US 3653185 A
Apparatus and method are disclosed for reducing or removing particulate solid matter as well as admixed gaseous contaminants from a main stream of gas, as for example removing solid and gaseous contaminants from air. The removal is effected by the combined action on the gas stream of high intensity electrical field and electromagnetic radiation, whereby to cause electrostatic precipitation of solid contaminants and electrochemical and photochemical transformation of gaseous contaminants to elemental or non-contaminant form. The field is induced by oppositely charged electrodes causing excitation of the particulate and gaseous contaminants to a state or condition causing dark current flow and/or glow discharge between the electrodes while avoiding field-disrupting arc discharge. Concurrently with such high voltage excitation, the fluid is subjected to electromagnetic radiation, more particularly in the ultraviolet range, to produce photoionization to sustain the electrochemical and photochemical transformation.
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Description (OCR text may contain errors)
United States Patent Scott et al. Apr. 4, 1972 s41 AIRBORNE CONTAMINANT REMOVAL 3,053,028 9/1962 Kayko ..55/103 BY ELECTRO.PH()T()I()NIZATI()N 3,109,720 11/1963 Cummings et a1. ..55/1 12 X 3,469,371 9/1969 Gelfand ..55/110  Inventors: Harold W. Scott, Ridgefield; Avery B.
Smith, Wallingford, both of Conn. FOREIGN PATENTS OR APPLICATIONS  Assignee: Resource Control, Inc., West Haven, 155,913 3/1954 Australia ..55/145 Conn, 931,625 7/1963 Great Britain... ....55/139 377,343 6/1923 Germany ..55/105  Filed: Oct. 8, 1968 ] App! No: 765,763 OTHER PUBLICATIONS German Printed Application No. 1,083,786, printed June 23, 52 u.s.c| ..55/l03,21/74R,21/D1G. 2, sht'dwg'zppspec'l 23/2 55,108 55,110 55/112 55/121 55,139 Primary Examiner-Dennis E. Talbert,.lr.
55/146, 55/150, 55/154, 55/155, 55/220, 55/447, Atmmey CumS Morris & Safford 55/522, 55/523, 55/527, 55/D1G. 30, 55/D1G. 41, /29, /119, 204/312, 250/42, 310/6, 313/231,  ABSTRACT 313/325 [51 Int. Cl. ..B03c 3/30 Apparatus and method are disclosed for reducing or removing  Field of Search ..55/2, 101, 102, 103, 105, 108, particulate solid matter as well as admixed gaseous contami- 55/109, 112, 113, 114, 136-139, 143, 145, 154, nants from a main stream of gas, as for example removing 155 110 121 146 150 220 447, 522 523, 527, solid and gaseous contaminants from air. The removal is ef- [)[G 30 mg 41; 21 74 R, mg, 2; 23/2 15; fected by the combined action on the gas stream of high inten- 1 0 1 19; 204 312; 250/42; 310/ 313/231 325 sity electrical field and electromagnetic radiation, whereby to cause electrostatic precipitation of solid contaminants and 5 R f e e Ci electrochemical and photochemical transformation of gaseous contaminants to elemental or non-contaminant form. The UNITED STATES PATENTS field is induced by oppositely charged electrodes causing excitation of the articulate and aseous contaminants to a state 2,019,485 11/1935 Deutsch.. ..55/139 x condition czfusing dark currgem flow and/Or glow discharge 2'244'279 6/1941 whlte 3O UX between the electrodes while avoiding field-disrupting arc 2375901 3/1942 Anderson "SS/137 X discharge. Concurrently with such high voltage excitation, the 2,381,455 8/1945 F "SS/102 fluid is subjected to electromagnetic radiation, more particu- 2'449'681 9/1948 "SS/102 larly in the ultraviolet range, to produce photoionization to 21682313 6/1954 whlle 55/139 X sustain the electrochemical and photochemical transforma- 2,788,859 4/1957 Eron.... ..55/304 mm 2,824,343 2/1958 Glass .....21/74 2,978,066 4/1961 Nodolf ..55/l45 2 Claims, 5 Drawing Figures PATENTEQ APR 4 I972 SHEET 1 OF 2 CE I Hi6 VOLTAGE SOUR VOLTAGE CONTROL M 7 HA f0 0 o o o o o o 0 0 0 0 1 1 u M h IV u J m L M a w INVENTOR S T H wn w W8 m ME Hm PATENTEDAPR 4 I972 SHEET 2 BF 2 m m 52 EM N 0 9 QC w E mm m WITH FIELD INTENS/T/ES VELOCITV= CM/SEC d a l a z E M l w Mm Mi R 0 r1 w M M K [L R A 2 w (AAIr Ps) AVERY B. SMITH TTORNI-iYS AIRBORNE CONTAMINANT REMOVAL BY ELECTRO- PHOTOIONIZATION This invention is directed to the reduction or removal of solid and gaseous contaminants from various gas streams and is particularly concerned with reducing atmospheric pollution occasioned by discharge to the air of combustion products from the operation of automobiles, aircraft and other vehicles, incinerators and domestic, industrial and commercial heating plants, and like producers of airborne pollutants.
With continuing increase in population density and escalation in use of domestic, commercial and industrial combustion devices and other exhaust gas generating activities involving the spewing of literally millions of tons of harmful pollutants and contaminants into the atmosphere, pollution alerts are becoming regular occurrences in many areas and control of the emission of such pollution is essential to public health, safety and agriculture.
Current methods for the control of airborne contaminants include inertial separation, scrubbing, filtration, and electrostatic precipitation Cyclone separators producing an abrupt change in direction of rapidly flowing gas streams effect separation of entrained solids by differences in the inertial forces acting on such solids as compared to the entraining gas. Cyclones have the advantage of simplicity of design, high capacity and easy maintenance. At best, however such inertial separation devices are efficient only in extracting relatively large particles from the entraining gas and of course they are completely unable to separate contaminant gases present in the main body of gas or air being treated. Similarly, scrubbing of a gas by contacting it with a fine spray of liquid such as water has the advantage of relatively low equipment cost. But there are operational disadvantages with scrubbers, including handling of resultant slurry or sludge, the corrosion of equipment and microbiological growth problems. In most instances, such devices are limited in practice to the removal of relatively coarse particles from a gas, and any separation of a gaseous pollutant is dependent upon the relative solubilities of the pollutant and main gas components in water or other liquid. Most scrubbing devices are quite inefficient for the removal of particles below 5p. in size. Likewise filtration of contaminated gases through mechanical entraining media such as loosely packed mats of long strand glass fibers, open cell plastic foam, sintered metal compacts, or cloth, paper or other fabric material, offers advantages of simplicity and economy in respect to initial equipment cost. But again, such devices are capable of removing only particulate matter from the gases, and in general they are not economically suitable for operation where the aerosol content of the gas to be filtered is high. Electrostatic precipitation is widely used in such applications and in spite of high initial equipment cost and operating expense, this system many times represents the only practical procedure for obtaining acceptably low solid airborne particulate levels in gas or air streams exhausted to atmosphere. The procedure employed, of course, involves the application of high voltages to electrode arrays such that the gas near the electrodes is ionized and the particles suspended in the gas acquire a charge from contact with the gas ions. Such charged particles then migrate to an electrode of opposite charge and, as the gas flows over the electrode array, the charged particles attach themselves to the electrodes. Removal of the accumulated solid particles in most cases is accomplished by mechanically vibrating the electrodes to discharge the cakes of collected dust into a collection bin. Although the system is versatile and efficient in removing small solid particles from an atmosphere where the particle size is extremely small, it does have some important limitations, chief of which is the fact that only particulate matter can be precipitated. In addition, the physical and electrical characteristics of some particulate materials prevent them from being collected efficiently by an electrostatic precipitator. One example is zinc oxide fume which has a tendency to quench the corona discharge. Since the corona discharge is necessary in electrostatic precipitation to effect ionization of the gas, the system is not well suited to such particulate contaminants. Various other materials also have such high electrical resistivity that they tend to coat the collecting electrodes with a highly insulating coating, interfering with the corona discharge and causing considerable reduction in the collecting etficiency of the electrode array. As has already been mentioned too, an electrostatic precipitator may well cost substantially more than other devices due to the size and complexity of the electrical components. Furthermore, power consumption may run from as high as 50 kw. for a 5,000 c.f.m. unit to as low as 15 kw. for a 500,000 c.f.m. unit. In general, however, mean power requirements are approximately 15 kw. for a 100,000 c.f.m. unit.
It is obvious from the foregoing discussion that previous methods for contaminant removal from a gas stream are generally effective only for the removal of particulate matter and such devices and methods do not take into account contaminant gases such as the gaseous oxides of nitrogen or sulfur or even of carbon, which present very serious problems in atmospheric pollution control.
The concept of the present invention is directed to a method and apparatus for removing gaseous as well as solid particulate contaminants from an air stream or other fluid stream, and utilizes the sciences of induced high voltage fields, quantum mechanics and electromagnetic radiation. It is the combination of these instrumentalities used in conjunction that is essential to and characteristic of the invention. In comparison to earlier known methods and equipment for atmospheric decontamination the method and devices of this invention offer advantages of greater compactness in equipment size, reduction in number of components, lower maintenance and operating costs, and high degree of effectiveness, reliability and efficiency. One of the most important advantages realized is that the method and apparatus here disclosed are effective in removing admixed contaminant gaseous components from an atmosphere, as well as the fine particulate matter that may be entrained in such atmosphere.
In brief, the invention involves providing a treatment chamber through which the atmosphere is caused to flow, and while it is within the chamber subjecting it to unidirectional high voltage field or fields produced between spaced electrodes and concurrently therewith to electromagnetic radiation, preferably radiation in the ultraviolet light range of frequencies. The basic steps in the process comprise, in general, electrically charging by means of the high voltage field both suspended contaminant particles as well as contaminant gases in the atmosphere undergoing treatment to induce an incipient ionizing or excited particle state, collecting the charged particles at the charging electrodes, introducing electromagnetic radiation as by subjecting the excited components to ultraviolet light to effect oxidation and reduction along with other photochemical reactions of the contaminant gas or gasses and conversion to elemental or at least noncontaminating form, and finally removing precipitated particulate material adhering on the electrodes.
It is apparent that during the course of the decontamination treatment quite a number of reactions are occurring simultaneously. Not all of these are fully understood at present but it is believed that the following postulation may be helpful in explanation of what does occur, whereby to afford a better understanding of the present invention and its practical application in the elimination or control of atmospheric contamination more particularly.
A gas in its normal state at atmospheric pressure is an excellent electric insulator, and in everyday life is of course widely used in that role. However, when an electrical force of sufficient intensity is established in the gas between opposing charged electrodes, a gas can and does become a conductor of electricity. The voltage or potential difference at which the transition occurs from an insulating to a conducting state is called the electrical breakdown voltage for that particular gas. This breakdown, when it occurs, is essentially a current flow established by gas ionization between the opposing electrodes and this flow is generally rather violent, being commonly referred to as arc discharge. However, before a large current can be developed in a previously insulating or non-conductive gas, it is clear that the current must build up by someionization mechanism from an extremely small value initially, probably due to the existence of a few free electrons in the gas in its normal state. Atmospheric air and gas as emitted by combustion processes normally contains a relatively small number of both negatively and positively charged gas molecules and very fine solid particles. These individual particles and/or molecules can accumulate a charge from several sources, as for instance from a flame in a combustion chamber since a flame is an area of high ionization. Thus small particles emanating from the flame generally have both negative and positive charges. Similarly, such charges can be developed by frictional forces between two materials with difierent dialectric properties. Particles can also receive a charge from frictional sources during aerosol generation and conceivably can accumulate some charge by their movement through a gas. Radiation emanating from a variety of sources in space constantly creates ions within the atmosphere. These ions can, via a complex mechanism, impart a charge to particles suspended in the atmosphere. The nature of this process is such that particles suspended will be positively or negatively charged in a distribution such that the net charge of the aerosol is zero. Individual particles, however, may exhibit a considerable charge even though the overall charge is zero. All of these charged particles, ions, molecules or even free electrons, when subjected to an electric field can initiate further ionization by means of single electron impact, double electron impact, ionatom collisions, excited atom-molecule collisions and atomatom collisions, and combinations of these processes. Therefore it is quite evident that the conditions for the initation of ionization are generally present naturally in most atmospheres and come into play under the effect of a high potential electric field. However, because of the quantum nature of the ionization process, only atomic particles and light quanta are capable by themselves of interacting with atoms and molecules to produce ions. Thus the actual ionization agents are free electrons, atomic or molecular ions, excited atoms, neutrons and photons. As used herein, the term excited state in reference to an atmosphere under-going treatment is intended to refer to that state or condition in which the ionization process has been initiated.
In the corona discharge process employed in conventional electrostatic precipitator installations, the dominant ion production mechanism is ionization by electron impact in which free electrons in the gas acquire energy from the applied electric field and collide violently with gas molecules, literally knocking electrons off of the molecules. The net result is the creation of additional free electrons and positively charged gas ions. For the process to occur, the colliding electron must possess a certain minimum energy which is characteristic of the molecule or atom bombarded and is known as the ionization energy. For most atoms and molecules encountered in air decontamination processes, the ionization energy is in the range of 4 to 25 electron volts (ev.). Electrons are singularly effective ionizers because they gain relatively high energies from the electrical field as a result of their long meanfree paths between collisions with gas molecules; and they retain virtually all their kinetic energy when they make elastic collisions with gas molecules, yet transfer virtually all of their kinetic energy when they make inelastic (ionizing) collisions.
Although electron-impact ionization is the primary mechanism for ion production in most are and corona discharge processes, it cannot alone account for electric break down of the gases. It is true that large amplification of the gaseous currents are possible by electron ionization alone but for the discharge to become self-maintaining, a regeneration or feed-back mechanism is required to supply the initating electrons. In the cases of corona and are discharge, most likely the feed back mechanisms are (a) release of electrons at the cathode by positive ion impact, (b) photoelectric emission of electrons at the cathode by ultraviolet radiation produced by the arc and corona discharge, (c) photoionization of the gas by ultra-violet radiation from the corona or arc discharge, and (d) ionization by metastable gas atoms.
The effect of photoionization in the ionization process is important. Technically, its importance lies in the fact that it is a process taking place entirely in the gas, quite independent on any electrode effect. For example, in simple (pure) gases, photons are not produced and the photoionization process plays no significant role in the ionization of such gases. In air or combustion exhaust gases on the other hand, photoionization does take place. There is experimental evidence'to indicate that this occurs as the result of electrons exciting nitrogen molecules at a level of about 15 ev., resulting in the production of high energy photons which subsequently ionize oxygen molecules whose ionization potential is lower than 15 ev.
In the conventional electrostatic precipitator, any photoionization is intermittent at best because of the nature of the high voltage field, and its occurrence is purely incidental to the principle objective sought, namely that of inducing very high charges in entrained particulate matter present in the atmosphere in order to effect a rapid migration of the particulate material to, and collection of it at the electrodes. Under the conditions existing in the corona discharge occurring in conventional electrostatic precipitation, where the air, combustion exhaust, etc., contains substantial proportions of nitrogen, a variety of nitrogeneous oxides are formed by reaction between the nitrogen and oxygen present in the atmosphere. Such oxides are themselves undesirable pollutants which can cause chemical corrosion, severe irritation to breathing or other harmful effects.
In contrast to the electrostatic precipitator or corona discharge process, the present invention controls the application of unidirectional high voltage to opposed electrodes at levels producing high electric field strength with excitation but avoiding disruptive arcing. This is made possible since the high voltage forces are not the sole influence relied upon in treating the atmosphere. And whereas in the operation of conventional electrostatic precipitators some photoionization is inherent but is merely an incidental and uncontrolled side effect, the present invention makes specific use of photoionization under controlled conditions to supplement the effect of the high voltage field in a manner causing a unique plasma condition to be established and maintained and full ionization to occur free of disruptive arc discharge and its undesirable side effects.
The invention is illustrated further by the following descriptions of certain practical embodiments, with references to the accompanying drawings in which 7 FIGS. 1 and 2 are top plan and end elevational views, respectively, schematically illustrating a simple form of apparatus embodying the teaching of the invention;
FIG. 3 is a graph plotting current flow vs. applied electrode potential in a typical gas such as air;
FIG. 4 is a plot of particle drift velocity vs. electric field strength for several of the most difficult particle sizes or diameters encountered in practical effluent discharges; and
FIG. 5 is a fragmentary elevational view partially in section of a cathode structure incorporating within itself high voltage generating means.
The illustration of FIGS. 1 and 2 show schematically a contaminated air treatment chamber 10 enclosed at its opposite sides, ends, top and bottom, and having inlet and outlet openings l2, 14, respectively, located in opposite ends for the admission and escape of the air. Suitable ducts 16, 18 are attached to the opposite ends for introducing and removing the air which is pumped by any suitable means, not shown, such as a blower. Within chamber 10 there is centrally mounted a plate-like cathode 20 which is electrically insulated from the side walls and other components of the treatment chamber. A terminal 22 makes electrical connection to the cathode within the chamber and provides means for connecting the negative side of an external high voltage potential source 23 and voltage control means 25 to the cathode. Also located within the treatment chamber is an anode 24 which is generally of open wire mesh or grid construction, laterally enclosing the opposite faces of cathode 20 in spaced relation thereto. Anode 24 may be mounted so as to be insulated from other components in the treatment chamber but preferably is grounded, and is provided with an external high voltage terminal 26 to provide electrical connection of the anode with the positive side of the high voltage source. There is also positioned within chamber a series of light tubes 30 which are spaced along the opposite side walls of the treatment chamber, between such walls and the adjacent anode screen 24.
In the operation of the device, contaminated air containing solid particulate contaminants as well as gaseous contaminant components in a main body or stream of entering air is introduced through inlet duct 16 and this air passes through inlet openings 12 of chamber 10, flowing along opposite sides of cathode, and out through exit openings 14 to be discharged through duct 18. A high potential field is induced between cathode 20 and anode 24 by application of a unidirectional high voltage potential from high voltage source 23 to terminals 22, 26, with the negative side of the voltage source connected to cathode 20 and the positive side of such source connected to anode 24. This potential source 23 may be any one of numerous available standard devices such as a Van De Graaff generator, Cockcroft-Walton device, Wimhurst machine, transmission line generator, the rectified output of a high voltage transformer or the like. Light tubes 30 supply the electromagnetic radiation, and for reasons to be indicated hereinafter it is preferred to use ultraviolet lamps or similar devices having peak spectral emission bands in the range of from 1,500 A. to 4,000 A. The voltage applied to the electrodes and the dominant wave lengths emitted by the light source will be selected in accordance with the characteristics of the gas stream, the initial contaminant content and the volume flow per unit of time. These matters will be discussed more fully hereafter.
In the operation of the device, as the anode potential is made progressively more positive with respect to the cathode by conventional control means 25, a current flow takes place between the electrodes due to ionization of the gas between these members. Reference is made to FIG. 3 of the drawings which is a typical plot of applied voltage against amperes or current flow occurring between spaced oppositely charged electrodes in an atmosphere of air or similar gas mixture. At zero potential difference between the electrodes no current flows between them of course, but as the potential difference is increased a measurable current flow results which increases generally proportionately to increase in the potential between the electrodes up to point a corresponding to voltage E, as seen in FIG. 3. This initial current is known as dark current because under these conditions there is no visible glow in the gas. However at point a, ignition occurs due to ionization of the gas and the ionized gas thereafter conducts current readily between the electrodes. Assuming a current limiting device is located in series with the voltage supply, the potential difference between the anode to cathode falls back at this point to an intermediate value determined by the characteristic of the gas as well as the cathode material. The current flow remains very nearly constant in this transition which is represented in FIG. 3 by the dotted line from point a to point b. The voltage difference between the electrodes at point b is E Once the ignition has occurred, very small changes in applied voltage produce a very great change in current flow, as is seen by the very steep slope of the amplification curve between points b and c on FIG. 3. This region of operation is known as the glow discharge condition and it is in this region that the present system is designed to operate. Beginning at point c on the amplification curve, however, corresponding to a potential difference of E the rate of increase in current flow drops off very rapidly to point d at which the glow discharge changes to an arc discharge and the potential difference then drops off extremely rapidly to point e. Further increase of current flow to very high amperage values then occurs at relatively low potential differences between the electrode, but this is accompanied by a power arc with high rate of power consumption.
Voltage applied to the anode and cathode causes positive ions present in the atmosphere travel to the cathode and negative electrons to travel to the anode. During this travel, the electrons are accelerated by the potential gradient between the electrodes, and in traveling to the anode the electrons strike other gas molecules forming more positive ions and releasing other free electrons. The process is regenerative and self-sustaining in the glow discharge region of operation referred to above so long as electromagnetic energy is properly supplied to the gas molecules in this excited state, and this is accomplished in accordance with the teaching of this invention without the accompanying disruptive effect of arc discharge. Such electromagnetic energy is supplied by radiation from the light source 30 seen in FIGS. 1 and 2.
In operation a balance must be maintained between the potential difference at the electrodes and the frequency of the electromagnetic radiation so that the contaminant gas atoms or molecules present in the air stream will be excited to their first level ionization state which, as mentioned earlier, will range from about 4 to 25 ev. for most atoms and molecules encountered. With very low voltage gradient between the electrodes, as for example volts/cm, the photoionization effects supplied by the light source must be of extremely high frequency, on the order to X-ray frequencies or greater. As the voltage gradient across the electrodes is increased, the photoionization frequency required becomes lower and may even reach visible light or infrared frequencies at extremely high potential differences. Practical considerations, obviously, enter into a determination of the selection of operating conditions. For example, the breakdown voltage for air occurs at about 30 kv./cm. under normal atmospheric pressures and temperatures. Thus, without some means to prevent such breakdown and arc-over occurring within the treatment chamber, it must be designed with regard to electrode spacing so as not to exceed this condition. Other factors which must be taken into consideration include velocity of flow through the treatment chamber and amount of contaminant components in the air stream since the size of the unit including the electrode surface areas, the cathode surface length, etc. must be calculated to account for rate of charged particle migration under the applied voltage to insure that the particles reach the electrode before being swept out of the treatment chamber. Simultaneously the selection of the frequency of the electromagnetic energy supplied by the light source must take into consideration not only the voltage potential applied to the electrodes but the availability and cost of sources of such radiation and the effect that particular frequencies may have on different components of the gaseous stream. For example, utilization of a light source in the ultraviolet range of approximately 2,500 A. in combination with an electrode potential gradient of approximately 9.5 kv./cm. in the treatment of industrial smokes or exhausts results in converting contaminant oxide gaseous components present in the smoke to nascent oxygen and nitrogen without substantial reaction between the two. This is in sharp contrast to the production of nitrogeneous oxides when arc discharge occurs with conventional electrostatic precipitation treatment of such gases.
In discussing the two major actions occurring in the treatment chamber, namely (a) finite particle charging and migration to the electrodes and (b) photoionization of molecular gaseous contaminate components, the later will be treated first.
The mechanism of the photochemical reaction is best described in two stages; first, photon or energy absorption, the primary process, occurs followed by more or less clearly separable ensuing secondary process which are essentially chemical in nature. Except in rare instances the later are quite uninfluenced by the presence of light and would occur in its absence if the primary products were formed in some fashion other than by light initation. The photon or light energy absorption act is a matter of pure physics in accordance with known concepts of the theory of matter. Light as spoken of here means a portion of the electromagnetic spectrum con siderably more extensive than that occupied merely by visible light. The range actually referred to here extends from at least 1,500 A., to 4,000 A. in wave length and occasionably to 7,200 A. Radiation having wave lengths greater than 7,200 A. are only rarely of photochemical consequence.
The effectiveness of light absorption for initiating a chemical reaction can be understood on the basis of the quantum theory. Light absorption provides more than enough energy to initiate most chemical reactions and in fact most photochemical reactions involving the absorption of light merely accelerate a transformation which otherwise would proceed very slowly. In some cases the effect of the light is that of a trigger wherein the illumination initiates a reaction which then runs to completion without the assistance of additional light. In a sense, activated reactants in the photochemical reaction are so highly excited that they may be regarded as extremely hot" entities reacting in a cold environment. a
It was shown first by J. Frank (1925) that light absorption can dissociate a molecule into smaller fragments, and that for gases of diatomic and polyatomic molecules this occurs when the absorbed light has a wave length lying in a region of continuous absorption.
This type of continuous absorption must be distinguished from that giving rise to ionization, the latter occurring with molecules as well as atoms but is almost always confined to the extreme ultraviolet spectral region. In addition to regions of continuous absorption, molecules also possess absorption bands which are simply groups of closely spaced absorption lines. The immediate effect of absorption in such a band is excitation, just as for an atom, but if the absorbed energy is greater than the bond energy, spontaneous dissociation of the excited molecule may nevertheless ensue. This dissociation may occur within lO'lO sec. after the absorption event. If it does, the individual absorption lines of the band will diffuse and the behavior is called predissociation. If the components of the absorption band are sharply defined, this spontaneous dissociation usually does not occur but even here, if the excitation energy is sufiicient, dissociation can often be induced by collisions of the excited molecules with atoms or other molecules as in the case when the components are exposed to a high intensity electrical field.
As previously indicated, the photochemical action is essentially completed with the formation of the primary dissociated products, i.e., free atoms, radicals, excited atoms or molecules and the further course of the reaction depends on the interaction of these with each other and with additional entities or components present.
In the case of some contaminant gases such as carbon monoxide, it has been found that ultraviolet radiation causes the desired photochemical oxidation process directly without the need of anode and cathode surfaces. The reaction taking place can be represented as follows:
C0 C0 UV'CO C The carbon released by this reaction collects at the cathode and is subsequently removed as a solid deposit, while the carbon dioxide is simply released as a gas and commingles with the effluent air stream. Its presence is of course not objectionable. In the case of the sulfur dioxide, the chemical reaction for this process is represented as follows:
SO: UV S 20' (Al: the cathode) S++++ 45 S" (solid) (At the anode) 0- 0- 0 0 4 Concurrently with the photochemical or photoionization reactions just described, there is also taking place in the treatment chamber of apparatus designed in accordance with this invention the electrostatic precipitation of finite particulate matter entrained in the entering gas stream. As such particles enter the highly ionized area or region of the treatment chamber between the anode and cathode, these particles become highly charged at an extremely rapid rate, usually within about 10 sec. or less. This means that the particles receive essentially their full charge in the first few inches of travel for any airstream velocity usually encountered. It is believed that three distinct particle-charging mechanisms are involved in the type of treatment apparatus herein disclosed. One is generally designated field or impact charging, while the second is known as ion diffusion charging. The third is the photoelectric effect which takes place when the photon energy exceeds the work function of a particular material. When this occurs, electrons are emitted from the surface of that material, thus of course resulting in a charge on the material. In practice, the field charging process predominates for particles larger than 0.5;. in diameter, while the diffusion process is dominant for particles smaller than about 0.2 t, and both processes are involved for particles in the intermediate size range. As might be expected and as is the fact, the larger the particle, the larger the number of charges it will take on. While charged particles smaller than 0.01 1.1. have been detected, their lifetime is apparently very short because of coagulation with larger particles and also it is possible that such small particles react to the field strength and ultraviolet radiation in the same manner as a gas molecule, whereby they assume the characteristics of an ion and not of a charged particle.
Physical collection of the charged entrained or suspended particles may be effected by passing them through a continua tion of the highly ionized field used to impart the initial charge, or this may be effected by a separate high potential electric field. The two arrangements are designated as singlestage and double-stage systems, respectively, and although there are application differences between the two, the electrical forces acting on the charged particles are basically the same and are governed by Coulombs Law of electrostatic force. That is, rate of particle collection is proportional to the Coulomb force and, therefore, to the product of particle charge and collection field intensity. Individual particle separation forces are large, even for submicron particles, which explains in large part the great effectiveness as well as the broad range of application of the present invention. Other factors important in the design considerations are interference from particle re-entrainment, disturbance of the ionization, non-laminar gas flow and voltage breakdown. Only when a particle moves close enough to the surface of a collecting electrode to be trapped by the electric field, therefore, will actual particle collection be effected. Particle trajectories are thus dependent on the combined effects of gas velocity and particle drift velocity in the electric field, and the drift velocity is in turn a function of the number of charges, the diameter of the particle and the field strength. Therefore if the particle collection apparatus is to be effective, the electrode surfaces must be designed with the lowest drift velocity of any significant particle in mind and from this the maximum length of time needed for such particle to arrive in the area of influence of the electrode must be calculated to determine the length of cathode needed. In a device of the general character illustrated in FIGS. 1 and 2 of the drawings, therefore, the length of the electrode in the direction of gas flow through the apparatus will depend on the various factors just mentioned. The area of the collecting electrode will be determined by the volume of gas flow per time unit and the average particulate content to provide the collecting surface capacity needed to accept and hold the particles for any given period of operation before shut down and discharge of the accumulated cake." 7
Upon reaching the surface of the collecting electrode, the particles tend to adhere strongly to the surface by virtue of the charge effect at the surface. This occurs with plain metal electrodes but it has been frequently observed that polymer films at the surface of the electrode tend to enhance this action. That is, electrodes having charged polymer films at their surfaces tend to accummulate particles at a higher rate than do similar uncoated electrode surfaces. The charged particles collected on the electrode tend to act as dipoles and typically it will be noted that the agglomerates are formed in a chainlike fashion, depositing head-to-tail on the collecting electrode. It has also been found that particles of 0. 1p. and smaller tend to coagulate somewhat faster than the larger particles, and this in some cases tends to offset their lower charging capabilities, thus helping the collection process.
Once the contaminant particles have been collected at the electrode, some means must be provided for removing the accummulation at least periodically if optimum collection conditions are to be maintained. In a small unit useful for domestic or institutional air decontamination, the removal of collected particulate matter can be relatively simply handled. In such cases, the operation of the unit can be temporarily discontinued and, by suitable access to the interior of the unit, the collected particles or cake may be removed by wiping or brushing the electrode surface or by mechanically vibrating the unit to dislodge them. In larger units generally similarly methods can be employed but it is preferred to build directly into the unit some sort of vibrating mechanism mechanically attached to the collection electrode, as for example magnetic or ultrasonic vibrator devices. This dislodgement of the collected deposits can be most effectively obtained where vibration is produced at the natural frequency of the mechanical structure of the cathode. Alternatively, particularly where the electrode surface is covered by a separable plastic film to take advantage of the charge effect mentioned above, means may be incorporated for pneumatically expanding and contracting the film relative to the electrode in order to dislodge the collected material.
Once the deposited material is dislodged from the electrode, it can be collected in a hopper or similar device and removed periodically. Obviously, in order to prevent re-entrainment of the collected particulates in the gas stream, the collection process must be temporarily discontinued for the particular unit during dislodgment but the decontamination process can be made continuous simply by providing an alternate processing unit to which the gas flow is diverted.
Various modification can be incorporated in the basic apparatus disclosed in FIGS. 1 and 2, depending on the particular requirements of the air stream or other fluid to be treated. For example, the apparatus may incorporate either an open loop control system or a closed loop system. In the open loop system, no means is provided for comparing the output or effluent air stream with the input stream for contaminant content. In the closed loop system, on the other hand, one or more feed back control loops, in which functions of the control signals are combined with functions of the commands for maintaining prescribed relationship between the commands and control signals may be provided. Such open or closed loop systems differ only in the design of the high voltage supply and feed back control systems employed. In either system a third element or grid 40 can be introduced in the basic unit described in FIGS. 1 and 2. This control grid is located between the anode and cathode, close to the anode, and a negative potential applied to such grid through terminal 42. Under this arrangement, grid-anode breakdown cannot occur until the grid-anode potential is made much higher than the grid-cathode potential required for breakdown. In this manner the grid electrostatically shields the cathode from the anode and prevents anode-cathode breakdown where the potential applied to those electrodes would normally exceed the breakdown voltage. From this it is evident that with a control grid of this type, a higher voltage gradient may be impressed across the anode-cathode area without causing breakdown than would be possible with the simple cathode-anode arrangement. Various advantages flow from this since the arrangement allows for a higher velocity gas stream which, in turn, reduces the overall size of the device for a given flow capacity.
In the closed loop system, the voltage applied to the cathode or anode is regulated through an electrical or electronic feed back mechanism of any conventional known type. The entering gas stream is measured for both conductivity and resistivity, and the control signals generated are matched with particular command signals determined by the decontamination requirements of the efiluent stream. If the relationship between the control signals and the command signals differ, the voltage will either increase or decrease until the proper match is accomplished. Essentially, a high resistive gas stream will cause the voltage gradient to increase and by the same token a high conductive gas stream will lower the voltage gradient. The procedure, being automatic, is rapid and precise and provides further assurance against an arc discharge occurring in the treatment chamber due to sudden changes in contaminant content of the input air stream. Such a condition is frequently encountered in flue gas emanating from incinerator operation.
The open loop system on the other hand has the advantage of greater simplicity and is usually entirely satisfactory where the entering gas stream remains fairly constant in its contaminant content. Such conditions are often encountered with gases from furnaces in power generating plants, as well as in simple room atmosphere decontaminating units for domestic or institutional use.
Other design modifications relate to the physical construction of the anode and cathode in the treatment chamber. For the smaller units, the cathode which is the collector for virtually all of the solid particulate contaminants, is preferrably a retangular plate member for the simple type of domestic or institutional apparatus described above. The anode is either a metal screen or a series of wires arranged parallel to the cathode in spaced relation to opposite sides. In other cases as where very large installations for industrial use are involved, the cathode preferably is formed as a hollow prism and the high voltage generating device of whatever nature is incor' porated directly within the cathode, thus eliminating insulation problems attendant upon the use of an external high voltage generating source. One such arrangement is shown in FIG. 5 in which the cathode structure comprises spaced, opposed metal plates 50 comprising the broad faces of the cathode which are joined about their edges by relatively narrow metal side panels 52 to form a hollow rectangular prism. The cathode is supported on a flange 54 of an insulating tubular post 56 which extends part way up into the interior of the cathode through an opening 58 in the bottom edge panel of the cathode. An electrostatic generator is mounted within post 56. This may be of any type but the one illustrated schematically in the drawing employs a belt 62 passing around spaced pulleys 64 and driven by a motor 66. A charging screen 68 is positioned adjacent the belt surface at one end of its run, and is supplied with excitation voltage by a suitable transformer 70. A terminal collector screen 72 is positioned at the opposite end of the belt run to pick up the static charge developed on the belt and transfer it to the cathode surfaces. In the illustration in FIG. 5, the cathode surfaces are enclosed in a close fitting flexible plastic film or bag 74 which, as previously mentioned, can be temporarily inflated to effect dislodgement of the collected particles.
In order to augment the effectiveness of the light source used in producing the photoionization effect, the interior surfaces of the treatment chamber may be made reflective for the particular frequency of the lamp output.
The particular structural details depicted in the drawings are intended to be merely illustrative, as it will be apparent that many modifications and embodiments may be employed in achieving the basic concept of this invention.
What is claimed is:
1. Apparatus for the treatment of gases to remove both admixed gaseous and particulate contaminates therefrom, which comprises in combination,
an enclosed treatment chamber having inlet and outlet means for introduction of the gas to and discharge of it from said chamber, respectively;
insulated anode and cathode electrodes spaced apart in said chamber and disposes substantially centrally thereof in the path of fluid flow from said inlet to said outlet, and a source of high voltage unidirectional current connected between said anode and cathode, said cathode being of generally plate-like configuration with its opposite faces generally parallel to the path of gas flow from said inlet to said outlet, said anode being of open mesh configuration and spaced about said cathode, said high voltage source producing a potential gradient between said anode and cathode of from about lkv./cm. to 30 kv./cm., and means for variably selecting said potential gradient;
a light source disposed in said chamber to produce electromagnetic radiation in the area of the high voltage field between said electrodes, said light source comprising a plurality of elongated lighting tubes spaced along opposite side walls of said chamber and having an output in the spectral range of from 1,50OA. to 7,200A.;
said cathode further being a hollow prism having generally rectangular faces joined about their peripheries by an enclosing wall, a tubular insulator mounted in said chamber and having an external shoulder intermediate its length, said enclosing wall of said cathode having an opening to receive said insulator and said cathode being supported on said shoulder;
an electrostatic generator including a source of high voltage positioned within said chamber, an endless transmission belt, and pulleys positioned at either end of said tubular insulator supporting said belt for travel within said insulator, means driving one of said pulleys to advance said belt about said pulleys, a charging screen connected to said source of high voltage for transferring charges therefrom to the surface of said belt outside said cathode, and a terminal collector screen within said cathode for transferring charges from said belt to said cathode.
2. Apparatus as defined in claim 1, which further includes an open mesh conductive grid adjacent said anode in spaced insulated relation thereto and between it and said cathode.