al. disclose suppression of DC corona discharge where ultrasound or ultrasound combined with an airflow is used in connection with a hollow needle-to-plate electrode system to activate the corona discharge for ozone production. It was also found that the application of ultrasound waves increases ozone generation.
The article ‘Improvement of Charging Performance of Corona Charger in Electrophotography by Irradiating Ultrasonic Wave to Surrounding Region of Corona Electrode’ (Kwang-Seok Choi, Satoshi Nakamura and Yuji Murata Jpn. J. Appl. Phys. 44(5A) (2005) 3248-3252.) discloses improvement of the charging speed of a corona charger in electrophotography using an ultrasonic wave where the ultrasonic wave increases the charge density on an insulator layer of a coated aluminum drum used instead of a photoreceptor drum used for printing. At least some of the findings in the articles have also been disclosed in patent application CZ 295687.
The ultrasonic generators disclosed in the above-mentioned articles are based on piezoelectric transducers. No mention is given of specific or preferred sound pressure levels of the emitted acoustic waves or ultrasonic waves or the advantages thereof.
Furthermore, the articles mention that the acoustic pressures developed by ultrasonic layouts are, respectively, of the order of 2 and 10 kPa near the emitting surface of the transducer at the frequency of generated ultrasound of 20.3 kHz. In the fourth article, the ultrasonic generator is a 28-kHz 50-mm-diam and 80-mm-height bolt-clamped Langevin-type piezoelectric transducer. The maximum input power is 50 W. These values give an estimation of the emitted acoustic pressure value to be approximately 2 kPa. The pressure values of 2 and 10 kPa correspond to very high sound pressure levels of 160 and 174 dB above the reference pressure of 20 μPa. It can be estimated that the above-specified acoustic pressures at the above frequency correspond to ultrasound intensities of 4.4 and 20 kW/m2 or the sound intensity levels of 156 and 163 dB above the reference intensity of 1 pW/m2. This characterizes the ultrasonic acoustic waves being applied in at least some of these articles as high-intensity. However, the acoustic power provided according to the articles is in fact too low and too localized to allow for efficient and high-volume enhancement of the gas-phase in a plasma.
A similar situation can be outlined regarding the patent specification U.S. Pat. No. 6,391,118. It discloses a method for removing particles from the surface of an article in an apparatus using corona discharge. The particles are supplied with an electric charge and subsequently an ultrasonic wave or gas stream is applied onto the surface of the article while an electric field is applied for driving away the electrically charged solid particles from the surface. The application of an ultrasonic wave and/or gas further facilitate the removal of the electrically charged solid particles. The variety of ultrasonic generators (oscillators) here includes a piezoelectric oscillator, a polymer piezoelectric membrane, an electrostrictive oscillator, a Langevin oscillator (that is as mentioned above just a special type of piezoelectric transducers), a magnetostrictive oscillator, an electrodynamic transformer, and a capacitor transformer. Use of such oscillators provides acoustic power that is low (no more than 50 W) and localized. It is too low and localized to allow for efficient and high-volume enhancement of the gas-phase in a plasma. Moreover, no disclosure is given of a specific or preferred sound pressure level of the emitted ultrasonic waves or the advantages thereof.
The article ‘Current Waveforms of Electric Discharge in Air under High-Intensity Acoustic Standing Wave Field’, Japanese Journal of Applied Physics, Vol. 43, No. 5B, 2004, pp. 2852-2856, Nakane et al. also discloses an electric discharge phenomenon in a high-intensity acoustic field. Here standing waves with a frequency of 660 Hz are used, which limits a discharge volume for ozone production. The enhanced ozone production according to this article can be explained by Paschen's law signifying that the lower the static pressure, the higher the growth rate of the discharge streamers. Therefore, the streamers should grow more intensively in the nodes of standing waves. Another part of the proposed mechanism is that streamer channels oscillate in the acoustic field.
Patent application US 2003/0165636 discloses a process for atmospheric pressure plasma surface modification of an object's surface where excitation of the surface to be treated is done so that it vibrates and undulates thereby activating the application of plasma. The energy for excitation of the surface may come from the process of creating the plasma, from an external source, or from a combination thereof. The energy for excitation of the surface may come from a vibration generator brought in contact with the object to be treated or by indirect contact from a vibration generator emitting acoustic waves, e.g. ultrasonic waves, to the object to be treated so that it provokes turbulent plasma. No disclosure is given of a specific or preferred sound pressure level of the acoustic waves or the ultrasonic waves or the advantages thereof. Therefore, exciting surface vibrations and undulations, or in other words, generation of guided and surface acoustic waves on the object is suggested in order to intensify a plasma treatment. Correspondingly, it is disclosed that the vibration of the surface to be treated can be the result of excitation at one or several eigenfrequencies and their harmonics associated with the body of the object to be treated. Thus, either the range of the characteristic dimension of the modified object (primarily its thickness) is strictly limited by the operating frequency of the used source of acoustic energy, or the said frequency is strictly determined by the dimension of the object. It is also disclosed that the vibration of the surface can also result from forced frequencies when an external generator of acoustic waves emits frequencies that are not harmonics of the eigenfrequencies of the object to be treated. This signifies generation of surface acoustic waves (primarily the Rayleigh surface waves).
The following procedures of transfer of acoustic power into ambient gas/plasma are mentioned:
Both procedures require acoustic waves to overpass the solid/gas interface at least once. However, due to more than four-orders-of-magnitude difference in acoustic impedance for a solid and a gas, most of generated acoustic power cannot be emitted (and especially re-emitted) into the gas atmosphere and remains in a solid being ultimately converted into thermal energy. Thus, it is not possible in this way to generate sound or ultrasound in the air with a power that would be enough to enable efficient and high-volume enhancement of a gas-phase in a plasma. Therefore, it is of prime importance not simply to “shake” the surface up and provoke uncontrolled turbulent plasma with unknown efficiency and spatial distribution in such a way, but rather to enable efficient and high-volume enhancement of the gas-phase in a plasma.
The generated acoustic power is relatively low (at any rate, below 100 W) because the power applied to the acoustic wave transmitter is actually 100 W, and the efficiency of sound generation in a gas atmosphere cannot exceed ˜30% even for the most effective gas-jet ultrasonic transmitters, not to mention other methods.
A principal impediment to the generation of a plasma or other plasma processes are how to enable a very efficient gas-phase reaction over large volumes in a plasma.
None of the mentioned prior art disclosures specify an acoustic power level sufficient to efficiently allow for efficient and high-volume enhancement of the gas-phase in a plasma.
Furthermore, the prior art involving piezoelectric transducers or other transducers involving a solid to transfer the energy, only provides the energy in a very localized fashion, e.g. very close to the piezoelectric transducer (or other solid transducer) and is therefore not suitable for high volume gas-phase enhancement.
It is an object of the invention to provide a method of and a system for enhanced gas-phase reaction in a plasma that alleviates the above-mentioned shortcomings of prior art at least to an extent.
It is another object to enable enhancement of a plasma gas-phase reaction in a large volume.
An additional object is to speed a plasma process up.
A further object of some embodiments is to enhance a generation process involving a plasma—e.g. hydrogen, ozone or syngas generation using plasma, exhaust gas cleaning, pollution control, odor removal, fuel conversion, liquefaction, sterilization, oxidation, etc. using a plasma.
These objectives are obtained at least to an extent by a method of enhancing a gas-phase reaction in a plasma comprising: creating plasma by at least one plasma source, and wherein that the method further comprises: generating ultrasonic high intensity and high power acoustic waves having a predetermined amount of acoustic energy by at least one ultrasonic high intensity and high power gas-jet acoustic wave generator, where said ultrasonic high intensity and high power acoustic waves are directed to propagate towards said plasma so that at least a part of said predetermined amount of acoustic energy is absorbed into said plasma, and where a sound pressure level of said generated ultrasonic high intensity and high power acoustic waves is at least substantially 140 dB and where an acoustic power of said generated ultrasonic high intensity and high power acoustic waves is at least substantially 100 W.
In this way, a high sound intensity and power are obtained that efficiently enhances a gas-phase reaction in the plasma, which enhances the plasma process, e.g. enabling more efficient ozone or syngas generation using plasma, exhaust gas cleaning, pollution control, odor removal, fuel conversion, liquefaction, sterilization, oxidation, etc using a plasma in relation to reaction speed and/or obtained concentration of the generated compound.
Increasing the concentration of aimed products, reducing the concentration of pollutants, odor etc. and/or reducing the reaction time, reduces the cost of the process since the process is expensive and requires a lot of energy.
As a comparison, the sound-emitting surface area of transducers, e.g. like the ones described in the previous mentioned prior art articles (‘Ozone generation by hollow-needle to plate electrical in an ultrasound field’; ‘Ultrasound and airflow induced thermal instability suppression of DC corona discharge: an experimental study’; and ‘Improvement of Charging Performance of Corona Charger in Electrophotography by Irradiating Ultrasonic Wave to Surrounding Region of Corona Electrode’) is of the order of 2×10−3 m2 and the emitted acoustic power is 10-40 W.
The acoustic power provided by a high-power gas-jet generator is capable of much higher acoustic power outputs along with the high sound pressure and intensity levels of 140 or 150 to 170 dB (see e.g. Y. Y. Borisov, Acoustic gas-jet generators of Hartmann type, in L. D. Rozenberg (ed.) Sources of High-Intensity Ultrasound (New York: Plenum: 1969) part I. and Levavasseur, R. High power generators of sound and ultrasound. US patent, book 116-137, U.S. Pat. No. 2,755,767 (1956).).
It has been shown that an acoustic power of about 100 W or more enhances the gas-phase reaction in a plasma.
A high-power gas-jet generator is normally capable of an acoustic power of several hundreds watts (i.e. approximately one order of magnitude higher than the piezoelectric transducer acoustic power output) and typical SPL (sound pressure level) of 160 dB at 10 cm from the generator orifice at the frequency of 20-30 kHz. Even an acoustic power of 1-2 kW is attainable.
The main physical reason for such a dramatic difference in acoustic power outputs of piezoelectric (or other solid-state acoustic transducers) and gas-jet generators is that a piezoelectric transducer works by vibrating (using sound) a solid being in contact with a gas and thus transfers the vibrations to the gas. Due to the tremendous difference in acoustic impedance for a solid and a gas (a so-called acoustic impedance mismatch), most of generated acoustic power cannot be emitted in the ambient gas and remains in a solid. It is converted into thermal energy and results in a transducer warming-up.
Consequently, it is not possible in this way to generate sound or ultrasound in the gas with a power that would be enough to enhance a gas-phase reaction in a plasma sufficiently. In fact, a single piezoelectric transducer provides the high-intensity ultrasound radiation only nearby its emitting surface and irradiates a limited surface area that is comparable with the area of its emitting surface. That is because of the acoustic wave diffraction, which is significant when the transducer diameter is comparable with the acoustic wavelength. Indeed, for ultrasound frequency of 20-30 kHz in the air the wavelength is about 10-20 mm that is of the order of the actual transducer diameter. In the case of gas-jet ultrasonic transmitters, a vibrating media is not a solid but a gas. It is clear that there is no any impedance mismatch and high enough acoustic power can be emitted in the ambient gas. Moreover, intensity and sound pressure levels of ultrasound radiation remain very high at several tens of centimeters from the gas-jet transmitter orifice while the acoustic wave front is broad (it is sometimes just a spherical wave front). In this way, it is possible to expose large volumes to high-intensity ultrasound (sound intensity and sound pressure levels of substantially 140 dB and above at approximately 10 cm from the generator's orifice) and enable an efficient gas-phase reaction in a plasma.
In one embodiment, the acoustic pressure level of said generated ultrasonic high intensity and high power acoustic waves is
In one embodiment, the acoustic power of said generated ultrasonic high intensity and high power acoustic waves is
It is to be understood, that if several acoustic generators are used even higher powers may be obtained.
In one embodiment, the plasma source comprises at least one source selected from a group of: a dielectric barrier discharge (DBD) plasma source, a surface discharge (SD) plasma source, a volume discharge (VD) plasma source, a plasma torch source, an arc plasma torch, a gliding arc plasma torch, a cold plasma torch, a pencil-like torch, a direct current plasma source, a capacitively coupled plasma source, a pulsed plasma source, a magnetron plasma source, an electron cyclotron resonance plasma source, an inductively coupled plasma source, a helicon plasma source, a helical resonator plasma source, a microwave plasma source, an atmospheric pressure plasma jet (APPJ) source, a barrier torch, an arc microwave torch, a corona discharge plasma source, a micro-plasma source, a low pressure plasma source, and a high pressure plasma source.
In one embodiment, a working gas pressure at an inlet of the at least one ultrasonic high intensity and high power gas-jet acoustic wave generator is between approximately 1.9 and approximately 5 bar.
In one embodiment, the plasma is created at atmospheric pressure.
In one embodiment, the plasma source comprises at least one electrode and wherein one electrode of said at least one electrode is a mesh type of electrode.
This allows the gas/energy to pass through the ‘upper’ electrode in a very simple and efficient way.
In one embodiment, the generated ultrasonic high intensity and high power acoustic waves are propagated towards a membrane so that any gases used by the at least one ultrasonic high intensity and high power acoustic wave generator is not mixed with one or more gases used by said plasma source to create said plasma.
In one embodiment, the generated ultrasonic high intensity and high power acoustic waves are generated using a gaseous medium and where the acoustic waves are directed towards said plasma and wherein said gaseous medium after exit of said at least one ultrasonic high intensity and high power gas-jet acoustic wave generator is directed away from said plasma.
In one embodiment, the generated ultrasonic high intensity and high power acoustic waves do not spatially overlap with the working gas flow outgoing from the generator orifice. Moreover, since the generated ultrasonic high intensity and high power acoustic waves are directed toward the plasma and the gas outgoing from the ultrasonic high intensity and high power acoustic wave generator do not overlap in space, the said outgoing working gas is not mixed with one or more gases used by said plasma source to create said plasma.
In these ways, control of the gas environment for the plasma generation process is enabled.
In one embodiment, a gas mixture, which is used for creating the plasma, is supplied to at least one electrode of the plasma source substantially in a direction that said ultrasonic acoustic waves propagate towards said plasma.
In one embodiment, at least one of the ultrasonic high intensity and high power gas-jet acoustic wave generators are selected from the group of:
In one embodiment, a food item is subjected to the plasma where the creation of the plasma generates chemical radicals and sterilizes the food item.
In one embodiment, the generating ultrasonic high intensity and high power acoustic waves comprises:
In one embodiment, the plasma is used in a process selected from the group of:
The present invention also relates to a system corresponding to the method of the present invention. More specifically, the invention relates to a system for enhancing a gas-phase reaction in a plasma comprising: at least one plasma source adapted to create plasma, wherein that the system further comprises: at least one ultrasonic high intensity and high power gas-jet acoustic wave generator adapted to generate ultrasonic high intensity and high power acoustic waves having a predetermined amount of acoustic energy and being directed to propagate towards said plasma so that at least a part of said predetermined amount of acoustic energy is absorbed into said plasma, and where a sound pressure level of said generated ultrasonic high intensity and high power acoustic waves is at least substantially 140 dB and where an acoustic power of said generated ultrasonic high intensity and high power acoustic waves is at least 100 W.
Advantageous embodiments of the system are defined in the sub-claims and are described in detail in the following. The embodiments of the system correspond to the embodiments of the method and have the same advantages for the same reasons.
These and other aspects of the invention will be apparent from and elucidated with reference to the illustrative embodiments shown in the drawings, in which:
a schematically illustrates a (turbulent) flow without application of ultrasonic high intensity and high power acoustic waves;
b schematically illustrates a flow where the effect of applying ultrasonic high intensity and high power acoustic waves to/in air/gas is illustrated;
Throughout the figures, same reference numerals indicate similar or corresponding features.
Further shown are two electrodes (103) placed apart with a discharge gap between them, where at least one of the electrodes (103) is adjoined or covered with an insulating or dielectric material (105) on a side of the electrode facing the other electrode in order to avoid arcing. In the figure, both electrodes (103) are adjoined or covered with dielectric material (105). One electrode (103) is connected to a suitable power supply (114), being connected to ground and supplying AC high voltage, e.g. 0.1 kHz-500 kHz, between the electrodes (103).
Further, shown is a high voltage probe (113) connected to the power supply (114) and an oscilloscope connected to the high voltage probe (113). The high voltage probe is used for monitoring the applied voltage, but is not relevant for and does not influence plasma process.
The gas mix is supplied from the gas-mixing unit (110) to the discharge area between the two electrodes (103) and, as a result, plasma (104) is created when voltage is applied to the electrodes (103). A specimen or object (not shown) may be located in the plasma (104) e.g. for surface modification, treatment, processing, etc.
The plasma process taking place may e.g. involve ozone generation, hydrogen production, syngas production, exhaust gas cleaning, pollution control, odor removal, fuel conversion, sterilization, oxidation, etc.
Ozone can be formed by recombination in triplets of single oxygen atoms which are split from the diatomic oxygen in a plasma. Fuel conversion is the chemical or physical transformation of a naturally occurring or already modified fuel to improve the quality of the fuel. A fuel conversion process may result in one or more upgraded fuel products which may be solid, liquid, or gaseous, and may generate chemicals or raw materials for chemical manufacture. For example, coal generally requires size reduction, washing, and removal of inert species. Natural gas may need removal of H2S and CO2 with separation of gas liquids and C2 compounds. However, “gas-to-liquid” fuel conversion (liquefaction) is also of significant importance. Here CH4, which is the major component of natural gas, is converted to liquid fuels in one or more than one step chemical reactions. One well-known process is first synthesize a syngas (CO+H2);
CH4+H2O→CO+3H2
CH4+C2O→2CO+2H2
or
CH4+O2→CO+2H2
and subsequently synthesize liquids;
CO+H2→gasoline, diesel, alcohol, etc.
Plasma can also synthesize liquids (methanol, ethanol, benzene etc.) and other molecules (ethane, propane, ethylene, acetylene, propylene benzene and heavier hydrocarbons) (K. V. Kozlov, P. Michel, H. E. Wagner “Synthesis of organic compounds from mixtures of methane with carbon dioxide in dielectric barrier discharge at atmospheric pressure” Plasmas and Polymers, 5(¾) (2000) 129-150, S. Kado, Y. Sekine, T. Nozaki, K. Okazaki “Diagnosis of atmospheric pressure low temperature plasma and application to high efficient methane conversion” Catalysis Today 89 (2004) 47-55 etc.). Produced hydrogen gas can also be used for other purposes such as an environmentally friendly fuel for fuel cells. Petroleum refining and coal gasification are also examples. Examples of quality upgrades are the manufacture of automotive gasoline by cracking of petroleum components, and removal of sulphur and nitrogen from liquid fuels by reacting the fuel with hydrogen.
Exhaust gas cleaning can be performed by passing through a plasma or injecting activated species in to the exhaust. Its examples are NOx and SO2 reduction.
Plasmas are useful for decomposing toxic gas components such as volatile organic compounds (VOCs) (K. Urashima, J. S. Chang “Removal of volatile compounds from air streams and industrial flue gases by non-thermal plasma technology” IEEE Transactions on Dielectrics and Electrical Engineering 7(5) (2000) 602-614) and odor such as NH3 (L. Xia, L. Huang, X. Shu, R. Zhang, W. Dong, H. Hou “Removal of ammonia from gas streams with dielectric barrier discharge plasmas” 152 (2008) 113-119). They are generally decomposed by oxidation in a plasma.
A gas-phase reaction in the plasma (104) can be enhanced as shown and explained in the following.
Further illustrated are one or more ultrasonic high intensity and high power acoustic wave generators (101) generating high intensity and high power ultrasound (102). According to the present invention, the generated ultrasound (102) is applied to at least a part of the plasma (104) whereby at least a part of the acoustic energy is absorbed by the plasma (104). The addition of energy to the plasma (104) will enhance the gas-phase reaction and will enhance the plasma process taking place. The application of high intensity and high power ultrasonic acoustic waves with a sound pressure level of at least substantially 140 dB and an energy of at least substantially 100 W will enhance the gas-phase reaction and thereby the plasma process significantly, as explained in the following in connection with
The working gas pressure at the inlet of the ultrasonic high intensity and high power acoustic wave generators (101) may be optimized so that high acoustic pressure can be generated. It is preferably between 1.9 and 5 bar or between 2.5 to 4 bar and will typically depend on the type generator used. The gas pressure at the outlet of the high-power gas-jet generators is lower than that at the inlet, and can be practically nearly equal to the gas pressure for the plasma process.
The air-pressure required for operation of gas-jet high-intensity and high-power ultrasonic generators is at least over 1.9 bar for operation under normal conditions and the pressure required for optimal operation providing stable generation of ultrasound with a SPL over 140 dB at 10 cm from the generator orifice is 2.5 to 4 bar depending on a generator type.
The one or more plasma sources (106) may be any plasma source suitable for the given plasma process, e.g. such sources as explained earlier and in the following and/or combinations thereof or such sources e.g. using one or more gases, as explained earlier, in creating the plasma. The plasma source(s) (106) can be chosen among any existing ones (both low and high pressure plasmas), and more specifically may be e.g. direct current plasmas, capacitively coupled plasmas, pulsed plasmas, magnetron plasmas, electron cyclotron resonance plasmas, inductively coupled plasmas, helicon plasmas, helical resonator plasmas, microwave plasmas, DBDs, SDs, plasma torches such as arc plasma torches, cold plasma torches, APPJs, pencil like torches, barrier torches, arc plasma torches, microwave torches, gliding arc, corona discharge, and microplasmas.
The gas pressure for the plasma process is preferably higher than 0.4 bar and may be around atmospheric pressure or more, so that the acoustic energy can be delivered efficiently. One the other hand, it is easier to generate plasmas at lower pressures. Therefore, the gas pressure for the plasma process is preferably more than 0.4 bar and less than the pressure at the inlet of the high-power gas-jet generators. More preferable plasma source may be DBDs, SDs, plasma torches such as arc plasma torches, cold plasma torches, APPJs, pencil like torches, barrier torches, microwave torches, gliding arc, corona discharge, and microplasmas, which can be operated at the pressures mentioned above.
The one or more ultrasonic high intensity and high power acoustic wave generator (101) is a gas-jet acoustic wave generator and may e.g. be one or more Hartmann type gas-jet generators, one or more Levavasseur type gas-jet generators, etc. or combinations thereof and as explained in the following and as shown in
The use of a gas-jet acoustic wave generator has advantages like described earlier in terms of acoustic power, high intensity, acoustic impedance, etc.
If more than a single ultrasonic and high intensity and high power acoustic wave generator is used they need not be of the same type although they can be.
The plasma process may e.g. be ozone generation, hydrogen production, exhaust gas cleaning, pollution control, fuel conversion, sterilization, oxidation, etc.
In one embodiment, a food item is subjected to the plasma process where the process will generate chemical radicals and sterilize the food item in a very efficient way.
a schematically illustrates a (turbulent) flow without application of ultrasonic high intensity and high power acoustic waves.
Shown is a surface (314) of a solid object (100) where a gas or a mixture of gases (500) surrounds or contacts the surface (314).
Thermal energy can be transported through gas by conduction as well as by the movement of the gas from one region to another. This process of heat transfer associated with gas movement is usually referred to as convection. When the gas motion is caused only by buoyancy forces set up by temperature differences, the process is normally referred to as natural or free convection; but if the gas motion is caused by some other mechanism, it is usually referred to as forced convection. With a condition of forced convection, there will be a laminar boundary layer (311) near to the surface (314). The thickness of this layer is a decreasing function of the Reynolds number of the flow, so that at high flow velocities, the thickness of the laminar boundary layer (311) will decrease. When the flow becomes turbulent the layer are divided into a turbulent boundary layer (312) and a laminar boundary layer (313). For nearly all practically occurring gas flows, the flow regime will be turbulent in the entirety of the streaming volume, except for the laminar boundary layer (313) covering the surface (314) wherein the flow regime is laminar. Considering a gas molecule or a particle (315) in the laminar boundary layer (313), the velocity (316) will be substantially parallel to the surface (314) and equal to the velocity of the laminar boundary layer (313). Heat transport across the laminar boundary layer will be by conduction or radiation, due to the nature of laminar flow.
Furthermore, mass transport across the laminar boundary layer will be solely by diffusion. The presence of the laminar boundary layer (313) does not provide optimal or efficient increased mass transport. Any mass transport across the boundary layer will be solely by diffusion, and therefore often be the final limiting factor in an overall mass transport.
One impediment to the transfer or transmission of energy and/or mass from a gas to a solid surface is the boundary layer of the gas, which adheres to the solid surface. Even when the motion of the gas is fully turbulent, the laminar boundary layer exists that obstructs mass transport and/or heat transfer. While various methods and types of apparatus have been suggested for overcoming the problem such as by means of driving the gas with sonic waves and vibrating the solid object (100) with external vibration generators, these methods while being effective to some extent, are inherently limited in their ability to generate an effective minimization of the laminar boundary layer and at the same time covering an area large enough to make the method efficient.
b schematically illustrates a flow where the effect of applying high intensity and high power ultrasonic acoustic waves to/in air/gas (500) is illustrated.
More specifically,
Again consider a gas molecule/particle (315) in the laminar layer; the velocity (316) will be substantially parallel to the surface (314) and equal to the velocity of the laminar layer prior applying ultrasound. In the direction of the emitted sound field to the surface (314) in
The thinning or destruction of the laminar boundary layer has the effect that heat transfer and mass transport from the surface (314) to the surrounding or contacting gas (500) greatly is increased, as the presence or the reduced size of the laminar boundary layer no longer will hind heat transfer and/or mass transport to the surface of the solid object(s) (100) being subjected to plasma surface modification, i.e. the plasma will more efficiently influence the surface of the object.
Furthermore, the high intensity and high power ultrasonic acoustic waves is absorbed in the plasma so that the reaction is enhanced.
Various embodiments are described in connection with the following figures.
Shown is at least one ultrasonic high intensity and high power acoustic wave gas-jet generator (101) generating high intensity and high power ultrasonic acoustic waves (102) propagating towards and reaching a plasma (104), which will absorb the substantial acoustic energy whereby a gas-phase reaction in the plasma (104) will be enhanced due to the received energy.
A plasma (104) is created by a plasma source (106) using the shown gas flow, the shown electrodes (103; 103′), and an insulator or dielectric material (105) e.g. as explained in connection with
The insulator or dielectric material (105) may e.g. comprise Al2O3 or in general material having a dielectric property or any kind of insulators such as ceramics, polymers and glasses. Ceramics and glasses are more durable against plasma since they have relatively high temperature resistance. They are often preferred, since they typically have high dielectric constants and thus plasma can be generated and sustained at lower AC voltages.
Further shown is a horn or the like (402) or sound guiding or directing means that ensures that the sound intensity and power is contained and focused towards the plasma/object.
In one embodiment, a membrane (401) or similar is located between the high intensity and high power ultrasonic acoustic wave generator (102) and the plasma. This enables control of the gas environment for the plasma generation process so that only the received gas flow is used in creating the plasma. This may be useful for gas driven generators (102) so that the gas from such generators do not interfere with the gas mix used for plasma creation. Other embodiments may exclude the membrane (401). The membrane (401) is preferably relatively thin and relatively transparent to ultrasound. The thickness, size, and/or shape of the membrane (401) and tension applied to it may be optimized for decreasing a loss of ultrasound.
In some embodiments, the membrane can be dispensed with even though it is not preferred that a mix of the gaseous medium used for generating the high intensity and high power ultrasonic acoustic waves and the gas(es) used for creating the plasma occurs. This can be achieved by having a high intensity and high power acoustic wave generator where the generated acoustic waves propagates generally in another direction than the general direction of the gaseous medium after exiting the acoustic wave generator.
In
Generators can be designed so that the two directions are about opposite. For instance, stem-jet Hartmann generators radiating ultrasound in the so-called high-frequency regime allow such “natural” spatial separation of the ultrasound field and the outgoing gas flow (see e.g. Y. Y. Borisov, Acoustic gas-jet generators of Hartmann type, in L. D. Rozenberg (ed.) Sources of High-Intensity Ultrasound (New York: Plenum: 1969) part I.). Such generators can be very useful in avoiding the use of a membrane as the gaseous medium may directed away from the plasma. In this way, no gas(es) used for generating the acoustic waves will influence the plasma gas(es). It is to be understood that even in such an arrangement a membrane may still be useful (although it may be of a different design) since it can contain the gas(es) used for creating the plasma contained so they do not diffuse into the surrounding environment, which may be useful since some have a significant cost.
Any kinds of membranes can be used, as long as there is neither significant loss of ultrasound nor significant gas leakage. As long as they can form thin films, their materials can be chosen from any thermoplastic and thermoset polymers such as polyesters, polyethylene terephthalate, polyolefins (low density (LD) polyethylene (PE), high density (HD) PE, ultrahigh density PE, ultrahigh molecular weight PE, polypropylene, poly(vinyl chloride), poly (vinylidene chloride), polystyrene, polyimide, polyamide, poly (vinyl vinyl ether), polyisobutylene, polycarbonate, polystyrene, polyurethane, poly (vinyl acetate), poly-acrylonitrile, natural and synthetic rubbers, polymer alloys, copolymers, and their laminates. They can be coated with organic and/or inorganic materials using any existing techniques. Among them, lower density materials such as PE can be used. Furthermore, a metal foil may be used as a membrane. Other examples are metal coated (or material coated with inorganic material) or laminated polymer membranes.
As an alternative, the membrane may comprise or consist of Aerogel.
In one embodiment, the electrode located between the plasma (104) and the generator (101) is a mesh type of electrode (103′) or another type of perforated electrode. This enables the generated ultrasound to virtually pass unhindered to the object(s) (100) without loosing a significant amount of energy whereby as much energy as possible is present for influencing the laminar sub-layer around the object(s) (100). Other embodiments may exclude the mesh type/perforated electrode (103′).
The direction of outgoing gas/gas mixture, used for creating the plasma (104), and the ultrasound (102) is quite controllable and the angle between their principal directions may vary. In the shown embodiment, the angle is about 90°. But the angle may in principal be any angle. In
The gas or gas mixture used for creating the plasma (104) may also be used for driving the gas-jet acoustic generator(s) (101). The plasma (104) may be generated before, in or after the acoustic generator (101).
The torch plasma source could e.g. be a barrier torch design or cold plasma torch design as well-known in the art.
The high intensity and high power acoustic waves (102) generated by the first generator (101) propagate in a principal direction as schematically indicated by arrows (B) that is different than the general direction of the gaseous medium (A) from the first generator (101) due to the design of the high power acoustic wave generator (101).
The high intensity and high power acoustic waves (102) generated by the second generator (101′) propagate in a general direction as schematically indicated by arrow (B′).
One example of a high intensity and high power acoustic wave generator operating in a way like this is shown and explained in connection with
The first (101) and the second high power acoustic wave generator (101) are located in relation to each other so that at least a part of the generated high intensity and high power acoustic waves (102) from the second acoustic wave generator (101′) has a general direction (B′) that is directed towards at least a part of the gaseous medium (121) from the first acoustic wave generator (101) and that at least a part of the generated high intensity and high power acoustic waves (102) from the first acoustic wave generator (101) has a general direction (B) that is directed towards at least a part of the gaseous medium (121) from the second acoustic wave generator (101′).
By directing high intensity and high power acoustic waves generated by the second generator (101) directly towards the gaseous medium (121) from the first generator (101), energy is supplied in as a direct way as possible so that it directly influences the gaseous medium (121) thereby increasing the efficiency or turbulence of the gaseous medium.
This gives a very compact and efficient setup as the gaseous medium of each generator is enhanced by the high intensity and high power acoustic waves of another generator using a total of only two generators.
If only a single generator (101) was used, the difference between the general directions of the high intensity and high power acoustic waves (B or B′) and the general direction of the gaseous medium (A or A′) for a single generator (101) would cause a loss in efficiency since the acoustic waves do not coincide with the gaseous medium (121).
The location of the generators (101; 101′) in relation to each other may vary. One example is e.g. where the two generators are facing each other displaced or shifted but where the high intensity and high power acoustic waves still directly influences the gaseous medium of the other generator.
In the figure, the shown sizes, directions, etc. of the cones (121; 102) do not relate to any specific physical properties like acoustic wave intensity, etc. but merely serve for illustrational purposes. The intensities and/or power of the two generators (101) may be equal or different (with either one being greater than the other is). Furthermore, the shapes, sizes, and directions may vary from application to application.
The specific location of one of the generator (101; 101′) may also vary in relation to the other generator and may e.g. be placed above or higher than and/or e.g. facing, the other generator (101); as long as the acoustic waves (102) of one generator (101) directly influences the gaseous medium (121) of the other generator (101) and vice versa.
Although this particular example shows two generators it is to be understood that a given arrangement may comprises additional generators.
The gaseous medium (102) may in general be any gaseous medium. In one embodiment the gaseous medium (102) is steam. In an alternative embodiment the gaseous medium (102) comprises one or more gases used for creating the plasma.
It is to be noted, that one or more of the acoustic generators shown in connection with
It should be emphasized that the term “comprises/comprising” when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components and/or groups thereof.
In the claims, any reference signs placed between parentheses shall not be constructed as limiting the claim. The word “comprising” does not exclude the presence of elements or steps other than those listed in a claim. The word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements.
The invention can be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer or processor. In the system and device claims enumerating several means, several of these means can be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.
| Number | Date | Country | Kind |
|---|---|---|---|
| PA 2007 00717 | May 2007 | DK | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2008/055801 | 5/13/2008 | WO | 00 | 11/12/2010 |
