trouble free scaleup of chemical reactions

8/6/2019 Trouble Free Scaleup of Chemical Reactions

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Doble MukeshGE India Technology Centre Pvt. Ltd

ncc thc viabilitv 01'a nurv l)roccssis e-.tablished at thc gram lcvciin tlre laboraton,. the proccssundergoes various scaleup stepsbefore it reaches the manufacturingplant. Because the capital investmentrequired for tran-qforming a raw mate-rial to a finiShed product i,s propor-tir-rnal t0 ther caltacitv of'tiic planlrliistil t o thi' i) 6 -0.; I)o\\-('r. it r. rlcsu'ebic trl itrrlrl iis lrrllc ii plnnt as is t:on-srst,cnt rr,ith tht- cxpecttrci futrrrc tlar-kct fol the product [1].

-st:aler.rll rs thus a criticai activityduring the development of a chen-ricalproccss. This article {bcuse-s main}v onst:alcup of thc rcur:tion stcyt, rvhichn'pir:ailv represer-rts lhr: ireal't r)f lr ncu'J)r'{)c(rss. Thr: articlc as-st:mblcs a ri'idtrangc of practical rcaction-scaleupguidelincs that previousll' havc beendilfuserd throughout the en$neeringlitr:raturt'. Included are the key ele-rntnts in developing :r scaleup -slrat-r,rr'. and ti'rc unknori.ns tirat thc r:ngi-lrr,r,r nlust atldrcss carlv in thc scaleult, il,rrt i: ,;i .l',:,tr',i iillclttio jt is iiivi nir, scalrLtps irrvrrlving lgtllrl iolt tincllliring. b:caus{' thcr rilLt,r-r p1'cs('nL:r'rr cial chlrllcngcs [,']1.Intelirgcnt sr:aleup cntails finrlingrind coliating all the infornration andrii'tii r^t:rlr.rirld lbr tlrr: dc'sign. cor-rst.r'trc:-i;rin. :inrl siarlr.lp ol lhc nttrv full-srzt:irjrrri'. ii tlu.ii rtlso t;tkr'into lir,:('orrnt

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A chemical-reaction step is II;tr*\;flowsheet for a new procls5i.':* iiTaking that rsaction from th6,

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safctl' and hazar4 issues, as '*'cll asthe irandling of rnaterials on a largescale [61.Keep in mind that there is usually aknowledge gap between benchchemists and process chemists on theone hanC, and chemical engineers andchemical technologists on the other. Ifthe scaleup activitY cannot success-{ull.r' I'rridge L}ris ga;t. t}rc rcsult-" in theltrli-scalc plant rnay inclucle a reduccdf ield, longer batch c1'clc Limes, great,cranounls of effluents or undesirablebyproducts, unneccssarily hazardousoperations, or unduly high consump-tion ofenergy.Regardless of the typer of proces-sinvolvcrd, the problems of scaleup arehighil' sinrilar. Accordingjy, the infor-mation presented in this articleshould apply to virtually all of thechernical process industries, includ-ing such seemingly diverse activitiesas biochemical s'r'ntheses, productionof paper pulp, and processing ofmetal orL's.Iinori'lccigcnirle rcaction scalcup en-cr()r'n])asses sevr-:rirl ar-ipects o{- cht't-n-istn'. cl-icnrrgal t;ngiri,:r'r'ing and flrrirlnrcrchnlricrs. I^rsur.s lrrising fronr kinct-ics, themodvnnnrics and hi'drodv-nanric-. alfect lhe sclection of the herstt1'pe of reactor, it.s intc.rnal design anriits operat.ing rr:gimt:. Thcsc, in turn,x'ill clictatc reilct,or f icid, dergree of

convcrsion, distribution betri'et'n tirt,'main product and byproducts. lrcat ef -fects, florv patterns, backnixing. gas-liqtrid-solid interaction and rssucs in-volving process controi.THE PLANT IS NOTTHE LABORATORYScveral diflerences exist betu'et'n Liit,,w:lv ?r proccss is developed in tlrc irib,r-ratorv ar-rd the rr'av it is carrierci r.rr-rt irithc lrlant. As a sirnple exanrple. ir lerilc-tion carried out in a rouncl-ltotl onrflask in the research laboralon, ar .rvery high speed of agitation might givlentirely different yields in a plant'svcrticai-sided kettle outfitted rr'ith nslolr'lv revolving mixer. Among thi:reasons are differences in itatch cvclt:tinic, the degree of agitation. rvall t'f-fect-s, and tl-re surface-to-r'olume ratit-rof the container. We focus successrvclr'on several such issues.Batch cycle timeTi're overall processing time inciudersnot onlv the rcaction tinti.i-rrtr st,. lrrrlalsn thc t,inrc nr-.cded frrr foecling tlr,rl*r' nlrl.('r'rals, hcating thr' rll('lrutl>,cooling titc lrrrrcluct,s, rrncl tii:r'irrrl'r. rr.rand clcraning. Thesc time rciluir'()r'n(,nisare generaill' negligrbie in tlrt' labolllory:. [.tt, nia\' bc on the orcier o{'.*e-r'crir]hours in thc full-scale plant.For cxample, it t1picail1' takc,s aborrl

46 3;iII",1ICi..i- ENGII']iI-RIN'i V/WVJ CHE COI/ JANUARY 2OO2


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in moreas weil detail in theas later in the nextarticle.

ratioratio of surface area to volume de-as a reactor vessel is scaledFor instance, for a spherical ves-a r.olumetric scaieup factor ofri'ould mean a reduction in sur-ratio by a factorThcrcfore, to input or dissipate theamount of heat, the heat-trans-r i'atc must be increased in propor-i.r tire. rise in vessel capacity. Thiscan be achieved by providingheat-transfer area or usingsher heating-medium temperatures.the latter tactic can lead to

ski.n temperature and local-which may raisehazards or lead to the formationtar-n' or otheru'ise polymericAnother approach is to de-the reaction rate so as to nratchnraximum possible available heat-or" cooling rate, but this optiona longer processing time.

and mixingaleup of mixing operations involvesx"rdr: rang'e of issues. First, rve set..tage ivith some basic points.in the first place, agitation must, asi)nlctlcal matter, differ rvith reactor;rlr, For instance, if we instead in--ir,,cl on maintaining the same

crrculation time in the full-, :r jt: \'.:SSl as rve have in a srnallerr r. \\'i. r','ould liave to have highcr ve-

rr ri,s. because the liquid n'ouid havetravei a greater distance. Accord-. . i ; Ii1. f irr\\'e r requ irctnt,nt 1_rel unit:un,,ol.r the fuil scale g'ould have to'rl; in prcportion to the square of' ril;rnrcter of the tank.ir,rrt -satisfi'ing that requirenrent is' lrnr.rrilr not feasjblt. economically.,:' r,siilrpi. iithc po\\'er per unit vol-

':'r r,\\'r.rl'{:, tO }tr: kept COnSlant befrveen;,-;:ril pilot plant vessel and a 625-

lh to fill a 10-kI vessel via a 5-pipe. Cooling the contents

rr lLil-scaie reactor could requirethe order of 1-2 h. Such ex-times may posefor temperature-sensi-Heal-transfer is-relaled to scaleup are dis-

TABLE 1. vA[uES OF EXPONENT,FOR THE IMPEILER.SPEED EOUATION, f,,IN SCALEUP BY GEOMETRIC SIMII.ARIW

scale mixing. For instance, for dispersing a gas into a liquid, the suggested ratio of impelier diameter trtank diameter is 0.25; for bringinltwo liquids into contact (as in liquidJiquid extraction), the figure i0.4; and for blending of low-viscosity liquids, it is equal to or greate

For equol liquid motion or conslontFroude number, ogiiotor-tip speed,sheor rote or grovitotionolefiects, D = IFor equolsolid suspension, n = 314For equol moss-tronsler rotes or powelper unil volume, n = 213Forequolsurfocomotion, n = l12Fot equolblend lime, n = 0For constont Reynolds number orconstont hydrodynomic similority, n = 2.For conslont pumping copocity, D = 3

gal, fuil-scale vessel, then the powerwould have to be increased by a factor 'of I25, which is unrealistic. Similarly,if one rvanted to maintain the sameagitator tip velocity during scaleup, itwould be necessary to raise the agita-tor power by a factor of 25 [7], which islikewise impractical.So, instead, the elapsed liquid-cir-culation time in a full-scale vessel isusually longer than in a small one.The question arises as to whether thelonger mixing period affects the reac-tion process. A good test for the sensi-tivity of the reaction to the elapsedbulk mixing time is to perform the re-action in a geometrically scaled downmodel of the full-scale vessel, with theagitation speed the same as in thefull-scale one. The results obtained atthese conditions are a good indicationof the fuil-scale performance.Here is another example regardingthe difference between small-scale andfull-scale agitation. In the laboratory,one sometimes agitates the flask (orother vessel) itself, rather than insert-ing a mixer per se. In fermentationstudies, for instance, most screeningsare done in "shake-flasks" mounted onrotational, shaking or rocking devices.There is no relationship between the"pumping" capacities and shear ratesachieved within these flasks and thecorresponding capacities and rates ex-perienced in a fuil-scale fermenter out-fitted with a conventional agitator.In short, it is unrealistic or impossi-bie to make all the mixing parametersof the full-scale vessel equal to the in-dividual fluid-mixing and fluid-me-chanics variabie in a small-scale tank.The eng:neer responsible for scaleupmust accept this as a fact of life, whichis to be kept in tnind when appiyinglhe purely mathematical scaleup rela-tronships presented below.Certain practical guidelines havebecomc common as regards relativeimpeller and tank diameters in full-

CHEMICAL ENGINEERING WWW.CHE.COM JANUARY 2OO2 47

, DuE,-ter tonglngn liq-.rre isiscos-'eaterthan 0.6.Similarly, operations that depend

on large velocity gradients but low cir-culation rates, such as gas dispersion,are accomplished at full scale withhigh-speed small-diameter impeilers.Operations that require high circula-tion rates are best done with large-di-ameter, slow-moving impellers.As an approximation relevant forlow-viscosity (waterlike) iiquids, itcan be said that a mixer-power inputof 0.5 to t hp per 1,000 gal of liquidgives "mild" agitation, 2 to 3 hp per1,000 gal gives "vigorous" agitation,and 4 to 10 hp per 1,000 gal gives "in-tense' agitation (as might be neededduring emulsification, for instance).These figures refer to the power thatis actually delivered to the iiquid anddo not include friction and gear losses.Scaleup rules: At least three differ-ent kinds of scaleup rules for mixinghave been published. Summar)' de-scriptions of them are as follorvs, rvitirmore details available from the refer-ences cited:Scaleup based an similarity concept[8]: In this approach, geometric simi-larity fixes the ratio of various lengthswithin the system. These lengths in-clude impeller diameter D, tank diam-eter T, blade width b and liquid levelH. During scaleup (for instance. inscaling up Diameter D 1 to D) under'this concept, the lengths are chosensuch that the "before and after" ratio,R, is the same for each pair of lengths:D2/D t = Tz/T t = bzlb t = HzlH t = R

The impeller rotational speeds, ly';,of the large vessel and the small ves-sel are related as foliolvs:I'z = l{,'( llR)nBased on theoretical and empiricalanalyses, suggested values of the ex-ponent, n, for different types of agita-tion situations appear in Table 1.Scaleup based. on friction latu of corre-sponding states I9l: Under this theory,


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all mixing situations in the laminarregime are characterized in such arvay that the power number (a dimen-sionless variable that relates impellerpower to other process variables, suchas N, D and liquid density p) remainsinversely proportional to the Reynoldsnumber. (Under turbulent conditions,the power number is instead con-stant.) The relevant Reynolds numberis Duplpt,, where z is the tip velocity(equal to AD) and pr is the iiquid vis-cositl'. All the scaleup transitions arecharacterized rvith reference to thisreference rel adionship.In applyrng this analysis, the flowregin're is held fixed, whereas the val-ues of N, D, and the power input, P,are determined simultaneously. Tomaintain flou' similarity (or the samedegree of ,nixing), rvith a given im-peller geometr.v, the parameters arerelated as fojiows:.^y' varies as U(D|T)2P varies as Nv2 and I/(D/T)PIQ varies as I|(D/T)2n'here A represents the flowratet}'rrough the impeller.St:aleup basecl ort agitati.on intensitt,l/01. Thc nrore intense the agitation ofthe contents of a vessel, the fastert]'rose contents become turned over.The vessel turnover rate, which is de-fined as QN rvhere V is the vessel vol-ume. can therefore serve as a criterionfor tire intcnsit-"- of agitation. This ap-proach en-rploys an agitation number,rV7. rvhich i-s scaled between 1 to 10 forrlverage btrlk-fluid velocities of 6 to 60It/min (in other rvords, mild to violentagitation ), respectively.For s1'stems that are geometricallysirnilar. the impeller tip speed, ND, oflhe scaled-up vessel equals3.S:r2lrttl(DlT\1.5, and the torque perunil volume is constant if A'iDl =ItzD,:. Accordinglr', torque per unitvolume equals 3.27 Nppl'{ fllg,(HlT)1,where Np is the power number and g.is the conventional conversion factorbetu'een force uni[s. And the vesse]turnover rate. Q^', equals i0.8.\'//l fr,'.t, I] {f P.:'.r1. .Pou'er requirements: For a scaled-up reactor, tiie po\\'er requirementper tinit volurne, PN, can be deter-mrned from the agitator speed. Forttrrbulent conditions, PN is propor-48 CHEI'"4ICAL ENGINEERING WWW.CHE.COM JANUARY 2OO2

tional to AFD2; for laminar condi-tions, it is proportional to N2. IAccordingly, if the power per unitvolume is to be held constant during ascaleup, then N2 equals N/D1lD)a3under turbulent conditions. And underlaminar conditions, N2 equals N7.Scaleup u:ith gas-liquid mixing: Inspecifying rnixing equipment, a majorprinciple is that. the impeller blademust be two or three times larger thanthe largest bubble, particle or fluid"clump" that is of significance to theprocess. So, when scaling up gas-liq-uid systems, be aware that a largevessel tends to have a wider bubble-size distribution than a smaller vessel[,l/]. Another consideration is that thesize of the bubbles should not exceedone-half to one-third of the verticalheight of the impeiler blade.In the presence of gas bubbles, therequired blend time for the continuousphase increases. Accordingly, the per-centage of gas holdup is an importantfactor in scaleup. The linear superfi-cial gas velocity increases on scaleup,and there is usually a greater volumeof gas hold-up in big tanks than insmall tanks. So the larger-scale sys-tem entails longer blend times.The impeller rotational speecl, N,required for completely dispersing agas in a flat-blade turbine assemblycan be found from the relationshipN = QeTgo.tp.2D4rvhere Q, is the gas florvrate and g isthe acceleration due to gravity. Thisequation indicates that the rotationalspeed required is directly proportionalto the square root of the gas flowrateand tank diameter, and inversely pro-portional to the square of the agitatordiameter. Of course, the power re-quirement for agitation decreaseswith the increased presence of gas.Solid suspension.. In a suspended-solids system in which the solids ac-count for up to about 30Vc of the totalrveigl'rt, scaleup of the mixer thatkeeps the solids in suspension posesno problem 1121. On the other hand,concentrated slurries start behavinglikc viscous pseudo-plastic material,rvhich complicates the scaleup task.The minimum impeller rotationalspeed required for suspending solids,Nrp^, is proportional to d00.2 1D0.85,

where do is the particle diameter, andthe power required per unit volume ofthe liquid is proportional 16 p-{.55.Liquid-liquid emulsion In scalingup Iiquid-liquid-emulsion systems,keep in mind that the shear-rate para-meters (viscosity and surface tension)'affect emulsioir formation. Both theblend time and the standard deviationof the circulation times for the individ-ual droplets influence the size of theemulsion droplets. The average emui-sion droplet size is proportional toN-2.56D4.17?1.88, and to achieve uni_form dispersion U3l, N should be pro-portional 1o 2-2'15.The position of the agitator plavs acrucial role in the formation of liquid-liquid emulsions. To disperse higher-density liquid into a iower-density liq-uid the agitator has to be located inthe latter, and vice-versa,Blending: If two or more liquids witirreiatively low viscosity are beingblended, so that the process is not af-fected by liquid shear rates, the differ-ence in blend time and circulation be-tween small and large tank is usuallvthe only scaleup factor involved. Hou'-ever, if there are large differences indensity or surface tension between theliquids, then extra shear forces (and.thus, more input power to the mixer-)are required to achieve the ultimat,elevei of uniformity.During blending, there is a relativeincrease in Reynolds number ri'ithscaleup, regardless of the relativemagnitudes of the incoming-fluidproperties. The time needed for blend-ing, l, is proportional Lo TH|ND2. Andin geometrically similar tanks. / isproportional to 1/l/.Agitation plus heat transferIn the scaiing-up of manv homoge-neous, stirred systems, the rate ofheat transfer controls the design ofthe heat-transfer equipment. In suchsystems, h is proportional 16 Jrnrlnr-1.rvhere /i is a heiit-tlansfer cor,l'flcient, u is the fluid circulation veloc-it1',.d is the characteristic dintcnsirrnof bhe system (for instance. the agitrr-tor diameter) and m is an entpiri-cally or experimentally deterniineCscaleup exponent whose value is gen-erally around 0.67.A scaleup based on holding the


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ITABLE 2. EXAMPLES OF INNOVATIVE REACTOR DESIGNFOR SCAtE.UPS

Process lssues Smoll scolesetup Scoled up versioni. Cotolylic oxidotion ofbenzene lo moleic onhydride . Highly exothermic. Needs efficienl heol removol U-tubeconverter Chombered multi-tubereoctor with cooling iockei (l9J2. Pencillin by fermentotion . Mold growth smolhers orgonism. Need conirolled surfqce growth Petd dish Submerged fermenlotion lechnology,lor mold lo grow submerged in theliquid food source (l9J3 Methyl lerl-butyl ether fromisobulene ond melhonol wilhion-exchonge cololysl

.Exothermic'Cololysl deoclivotion obove I '10"C.Swelling of cololyst

StinedvesselFixed bed

Fluidized-bed reoclor to overcomehot spots ond occounl for swellingta anl

4. TAX (l -ocetylhexohydro-3,5dinilro- 1 ,3,5 triozine). Exothermic. Eosily delonoble moleriol Slirredvessel Low-dio. tubulor reoctor with coolingiockel (fost heol removol, ond omounof delonoble moteriol is smoll) (21)5. Sevofluorone (non-explosive,inholing onoeslhetic) from

h e x of luoroisoproponol,HCHO, onhydrous HF, oleumExothermicDetonobleLow volotility ol producl ondqos releose

Stinedvessel Plug-flow reoclor wilh cooling iocket,slightly lilled upwords from horizontqlto ollow gos escope f2lJ5 Sullolion ol mixed lollow-derived olkonolomides withchlorosulfonic ocid(a phormoceulicol product)

. Highly viscous.Gos releose.Connol use onlifooming ogentsStirredvessel Slotic mixer reoclor (21)

7. Hydoxyl ommoniumphosphole fromN HaNOr+H3PO6+Pd+H2

.Cololysi ottrition due lo ogilotion.Gos leok ol ihe glonds due tohigh pressureCSTR(see mointexl)

Continuous, spqrged bubble column(2t)Aulo-oxidotion ol corene withhomogeneous cololyst . Exothermic. Low gos-fo-liquid moss tronsfer Gos-sporgedstirred vessel

Pocked column {or gqs-liquidconloct ond exlernol heot exchongerin lhe recirculolion loop. (22'j'?. Suspension polymerizoiionformed by feeding inilioledrnonomers (micron-sizedpolymer dispersion formed!n non-oqueous medium)

. Lumps of polymersformed, needing inline productfiller in lorger scoleSemi-bolchslirredreoclor

Locote monomer feedinlet pipes directly ol the impellerzone to improve ogilotion (23)

3. Suspension polymerizotion lmproper ogitolor design leoding lolormotion of woter droplets in mono-mer (insteod of monomer in woter),cousing o decreose in heot trqns-fer coefficienl form 50 lo2 Btu/(h)(fiz)('F)

Semi botchsiinedreociorSecond impeller in theupper woler loyer (2j)

Trcnseslerficotion ol fottyccid ond sorbilol, loproduce surfoclonlReduced yieldFormotion of oleic ocid droplets insorbilol, insteod of sorbilol in oleicocid os desired

StirredbotchreoctorRoising ihe impeller lolocole il in lhe oleic ocid phose

:i rIirlrsii,r ctielficient c0nstant is- : rlr i,lt in cascs rvhere pilot plant; ii,.- inclic:rle that tiic heat-triinsfcrr-t;itt,',, ilt, tt'ltst's ::l()\\'l\' {)\'cf il'. ilr:r 1rr. lirl instlncr'. thc firruta-: rri lltrlr lttcrtc or lat'rv dcPosiLs::.l t.hr heat lran,sler are:r. (Onc- -.i'i, ruLrsc of' snch dcposits is arii/,r';liulo sr.lnsilir-itl in the mater-'l'l.,rt-'. 1i,r lLrt agitatcrl svsl:cnt.' . D-i l), 'r I rr' '/'r,1.,i .. ,j t.,:l) .1 i ! " "i

;.,.1\\'t,r I)i,r unit voluntt tn Strcrlt ar',1 rvrii.s;1ll Lrit rrs lollorvs;, [',ii,-: . -. D,l'-.trt.t!!t] f I) tt:l-'1tn\itrl'' ";.; '. ', t'til.'l','rr'''l pt't'rttiil, i,,,r,,,111'. :(.irlr,(l LII) ilt ltrrllti'r'ii, .,:r{'llttltl l.r,r'.\'r.l' (ll tirt irgi-'i:,rt-'r,1, l' i,:;1 '. i' rnf,'r'.rlrtt0-:, tlsilirr'

material, it is olten considered essen-rial to assurc that lhe same rabe ofhcat transfer is maintained in tl'rclalgt:-sc:rlo urijt. t,c, avoiil r-nirtct'itrldt:gr:rclation. l'rtr Lhrrsc cascs, lr2l/r1rtrtrsl eqrlal D2lD j, rv[ic]r me:rrts Lhat,for geometrically similar vessels,Nc/.ly'7 equals (D clD rlr2'2nttt m .fiervevcr, this latter relatior.rship in-dicates thiit the inrpcllcr rotationalspecd in the largcr r-init should bc,.qtu'l to {rl {r('.ttcr- tltar tliat rt thr'srniil.I-st:l l': urr rt i clcpr:ncli n g ulton thr:i alrrt' oi lt ). J'huL stipulirtron is con-lrarl' lo nornral practice - gcnt:rallv,irgita tol s peecl ciecrreases rvi t.h st:aIeup.Furt,irermore, such a scaleup principleis not vr:n' practical (even u'hcrr con-sidclt:ri esscntial ). ber:ausr: lltc grc:rlcr'1)l' r'riirtl lr)tirti,)lral spt'r,d irr thc,sclrlt,rl'up vlssr,l, along * itir thr,::i r.lltlr agitet,;r riiarrrr:tcr. lcads to n0-tablv high po\r'er costs.

Scaleup can instead be baserd onmainlaining a const,ant imptrller-tipslrt:cd irr thc agitated vessel. In lhetcasr-'. /r,/h 7 cqual-" (DelD t)"r-t .1'his a1r-proach n'ray be rclcvant wl'ren bubblebreakagc is an issue.Maintaining dr.namic sjrnilaritltlnd(')' tulhulettt flrlu rcqrtir,,s c,'n--qtant, values ol the Retrrold,q nunrircr'at Lire inrpcllel ttp. Sut:h sintrlalitr'..,vlriclr i-.. ibr rnst,ancc. usclirl rr-it lrn()n-\\-\\'Ioniill] flLrid-.. gu.rriitit(,r.:-.inrilur nrappirlg-q (fron small toscirlcd-u1l vc-sselsi of lht: r'atios rrl localto averagc rate-q olencrg-'- clissiJration.As a rtrstrlL, A-.2/AIi ttquals \,D ti Dri-.PUTTING THE PRINCIPLESTO WORKil,.':, iclt's l pplr-r n g r.l rc grlrrrrr l r',.1 r t ir,n.lr t 1t. .j' 151 ri rscrrssr-'1. I irc cr rgrrr,'t.r' tlr r'-n'ing orrt a -scaleulr must. aiscr kcep inrnind a varict.v of'l.rractical considera-

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tions. Many pertain to specific reac-tors (see below). Others are morewidely applicable:. Botch uersus continuous opera-fron: Although the laboratory rvorkmay have taken place on a batchscale, a continuous reactor (such as astirred-tank design) is iikely to besuperior for high-throughout full-scale processing, especially in casesinvoiving a single reaction step andrequiring only a short residence timeo Temperatures, concentrations,residence tim-es: Temperaturc andconccntration gradients, as rveli asresidence-time distributions, willnot necessarily be the same on alarger scale. The differences mayadversely affect the yield and prod-uct purity. Such considerations maylead to the selection of rsothermaloperation for the scaled-up plant. Prouision for heat transfer: Theengineer must decide between usingjacketing or a coil for the heat addi-tion or removal. Coils are attractivervhen a high heat-transfer area orcoefficient is important; conversely,coils should be avoided and jacketsused rvhen, for instance, the processliquid is highl"v viscous or prone tocause fouling, or contains a highpercentage of solidst Reactant purity, and once-through uersus recycle: Take carethat in the full-scale plant, the reac-tants entering the vessel are pureenough to avoid poisoning the cata-l1,st, or fouling the catalyst bed orheat-transfer surfaces. In some sys-tems, similar concerns may arisewith using recycle rather than once-through operation. The catalyst: In a full-scale plant, an'ide range of catalyst-handling is-sue-s can arisc that are not necessar-ilv olrvrous at Lirc laboraton' ,sc:tlebnt can afferct the catalv-st activity.lif'e and regenerability. Exarnples in-clude the catal5ust shape and dirnen-sion-s (for heterogeneous catalysis),anil ]ro*' the catalysl is prepared.Also to be considered i-s horv thc cat-alvst is charged and di-"chargecl. Stirred-tanh ler.su.s tubul.ar re-actor: ])er unit of volrrmc, a full-scale strrred tank is less expensivethan a tubular (nominally plug-flor,r')reactor. Horvever, the choice is not a

simple one. And when series or com-plex reactions are carried out, theproduct distributions for the two op-tions differ. If a tubular reactor ischosen, its diameter affects the ra-dial and axial temperature gradi-ents, which can introduce heat-transfer and flow-distributionchallengesScaling up frxed-bed reactorsFor the scaleup of a fixed-bed catalyticreactor in which a first-order reactionis taking place under isothermal con-ditions, proper scalcup demands thatcatalyst particle size and bed depth(or height) be kept constant while thecross-section is multiplied by thescaleup factor.In the laboratory and pilot plant,fixed-bed reactors operate under plug-flow conditions (ust as stirred-tankreactors operate under weil-mixedisothermai conditions). But it will notbe so in the full-scale plant. The tem-perature and concentration gradientscan never be made to be uniform,which leads to a difference in effectiveheat conductivity.For constant fluid velocity; the reac-tor throughput increases as the squareof diameter of the tube. But with theincrease in diameter during scaleup,temperature-distribution effects wiilbe more pronounced. With exothermicreactions, the center of the tube willexperience the highest temperatures;with endothermic reactions, the tem-perature there will be the lowest. Theresulting radial temperature gradi-ents, like hot spots, can cause loss ofselectivity, coking or fouling in the cat-alyst bed, and, sometimes, mnawayconditions that may lead to unsafe sit-uations or destroy the bed.In pilot plants, small catalyst parti-cics are generally preferred; for onething, they minimize mass-transfereffects when developing the rate equa-tions. But for full-scale plants, theusual tendency is to opt for larger par-ticles, to reduce pressure drop acrossthe bed. The engineer must ke'ep inn'rind that the los'cred prc-ssurer dropconles at the cost ofa lou'ered reactionrate, due to intra-pellet heat- andmass-transfer resistances. The actir'-ity or effectiveness of larger pelletsmust be predicted beforehand.

Fluidized-bed reactorsBecause the circulation of the catalystparticles minimizes the temperatureand concentration gradients, thechoice of a fluidized-bed reactor side-steps the problems, just discussed,that are inherent in the scaleup ofpacked-bed reactors. On the othe'rhand, the effect of the carrier-fluid ve-locity, and of the physical properties ofthe catalyst, upon bed activity, bed ex-pansion and catalyst attrition mustfirst be studied thoroughly.When one scales up a fluidized-bedreactor, it is not suf{icient solely to un-derstand the effect of increasing thevessel size _- the interaction of thesolid catalyst with the fluid alsochanges. A ruie of hydrodynamic simi-larity for a scaie change in fluidizedbeds has been developed, based on thegoverning equations of bubbie and in-terstitial gas dynamics []41.In addition to the maintaining ofgeometric similanty, scaleup of a flu-idized bed requires maintaining h1,-drodynamic similarity. This lattersimilarity rule requires that the foi-iowing trvo conditions be satisfied:(U o - U *l = mu2(tJ u - U rriiandU^f = mu2(U ^)iwhere Uo is the superficial gas veioc-ity, U6 is the minimum veiocity re-quired for fluidization, nL is thescaleup factor, and Subscript i repre-sents the base-case (before scaleup)condition. The first condition assuresgeometric similarity in bubble coales-cence; the second assures the geontet-ric similarity in the flou,field.These two conditions can be thoughtof the necessary and sufficient, condi-tions for hydrodynamic similaritv. F'orscaleup and development probk'nis. itis suggested that both the hydrodr'-namic-similarity rule and the identitr'rule, described in the next section, bcapplied.Identity rule: The identitl, r'ule dcalsonly with increasing the diameter of afluidized bed. It is especiailv useftrl f'osludying chemical reactions; that isfor cases in rvhich the fluidize'd parti-cles consist of catalyst.According to this rule, the catall'st-particle size, bed height and gas ve

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TABLE 3. SOME ISSUES ARISING DURING REACTOR SCAIE.UPProcess Setup lssues Observqlions during scole-upi. Polybutyleneterephtholotefrom bulonediolond cotolyst

Stinedvessel. High eslerificotion(polymeizotion) rqte will. Heol buildup in loler stogesleods lo polymerdegrodotion

. High heoting role ochlevoble ln smollscole, not in lorge scole, due less heot-lronsfer oreoper unit of reoctor volume. High ogilolor tip speed in lorge scolegives more of degroded producl (24)2. Bioreoctors in wosle-woler lreotment Airliflreoclor . Mixing. O2-lronsfer tote lo liquid k1o does not depend on reoctor size but is proportionolto U,ro. no* 0.86 (25)3. Rodicol-initioted

suspensionpolymerizotion ofstyrene with butylmethocrylole

Slinedvessel Moleculor weight ondpolydispersionPorticle sizeAuto-occelerqlionbehqvior

.lncreosing reoctor size leods to- increose in moleculor weighl ond polydispersion- increose in initiotor quontity to mqinloin somemoleculor weight- longer time needed to ochieve oulo-occelerotion(26)4. Suspensionpolymerizolionof vinyl chloride

Stirredtonk . Porticle size. Solid suspension . Size vories with rpm -0.e, height-0'I3, ogitotor dio. -0.e. Minimum rpm vories with ogitotor dio. -l.l(27)5 Finol slep to vitomin 86(diozotizotion of nitroto omino compound)Stirredlonk . Drop in yield . Lorger liquid surfoce leods lo higher liquidevoporotion rote ond chonge in concentrotion(28)

,5 I'Jeutrolizing on esterprepored in dilule,olkoline, olcoholicsolution using ocidSlirredlonk . Excess of hydolyzedside product . Longel oddition time of ocid leods to reoctionbetween woler ond ester

/ lieutrolizotion o{ lobileester using NoOH

Slirredlonk

Bose-coiolyzed decom-position of side producl

Longer oddition time of olkoli leods lo decom-posed product, due io high concenlrolion grodienlsUse of No2CO3 os neulrolizing ogent ond increose inogilolor rpm, to decreose pH grodienlsAddition of olkoli to oqueous-rich porfion leods lodilution [28)

3. Biochemicol reoctionusing onimolcells Slirredlonk Teoring of bio-orgonism. Use of shoker ogitotion poses no problem in smollscole. High tip speed in ogitoted vessels leod to teoring ondbreokup of cells

9. Production ofsurfoclonl using oirfor oxidotionStirredbotchwith oirbubbting

FoomExcessive heot generotionlncreosed botch time alncreose in viscosity during reoction prevented N2 inoir frorir escoping; use of puie 02 obvioted this prob-lemDecreose in heol-tronsfer coefficient due to foomincreosed the bolch lime (5)

a

l0 Gos-phose oxidolion CSTR . Heol removol Use ol diluents con reduce reoction roleDecreose the process volumeMoinloin the low conversion (6)I I Three-phoseequilibrium reoctionrvith 45% solids

CSTR Low conversion due to inode-quole removol of secondgoseous product, NH3Use of N2 sporging lo drive the gos (2JJ

12 Fine-chemicolproduction with twormmiscible liquidsStirred-tonkreoclor

lnsutficienl liquid dropletsheoring, leoding lo cooles-cence ond formolion oflorge droplets. Use of second impeller lo oller the regime fromcoolescence lo dispersion-controllin g (23)

3 Crystollizotion process(suspending solids inliquid lo ochieve goodblending ond ovoidcrystol breokoge)

Stirred-tonkreociorNon-uniform, brooder sizedistribution in lorge scole

. Locote diluent oddition neqr the intoke ofimpeller to ollow crystol growth rotherthon nucleoiion to dominote f23l

rr'. nrrrst all be hcld constant dur-.';1lr,i1r'r Srr ntrrst the clintensionsr l i. ii,rv, -rlistriltLrtion cquipnrent.

r; ;r: ljrr,,rrifjce rlirtnrr;tcrs. and thc' rJ'rii {it)r)tl llt'i,il oll titl h:rst. Ol'li:- Iiir, :: :irit:ii,'tj. iirt 1t;r('ilr)ll

r ; !' f ,],,.t .lr.\'(rI(JI)i n g an rl tltt- il \'(\rti g,-(lI 'v' r jil iltrrittr' 1()r olh('r qtar:ttttetr:r:' :r',: i,, ilrr', loncituclinnl clistribtr-. ',1' : itr, l,rrirhle'sI can l-loLh hc kept' i: rr)rrir.lr- cortstirnt. ;tnrl llrcrt:-'::,, lri.rrtr,:rl rr';rc'tion lrt'hltvirtr1,, ,,r'i;,.,'ii,tj 1l lrr. irl,'ltiir:;rl ()n

bot.h the small and the largc scale.The ir-rcrease in the capacity of thesvstem is purely through the increasein t,he cross-scctional area of t,he {lu-irliz.r'cl hrrrl .Sirrri/rrrilr' r'ttlc:'l'lris rrrlr' nt'r'liitt'ts tcritltilllurlq gt,onrctlicitilv stntilat' flLr-idiz.ing cortclitions betq'ccn lteds of dif:{'crt:nt scirles.'lhe fltridizing condiliortin a geom-etrically similar rnodel nttinres larger than the base ntodel as-sumes that the bcd ircight H, the col-irnrn di;inrt'1et' Z, the clistrilltrtion-ori-{lr'


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\

scale ratio. A consequence of this isthat the sintilaritv rule does not re-qtrire geometrical similarity for theparticies themselves.Stirred-tank reactorsFor scaleup of a continuous stirred-tank reactor (CSTR) with multiple im-peilers and a large height-to-diameterratio. studies have shown that theratio r of the internal circuiation rateto the vessel throughput q should bekept constant [15, /6]. The internalcirculation rate for geometrically sim-ilar svstems is assumed to be equal tothe impeller pumping capacity Q, sor=Qlq

Q is proportional to l/D3. The ratioof pou'er in sniail and large scales forgeometricaily similar systems (that is,D is proportional to T).and equal den-sily on both scales will be as follows:P(.I P" = (?., lT,)4 (qrl qr)3s'here Subscript c is for large scaleand .s for sn'rall scale. Also, for geomet-rically similar systems and equalmean residence times in both thescales, the vessels throughput will berelated as,i'q1q)r/3 = TJT,These trvo equations lead toP,JP. = (T,J'l r\L = (q rlq.)5/3

Accordingly, a tenfold scaleup of thetank diameter means an increase inpower requirement of about 105 forthe same process result, which is un-rt:asonablc to achieve.Successful scaleup has beenachievr.d for the continuous biendingof loiv-viscrisitv fluids using the crife-rir.rn t,hat q/Q is held consLanL 1171.This crirerion rs similar to the one dis-cussed earlier in this section.The relationship PJPr= (?"/?r)5 hasLreen found to be valid for a continuousstirred-tank reactor by other authors,for rrierrtical reaction yields and mix-lr){ lr lnts iilld t'ortstitnt, turrtovcr tintt,lt iroth small and large scale.

G a s/Ii q ui d/s o I i d:re a cto rs'llhe conver-sion, selectivity and stabil-rt-v of catall'zed reactions in fluid-solidreactors can be significantly modifiedl.r',' the iniluence of physical transport

resistance. The modifying effect de-pends on reactor type and size. There-fore, transport resistances are criticalin the scaleup.Three-phase contact is achieved ineither a stirred slurry reactor, wheremixing and solids suspension areachieved through agitation, or atrickle-bed reactor, where the catalystis held firm in a packed bed while theliquid trickles down the column slowlyand the gas rises from the bottom. Theslurry option is the more popular.The fundamental assumption in aslurry reactor is that the solid is ho-mogeneously dispersed in the liquid.In developing and scaling up thistype of ieactor, the most critical ele-ments are understanding the hydro-dynamic issues, the liquid circulationpattern and the slip between thesolid and the liquid.The size of the gas bubbles dependson the dynamic equilibrium betweenthe bubble coaiescence and breakupthroughout the column. The specificsof those two processes in a given casedepend on the physical properties, op-erating conditions and localized hy-drodynamic behavior of the system.The presence of solids affects the co-alescence and the breakup of bubbles.The bigger the particle, the more thebubble breakup is promoted. Theholdup of solids-free gas increaseswith increasing reactor diameter, andliquid holdup correspondingly de-creases. The liquid holdup governs theliquid residence time, which is theprincipal scaleup parameter for pre-dicting product yield. The bubbleshape and prevalence, and the resul-tant rise velocities, are also importantfbr reactor performance.The minirnum impeller rotationalspeed, N, required for lifting thesolids in a siurry rector is inverselyproportional to the impeller diameter,D, raised to the 0.67-0.85 power. Butto obtain a satisfactory degree of ho-rnogcneity, the particles must notonly be lifted {r'orn the bottom buL alsocarried throughou[ the volume of'thevcssel in sufficient quantities. A rele-vant relationship is:NDzlde = constant

The stirrer-blade thickness, b, alsohas an effect on the suspension of the

solids. Based on laser-Doppleranemometry studies U2l, it has beenfound thatN Dru 12 I (bU 4 d pu 6) =constan r

In general, when geometric similar-ity is maintained, lower stirrer speedsare needed for suspending solids inlarger tanks. However, small differ-ence in the exponent on b lead to largedifferences in the power consumptionper unit volume. For given impeller-size ratio and clearance ratio, the cnt-ical speed is almost the same for tur-bine and flat-paddle impellers - butthe turbine version draws twice asmuch power as the paddle (and 15 to20 times as much as a propeller) forcomplete suspension of solids.Biotechnology-process scaleupScaleup of agitated-vessel processesthat use animal cells poses specialproblems. For one thing, the tips oflarger agitator blades tear at in-creased shear rates. Conditioning thecells to withstand the required sirear,or encapsulating the cell organism inor on micro-scale particles, are trvo ofmany strategies being tried to side-step that problem.Both the shear rate and the circula-tion rate affect the solid-liquid masstransfer rate. Larger particles tend tosiip behind the liquid motion, whereassmalier particles tend to follow theflow pattern. In aerobic biochemicalprocesses, the living organism shouldhave access to dissolved oxygenthroughout the tank. Top-to-bottomblending to meet that need is espe-cially important in fermentation reac-tors having height-t,o-diamerer (.1'Il'ftratios of about 2:1 or greilt,er. F-er-nrenters withHlT greater tiran 1.5 re-quire dual impellers: the lorver canprovide the shear for air bubbles, rvitirthe higher providing axial and btrlkmixing tl8l. scaleup of aerobic fer-menters is based on the criterion thatthe oxygen-transfer rate rnust l.r('gleater than the pcruk ox1'gr-'r'r-u1ltakt,rate of the organism.Sdccess storiesTabie 2 lists examples of reactor clc-signs that have been adopted rvhilemoving from bench to larger scale ofoperation. The laboratory-scale de-

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trouble free scaleup of chemical reactions

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