CA1070250A - Method and apparatus for precipitating particles from a gaseous effluent - Google Patents
Method and apparatus for precipitating particles from a gaseous effluentInfo
- Publication number
- CA1070250A CA1070250A CA234,371A CA234371A CA1070250A CA 1070250 A CA1070250 A CA 1070250A CA 234371 A CA234371 A CA 234371A CA 1070250 A CA1070250 A CA 1070250A
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- particles
- medium
- region
- ions
- electrode
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B03—SEPARATION OF SOLID MATERIALS USING LIQUIDS OR USING PNEUMATIC TABLES OR JIGS; MAGNETIC OR ELECTROSTATIC SEPARATION OF SOLID MATERIALS FROM SOLID MATERIALS OR FLUIDS; SEPARATION BY HIGH-VOLTAGE ELECTRIC FIELDS
- B03C—MAGNETIC OR ELECTROSTATIC SEPARATION OF SOLID MATERIALS FROM SOLID MATERIALS OR FLUIDS; SEPARATION BY HIGH-VOLTAGE ELECTRIC FIELDS
- B03C3/00—Separating dispersed particles from gases or vapour, e.g. air, by electrostatic effect
- B03C3/34—Constructional details or accessories or operation thereof
- B03C3/38—Particle charging or ionising stations, e.g. using electric discharge, radioactive radiation or flames
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- Physical Or Chemical Processes And Apparatus (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
Apparatus and a method for electrically sweeping particles from a gaseous effluent are disclosed which are particularly efficient in removing small as well as large particles. A voltage is applied across two electrodes in such a way that a strong electric field can be generated between them. A source of ions is provided by bombardment of the effluent gas stream with electrons. A strong electric field established between the electrodes creates at least one region of ions having only one polarity and moves these ions towards the oppositely charged electrode. In the region having ions of one sign, these ions rapidly charge the particles, especially small sized particles because of the strong electric field. The charged particles are moved by the field and deposited on the oppositely charged collection electrode where they agglomerate in preparation for collection and disposal.
Apparatus and a method for electrically sweeping particles from a gaseous effluent are disclosed which are particularly efficient in removing small as well as large particles. A voltage is applied across two electrodes in such a way that a strong electric field can be generated between them. A source of ions is provided by bombardment of the effluent gas stream with electrons. A strong electric field established between the electrodes creates at least one region of ions having only one polarity and moves these ions towards the oppositely charged electrode. In the region having ions of one sign, these ions rapidly charge the particles, especially small sized particles because of the strong electric field. The charged particles are moved by the field and deposited on the oppositely charged collection electrode where they agglomerate in preparation for collection and disposal.
Description
` ` ~10'~()250 . ~ .
.
The present invention generally relates to electrostatic - -precipitators, and more specifically, to a method and apparatus for electrostatically precipitating particles of different sizes from a gaseous medi~, including those having a diameter less than 5 microns.
The increased emphasis being given to reducing levcls of air pollution has culmina~ed in a wealth of local, state, and fed~ral legislation setting rigorous standards ~or particle removal from industrial and other gaseous emissions. Since the problems of air 10 pollution directly affect a vast majority of the public, particularly in those urban areas where industry is concentrated, it is assumed that the standards may become even more rigorous in the future. While improvements continue in the design and effectiveness of particle removal apparatus, including electrostatic precipi*ators, the rigorous 15 standards that are now being adopted have shown that many present types of precipitators are relatively ineffective in removing very small particles. This is coupled with the recent realization that the greatest number of particles in industrial gaseous effluents are in the range of about 0.1 to 10 microns in diameter, and also 20 that the smallest particles remain suspended in the air for the longest time. ~oreover, the greatest health ha~ard is posed by particles in the range of about 0.1 to 5 microns in diameter, according to the National Bureau of Standards Technical News Bulletin, dated December 1972.
All electrostatic precipitators use two charging mechanisms to build up the charge on a dust particle. These two mechanisms are diffusion charging and field charging. In field charging, ions are accelerated by the electric field of the precipitator. These accelerated ions strike a dust particle and combine with it. As 30 the dust particle accumulates these charges, it takes on the same . - ' ~ - -107V2S~
charge as the ions. Whon the ~u~t particla ~ecome~ chargod and hss the s~me charge A~ tha ion, th~ lon and chargcd partlcle tend to repel each other, which makaq it more ~lfflcult ~or other ions to add add$tlonal ¢harg~ to the pnrtlcl~. For a glva~ ~loctric iol~
5 ~trength and a giYen ulze of du~t pnrticlo, th~re will b~ n lLmlt beyond which the ~u~t partlcl~ will no loncJer Acoept ndditlonal charga~
by ~ld ch~rglng. Fox ~mall partlol~ ln aonventlo~al preolpltators~
thi8 llmit 18 v~ry qulokly ~eaohad. Th~ other chargln~ machanlsm, dl~fu~ion chnrging, utllize3 thermally aa~lvated ion~ that pO~9~9S `
su~flclent energy to pan~tra~e the r~p~lling ~ield and add additional charges to the dust particle~ This aharg~ng mechanl~m will charge ~mall pArticl~s~ but ls ~ulte ~low compared to the mechanism o~ ~ield chargincy.
It is gen~rally Xnown that presently used indu3trial 2reoip~-lS tators are relati~aly inef~ectiv~ in removlng partlcles havin~ a 9ize rang~ of nbout 0,1 to 3 micron3. Conventional electro~tatlc precipl-tator~ fail to colleot thesa ~ine particle~ as rapldly as the larser particles because the dif~u~io~ mechani~m 1~ th~ mechani~m ~lat is us~d to depo~it el~ctff cal charge on the ~mall particle~ and it operatq~
too 910wly for such pastlcle~. Ion~ drift onto the partlcl~ by thermal mot~on ~o that n~ ~ particle begln~ to acquire a charqe, lt repels th~ ~lower ~ovlng lon~ whlch could brinq further charge~ to the ;
partlcle. Stated ~n other worda, larqe particl~s are predomlnately charqed by the char~inq mechani~m o~ field charging w~ich is ~ub~ect to a limlt based upon ~he electro~tatlc repulsion ~ the charged pastlcle aq~lnst furSh~r charges approaahlng lt. ~ho~e charges are typically dri~en by an electrio fiel~ applle~ by remote alectrodes. - -Thus~ in the prlor ~rt ~pparatus and 1~ the pre~ent iAveAtion tha bal~nce between ~hd drl~ng and repelllng orce~ aetermines the 30 m~x~mum oharqe whioh can be aaqulre~ N8- ;
N - S2 ~e ~D
+ 2 .
.
The present invention generally relates to electrostatic - -precipitators, and more specifically, to a method and apparatus for electrostatically precipitating particles of different sizes from a gaseous medi~, including those having a diameter less than 5 microns.
The increased emphasis being given to reducing levcls of air pollution has culmina~ed in a wealth of local, state, and fed~ral legislation setting rigorous standards ~or particle removal from industrial and other gaseous emissions. Since the problems of air 10 pollution directly affect a vast majority of the public, particularly in those urban areas where industry is concentrated, it is assumed that the standards may become even more rigorous in the future. While improvements continue in the design and effectiveness of particle removal apparatus, including electrostatic precipi*ators, the rigorous 15 standards that are now being adopted have shown that many present types of precipitators are relatively ineffective in removing very small particles. This is coupled with the recent realization that the greatest number of particles in industrial gaseous effluents are in the range of about 0.1 to 10 microns in diameter, and also 20 that the smallest particles remain suspended in the air for the longest time. ~oreover, the greatest health ha~ard is posed by particles in the range of about 0.1 to 5 microns in diameter, according to the National Bureau of Standards Technical News Bulletin, dated December 1972.
All electrostatic precipitators use two charging mechanisms to build up the charge on a dust particle. These two mechanisms are diffusion charging and field charging. In field charging, ions are accelerated by the electric field of the precipitator. These accelerated ions strike a dust particle and combine with it. As 30 the dust particle accumulates these charges, it takes on the same . - ' ~ - -107V2S~
charge as the ions. Whon the ~u~t particla ~ecome~ chargod and hss the s~me charge A~ tha ion, th~ lon and chargcd partlcle tend to repel each other, which makaq it more ~lfflcult ~or other ions to add add$tlonal ¢harg~ to the pnrtlcl~. For a glva~ ~loctric iol~
5 ~trength and a giYen ulze of du~t pnrticlo, th~re will b~ n lLmlt beyond which the ~u~t partlcl~ will no loncJer Acoept ndditlonal charga~
by ~ld ch~rglng. Fox ~mall partlol~ ln aonventlo~al preolpltators~
thi8 llmit 18 v~ry qulokly ~eaohad. Th~ other chargln~ machanlsm, dl~fu~ion chnrging, utllize3 thermally aa~lvated ion~ that pO~9~9S `
su~flclent energy to pan~tra~e the r~p~lling ~ield and add additional charges to the dust particle~ This aharg~ng mechanl~m will charge ~mall pArticl~s~ but ls ~ulte ~low compared to the mechanism o~ ~ield chargincy.
It is gen~rally Xnown that presently used indu3trial 2reoip~-lS tators are relati~aly inef~ectiv~ in removlng partlcles havin~ a 9ize rang~ of nbout 0,1 to 3 micron3. Conventional electro~tatlc precipl-tator~ fail to colleot thesa ~ine particle~ as rapldly as the larser particles because the dif~u~io~ mechani~m 1~ th~ mechani~m ~lat is us~d to depo~it el~ctff cal charge on the ~mall particle~ and it operatq~
too 910wly for such pastlcle~. Ion~ drift onto the partlcl~ by thermal mot~on ~o that n~ ~ particle begln~ to acquire a charqe, lt repels th~ ~lower ~ovlng lon~ whlch could brinq further charge~ to the ;
partlcle. Stated ~n other worda, larqe particl~s are predomlnately charqed by the char~inq mechani~m o~ field charging w~ich is ~ub~ect to a limlt based upon ~he electro~tatlc repulsion ~ the charged pastlcle aq~lnst furSh~r charges approaahlng lt. ~ho~e charges are typically dri~en by an electrio fiel~ applle~ by remote alectrodes. - -Thus~ in the prlor ~rt ~pparatus and 1~ the pre~ent iAveAtion tha bal~nce between ~hd drl~ng and repelllng orce~ aetermines the 30 m~x~mum oharqe whioh can be aaqulre~ N8- ;
N - S2 ~e ~D
+ 2 .
-2 .
.~ , . .
10'7t)ZS0 whero N~ 18 the ~Aturatlon number o~ el~c~ronlc magnltuae charg~, E lo the appl~ed elea~rlc fleld ln ~llovelts per centim~tor~ D 1B
tho partlcle dlameter ~n mlcrometur~ and ~ i~ t2~e partialo dielsctrlo constant. ~lowev~r~ in conventlonsl ~l~otro~tatic prcclpitato~s, the mean char~lng and collectlon fiold 1~ limited ~o ~bout ~ kV~cm becaus~ 1~ i8 lln~d to a hl~her fleld whlch ~upport~ ~ coron~
dls¢harge Ad~acent n smAll~ ld 0nh~ncln~ ol~c~rod~ an~ hlgher ~loldu tend ~o cau~ spArk broakdown in th~ gns. ~hun~ ~or A 0~3 mlcron dl~mo~or partiole~ the n~AxlmUm ~for l~r~o ~) s~ ratlon chArge produced by the elea~rla ~lald in nn ordlnary electro~t~tlc precipitator i~ about 20 olactron chargcs.
In conventional ~lectro~tatlc preclpltators, the only e~ectiv~ oharglng method $or charging small partlcle~ 1~ by dl~fu~lon ch~rging because o~ the low electrla fleld. ~he number of charges added ~ 8 given approxlmately by the ~ollowinq aquationt N - 0.03 T D ln (1 * 7.6 x 10 4 No D t/T1~2) whsro T is lon ~netlc temperature ln degreee Kelvin, No 18 the ambien~
concentration of ~on~m3 and t i3 the tlme ~n s~conds after the ~ield charglng has been completed. Since the charye attalned a~er a long tlme by dlf~uslon 18 proportlonal to D ln D~ lt will exceed the fleld produced charga for small particlas. In typical Cottrell pseclpltatore! for ex~mple~ ion den~l~ie~ ~re aeveral tlme~ 107~cm3.
At thls lon density, about 0~3 second is reqnired to deposlt 20 charge on a 0.3 ~lcron dlameter pasticle whlle 24 second~ would be require~ to doubla th~s charge ana the tran~it t1me of g~ through ~ypl¢al preclpltators i8 only ~bout 8 s~condsO
In other woxd~ con~ent~onal electrosta~lc precipitator~
oper~ta by prod~cing ions of both polariti~o in a GOrOna dl~ah~-ge pln~ma ne~r one 8~11 elQct~od~ aro-~nd whlch the electrlc ~leld concen-trat~. Tho ~tr~ngth of the ~i81~ i8 guit~ high noar the e~ectrode~n~ drop~ dramAtlcally away fro~ th0 elactrode ana thereby provlda~
n nonunlfon~ lA, Ions o~ ona pol~rity ¦usuallY negative) ~re 107V2S~
withdra~n ~rom thl~ roglon and ~ th~y drlft townr~ th~ other olectrods, th~y beco~ att~ched to the aoroDol p~xticlo~ in th~ e~luent~
~o produce th~ flel~ onhancement noc~aary for coron~ di~ch~xga at one elQctrode wlthout cau~ln~ olectrlcal bre~lcdown bo~woen ~he two ~lectroda~, conv~ntion~l preclpitntor~ o~ton ma~o uaa of co~xlal geQm~try wlth a amall dla~Q~ar wlre a~ tho c~nter elootrodo and a largo diamoter outer cylindar. Tho ~rl~t o~ ~ho ion~ i3 c~u~ by the ln~ractlon o~ th~ chnrge on tha ~on and the nonunl~onm, gonorally wonk aloctric ~ield. ~a ~h~ ion~ dxlXt, they charge th~ partl~le~
by ~ttnchlng to them, thexeby cAu~inq the parti~'le~ to be drlven by the olootrlo 101d toward nnd ~ttachod to tha collectln~ electxode.
The officl~ncy of all oleat~os~ntlc precipltators lncluding tho8a 0~ the prior art ~nd al~o of the pre~en~ ~nvention 1~ limlted by three ma~or factor~9 ~speclally ~or the a~roaol partlcles whlch are lo~ than flve mlc~ons in dlameter. The ~ir~t ur~ s~a bQcau~e the charglng rate of the anroaol par~lcl~ decrenses rapidly R~ the radius of the particl~s decrea~as~ Thus~ a9 the slze of the pas-tlclas decsea~es, the paxtlcle charge 1~ le~ ~nd the dri~t velocity, l.e., the component o~ tha ~vex~ge velocl~y e~ the par~lcle~ direc~ed toward the ~lectrod~s, decsea~e~, The second actor i~ tbat or a given charge the drift velocity decresses a~ the electric leld stxength decrease~ Thus~ thQ dr~ft velocity of a glven 8ize particle decr~aaes as it move~ in the direction toward th~ collecting electrode bacause of the decre~slng ~eld in tha coaxial electrode co~flguratlon.
ThQ third ~actor 1~ the att~chment e~iclency of the collecto~ elec~
txode~ i.e., the partlcles which are ~rl~ted to ~ho collec~or electrGde may rebound o~ be dislodged by the lmpaat o~ o~her paxtlclcB
o~ b~ ~wcpt away by ~he tuxbulent flo~ o th~ gaxeou~ e~ luent n~t~r they have been ~nltlally collec~a upon lt becau~ the charge on the part~cl~0 and the electrlc fteld they expe~ienc~ are not 8uf~
o~ently l~rge.
It h~3 genarnlly been recognlæ~d that lmpro~d opera~ion o~ an electro~tatic p~clpltator rQsult~ ~rom increa~lng the electrlc - field str~n~th provld~d, however, that el~ctrlcsl break~own or arcing do~ not renult fro~ the hi~her eloctria ~l~ld ~tr~ngth~
The prior art al~o disclos~s pr~clpi~atin~ ~ppnr~tu~ ~1hlch ind~p~ndontly produ~o tho ~onu and the electric fiald xath~r tbnn a configuration that u~e~ ~ ~mall wiro cantral nloatrode ~n~ outor cylindrical electrodo to ~imultanoou~ly croa~o the ion~ ~nd ~he ~l~ct~la ~ield. Whllo rndloactlvo ~Atorial~ and photoionlzation ~ource~, o.~. light tubes such a~ ultraviolet l~mps, have been di3clo~ed to provide a sourae o~ lon~ lndependently of the produc~lon o~ th~ electrlc f~eld, these ion source~ hav~ practical operational and other disa~vantage~ and it i~ not believed that any commerclal apparatus have been developed.
~ disadvantage of radloactive ~ources ~8 the difficulty in varying thP energy and quantity of par~icle~ e~ltted by such nources~ Yurther, the psychologlcal i~pact of u~ing a radioactlve source o~ lon3 in a precipitating apparatuq, particularly in an urban area, would b~ ~
quite negat~ve. .~oreo~er, there could be a signifi~ant problem of ~ -radioactiv~ contamlnatlon of the atmo~phere if a rupture or br~akdown of 60me portion of the apparatus occurred. Preclpi~a~ors that U8 ul raviolet or other lamps to pro~ide pho~ons ~or creatlng *ha neceY-sary ionlzatlon within the prec~pltator ar2 also ~ub~ect to many practlcal oparational d~sadvantage~. The lamps are sub~ect to duætlng o~ uding over by the part~cle~ in th~ gaseou3 medium or effluent 25 ~nd will becoma d~rty qulte r~pidly. Thi~ dustlng o~er ~ay easily ~ ~
-occur ln only a few seconds and greatly decreases th~ ef~clen~y a~
their operatlon. Moreov~r, the photon energy created by ~uch la~p~
cannot be continuously and convenlently controlled~
~owaver, the pre~ent inventlon does not ~u~f~r ~r~m ~ha -~
disndvantages of these rcadioactiYe a~d p~otoionlzation source~ and, ln fact, exhi~ito many de~lr~ble at~ributes th~t e~ables lt to ~chieve th~ result~ sough~ by the abo~o ~ource~ ln additlon to other ~lgnllcant , 1~70%5~
advantages.
More particularly, the present invention utilizes an electron generating source (often also referred to as an elec-tron beam generator, E-beam generator or the like) to bombard the gaseous medium within the pxecipitator with high energy electrons and produce a plasma region therein. The electron generating source has the advantages of being able to accurakely control the penetration and density of the electrons that are injected into the gaseous medium and th~reby control the extent of the plasma region. Further, the "window" or surface through which the electrons are injected into tbe medium, i.e. the surface through which the electrons pass which is in contact with the gaseous medium, is self cleaning and will not dust up or become dirty from the particl~s within the gaseous medium or effluent. These and other advantages will be described in detail hereinafter.
Accordingly, it is an object of the present invention to provide an improved method and apparatus for precipitating particles from a gaseous medium such as a gaseous effluent, which method and apparatus are effective to remove extremely small particles, i.e., those particles between about 0.1 and 5 microns in diameter and particularly those less than 1 micron in diameter. -Yet another object of the present invention is to provide an improved method and apparatus for removing particles from gaseous effluents with high volume throughput, high efficiency, and only moderate power re~uirements.
The above objects are met by the present invention which provides a method of electrostatically precipitating particles from a gaseous medium carrying the same, comprising:
passing the medium through a channel in a precipitating station wherein the particles are brought into a first region containing Y( 25(~
ions of only one sign; s~ecting the medium to a supply of electrons from an electron beam generator to generate a supply of ions of both signs in a second region, the ions of one sign in the first region being supplied from the second region; sub-jecting the medium to a generally uniform, strong electric fieldto drive the ions of one sign onto the particles, the average field strength of the electric ield approachiny ~he maximum field strength therein; the electric field causing attraction of the charged particles to one or more electrodes hav:Lng a charge of opposite polarity relative to the charged particles to there-by precipitate the particles out of the medium.
The above objects are also met by the present inven-tion which provides apparatus for removing particles from a gaseous medium passing therethrough comprising: an inlet for receiving and an outlet for expelling the medium; a central portion located between and being in communication with the inlet and outlet, the central portion guiding the medium through the apparatus; one or more positively charged electrodes being located in the central portion; one or more negatively charged electrodes located in the central portion for attracting par-ticles having a net positive charge from ~he medium; means for charging the electrodes in the central portion to provide a uniform, high electric field in the central portion of the apparatus; an electron beam energy source means for injecting high energy electrons into the central portion for producing a supply of positive ions which bombard particles and cause them to be attracted to the negatively charged electrode.
The inventive aspects of the invention wi11 become ap-parent from the following description and accompanying drawing, in which:
FIGURE 1 is a diagrammatic representation of precipi-tating apparatus embodying the present invention and which is - ~ -6a-1~)';'0250 .
useful for practicing tha method of the present invention;
FIGURE 2 is a perspective view of one :Eorm of the apparatus that may be used to practice the method of the present invention; and, -6b-~0~ 50 FIGUnE 3 1B a ~Chemat10 illu~tratlon o~ anoth~r embodl~.en~ o~
the present inventlon.
~ roadly sta~ed, tho pre~nt inventlon iB dlrectod to Appar~tus ~ well A9 a method ~or preclplt~tlng or romovlng particla~ ~rom n ~tre~m of gA~eOu~ ~Pluent whtch prof~rably u~ n gencrally wlL~orm, ~tron~ electric fl~ld for chargln~ the p~rticle~ wlth lonu, with the ion~ belng supplled lndepend~ntly o~ the uourc~ of tho olQatrla ~leld ~rom a plasma ~hat i~ ~oxmod by hlyh enorl~y alectron~. ~ pre~ipitatl~g ~tation includeu at l~A~t one po~itlvoly nnd one negatively char~ed el~atrode for s0tting up ths elaatrla flold, and a source o~ ion~
~hich charge tbe pRrticlo~. ~he particles charged ln the pro~en~e o~
th~ el~ctrla field are therbby precipih~0d or rem~Y~d fro~ the g~seou~
o~fluent and collected at ons o~ ~be electrode~. ~lgh-energy electron~
nro directed ~o a8 to produce a plas~a in thc g~eeou~ medlum or effluent 15 near one o~ the electrode~ and the partlcle~ have no net positlve or negatiue charge withln tb~ neutral region of pl~sma. ~owever, ~he charqed el0ctrode~ and pla~ma produce a charged rsglon between the plasma and the collectlng electrode, 80 that once thc partlcle3 ara withln tbe chnrged electrlcal region, they will acqulre a net chsrge, und be AttrA~-ted to ~he opposi~ely charged collectlon electrodeO
Refarring to the dr4wings and partlcularly F~GUR~ 1, an ideal~zed s~hematlc cro~s-~ectional ~i~gram o~ apparatu~ which ~ay be used to casry out the method o~ the present inventlon 1~ ~hown. The appar~tu~, lndicated generally at 10, communlcatea ~ ga~eou~ madlum or ef~luent from ~he lower lnlet 12 throuqh tD the outlet 14 ln an upward dlrectlon a~ ~hown. Slde wall~ 16 ~nd 18 dlr~ct the ~low ~hrough the apparatu~. An eleotron gen~r~tlng ~ourc~ 20 1~ po31tioned wlt~ln an openlng ln ~he side wall 18 ~nd produces high enargy elactron0 indl-cated by the arso~s 22 ~hlch penetrat~ ~ ~hln tr~nsml~slon window 24 ~s well a~ a positlvely oharged elect~ode or ano~3 26 lnto th8 ga8~eUO
~e~lum. A neqatlvely ~harged electro~e 28 1~ po~tloned ad~acent ~e slde w~ll 16 ~o tha an alectrlc ~leld ~9 s~t iup b~waen th~ anod~
~70Z~;iO
and the cathode aeroa~ ~ubstRntlally the entire chann~l wlath a~ ~ho~n.
~he anode 26 and cathode 28 are charg~d by a direct current ~ource 30 havlnq it~ poBit~ ve termlnal connected to the anodo 26 through llne 32 and lts negatlva termlnal connect~d to the catho~e 2B thxough llne ~a. ~B i~ deplated by the curv~d ~rrow~ withl~ the oha~nel os nrea ln~ld~ betw~en the lnlet and outlet o~ tho ~pparAtu~ the a~fluent pr~ferably has 80mo turbulence ~o thAt lnrg~cal~ mlxin~ o~ tho particle~ occurs a~ it p~3~e8 ~hrough ~ha app~ratu~. Becau~e a~
the mixing actlon, virtually n~ particle~ will remain for any length o~ tima ln the re~ion contalnin~ lons of both ~igna clo~c to the po~ltively aharged electrode 260 The partlale~ wlll b~ s~ept into the r~gion batwaen alectrode~ 26 and 2B dusing thls pas~age. The electrodes 26 and 28 are preerably generally fla~, planar membess havlng arcuate edges ~hat are charged by the external ~ource 30 to positlve and negatl~e potential~, se~pQctively. ~ha ~nslde surface of electsode 26 1~ sho~n to ba generally coplanar with ~ide wall 18 slnce ~he flat electrode fits an opening in the right slde wall.
~he generally flat configuration3 and curved edges of ths cath~de ~nd anode are preferred to minimize electx~c ~leld ma~ima, i.e. lt 18 ds~irable that the average field strength approach the maxlmum field strangth withln the apparatu~. 5tated ~n othcr word~, it 1~ -deslrabl~ tha~ the electric fleld be uniform 80 that i~ oan be ma~imlzed ~ithout experienclng electrlcal braakdown or arcing. The electsode 26 separates tho stre~m o~ gaseou~ medium on lts left ~ide a~ ~hown in thu drawing ~rom a quiesce~t g~seous medium on it~ rlght ~lde that is prefer~bly sealed from the left ~ide to prevent du~t to accumulate - -between the ~lectrode 26 and the w~ndow 24. ~he thin wall or window 24 separata~ the qulascent meaium from a region o~ vesy lo~ pre~ur~
l.e, as ~uch as 3 x 10 4 tor. The wlndow 24 be fabrlcated from ~ny material that will tran3~1t electron~ ~herethrough that l~ o capablo o~ ~eparating the lo~ pre~sure ~ in the electron generator 20 ~ro~
tha ext~rlor pre~ure. Thus, the ~indow 2~ ~an ~e mado ~om tit~nlum, - . . . . ~ . -~07(~250 alhmlnum, ~talnle~ st~el, nylon ~nd th~ llke. ~he anode plnt~ 26 may be mad~ o~ ~ tbln she~t o~ conduotiv~ mAt~lnl such ~ t~nl~, ~luminum, or ~tainla~s sta~l wlth th~ combln~d ~hickne~ o~ tho nnodo and window bolng pre~ara~ly le~ than nhou~ two mllo.~,002 lnoh~o) to perml~ p~natration o~ the olectronu throu~h th~m. When th~ wlndow 24 L8 madn of conductiv~ mnterl~l, it ~nn b~ dn~igned to nl~o ~erv~ A~
the anode 26. Ilowov~r, 1~ gonexnl, it m~y ba pxofoxred to we A ~parate Anod~ 26 to ~lmpli~y tho ~ervlclng of tha el~ctron boam ~onerator.
Further, ln c~rtAin embodlment~ it may be pre~esred to orm the electrod~
26 ~rom A ~creen ma~erial o~ ~rom rod~. Means for constrlc~lng or expandlng the gaa ~low to ad~ust lto velocity and to control the mlxing action or to control turbulence of the flow are not shown. ~he an~de 26 L~ preforably chargod to produce a8 hl~h a field strength a~ posslble, generally of about 12 to 18 kLlovolts per contimeter in the gaseous medlum. i~owaver, any potentlal up to the breakdown po~ential of the gaseous medium may be us~d. The elactron beam ~enerator 20 i~ posi~loned between the inlet 12 and outle~ 14 ~o as to irradlate tha gaseou~
medlum or effluent wlth elec~ron~ paseing through window 24 and anode 26 and tho generator prefera~ly ha~ pow~r to provide a~ eleatron bea~
havlng an enesgy den~ity ~u~ficlent to gener~te enough lons to charge all the particle~ ~n the ga~eous medium to nearly saturat~on.
electron gen~rator i8 pre~erably po~itioned so tha~ it lrradi~te~ only th~ volum0 immediately ad~acent the anode ~urfaca. This ~ achi~ved by using electron~ that can only penetrate a ~hort di~tanc~
into the ga~eou~ medium. Tha el~ctron generator 20 pr~ferably operate~
at ~ufficient vDlt~ge to produce lon~zatlon and ~u~ficlen~ ourrent to generate tbR quant~ty o2 lon~ that are ¢apabie o~ c~arging the partlcle~
in ~he gassous streRm. In thi~ regard, the el~c~ron g~nsrator prefer-- ably operate3 to provide eleotron~ Rntesing th~ gs~eou~ medium wlth an energy o~ betwaen about 1 ~8V and about 12 KeV p~r contimeter of plato ~ep~r~tion ana at a current lqval~ o~ ~bDut ono mlcroamper~ per ~et~r o2 eluctrod~ width perpen~$culnr to the ga~ ~lo~. For a con~lg~r~tlon ~070ZS~) havlng n .5 mil thlck t1tanlum wlndow whero the wl~d~w al~o ~unctlons a9 the anod~ Rnd wlth a 10 cm ~p~clng between the ano~e an~ ca~hode, the ~ppar~tu~ per~Qrmed ~atlsfActorlly wlth alectron~ o~ betweon 100 XeV ~nd 115 XeV ~nt~r1n~ the wlnflow.
In Accor~nnc~ with nn lmpoxt~nt ~ap~c~ ~ thn pres~nt lnv~n-tion, ~ the window ~ of tho electron boam qenor~tor nl30 ~at~ a~ the olectrode 26, th~ wlndow 24 oxpo~d t~ the partiale lnd~n qaBeOUB modlum 1B ~al~ ¢lQaning. ~ n sepnrAto ~leakrode 26 i~ usud, the ele~tron b~am from tho qenerAtor ~dt~ to prevent particle bulldup on the anod~ 26.
While there may be some partloles on the expo~ed ~ur~ace o~ elther confl~urnt~on at nny one tlme, thera 1~ no bulldup o~ particles on it dua to lt~ s~l~ cleanlng operatlon. ~he expo~ed surfAce doe~ not ex-perlance any accumulation of small partlcles beQau~e they axe repelled be~ora they can reach the sur~ac~. A$ ~he qmall partlcles ~re bombards~
-by the electron~ produced by the electron generator, the ~lectrona go completely through them causing ~econdary emls~ion and tha smalI part~-cl~ becGmes posltlvely charged and ~ rep~lled by the po~itively charged Qurface. ~hu~, ~mall particle~ never reach the ~urf~se and cannot accumulate on ~t.
Wlth reApQct ~o larger partlcle~, however, the electro~
bomhardlng ~he particle wlll not travel through the partlcle and s0cond-Rry .em~s~ion affect~ wlll not be ~igniflcant compared to the piling up of electron~ withln the part~cle. Thu~, voltage on the lnslde d the part~cle build~ up w~thln the partlcle and lt becomes qulte 25 negat~Ye. If the partlcle 1~ in contac~ with the ~rf.ac~ lt wlll ~i~charge to th~ polnt o~ con~aat b~tween the partlcle and ~ho surfac~. -Thia dl6charge pro~uco~ ~ dlscharg~ path that c~n ~Q analOgl%0d to the ~h~pe o~ a tree, 1~ h~ d~charge path goes from the branche~
~nd comblnes ~n a large~ trunk portlon wher~ lt contacta ~he ~urface.
~he paths ar~ hol~s ln the part~ala cau~ed b~ vaporl~ing th~ solld 9 the p~rtlcle to ~ g~s, ~h~ ~aporl~atlon produce~ a ~housa~d fold Yolume lncreaae whlch e~c~pes through ~he dl~charg~ pAth8~ ~h~
:
~ 0702~0 vaporlzatlon procQ~a producas a grP~t f~rc~ that blows th~ particle from the surfacQ or d~stroy~ th~ pnrtiqle lt~l4, ~lther re~ult being e~eative to rld the ~urface of the partlale. ~or~ov~r, the ~orco o~ ona partlcl~ be~ng removed will e~ctlv~ly xemove ~ovoral othera as well, Thi~ cl~aniny not~on can b~ inarea3ad b~ incr~Aslng th~
op~rating volt~g~ o~ the ol~ctron ~on~ra~or. It ~hould therePor~
be understood that tha operating voltage c~n ba varlad, perhap~ p~rlod-1CA11Y~ tO control the cleanin~ ~ctlon. ~n op~lmum duty cycl~ can 10 be eotablished tl~at woul~ eff~at ad~quate cloaning and minlm~ze tha . .
povQr r~ulrements for th~ ~va~all opera~ion o~ the apparatus.
The upper ~lQctria field ~tr~ngth limit ls deter~ined by the dlel~ctric strength of ~ha ga~ou~ ~edium at operating pre~ure.
~or a ten centlmetsr separation di~tanco b~tween cathode and anode, a separation distance used ~n one ambodlment of the apparatu~, tha unlfonm field breakdown ~rength of alr at normal dan~lty i~ about 25 kV/cm. Since the ab301ute tcmperaturc il~ a typical gaseous e~fluent ~ -will be ln the rangc of about 400K ~o 500~K, the gas density vill . ;.
bs about a factor o~ two lower than no~mal atmospherlc den~ty, and the llmltlng field ~tr~ngth would be about 13 kV/cm. ~owever, electron-~ttaching ga~e~ ~uch a~ sulfur dioxide for example~ will o~ten be pre~ent in a gaseou~ ef~lueDt, and the pxes~nce of Shese ga~e~
may~enable operatlon at a higher electrlc ~eld value ~han the descri~ed 13 kV/c~.
It ~hould al80 be under~tood that the electron genera~or ~ay generatu ~ singla b~oad steady be~m or one ox morQ narrow beam~
~nd may al~o be adapted to ~can the area withln the apparatu~ in a pradeterm~ned p~ttern. For examplo, the pattern may have the beam gollow a moving ga~oou~ medium through a volume for an average dwell time for partlcle~ w~thin ~hat Yolume, then ts~at other volume8s~cco~siveIy in llke manner an~ then nf~er an average dl~fu~lo~ tlmc require~ to repopulate the ~irst xffglon wlth partlelen~ seturn to --11~
~070250 thAt fir~t volume.
Tha rQsldu~l, ambi~nt mlxing nctlon or turbulence o~ the flow of tho gns~ou~ m~d~um through tho nppnr~tus carrie~ ~lo p~rtlclu-l~don gaseous modlum to wlthln n dlatanoa d~lnlng the lmnln~r ~low S bvundary ~ubl~yor of th~ c~rgod eloutrodou. ~llthln n r~tglol- o~
thlcknuo~ compnr~blu wlttl tho rnngo Or tho ol~otronrl ln th~ modlum, ~hn cllnrgo oll du~t p~tlcla(l la nenxly noutrnll-o~ ~onAu~o o~ ~.h~
pro3onad o~ lon~ o~ bot~ nlgn~, In ~ha r~t ~f thu volum~ however, tholr ch~rqng r~t~ nre no lonqcr nautrAll~ed nnd bui~d xapidly 0 80 that by the t~me the oddy ~otlon carries the ga~eous ~low to and thsn nw~y from tbe cnthode 28, dust particles which have positlve cb~xge rsmain becau~e o~ the electrical force that is exerted upon the charges. ~he partlcle~ may acquire additional charges by i~pinge~ent of qaaeous lons wh~le tbsy are attached to the csthode. Thii would l~ lncrea3e the holding force 90 that they uould not be inclined to be diYlodged. ~o~ever, lf the du~t is of vary high sesistiv~ty, excesslve local field strength can result from thiY chsrge buildup ~nd cause harmful local breakdown. Thl3 local breakdown can be prevented by keeplng the ion denRlty in the qaseous ~edium low except in the rogion o~ lnltla~ partlcle chasging. Partlcles of all slzes rapldly -collect on ths cathode 28 because the electric ~leld, no longer limited to about 4 kV~c~ in the bulk of the qas by th~ requirement of corona generation at one edge, ~an be rai~ed to between 13 a~d about 18 kV/cm. In thi-~ preferred form of operation, the hign fleld covers 25 virtually all the dlatance betveen the electrodes. It ~hould be ~nder- -stood that while the above de~crlptlon deals wlth p~lllng po~ltive ions fro~ th8 reglon of neutrol plasma, the present lnvention ls aPP1$Cab1e tO 10D~ of negative polarlty. ~o~ever, ~he use of posltive ion~ ha~ the advantage ~n that elsctrons and negative lons are pulled bac~ toward the anode ~nd the thlckneaa o~ the reglon of neutral pla~sa 1~ ~inlmlzed, a~ lo de~lred. As prevlously mentioDed~ the satu~atlon ch~rge by the u~ual mech~nl~m of ~leld cb~rglng 1~ ~ub~ect to a limlt cnu~d by the el~ctro~tatlc repul~ion b~twsen the pnrtlcles that have ~cquir~d ~ ch~rge ~nd additlonal chorge~ which ~pproAch ~t~
~n nccord~nco wLth tho pros~nt lnvontlon, howovor, tho Aatur~tlon ohnrge on ~11 p~rtlol~n 1~ muoh ~treAtor b~aau~o ~u ~n oluctrlo ~iold ~tr~n~ oAn bo rnlood by nbout n ~nctor o~ botw~on ~bout 3 nnd 5. ~hu~ a mnxlm~ o~ botwoon nbout G0 ~nd ~0 ahA~o~
uould bo dnl)oD~to~l ~n ~ 0,3 mloron pn~tlcl0 ln nn 18 kV/~m ~lol~, ~)~llo only nbout 20 eo 30 ch~r~ Ar~ ~ypi~lly depo~it~d durlng tho tr~n~lt o~ such ~ pArtlolo throu~h ~n ordinary eleatro~tatio preoipi~ator. Wlth re~poct to Pleld oharglng, the ini~ial charglng rnte 18 glven by !dt 4~7 x 10 5 ~ No~ D2 E ~ 2 wherfl D i~ the particle diameter ln mlorons, ~ is the electric field strength in kllovolts per centimeter, F LS the dlelectric constant of the partlole, and No i3 the amblent ion concentratlon in nu~ber per cublc centimeter. Values ~or ND- are nbout 3 x 107 per cublc cent~meter in conventional preclpitators. In the present lnvention, No iY controlled lndep~ndently o~ tho field stsength E, wherca3 thsse two values are interllnked ln conventlonal pr~clpitator~. The field ~trength can be controlled independently of ~O to achieve particular advantages, ~.e., the ~leld strength can be reduced to mLnimize power consu~ptlon or ~ncreased to maximize the charging rate. ~or example, ln an 18 ~v~cm fleld, wlth an No o~ 3 x 107/cm3, dN/dt equal~ between abo~t B00 and 2200 per second for ~ 0.3 mlcron partlcle BO that the pnrtLcle very rapidly approache~ its saturation chnrgs of about 60 to 80. I~ for other reasons, lt 18 necessary to reduco the field, the chsrglng rat~ c~n be ~alntalned by increaslng No~
From the aboYe, it ~hould be under~tood that a l~rge decre~e ~n charqing tlme ~8 well ns a lnrge lncre~se ln totnl charge occurs for 0-3 ~lcron dL~m ~ter pnrtlcle belng-charged in the lnrge electrla flelds thnt c~n exlst in ~ ch~nnel where lon~ are nuppli~d by the agency -~3-1~70~S0 of hlgh-energy electron3 ~rom an ~l~ctron generator, for ex~mple, rnther than ln ~he small~r overall ~lelds typicnl of Q convQntlon~l preclpl-t~tor, Thus, electron beam supportod charglng may exceed charging cur-r~ntly u~Qd ln prlor nrt preclpltAtor~ ln both tho rnto an~ mnxlmum charge attnlnnblo ln roA~onAblo dwoll tlm~l o~ pnrtl~lo~ ln tho I)ro~lp-lt~tox, ~nd ~l~o mny roqulro loo~ pow~r ~urln~ oporAtlon~ Purthen~ro, tho olectrlc ~lold nctln~ on onch o~ t~Q~o ch~r~o~ lo l~r~or by n fac~or o~ nbout 4 nnd wlll provido nn nverAqo preal~it~lon vqloalty that will bo abo~t 12 ~ln~e~ l~rcJer thnn th~t whlch would~e~perlenced by 0.3 mlcron par~lol~ ln pre~ent conventlon~l pxcclpltator~. ~lowev~r, ~lnce only ono o~ the two aur~Aces colleat3 thefle partlcle~, the effectlve oollection rate per unlt AreA wll~ only be lncrea3ffd by a ~actor of about 81x. An alternatlve deslgn uslng two electron guns on opposlte side3 of a central colleotlng cathode would lncrease the collection rate by a,factor o~ about twelve. Alternatively, an electron beam may be pro~ected down the center of the preclpltator to produce a plasma. ~ach electrode would then attract it3 respective opposltely charged partlcles and would be preclpitated out o~ the gaseou~ medium.
- ~eferring to FIGURE 1, it should be understood that the thln curved electron beam wlndow 24 is preferably covered wth the thin metal anode 26 to protect the ~tressed window 24 from corroslve gases and largu particles in the gaseous medium or effluent. The thin flat protectlve cover anode 26 also produces a smoother electrlc field distribution and thereby allows a higher average fleld strength.
Turning now to FIGURE 2 which illustrates one form of app~-atus th~t i~ useful in practicing the method of the prQsent ~nvention, the apparatus 40 ha~ ~n inlet 42 at lts lower end and ~n outlet 44 at its upper end, with gaseous medium or effluent flowlng Yert~cally upwardly a~ shown by the arrows. The dust laden gaseou~
effluent preferably flows in the prealpitation channel at 5 to 10 meters per ~econd, An electron generator 4~ 19 posltloncd to lrradlate the effluent whlle ~t i8 wlthin the channel 46. ~ cathode 18 provldod 1070~2S0 an~ mny be ln th~ for~ o~ a ~lexlble ~t~lnlo~a ~teel b~lt 50 ~8 ~ho~n which travel~ around upper and lowur roller~ 52 and 54, raspectl~ly, ~lth on~ o~ tha lowor rollers belng drlven by a motor 56. ~he belt ., ha~ n frone ~ld~ axposed to the qa~eous medlum or e~luent cont~nlng S hlqh ros13tlvlty duu~ pas~1ng through tho ahannol ~nd n hnok Hldo eh~t 1~ out~1do o~ the ohnnnol~ on~blln~ th~ pnrt1closl to ~n roll~ov~
from eho bolt bofor~ tha bolt roontoru tho oh~nnol an~l n~Jaln b~com~n oxpoood to tno ~luont. Ono ~vnn~ngo o~ tho ~pp~rntun nhown ln F~CUR~ 2 ln ~hn~ lt l~ o~ ~ rolntlvaly llm~ll hel~ht aompAred wltll 10 laas ~f~a~lv~ prlhr n~t pr~lpi~or~ ~or A ~lven througl~put rate.
~ n aooord~nco with Another A0peCt o~ the presant lnventlon and referrlng to the cro~s-~ectlon~l vlew ~hown ln ~IGUI~ 3, appnratus, lndlcAted generally At 6~, nnd al~o embodying ~ho present lnvention, communlcntes a qasaoUs me~ium or effluent ln A dlrection tow~rd the re~der. A~ l~ deplcted by tho curve~ arrows with~n th~ apparatus, th~ effluent l~ preferably glven Bome turbulence 80 that large scale ~lx1ng of the partlcle~ occurs as it passes through the apparatus.
Because o~ the mlxlng act~on, the particles wlll be swept around ~nd brought ln close proxlmlty to negatlvely charged cathode~ 62 as well a8 the posltlvely charged anode 64 durlng thls pa~age. The turbulent actlon se~oves p~rtlcles from the reglon o~ neutral charge den~ity noar tha electron ~ea~ wlndow, brlnglng them through the reglon of posltlve charge d~nslty to ulthln close range o~ the aathodes. This ~110~B all partlcles to be attracted to tha cathode~ ~o that they may bo sub~ect to prwlpitation out o~ the gaseous medlum before lt 1Q dis-charged at ths outlet. It ~hou~a be understood that while the dlagrHm-mat~c representation sh~n ln FIGURE 3 does not lllustrate elther the ~de or end exterlor ~nllJ o~ the Appnratus, the eleotrode~ 62 and 64 - wlll be posltloned wlthln tha outer slda wall~ whlch guide the ~low 33 o~ a~luant through tha apparntu~. The eloctron generatorn pre erably comprlsQ a number o~ thln wlres or roughened rod~ 66 enclo~d wlthin ovacuated tub-~ 68 ln the ~node our~Ace 64. ~hese ~ro3 nra small and ~Q70250 ~harged to a su~lclently lArg~ n~gntlv~ potentl~l that they emlt electrons by fleld emi~lon, Altornatlvoly, the wlros 66 mAy bo he~te~
and ~mlt electxons thermionlcally. ~hese electron~ are attr~cted to the thin u~ode wall tubes 68 nnd, becau~e of the hlgh voltage dl~ference, S hn~o sufrlcient enor~y to penetrate the thln Metal ~node 64. ~node supports (not ~hown~ con~l~t Or atructurnl ~oln~orclng loop~ o~ motal t~nt nro ~pnco~ porlo~onlly ~ltllln tho tub~n G3. Tl~o oporntlon lo ~ub8t~nti~11y slmllnr to thnt doocxlbo~ wlth roapoot to tho ~ppnrntuo ot FlGURE 1. ~ ~v~ntn~ o~ thu con~l~urntlon Or FS~Un~ 3 ln tl~At l~
th~ ~aouum soal no~ ono o~ tho wl~ 66 l~ bxoXon~ v~lta~e c~n b~
romoved ~rom the brokon wlro 66 wlthou~ Aub~t~ntiAlly ndver~ely arfqctlng the op~ration o~ the npparatu~.
From the to~egolng detAlled d~crlptlon, lt ~hould be under-~tood that ~ method ~nd flpparatus for electro3tatically preoipltating partlcles from ~ partlcle carrylng gaseous medlum has been illustrated and described whlch l~ more efflclent than conventlonal designs and 18 e~ectlve ln removlng extremely ~mall partlcles, even to such ~mall slzes as 0.1 micron in dlameter. In addition to effectlvely preclpl-tating ~uch small particle~, the pr~sent lnvention provides rapld charqlng and rapid preclpl~atlon of ~uch small aR well as larger par-tlole~, and enables fa~t throughput of the gaseou~ medlum or ef$1uent.
Although particular embodlment~ of the present lnventlon have been lllustrated and de~crlbed, var~ou~ modiflcation.s, ~ubstltutlona ~nd Alternativ~s wlll be apparent to tho~e ~klllcd ln the art, and 25 accordlngly, the scope o~ the inventlon ~hould be only de~insd by the ~ ~
nppended claims and equ~valents th~reof. - -Various ~eatures of the inventlon are ~et forth in thQ
follo~ing clAims.
.~ , . .
10'7t)ZS0 whero N~ 18 the ~Aturatlon number o~ el~c~ronlc magnltuae charg~, E lo the appl~ed elea~rlc fleld ln ~llovelts per centim~tor~ D 1B
tho partlcle dlameter ~n mlcrometur~ and ~ i~ t2~e partialo dielsctrlo constant. ~lowev~r~ in conventlonsl ~l~otro~tatic prcclpitato~s, the mean char~lng and collectlon fiold 1~ limited ~o ~bout ~ kV~cm becaus~ 1~ i8 lln~d to a hl~her fleld whlch ~upport~ ~ coron~
dls¢harge Ad~acent n smAll~ ld 0nh~ncln~ ol~c~rod~ an~ hlgher ~loldu tend ~o cau~ spArk broakdown in th~ gns. ~hun~ ~or A 0~3 mlcron dl~mo~or partiole~ the n~AxlmUm ~for l~r~o ~) s~ ratlon chArge produced by the elea~rla ~lald in nn ordlnary electro~t~tlc precipitator i~ about 20 olactron chargcs.
In conventional ~lectro~tatlc preclpltators, the only e~ectiv~ oharglng method $or charging small partlcle~ 1~ by dl~fu~lon ch~rging because o~ the low electrla fleld. ~he number of charges added ~ 8 given approxlmately by the ~ollowinq aquationt N - 0.03 T D ln (1 * 7.6 x 10 4 No D t/T1~2) whsro T is lon ~netlc temperature ln degreee Kelvin, No 18 the ambien~
concentration of ~on~m3 and t i3 the tlme ~n s~conds after the ~ield charglng has been completed. Since the charye attalned a~er a long tlme by dlf~uslon 18 proportlonal to D ln D~ lt will exceed the fleld produced charga for small particlas. In typical Cottrell pseclpltatore! for ex~mple~ ion den~l~ie~ ~re aeveral tlme~ 107~cm3.
At thls lon density, about 0~3 second is reqnired to deposlt 20 charge on a 0.3 ~lcron dlameter pasticle whlle 24 second~ would be require~ to doubla th~s charge ana the tran~it t1me of g~ through ~ypl¢al preclpltators i8 only ~bout 8 s~condsO
In other woxd~ con~ent~onal electrosta~lc precipitator~
oper~ta by prod~cing ions of both polariti~o in a GOrOna dl~ah~-ge pln~ma ne~r one 8~11 elQct~od~ aro-~nd whlch the electrlc ~leld concen-trat~. Tho ~tr~ngth of the ~i81~ i8 guit~ high noar the e~ectrode~n~ drop~ dramAtlcally away fro~ th0 elactrode ana thereby provlda~
n nonunlfon~ lA, Ions o~ ona pol~rity ¦usuallY negative) ~re 107V2S~
withdra~n ~rom thl~ roglon and ~ th~y drlft townr~ th~ other olectrods, th~y beco~ att~ched to the aoroDol p~xticlo~ in th~ e~luent~
~o produce th~ flel~ onhancement noc~aary for coron~ di~ch~xga at one elQctrode wlthout cau~ln~ olectrlcal bre~lcdown bo~woen ~he two ~lectroda~, conv~ntion~l preclpitntor~ o~ton ma~o uaa of co~xlal geQm~try wlth a amall dla~Q~ar wlre a~ tho c~nter elootrodo and a largo diamoter outer cylindar. Tho ~rl~t o~ ~ho ion~ i3 c~u~ by the ln~ractlon o~ th~ chnrge on tha ~on and the nonunl~onm, gonorally wonk aloctric ~ield. ~a ~h~ ion~ dxlXt, they charge th~ partl~le~
by ~ttnchlng to them, thexeby cAu~inq the parti~'le~ to be drlven by the olootrlo 101d toward nnd ~ttachod to tha collectln~ electxode.
The officl~ncy of all oleat~os~ntlc precipltators lncluding tho8a 0~ the prior art ~nd al~o of the pre~en~ ~nvention 1~ limlted by three ma~or factor~9 ~speclally ~or the a~roaol partlcles whlch are lo~ than flve mlc~ons in dlameter. The ~ir~t ur~ s~a bQcau~e the charglng rate of the anroaol par~lcl~ decrenses rapidly R~ the radius of the particl~s decrea~as~ Thus~ a9 the slze of the pas-tlclas decsea~es, the paxtlcle charge 1~ le~ ~nd the dri~t velocity, l.e., the component o~ tha ~vex~ge velocl~y e~ the par~lcle~ direc~ed toward the ~lectrod~s, decsea~e~, The second actor i~ tbat or a given charge the drift velocity decresses a~ the electric leld stxength decrease~ Thus~ thQ dr~ft velocity of a glven 8ize particle decr~aaes as it move~ in the direction toward th~ collecting electrode bacause of the decre~slng ~eld in tha coaxial electrode co~flguratlon.
ThQ third ~actor 1~ the att~chment e~iclency of the collecto~ elec~
txode~ i.e., the partlcles which are ~rl~ted to ~ho collec~or electrGde may rebound o~ be dislodged by the lmpaat o~ o~her paxtlclcB
o~ b~ ~wcpt away by ~he tuxbulent flo~ o th~ gaxeou~ e~ luent n~t~r they have been ~nltlally collec~a upon lt becau~ the charge on the part~cl~0 and the electrlc fteld they expe~ienc~ are not 8uf~
o~ently l~rge.
It h~3 genarnlly been recognlæ~d that lmpro~d opera~ion o~ an electro~tatic p~clpltator rQsult~ ~rom increa~lng the electrlc - field str~n~th provld~d, however, that el~ctrlcsl break~own or arcing do~ not renult fro~ the hi~her eloctria ~l~ld ~tr~ngth~
The prior art al~o disclos~s pr~clpi~atin~ ~ppnr~tu~ ~1hlch ind~p~ndontly produ~o tho ~onu and the electric fiald xath~r tbnn a configuration that u~e~ ~ ~mall wiro cantral nloatrode ~n~ outor cylindrical electrodo to ~imultanoou~ly croa~o the ion~ ~nd ~he ~l~ct~la ~ield. Whllo rndloactlvo ~Atorial~ and photoionlzation ~ource~, o.~. light tubes such a~ ultraviolet l~mps, have been di3clo~ed to provide a sourae o~ lon~ lndependently of the produc~lon o~ th~ electrlc f~eld, these ion source~ hav~ practical operational and other disa~vantage~ and it i~ not believed that any commerclal apparatus have been developed.
~ disadvantage of radloactive ~ources ~8 the difficulty in varying thP energy and quantity of par~icle~ e~ltted by such nources~ Yurther, the psychologlcal i~pact of u~ing a radioactlve source o~ lon3 in a precipitating apparatuq, particularly in an urban area, would b~ ~
quite negat~ve. .~oreo~er, there could be a signifi~ant problem of ~ -radioactiv~ contamlnatlon of the atmo~phere if a rupture or br~akdown of 60me portion of the apparatus occurred. Preclpi~a~ors that U8 ul raviolet or other lamps to pro~ide pho~ons ~or creatlng *ha neceY-sary ionlzatlon within the prec~pltator ar2 also ~ub~ect to many practlcal oparational d~sadvantage~. The lamps are sub~ect to duætlng o~ uding over by the part~cle~ in th~ gaseou3 medium or effluent 25 ~nd will becoma d~rty qulte r~pidly. Thi~ dustlng o~er ~ay easily ~ ~
-occur ln only a few seconds and greatly decreases th~ ef~clen~y a~
their operatlon. Moreov~r, the photon energy created by ~uch la~p~
cannot be continuously and convenlently controlled~
~owaver, the pre~ent inventlon does not ~u~f~r ~r~m ~ha -~
disndvantages of these rcadioactiYe a~d p~otoionlzation source~ and, ln fact, exhi~ito many de~lr~ble at~ributes th~t e~ables lt to ~chieve th~ result~ sough~ by the abo~o ~ource~ ln additlon to other ~lgnllcant , 1~70%5~
advantages.
More particularly, the present invention utilizes an electron generating source (often also referred to as an elec-tron beam generator, E-beam generator or the like) to bombard the gaseous medium within the pxecipitator with high energy electrons and produce a plasma region therein. The electron generating source has the advantages of being able to accurakely control the penetration and density of the electrons that are injected into the gaseous medium and th~reby control the extent of the plasma region. Further, the "window" or surface through which the electrons are injected into tbe medium, i.e. the surface through which the electrons pass which is in contact with the gaseous medium, is self cleaning and will not dust up or become dirty from the particl~s within the gaseous medium or effluent. These and other advantages will be described in detail hereinafter.
Accordingly, it is an object of the present invention to provide an improved method and apparatus for precipitating particles from a gaseous medium such as a gaseous effluent, which method and apparatus are effective to remove extremely small particles, i.e., those particles between about 0.1 and 5 microns in diameter and particularly those less than 1 micron in diameter. -Yet another object of the present invention is to provide an improved method and apparatus for removing particles from gaseous effluents with high volume throughput, high efficiency, and only moderate power re~uirements.
The above objects are met by the present invention which provides a method of electrostatically precipitating particles from a gaseous medium carrying the same, comprising:
passing the medium through a channel in a precipitating station wherein the particles are brought into a first region containing Y( 25(~
ions of only one sign; s~ecting the medium to a supply of electrons from an electron beam generator to generate a supply of ions of both signs in a second region, the ions of one sign in the first region being supplied from the second region; sub-jecting the medium to a generally uniform, strong electric fieldto drive the ions of one sign onto the particles, the average field strength of the electric ield approachiny ~he maximum field strength therein; the electric field causing attraction of the charged particles to one or more electrodes hav:Lng a charge of opposite polarity relative to the charged particles to there-by precipitate the particles out of the medium.
The above objects are also met by the present inven-tion which provides apparatus for removing particles from a gaseous medium passing therethrough comprising: an inlet for receiving and an outlet for expelling the medium; a central portion located between and being in communication with the inlet and outlet, the central portion guiding the medium through the apparatus; one or more positively charged electrodes being located in the central portion; one or more negatively charged electrodes located in the central portion for attracting par-ticles having a net positive charge from ~he medium; means for charging the electrodes in the central portion to provide a uniform, high electric field in the central portion of the apparatus; an electron beam energy source means for injecting high energy electrons into the central portion for producing a supply of positive ions which bombard particles and cause them to be attracted to the negatively charged electrode.
The inventive aspects of the invention wi11 become ap-parent from the following description and accompanying drawing, in which:
FIGURE 1 is a diagrammatic representation of precipi-tating apparatus embodying the present invention and which is - ~ -6a-1~)';'0250 .
useful for practicing tha method of the present invention;
FIGURE 2 is a perspective view of one :Eorm of the apparatus that may be used to practice the method of the present invention; and, -6b-~0~ 50 FIGUnE 3 1B a ~Chemat10 illu~tratlon o~ anoth~r embodl~.en~ o~
the present inventlon.
~ roadly sta~ed, tho pre~nt inventlon iB dlrectod to Appar~tus ~ well A9 a method ~or preclplt~tlng or romovlng particla~ ~rom n ~tre~m of gA~eOu~ ~Pluent whtch prof~rably u~ n gencrally wlL~orm, ~tron~ electric fl~ld for chargln~ the p~rticle~ wlth lonu, with the ion~ belng supplled lndepend~ntly o~ the uourc~ of tho olQatrla ~leld ~rom a plasma ~hat i~ ~oxmod by hlyh enorl~y alectron~. ~ pre~ipitatl~g ~tation includeu at l~A~t one po~itlvoly nnd one negatively char~ed el~atrode for s0tting up ths elaatrla flold, and a source o~ ion~
~hich charge tbe pRrticlo~. ~he particles charged ln the pro~en~e o~
th~ el~ctrla field are therbby precipih~0d or rem~Y~d fro~ the g~seou~
o~fluent and collected at ons o~ ~be electrode~. ~lgh-energy electron~
nro directed ~o a8 to produce a plas~a in thc g~eeou~ medlum or effluent 15 near one o~ the electrode~ and the partlcle~ have no net positlve or negatiue charge withln tb~ neutral region of pl~sma. ~owever, ~he charqed el0ctrode~ and pla~ma produce a charged rsglon between the plasma and the collectlng electrode, 80 that once thc partlcle3 ara withln tbe chnrged electrlcal region, they will acqulre a net chsrge, und be AttrA~-ted to ~he opposi~ely charged collectlon electrodeO
Refarring to the dr4wings and partlcularly F~GUR~ 1, an ideal~zed s~hematlc cro~s-~ectional ~i~gram o~ apparatu~ which ~ay be used to casry out the method o~ the present inventlon 1~ ~hown. The appar~tu~, lndicated generally at 10, communlcatea ~ ga~eou~ madlum or ef~luent from ~he lower lnlet 12 throuqh tD the outlet 14 ln an upward dlrectlon a~ ~hown. Slde wall~ 16 ~nd 18 dlr~ct the ~low ~hrough the apparatu~. An eleotron gen~r~tlng ~ourc~ 20 1~ po31tioned wlt~ln an openlng ln ~he side wall 18 ~nd produces high enargy elactron0 indl-cated by the arso~s 22 ~hlch penetrat~ ~ ~hln tr~nsml~slon window 24 ~s well a~ a positlvely oharged elect~ode or ano~3 26 lnto th8 ga8~eUO
~e~lum. A neqatlvely ~harged electro~e 28 1~ po~tloned ad~acent ~e slde w~ll 16 ~o tha an alectrlc ~leld ~9 s~t iup b~waen th~ anod~
~70Z~;iO
and the cathode aeroa~ ~ubstRntlally the entire chann~l wlath a~ ~ho~n.
~he anode 26 and cathode 28 are charg~d by a direct current ~ource 30 havlnq it~ poBit~ ve termlnal connected to the anodo 26 through llne 32 and lts negatlva termlnal connect~d to the catho~e 2B thxough llne ~a. ~B i~ deplated by the curv~d ~rrow~ withl~ the oha~nel os nrea ln~ld~ betw~en the lnlet and outlet o~ tho ~pparAtu~ the a~fluent pr~ferably has 80mo turbulence ~o thAt lnrg~cal~ mlxin~ o~ tho particle~ occurs a~ it p~3~e8 ~hrough ~ha app~ratu~. Becau~e a~
the mixing actlon, virtually n~ particle~ will remain for any length o~ tima ln the re~ion contalnin~ lons of both ~igna clo~c to the po~ltively aharged electrode 260 The partlale~ wlll b~ s~ept into the r~gion batwaen alectrode~ 26 and 2B dusing thls pas~age. The electrodes 26 and 28 are preerably generally fla~, planar membess havlng arcuate edges ~hat are charged by the external ~ource 30 to positlve and negatl~e potential~, se~pQctively. ~ha ~nslde surface of electsode 26 1~ sho~n to ba generally coplanar with ~ide wall 18 slnce ~he flat electrode fits an opening in the right slde wall.
~he generally flat configuration3 and curved edges of ths cath~de ~nd anode are preferred to minimize electx~c ~leld ma~ima, i.e. lt 18 ds~irable that the average field strength approach the maxlmum field strangth withln the apparatu~. 5tated ~n othcr word~, it 1~ -deslrabl~ tha~ the electric fleld be uniform 80 that i~ oan be ma~imlzed ~ithout experienclng electrlcal braakdown or arcing. The electsode 26 separates tho stre~m o~ gaseou~ medium on lts left ~ide a~ ~hown in thu drawing ~rom a quiesce~t g~seous medium on it~ rlght ~lde that is prefer~bly sealed from the left ~ide to prevent du~t to accumulate - -between the ~lectrode 26 and the w~ndow 24. ~he thin wall or window 24 separata~ the qulascent meaium from a region o~ vesy lo~ pre~ur~
l.e, as ~uch as 3 x 10 4 tor. The wlndow 24 be fabrlcated from ~ny material that will tran3~1t electron~ ~herethrough that l~ o capablo o~ ~eparating the lo~ pre~sure ~ in the electron generator 20 ~ro~
tha ext~rlor pre~ure. Thus, the ~indow 2~ ~an ~e mado ~om tit~nlum, - . . . . ~ . -~07(~250 alhmlnum, ~talnle~ st~el, nylon ~nd th~ llke. ~he anode plnt~ 26 may be mad~ o~ ~ tbln she~t o~ conduotiv~ mAt~lnl such ~ t~nl~, ~luminum, or ~tainla~s sta~l wlth th~ combln~d ~hickne~ o~ tho nnodo and window bolng pre~ara~ly le~ than nhou~ two mllo.~,002 lnoh~o) to perml~ p~natration o~ the olectronu throu~h th~m. When th~ wlndow 24 L8 madn of conductiv~ mnterl~l, it ~nn b~ dn~igned to nl~o ~erv~ A~
the anode 26. Ilowov~r, 1~ gonexnl, it m~y ba pxofoxred to we A ~parate Anod~ 26 to ~lmpli~y tho ~ervlclng of tha el~ctron boam ~onerator.
Further, ln c~rtAin embodlment~ it may be pre~esred to orm the electrod~
26 ~rom A ~creen ma~erial o~ ~rom rod~. Means for constrlc~lng or expandlng the gaa ~low to ad~ust lto velocity and to control the mlxing action or to control turbulence of the flow are not shown. ~he an~de 26 L~ preforably chargod to produce a8 hl~h a field strength a~ posslble, generally of about 12 to 18 kLlovolts per contimeter in the gaseous medlum. i~owaver, any potentlal up to the breakdown po~ential of the gaseous medium may be us~d. The elactron beam ~enerator 20 i~ posi~loned between the inlet 12 and outle~ 14 ~o as to irradlate tha gaseou~
medlum or effluent wlth elec~ron~ paseing through window 24 and anode 26 and tho generator prefera~ly ha~ pow~r to provide a~ eleatron bea~
havlng an enesgy den~ity ~u~ficlent to gener~te enough lons to charge all the particle~ ~n the ga~eous medium to nearly saturat~on.
electron gen~rator i8 pre~erably po~itioned so tha~ it lrradi~te~ only th~ volum0 immediately ad~acent the anode ~urfaca. This ~ achi~ved by using electron~ that can only penetrate a ~hort di~tanc~
into the ga~eou~ medium. Tha el~ctron generator 20 pr~ferably operate~
at ~ufficient vDlt~ge to produce lon~zatlon and ~u~ficlen~ ourrent to generate tbR quant~ty o2 lon~ that are ¢apabie o~ c~arging the partlcle~
in ~he gassous streRm. In thi~ regard, the el~c~ron g~nsrator prefer-- ably operate3 to provide eleotron~ Rntesing th~ gs~eou~ medium wlth an energy o~ betwaen about 1 ~8V and about 12 KeV p~r contimeter of plato ~ep~r~tion ana at a current lqval~ o~ ~bDut ono mlcroamper~ per ~et~r o2 eluctrod~ width perpen~$culnr to the ga~ ~lo~. For a con~lg~r~tlon ~070ZS~) havlng n .5 mil thlck t1tanlum wlndow whero the wl~d~w al~o ~unctlons a9 the anod~ Rnd wlth a 10 cm ~p~clng between the ano~e an~ ca~hode, the ~ppar~tu~ per~Qrmed ~atlsfActorlly wlth alectron~ o~ betweon 100 XeV ~nd 115 XeV ~nt~r1n~ the wlnflow.
In Accor~nnc~ with nn lmpoxt~nt ~ap~c~ ~ thn pres~nt lnv~n-tion, ~ the window ~ of tho electron boam qenor~tor nl30 ~at~ a~ the olectrode 26, th~ wlndow 24 oxpo~d t~ the partiale lnd~n qaBeOUB modlum 1B ~al~ ¢lQaning. ~ n sepnrAto ~leakrode 26 i~ usud, the ele~tron b~am from tho qenerAtor ~dt~ to prevent particle bulldup on the anod~ 26.
While there may be some partloles on the expo~ed ~ur~ace o~ elther confl~urnt~on at nny one tlme, thera 1~ no bulldup o~ particles on it dua to lt~ s~l~ cleanlng operatlon. ~he expo~ed surfAce doe~ not ex-perlance any accumulation of small partlcles beQau~e they axe repelled be~ora they can reach the sur~ac~. A$ ~he qmall partlcles ~re bombards~
-by the electron~ produced by the electron generator, the ~lectrona go completely through them causing ~econdary emls~ion and tha smalI part~-cl~ becGmes posltlvely charged and ~ rep~lled by the po~itively charged Qurface. ~hu~, ~mall particle~ never reach the ~urf~se and cannot accumulate on ~t.
Wlth reApQct ~o larger partlcle~, however, the electro~
bomhardlng ~he particle wlll not travel through the partlcle and s0cond-Rry .em~s~ion affect~ wlll not be ~igniflcant compared to the piling up of electron~ withln the part~cle. Thu~, voltage on the lnslde d the part~cle build~ up w~thln the partlcle and lt becomes qulte 25 negat~Ye. If the partlcle 1~ in contac~ with the ~rf.ac~ lt wlll ~i~charge to th~ polnt o~ con~aat b~tween the partlcle and ~ho surfac~. -Thia dl6charge pro~uco~ ~ dlscharg~ path that c~n ~Q analOgl%0d to the ~h~pe o~ a tree, 1~ h~ d~charge path goes from the branche~
~nd comblnes ~n a large~ trunk portlon wher~ lt contacta ~he ~urface.
~he paths ar~ hol~s ln the part~ala cau~ed b~ vaporl~ing th~ solld 9 the p~rtlcle to ~ g~s, ~h~ ~aporl~atlon produce~ a ~housa~d fold Yolume lncreaae whlch e~c~pes through ~he dl~charg~ pAth8~ ~h~
:
~ 0702~0 vaporlzatlon procQ~a producas a grP~t f~rc~ that blows th~ particle from the surfacQ or d~stroy~ th~ pnrtiqle lt~l4, ~lther re~ult being e~eative to rld the ~urface of the partlale. ~or~ov~r, the ~orco o~ ona partlcl~ be~ng removed will e~ctlv~ly xemove ~ovoral othera as well, Thi~ cl~aniny not~on can b~ inarea3ad b~ incr~Aslng th~
op~rating volt~g~ o~ the ol~ctron ~on~ra~or. It ~hould therePor~
be understood that tha operating voltage c~n ba varlad, perhap~ p~rlod-1CA11Y~ tO control the cleanin~ ~ctlon. ~n op~lmum duty cycl~ can 10 be eotablished tl~at woul~ eff~at ad~quate cloaning and minlm~ze tha . .
povQr r~ulrements for th~ ~va~all opera~ion o~ the apparatus.
The upper ~lQctria field ~tr~ngth limit ls deter~ined by the dlel~ctric strength of ~ha ga~ou~ ~edium at operating pre~ure.
~or a ten centlmetsr separation di~tanco b~tween cathode and anode, a separation distance used ~n one ambodlment of the apparatu~, tha unlfonm field breakdown ~rength of alr at normal dan~lty i~ about 25 kV/cm. Since the ab301ute tcmperaturc il~ a typical gaseous e~fluent ~ -will be ln the rangc of about 400K ~o 500~K, the gas density vill . ;.
bs about a factor o~ two lower than no~mal atmospherlc den~ty, and the llmltlng field ~tr~ngth would be about 13 kV/cm. ~owever, electron-~ttaching ga~e~ ~uch a~ sulfur dioxide for example~ will o~ten be pre~ent in a gaseou~ ef~lueDt, and the pxes~nce of Shese ga~e~
may~enable operatlon at a higher electrlc ~eld value ~han the descri~ed 13 kV/c~.
It ~hould al80 be under~tood that the electron genera~or ~ay generatu ~ singla b~oad steady be~m or one ox morQ narrow beam~
~nd may al~o be adapted to ~can the area withln the apparatu~ in a pradeterm~ned p~ttern. For examplo, the pattern may have the beam gollow a moving ga~oou~ medium through a volume for an average dwell time for partlcle~ w~thin ~hat Yolume, then ts~at other volume8s~cco~siveIy in llke manner an~ then nf~er an average dl~fu~lo~ tlmc require~ to repopulate the ~irst xffglon wlth partlelen~ seturn to --11~
~070250 thAt fir~t volume.
Tha rQsldu~l, ambi~nt mlxing nctlon or turbulence o~ the flow of tho gns~ou~ m~d~um through tho nppnr~tus carrie~ ~lo p~rtlclu-l~don gaseous modlum to wlthln n dlatanoa d~lnlng the lmnln~r ~low S bvundary ~ubl~yor of th~ c~rgod eloutrodou. ~llthln n r~tglol- o~
thlcknuo~ compnr~blu wlttl tho rnngo Or tho ol~otronrl ln th~ modlum, ~hn cllnrgo oll du~t p~tlcla(l la nenxly noutrnll-o~ ~onAu~o o~ ~.h~
pro3onad o~ lon~ o~ bot~ nlgn~, In ~ha r~t ~f thu volum~ however, tholr ch~rqng r~t~ nre no lonqcr nautrAll~ed nnd bui~d xapidly 0 80 that by the t~me the oddy ~otlon carries the ga~eous ~low to and thsn nw~y from tbe cnthode 28, dust particles which have positlve cb~xge rsmain becau~e o~ the electrical force that is exerted upon the charges. ~he partlcle~ may acquire additional charges by i~pinge~ent of qaaeous lons wh~le tbsy are attached to the csthode. Thii would l~ lncrea3e the holding force 90 that they uould not be inclined to be diYlodged. ~o~ever, lf the du~t is of vary high sesistiv~ty, excesslve local field strength can result from thiY chsrge buildup ~nd cause harmful local breakdown. Thl3 local breakdown can be prevented by keeplng the ion denRlty in the qaseous ~edium low except in the rogion o~ lnltla~ partlcle chasging. Partlcles of all slzes rapldly -collect on ths cathode 28 because the electric ~leld, no longer limited to about 4 kV~c~ in the bulk of the qas by th~ requirement of corona generation at one edge, ~an be rai~ed to between 13 a~d about 18 kV/cm. In thi-~ preferred form of operation, the hign fleld covers 25 virtually all the dlatance betveen the electrodes. It ~hould be ~nder- -stood that while the above de~crlptlon deals wlth p~lllng po~ltive ions fro~ th8 reglon of neutrol plasma, the present lnvention ls aPP1$Cab1e tO 10D~ of negative polarlty. ~o~ever, ~he use of posltive ion~ ha~ the advantage ~n that elsctrons and negative lons are pulled bac~ toward the anode ~nd the thlckneaa o~ the reglon of neutral pla~sa 1~ ~inlmlzed, a~ lo de~lred. As prevlously mentioDed~ the satu~atlon ch~rge by the u~ual mech~nl~m of ~leld cb~rglng 1~ ~ub~ect to a limlt cnu~d by the el~ctro~tatlc repul~ion b~twsen the pnrtlcles that have ~cquir~d ~ ch~rge ~nd additlonal chorge~ which ~pproAch ~t~
~n nccord~nco wLth tho pros~nt lnvontlon, howovor, tho Aatur~tlon ohnrge on ~11 p~rtlol~n 1~ muoh ~treAtor b~aau~o ~u ~n oluctrlo ~iold ~tr~n~ oAn bo rnlood by nbout n ~nctor o~ botw~on ~bout 3 nnd 5. ~hu~ a mnxlm~ o~ botwoon nbout G0 ~nd ~0 ahA~o~
uould bo dnl)oD~to~l ~n ~ 0,3 mloron pn~tlcl0 ln nn 18 kV/~m ~lol~, ~)~llo only nbout 20 eo 30 ch~r~ Ar~ ~ypi~lly depo~it~d durlng tho tr~n~lt o~ such ~ pArtlolo throu~h ~n ordinary eleatro~tatio preoipi~ator. Wlth re~poct to Pleld oharglng, the ini~ial charglng rnte 18 glven by !dt 4~7 x 10 5 ~ No~ D2 E ~ 2 wherfl D i~ the particle diameter ln mlorons, ~ is the electric field strength in kllovolts per centimeter, F LS the dlelectric constant of the partlole, and No i3 the amblent ion concentratlon in nu~ber per cublc centimeter. Values ~or ND- are nbout 3 x 107 per cublc cent~meter in conventional preclpitators. In the present lnvention, No iY controlled lndep~ndently o~ tho field stsength E, wherca3 thsse two values are interllnked ln conventlonal pr~clpitator~. The field ~trength can be controlled independently of ~O to achieve particular advantages, ~.e., the ~leld strength can be reduced to mLnimize power consu~ptlon or ~ncreased to maximize the charging rate. ~or example, ln an 18 ~v~cm fleld, wlth an No o~ 3 x 107/cm3, dN/dt equal~ between abo~t B00 and 2200 per second for ~ 0.3 mlcron partlcle BO that the pnrtLcle very rapidly approache~ its saturation chnrgs of about 60 to 80. I~ for other reasons, lt 18 necessary to reduco the field, the chsrglng rat~ c~n be ~alntalned by increaslng No~
From the aboYe, it ~hould be under~tood that a l~rge decre~e ~n charqing tlme ~8 well ns a lnrge lncre~se ln totnl charge occurs for 0-3 ~lcron dL~m ~ter pnrtlcle belng-charged in the lnrge electrla flelds thnt c~n exlst in ~ ch~nnel where lon~ are nuppli~d by the agency -~3-1~70~S0 of hlgh-energy electron3 ~rom an ~l~ctron generator, for ex~mple, rnther than ln ~he small~r overall ~lelds typicnl of Q convQntlon~l preclpl-t~tor, Thus, electron beam supportod charglng may exceed charging cur-r~ntly u~Qd ln prlor nrt preclpltAtor~ ln both tho rnto an~ mnxlmum charge attnlnnblo ln roA~onAblo dwoll tlm~l o~ pnrtl~lo~ ln tho I)ro~lp-lt~tox, ~nd ~l~o mny roqulro loo~ pow~r ~urln~ oporAtlon~ Purthen~ro, tho olectrlc ~lold nctln~ on onch o~ t~Q~o ch~r~o~ lo l~r~or by n fac~or o~ nbout 4 nnd wlll provido nn nverAqo preal~it~lon vqloalty that will bo abo~t 12 ~ln~e~ l~rcJer thnn th~t whlch would~e~perlenced by 0.3 mlcron par~lol~ ln pre~ent conventlon~l pxcclpltator~. ~lowev~r, ~lnce only ono o~ the two aur~Aces colleat3 thefle partlcle~, the effectlve oollection rate per unlt AreA wll~ only be lncrea3ffd by a ~actor of about 81x. An alternatlve deslgn uslng two electron guns on opposlte side3 of a central colleotlng cathode would lncrease the collection rate by a,factor o~ about twelve. Alternatively, an electron beam may be pro~ected down the center of the preclpltator to produce a plasma. ~ach electrode would then attract it3 respective opposltely charged partlcles and would be preclpitated out o~ the gaseou~ medium.
- ~eferring to FIGURE 1, it should be understood that the thln curved electron beam wlndow 24 is preferably covered wth the thin metal anode 26 to protect the ~tressed window 24 from corroslve gases and largu particles in the gaseous medium or effluent. The thin flat protectlve cover anode 26 also produces a smoother electrlc field distribution and thereby allows a higher average fleld strength.
Turning now to FIGURE 2 which illustrates one form of app~-atus th~t i~ useful in practicing the method of the prQsent ~nvention, the apparatus 40 ha~ ~n inlet 42 at lts lower end and ~n outlet 44 at its upper end, with gaseous medium or effluent flowlng Yert~cally upwardly a~ shown by the arrows. The dust laden gaseou~
effluent preferably flows in the prealpitation channel at 5 to 10 meters per ~econd, An electron generator 4~ 19 posltloncd to lrradlate the effluent whlle ~t i8 wlthin the channel 46. ~ cathode 18 provldod 1070~2S0 an~ mny be ln th~ for~ o~ a ~lexlble ~t~lnlo~a ~teel b~lt 50 ~8 ~ho~n which travel~ around upper and lowur roller~ 52 and 54, raspectl~ly, ~lth on~ o~ tha lowor rollers belng drlven by a motor 56. ~he belt ., ha~ n frone ~ld~ axposed to the qa~eous medlum or e~luent cont~nlng S hlqh ros13tlvlty duu~ pas~1ng through tho ahannol ~nd n hnok Hldo eh~t 1~ out~1do o~ the ohnnnol~ on~blln~ th~ pnrt1closl to ~n roll~ov~
from eho bolt bofor~ tha bolt roontoru tho oh~nnol an~l n~Jaln b~com~n oxpoood to tno ~luont. Ono ~vnn~ngo o~ tho ~pp~rntun nhown ln F~CUR~ 2 ln ~hn~ lt l~ o~ ~ rolntlvaly llm~ll hel~ht aompAred wltll 10 laas ~f~a~lv~ prlhr n~t pr~lpi~or~ ~or A ~lven througl~put rate.
~ n aooord~nco with Another A0peCt o~ the presant lnventlon and referrlng to the cro~s-~ectlon~l vlew ~hown ln ~IGUI~ 3, appnratus, lndlcAted generally At 6~, nnd al~o embodying ~ho present lnvention, communlcntes a qasaoUs me~ium or effluent ln A dlrection tow~rd the re~der. A~ l~ deplcted by tho curve~ arrows with~n th~ apparatus, th~ effluent l~ preferably glven Bome turbulence 80 that large scale ~lx1ng of the partlcle~ occurs as it passes through the apparatus.
Because o~ the mlxlng act~on, the particles wlll be swept around ~nd brought ln close proxlmlty to negatlvely charged cathode~ 62 as well a8 the posltlvely charged anode 64 durlng thls pa~age. The turbulent actlon se~oves p~rtlcles from the reglon o~ neutral charge den~ity noar tha electron ~ea~ wlndow, brlnglng them through the reglon of posltlve charge d~nslty to ulthln close range o~ the aathodes. This ~110~B all partlcles to be attracted to tha cathode~ ~o that they may bo sub~ect to prwlpitation out o~ the gaseous medlum before lt 1Q dis-charged at ths outlet. It ~hou~a be understood that while the dlagrHm-mat~c representation sh~n ln FIGURE 3 does not lllustrate elther the ~de or end exterlor ~nllJ o~ the Appnratus, the eleotrode~ 62 and 64 - wlll be posltloned wlthln tha outer slda wall~ whlch guide the ~low 33 o~ a~luant through tha apparntu~. The eloctron generatorn pre erably comprlsQ a number o~ thln wlres or roughened rod~ 66 enclo~d wlthin ovacuated tub-~ 68 ln the ~node our~Ace 64. ~hese ~ro3 nra small and ~Q70250 ~harged to a su~lclently lArg~ n~gntlv~ potentl~l that they emlt electrons by fleld emi~lon, Altornatlvoly, the wlros 66 mAy bo he~te~
and ~mlt electxons thermionlcally. ~hese electron~ are attr~cted to the thin u~ode wall tubes 68 nnd, becau~e of the hlgh voltage dl~ference, S hn~o sufrlcient enor~y to penetrate the thln Metal ~node 64. ~node supports (not ~hown~ con~l~t Or atructurnl ~oln~orclng loop~ o~ motal t~nt nro ~pnco~ porlo~onlly ~ltllln tho tub~n G3. Tl~o oporntlon lo ~ub8t~nti~11y slmllnr to thnt doocxlbo~ wlth roapoot to tho ~ppnrntuo ot FlGURE 1. ~ ~v~ntn~ o~ thu con~l~urntlon Or FS~Un~ 3 ln tl~At l~
th~ ~aouum soal no~ ono o~ tho wl~ 66 l~ bxoXon~ v~lta~e c~n b~
romoved ~rom the brokon wlro 66 wlthou~ Aub~t~ntiAlly ndver~ely arfqctlng the op~ration o~ the npparatu~.
From the to~egolng detAlled d~crlptlon, lt ~hould be under-~tood that ~ method ~nd flpparatus for electro3tatically preoipltating partlcles from ~ partlcle carrylng gaseous medlum has been illustrated and described whlch l~ more efflclent than conventlonal designs and 18 e~ectlve ln removlng extremely ~mall partlcles, even to such ~mall slzes as 0.1 micron in dlameter. In addition to effectlvely preclpl-tating ~uch small particle~, the pr~sent lnvention provides rapld charqlng and rapid preclpl~atlon of ~uch small aR well as larger par-tlole~, and enables fa~t throughput of the gaseou~ medlum or ef$1uent.
Although particular embodlment~ of the present lnventlon have been lllustrated and de~crlbed, var~ou~ modiflcation.s, ~ubstltutlona ~nd Alternativ~s wlll be apparent to tho~e ~klllcd ln the art, and 25 accordlngly, the scope o~ the inventlon ~hould be only de~insd by the ~ ~
nppended claims and equ~valents th~reof. - -Various ~eatures of the inventlon are ~et forth in thQ
follo~ing clAims.
Claims (33)
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A method of electrostatically precipitating particles from a gaseous medium carrying the same, comprising:
passing the medium through a channel in a precipitating station wherein said particles are brought into a first region containing ions of only one sign;
subjecting the medium to a supply of electrons from an electron beam generator to generate a supply of ions of both signs in a second region, said ions of one sign in said first region being supplied from said second region;
subjecting said medium to a generally uniform, strong electric field to drive said ions of one sign onto said particles;
said electric field causing said charged particles to be attracted to one or more electrodes having a charge of opposite polarity to thereby precipitate said particles out of the medium.
passing the medium through a channel in a precipitating station wherein said particles are brought into a first region containing ions of only one sign;
subjecting the medium to a supply of electrons from an electron beam generator to generate a supply of ions of both signs in a second region, said ions of one sign in said first region being supplied from said second region;
subjecting said medium to a generally uniform, strong electric field to drive said ions of one sign onto said particles;
said electric field causing said charged particles to be attracted to one or more electrodes having a charge of opposite polarity to thereby precipitate said particles out of the medium.
2. A method as defined in Claim 1 wherein an electron source produces said supply of electrons.
3. A method as defined in Claim 2 wherein said electron source has sufficient voltage to produce ionization and sufficient current to generate a quantity of ions capable of charging said particles.
4. A method as defined in Claim 2 wherein said electrons have an energy of between about 1 KeV and about 12 KeV
per centimeter of electrode separation and about one microampere per meter of electrode width perpendicular to the gas flow.
per centimeter of electrode separation and about one microampere per meter of electrode width perpendicular to the gas flow.
5. A method as defined in Claim 1 wherein said second region is adjacent one or more electrodes.
6. A method as defined in Claim 1 wherein said electric field has an average field strength approaching the maximum field strength therein.
7. A method as defined in Claim 1 wherein said station includes at least one negatively charged electrode, said charged electrodes attracting oppositely charged particles.
8. A method as defined in Claim 1 wherein the volume of said second region is small relative to the volume of said first region.
9. A method as defined in Claim 8 wherein the volume of said second region is less than about 10% of the volume of said first region.
10. A method of electrostatically precipitating particles from a gaseous medium carrying the same, comprising the steps of:
passing the medium through a precipitating station in a manner whereby the medium passes near at least one positively and at least one negatively charged electrode located at said station, said electrodes being charged to produce a strong electric field within said precipitating station;
subjecting said medium adjacent the positively charged electrode to high energy electrons from an electron beam generating means;
said electrons being effective to produce a plasma of predetermined thickness in a region adjacent said positive electrode, the positive ions traveling outside of said region bombarding the particles of the medium, thereby resulting in said particles acquiring a net positive charge so that the magnitude of the attractive force between the particles and said negative electrode increases sufficiently so that the particles move towards said negatively charged electrode.
passing the medium through a precipitating station in a manner whereby the medium passes near at least one positively and at least one negatively charged electrode located at said station, said electrodes being charged to produce a strong electric field within said precipitating station;
subjecting said medium adjacent the positively charged electrode to high energy electrons from an electron beam generating means;
said electrons being effective to produce a plasma of predetermined thickness in a region adjacent said positive electrode, the positive ions traveling outside of said region bombarding the particles of the medium, thereby resulting in said particles acquiring a net positive charge so that the magnitude of the attractive force between the particles and said negative electrode increases sufficiently so that the particles move towards said negatively charged electrode.
11. A method as defined in Claim 10 wherein said electrons are produced by an electron generator that has sufficient voltage to produce ionization and sufficient current to generate a sufficient quantity of ions to charge said particles passing through said station.
12. A method as defined in Claim 11 wherein said electrons have an energy of between about 1 KeV and about 12 KeV
per centimeter of electrode separation and about one microampere per meter of electrode width perpendicular to the gas flow.
per centimeter of electrode separation and about one microampere per meter of electrode width perpendicular to the gas flow.
13. A method for electrostatically precipitating particles from a gaseous medium carrying the same, comprising the steps of:
passing the medium through a precipitating station in a manner whereby the medium passes near at least one positively and at least one negatively charged electrode located at said station;
injecting high energy electrons generated by an electron beam generating means into said gaseous medium at said precipitating station so that said medium adjacent a negatively charged electrode is subjected to high energy electrons;
the application of said electrons being effective to produce a plasma in a region adjacent said negative electrode, the negative ions traveling outside of said region bombarding the particles of the medium, thereby resulting in said particles acquiring a net negative charge so that the magnitude of the attractive force between the particles and the positive electrode increases so that the particles move towards the positively charged electrode.
passing the medium through a precipitating station in a manner whereby the medium passes near at least one positively and at least one negatively charged electrode located at said station;
injecting high energy electrons generated by an electron beam generating means into said gaseous medium at said precipitating station so that said medium adjacent a negatively charged electrode is subjected to high energy electrons;
the application of said electrons being effective to produce a plasma in a region adjacent said negative electrode, the negative ions traveling outside of said region bombarding the particles of the medium, thereby resulting in said particles acquiring a net negative charge so that the magnitude of the attractive force between the particles and the positive electrode increases so that the particles move towards the positively charged electrode.
14. A method as defined in Claim 13 wherein said electrons are produced by an electron generator that has sufficient voltage to produce ionization and sufficient current to generate a sufficient quantity of ions to charge said particles passing through said station.
15. A method as defined in Claim 14 wherein said electrons have an energy of between about 1 KeV and 12 KeV per centimeter of electrode separation and about one microampere per meter of electrode width perpendicular to the gas flow.
16. A method for removing particles from a gaseous medium at a precipitating station having a plurality of electrodes including at least one anode and at least one cathode, the cathode being adapted to attract particles having a net positive charge, comprising the steps of:
charging said electrodes to produce a strong electrical field within said precipitating station;
passing the particle containing medium through at least one channel in the precipitating station with sufficient mixing action to sweep the said particles out of any region having a plasma with a predominately neutral net charge therein;
subjecting the medium to electrons generated by an electron generator as said particles enter said precipitating station; said generator being effective to produce a plasma region having positive and negative ions, said plasma region being small relative to the volume of said channel, the positive ions passing, out of said plasma region bombarding particles of said medium and causing them to acquire a net positive charge, the mixing action and the high electric field therein effecting said positively charged particles to be attracted to said cathode.
charging said electrodes to produce a strong electrical field within said precipitating station;
passing the particle containing medium through at least one channel in the precipitating station with sufficient mixing action to sweep the said particles out of any region having a plasma with a predominately neutral net charge therein;
subjecting the medium to electrons generated by an electron generator as said particles enter said precipitating station; said generator being effective to produce a plasma region having positive and negative ions, said plasma region being small relative to the volume of said channel, the positive ions passing, out of said plasma region bombarding particles of said medium and causing them to acquire a net positive charge, the mixing action and the high electric field therein effecting said positively charged particles to be attracted to said cathode.
17. A method as defined in Claim 16 wherein said plasma region is located adjacent said anode.
18. A method as defined in Claim 16 wherein said plasma region occupies less than about 10% of the volume of said channel.
19. A method as defined in Claim 16 wherein said electrodes are in the form of generally flat members having curved edge portions.
20. A method as defined in Claim 16 wherein said electron generator has sufficient voltage to produce ionization in said plasma region and sufficient current to generate a sufficient quantity of ions to charge said particles passing through said station.
21. A method as defined in Claim 20 wherein said electrons have an energy of between about 1 KeV and about 12 KeV
per centimeter of electrode separation and about one microampere per meter of electrode width perpendicular to the gas flow.
per centimeter of electrode separation and about one microampere per meter of electrode width perpendicular to the gas flow.
22. A method as defined in Claim 16 wherein said electrodes create a high electric field wherein the average field strength approximates the maximum field strength.
23. A method as defined in Claim 22 wherein said average field strength is up to the range of about 12 KV/cm to about 18 KV/cm.
24. A method for removing particles from a gaseous medium at a precipitating station having a plurality of electrodes including one or more positively charged anodes and one or more negatively charged cathodes, the anodes being adapted to attract particles having a net negative charge, comprising the steps of:
charging said electrodes to provide a strong electrical field within said precipitating station;
passing the medium through at least one channel in the precipitating station with sufficient mixing action to sweep the said particles out of any region of plasma with a predominately neutral net charge therein;
subjecting the medium containing the particles to electrons generated by an electron beam generator as said particles enter said precipitating station, said generator being effective to produce a plasma region having positive and negative ions, said plasma region being small relative to the volume of said channel, the negative ions passing out of said plasma region bombarding particles of said medium and causing them to acquire a net negative charge, the mixing action and electrical influence therein effecting said negatively charged particles to be attracted to said anode.
charging said electrodes to provide a strong electrical field within said precipitating station;
passing the medium through at least one channel in the precipitating station with sufficient mixing action to sweep the said particles out of any region of plasma with a predominately neutral net charge therein;
subjecting the medium containing the particles to electrons generated by an electron beam generator as said particles enter said precipitating station, said generator being effective to produce a plasma region having positive and negative ions, said plasma region being small relative to the volume of said channel, the negative ions passing out of said plasma region bombarding particles of said medium and causing them to acquire a net negative charge, the mixing action and electrical influence therein effecting said negatively charged particles to be attracted to said anode.
25. a method as defined in Claim 24 wherein said electrodes are in the form of generally flat members having curved edge portions.
26. A method as defined in Claim 24 wherein said plasma region occupies less than about 10% of the volume of said channel.
27. A method as defined in Claim 24 wherein said electron generator has sufficient voltage to produce ionization in said plasma region and sufficient current to generate a sufficient quantity of ions to charge said particles passing through said station.
28. A method as defined in Claim 27 wherein said electrons have an energy of between about 1 KeV and about 12 KeV
per centimeter of electrode separation and about one microampere per meter of electrode width perpendicular to the gas flow.
per centimeter of electrode separation and about one microampere per meter of electrode width perpendicular to the gas flow.
29. A method as defined in Claim 24 wherein said region of plasma comprising positive and negative ions is adjacent said cathode.
30. Apparatus for removing particles from a gaseous medium passing therethrough, comprising:
an inlet for receiving and an outlet for expelling the medium;
a central portion between said inlet and outlet for guiding said medium through the apparatus;
one or more positively charged electrodes located in said central portion;
one or more negatively charged electrodes located in said central portion for attracting particles having a net positive charged from the medium;
means for charging said electrodes to provide a high electric field in said central portion of said apparatus;
an electron beam energy source means for producing a supply of positive ions which bombard particles and cause them to be attracted to the negatively charged electrode.
an inlet for receiving and an outlet for expelling the medium;
a central portion between said inlet and outlet for guiding said medium through the apparatus;
one or more positively charged electrodes located in said central portion;
one or more negatively charged electrodes located in said central portion for attracting particles having a net positive charged from the medium;
means for charging said electrodes to provide a high electric field in said central portion of said apparatus;
an electron beam energy source means for producing a supply of positive ions which bombard particles and cause them to be attracted to the negatively charged electrode.
31. Apparatus as defined in Claim 30 wherein said electrons have an energy of between about 1 KeV and about 12 KeV
per centimeter of electrode separation and about one microampere per meter of electrode width perpendicular to the gas flow.
per centimeter of electrode separation and about one microampere per meter of electrode width perpendicular to the gas flow.
32. Apparatus as defined in Claim 30 wherein said electron energy source means produces said supply of ions adjacent said positively charged electrodes.
33. Apparatus as defined in Claim 30 wherein said electrodes are generally planar and parallel to one another and have arcuate edges to provide a generally uniform strong electric field therebetween.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US50210374A | 1974-08-29 | 1974-08-29 | |
| US05/602,730 US4071334A (en) | 1974-08-29 | 1975-08-07 | Method and apparatus for precipitating particles from a gaseous effluent |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1070250A true CA1070250A (en) | 1980-01-22 |
Family
ID=27054016
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA234,371A Expired CA1070250A (en) | 1974-08-29 | 1975-08-28 | Method and apparatus for precipitating particles from a gaseous effluent |
Country Status (9)
| Country | Link |
|---|---|
| JP (1) | JPS5150068A (en) |
| CA (1) | CA1070250A (en) |
| CH (1) | CH617363A5 (en) |
| DE (1) | DE2537931A1 (en) |
| FR (1) | FR2282943A1 (en) |
| GB (1) | GB1500154A (en) |
| IT (1) | IT1041549B (en) |
| NL (1) | NL7510240A (en) |
| SE (1) | SE404659B (en) |
Families Citing this family (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN102962136A (en) * | 2012-12-04 | 2013-03-13 | 武汉纺织大学 | High-voltage discharge oil smoke purification method |
| CN107120160B (en) * | 2017-06-01 | 2024-05-28 | 山东大学 | Tail gas purification device based on high-voltage electrostatic dust removal principle and application thereof |
| CN113477080A (en) * | 2021-08-05 | 2021-10-08 | 北京航天方达科技有限公司 | Light quantum beam generating device |
| CN113440970A (en) * | 2021-08-05 | 2021-09-28 | 北京航天方达科技有限公司 | Light quantum beam flue gas whitening and low-nitrogen system of gas boiler |
Family Cites Families (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE348378C (en) * | 1922-02-07 | Elga Elek Sche Gasreinigungs G | Process for removing very fine dust particles from gases during electrical gas cleaning | |
| DE833798C (en) * | 1950-07-05 | 1952-03-13 | Metallgesellschaft Ag | Process for preventing disruptive space charges in electrostatic precipitators |
| JPS4967270A (en) * | 1972-10-31 | 1974-06-29 |
-
1975
- 1975-08-20 GB GB34638/75A patent/GB1500154A/en not_active Expired
- 1975-08-26 DE DE19752537931 patent/DE2537931A1/en not_active Withdrawn
- 1975-08-28 IT IT51104/75A patent/IT1041549B/en active
- 1975-08-28 CA CA234,371A patent/CA1070250A/en not_active Expired
- 1975-08-28 SE SE7509584A patent/SE404659B/en unknown
- 1975-08-29 JP JP50104858A patent/JPS5150068A/ja active Pending
- 1975-08-29 FR FR7526583A patent/FR2282943A1/en active Granted
- 1975-08-29 NL NL7510240A patent/NL7510240A/en not_active Application Discontinuation
- 1975-08-29 CH CH1121475A patent/CH617363A5/en not_active IP Right Cessation
Also Published As
| Publication number | Publication date |
|---|---|
| CH617363A5 (en) | 1980-05-30 |
| JPS5150068A (en) | 1976-05-01 |
| FR2282943A1 (en) | 1976-03-26 |
| IT1041549B (en) | 1980-01-10 |
| SE404659B (en) | 1978-10-23 |
| DE2537931A1 (en) | 1977-02-24 |
| GB1500154A (en) | 1978-02-08 |
| FR2282943B1 (en) | 1982-10-08 |
| SE7509584L (en) | 1976-03-01 |
| NL7510240A (en) | 1976-03-02 |
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