Investigação de Concentração de Sólidos em Sistemas Fluidizados usando Radiação Gamma, Notas de estudo de Química Industrial

Baixe Investigação de Concentração de Sólidos em Sistemas Fluidizados usando Radiação Gamma e outras Notas de estudo em PDF para Química Industrial, somente na Docsity! J _r I F:- L(cGJf'Y' ( vL"a' a -'. / _::ii J4v.>.L.. R. N. BARTHOTOMEW cnd R. M' CÂSAGRANDE Shcll Oil Co., lnc., Houston, Tox. ,-õ { rj'i,l i Io, "*oos co:{TRoL' recent availabiliry ol suitabie radioi:otopes has focused at- tcntion on radiarion absorption for meas- uring dcnsities and dctecting densiry di-ffercnces in opaqr.rc matcrials. A grorr'ing nuobcr oÍ indusria'l ap- piicacioos use bcta or garrlma raoiatioo; úc radioactivc sourccs arc chcap, com- pact, an6t nccd no adjusrment or calibra- too- Howcver, spccial handüng and storage facilities arc rcquircd for person- ncl protection Gam:ra-ra:- absorpcion has bcen used successfuüy to mcasurc densiry of con- crcte slabs and consol pioelinc move- ments and lcvels in storage and ,surge tanb (4, 9); a bcta-ray dcvice is uscd to conroi úc üicknes of papcr during is manuÍacturc (n. This mcúod has gcneraÌ utüry and , bccause the equig ment i.s mountcd cxtcrna-Ìly, it docs not intcrfc:'c with úc slstelrl under srudy-a' fcaturc of particuìar valuc for rcsearú' K-oowledge of solids conccntradon is oftca nccessary for úe design and opcra- tion of prc\cõscs embodying fluidizatios of parcicles undcr satic or traülPort con- diúons. Io a fluidized fixcd bed, úe av€ragt solids holdup is approximatdy equal to the Pressurc droP Pcr unit length thrcugh thc bcd (2' 8), úough oothlog is said about solids dbÈribution within the bcd. Poor solids disribu- tion in a catãJ',Tic rcacrion Process rnay cause undë'i;blc effects zuú as high ,=râlFr i:rventory, low convcrsion, ex- i:asivc side reactioru, or rÊduced ca- pacity.^ Ertimates of solids holdup in fluidizeC transPoli slstcstr frorD pressurc'drop Ele'a:rdrc:EenEt over vcrtical scctions arc u-ruaÌly ef limitcd reliabiiiry bccause a ' rubstantial part of üe pressurc drop oc- orn tlrough encrgy consunpdon -for particlc aiclcradon and various fric' ic:al effccts. Space tt lo.iry, an im' porrant catalvric rcacdon proccss vari- ^Ut., it especially dificult to cictcrmine u-ndcr conditions of caulltr FansPort wiúout a dircct rncasurcmcnt of cata- Ìr'st holdup and disrriburion. FÌuid dlrramics srudies of hctcrogcne- ous flow ,Yra.*t 3çç f3eiÌiç213d if $6 ohasc conccntradons arc kaown, i:ccausc I -o*.t ,u- balancc givcs úc value of úe wall friction coc6cicot' The gam-ma- ray absorption tcchnique has !6s;1 6;a- tcnded to- quantitativc mcasurcmcnt of solids conccntradon and disrribudon in fluidizcd s).stems, pardcularlv thc dis- pcned phasc, but the applicable equa- Lottt "t. valid for an1- hcterogeneous s.ystcm. An investigation of dense phase fiuidization by means of x-ravs has re- ccntly bcen rcPoncd (5). Gamma radiation, being eÌcctromag- netic, is assumed to obc.v thc wcü-knolrn Lamben-Bcer's law of orPoncndal ab-' sorpdon in a homogencous medium (J, 6),' This law is cxprcsscd mathcnad- c- lly íor monocoergetic radiatioo as !,: f4-yL (1) wherc -I, is irrcident intcosity; l, sub' sidcnt intcssitYi L, absorbcr thickncss; anri p, ünear absorption cocficient' The comparabie expression íor muÌdenerçtic radiation is morc complcx and docs not lcad to thc dcsüed simple rclationship' The numcrical value of Ë dePcnds on tle phwicai and úcoicaÌ sratc of úc absorbcr. R-alo v/P, where P is ab' aorbcr densiry, elied mass absorprion cocficienr' a, is indepcndeot of starc but oot compoiirion of thc absorbcr' Equa- tion 1 thus bccomc: !, : l4-*L (2) For rcvcral homogtneous abcorbi'g mcdia in rcries, rclationships arc dcrivcd ar foüowr: .Aftcr ndiation Pa$€t tÀrough üc 6nt I l' Ì. *r 128 Drou!Ìü t Âlo DloDlgtÌ'o cxurÌÍfü Messuring Solids Concenïrotion in Fluidized Syslems by Gammo-RoY AbsorPrion ln Drocess conlrot, rodioisotopes con .be usêd to meosure density oÍ ;;J;;oúi"lr ond solids concentroiion in fìuids. They ore compoct ;;J .i,=;p, ond need no odiustmenl or colibrotion absorbcr, subsidcnt intersiq' givcn br Equarion 2 is Ir : I*-:AIa ('i. AÍrcr pt."igg through thc second'at'- sorbcr, radiation inrersiq' from Equa- tiors2and3is lr: f ,2-aIa : It- ePJa + 'sI4) (4) Similarì1', for r absorbing media in scrics' J, : IJ-tdí2'14 (5) Equation 5 rcprcscnu the general cast' for a'bsorpdon ol radiarion in a scries oi diffcrcnt homogeneous media. A het- erogcncous srrte.ú mav bc treated as a serics of homogeneous mecìia if thc mass absorpdon co.ffi.i.t t for anv phase is assumed to bc indcpendenr of ics posi- tion in the stst.-. This assumpcion is valid *'h'-n highìy coÌlinatcd (narrorr bcam) sourccs of monocncrgctic raciia- don arc uscd; in oihcr cascs, a correcoon factor nay bc appiied to Equation 5' Dcrcrmination of the numcrical value of .ío is avoidcd by mcans of "zcro" ma?s- urcrncnts. Coosider a systes corsist- ins of soüd pa;tides fluidizeci by a gas in; üúìüãG vcascl- For a zcro meas' urcmcnt, the intensiry of radiation which ha: passcd through the ernpry vcsscl along any choscn Paú is obscrvcd. l, : [4-(o.l.La * c*,L.a etxL;) . (ó) where subccripts a, u,and i rcfer to air' vesscl wall, and insulation' rcspectlvcl1" Thc radiation intensiry rDCâliürÊlircrr! l5 rcpcated rrith thc fluidized swtcm in op" .r"aioo. Undcr flow conditions. air iras bcen rcplaced by gas anci parddes; tle paü lengü through thc gas is rcduc':c from l. to Ir. , It - lt- Gnb * aonLt *qt'Lo*a-Li) (7) o.14 The ratio oí I, lo I t i'a 'Jren I: - e o*zLt * qttLt - sjliit Ir Howertrt L. :!:r' L" + Lt Combinadon of Equations I and 9 {: : c(o*t - osl.s) f'" + (aceg - o,',',)Lo I1 I." OF RADIATION ATOMIC NUMBãR OF ABSORSING I'{EDIUIT nergy of rodialion on moss obsorptic ,q' scincillarion ccunter (crvsal Pi'- phoromutdplicr rubc) is parcicuJarlv r:st hrl urirh a iinear "..'pli6er and a pul: height analyzer-üe endre aPPararL constirutcs a garnma tay sPectror[ctc' Adjustmcnt of th. irstrument to Cete: mi"e onlv attenuadon of the Primar photoelectric peaks eiiminates thc nee for collimadon<nlv necessan' sour( shielding is required- Jn fieid appiie tion, usJ of tt itrpttt-line voirage regd;J tor ,*,ill improve rhe r=uks materiail' r in some instances' additionai' excrcme' fine rcgulation of the high voltaq" :uP.q to *rc-ptrotorouldplier nrbc is desirabi Tcmpcrarure control is mainaineci i p€ns of a jackt't, heated or cooied necësar)', piaced around rhc scinriil ,igt counter. Experimenfol R'esutts Sincc 1 951, üt g"rnrna-ra)' absorpric pri:rciple has been r:sed to determi: ãcrxity pal..rns in transa'erse sccrio of fluidized sr:sterns in rarsfer lines a: vcssels ranging frona ó inches to 40 feet diameter. -Snrdies have bccn made ( pilot plant and cornrnercial units in bc à.*. and dispened phase fluidizaric ; Results of thesc cbemica! cngineeri:t snrdics made ì^'ith the gamma ra,v tec niquc udtl tË pullished scparatel-v' ord.t to demorYf,rate this meúod, c,ïanPle of its application ro a particui unit is gitcn' Data-for calculadng the cross-sccdor ' YG'|J'4?, Ìlc). 3 . nAlcH IttT' 1'. (e) gives (10 ) o.to o.o 6 I J lr. d a - l- L lrj q ttt! u.,oo 3o ç o- É.oa co U'' a s The meãn soücis densiry over úe chosen path is Lepa: PoT Elimination oí Lrfrorn Equarions 10 and 1 1 and soludon for P- gives q\ O ''-. : $ (11) 1 , I, , doga -:tJì--T.- d.D a't. a d- , (12) nGB ì.o2 r.36 ?.o4 o&o-'It ' o, P'-: . QçPgI_ q*P .çhich is a ge neral form valid for solid- liquid or tiquid-gas s\:sl'ems as *'eil as for toiid-gas s\stems- The terrn for garnma- ra-v uÉrorprion in air can bc neglected r+'ithout aPPrcciable error' l\rhen den- siq. of the'fiuidizing medium is negligibÌc and d.pc is approrimatciy cqual to arPm üe súplifi.ed equarion üus obtained is usuall-v satisfactory as a basis for ex- pcrimcntal rvork- Grohse (5) also uscd zera and florv measurements in his x-ray stuciies of fiuidized beds- Hor+'ever, he rnade a series of zero rneasurements for van{ttg thickncsse s of a standard absorber of knor*,n densir insened in the radiation paü, to esÉblish a dcnsíw calibradon cr.rrve with the aid of Equacion 4' - Av- erage bcd densiris were determined di- ,..úy from flor*- measurcments b-v rnea's of úe calibrarion curve- This procedure is equivalent to using Equation 13 but requires more phrsical measurernents' Èardcle mass absorpdon coefficient dp F-a funcrion of g2rnrna energy used' de- gtee of source and detector colli'na rion' ãnd gcomecry of thc system, it addidon to coulPosidon of üe absorbing me- . dinrn. Wiú highly collirn^ted arrange- 'ú *.o* ald monoenergeric sources, &? ï i** thc data of Davl"son and Evans i (/), can bc estimated within *37o' fypicaf curves of mass absorption cocf- ficient as a funcrion of atomic numbcr and gamna energy lPPcar in Figurc 1' The effect of geoúcuy is associated with back-scatrcrcd radiadon from üe Figure 1. tfect of qtomic number ond e co e fíicient surroundioç and Parts of thc s\:tern not' clirccdy ia che absorpdon Path' It is rrsuallv adl'isable td determine ot c'xPcri- mentally in order to rainimizc erToni caused by various assunPcions -"'de in thc dcrir adon of Equation 13 ; dcvia- ão* of 20/s or morc ito* úe theorcrical value of armay occur under some expcri- mental condirions. An idcal g2T rma source for dcrsiry measur.*.r,r! shouid have a long h.aif lifc and ernit rnonoenergetic radiàtion in ã. ,"nge of 0.5 to 1'5 m'e'v' Bclow 0'5 m.e.v., srong sources are required for su-fficienr ,"ái^,iott to pcneraie thc walls of a large aPParatus; and in'addi- tion, c" vario .o*iderably r*ith l-toÏtt ; n,-,mber; above 1'5 rrì'e'v'' radiarion absorprion is so sm^ ll that obtaining precise ciata under. t]"ical cxpcrimenal conditions is di-fficult' Cobalt-60 (1'1 and 1 .3 m-e-v.) and cesium' -137 (0'66 m.e.v.) are readily available and have proven saúfactory' Râdiation intensiry is measured by any pracricable means' Both Gciçr- Mutter nrbcs and scindlladon ssyst-lq are suirabie detectors, but spcciaf corrsidera- don of úeir lirniratiora is necessary for each application- Sensitiviry to ganna rals, temPcrzlfürc, volt'Ee fl-uctuadons' ."d vibracion are the chief factors' but iniEial cosr, bulh and uscfirl lifc also "'Ì--v a-ff'ect the choicc- In gencral, Gciçr- Mütcr übõ have the advantage of 3im'- fúã+; but scintiUation crlnt-lc ha'e Luch highcr g?Ínrn? scruitivirY' # :" 1 , I, Ds ^i-rà a*o 11 (13) l,