Measurement of liquid holdup and axial dispersion in trickle bed reactors using radiotracer technique

Transcription

1 NUKLEONIKA ;45(4):35 41 ORIGINAL PAPER Measuremen of lqud holdup and axal dsperson n rckle bed reacors usng radoracer echnque Harsh Jaga Pan, Anl Kumar Saroha, Krshna Deo Prasad Ngam Absrac The holdup and axal dsperson of aqueous phase has been measured n rckle bed reacors as a funcon of lqud and gas flow raes usng radosoope racer echnque. Expermens were carred ou n a glass column of nner dameer of m column for ar-waer sysem usng hree dfferen ypes of packngs.e. non-porous glass beads, porous caalyss of able and exrudae shape. The range of lqud and gas flow raes used were m 3 /s and m 3 /s, respecvely. Resdence me dsrbuons of lqud phase were measured and mean resdence mes were deermned. The values of lqud holdup were calculaed from he measured mean resdence mes. I was observed ha he lqud holdup ncreases wh ncrease n lqud flow raes and was ndependen of ncrease n gas flow raes used n he sudy. Two-parameer axal dsperson model was used o smulae measured resdence me dsrbuon daa and values of mean resdence me and Pecle number were obaned. I was observed ha he values of Pecle number ncreases wh ncrease n lqud flow rae for glass beads and ables and remans almos consan for exrudaes. The values of mean resdence me obaned from model smulaon were found o be n good agreemen wh he values measured expermenally. Key words axal dsperson model holdup mean resdence me Pecle number resdence me dsrbuon rckle bed reacor Inroducon H. J. Pan! Isoope Applcaons Dvson, Bhabha Aomc Research Cenre, Trombay, Mumba 4 85, Inda, Fax: 91-/ , , e-mal: hjpan@apsara.barc.erne.n A. K. Saroha Deparmen of Chemcal Engneerng, Thapar Insue of Chemcal Engneerng and Technology, Paala 1441, Inda K. D. P. Ngam Deparmen of Chemcal Engneerng, Indan Insue of Technology, Hauz Khas, New Delh 11 16, Inda Receved: 9 February, Acceped: 14 July Trckle bed reacor (TBR) mples a reacor n whch a lqud phase and a gas phase flow cocurrenly downward hrough a fxed bed of caalys parcles whle he reacon akes place. In ceran cases he wo phases also flow cocurrenly upward. The cocurren upward flow operaon provdes beer radal and axal mxng han he downward flow operaon due o whch he hea ransfer beween he lqud and sold phases s beer. Ths s useful n hghly exohermc reacons n whch producs needs o be removed from he reacor connuously. However due o hgher axal mxng n he upward flow operaon, he degree of converson, whch s a crucal facor n he operaon of a reacor, s low. Therefore, cocurren downward operaon s preferred due o beer mechancal sably, lower axal mxng and less floodng, hus faclang processng of hgher flow raes and ncreased reacor capacy. In he las few decades he TBRs have been suded exensvely by chemcal engneers due o her suably for many operaons n peroleum refnng, chemcal, pero-chemcal and bo-chemcal processes. The major processes carred ou n TBRs are hydroreang, hydrocrackng, hydrodesulfurzaon, hydrodenrogenaon, hydrodewaxng, hydrodemeallsaon and hydrofnshng. In effluen reamen plans, he rckle bed reacors are used for removal of organc maer from

2 36 H. J. Pan, A. K. Saroha, K. D. P. Ngam Fg. 1. Schemac dagram of expermenal seup. 1 surge ank, pressure gauge, 3 ar roameer, 4 waer ank, 5 lqud feed pump, 6-7 lqud roameers, 8 racer njecon por, 9 lqud dsrbuor, 1 rckle bed reacor, 11 gas-lqud separaor, 1 ar oule, 13 lqud oule, 14 couner, 15 daa acquson sysem, 16 lapop compuer; D 1, D, D 3 collmaed radaon deecors; V1...V8 valves. wasewaer sreams by aerobc baceral acon. The poenal applcaons, advanages and dsadvanages of he TBRs have been revewed n wo recen arcles [3, 8]. The knowledge of mean resdence me, holdup and axal dsperson s a basc requremen o evaluae he reacor performance, s opmal sze, he physcal and chemcal neracons and he pumpng requremens. Lqud holdup and axal dsperson are wo key parameers o descrbe he performance of a TBR. Resdence me dsrbuon (RTD) analyss faclaes he deermnaon of hese parameers. Radoracer echnques are wdely used o measure RTD of process maeral n ndusral process sysems because of her varous advanages over convenonal racer echnques [8]. Ths paper descrbes measuremen of RTD, deermnaon of mean resdence me (MRT), holdup and axal dsperson of aqueous phase usng radoracer echnque. Expermenal The schemac dagram of he expermenal seup s shown n Fg. 1. A seres of radoracer expermens was performed o measure he RTD of lqud phase n a glass column of m nner dameer. The expermens were carred ou wh hree dfferen ypes of packngs.e. non-porous glass beads, porous caalyss of able and exrudae shape wh ar-waer flow a amben condons. The ar (densy: 1.93 kg/m 3, vscosy: Pa.s) and waer (densy: 997. kg/m 3, vscosy: Pa.s) flowed cocurrenly downward hrough he column whch had a packed hegh of 1.5 m. Table 1 lss he deals of he packngs and range of Reynolds number used. The lqud sored n a ank of. m 3 capacy was connuously pumped no he column from he op hrough a dsrbuor mouned.1 m above he packng. The lqud dsrbuor consss of a sanless seel ube of dameer m o whch ubes of dameer of m were aached. There were 37 holes of he sze of m arranged n a square pch of. 1 m [9]. The ar was connuously nroduced no he column from he op from a compressor afer passng hrough an ar sauraor (surge ank). Afer passng hrough he packed bed, he lqud and gas phases were separaed n a gas-lqud separaor a he boom of he column. Two precsely calbraed roameers were used o measure he lqud and gas flow raes. The ar was allowed o escape no he amosphere whle he waer was dscharged no a dranage ppelne. 99m Tc (half lfe: 6 h and gamma energy.14 MeV (91%)) as sodum perechnaae was used as a racer. 99m Tc was exraced from a 99 Mo/ 99m Tc-generaor and abou 1 MBq acvy was used n each run. The expermens were performed a dfferen combnaons of gas and lqud flow raes. The racer was njeced nsananeously no he nle feed lne hrough an njecon por a he op of he column usng a calbraed glass syrnge. The racer was njeced afer achevng seady sae flow. The racer movemen was monored a he nle (D 1 ) and oule (D ) of he column usng collmaed NaI(Tl) scnllaon deecors (M/s Bcron Corporaon, U.S.A.) separaed by a dsance of 1.5 m. In order o nvesgae he radal dsrbuon of lqud, an addonal deecor D 3 was also mouned damercally oppose o deecor D a he reacor oule. The deecors were conneced o a mulchannel daa acquson sysem (DAS) suppled by M/s Elecronc Enerprses Pv. Ld., Mumba, Inda. The DAS was se o record 1 daa pons a an nerval of. seconds. In he nal few expermens wh glass beads as packng maeral, he nerval was kep.5 seconds. The racer concenraon was recorded unl he radaon level a he oule comes o he background level. The recorded daa was ransferred o he compuer for subsequen analyss. Daa analyss Deermnaon of mean resdence me and holdup The daa recorded were reaed and analysed usng a Resdence Tme Dsrbuon analyss sofware provded by he Inernaonal Aomc Energy Agency Venna, Ausra [7]. The daa reamen ncludes background subracon, zero shfng and al correcon. Fgs. a, b and c show hree ypcal reaed normalsed resdence me dsrbuon curves. Type of packng *Parcle dameer, Bed vodage Reynold No. for Reynold No. for d p (m) (ε) lqud (Re L ) gas (Re G ) Glass beads Tables Exrudaes Table 1. Deals of he packngs used. * Parcle dameer s defned as he dameer of a sphere of he same volume as a parcle. d p qu Re = µ where: q densy (kg/m 3 ), u superfnal velocy (m/s), µ vscosy (Pa.s)

3 Measuremen of lqud holdup and axal dsperson n rckle bed reacors usng radoracer echnque 37 (3) τ = V Q Fg. a. Comparson of expermenal and model smulaed RTD curves. (Packng: glass beads, Q g = m 3 /s, Q l = m 3 /s, Pe = 15, ADE =.57). where: V volume and Q flow rae. For a normally operang closed sysem he heorecal and he expermenally measured MRT should be he same. Based on he calculaed MRT, he lqud holdup was calculaed usng he followng relaon: QL (4) HT = VR where: H T lqud holdup, expermenally deermned MRT, Q L volumeerc lqud flowrae, V R effecve reacor volume. The values of MRT and lqud holdup a dfferen operang condons used are gven n Tables, 3 and 4 for glass beads, ables and exrudaes, respecvely. Resdence me dsrbuon and model smulaon Fg. b. Comparson of expermenal and model smulaed RTD curves. (Packng: ables, Q g = m 3 /s, Q l = m 3 /s, Pe = 65, ADE =.388). The resdence me dsrbuon (RTD) s a characersc funcon of connuous process sysems and provdes nformaon on malfuncon(s), f any, and flow paern.e. degree of mxng. The RTD s defned as a normalsed response of he sysem o an deal mpulse njecon of smulan n he form of δ-drac dsrbuon [1]. If an deal mpulse of racer s njeced a he nle of he sysem a me = and s concenraon s measured as a funcon of me a he oule, hen E()d represens he fracon of he racer havng resdence me beween me nerval (, +d) or as oherwse saed he probably for a racer elemen o have a resdence me beween nerval (, +d). (5) E() = c() c()d such ha: Fg. c. Comparson of expermenal and model smulaed RTD curves. (Packng: exrudaes, Q g =. m 3 /s, Q l = m 3 /s, Pe = 3, ADE =.81). Frs momens (M ) of he npu and he oupu racer concenraon curves were deermned usng he followng relaon [8]: (1) where = 1 for npu curve, = for oupu curve. The dfference of frs momens of he wo curves gves MRT of process maeral n he sysem. Thus: () M = C ( )d C()d (MRT) = M M1 where, M 1 and M are values of he frs momens of npu and he oupu curves, respecvely. The heorecal MRT (τ) of he maeral n a closed sysem s gven as: (6) E ()d = 1 where: = 1,..n, c () racer concenraon and E () resdence me dsrbuon funcon. The ndusral TBRs are operaed eher n he ranson regon from rckle o pulse flow regme or n pulse flow regme. As descrbed earler a TBR s one n whch lqud and gas phases flow cocurrenly downward hrough a fxed bed of caalys parcles. Ideally, he flow should be plug flow bu some axal nermxng s always nevable. The resdence me of racer n he column s small as compared o he me requred for racer o dffuse no he caalys pores. Therefore, s assumed ha he porosy of caalys has a neglgble affec on he racer movemen n he column. Based on he above consderaons, he obaned RTD daa were smulaed usng a wo-parameer axal dsperson model (ADM) wh open-open boundary condons. The one-dmensonal dfferenal equaon of ADM for flud flow s gven as [, 5]

4 38 H. J. Pan, A. K. Saroha, K. D. P. Ngam Run No. Q g (m 3 /s) Q 1 (m 3 /s) (s) H T m (s) Pe 1/n Σ Y() Y m () Table. Holdup and Pecle number for glass beads. Run No. Q g (m 3 /s) Q 1 (m 3 /s) (s) H T m (s) Pe 1/n Σ Y() Y m () Table 3. Holdup and Pecle number for ables.

5 Measuremen of lqud holdup and axal dsperson n rckle bed reacors usng radoracer echnque Run No. Q g (m 3 /s) Q 1 (m 3 /s) (s) H T m (s) Pe 1/n Σ Y() Y m () Table 4. Holdup and Pecle number for exrudaes. (7) where: C dmensonless racer concenraon = c()/c(), Pe Pecle number = ul/d, X dmensonless axal co-ordnae = x/l, u mean lnear velocy, D axal dsperson coeffcen, c( ) racer concenraon a me, c() nal concenraon. The soluon of he above equaon for open-open boundary condon of equaon n dmensonless form s gven as [, 5] (8) The MRT and varance of mpulse characerscs, E(q) are gven by he followng relaons: (9) (1) C 1 C C = θ Pe X X Pe Pe(1 θ ) E( θ ) = exp. 4πθ 4θ = 1+ τ Pe σ () 8 = Pe Pe The response of a sysem o an deal mpulse drecly gves RTD of flud flowng n he sysem, whch s drecly compared wh he model RTD. However, s no always praccally feasble o njec racer as an deal mpulse and n such suaons he racer s njeced as a pulse. The model response, Y m () of a lnear sysem o an arbrary pulse of racer s obaned by convolung he npu funcon, X() wh mpulse response of he model, E(). Thus [4, 5]. (11) For dscree me nerval, he above convoluon negral can be wren as: (1) m Y () = X( T) E() dt m Y() = X( T) E() T One of he oldes and smples echnques of parameer esmaon s he momen's mehod, whch nvolves he comparson of varances of he model and expermenal dsrbuon funcons. Unforunaely here are some nheren compuaonal errors nvolved n he varance of he measured response curves. The varance s compued from he frs and second momens abou he orgn of he expermenally measured racer concenraon dsrbuon curve. In he Fg. 3. Comparson of lqud holdup as a funcon of lqud flow rae for dfferen packngs. Fg. 4. Comparson of lqud holdup as a funcon of gas flow rae for dfferen packngs.

6 4 H. J. Pan, A. K. Saroha, K. D. P. Ngam Fg. 5. Comparson of Pecle number as a funcon of lqud flow rae for dfferen packngs. Fg. 6. Comparson of Pecle number as a funcon of gas flow rae for dfferen packngs. esmaon of momens, he values of concenraon c() a large me () are weghed heavly. The weghng facor consss of for he frs momen and for he second momen. Snce he 'al' of a racer response curve s he leas precsely recorded poron due o he small value of concenraon nvolved, he compued momens wll have a large error. In addon o he above dsadvanage, he mehod assumes ha he model s an exac descrpon of he flow sysem and no check of model applcably s provded. The values of model parameer esmaed by he momen mehod can be consdered only as a rough esmae. The dsadvanage of he momen mehod could be avoded by fng he complee model RTD curve wh he expermenal RTD curve. The leas squares curve-fng mehod usng he well-known Marquard-Levenberg algorhm s used o f he wo curves and oban he opmum model parameers. The qualy of he f s judged by choosng he model parameers o mnmse he sum of he squares of he dfferences beween he expermenal and model compued curves [6]. The values of he model parameers (MRT and Pe) correspondng o he mnmum value of roo mean square error (RMS) or absolue devaon error (ADE) are chosen as he opmum values. Thus: (13).5 1 RMS = Y() Y m(, parameers) = Mnmum n Fg. 7. Comparson of expermenal and model MRTs. or (14) 1 ADE = Y() Y m(, parameers) = Mnmum n The hree represenave plos of model smulaon for glass beads, ables and exrudaes are shown n Fgs. a, b and c, respecvely. The values of MRT ( m ) and Pecle number obaned by model smulaon, a dfferen operang condons used, are gven n Tables, 3 and 4 for glass beads, ables and exrudaes, respecvely. Resuls and dscusson The racer concenraon dsrbuons recorded by deecor D and D 3 were compared o analyse he radal dsrbuon of lqud phase n he column. Effcences were so adjused ha boh he deecors have he same response. The supermposon of racer dsrbuon curves recorded n dfferen runs ndcaed no sgnfcan radal maldsrbuon and s hus negleced because of small dameer o lengh rao of he reacor. I can be observed ha lqud holdup ncreases wh ncrease n lqud flow rae for all he hree ypes of packngs. One of he represenave plos of lqud hold up vs. lqud flow rae s shown n Fg. 3. The lqud holdup s found o be ndependen of he gas flow rae. Fg. 4 shows he varaon n holdup wh gas flow rae for hree dfferen ypes of packng. I s observed ha Pe ncreases wh ncrease n lqud flow rae n general for glass beads and ables. For exrudaes s almos consan. A represenave plo of varaon of Pe wh lqud flow raes s shown n Fg. 5. No specfc rend n Pe wh varyng gas flow rae has been observed. One of he plos showng he varaon n Pe wh gas flow raes for glass beads, ables and exrudaes s shown n Fg. 6. The values of Pe for glass beads are hgher han he correspondng values for ables and exrudaes. The values of Pe wh ables are hgher han ha for exrudaes as can be seen n Fg. 6. The values of MRT esmaed by model smulaon are almos same as expermenally measured values. A comparson of he wo s shown n Fg. 7.

7 Measuremen of lqud holdup and axal dsperson n rckle bed reacors usng radoracer echnque Conclusons From hs sudy he followng conclusons have been drawn: 1) No sgnfcan radal maldsrbuon was observed. ) The lqud holdup ncreases wh ncrease n lqud flow rae and s almos ndependen of gas flow rae used n he sudy for all he packngs. 3) For glass beads and ables he axal dsperson (D) of lqud phase decreases wh ncreasng lqud flow rae and s consan for exrudaes. However, no specfc rend n axal dsperson (D) s observed wh respec o varyng gas flow raes. 4) The model esmaed MRTs are n good agreemen wh MRTs measured expermenally. Ths jusfes ha he axal dsperson model s suable o descrbe he dynamcs of lqud phase n TBRs flled wh non-porous caalys parcles. 5) The lqud holdup and degree of axal dsperson of lqud (Pe) are srongly dependen upon shape and sze of he caalys used n TBRs. The degree of axal dsperson s less wh sphercal caalys han he caalys of able and exrudae shapes. 6) The resuls obaned n hs sudy wll be useful for scale-up, desgn and o opmse he performance of full scale ndusral TBRs. 7) Radoracer echnques provde an excellen ool o sudy he holdup and degree of axal mxng of flowng meda n TBRs. References 1. Danckwers PV (1953) Connuous flow sysems, dsrbuon of resdence mes. Chem Eng Sc :1 13. Fu Meng Shyang, Chung-Shung Tan (1996) Lqud holdup and axal dsperson n rckle bed reacors. Chem Eng Sc 51: Ganeo A, Speccha V (199) Trckle bed reacors: sae of ar and perspecves. Chem Eng Sc 47: Gudebook on radosoope racers n ndusry (199) Techncal Repor Seres No IAEA, Venna 5. Levenspel O (1996) Chemcal reacon engneerng. nd edn. Wley, New York 6. Mchelsen ML (197) A leas-squares mehod for resdence me dsrbuon analyss. Chem Eng J 4: Resdence Tme Dsrbuon sofware analyss (1996) Compuer Manual Seres No. 11. IAEA, Venna 8. Saroha AK, Ngam KDP (1996) Trckle bed reacors. Revews n Chemcal Engneerng 1: Saroha AK (1997) Sudes on mulphase reacors. Ph.D. Thess, Indan Insue of Technology, Delh, Inda 41 Acknowledgmens Auhors are graeful o Dr. S.M. Rao and Dr. S.V. Navada for her help and suppor durng he course of hs work.