Residence time distribution of solids in a fluidised bed

Transcription

1 Indian Jurnal f Chmical Tchnlgy Vl. 6, March 1999, pp Rsidnc tim distributin f slids in a fluidisd bd Samir S Chapadgankar' & Y Pydi Sttyb 'Dpartmnt f Chmical Enginring, Dharmsinh Dsai Institut f Tchnlgy, Nadiad , India bdpartmnt f Chmical Enginring, Rginal Enginring Cllg, Warangal , India Rcivd 1 Nvmbr 1997; accptd 3 Fbruary 1999 Th prsnt wrk invlvs an xprimntal invstigatin n rsidnc tim distributin f slids in a cntinuus fluidisd bd fr unifrm siz and fr binary slid mixtur at diffrnt dilutins. Th clumn was tstd fr imprvmnt in prfrmanc with incras in dilutin. Th ffct f gas flw rat, slids flw rat and dilutin n th first, scnd and third mmnts and n F-curvs hav bn dtrmind. Fractinal tank xtnsin mdl fits th xprimntal data. High valus f N as dilutin incrass, indicats rducd slids mixing, thus rducing sprad f rsidnc tims, giving ris t mr unifrm quality f prduct. Th wrk finds imprtanc in that similar imprvd prfrmanc f th clumn using intrnals insid th bd can b btaind by :'~ i n g a mixtur f slids rathr than unifrm siz. Fluidisd bds find wid applicatins du t unifrm tmpratu~ thrughut th bd bcaus f slids mixing. But this rapid slids mixing in th bd lads t nn-unifrm rsidnc tims f slids in th ractr. Fr cntinuus tratmnt f slids, this givs nn-unifrm prduct and lwr cnvrsins, spcially at highr cnvrsin lvls. Dspit its srius drawbacks, fluidisd bds hav bn widly usd by maki;1g mdificatins insid th bd. On such mdificatin is t sctin th bd hrizntalli r vrtica lll, knwn as a multistag fluidisd bd r using a spiral intrnal t mak it a spiral fluidisd bds. Ths mdificatins hav bn mad t rduc th grss irrgularitis and instability f th bd inducd by th gas flw. Th prsnt wrk is aimd at rducing th sprad f rsidnc tims in a singl stag flat fluidisd bd, which is achivd by using a binary slid mixtur. Th unifrm sizd particls giv ris t mr nnunifrm rsidnc tims fr slids. Hnc a binary mixtur has bn usd, whr th fin siz frmd th majr po/iin and th carsr siz frmd th minr prtin. In thr wrds, th fin sizd slids ar dilutd with th carsr sizd particls and th prcntag f dilutin (sam as th prcntag f cars siz) is varid. Exprimnts wr carrid ut t tst th imprvd prfrmanc f th bd with incras in dilutin. Similar bsrvatins f bttrmnt in fluidisatin phnmna wr bsrvd by Chn and Pi, whr carsr particls wr addd t a bd f fin particls and an incras in fluid t particl hat transfr cfficint was bsrvd. Exprimntal Prcdur Fig. 1 shws th xprimntal st-upi D. Th unit cnsists f a fluidisatin clumn and an air chambr bth f idntical dimnsins and mad f mild stl. Th air chambr cnsists f tw prfratd plats f 3 mm prfratins with 6 mm triangular pitch. Th chambr is cnnctd fr air supply t a blwr. Th fluidisatin clumn is prvidd with a prfratd plat f 3 mm prfratins and 6 mm triangular pitch with a stainlss stl wir msh f 40 msh siz fixd t it. Th plat is prvidd with a dwncmr tub which xtnds dwn thrugh th air chambr t facilitat th xit f slids. Th dwncmr wir hight is takn as th bd hight which is kpt cnstant at 5 mm fr all th xprimnts. Slids wr fd cntinuusly thrugh a hppr as shwn in th figur. Exprimnts wr prfrmd fr th rsidnc tim distributin f slids at diffrnt gas flw rat, slids flw rat and dilutin f th matrial. Exprimnts wr prfrmd using slids f unifrm siz as wll as fr diffrnt binary slid mixtur, ach f diffrnt dilutin. Clurd matrial was usd as th tracr during th xprimnts. Tabl I silpws th diffrnt slid sampls usd in th prsnt wrk and th xprimntal cnditins. On sampl f unifrm particl siz and fiv diffrnt sampls f binary slid mixturs with dilutin ranging frm 10 t 50 % wr

2 CHAPADGAONKAR t al.: RESIDENCE TIME DISTRIBUTION OF SOLIDS IN A FLUIDISED BED 101 Tabl I-Exprimntal cnditins f th prsnt in vstigatin!i SOLIDS DISCHARGE 7 1) FLUI DIZA TION SECTION E u '" N... ) AIR CHAMBER 3) DI STRIBUTOR PLATE 4) CONTROL VALVES 5) PRE 55 URE TAPPINCJS 6) FEED HOOPER ~ 7,) FEED PIPE B) DOWN COMER PIPE 9) AIR INLET PI PE u 10) SOLID FLOW : CONTRO LLING PLATE 4 11) PERF~ATED PLATE 4 I Fig. I-Exprimntal st-up - AIR INLE T usd. Th tracr was idntical t that f th riginal matrial fr th xprimnts using unifrm sizd particls. Fr th xprimnts usi ng slid mixturs th tracr prpard was that crrspnding t th majr prtin (fin siz). Hnc this xprimntal wrk givs infrmatin abut ffct f dilutin n slids mixing f fin sizd particls, whn a bd f ths particls ar dilutd by cars sizd particls. Tracr cncntratin analysis has bn dn by physical sparatin mthd by hand - picking and wighing th clurd matrial. Th mixtur f si zs wr mad utsid th clumn and again thy wr allwd t mix unifrmly insid. th clumn in th initial stags f fluidisatin. Aftr stady-stat was btaind, a sampl f mixtur was analysd fr th fractins and th analysis indicats n sgrgatin f diffrnt sizs insid th bd. A mixtur f 10% dilutin cntains 10% f cars si z (d p =.03) and 90% f fin siz (d p = 1.44). Sam cnvntin f prcntag di lutin is f llwd fr all th mixturs. Undr stady stat cnditins at knwn gas and slids flw rat th tracr was admittd int th bd in th frm f a puls. Sampls wr cllctd at diffrnt lim li1trvals and analysd fr clur cncntratin by hand - picking. Rsults and Discussin Th mmnts f RTD curvs and th ag di stributin functins, C(8) and F(8) wr f und 4. 6 as, Clumn di amtr, m : Clumn thicknss, m : Dwncmr tub diamtr, m : Clumn lngth, m : (sam as air-chambr lngth) Fluidisd bd hight, m Dnsity f slids, kg / m 3 Matrial Charactristics Sampl A B C D 0% E 30% F 40% G 50% Typ Unifrm Siz (d p =.03 mm) Unifrm Siz (d p = 1.44 mm ) 10% S lids Flw Rat, G s = t kg / m s Air Flw Rat, Gr = t kg / m s t = Tctdl / Td! 0 c!dt / T 0/ =. (' = 0/ 1 ( t/ TCl 3 dt cdt ) - ( t) Tct dt _ 0 _ ).1' 3 = I ( 1)3 T tcft v' 3 = ~l ' 3 /(t) 3 Tcdt... ( 1)... ()... (3)... (4)... (5) Fr ach xprimntal c(t) vs 1 curv, all th abv mmnts hav bn fund alng with th nrmalizd c(8) va lus at diffrnt 8. Th F(8) vs 8 data was btaind by intgrating c(8) vr 8. Effct f th fllwing variabls has bn invstigatd: (i) Effct f gas fl w rat (i i) Effct f slids flw rat (i ii) Effct f dilutin. Tabl shws th valus f mmnts crrspnding t all th systm variabl s. It was bsrvd that Man rsidnc tim, i btaind frm th RTD curv was fund t dcras with incras in slids flw rat and was littl ffctd wi th incras in gas flw rat. Man rsidnc tim was fund t dcras with incras in dilutin rat. With incras in slids flw rat, slids cncn-

3 10 INDIAN J CHEM. TECHNOL., MARCH 1999 Tabl -Exprimntal rsults and mdl paramtr in th prsnt wrk Sampl G s G r t G v' 3 I:!,.Ps N R.M.S Dviatin Effct f gas flw rat C Effct f slids flw rat D E Effct f dilutin C D E F G C D E F G C D E F G tratin incrass but it ds nt incras in prprtin t slids flw rat. Incras in slids cncntratin is lss as cmpard t incras in slids flw rat and hnc j which is qual t WIAG s dcrass with incras in slids flw rat. It was furthr bsrvd that, at fixd slids flw rat, as th gas flw rat is incrasd, bubbling incrass, thus incrasing tr slids mixing. Agitatin f th bd incrass incrasing th randm mtin f particls. This is spcially tru whn ffct f gas flw rat at high slids flw rat was cnsidrd. With in:;ras in dilutin frm zr t 50%, j was fund t dcras. It was fund that with incras in dilutin, slids mixing dcrass i.. th randm mtin and grss circulatin pattrn f particls dcrass with incras in dilutin and slids will b mr and mr diffusin rintd in mtin. As dilutin is incrasd, di ffusin f particls is mr, which rducs man: rsidnc tim. It was als fund in th prsnt invstigatin that th dimnsinal scnd mmnt, 0/ incrass with incras in gas flw rat. With incras in slids flw rat, it was ntd that cr/ dcrass t a crtain xtnt and thn incrass. cr/ vs Gs data shws a minimum. Abuzid t al. I ntd that th varianc dpcnds n particl cllisin frquncy and th distanc travlld

4 CHAPADGAONKAR t al.: RESIDENCE TIME DISTRIBUTION OF SOLIDS IN A FLUtD~D BED ' 00 ' 0 loa (0) ~ CD i.l 0' SAMPLE : E Gs:O 8140 LEGEND Gf <:;J CD IL ' SAMPLE '. C Gf : lj919 LEGEND G V U6li A Fig. -Effct f gas rat n RTD f slids Fig. 3a-Effct f slids rat n RTD f slids at lw dilutin du t cllisin. At lw slids flw rats, th cllisin frquncy will b lss, whil th distanc du t cllisin will b mr. Thrfr th particls will b travlling t th xit rapidly and lssr travl path will b ncuntrd. Th phnmna will b rvrs at high slids flw rat, whr th cllisin frquncy will b vry high and th distanc t th cllisin will b vry lss. Hnc th travl path will b incrasd du t mr hindranc fr th particl t travl frm th ntranc t th xit. Th dimnsinlss varianc, cra was fund t incras with incras in slids flw rat at lw dilutin (up t 0% ). At lw dilutin, th dcras in cr/ is lss cmpard t dcras in t with incras in slids flw rat. Hnc cra incrass. At high dilutin and lw slids flw rats rductin in cr/ is mr cmpard t rductin in t as slids flw incr,,:ss. At high dilutin and high slids flw rats, cra incrass bcaus cr/ incrass and t dcrass. Pydi Stty t al. 8 rprtd a dcras in slids mixing with incras in slids flw rat in a spiral fluidisd bd. Owing t th spiral gmtry f th bd, th slids hav mr dirctinal mvmnt which lads t mr diffusinal mvmnt f slids. cr was fund t incras with incras in gas flw rat which is in gd agrmnt with arlir wrk 8. Incras in mixing with incras in gas flw rat is du t incrasd htrgnity bcaus f mr bubbl phas, which incrass sprad in rsidnc tim. ( b) SAMPLE: E' Gf : 1-3 Q()7 LEGEND G v c , \100.c. 4 6 Fi g. 3b--Effct f slids rat n RTD f slids at high dilutin With incras in dilutin, sprad f rsidnc tim was fund t dcras. Chn and Pi wrkd n a fluidisd bd using a binary slid mixtur whr a fractin f cars particls wr addd t a bd f fin particls. It was ntd that such additin imprvs th prfrmanc f th bd and with incras in dilutin, hat trasnfr cfficint incrass. Im-

5 104 INDIAN J CHEM. TECHNOL., MARCH 1999 ' 0 (a) 1.0', , (a) ' <D 0 60 Gs =O 4579 LL Gf::, LEGE ND SAMPlE B V C D 0 E A F 0 G 3 4 () ' SAMPLE : B <>.. 0 '6868 Gf l 1919 NO OF TANKS: 4'053 R M S DEVIATION : A EXPERIMENTAL ' MODEL 3 4 ( b) 1 00 (bl 0,80 G s :: Gf= ' 1919 LEGEND SAMPLE B <:J C D 0 E A F 0 G SAMPLE : C Ga : G<J : N OF TANKS : 3-0 R M S. DEVIATION : C.005 A EXPERIMENTAL MODEL O. OO~'----L-_-_L----L---...J (d 1 0 tj. EXPERIMENTAL MODEL Fig. 4-Effcct f dilutin n RTD f slids prvcmnt was als bsrvd in th study' whr slids mixing rducs with incras in dilutin thus rducing th sprad in slids rsidnc tim in th cmplt rang f dilutin up t 50 %. Th RTD curvs at varius xprimntal cnditins wr als cmpard fr dimnsinlss third cntral mmnt. It was fund that th curvs gt mr skwd as gas n w rat incrass fr th cmplt ~ () SA MPLE : G G. : Gt: N OF TANKS '. 6'~ R M S OEVIATION Fig. 5-Cmparisn f th :-.primntal data with th Mdl 9 3

6 CHAPAOGAONKAR ('I al. RESIDENCE TIME DISTRIBUTION OF SOLIDS IN A FLUIDISED BED 105 rang f xprimntatin. With incras in slids flw rat, thr is n ffct n th skwnss f RID curvs at lw slids flw rat, but it incrass at high slids flw rat du t incrasd randmnss f slids mvmnt. Th third cntral mmnt, skwnss dcrass with incras in dilutin. This is als du t. rducd slids mixing as dilutin incrass. This indicats that th grss circulatin pattrn and randm mvmnt f particls dcrass and flw bcms mr and mr diffusin rintd as dilutin incrass. Mdlling f RTD curvs Th fractinal tank xtnsin mdl I I has bn fittd t th prsnt xprimntal data. Th mdl paramtr, N, th numbr f tanks fr diffrnt xprimntal cnditins has bn dtrmind. Th paramtr, N was valuatd fr ach xprimnt by minimizing th rt man squar dviatin btwn th xprimntal and mdl F() valus at ach intrval f tim fr ach xprimntal run. RMS dviatin is givn by: M 0" = [(1 / M) L {(Fk )xptl - (Fk )mdl} f... (6) k=1 Th dviatin is minimizd. with rspct t th paramtr using th Fibnacci sarch mthd. Th numbr f tanks is fund t dcras with incras in gas flw rat. This is du t incrasd slids mixing which rducs numbr ftanks. At lw dilutin, N dcrass with incras in Gs, whil at high dilutin, N shws a maximum. It incrass up t 0%. Abv 30% dilutin N incrass at lw Gs and dcrass at high Gs indicating diffusinal mvmnt at lw slids flw rats and randm mvmnt at high slids flw rat, which was als bsrvd frm th valus f O" fr highr dilutin f fin siz. Numbr f tanks, N, incras with incras in dilutin at any slids flw rat r gas flw rat, an bsrvatin f imprtanc t rduc th sprad f rsidnc tims withut using any intrnal in a singl stag fluidisd bd. Incrasing dilutin Incrass plugflw tndncy fr th bd matrial. Cnclusin In th prsnt wrk, it was fund that, slids mixing incrass with incras in gas flw rat. At lw dilutins, slids mixing incrass with incras in slids flw rat but at highr dilutin, it dcrass initially and thn incrass with slids flw rat. Sinc, in practic, th actual matrial will b a mixtur rathr than a matrial f unifrm siz, th prsnt wrk finds imprtanc practically. Nmnclatur A = crss-sctinal ara f th clumn, m c = cncntratin f th tracr, kg tracr/kg matrial C = rsidn:: tim dnsity functin d p = particl diamtr, mm Fc = rsidnc tim distributin functin frm th xprimnt FM = rsidnc tim distributin functin frm th mdl G s = mass vlcity f slids, kg/m - s G f = mass vlcity f air, kg/m - s II = dwn cmr wir hight, m k = indx f summatin M = numbr f intrvals N = quivalnt numbr f idal stags /1P s = prssur drp du t slids, mm t = tim, s t = man rsidnc tim, s tk = tim crrspnding t. K' th tim intrval, s T = ambi nt tmpratur, 0 C w = hldup f slids, kg B= dimnsinlss ti m (= tl i) J.1 '3 = dimnsinal third cntral mmnt f th RTD curv, s, V3 = dimnsinlss third cntral mmnt f th RTD curv ( = 11'/ i -' ) Ps = dnsity f s lids, kg I m; a/ = dimnsinal vari anc f th RTD curv, S a = dimnsin lss varianc f th RTD curv ( = a/ I i ) FTEM = fractinal tank xtnsin mdl RMS = rt man sq uar dviatin RTD = rsidnc tim di stributin Rfrncs I Abuz id A Z M A, Mika T S, Sastry K V S & Furstnau D W, POII'dr Tch II 01, 10 ( 1974 ) 73 Chn P & Pi D C T, Call J Clilll Eng, 6 (4) (1984) Kri shnaiah K, Pydi Stty Y & Varma Y B G, Chm Eng Sci,37 (9) ( 198) Lvnspil 0, Chlllical Ractill Ellginring, nd d. (Wi ly Eastrn Limitd, Nw Dlhi), MOD'is D R, Gubbi ns K E & Watkins S B, Trans fnst Chm Ellg, 4 (1964) T 33 6 auman E B & Buffh am B A, Mixillg ill COlltilllllis FllV,~)'S I (, I/I S ( Jh n Wil y & Sns, Nw Y rk ), Pyd i Stty Y, Axial Mixillg j Slids in Spiral Fluidisd Bds, Ph. D Thsis, Ind ian In stitut f Tchnlgy, Madras,

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