CA1335806C - Reactive bed plasma air purification - Google Patents
Reactive bed plasma air purificationInfo
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- CA1335806C CA1335806C CA 588339 CA588339A CA1335806C CA 1335806 C CA1335806 C CA 1335806C CA 588339 CA588339 CA 588339 CA 588339 A CA588339 A CA 588339A CA 1335806 C CA1335806 C CA 1335806C
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Abstract
A plasma air purification device for producing a breathable effluent from contaminated air is disclosed which overcomes the limitations of known air purification devices. The present invention comprises an alternating current plasma device with a porous spherical or granular material packed into the plasma zone. The device of this invention provides air purification by decomposiing and deactivating toxic contaminants.
The contaminants that can be successfully processed include toxic vapors, gases and aerosols. In addition, the air by-products such as NOx, O3 and CO are controlled to below toxic levels. The presence of the porous packing enhances the performance of the device by further reducing air by-products formation, maximizing processing efficiency reducing power consumption, and increasing volume flow rate (i.e. reducing carrier gas residence time).
The contaminants that can be successfully processed include toxic vapors, gases and aerosols. In addition, the air by-products such as NOx, O3 and CO are controlled to below toxic levels. The presence of the porous packing enhances the performance of the device by further reducing air by-products formation, maximizing processing efficiency reducing power consumption, and increasing volume flow rate (i.e. reducing carrier gas residence time).
Description
~ 1 335806 The invention relates to the removal of toxic vapors from flowing air streams and relies upon adsorption, and catalysis. To date, charcoal adsorption has been the proven technology for the purification of contaminated air streams.
However, charcoal filters suffer from a short lifetime, limited adsorptive capacity for toxic compounds, desorption of toxic compounds from the filter, and selectively for only certain classes of compounds. Additionally, a charcoal filter contains a particulate filter which collects toxic aerosols. These toxic aerosols would pass through an unprotected charcoal filter system.
Particulate filters also have a limited lifetime resulting from clogging which restricts air flow. A plasma discharge device can solve many of the problems associated with the use of charcoal-based filtration systems. For example, plasma devices have air purification applications because of the ability to decompose most organic and inorganic toxic compounds, to control the reaction chemistry by altering the operating conditions of the reactor such as flow rate and applied power, to achieve long-term operation by the selection of suitable reactor components, to process aerosols via electrostatic precipitation, interception, and other aerosol removal mechanisms as well as decomposition or deactivation of particulates such as aerosolized biological materials, and to attain an infinite capacity for toxic compounds by fabricating an efficient toxic chemical decomposition reactor.
The purpose for utilizing a plasma reactor for air purification is to produce a breathable effluent from contaminated air. It was recognized that any plasma device capable of sustaining an air plasma at atmospheric pressure could produce -- 1 -- *
~ 1 335806 by-products such as nitrogeneous oxides (NOX), ozone (03), and carbon monoxide (CO). A plasma air purification device would have to efficiently decompose and deactivate toxic materials while demonstrating the ability to control air by-product formation. It was found that increased residence time of the air in the plasma zone resulted in the decomposition of O3, and increased the formation of NOX. However, when water vapor was added to the system, the NOX production was minimized. ~t high powers, O3 was decomposed and the amount of NOX produced increased proportionally to the power applied to the plasma reactor. Once again, the amount of water present altered the amount of ~x found in the effluent air. The existence o a compromise between residence time and applied power was found which maintained high reactor performance including the elimination of O3 while minimizing NOX formation. Thus, one could vary flowrate or applied power in conjunction with water vapor to control the by-products formed.
One method on the use of packed electrical discharges for the decomposition of toxic compounds uses a plasma device for decomposing nitrogenous oxides (NOX) in a gas stream composed predominantly of nitrogen. The chemical conversion eficiency of such a process is low. Another method utilizes a fluidized bed silent electrical discharge for the conversion of oxygen to ozone.
Such devices would not be useful as plasma air purification systems because as the oxygen concentration is increased to levels found in breathable air, the plasma-formed by-products such as NOX and O3 increase to toxic levels in the reactor effluent.
In addition, their ability to efficiently process chemical and 1 335~06 biological contaminants is undetermined. Therefore, the successful plasma air purification system will have to efficiently decompose toxic compounds without creating unmanageable amounts of by-products such as NOX and 03 at atmospheric pressure and high flow rates.
The above-mentioned and other problems are solved by the present invention which comprises an alternating current plasma device with a porous spherical or granular material packed into the plasma zone. The device of the invention overcomes limitations of previously described air purification systems by decomposing and deactivating toxic contaminants and producing breathable air. The contaminants that can be successfully processed include toxic vapors, gases and aerosols. In addition, the air by-products such as NOX, O3, and CO are controlled to below toxic levels. The processing efficiency of toxic compounds is high while maintaining air by-product control to produce a breathable effluent. The presence of the porous packing enhances the performance of the device by further reducing air by-product formation, maximizing processing efficiency, reducing power consumption, and increasing volume flow rate (i.e. reducing carrier gas residence time).
The reactor can be operated at low pressure (~20 torr) and up to higher pressures (80 psia) in predominantly oxygen, nitrogen, argon, air, helium gases or mixtures of these and other gases. It has been established that high relative humidity conditions are favorable to the operation of the device. It is possible to optimize the system performance for highly efficient processing of a wide variety of toxic materials while minimizing air by-product formation consistent with producing a breathable effluent.
Figure 1 illustrateæ the apparatus in block diagram form.
Figures 2 and 3 illustrate reactor configurations with active cooling methods.
Figure 4 illustrates a reactor configuration without active cooling methods.
The plasma air purification reactor is composed of several distinct components. Figure 1 illustrates the apparatus in block diagram form. The principal component is the reactor 1 and its associated power 2 and cooling systems 3. The measurement of power applied and deposited into the reactor is accomplished by use of voltage 4 and current 5 probe signals displayed simultaneously on an oscilloscope 6. The descriptions of the various components of the plasma air purification system and their operating characteristics are discussed in detail below.
An embodiment for the plasma reactor 1 used for the above-mentioned toxic compound decomposition may comprise an arrangement of concentric dielectric (quartz, pyrex, ceramic, or other dielectric materials) tubes 7, 8 with the inner cylinder 7 enveloping one electrode 9 (smooth steel, aluminum, or other conductive materials including ionic solutions) while the second electrode 10 (steel, aluminum, or other conductive materials) enveloped the outer dieletric cylinder 9. The dielectric tubes 7, 8 are coaxially positioned by insulating end-pieces 11, 12. The selection of a suitable tube 7, 8 depends on the dielectric strength of the material. A high dielectric strength will ~,7~d~ k preclude catastrophic arcing at high electric field strengths.
The monolothic, nonconductive end-pieces ll, 12 are composed of two pieces such that o-ring seals 13, 14, 15, 16, 17, 18 maintain leakproof operation. The reactor l can be operated without or with dielectric barriers 7, 8 to isolate the electrodes 9, lO from the contaminated gas stream. However, for many air purification applications, it is desirable to incorporate the dielectric barriers 7, 8 to preclude chemical reactions at the surfaces of the electrodes 9, lO and to provide an increase in dielectric strength. In two reactor configurations, Figure 2, Figure 3 the electrodes 9, 10 are cooled. Several cooling methods are successfully incorporated utilizing countercurrent coolant flow to maximize heat transfer. In one method, cooling air 22 is blown through the inner electrode 9 while the outer electrode 10 is cooled with cold water 22 flowing through a metal coil 23 (Figure 2) or ~acket 28 (Figure 3). The cooling effect is maximized by directing the water flow countercurrent with respect to the contaminant air flow. Another method utilizes liquid cooling of the inner, 9 and outer lO, electrodes (Figure 3). Air can also be used for cooling of the inner, 9 and outer lO, electrodes. It should be mentioned that the proper selection of materials for the dielectric 7, 8 and end pieces ll, 12 precludes the need for cooling electrodes 9, lO. The reactor l (Figure 4) does not utilize active cooling methods.
The packing 20 is placed in the annular volume l9 of the plasma reactor. The form of the packing material can be fibrous, granular, tubular, ring, spherical, or spheroidal shaped.
The packing can be porous or nonporous but it should be composed 1 33580~
of a material with a surface resistivity of approximately 102 ohms-square centimeter per centimeter (ohm cm2/cm) or greater. The packing surface may be inert or catalytic in nature.
Surfaces impregnated with active metal catalysts have been demonstrated to be as effective as inert or unimpregnated packings.
Pyrex beads, pyrex Raschig rings, platinum-palladium-rhodium catalyst spheroids, alumina spheroids, and other materials have been successfully utilized as packings. Greater adsorptive capability is preferred for packings used in high performance reactors. The concept of altering residence time is critical to understanding one of the principal mechanisms of reactor operation.
Characteristic of gas chromatography, a contaminant is introduced into a carrier gas at elevated temperature and passed through a packed column of granular material. The contaminant interacts with the packing sufficiently to slow its procession through the packed column relative to the carrier gas. Thus, while both carrier gas and contaminant molecules continuously enter and exit the packed column, the individual molecules of the contaminant re~uire more time to traverse the packed column than the carrier gas molecules. In the Reactive Bed Plasma reactor 1, this chromotographic effect permits higher carrier gas flow rates to be attained while maintaining a very high processing efficiency for the contaminant which resides in the plasma for a longer period of time. This relative time difference in plasma treatment increases the processing time of the contaminant resulting in higher efficiency and decreases the processing time of the air, resulting in low air by-products distribution. Another important mechanism involves the activation of the surfaces of the packing material by .
the plasma. This plasma activation promotes chemical reactions (i.e. surface catalysis) on the packing in addition to the gas phase chemical reactions. A critical aspect of plasma activation in particular is the characteristic of surface cleaning. The continual cleaning of the surfaces by the plasma prevents saturation or poisoning of the packing. This cleaning process insures optimum performance of the Reactive Bed Plasma reactor 1.
It should be noted that the usable lives of systems using conventional adsorbent and catalyst technologies are severely limited by saturation or poisoning.
The power required to generate a plasma in the packed annular space 19 between the concentric cylinders 7, 8 of the reactor 1 is accomplished by coupling a variable frequency power supply 24 to a high voltage transformer 25. The applied and deposited powers are ascertained by displaying secondary voltage probe 5 signals, and simultaneous display of voltage and current waveforms on an oscilloscope 6 permit the measurement of phase angles. The frequency of the system is tuned so that the voltage and current signals coherently interfere producing values for the cosine of the phase angle which are close to one. This is also known as tuning to the resonant frequency of the plasma system.
The significance of the phase angle is that the applied power to the reactor is calculated by P=I*V*cos(phase angle). The area of a Lissajous figure formed by the display of the current versus voltage signals indicates deposited power. Additionally, the power system maximizes the power transferred to the reactor with the inclusion of an impedence matching network 24 to minimize the reflected power. Every component of the power system is designed to insure that the power applied and deposited into the reactor 1 is maximized. The packing 20 in the reactor 1 augments the power transfer into the annular space 19 by decreasing the electrical resistance between the electrodes 9, 10 while maximizing the strength of localized electric fields.
The power system operating parameters of the device include frequency, voltage, and current. The operational frequency ranges from 50 Hertz to 40 Kilohertz. The operational voltage ranges from 4 Kilovolts to 28 Kilovolts. The operational current ranges from 1 milliampere to 0.2 ampere. The electric power deposited into the reactor 1 is an important operating characteristic that describes the effectiveness of the plasma to decompose toxic materials. This device incorporates well developed techniques for measuring power in a corona device.
The contaminant-bearing gas 26 can be predominantly oxygen, nitrogen, air, argon, or helium. However, the main objective of this system is to efficiently decompose toxic materials. Therefore, a most favorable performance of the s~stem is for operation in air. The contaminant-bearing air or gas enters the reactor through the gas inlet 26, fLows through the plasma zone comprised of the packing 20 in the annular volume 19, and exits through the gas outlet 27. The plasma is initiated at atmospheric pressure. The plasma formed within the annulus 19 and throughout the packing material spanning the length of the electrodes 9, 10 is a highly ionized gas in which energy is deposited into the atoms and molecules by interaction with energetic electrons (i.e. electron impact). Electron impact is the driving force of plasma-induced decomposition because it creates more free electrons, ions, reaç~ive neutrals and radicals.
The contaminant molecules are decomposed via several possible mechanisms including electron impact dissociation or ionization, photodissociation or ionization, secondary ionization, ion-molecure reaction, radical and neutral species reactions.
These electrongenerated species are generally highly reactive and cause further rearrangement of the contaminant molecules passing through the plasma device 1. The modelling of fundamental processes of pLasma device 1 indicate the importance of free oxygen radicals for efficient decomposition of toxic molecules as well as the significance of the air by-products distribution in assessing the performance of the plasma device. Further, the air by-products distribution observed experimentally has been predicted by a chemical reaction model for the system.
The consummate interest of utilizing a plasma device 1 for air purification is the decomposition of toxic molecules in a flowing air stream. The plasma device 1 was evaluated for the decomposition and conversion efficiencies of toxic gases and vapors. Cyanogen chloride and phosgene were among the gases tested as air stream contaminants. The relative retention time of cyanogen chloride was found to be greatly increased by the reactor packing material. The decomposition efficiency of the cyanide gas was greater than 99.6% with an air flow rate of 2.6 standard cubic feet per minute (scfm). At this flow rate, the air residence time was 0.44 second while the residence time of the cyanogen chloride molecules was experimentally determined to be 7.3 seconds. The decomposition efficiency of phosgene was greater than 99.84% with an air flow rate of 5.5 scfm corresponding to an air residence time of 0.31 second. The reactor effluent monitoring revealed that parent toxic molecules were reduced to below hazardous concentrations. In the course of phosgene decomposition, chlorine gas was formed. This reaction product was readily removed by gas phase reaction with ammonia. Other commercial methods available for the removal of acid gas reaction products such as chlorine include fixed bed adsorbers and liquid scrubbers. Implementation of these specific post-treatment methods results in the production of breathable air.
The control of air by-products such as NOX~ 3, and CO is required to produce a breathable effluent. The choice of operating conditions such as humidity, flow rate, and applied power affects the distribution of these by-products of air processing. However, the operating conditions that facilitate the control of air by-products must result in efficient decomposition of toxic materials. The operating conditions that produce substantial amounts of O3 and sub-ppm concentrations of NOX
and CO do not result in the efficient decomposition of toxic materials. For efficient chemical decomposition, an unpacked AC
plasma reactor has a dry air by-product distribution that contains several hundred ppm of NOX, sub-ppm levels of CO, and sub-ppm levels of O3. The humidification of the air stream prior to discharge actually reduces NOX to low ppm levels. At sufficient applied power to the reactor 1, the concentration of O3 found in the effluent is sub-ppm. Fortunately, typical power levels for operation of this device are too low to produce the high thermal temperatures responsible for reduction of CO2 to CO. In fact, Co introduced at the influent of the reactor 1 or formed during ~ 1 335806 hydrocarbon decomposition within reactor 1 is efficiently converted to carbon dioxide with sufficient residence time in the active plasma zone. Thus, the regulation of humidity, flow rate (i.e. residence time in the active plasma zone), and applied power dramatically reduce the air by-products concentrations. The ability of the reactor to decompose toxic materials under similar operating conditions was established when benzene vapor was passed through the plasma with the air stream at different relative humidities. In an early experimental version of plasma device I, benzene was decomposed to carbon dioxide and water with an efficiency of 97.85% at 30% relative humidity, and greater than 95% at 80~ relative humidity. In this testing, the air flow rate was 2.0 scfm corresponding to a residence time of 0.92 second.
Thus, the technical feasibility of utilizing a plasma for air purification has been illustrated.
Contributions of this invention include the ability to efficiently process contaminated air streams: at scfm flow rates, at atmospheric and higher pressure, at low and high relative humidities, and with efficient power usage. A significant advantage of the Reactive Bed Plasma system 1 is the ability to decompose with very high efficiencies the myriad of highly toxic materials which through accidental or deliberate release pose a serious environmental and health threat by contaminating air, water and soil.
~ hile the invention may have been described with reference to one particular embodiment or embodiments, our invention also includes all substitutions and modifications within the spirit or scope of the invention, as will occur to those skilled in this art.
However, charcoal filters suffer from a short lifetime, limited adsorptive capacity for toxic compounds, desorption of toxic compounds from the filter, and selectively for only certain classes of compounds. Additionally, a charcoal filter contains a particulate filter which collects toxic aerosols. These toxic aerosols would pass through an unprotected charcoal filter system.
Particulate filters also have a limited lifetime resulting from clogging which restricts air flow. A plasma discharge device can solve many of the problems associated with the use of charcoal-based filtration systems. For example, plasma devices have air purification applications because of the ability to decompose most organic and inorganic toxic compounds, to control the reaction chemistry by altering the operating conditions of the reactor such as flow rate and applied power, to achieve long-term operation by the selection of suitable reactor components, to process aerosols via electrostatic precipitation, interception, and other aerosol removal mechanisms as well as decomposition or deactivation of particulates such as aerosolized biological materials, and to attain an infinite capacity for toxic compounds by fabricating an efficient toxic chemical decomposition reactor.
The purpose for utilizing a plasma reactor for air purification is to produce a breathable effluent from contaminated air. It was recognized that any plasma device capable of sustaining an air plasma at atmospheric pressure could produce -- 1 -- *
~ 1 335806 by-products such as nitrogeneous oxides (NOX), ozone (03), and carbon monoxide (CO). A plasma air purification device would have to efficiently decompose and deactivate toxic materials while demonstrating the ability to control air by-product formation. It was found that increased residence time of the air in the plasma zone resulted in the decomposition of O3, and increased the formation of NOX. However, when water vapor was added to the system, the NOX production was minimized. ~t high powers, O3 was decomposed and the amount of NOX produced increased proportionally to the power applied to the plasma reactor. Once again, the amount of water present altered the amount of ~x found in the effluent air. The existence o a compromise between residence time and applied power was found which maintained high reactor performance including the elimination of O3 while minimizing NOX formation. Thus, one could vary flowrate or applied power in conjunction with water vapor to control the by-products formed.
One method on the use of packed electrical discharges for the decomposition of toxic compounds uses a plasma device for decomposing nitrogenous oxides (NOX) in a gas stream composed predominantly of nitrogen. The chemical conversion eficiency of such a process is low. Another method utilizes a fluidized bed silent electrical discharge for the conversion of oxygen to ozone.
Such devices would not be useful as plasma air purification systems because as the oxygen concentration is increased to levels found in breathable air, the plasma-formed by-products such as NOX and O3 increase to toxic levels in the reactor effluent.
In addition, their ability to efficiently process chemical and 1 335~06 biological contaminants is undetermined. Therefore, the successful plasma air purification system will have to efficiently decompose toxic compounds without creating unmanageable amounts of by-products such as NOX and 03 at atmospheric pressure and high flow rates.
The above-mentioned and other problems are solved by the present invention which comprises an alternating current plasma device with a porous spherical or granular material packed into the plasma zone. The device of the invention overcomes limitations of previously described air purification systems by decomposing and deactivating toxic contaminants and producing breathable air. The contaminants that can be successfully processed include toxic vapors, gases and aerosols. In addition, the air by-products such as NOX, O3, and CO are controlled to below toxic levels. The processing efficiency of toxic compounds is high while maintaining air by-product control to produce a breathable effluent. The presence of the porous packing enhances the performance of the device by further reducing air by-product formation, maximizing processing efficiency, reducing power consumption, and increasing volume flow rate (i.e. reducing carrier gas residence time).
The reactor can be operated at low pressure (~20 torr) and up to higher pressures (80 psia) in predominantly oxygen, nitrogen, argon, air, helium gases or mixtures of these and other gases. It has been established that high relative humidity conditions are favorable to the operation of the device. It is possible to optimize the system performance for highly efficient processing of a wide variety of toxic materials while minimizing air by-product formation consistent with producing a breathable effluent.
Figure 1 illustrateæ the apparatus in block diagram form.
Figures 2 and 3 illustrate reactor configurations with active cooling methods.
Figure 4 illustrates a reactor configuration without active cooling methods.
The plasma air purification reactor is composed of several distinct components. Figure 1 illustrates the apparatus in block diagram form. The principal component is the reactor 1 and its associated power 2 and cooling systems 3. The measurement of power applied and deposited into the reactor is accomplished by use of voltage 4 and current 5 probe signals displayed simultaneously on an oscilloscope 6. The descriptions of the various components of the plasma air purification system and their operating characteristics are discussed in detail below.
An embodiment for the plasma reactor 1 used for the above-mentioned toxic compound decomposition may comprise an arrangement of concentric dielectric (quartz, pyrex, ceramic, or other dielectric materials) tubes 7, 8 with the inner cylinder 7 enveloping one electrode 9 (smooth steel, aluminum, or other conductive materials including ionic solutions) while the second electrode 10 (steel, aluminum, or other conductive materials) enveloped the outer dieletric cylinder 9. The dielectric tubes 7, 8 are coaxially positioned by insulating end-pieces 11, 12. The selection of a suitable tube 7, 8 depends on the dielectric strength of the material. A high dielectric strength will ~,7~d~ k preclude catastrophic arcing at high electric field strengths.
The monolothic, nonconductive end-pieces ll, 12 are composed of two pieces such that o-ring seals 13, 14, 15, 16, 17, 18 maintain leakproof operation. The reactor l can be operated without or with dielectric barriers 7, 8 to isolate the electrodes 9, lO from the contaminated gas stream. However, for many air purification applications, it is desirable to incorporate the dielectric barriers 7, 8 to preclude chemical reactions at the surfaces of the electrodes 9, lO and to provide an increase in dielectric strength. In two reactor configurations, Figure 2, Figure 3 the electrodes 9, 10 are cooled. Several cooling methods are successfully incorporated utilizing countercurrent coolant flow to maximize heat transfer. In one method, cooling air 22 is blown through the inner electrode 9 while the outer electrode 10 is cooled with cold water 22 flowing through a metal coil 23 (Figure 2) or ~acket 28 (Figure 3). The cooling effect is maximized by directing the water flow countercurrent with respect to the contaminant air flow. Another method utilizes liquid cooling of the inner, 9 and outer lO, electrodes (Figure 3). Air can also be used for cooling of the inner, 9 and outer lO, electrodes. It should be mentioned that the proper selection of materials for the dielectric 7, 8 and end pieces ll, 12 precludes the need for cooling electrodes 9, lO. The reactor l (Figure 4) does not utilize active cooling methods.
The packing 20 is placed in the annular volume l9 of the plasma reactor. The form of the packing material can be fibrous, granular, tubular, ring, spherical, or spheroidal shaped.
The packing can be porous or nonporous but it should be composed 1 33580~
of a material with a surface resistivity of approximately 102 ohms-square centimeter per centimeter (ohm cm2/cm) or greater. The packing surface may be inert or catalytic in nature.
Surfaces impregnated with active metal catalysts have been demonstrated to be as effective as inert or unimpregnated packings.
Pyrex beads, pyrex Raschig rings, platinum-palladium-rhodium catalyst spheroids, alumina spheroids, and other materials have been successfully utilized as packings. Greater adsorptive capability is preferred for packings used in high performance reactors. The concept of altering residence time is critical to understanding one of the principal mechanisms of reactor operation.
Characteristic of gas chromatography, a contaminant is introduced into a carrier gas at elevated temperature and passed through a packed column of granular material. The contaminant interacts with the packing sufficiently to slow its procession through the packed column relative to the carrier gas. Thus, while both carrier gas and contaminant molecules continuously enter and exit the packed column, the individual molecules of the contaminant re~uire more time to traverse the packed column than the carrier gas molecules. In the Reactive Bed Plasma reactor 1, this chromotographic effect permits higher carrier gas flow rates to be attained while maintaining a very high processing efficiency for the contaminant which resides in the plasma for a longer period of time. This relative time difference in plasma treatment increases the processing time of the contaminant resulting in higher efficiency and decreases the processing time of the air, resulting in low air by-products distribution. Another important mechanism involves the activation of the surfaces of the packing material by .
the plasma. This plasma activation promotes chemical reactions (i.e. surface catalysis) on the packing in addition to the gas phase chemical reactions. A critical aspect of plasma activation in particular is the characteristic of surface cleaning. The continual cleaning of the surfaces by the plasma prevents saturation or poisoning of the packing. This cleaning process insures optimum performance of the Reactive Bed Plasma reactor 1.
It should be noted that the usable lives of systems using conventional adsorbent and catalyst technologies are severely limited by saturation or poisoning.
The power required to generate a plasma in the packed annular space 19 between the concentric cylinders 7, 8 of the reactor 1 is accomplished by coupling a variable frequency power supply 24 to a high voltage transformer 25. The applied and deposited powers are ascertained by displaying secondary voltage probe 5 signals, and simultaneous display of voltage and current waveforms on an oscilloscope 6 permit the measurement of phase angles. The frequency of the system is tuned so that the voltage and current signals coherently interfere producing values for the cosine of the phase angle which are close to one. This is also known as tuning to the resonant frequency of the plasma system.
The significance of the phase angle is that the applied power to the reactor is calculated by P=I*V*cos(phase angle). The area of a Lissajous figure formed by the display of the current versus voltage signals indicates deposited power. Additionally, the power system maximizes the power transferred to the reactor with the inclusion of an impedence matching network 24 to minimize the reflected power. Every component of the power system is designed to insure that the power applied and deposited into the reactor 1 is maximized. The packing 20 in the reactor 1 augments the power transfer into the annular space 19 by decreasing the electrical resistance between the electrodes 9, 10 while maximizing the strength of localized electric fields.
The power system operating parameters of the device include frequency, voltage, and current. The operational frequency ranges from 50 Hertz to 40 Kilohertz. The operational voltage ranges from 4 Kilovolts to 28 Kilovolts. The operational current ranges from 1 milliampere to 0.2 ampere. The electric power deposited into the reactor 1 is an important operating characteristic that describes the effectiveness of the plasma to decompose toxic materials. This device incorporates well developed techniques for measuring power in a corona device.
The contaminant-bearing gas 26 can be predominantly oxygen, nitrogen, air, argon, or helium. However, the main objective of this system is to efficiently decompose toxic materials. Therefore, a most favorable performance of the s~stem is for operation in air. The contaminant-bearing air or gas enters the reactor through the gas inlet 26, fLows through the plasma zone comprised of the packing 20 in the annular volume 19, and exits through the gas outlet 27. The plasma is initiated at atmospheric pressure. The plasma formed within the annulus 19 and throughout the packing material spanning the length of the electrodes 9, 10 is a highly ionized gas in which energy is deposited into the atoms and molecules by interaction with energetic electrons (i.e. electron impact). Electron impact is the driving force of plasma-induced decomposition because it creates more free electrons, ions, reaç~ive neutrals and radicals.
The contaminant molecules are decomposed via several possible mechanisms including electron impact dissociation or ionization, photodissociation or ionization, secondary ionization, ion-molecure reaction, radical and neutral species reactions.
These electrongenerated species are generally highly reactive and cause further rearrangement of the contaminant molecules passing through the plasma device 1. The modelling of fundamental processes of pLasma device 1 indicate the importance of free oxygen radicals for efficient decomposition of toxic molecules as well as the significance of the air by-products distribution in assessing the performance of the plasma device. Further, the air by-products distribution observed experimentally has been predicted by a chemical reaction model for the system.
The consummate interest of utilizing a plasma device 1 for air purification is the decomposition of toxic molecules in a flowing air stream. The plasma device 1 was evaluated for the decomposition and conversion efficiencies of toxic gases and vapors. Cyanogen chloride and phosgene were among the gases tested as air stream contaminants. The relative retention time of cyanogen chloride was found to be greatly increased by the reactor packing material. The decomposition efficiency of the cyanide gas was greater than 99.6% with an air flow rate of 2.6 standard cubic feet per minute (scfm). At this flow rate, the air residence time was 0.44 second while the residence time of the cyanogen chloride molecules was experimentally determined to be 7.3 seconds. The decomposition efficiency of phosgene was greater than 99.84% with an air flow rate of 5.5 scfm corresponding to an air residence time of 0.31 second. The reactor effluent monitoring revealed that parent toxic molecules were reduced to below hazardous concentrations. In the course of phosgene decomposition, chlorine gas was formed. This reaction product was readily removed by gas phase reaction with ammonia. Other commercial methods available for the removal of acid gas reaction products such as chlorine include fixed bed adsorbers and liquid scrubbers. Implementation of these specific post-treatment methods results in the production of breathable air.
The control of air by-products such as NOX~ 3, and CO is required to produce a breathable effluent. The choice of operating conditions such as humidity, flow rate, and applied power affects the distribution of these by-products of air processing. However, the operating conditions that facilitate the control of air by-products must result in efficient decomposition of toxic materials. The operating conditions that produce substantial amounts of O3 and sub-ppm concentrations of NOX
and CO do not result in the efficient decomposition of toxic materials. For efficient chemical decomposition, an unpacked AC
plasma reactor has a dry air by-product distribution that contains several hundred ppm of NOX, sub-ppm levels of CO, and sub-ppm levels of O3. The humidification of the air stream prior to discharge actually reduces NOX to low ppm levels. At sufficient applied power to the reactor 1, the concentration of O3 found in the effluent is sub-ppm. Fortunately, typical power levels for operation of this device are too low to produce the high thermal temperatures responsible for reduction of CO2 to CO. In fact, Co introduced at the influent of the reactor 1 or formed during ~ 1 335806 hydrocarbon decomposition within reactor 1 is efficiently converted to carbon dioxide with sufficient residence time in the active plasma zone. Thus, the regulation of humidity, flow rate (i.e. residence time in the active plasma zone), and applied power dramatically reduce the air by-products concentrations. The ability of the reactor to decompose toxic materials under similar operating conditions was established when benzene vapor was passed through the plasma with the air stream at different relative humidities. In an early experimental version of plasma device I, benzene was decomposed to carbon dioxide and water with an efficiency of 97.85% at 30% relative humidity, and greater than 95% at 80~ relative humidity. In this testing, the air flow rate was 2.0 scfm corresponding to a residence time of 0.92 second.
Thus, the technical feasibility of utilizing a plasma for air purification has been illustrated.
Contributions of this invention include the ability to efficiently process contaminated air streams: at scfm flow rates, at atmospheric and higher pressure, at low and high relative humidities, and with efficient power usage. A significant advantage of the Reactive Bed Plasma system 1 is the ability to decompose with very high efficiencies the myriad of highly toxic materials which through accidental or deliberate release pose a serious environmental and health threat by contaminating air, water and soil.
~ hile the invention may have been described with reference to one particular embodiment or embodiments, our invention also includes all substitutions and modifications within the spirit or scope of the invention, as will occur to those skilled in this art.
Claims (9)
1. A packed, alternating current electrical discharge plasma device to decompose toxic contaminants in air for the purpose of air purification and general toxic and hazardous material processing, including removal of ozone, carbon monoxide, and nitrogenous oxides from air, comprising:
a. a first electrode facing a second electrode;
b. a non-conducting packing material in a bed residing between said first and second electrodes;
c. an alternating current power supply to include but not limited to a frequency range of 0.5 kilohertz to 40 kilohertz frequency;
d. said power supply being connected to a transformer to produce high voltage alternating current to include but not limited to a voltage range of 4 kilovolts to 28 kilovolts;
e. said transformer being connected between said first and second electrodes to produce an air plasma throughout said bed of packing material;
f. said power supply, said transformer, said electrodes and said packing material comprising a complex impedance wherein impedance matching is provided;
g. said power supply, said transformer, said electrodes and said packing material comprising a resonant electrical circuit wherein frequency tuning is provided; and h. said air plasma produced by said device having applied thereto a power necessary to achieve high decomposition efficiencies of toxic contaminants at both trace and percent concentrations, and flow capacities of 1 cubic foot per minute or greater.
a. a first electrode facing a second electrode;
b. a non-conducting packing material in a bed residing between said first and second electrodes;
c. an alternating current power supply to include but not limited to a frequency range of 0.5 kilohertz to 40 kilohertz frequency;
d. said power supply being connected to a transformer to produce high voltage alternating current to include but not limited to a voltage range of 4 kilovolts to 28 kilovolts;
e. said transformer being connected between said first and second electrodes to produce an air plasma throughout said bed of packing material;
f. said power supply, said transformer, said electrodes and said packing material comprising a complex impedance wherein impedance matching is provided;
g. said power supply, said transformer, said electrodes and said packing material comprising a resonant electrical circuit wherein frequency tuning is provided; and h. said air plasma produced by said device having applied thereto a power necessary to achieve high decomposition efficiencies of toxic contaminants at both trace and percent concentrations, and flow capacities of 1 cubic foot per minute or greater.
2. A device as claimed in claim 1 wherein said first and second electrodes comprise electrically-conductive, metal or non-metal, rods, tubes, pipe, foils or films.
3. A device as claimed in claim 1 wherein said packing material placed between said electrodes is a monolithic aggregate.
4. A device as claimed in claim 1 wherein said packing material placed between said electrodes comprises fibrous, granular, tubular, ring-shaped, spherical-shaped, or spheroid-shaped material, in a porous or non-porous form, being inert or catalytic in nature.
5. A device as claimed in claim 1 further comprising means for operation with chemically protective coatings on said first and second electrodes, said coatings consisting of a dielectric material.
6. A device as claimed in claim 5 wherein the dielectric coating comprises a material selected from the group consisting of polymeric, glass, and ceramic materials.
7. A device as claimed in claim 1 wherein said electrodes are cylindrically-shaped and coaxial.
8. A device as claimed in claim 1 which provides a means for operation with cooling of said first and second electrodes wherein said first and second electrodes are heat pipes.
9. A device as in claim 1, wherein the operation of the device at atmospheric pressure or above, has the features of:
a. a combination of several mechanisms for decomposition including plasma-induced decomposition, combustion, photoionization, electron impact dissociation, ion-molecule reaction, radical and neutral species reactions, and plasma etching processes simultaneously and synergistically operating within said device;
b. efficient precipitation of aerosols, including biological spores, which are simultaneously deactivated in the plasma;
c. surface cleaning of said packing material occurring at atmospheric pressure; and d. the residence time of contaminants in the active plasma zones is increased, due to the presence of the packing material.
14.
a. a combination of several mechanisms for decomposition including plasma-induced decomposition, combustion, photoionization, electron impact dissociation, ion-molecule reaction, radical and neutral species reactions, and plasma etching processes simultaneously and synergistically operating within said device;
b. efficient precipitation of aerosols, including biological spores, which are simultaneously deactivated in the plasma;
c. surface cleaning of said packing material occurring at atmospheric pressure; and d. the residence time of contaminants in the active plasma zones is increased, due to the presence of the packing material.
14.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US18484888A | 1988-04-22 | 1988-04-22 | |
| US07/184,848 | 1988-04-22 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1335806C true CA1335806C (en) | 1995-06-06 |
Family
ID=22678613
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 588339 Expired - Fee Related CA1335806C (en) | 1988-04-22 | 1989-01-16 | Reactive bed plasma air purification |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1335806C (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP1910745A1 (en) * | 2005-07-20 | 2008-04-16 | Alphatech International Limited | Apparatus for air purification and disinfection |
| CN109126401A (en) * | 2017-06-15 | 2019-01-04 | 浙江大学 | Charcoal circulator and purification device |
-
1989
- 1989-01-16 CA CA 588339 patent/CA1335806C/en not_active Expired - Fee Related
Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP1910745A1 (en) * | 2005-07-20 | 2008-04-16 | Alphatech International Limited | Apparatus for air purification and disinfection |
| EP1910745A4 (en) * | 2005-07-20 | 2011-11-30 | Alphatech Internat Ltd | Apparatus for air purification and disinfection |
| CN109126401A (en) * | 2017-06-15 | 2019-01-04 | 浙江大学 | Charcoal circulator and purification device |
| CN109126401B (en) * | 2017-06-15 | 2023-11-24 | 浙江大学 | Carbon circulation device and purification device |
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