WO1994017899A1

WO1994017899A1 - Tunable compact electron beam generated plasma system for the destruction of gaseous toxic compounds

Info

Publication number: WO1994017899A1
Application number: PCT/US1994/001214
Authority: WO
Inventors: Richard M. Patrick; Leslie Bromberg; Daniel R. Cohen; Paul Thomas; Mathias Koch
Original assignee: Massachusetts Institute Of Technology
Priority date: 1993-02-05
Filing date: 1994-02-02
Publication date: 1994-08-18

Abstract

A method and system for destruction of gaseous toxic compounds includes introducing an electron beam into a reaction chamber through an electron beam-pervious window, the chamber having an inlet port and an outlet port for the transport of a gas stream therethrough, the electron beam having a current selected to generate a plasma with a predetermined density of secondary electrons, the secondary electrons having an average electron energy which promotes decomposition of at least one gaseous compound; applying an electric field to the chamber at a field strength selected to generate a predetermined average electron energy of secondary electrons; introducing a gas stream comprising at least one gaseous toxic compound into the chamber at a selected mass flow rate, whereby the gas stream is irradiated with secondary electrons and the gaseous toxic compound decomposes; analyzing the gas stream at a detector downstream from the chamber to determine a residual gaseous toxic compound concentration; and adjusting at least one of the electron beam current, the electric field strength and the mass flow rate responsive to the residual gaseous toxic compound concentration to maintain the concentration at no more than a preselected value.

Description

Tunable Compact Electron Beam Generated Plasma System for the Destruction of Gaseous Toxic Compounds

The United States government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license to others on reasonable terms as provided for by the terms of contract No. 203401-A-L awarded by the Department of Energy.

Field of the Invention This invention relates to the destruction of gaseous toxic wastes that are convected in a waste gas stream. The invention further relates to the destruction of gaseous toxic compounds using electron-beam-generated plasma with the energy of the plasma (secondary) electrons controlled by a variable electric field imposed on the plasma. Further, the invention applies particularly to waste gas streams where the concentration of the toxic gas to be removed is at a relatively low initial concentration.

Background of the Invention Electron-beam technology for toxic waste removal from waste gas streams has several advantages over conventional systems such as incineration, steam reduction and capture and adsorption onto activated carbon, particularly when the input concentration of toxic gas is small. Electron-beam destruction of organic compounds can be done selectively by acting specifically, if indirectly, on the target toxic waste gas. Also, electron- beam remediation requires less energy than incineration or steam reduction and does not require regeneration of materials as does carbon absorption processes.

Electron-beam-generated plasmas have been used to create non- equilibrium excitations in gaseous media for high powered lasers, removal of sulfur and nitrogen compounds from stack gases and for destruction of organic molecules in waste water streams. Slater and Douglas-Hamilton in J. Appl. Phys. 52(9), 5820-5828 (1981) describe electron-beam-initiated destruction of gas streams containing vinyl chloride. Recently, Matthews et al. in Proc. Nucl. Hazard. Waste Manage. Spectrum '92, 649-652 ( 1992), have reported the remediation of vapor-extracted trichloroethylene (TCE) using an on-site electron accelerator. Electron beam technology, such as that developed by Energy Science,

Inc. with primary electron energies between 150 keV and 300 keV, have been demonstrated to be both portable and versatile. However, high power electron beam reactors (1.5-2 MeV) require heavy shielding, rendering on-site treatment awkward because of the bulk of the shielding. Further, prior art electron beam reactors have not provided portable on¬ line determination of downstream gas composition for optimization of the remediation process. For example, on-site waste streams will have varying toxic gas compositions and concentrations. Processing parameters set to handle remediation of average toxic gas concentrations will be unable to properly remediate "blooms" of toxic gas concentrations or sudden changes in overall waste stream composition.

Additionally, waste treatment must often occur under harsh conditions in remote locations. For example, a target site for treatment of vapor extracted halocarbons is the leaching fields of a former plutonium finishing plant in Hanford, WA, where there is little available water, no nearby communities and minimal support services. It is desirable that a remediation system require little or no maintenance or operator monitoring.

It is the object of the present invention to overcome these and other limitations of the prior art electron beam reactors and to provide for the mobile and versatile remediation of a wide range of waste gas compounds.

Summary of the Invention In one aspect of the present invention, remediation of waste gas compounds includes introducing an electron beam into a reaction chamber through a window pervious to electron beams. The electron beam current is selected to generate a plasma with a predetermined density of secondary electrons in the chamber. The secondary electrons have an average electron energy which promotes decomposition of at least one waste gas compound.

"Waste gas", "toxic gas" or "toxic waste gas" is used herein in the conventional sense to include any toxic compound having a sufficiently high vapor pressure to permit transport in a gaseous stream, with or without the aid of a carrier gas.

By "average electron energy" as that term is used herein, it is meant the average electron energy of the plasma secondary electrons after collision with the heavier particles in the chamber. The energy can be equivalently expressed as "electron temperature". An electron energy of 0.03 eV at ambient gas temperatures has an electron temperature equal to zero.

An electric field is applied to the chamber at a field strength selected to generate a predetermined average electron energy in the secondary electrons. The electric field need be applied only when it is desired to have an average electron energy greater than 0.03 eV at ambient gas temperature. Therefore, the electron beam furnishes the plasma secondary electrons and the applied electric field modifies the average electron energy to above the ambient gas temperature. The average electron energy is selected to decompose at least one gaseous toxic compound. As is discussed herein below, the field may be applied uniformly over the entire chamber or to a portion thereof.

A gas stream containing one or more waste gas compounds is introduced into the chamber at a selected mass flow rate. The waste gas stream is typically introduced after the initial electron beam and electric field conditions have been established; however, in some instances it may be desirable to first introduce the gas stream and then adjust the processing conditions accordingly. The gas stream is irradiated, thereby effecting the decomposition of the gaseous toxic compounds contained therein. After emergence from the chamber, the gas stream is analyzed downstream with detectors to determine a residual concentration for each of the component toxic compounds. It is particularly desirable that the detection method be capable of discriminating Δ among the various components of the toxic waste gas. Electron beam current, mass flow rate and/or electric field strength are adjusted responsive to the measured residual gaseous toxic compound concentration to maintain the residual concentration at no more than a preselected value. In preferred embodiments, any combination of the electron beam current, mass flow rate and electric field strength may be adjusted to optimize the process. Mass flow rate or electron beam current alone can be adjusted responsive to the residual gaseous toxic compound concentration. In other embodiments, combinations of two or more factors may be adjusted responsive to the residual gaseous toxic compound concentration.

In another preferred embodiment, the decomposition products are collected in a trap downstream from the chamber for disposal. In another preferred embodiment, the air stream is dried prior to its introduction into the chamber. This permits a more efficient remediation process in some cases since electrons can preferentially attach to water molecules. In yet another preferred embodiment, the composition of the gas stream is analyzed prior to its introduction into the chamber. Prior analysis is useful in setting initial conditions for the remediation system and for anticipating changes in gas stream composition that may require adjustment of the election beam current, mass flow rate and field strength. In preferred embodiments, the detectors used for analysis of the gas stream are solid state detectors which can withstand the harsh conditions typical of many waste removal sites.

In other preferred embodiments, an electron beam is introduced into the reaction chamber through a plurality of electron beam-pervious windows. The plurality of windows may all be supplied by a single electron beam source which is split and diverted to each window with a focusing grid structure. Alternatively, each window may be serviced by its own electron beam source. One or more electric fields may be applied to the reaction chamber. The field may be imposed on the entire reaction chamber or a portion of the reaction chamber. If more th.an one electric field and more than one window are used. the electric fields may be applied over a portion of the chamber comprising one or more windows. The selection of field strengths and number of electron beam sources and their respective currents) permit a finely tuned control over the plasma conditions within the reaction chamber. In another aspect of the present invention, a system for the remediation of waste gases is provided. The system includes a reaction chamber comprising at least one electron beam-pervious window and having an inlet port and an outlet port for the transport of a waste gas stream therethrough; an electron beam source adjacent to the electron beam-pervious wall; an electric field generator; a detector positioned downstream from the chamber; and a controller responsive to a residual gaseous toxic compound concentration and in communication with controls for the selected processing parameters, including mass flow rate, electron beam current and/or electric field strength. The electron beam source is positioned adjacent to the electron beam-pervious window and generates an electron beam having a current selected to generate a plasma with a predetermined density of secondary electrons. The secondary electrons have an average electron energy which promotes decomposition of at least one gaseous toxic compound.

The electric field generator applies an electric field to said chamber at a field strength selected to generate a predetermined average electron energy of secondary electrons. The electric field is applied only when it is desired to have an average electron energy greater than 0.03 eV (room temperature). Therefore, the electron beam furnishes the plasma secondary electrons and the applied electric field modifies the average electron energy to above the ambient gas temperature. The average electron energy is selected to decompose at least one gaseous toxic compound.

A gas stream comprising at least one gaseous toxic compound is introduced into the chamber at a selected flow rate and is irradiated with secondary electrons, whereby the compound decomposes. The detector is position downstream from the chamber for determination of a gaseous toxic compound concentration in the gas stream. In preferred embodiments, the detector may be located downstream and proximate to the reaction chamber. In another preferred embodiment, the detector may be located downstream and proximate to the scrubber. In yet another embodiment, detectors are located in both positions. The detector is preferably a solid state detector.

The controller is responsive to the residual waste gas concentration and is capable of adjusting at least one of following parameters: the electron beam current, the electric field strength and the mass flow rate. The controller adjusts these parameters to maintain the residual gaseous toxic compound concentration at no less than a preselected value.

In other preferred embodiments, the reaction chamber can have more than one electron beam-pervious window for introducing electron into the reaction chamber. The plurality of windows may all be supplied by a single electron beam source which is split and diverted to each window.

Alternatively, the plurality of windows may each be supplied by its own electron beam source. In other preferred embodiments, the reaction chamber possesses a plurality of electric fields with strengths optimized for different waste gases, which can be applied over a portion of the chamber comprising one or more windows. Heat exchangers can be used between reaction zones to control the overall temperature rise.

The above system can be used in conjunction with other known remediation systems, such as plasma arc heaters and incinerators.

The present invention provides a method and system for the effective remediation of a wide range of toxic waste gases that are highly versatile and responsive to changes in the remediation process. The process is particularly useful in the remediation of moderate to low pollutant levels in a gas stream (ca. less than 2000 pm). In addition, the system and method are portable and can be operated with little or no operator intervention and under unfavorable environmental conditions. Brief Description of the Drawing In the drawing;

Figure 1 is a graph of electron attachment rate constant v. mean electron energy for selected halocarbons; Figure 2 is a perspective view of prior art electron beam and electric field sources and reaction chamber;

Figure 3 is a schematic diagram of the remediation system of the present invention:

Figure 4 is a schematic illustration of another embodiment for the electron beam and electric fields of the present invention;

Figure 5 illustrates the operating region for a waste inlet stream; Figure 6 is a schematic diagram of the experimental remediation system illustrating the features, of the present invention; and^'

Figure 7 is a graph of CC1₄ mole concentration for different intake concentrations as a function of electron beam specific power deposition. Like features are indicated by the like numbers in all figures.

Description of the Preferred Embodiment The underlying principles of plasma destruction of waste gases is discussed using carbon tetrachloride as an example and is in no way intended to limit the scope of the invention. For further explanation of the electron beam-initiated destruction of other halocarbons; see, Slater and Douglas- Hamilton, herein incorporated by reference.

An ionizing reaction is induced in a reaction chamber where plasma treatment is to take place by the introduction of high-energy electrons (secondary electrons), thereby generating a "cold plasma" within the chamber. "Cold plasma", as that term is used herein, refers to a plasma created by secondary electrons (energies of less than 30 eV) consisting of ions, electrons (very small concentration) and neutral gas atoms and molecules in which the overall gas temperature rise is controlled by the residence time of the flowing gas in the chamber and the deposited electron beam power in the gas. The plasma is non-thermal, the electron density is controlled by the electron beam flux i which is a function of the electron beam current, see below) and the attachment and recombination rates for electrons in the gas system. Electron beam energies of no more than 300 keV are sufficient to generate a "cold plasma" in the reaction chamber.

By "electron beam flux", as that term is used herein, it is meant the number of electrons that pass through the wall into the reaction chamber per unit time and area.

The mechanism for the plasma-induced decomposition of CC1₄ is presumed to be that of dissociative electron attachment. As shown, in Fig. 1, carbon tetrachloride, designated by curve 10, has a larger electron attachment rate constant than a variety of chloro- and fluorocarbons, which are indicated by curves 11 through 15, particularly at low electron energy. However, other halocarbons and organic compounds have also significant electron attachment rates and the remediation of these compounds is contemplated in the present invention. The attachment process for carbon tetrachloride follows the reaction sequence as shown in egs. (1) and (2),

CCl₄ + e → CC1₃ + C1 → CC1₂ + Cl₂. ( 1)

CC1₂ + 0₂ → C0₂ + Cl₂ (2) The CC1₃ fragment is unstable and dissociates into CC1₂ .and a free chlorine molecule. Water, present as water vapor in the reaction chamber or in the scrubber used for collecting decomposition products, can react with the free chlorine to form HCl. At the anticipated low halocarbon levels, even an air stream which has initially been treated by a dryer may contain sufficient water vapor to form HCl from the free chlorine. The carbon-containing species are oxidized to form C0₂.

An enlarged perspective view of the electron beam source, electric field source .and chamber are illustrated in Fig 2. A schematic diagram of the system of the invention, including optional features, is shown in Fig. 3. Interested readers are referred to US 3,702,973 by Daugherty et al., herein incorporated by reference, for further explanation on electron beam technology coupled with a sustaining electric field.

Electron Beam Source The electron beam source used in the present invention may have electron beam energies of below 300 keV. Electron beam energy ranges preferably from 150 keV to 300 keV. Well established commercial technology exists which meet these requirements. For example, Energy Sciences, Inc. of Wilmington, MA manufactures a self-shielding, light-weight and portable electron beam processor capable of generating electron beams of energies in the desired range of 150 to 300 keV.

The electron beam current is typically produced by a triode electron gun. Referring to Fig. 2, electrons indicated by arrow 10 and generated at a filament 11 pass through a current control grid 12 and are strongly accelerated toward plate 14. The filament constitutes a cathode and is at a negative potential. The grid 12 is negatively biased with respect to the filament to the point where the grid passes the desired current due to electric field leakage. The plate 14 mechanically supports a thin metal foil 16 (typically aluminum or titanium) which is transparent to the electron beam, i.e., an electron beam-pervious window. The foil 16 constitutes the anode of the triode and is at zero potential. Electron beam current can be adjusted by varying the negative bias voltage to

The electron beam acceleration voltage (total energy of the primary electrons) is set so that the penetration depth of the primary electrons supplied by the electron beam to the chamber is very nearly equal to the chamber dimension in the direction of the electron beam flux. By "penetration depth", as that term is used herein, it is meant the distance the electrons travel into the gas stream medium before they expend their energy. The depth of the secondary electrons production in the chamber is determined by the energy of the primary electrons. Hence, for a given electron beam source or inlet dimension of the gas stream, the acceleration voltage is defined. The acceleration voltage depends upon the gas density and chamber dimension and is not responsive to the residual waste gas concentration.

Electron beam energies of 150 to 300 keV are well-suited for the intended function of plasma treatment of waste gas. In this range , the depth of secondary electron production is in the range of 15-30 cm. This is a reasonable dimension for the chamber itself in terms of gas transport into and out of the chamber and when considering the desired portable nature of the system. Since penetration depth a function of the square of the electron beam energy, higher electron beam energies (and up to 2 MeV is not unusual) have a much greater depth of secondary electron production, i.e, 1500-3000 cm.

Enormous reaction chambers are required to fully utilize the plasma, which has a deleterious effect on gas transport through the reaction chamber. After the electrons pass through the window (foil), they enter the reaction chamber 18 as primary electrons of slightly lower electron energy and produce secondary electrons as the primary energy deposition process. The electron energy of these secondary electrons may be further adjusted by the application of an electric field established between oppositely disposed anode plate 20 and cathode plate 22. A CW potential can be applied across the two plates, however, a AC, DC, RF or pulsed mode is also contemplated in the present invention . The required electric field strength depends on the on-site conditions .and types of compounds to be treated. The electric field strength can be zero. However, in all instances, it should not be strong enough to create a self-sustaining discharge.

The electric field is applied in all circumstances to control the average electron energy or electron temperature of the plasma electrons. As mentioned above, carbon tetrachloride has a maximum electron attachment rate at zero applied field (which corresponds to an average electron energy of 0.03 eV at ambient gas temperature). Halocarbons other than carbon tetrachloride or stable decomposition intermediates of carbon tetrachloride have maximum electron attachment rates at higher average electron energies and an applied electric field may be desired. Referring to Fig. 1, for example, chloroform (CHCI₃) curve 12 shows a maximum electron attachment rate at approximately 0.3 eV. An applied field will increase average electron to optimize electron capture by chloroform. The specific field strength is dependent upon the composition of the gas stre.am.

In instances where the composition of the inlet gas stream is mixed, it may be desirable to provide regions within the reaction chamber of different applied field to target the various components of the waste gas. This can be accomplished by using a plurality of anode plates 20 and cathode plates 22 with separately controllable potentials along the length of the reaction chamber or by choosing an optimum average field for total toxic gas destruction.

Integrated Electron Beam System Figure 3 illustrates the integration of the electron beam source with the other features of the system of the present invention. A waste gas stream indicated by arrow 30 is introduced into the system through an inlet 31 and passes into an optional dryer 32. The gas stream then passes into a reaction chamber 18, which is coupled to the electron beam source and electric field source 13 as described above. In the reaction chamber, the gas stream is irradiated with secondary electrons, thereby forming high energy intermediates which decompose into by-product compounds, such as chlorine gas, which can be safely collected and disposed. Upon emerging from the chamber 18 the gas stream enters an optional scrubber 33 which collects the decomposition products. Detector 35 is positioned in-line downstream from the reaction chamber 18. Detectors are located at one or both of sites 37 and 39. The detector at site 37 measures a residual waste gas concentration after electron beam irradiation and hence provides an indication of the effectiveness of the plasma treatment. The detector at site 39 measures the residual gaseous toxic compound concentration after passing through the scrubber and hence provides an additional indication of the scrubber efficiency as well. An additional detector 40 may be optionally located at a site 41 upstream from the reaction chamber for determination of the inlet waste gas concentration. Data from the detectors is input into the controller 45, which uses the data to adjust the electron beam current, mass flow rate and electric field strength need to be adjusted. An input signal 46 from the detectors 35 is provided to the controller 45, which in turn gives an output signal 47 to a mass flow valve 48 the current control grid 12 and the potential controlling the electric field.

The system of the present invention is flexible and can accommodate a wide range of inlet flow rates and inlet gas compositions. A typical flow rate is in the range of 10-400 cfm, however, higher flow rates up to 1000 cfm could readily be accommodated. For example, multiple inlet ports to a reaction chamber having a plurality of windows and electron beam sources.

Various modes of operation of the above integrated system are contemplated within the scope of the present invention. The electron beam source can be that described above and shown in Fig. 2. .Alternatively, the reaction chamber 18 may have a plurality of electron beam-pervious windows 50, each adjacent to an electron beam 10 (Fig. 4). The windows 50 are spaced sufficiently far apart from one another so as to define regions 52 containing secondary electrons and interdisposed regions 53 having little or no secondary electron and/or plasma population. Heat exchangers 54 may optionally be located in regions 53 for reducing the temperature of the gas stream. The electron beam for each electron pervious window 50 may be generated from a single electron gun which is directed towards each window using focusing grid structures. Alternatively, each window may have its own electron beam source, thereby allowing for independent control of electron beam current within each region 52 of the chamber 18. It is further contemplated as within the scope of the invention that the electric field may be applied to one, all or a subset of the regions 52. In this embodiment, further control of the electron beam environment is provided not only within the reaction chamber but within regions of the reaction chamber. A heat exchanger is used to remove heat generated in the gas stream upon electron beam irradiation and to maintain the desired "cold plasma" throughout the chamber. By keeping the gas stream temperature low, it is possible to prevent the undesirable formation of NO_x , which is favored at higher temperatures. It also permits a higher radiation dose per unit temperature rise over the entire chamber which is desirable to eliminate oxides of nitrogen, for example. "Radiation dose", as that term is used herein, means the amount of radiation (measured in kilorads or megarads) per concentration of toxic compound. Efficiency of the plasma treatment can be measured as the radiation dose required to reduce the residual gaseous toxic compound concentration to a predetermined level. The predetermined level can be a minimum acceptable level or it can be measured as a percent reduction relative to the inlet concentration, i.e., a two to three orders in magnitude reduction in toxic gas concentration.

Water can readily absorb electrons, which reduces the efficiency of the remediation process. In instances where the waste stre-am has unacceptably high moisture levels, it may be desirable to use an in-line dryer. Any dryer of the prior art can be conveniently incorporated into the system, including, but not limited to heat exchange systems, silica and membranes. A particularly preferred method of drying the inlet gas stream is to use a membrane counterflow exchanger, such as that available under the tradename of PermaPure Nafion Dryers from PermaPure Corp in New Jersey. The membrane is a material that is preferentially wetted by water. Moisture from the gas stream wets the membrane and is wicked away from the interior of the dryer to condense on the membrane outer wall. An air heater 49 removes the accumulated water from the outermost surface of the membrane. Such a process is preferred because it allows the preferential removal of water from the inlet gas without concomitant removal of the target toxic waste gas.

In most instances, the treated gas stream can not be safely vented to the ambient. In those cases, it is desired to use an in-line scrubber to trap the noxious decomposition products of the gas stream. The scrubber can be any conventional scrubber used to remove small halogenated and non-halogenated by-product molecules, such as Cl₂, HCl and C0₂. Suitable prior art scrubbers include, but are in no way limited to, solid absorbers and solvent scrubbers. Solid absorbers may be optima for certain waste gas by-product molecules. If the decomposition products have significant solubility in water, the scrubber is simply a water reservoir through which the gas stream is bubbled to form a dilute acid solution, chlorine gas and other chlorinated decomposition products react with water to form HCl.

A particularly preferred scrubber is an aqueous alkali solution, such as a NaOH, Ca(OH)₂ or KOH solution. Chlorine forms alkali metal salts in the alkali solution, which precipitate from solution at high concentration. Once the pH of the scrubber solution has dropped below a useful point, the solution is syphoned off of the precipitated salts, new alkali is added and the scrubber solution can be reused. Halogens are collected as alkali salts. This scrubber system allows for the efficient collection of halogenated decomposition products using minimal amounts of water. This is particularly useful when treatment sites are remote from running water or located where water is scarce.

The detector system can be any standard detecting means, including but not limited to gas chromatography, mass spectroscopy, UV-Vis spectrophotometry and infrared analysis. However, all the standard methods require operator maintenance and can not be readily remote controlled. It is thereby particularly desirable to use a solid state detector. Solid state detectors operate by monitoring a change in surface potential as particular species are absorbed on the surface. Solid state detectors useful for in-line determination of residual waste gas concentration and residual halogen concentration include semiconductor metal oxide gas sensors and ceramic- metallic gas sensors. Such sensors require little maintenance, operate on little electricity and are inexpensive, making them particularly desirable for remote operations and locations. In some instances, a contamination site may include more than one type of toxic gas for remediation. It is therefore desirable to use several detectors or a single detector which recognize several gases in a mixture, such as recently developed by Argonne National Laboratories (see, MRS Bulletin, December, 1992, p 10) to measure the concentration of several contaminants in the gas stream. It is contemplated that processes can be integrated into the present system. For example, it is known in electron beam processing art to add a gaseous compound to the reaction chamber, which can combine with the waste gas and promote decomposition. This results in a reduced radiation dose requirement. The above system can be conveniently assembled in the cargo area of a forty foot long trailer truck and requires only electricity for operation. The system therefore provides for optimal remediation of a target waste gas while at the same time providing versatility and portability required for remote operations. Controller Operation

In order to maintain optimal remediation of the toxic waste gases, it is desirable to operate the system within a defined operating region 50, the region defined by upper and lower residual waste gas concentrations as shown in Fig 5. Fig. 5 shows a desired upper bound 51 and a desired lower bound 53 of 1 pm and 0.6 pm, respectively for carbon tetrachloride; however, this value is will vary dependant on the toxic gas to be remediated. The upper bound is set by the maximum acceptable level of the toxic gas at issue. The lower bound is set to maximize the efficiency of the operation.

The operating region shown in Fig. 5 is for a particular mass flow volume flow of air with variable amounts of carbon tetrachloride in the input waste gas stream. The dose requirements for larger mass flows and larger reaction chambers will be smaller for the same removal efficiency because the ratio of surface area to reactor volume will be reduced, thereby decreasing dose requirements. Further, waste streams containing other components, such as trichloroethylene (TCE) and water vapor will be processed within similar but quantitatively different operating bounds.

The controller operates in the following manner. If for any reason, such as a change in the inlet conditions, the output of the reactor is above the upper bound of the operating region, then the dose applied to the gas in the reaction chamber must be increased. This can be done by either increasing the electron beam current or by decreasing the gas stream mass flow. The electron beam dose can be expressed by eq (3),

dose-HUz L , M

where / is the electron beam current, -AE is the electron beam voltage loss to the gas and M is the mass flow rate of the waste gas. The electron beam current is controlled by the relative bias voltage between the emitting filament 11 and the current control grid 12 (Fig. 2) in the electron beam source. The current is increased by making the grid 12 more positive with respect to the filament 11; the current can be reduced to zero by adjusting the grid bias voltage negative with respect to the filament. In general, additional grids can be used to help make the current density uniform over the entire foil 16 and support plate 14.

The second method of changing the electron beam dose is to change the gas mass flow rate. This can be done by adjusting an inlet valve 48. The whole flow can be maintained as slightly less th.an atmospheric pressure for safety considerations using a outlet pump 49.

The capacity of the flow system .and pumps are sized to process the expected volume of flow of gas stream at a given site. The electron beam current capability is sized for the most probable expected concentration of waste gas. The normal operating condition is specified to use the components near their rated capacity to maximize the amount of material processed for a given time or investment in components. The system responds to changes in the initial conditions in the following manner. Referring again to Figs. 3 and 5, if the concentration of waste gas (carbon tetrachloride) is low, say 62 pm, then for a maximum mass flow for a given valve 48 and pump 49, the controller will have the electron beam can be operated at a reduced current to optimize power consumption. If the concentration of the toxic gas increases, the electron beam current will increase, up to a value that corresponds to the capacity of the electron beam. For a further increase in toxic gas concentration the controller would have the mass flow rate decrease, thereby increasing the dose to maintain the preset limits on toxic waste gas removal.

The electric field is applied to change the average electron energy of the secondary electrons. As mentioned above, the control of the electric field can create secondary electrons whose energies are targeted to specific toxic gases. If the residual waste gas composition changes, as noted at the detector, the controller can change the strength of the applied field to improve the electron attachment rate for the new waste gas composition. Since CC1₄ has a maximum attachment rate at zero field, this would require application of an electric field when waste gas other than CC1₄ are to be targeted. Even in the case of CC1₄ remediation, it is possible that metastable or stable intermediates could form that would benefit from the increased average electron energy obtained with an applied field. Example

An electron beam of 100 keV energy is introduced into a reaction chamber through a 25 um (1/1000 inch) thick aluminum foil. The foil is mechanically supported by a water-cooled copper support grid. The electron beam current is produced by a triode electron gun. The filament constitutes its cathode and is at -100 kV. The Faraday cage constitutes its grid and is biased negatively up to -100 V with respect to the cathode. The foil constitutes the .anode of the triode and is at zero potential. All electron beam current is collected through ground. The electron beam current can be adjusted from 0 to 4 mA. The electron beam cross section upon entering the reaction chamber is square 2.5 x 2.5 cm². The depth of the reaction chamber in electron beam direction is 1.6 cm. For destruction of carbon tetrachloride, an electric field was not applied.

A combined experimented and theoretical approach was used to estimate the electron beam power deposition into the reaction chamber. A 2.5 x. 2.5 cπr aluminum plate of 1.5875 mm thickness was mounted in parallel to the luminum foil at a distance of less than 0.3175 mm. Thus, the plate approximately intercepts the entire electron beam cross section. The temperature rise of the aluminum plate for a given duration of irradiation represents the power deposition into the aluminum plate, and hence the electron beam power entering the reaction chamber. A 1-D Monte-Carlo simulation using the TIGER code of the Integrated Tiger Series was then performed to calculate the energy deposition into the aluminum foil and the aluminum plate as well as the energy deposition into the reaction chamber, all normalized to the electron beam current. The power deposition and the normalized energy deposition into the aluminum plate are then combined to yield the electron beam current entering the reaction chamber. This in turn can be combined with the energy deposition into the reaction chamber to yield the electron beam power deposition into the reaction chamber. Typically, 12.5 W were obtained for 1 mA of electron beam current. The ratio of this power deposition and the mass flow rate is referred to as the specific power deposition.

The CC1₄ intake mole concentration and the relative air humidity were controlled by means of volume flow meters and bubblers of 250 cm³ and 100 cm³ capacity for H₂0 and CC1₄, respectively. One fraction of a dry air stream was passed through these bubblers, another fraction bypassed them. All volume flow meters were calibrated at standard conditions, i.e., 70°F and one atmosphere, with a BIOS International Corporation DryCal DC-1H flow calibrator and a Hewlett-Packard 100 ml soap film flow meter.

For a given CC1₄ intake mole concentration, the air volume flow rate was adjusted so that a high destruction and removal efficiency could be obtained with the maximum available electron beam current. CC 1₄ intake mole concentrations ranged from nominally 10 pm to 700 pm, with relative air humidities between 0% and 45%. For a given setting, the CC1₄ exhaust mole concentration, the electron beam current, and the volume flow rates were measured two to four times within about five minutes to assure steady state conditions. The relative air humidity was re-checked after a complete electron beam current scan had been performed and the CC1₄ supply had been turned off.

The CC1₄ exhaust mole concentration was determined by a 1992 Hewlett - Packard Model 5890 Series II gas chromatograph. The chromatographic separation of CCL₄ was achieved by a Series 530 μ capillary column obtained from Hewlett-Packard. The overall experimental setup is presented in Fig. 6. The CC1₄ intake mole concentration was taken to be the CC1₄ exhaust mole concentration with the electron beam turned off. A small fraction of the reactor exhaust volume flow was passed through a bubbler containing either water or an aqueous solution of sodium hydroxide. By using a Spectrex halide detector, it was shown that all chlorine compounds were trapped in both solutions.

Figure 7 shows the CC1₄ exhaust mole concentration for different intake concentrations (and flow rates) as a function of the electron beam specific power deposition in the air stream. The results in this figure were obtained with dry air at atmospheric pressure. The electron beam specific power deposition is given in kilowatts per kg/sec of gas flow; 1 kW/kg/sec corresponds approximately to a 1 K adiabatic temperature rise in air. CC1₄ exhaust mole concentrations can be reduced to very low values (<1 pm).

The selectivity of the process is indicated by the dependence of the specific power deposition requirements upon the CC1₄ intake mole concentration. For 90% fractional decomposition, the specific power depositions were about 15 kW/kg/sec for 26 pm intake concentration, 60 kW/kg sec for 166 pm, and 160 kW/kg/sec for 663 pm. Thus the specific power deposition could be reduced by a factor about 3 as the intake concentration was reduced by a factor of about 4 (from 663 pm to 166 pm). The specific power deposition was decreased by a factor of about 10 for a reduction of intake concentration of -25 (from 663 pm to 26 pm). For a non-selective, thermal process (where all the molecules in the gas stream must be heated), there would have been no decrease in the specific power deposition as the intake concentration was reduced.

What is claimed is:

Claims

1. A method for the removal of gaseous toxic compounds, comprising the steps of: introducing an electron beam into a reaction chamber through an electron beam-pervious window, said chamber having an inlet port and an outlet port for the transport of a gas stream therethrough, said electron beam having a current selected to generate a plasma with a predetermined density of secondary electrons, said secondary electrons having an average electron energy which promotes decomposition of at least one gaseous toxic compound; applying an electric field to said chamber at a field strength selected to generate a predetermined average electron energy of secondary electrons, said electric field to be applied when it is desired to have an average electron temperature greater than ambient gas temperature, whereby said electron beam furnishes the plasma secondary electrons and said applied electric field provides the average electron energy to decompose at least one gaseous toxic compound; introducing a gas stream comprising at least one gaseous toxic compound into said chamber at a selected mass flow rate, whereby said gas stream is irradiated with secondary electrons and said gaseous toxic compound decomposes; analyzing said gas stream at a detector downstream from said chamber to determine a residual gaseous toxic compound concentration; and adjusting at least one of said electron beam current, said electric field strength and said mass flow rate responsive to said residual gaseous toxic compound concentration to maintain said concentration at no more than a preselected value.

2. The method of claim 1, wherein said mass flow rate is adjusted responsive to said residual gaseous toxic compound concentration.

3. The method of claim 1, wherein said electron beam current is adjusted responsive to said residual gaseous toxic compound concentration. 4. The method of claim 1, wherein said electron beam current and said flow rate are adjusted responsive to said residual gaseous toxic compound concentration.

5. The method of claim 1, wherein said flow rate, said electron beam current and said electric field strength are adjusted responsive to said residual gaseous toxic compound concentration.

6. The method of claim 1, wherein said flow rate and said electric field strength are adjusted responsive to said residual gaseous toxic compound concentration.

7. The method of claim 1, 5 or 6, wherein said electric field is applied to a portion of said chamber.

8. The method of claim 1, 5 or 6, wherein a plurality of electric fields having different field strengths are applied to different regions of said chamber.

9. The method of claim 1, wherein said electric field is selected from the group consisting of DC, RF, microwave and pulsed fields 10. The method of claim 1, wherein said gas stream is maintained a pressure slightly less than one atmosphere.

11. The method of claim 1, wherein said electron beam energy is less than 300 keV.

12. The method of claim 1, wherein said electron beam energy is in the range of 150 to 300 keV.

14. The method of claim 1, 2, 4, 5 or 6, wherein a mass flow valve in said inlet port is adjusted responsive to said controller.

15. The method of claim 1, 3, 4 or 5, wherein a negative bias voltage is applied to an electron-emitting filament and a current control grid of said electron beam source responsive to said controller.

16. The method of claim 1, 5 or 6, wherein a potential is applied to opposing anode and cathode plates to generate said electric field, said potential responsive to said controller.

17. The method of claim 1 further comprising the step of : collecting said decomposition products downstream from said chamber ? ^■ . for safe disposal.

18. The method of claim 17, wherein said decomposition products are collected in a water reservoir through which said gas stream passes.

19. The method of claim 17 wherein said decomposition products are collected in an aqueous alkali solution through which said gas stream passes.

20. The method of claim 1 or 17, further comprising the step of: drying said gas stream prior to its introduction into said chamber.

21. The method of claim 20, wherein drying is carried out using a membrane counterflow exchanger. 22. The method of claim 1 or 17, wherein analysis of said gas stream occurs downstream and proximate to said reaction chamber.

23. The method of claim 17, wherein analysis of said gas stream occurs after collection of said decomposition products.

24. The method of claim 1 or 17, further comprising step of: analyzing said gas stream at a second detector upstream from said chamber to determine an initial gaseous toxic compound concentration.

25. The method of claim 1 or 23, wherein said analysis is accomplished using a solid state detector.

26. The method of claim 24, wherein said analysis is accomplished using a solid state detector.

27. The method of claim 24, wherein said solid state detector selected from the group consisting of metal oxide gas sensors and ceramic-metallic gas sensors.

28. The method of claim 1, further comprising the step of: introducing a material into said chamber for enhancing the decomposition of said gaseous toxic compounds.

29. The method of claim 1, wherein said electron beam is introduced into said chamber through a plurality of electron beam-pervious windows.

30. The method of claim 1, wherein said electron beam for supplying all of said windows is generated from a single electron beam source. 31. The method of claim 1 , wherein said electron beam for supplying each of said windows is generated from a different electron beam source.

32. A system for the destruction of gaseous toxic compounds, comprising: a reaction chamber comprising at least one electron beam-pervious window, said chamber having an inlet port and an outlet port for the transport of a gas stream at a selected flow rate therethrough; an electron beam source for generating an electron beam, said electron beam source proximate to said window, said electron beam having a current selected to generate plasma with a predetermined density of secondary electrons, said secondary electrons having an average electron energy which promotes decomposition of at least one gaseous toxic compound; an electric field generator for applying an electric field to said chamber at a field strength selected to generate a predetermined average electron energy of secondary electrons, said electric field to be applied when it is desire to have an average electron energy greater than 0.03 eV, whereby said electron beam furnishes the plasma secondary electrons and said electric field provides said average electron energy to decompose at least one gaseous toxic compound; a detector downstream from said chamber for determination of a residual gaseous toxic compound concentration in said gas stream; and a controller responsive to said residual gaseous toxic compound concentration and capable of adjusting at least one of said electron beam current, said electric field strength and said flow rate to maintain said concentration at no more than a preselected value.

33. The system of claim 32, wherein said mass flow rate is responsiv to said residua gaseous compound concentration.

34. The system of claim 32, wherein said electron beam current is responsive to said residual gaseous compound concentration. 35. The system of claim 32, wherein said electron beam current and

StrøTTWt SHEET (RULE 26) said flow rate are responsive to said residual gaseous compound concentration. 36. The system of claim 32, wherein said flow rate, said electron beam current and said electric field strength are responsive to said residual gaseous compound concentration. 37. The system of claim 32, wherein said flow rate and said electric field strength are responsive to said residual gaseous compound concentration.

38. The system of claim 32, 33, 35, 36 or 37, wherein said inlet port comprises a mass flow valve responsive to an input signal from said controller.

39. The system of claim 32, 34, 35 or 36, wherein a negative bias voltage is applied to an electron-emitting filament and a current control grid o said electron beam source responsive to said controller.

40. The system of claim 32, 36 or 37, wherein said electric field generator comprises opposing anode and cathode plates across which a potential is applied, said potential responsive to said controller. 41. The system of claim 32, 36 or 37, wherein said electric field generator, applies a field to a portion of said chamber.

42. The system of claim 32, 36 or 37, wherein a plurality of electric field generators apply fields of different strength to different regions of said chamber. 43. The system of claim 32, wherein said electron beam energy is in the range of 150 to 300 keV.

44. The system of claim 32, 33, 34, 35, 36 or 37, further comprising: a scrubber for collecting said decomposition products downstream of said detector and controller for safe disposal. 45. The system of claim 44, wherein said scrubber comprises a water reservoir, through which said gas stream passes.

46. The system of claim 44, wherein said scrubber comprises an aqueous alkali solution, through which said gas stream passes.

47. The system of claim 32, further comprising: a dryer positioned upstream from said chamber for reducing water 9 \ content of said air stream.

48. The system of claim 47, wherein said dryer is a membrane counterflow exchanger

49. The system of claim 32, wherein said chamber has a high aspect ratio.

50. The system of claim 32, 33, 34, 35, 36 or 37, wherein said detector is located downstream and proximate to said chamber.

51. The system of claim 32, 33, 34, 35, 36 or 37, wherein said detector is located downstream and proximate to said scrubber. 52. The system of claim 32, 33, 34, 35, 36 or 37, further comprising: a second detector is positioned upstream and proximate to said reaction chamber for determination of initial gaseous compound concentration.

53. The system of claim 32, wherein said detector is a solid state detector. 54. The system of claim 53, wherein said solid state detector is selected from the group consisting of metal oxide gas sensors and ceramic- metallic gas sensors.

55. The system of claim 32, wherein said chamber comprises a plurality of electron beam-pervious windows. 56. The system of claim 55, wherein all of said windows are supplied electron beams from a single electron beam source.

57. The system of claim 55, wherein each of said windows is supplied electron beams from a different electron beam source.

58. The system of claim 32, wherein said secondary electrons have an average electron energy less than that capable of creating a self-sustaining discharge.