Mixing_Succeed at Scale Up

8/6/2019 Mixing_Succeed at Scale Up

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)

)

Succeed at Scale Up Scale-up

problems in industrial reactor

mixing can be costly, but are all too common.

Here's a proven procedure that

avoids them.

Douglas E. Leng,The Dow Chemical Co.

Failure to properly scale up mixing inbatch and continuously stirred vessels remains a persistent problem inthe process industries. Numerouscauses of difficulty exist, but a pattern isevident: A lack of understanding of the process undermines many efforts. Mixing problems are seldom recognized, in part becausethey are not well understood, but alsobecause they are not examined as quantitatively as other unit operations.

Correcting errors in scaling mixingoperations is costly and sometimes impossible. Errors can cause losses of productivity, quality, and profit, and they also canlead to safety problems such as reactivechemical incidents. Although technicalproblems cause most failures, nontechnicalreasons also contribute to difficulties.Scale up of mixing can be easy or complex. Most difficulties arise when potentialproblems have not been well thought out.Hardest of all are multiphase processes inwhich the chemistry depends on conditionof the phases.Good quantitative tools and measures ofperformance are required to solve suchproblems. (See the accompanying articleby Tatterson for an examination of theseneeds.) My 25 years of experience withindustrial mixing and scale-up problemshas led me to identify some general troublespots and to develop a procedure forsuccessful scale up.First of all, the engineer" must clearlyunderstand the role of the mixing. Is rapidattainment of uniformity critical to processsuccess? The following are typical chemical processes that depend on rapid attainment of uniformity: chemical reactors/polymerizers inwhich reaction kinetics are equal to orfaster than the ratc of mixing: competing chemical reactions wherepoor mixing affects yields; crystallizers thai depend on uniform

mixing to promote the growth of large uniform crystals; and reactions dependent on mass transport.such as coalescing and redispersing of liquid-liquid and gas-liquid mixtures.For such processes. desired results canbe achieved more easily in small equipment than in large equipment.Contrast those with applications that areless sensitive to the needs of uniform mixing. These include: heat transfer: blending of miscible fluids: reactors inyolved with slow chemicalreactions; suspension of solids.These foul can be considered noncriticalapplications, i.e . they can usually be scaledup with few difficulties.Many engineers are most familiar with

the latter, noncritical applications.Unfortunately. they get the impression thatmixing is simple and can be treated casually. Thus. it is not surprising that so manyscale-up problems occur.Avoiding problems

Fo r successful scaleup of mixing inindustrial processes, a designer should follow six distinct steps:1. define the process need;2. identify all of the operationalparameters;3. review the process history:4. select the imponant processparameters;S. choose an initial equipment designvessel design, impellers. impeller location.

bal'fles. and points of feed and exit~ t r e a m s : and6. test the design relative to the processneeds and assumptions and then fine tUlle itto meet the needs of the most importantvariables.

Many scale-up failures can he traceddirectly to the omission of one or more of

CHEMICAL ENGINEERING PROGRESS JUNE 1991 23


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MIX ING

thesl! six steps. We will considereach of the steps in turn and provideex:ampJes of problems.Defining the .process need. The d e s i g n / ~ c a l e - u p . processshould start with a thorough examination of all process needs.Requirements are often straightforward. Sometimes. however. theymay he complex and even contlicting: For example. it might be necessary to exceed minimum shear to sat- .isfy dem;)nds for flow anduniformity.

Typical process needsinclude: supplying a uniform slurryto feed another processingstep; facilitating a controlledchemical reaction; Feed from providing for mass or heat SecondStageReactortransfer: dispersing liquid drops fora suspension polymerizationor extraction; assuring continuous mixing

the impeller 10 distrihute the S,.uperheated hydrocarhon vapor. Th eimpeller was supposed. to dispersethe vapor into buhhles to prtwidemass transfer for the liberation ofammonia. driving the reaction ( 0 theright to form the dimer (2 Al.Onstart-up, however, solids accumulated at the base of the vessel. causingthe plant to shut down. The scale uphad been based on invalid criteria,which led to inadequate mixing forsuspension of the solid.

Vapor toCondenser! 85hp Driveof added reactants or Isopar VaporfromPetrochemmonomers.Avoid the temptation to Superheater Vapor Distributorquickly accept the obviouswithout evaluating Jess obvi- Iou s needs. Fo r instance,solids must be kept in sus

Figure 1. Sparged, lOO,OOO-gallon cofltinuously-stirredpension. but sometimes that

45' psr ImpellerI , ~ \

achieved by sparging gas into a3,SOO-gal pilot-plant reactor.Finally. it was shown 'that conditions had been different in the pilotplant vessel. Sparging was accomplished by p a s ~ i n g nitrogen into theslurry of suspended solids in minerai oil. Nitrogen bubbles had provided th e mass transfer needed toremove the ammonia in the smallreactor, but, in the plant reactor, calculations ot heat transfer from thebuobles to the surrounding fluidindicated tpat all bubbles had

collapsed due to condensation. The liberated heat fromcondensation was transportedto the surface by convection.where it caused the top layerto boil. Mixing, therefore,also had to rapidly circulatereacting solids through theboiling surface layer toremove ammonia. Manymixing designs were proposed. several were testedand pro\'ided improvements,bu t none was sufficientlyeffective to permit continuing to operate the plant.

Fortunately, problems ofsuch magnitude occurinfrequently.Identifying all of theoperational parametersList and then prioritize thereasons for mixing. These

tank reactor containing 45% solids.ma y no t be enough, asshown in th e followingexample.A complex, three-phase, continuous-stirred-tank-reactor system wasbeing scaled up from test dataderived in 3,SOO-gal vessels. Thereaction was an equilibrium dimerization, described by:2 A (solid) (2 A) (solid) + 2 NH.(gas) .'

The reactor was the last stage of athree-stage train. see Figure I, andcontained reacting solids suspendedin a light hydrocarbon. Melt from thesecond stage entered the 100,000-galstirred reactor and snliditled immediately to form a fasl-:-;ettling slurrycontaining 4S% solids. Agitation was' provided by a single. four-blade, 4So-pi lched-blade turhine 16 ft in diam1i eler operated at 26 rpm and driven

The major mistake was thinkingthat adequate suspension of solidswas sufficient to produce desiredproduction rates. The suspensionproblem was solved by doubling thepower per unit volume, as suggestedin tests, bu t that necessitated transferring the original drive to a standby SO,OOO-gal reactor. As therevised plant got back in production,tests revealed good solids suspension was provided in all regions ofthe smaller SO.OOO-gal reaclor. Th eproductivity, however, was onlyabout 2S% of the projected rate forthat size of vessel. The reaction ratewas an important problem that hadnot heen solved, Further tests alsorevealed that sparging the superheated hydrm.:arhon was not effective in

may include requirements for mixingtime (blending). micromixing, heattransfer, shear. solids suspension.mass transfer (including liquid-liquid, liquid-solid. and gas-liquid 1.introduction of gas into liquid, reactants, chain terminators, initiators.and reflux return.

Textbooks on mixing note thatthese operational parameters do notscale up equally. Oldshue ( I) illustrates this for scale up from a :::O-galto a 2,SOO-gal \essel: If the powerper unit volume (PlY) is held constant, circulation lime increasesthreefold, tip speed by 70%, and th\.'impeller Reynolds number by a factor of 8.S. If processes reqUire bothconstant PlY and shear to be mainlained, condition;. of geomelric simi

by a 38S-hp motor. A large, bell removing the ammonia, although larity need to he n:laxeJ.shaped distributor \\as located under ~ H . ' c e p t a h l c reaction rates had heen In miniplanh. nonsill1ilar geol11\.'24 JUNE 1991 CHEMICAL ENGINEERING PROGRESS


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try is used to simulate mixif'!!. in a large-scale reaclor:

), example, when blendtIme i, critical. miniplant.;tudies ca n be done in areactor having a smallerimpeller diameterltank diameter (OIT) ratio.

Identifying all of the keyparameters is, admittedly, notalways easy, but the problems that can be caused byskimping on the analysis canbe tougher. For instance,incomplete parameter identification led to the followingproblem:

Production of a finechemical was' being scaledup from a miniplant to a3,500-gal reactor. Agitationwas believed to be needed ill Figure 2. Schematic oj local relative turbulenceintensities.for solids movement equivalent to the just-suspendedstate. In reality, the reaction ratesdepended on several factors, perhaps the most important of whichwas mass transfer between the twocoalescing liquid phases. The proiuction facility was sized accordingreaction rates observed in miniplant vessels. Production ratesdetermined after plant start-up werefound to be more than an order ofmagnitude less than expected.At small scale, high shear andrapid circulation led to a dispersioncontrolled environment and smalldrops with a large interfacial area.At the large scale, shear rates werelower and circulation was slower,with both factors contributing toconditions classified as coalescencecontrolling. This led to large dropsand much less interfacial area.Because the kinetics were controlledby mass transfer, the change fromdispersion dominance to coalescencedominance explained the unexpectedbehavior on scale up.The scale-up problem might havebeen anticipated if the agitator hadbeen stopped during early laboratory work so that very rapid coalescence could have been seen. Th eproblem wa s greatly eased byadding a second impeller, which

the overall circulationrates and provided an additionalregion ror drop dispersion.

Input Intensity=1.0

Intensity=20

Intensity=5

Intensity=1

Intensity=O.2

Reviewing the process historyA process is often scaled up inincremental stages to meet growing

product-development needs. Thisprovides an opportunity to observethe development of problems due toscale up and can help minimize risk.Problems can be subtle, as seen inthe following example:A nonaqueous dispersion wa sbeing developed from laboratorythrough a mini plant, pilot plant,semiworks plant, and, finally, fuIlscale production. The product was tocontain a certain fraction of solids inthe form of micron-sized polymerparticles. The procedure developedwas to continuously feed initiatedmonomers into a vigorously agitatedhot fluid to rapidly disperse themonomers and provide uniformityand heat transfer. The rate of polymerization was governed by the rateof the addition of monomer, whichcontained free-radical initiators witha half-life of less than a second at thetemperatures in the reactor.Superior product was obtained inS-L glass reactors with the desiredviscosity, particle sizes, and percentage of solids. The results were notquite as good in the next size reactors, but cerlainly good enough tomaintain enthusiastic support frommanagement and customcrs. In the

2.0()()-gal semiworb plant.howe\cr. agglomerates andsmall lumps of polymerformed. which necessitated

' in-line product filtration.O t h ~ r problems a l ~ ( ) appearedbut were controllable.The full-scale plant was ang,OOO-gal multi-pitch-bladeturbine installation. whichincluded an elaborate recyclesystem through heaters andcoolers arranged in parallel.Problems greatly increased atthis scale and were unmanageable even with the installation of large in-line filters.

Calculations showed thatthe energy per unit volume , /was 120 hplLOOO gal at the'75- L s,cale and decreasedalmost linearly to 3 hpll,OOOgal in the production reactor.Meanwhile. product continued to coarsen as larger vessels wereused. Since the initiator half-life wasonly a fraction of a second, the sizes

of particles were established virtuallyinstantaneously when the monomersentered the reactor.. Intensemicromixing in the small vessel dispersed the monomers much morerapidly as polymerization proceeded.

. D. E. lENG is a senior research .scientist ill the CentralResearch Engineeringlaboratory of The DowChemical Co., Midland, MI(517/636-3387; Fax.: 517/6389674). He ha s worked for Dowat Midland for 35 years.During the last 25 years, hehas been involved primarily inmixing and multiphaseresearch, applications, prob- ,lem solving, and engineeringadministration. He obtained aBSc and an MSc from Queen'sUniv., Kingston, Ont., and a 1PhD from Purdue Univ. A fel- .{low of the AIChE and a mem- jber of th e American Chemical lSociety, he was th e chairman, '.of th e Engineering Foundation AConference on Mixing (1981) .. ';.i"- '1,',?.. :',tii

II

I,.1. ;J!I!11



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-MIXING

In larg('r s..:alc vessels. th emixing intensity was tw oorders of magnitude less. sothe dispcr;.ion of th emonomer \ \ ' a ~ 'slow relativeto the rat(' of reaction. Thisr('sulted in a big difference inthe Damkohler number.

Clearly. increasing th etotal power to the level provided in laboratory-scalework \\'as not a dable solution. Instead. attention wasturned to assessing whatcould be done to improveth e mixing intensity at themonomer inlet withoutincreasing the overall mixing energy. The location ofthe monomer inlet was at a Figure 3. Improper geometry for the dispersion ofregion of low turbulence,, monomer ill water.and local energy-dissIpationrates in agitated vessels areknown to be highly nonuniform.Recently published studies (2,3)have shown that regions close toth e impeller gave dissipation rates20 times greater than the average inth e reactor as a whole. Figure 2shows schematically that regionsnear th e impeller provide moreintense energy dissipation. Addingthe monomer through a cooled inletpipe directly to th e impeller discharge stream worked so well at theproduction scale that filtration wasobviated.

Thus, careful observation an danalysis gave warning that a problem, could occur, Fortunately, th esolution was fairly simple to implement and worked well.Selecting the important pro cess parameters

Textbooks generally treat mixingas a series of topics that have little incommon. These topics can, however.lead to an orderly checklist fo rselecting parameters that are important to the mixing process.Blending of lIliscihlt! liquids dealswith topics related to design and

I impeller and vessel dimensions, and! generally ends up with a discussionabout mixing time. 8m , A process isseldom scaled up oy holding mixingtime constant because this generallyrequires high power commitments.Processes that d.::pend on rapidly

26 JUNE 1991 CHEMICAL ENGINEERING PROGRESS

Interfacia IVortex

Baffles

attaining uniformity generally do notscale up well.

Mixing time was used as a criterion for the scale up of a polymerization process in which a blockcopolymer was being produced.Polymerization rates were fairlyfast, and locally poor mixing gaveinferior results, leading to a productwith a broad distribution of sidechain lengths. Calculations indicated what mixing t imes could beachieved reasonably in the plannedproduction reactors, and miniplantconditions were chosen that wouldduplicate these conditions. Testswere then conducted in th e miniplant to determine whether th eproduct quality was sensitive toagitation rates above and below thecalculated rate based on equal mixing time. In this case. scale up wassuccessful. due in part to the recognition that mixing time was a critical fac.tor fo r product quality.G a . ~ - l i q u i d mixing usuallyrelates to phenomena su(;h as ga s

holdup, interfacial area (kiP, th emass-transfer coefficient and interfacial area). and the ratio of powerwhile gassing to power fo r th eungassed state (P / P I f ~ ) ' Th e latter isexpressed as a ru'netion of the aeration number QIND\ where Q is thegassing rate, N is th e impellerspeed. and D is the impeller diameter. Sin(;e mixing has lillie effect on

kl.' rthe mass transfer coefficient). it is the interfacialarea. a"that is atTected in k1umeasurements, Hen(;c kl.a isa function of the superfi(;i:lga s velocity an d PIV. thepower pe r unit volume,For example. consider the

scaleup of a microbial fermentation. Th e role of mixing was to provide gas dis ,persion, supply nutrientblending, keep solids insuspension. and maintai'lreI ati ve\y low shear. Th:broth became rheologicallymore non-Newtonian withtime. The scale up was toprovide correlations for 803m vessels in which th edesired dissolved oxygencould be maintained by acombination of impeller

speed and gas rate. The criticnscale-up variable was selected to bethe kLa, so that equivalence in kLacould provide s imilar dissolvedoxygen levels at correspondingstages in the fermentation.

Studies were conducted in 30-Land 2S0-L laboratory vessels. ThekLa measurements were made by thesteady-state method: oxygen consumption was determined from thedifference between the inlet and outle t oxygen concentrations an d theairflow rate. Probes measured thedissolved oxygen in the liquid phase.The relationship between th e tw osizes of fennenters suggested a correlation of the fonn:k aa.(PAl:14(S )oAfi(lI)L r ~ ' where is the superficial gas velocity. It was necessary to correlate kLfrom a determination of th e zeroshear viscosity (11) of broth sampledfrom the fermenter. The correlationproved to be accurate, providedimpeIler flo\\' was sufficient toentrain ga s buhbles to the regionsbelow the impeller,

Many published studies ongas-liquid mixing have been done ;11low gassing rates. conditions underwhich mixing is fairly straightforward. Gas rates. however. arc usuallymuch highcr in industrial mixing often I to 3 \'olumcs of gas per volume of liquid pcr minute - and canlead to impclkr cavitation. W h ~ ' n


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that happens. impeller discharge catalYSt). form a suspension of such' as baffles. vessel \\alls. and) lWS diminish and the distribution monomer drops in water. heat to impellers. Conditions of shear lead to

,)j" g a ~ in the vessel can change dra polymerize. polymerize. and finally drop collisions, High local shearmatically from fairly uniform to cool, The monomers were more rutes also leud to drop di'persion. Inpoor. .-\5 discharge rates decrease. dense and initially formed the bot a coalescing and dispersing system.impeller flow can no longer entrain tom layer. while water and suspend coalescence occurs principally ingas and carry it to all regions. The ing agent formed the top layer. regions of low shear. v.hile disperregion below the impeller near the Agitation begun, and heat-transfer sion occurs in regions of high sheartank bottom is usually the first to coefficients were calculated online near the impeller. An agitated disperbecome void of bubbles. Under by the latest instrumentation. As the sion is both dynamic and periodic,flooded conditions. the impeller can polymerizer began its first run. the Large-scale eddies appear. disappear,often only distribute gas radially to heat-transfer coefficient feli from 50 and reappear from time to time. sugregions above it. The ratio P I P I t ~ to less than 2 Btu/h/ftC/oF. reactor gesting highly transient behavior.falls to less than 0.5 under these temperatures went out of control. and Colliding pairs of drops coalesce ifconditions. emergency venting to a condenser the forces pressing them together

Liqllid-liquid dispersiolls are was activated to provide evaporative endure long enough for drainage andcommonly associated with drop dis cooling and avoid serious reactive thinning of the continuous phase topersion and the generation of inter chemicals problems. occur between them. If insufficientfacial area. Various mechanisms The monomer layer, instead of the drainage occurs. the drop pair willhave been proposed for drop disper water layer, had become the continu separate as fluid forces diminish.sion. usually relating fluid forces ous phase. and heating had initiated Other factors. such as the mobility of(laminar or turbulent shear) to drop polymerization - resulting in a the liquid-l iquid interface. the deforsurface forces. The fluid forces can mess. Similar results were produced mation of the drops upon impact, andarise from impact at the walls. or in two further runs. The problem was foreign matter collected at the surrotational, dilational or turbulent solved by placing a second impeller face. also playa role in the drainage.shear. Resistance forces are com in the upper. water layer, which Our current ability to scale up liqmonly those due to interfacial ten caused monomer to be thrown up uid-liquid dispersions depends onsion or surface viscosity. The drop into that layer. Laboratory mixing whether the system is coalescing orbreaks when fluid forces exceed tests showed that the single lower noncoalescing. Coalescing systems, cohesi\e forces. impeller created a large interfacial are extremely difficult to scale up ifThe process of forming the d is vort ex tha t allowed water from the equal interfacial area or drop-sizepersion is also important. but it has upper layer to reach the impeller and distribution must be maintained.recehed little attention. Consider become dispersed, as shown in NOllcoalescillg systems oftentwo separated liquid phases. A and B. Figure 3 , Th e problem was no t involve suspens ion-polymerizationof diiferent density with A on top apparent in the 3,500-gal vessel, pos processes employing a water-solubleand B below. To create a dispersion sibly because relatively lower tan protective polymer that collects atof B in A, an impeller must be placed gential velocities existed, resulting in the drop surfaces and prevents coain A. For the reverse. the impeller a shallower interfacial vortex that did lescence. Mixin'g for these systemsmust be placed in B. This sounds not reach the impeller. need only provide for dispersion and !simple and logical. but its impor Coalescence of drops occurs upon suspension because coalescence is I " ~ tance was learned the hard way: collisions with other drops, or virtually absent. Figure -+ shows thatA suspension polymerization was between drops and solid surfaces mean drop sizes become smaller andbeing scaled up from 3,500-gal tolO.OnO-gal size. All factors about thevessel geometry were similar, Theprocess technology had been established for many years in the smaller suspensionreactor. Therefore. a prototype line, drops10.OOO-gal vessel was to be tested are suspended

Logirst: if it was successful, the remain DropSizeing pl)lymerizers would be installed.Agitation at both scales was provid

.....f------_l_ Dispersion:ed by a single. three-blade. retreat drops abovecuneo glussed-steel impeller located the dispersionat the bottom of the reactor. Th e line arepolymerizers had two finger baffles dispersed.placed 1800 apart.The polymerizer had to sequen Log Impeller Speedtially mix the phm;es (waler with suspl'nding agl'nt. and monomers with F i ~ l l r e -I. Dispersioll-suspensio/l relations/ri/ls ill slispensioll poIYllleri:.atio/l.CHEMICAL ENGINEERING PROGRESS JUNE 1991 27


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MIXING- - - - - - . - - - ~ ~ - - - - - - - - - - - - - - - - - - - - - - - - - - ~ ~ ~ - - - - - - - - - - - - - - - - - - - - - - - - - - -

drops arc hetter slispenued asimpeller speed" increase. Decreasingthe agilalor spe\.:d uo\.:s the opposite,The drop sile eorn.:sponciing. to theinter.scctipn Ill' the dispersion andsuspension line" is the largest meandrop size that can he produced underthose specific conditions. The average size of drops depends on eitherturbulent or laminar shear. whilesuspension depends on drop sizes.density differences. and agitationparameters (-J.).

Another factor to consider is thatthe time to reach a completely dispersed condition is short in smallequipment but long in large equipment. A light-transmission apparatusfor measuring interfacial area wasplaced on a I.OOO-gal suspensionpolymerizer to measure the timerequired for complete dispersion. Thenoneoalescing drops were still beingdispersed after 24 h of agitation.

Becaus.e proUllCliol1 schedulesusually do no! allow for allain111entof "complete" dispersion. scale upbecomes more complicated. Oneends up comparing completely disperscd drops made in sillall equipment with partly dispersed dropsproduced in large polymerizers.Therefore. agilation speed has to beincreased in the large vessel to produce drop size:.. equivalent to thosemade in the miniplant polymerizer.Coalescing systems are, by comparison. complicated to scale upfor a number of reasons. Rates ofchemical reactions often depend onthe interfacial area, which in turnhinges on two dynamic factors:coalescence and dispersion. Otherimportant points for these systemsare as follows:

1. Dispersion is localized nearth e impeller in regions of highshear; practically no dispersion

Table 1. Rules for scaling-up similar vessels underturbulent conditions

NDX =ConstantValue of X Rules ProcessesConstant tip speed,constant torque/volume

0.85 Solids suspension

0.75 Solids suspension

0.S7 Power/volume

0.5 Constant Reynoldsnumber

0.0 Constant speed

Same maximum shear,simple blendingUsed in Zweiteringequation for Nis, foreasily suspended solidsScale-up of average solidssuspensionsSuspension of fast-settlingslurries, turbulentdispersion, gas-liquidoperations where kLa'smust be scaled,refctions requiringmicromixingSimilar heat transfer,equal viscous/inertialforcesEqual mixing time, fastreactions

Note: Using these rules for scale-up requires a Reynolds number greater than 10',and geometry of similar proportions.28 JUNE 1991 CHEMICAL ENGINEERING PROGRESS

occurs elsewhere. except initiallv.. 2. Coalescence occurs e,erywhere drop c o l l i ~ i o n s can OCCllr.

3. Circulation times increasegreatly in large 'e,sels.The net effect of these three f ~ \ C t r ) r \ is that dispersion dominates in sma! lscale equipment and coalescem:cdominates in large-scale. commercial equipment. .

Solid-liquid dispersiolls commonly involve the suspension ofsolids' that are more dense than thesuspending phase. A reliable way topredict how much agitation is needed to suspend solids is provided b:the Zweitering equation (5;:

i.;N = [5 VOI d ~ . : ( g / ) . n l p p 4 ~ X


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:'1: IBatches were cooled to ambitcmperatures. vented to remove

flammable blowing agen!.,11en inspected prior to beingout of the rolymerizeLprovided a rough qualityin the

were never ideal. as eviby c ~ u s t i n g on the walls. baf

On one occasion. conditions at theof the cycle were different. and

stickier than usual.he was greeted with a

of water, beads. and blowingof the manway. A

of unknown origin ignited thewas engulfed in flames.

hun. but theA suspension failure ha d

to take place. This agglomeratformed a solid layer

of the liquid.Venting had

reduced pressure in the headabove the crust layer, no t

bw it. As the crust layer broke,ou t through th e opening.

of the panicles. Th eand baffle designs were

Research showed that beads near

conditions existed. wateran d agent suspension

In this prolonged plasticizedof the beads

polymerformed across the sur

of the contents. The level of liqis very critil in marginal cases of floating

and it may have beenof the failure. although this

Agitation requirements in crystalinvolve suspend

'g solids in liquids, providing low(so as not to fral..'llIre the crys

I -VV 'i

Coagulatedlayer of beadsjj ' \- ,"..,J -, Figure 5. Conditioll of suspellsionfailure caused by poor surface mixing.ing to minimize concentration andtemperature gradients. Processneeds are specific to the system.The usual objective in scale up oftank crystallizers is to producelarge. uniform crystals that can beeasily separated and washed.Conditions of uniformity are moredifficult to attain in large vessels,which tend to produce a broader,finer population of crystals. Thismakes subsequent processing moredifficult. If nonsolvents are used {O"salt out" th e crystals. the samemixing phenomena are encountered.There is one important difference the rate of approach to uniformitycan be influenced by the location of

the entry point for diluent addition.If diluenh are intniduccd near theintake of the impeller. uniformitycan often be reachl.'d in time, ,hortenou!!h to allow crystal !.!rowth tod o m i ~ a t e over n u c l ~ a t i ( l n ~ Becauseon e of the biggest unknowns isshear and its effect on c r y ~ t a h . variable-speed drives are commonlyused for tank crystallizers.Choosing an initial equipmentdesign

Approximately 75% of all newinstallations use contoured or hydrofoil impellers because they produceaxial tlow and develop a more orderly circulation pattern. create lowshear. and require less power thantraditional impellers. The effects onflow pattern produced by thesehydrofoils are ,shown in Figure 6.Most processes require these fea-,tures. Installations needing h i g h ~ shear mixing often couple a highshear turbine with a hydrofoilimpeller. thereby maintaining goodcirculation while providing highshear/turbulence.

The position of the impellers isimportant. in part because the totalvolume of liquid may change (usually increase) during many processes.More than one impellercan be used,even for normally shaped vessels,i.e..where the ratio of tank height totank diameter (HIT) is 1.2. In particular, paired hydrofoils offer advan

45 PST,/

\ /I . It, , j JI

j. u0Guv JFOi l / "

Figllre 6. Dijlerences il l ol'em/lfloll' pallerlls hetweell a ./5_pitclted-blade turbi/l e al/{I a 1t,I'dmflJiI.



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MIX ING

~ - - - - - - - - - - - - - - - - - - - - ~ - - - - ~ ~ Big Eddies Vortex

A 8 c

tages - their d i ~ c h a r g e pallernsmatch rather than conflict. as in otherdesigns. A second impeller is helpfulfor solids suspension to provide mixing even while draining the contentsof the vessel. The upper impeller of adual-impeller systel11 ,hould be positioned at a diswnce of about half animpeller diameter (D l from the liquidsurface while introducing solids, andeven for entraining gas into the system. Normally. the upper turbine isplaced about one impeller diameterfrom the surface.

The selection of the ratio of theimpeller diameter to diameter of thetank (D in is also important. In industrial applications. ratios from0.25 to 0.65 are used for commonimpellers, while larger ratios areused for anchors, gates. and helicaltypes of impellers. I f the processrequires flow rather than turbulence,\ DfT ratios of 0.4 to 0.6 are chosen.Fo r micromixing and processesrequiring turbulence. DfT ratios of0.25 to 0.35 would be selected andimpellers would be operated at higher speeds than when flow is required.The trend in solids suspension hasbeen to use smaller. higher speedhydrofoil impellers. These produce a'1coherent je t of liquid that impinges (on the bottom of the tank. resulting Jin effective suspension of solids andresuspension of settled solids.Although the Rushton turbine iscommonly referred to in publications, it is rarely used now for newindustrial gas-liquid installations.An improved design is the concavedisk, which resembles a Rushton buthas six concave, semicylindricalblades facing the direcliun of movement. This provides excellent gasdispersion. ca n handle much moregas before flooding. and lIses lesspower than the Rushton turbine. Anew hydrofoil with three wide bladesis also useful. It disperses gas effectively and has little tendency toflood. even at high gas tllnv rales.The four-blade, 45 c -pilch-bladeturbine is still used for systems ofmoderately high viscosity. e.g . thoseup to 40.000 cPo It pnnides morelocal shear than the ncw wide-bladeJ hydrofoil designs: the latll..' 1'. however. supply more flow and a bellerpattern and u ~ c less energy. At sti 11

Figure 7. Typical surface conditiolls: (A) Turbulellt large eddies promote goodsurface mixi/lg; (B) Laminar conditiolls with large ce1ltral vortex - good for addi tion; (C) Flat surface, IlOt recommended for chemical additions.higher viscosities, particularly whereheat transfer is a problem, specialimpellers - including anchor, gate.nelical. and double-helical typesshould be used. In cases where periodic sweeping by blades of vesselwalls facilitates heat transfer, theseimpellers can be designed to comevery close to the walls (or, in somecases. to actually scrape the walls).Some detail is given by Ulbrecht andPatterson (6).

Fluids up to 3,000-4,000 cPrequire some degree of baffling. The "standard configuration calls for fourbaffles 90 apart with a width ofTil 0 or Til 2 and located T172 fromthe tank wall. This arrangementworks well unless the presence ofsolids or non-Newtonian behaviorleads to regions of stagnation.Baffles placed midway between theimpeller tip and the vessel wall helpto eliminate problems of stagnation.particularly for thixotropic slurries.Full-length baffles sometimes lead toproblems because they create a relatively quiet. vortex-free surface- (making it more difficult to entrainsolids, liquids, and even gases.Baffles placed below the surfaceallow the fOf-mation of a free vortex.which assists the transfer of materi- :)als into the vessel.

Careful consideration must begiven to locations or the entries andexits in stirred vessels. Materialsarc often added directly to the freesurface to avoid using dip pipes.whit.:h can beconle plugged. Ir thereaction is fast, the free surfacemust be turhulent to adequatelycope with reaction kinetics. To fore

stall such problems. it generally ispreferable to add feed streams closeto the impeller inlet. Feed can sometimes be added to the bottom of thereactor where an impeller is usuallylocated. In continuous processing.avoid placing the feed and exitpoints near each other because thisleads to bypassing and distorts theresidence-time distribution.Designs often make use of steadybearings to help a\'oid mechanicalproblems associated with the use oflong, thin shafts. If steady bearingscannot be tolerated, however. analternative is to U$e a shorter. thickershaft and place the drive at the bottom of the vessel. Using bottomdrives eliminates clutter at the top ofthe vessel and allo\\s more options.

Motionless (or in-line) mixersshould be considered as an alternative to stirred vessel designs for certain types of applications. Fast chemical reactions often are idealapplications for motionless mixers.Also. such mixers excel for applications in which competing reactions I.take place and backmixing must beavoided. In-line mixers are also usedto improve heat transfer to exchangers and pipe walls. Simple distributive mixing depends on the length ofthe mixer and the number of elements. Dispersive mixing. such asdrop sizing. depends not only on thenumber of elements bu t also thepressure drop and !low rate, whichrclate to energy dissipation.An advantage of motionless mixing over stirred-\essel mixing is theelimination of bypassing. All Iluidspass through regions of uni form

30 JUNE 1991 CHEMICAL ENGINEERING PROGRESS

i


9/9

and axial mixing. whereas" in stirred vessels is random.

can lead to a problemthe process requires the intense

only in regions nearimpeller. These differences

account for why in-lineinvariably scale up successunlike stirred vessels, which

~ ) f t e n fraught with problems.),lotionless mixers come in a

of designs an d are usuallyto fi t inside standard pipeSystems can be designed forlaminar or turbulent mixing

Designs fo r in-line mixers areon the degree of nonmixing infeed and the desired degree of

for the product.quality of mixing is usually

as the ratio of the stan(o f unmi xedness ofadded species) in the feed to the

isconsidered to be complete

this ratio is 0.05. This isto the "95% mixed" conoften referred to in stirred)1 technology. Th e length need

for the motionless mixer dependsof mixer selected, the

and the final uniformityA comparison of typesbe found in Hamby et af. (5),with the design procedure. Italso important to take pressureinto account. Efficient mixers

appear to require ~ p o r t lengthsalso demand an excessive

drop.

It is important to check all of thehas been met by the proposed

would includepower to be used. the speed, thebox. and the ability to achieve

process result. Usingsense also helps. An

to overcome the solids setAn opportunity exists til the time'\ant s t ~ r t - u p to m e ~ s u r e and docf.:t performance. 01 course, there

~ l r o n g desire to begin proSl)on as possinle. If oper

aling management can be persuadedto permit the validation of thedesign. however. this will add to theknowledge base and pay big dividends fo r future operation anddesign. Routine testing is done withwater in the vessel. Speed, vibration.and power draw (using a wattmeter)are the first things to check.An inspection of the liquid surface tells how effective the bafflingis and how easily materials can beadded to the surface of the fluid.Figures 7A- C show typical surfaceconditions. Large-scale. high-energyeddies, Figure 7A, promote largesurface waves. A large central vortexis shown in Figure 7B, while staticconditions are shown in Figure 7C.Conditions in Figures 7A and Beanbe used effectively for adding reactants; faster mixing is achieved byadding reactants to th e impellerregion. Conditions shown in Figure7C should never be used to add reactants to a vessel. A camcorder canhelp to record motion, which ca nserve as a valuable future reference.

Mixing times can be measuredby a conductivity probe placedinside the vessel. The probe is connected to a high-speed recorder tomonitor changes in conductance(and, hence, concentration). A tracer, often salt water, is injectedwhile the vessel is in operation andthe conductivities are recorded, ifappropriate. Tests can be repeatedat different water levels and agitato r speeds.

Draining the vessel is importantto see if mechanical problems occuras the liquid passes through theplane of the impeller.Other pitfalls

Engineers also should guardagainst the following common trapsfo r the unwary in the scale up ofmixing.

1. Reliance on rules:Preoccupation with scale-up rules e.g .. "constant tip speed." "constantpower per unit volume," or "constanttorque per unit volume" - usuallymeans the engineer wanls an easyanswer. Depending on rules oftencreates problems because rules truncate thinking. Simple mixing (slichas the blending of nnnrc:li.ting homo

geneous liquids). however. can oftenbe treated via rules. If truly simplesystems are involved in scale up.consider the rules given in Table I.which have proven succe\\ful inour work.2. Acceptance ( ~ r eXiSlil1!{ condi tions: Energy is often wasted whenscale up is based on undefined needs.In the case of a fermentation process.it was found that reducing the originalmixing energy by 60% did not changethe process performance.3. Undue secrecy: Honest. competent equipment vendors can ,serveas important resources in helpingyou choose the most suitable mixerdesign. if they are given adequateinformation. Secrecy is important,but harm ca n result when onlyselected information is provided.

4. Rigidity about flexibilit}:System demands sometimes are toobroad for anyone design to handlewell. For instance, a single \es.selmay have to serve as a blend tank. apolymerizer, a devolatilizer. andeven a crystallizer. I t would beremarkable if any design could serveeach of those requirements equallywell. Therefore, it is essential toassess the trade-offs between flexibility an d performance.Occasionally, particularly at small,remote sites, multi task designs mustbe used regardless of consequences.Mixing is still largely an art. butit is becoming more scientific. In thefuture. quantitative tools such ascomputational fluid dynamicsshould provide a better link betweendesign and performance. 8D

Literature C i t e d .. ",-,'J1. Oldshue; J. y.P."Fluid Mixing',Technology," p.191, .;!,Vt.cGraw-HiIl,York (1983). .,:,:".J . . '.. .,'2. Cutter, L. A., AiChllJ.. 12; p. 3 5 ~ '(1966). . .:;1\'" . ; .' iij.3. Angst, W., J. Bourne, an d P.",'Dell'ava, Chem. Eng:.Sci .39 p: 3 3 5 , ~ (1984). ~ l ~ ; , ~ . : . . . . . '.4. Leng, D. E. , an d G. J ~ . Quarderer, 'iChern. Eng. Commun.,.. ~ 4 , p. 177 (982).:;;S. Hamby, N., M. F; Edwards, and A . :W. Nienow, "Mixing in the P r o c e s ( ~ Industries," p. '226. 306. B u l t e r w o r t h s . ~ Woburn. MA (1985). ; i , i , . l ' ~ , : Ulbrecht. J. J., a ~ d G. K. P a t t e r s o n ; ' ~

'. UMixing of Liquids 'by Mechanical ..;. Agitation." p. 93, Goro,o", & Breach. New::!. York ( I 9 8 5 ) . . , . , ~ " . _ ' ~ ' A


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Mixing_Succeed at Scale Up

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