Chapter 7 Liquid Filtration

8/13/2019 Chapter 7 Liquid Filtration

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Liquid ltEtion 228

ChaptersevenLiquid Filtration

7.I, INTRODUCTIONThe separationf solids rom a suspensionn a tiquid by means f a porousmediurnorscreen hich retains he solidsand aliows he iquid to passs tcrmed ilhation.ln general, tle pores of ihe medium are larger than the palticles which are to beiemoved, and thc filtcr works cflicicntly only aller an initial deposit has been trappedin fhe medium. ln fte labontory, filtration is oiler canied out using a folnl of Buc}nerfunnel,and thc liquid is suckcd brough bc thin laycr of parliclcsusing a source fvacLrum.n cvensimplercaseshe suspensions porred nto a conical Lrnnclitted widra fiiter paper.n the ndustdal quivalcnt, imcultiesarc encountercdn the mcchanicalhandlingof much argerquantities f suspensionnd solids.A thickcr ayer of solidshas to form and, in order to achieve a high rate of passase f liquid throush the solids,higherpressuresrcnceded, nda far grcater rea as o bc providcd.A rypical ilt|ationoperations illustratedD Figure7.1, which shows he ilter medium, D his casea cloth,iis supportand the layer of solids, or lilter cakc,which has already formed.Volumes of the suspensions o be handled vary fron the extemely large quantiticsinvolved in water purification and ore handling n ihe mining industry to rclativciy smallquantities,as n thc fu1echemical ndustry where he variery of solids is considerable.hmost industrial applications t is the solids thai are required aM their physical size andpropertiesare of paramount mportancc. Thus, the main factors to be consideredwhenselecting quipment nd opemting onditions rei

(a) Thepropertiesofthe fluid, particularly its viscosily, density and colrosiveFope ies.(b) Thenatureof thc solid its padiclesizeand shape, izedistribution, ndpackingcharacteristics.(c) Theconcentrationf solids n suspcnsion.(d) The quantity of material to be hardled, and its value.(e) whether he valuable roduct s theso1id,he lujd, or both.(0 Whether t is ncessaryo wash he ilteredsolids.(g) Wherher verJ slisht contamination causedby contact of the suspensionor filtratewith the various componentsof the equipment s detimental to the product.(h) Wlether the feed iquor maybe heated.(i) whether any form of pretreaimenrmight be he1ptul.Filtration is essentially a mechanical operation and is lessdemanding n cncrgy ihanevaporationr dryingwhere hehigh atentheatofihe liquid,which s usuallywater, asto beprovided.n therypicalopration hown n Figure7.1, hecakegradually uildsup


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229ChemilEngineeringrocesses

Slurry

,1,lisure 7 L Pimiple ofnlhdon

on the medilun and he resistarce o flow progressively ncreases.Dudng ihe initial periodofflow, particlesare deposited n lhe sudace ayers of the cloth to form the true filteringmedium.This initial deposii may be formcd from a special nitial flow of precoatmaterialwhich is discussed ater. The most important factors on which the rate of filtation ihendepeMswill be:(a) The drop in pressure rom the feed to the far sideof thc filter medium.(b) The area of the filtering surfacc.(c) The viscosity of tle filtraie.(d) The resistanceof the 6lter cake.(e) The resistanceof the fiIter medium and hitial layersof cakc.

lwo basrc y?e5 l filrmrtonproce.se( a) be idenlifico. ltbougb bere recaseswberethe two E/pesappear o merye. ln the first, frequently refened to as cake hrution, theparticles rom the suspension,which usually has a high proporrior of solids, medepositedon the swface of a porous septumwhich should ideally ofler only a small resistance oitow. As the solids build up on the septum, he iniiial laycrs folm the effective 6ltermedium, preventiry thc particles ftom embeddingthemselves r the filtef cloth, andensuring hat a particle-free filtrate is obtainedln the seco t)?e offilhatlon, depth or deep-bedfltratior, the pariicles penehatentothe pores of the filter medium, where impactsbetween he particles and the surface ofthe medium are largely responsible or their removal and retention. This configurationis comnonly used for the rcmoval of fine particles lom very dilute suspensions,wherethe recovery of the paiticles is not of primary importance.T)?ical exampleshere ncludeair and water filtation. The filter bed graduallybecomesclogged with particles,and ilsresistanceo flow eventually reachesan unacceptably igh level. For continuedoperatio&it is threfore necessaryo remove the accumulatedsolids, and it is importart that thiscanbe feadily achievd.For this reason, he filter conrmonly consistsof a bed of partic-ulate solids, such as sand, which can be cleanedby back-flushing,often accompanied y


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fluidisafiol ln this chapter, he emphasis s on cake ilhation although deep-bed iltration,whichhasbeendiscussedn detailby IvES(r.':)s consideredn the section r bed ilters.There are two pdDcipal modcs under which decp bed filtmtion may be caded out.Irr he tust, dead-e d fltratian ||hich is illustated in Figure 7.1, the slunl is filteredin such a way that it is fed perpendicularly o the fi1ter medium and there is littl flowparallel to the surface of the medium. ln the second, ermed .r'oss-Jla Jiltration whichis discussedn Section .3.5.andwhich s usedparticularlyor very dilutesuspensions,the slurry is contiruously recirculatedso that it flows essentiallyacross he surfaceof thefilter medium at a rate conslderably n exccssof thc frowratc through the 6lter cake.

7.2.FILTR,{TIONTHEORY7.2.1. ntrodLlction

Equations regiven n Chapter for thecalculation fthe Iateofflow ofa fluid througha bed of granular material, and ihcsc arc nolv applicd to thc flow of filtrate through afilter cake.Somedifferencesn gcncralbehaviourmay bc cxpcctcd. owever, ecausethe casesso far considered elate to unifom fixed beds, whereas n filtration the bedis steadily growiry in thickness. Thus, if tle filtration pressure s constant, the mte offlow progressivelyimjnisheswhcrcas.f the lowratc s to bc maintainedonstant,hepressuremust be $adually increased.The mecha cal details of the equipment, particularly of the flow channel and ihesupporl for the medium, innuence he way th cake s built up and the easewith whichit may be removed. A uniform structure s vry desirable or good washing and cakesformed rom paficlesofvery mixedsizes ndshapesresent pecial roblems. lthoughiilter cakes arc codplex in their structurc and camot fiuly be regardedas composedofrigid ron-deformable pafiicies, the method of relating the flow parametercdeveloped nChapter is useful n describinghe flow within the filier cakc.Thc generalhcory off i l t rarionrd iLs mpodancen design as een o'r. ide_edi S. | ' r . l l ma) benolcdthat there are nvo quite diferent methodsof operatingabatch ilter. lf thc prcssuros keptconstanthen he rateof flow progressivelyiminishcs, hercasf the flownte is keptconstanthen hepressurcmustbe graduallyncrcascd. ccauschc particlesorming hecake are small and the flow through the bed is s1ow,streamline conditions are almostinvariably obtained, and, at any instant, the flowrate ofthe filtrate may be represented ythe followins forrnulaI e 1 a P

Liqt'idltration230

(7 . r )wherey is thevolumeoffiliratewhichhaspassedn timer, ?4 s the otalcrcss-sectionalarea ofthe filter cake, r. is the superficial velocity of the filtrate, I is lhe cakethickness,S is the specific urface fthe palticies, s thevoidage, is theviscosiry fthe filtrate,andAP is the appliedpressureifference.ln deriving this equation i is assumed hat the cake is uniform and that the voidageis constanthoughout. n the dposition f a filter cake his is unljkcly to be the caseand the voidage, e will dependon the natureof thc support, ncluding its geometryand


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231Chemicalngineefingrocesses

surfacesfucture, and on the rate ofdeposition. The idtial stagesn the fomation of thecake are thereforeof special mportance or the ibllowing reaions:(a) For any filtration pressure,he rateofflow is greatestat the begiDningofthe plocesssince the resistances then a minimum.(b) High initial ratesoffilrrationmayrcsu1tn pluggtng f the poresofrhe filterclothand cause veryhigh resistanceo flow.(c) The orientation of the particle in the inirial layers may appreciably nfluerce thesfucture of ihe whoie fitrer cake.l- i l rer akcsmaybedividcd flo woclds.cs incompressibleales Itrd ompre.siblecakes,n the caseofan incompressibleakc, he esistanceoflow of a givenvolumeofcalr j r nor ppreciablyf lecredjrher y hepre.sure Fcrence"ros, ] i cake r b] rhemreotdeposir ionl md'er;dt. n rbeorhcjhcnd. i rh a compre\sibleake,ncre"sc fthe pressuredifferenceor of the rate of flow causeshe formation of a densercakewitha higher esistance.or incompressibleakes in equation _l maybe takenas constantand h.quantiry3lI5(1- r)2S21s thena property frhe particle; orming he cakeandshouldbe constant or a given material.

Thus: (7.2)(7.3)

l dvAd t rp l

5(1 - ) 'zS'?' tIt may be noted thai, when therc is a hydrostatic pressurecomponentsuch as with ahorizontalilter sudace,hisshouldbe ircluded r the calculatio;of _Ap.. Equation .2 s thebasic iltrationequation nd is termedhespecific csistance hichrssecn o depcnd n-e_aDd. of incomo-essib,eales. is takcn 5con5t t . a, lhoughrt depctrdsn -aleof depo.iLion.be nanfeof thc panic'er, ndon rbe otce.benre;nthe particles. r has the dimensionsof L : and the units m-2 in the Sl system.

7.2.2.Relationbetween lrickness fcake andvolumeof ltrateIn equation .2, the variables and y arc connected,nd the relatior between hemmay be obtainedby making a material balancebetwcen he solids in both the slurry andth cake as foliows.Massof soiids n filter cak (t - e),41p", herepI is ihe densi, of the solidsMassofliquid retaircd n the ilrer cake= rAlp, wherep is the dinsity ofthe fitrrate.If "/ is themass ractionof solids n the originalsuspensionhenl

t_aa,, :y l : )1 _ Jor: (1 J)( l e)Alp": JVp + AeJIpA1(1- t ) ( I - e)ps Jepl ('7.4)


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Liquidiiltralion32

and: Q.s)t

(7.6)Il D s the volulne of cake depositedby unit volume of filtrate ther

I p "O-e ) ( r - J ) -epJ lA I

lA uvu : V o r I : i

dvvv A,eLP)t rttvl)t rpx ,,v A 2 t L P )

alld iom equahon7.5:

Substituting or , in equatiotr7.2:

so that:

(7.8)Equation 7.8 may be regaxdedas the basic rclatioll between - AP, y, alrd l. Twoimportant twes of operation are: (i) \'r'herehe Fessure diffeleDce s maintained constantalld fi wherc the mte offiltation is rnaintaircd constatrt.

For afltration at constant rate

( r - t ) ( 1 -e )p " - J fp

I dv _ (-aP) AA d rp l)vdv A'z(-^P)dt TLLL,V

\. (1.7)

(7.9\(7.10)

( 7 . 1 1 )

Q.r2)

and-AP is directlyproportionalo V.For a ltration at constantessuredffercnce

v2 _ AZ(-LP) i2 rtt1)t f|t u

= 2Ar (_LP) ,Thus for a constantpressure iltration, there is a linear rclatiotr between V' and , orbenreen /Y and V.Filtration at cofftant pressrre is morc ftequendy adopted n pmctic, although theFessure diference is nomally gadualy built up to its ultimate value.ff this takes a time tr duing which a volume yl of filtrate passes,hetr integatiotr ofequation .12gives:

I . ^ Azt-LPl- ( v . _ v i ) : L l t t t )2 r&u (7.13)


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233Chemical ngineenngrocessesf p t . , r . , . . t w v )2 4 2 ( L P ) A 2 \ L P ) ('7.14)

Thus, herewhere s a linear elatlorberweenz, andr andbetween t _ ^)/(V _ Vt)and (y vr), where (r - rr) reFesents rhe time of ihe constantprc""*e ntt otion aoa(y - Vr) theco[esponding olumeof filtrateobtained.Ruru et al(a- 7)havemademcasurementsn the low in a filtcr cakeandhave concludedthat the resistances somewhatgreater han hat hdicated by equation7. . It was assunedthat pmt of the pore space s fendered nefective for the florv of filtrate becauseof theadsorptionof ions on the surfaceof the paricles. This is not born out by cRAcE(s)orby HoFFrNc nd LocKrARr(e)who determied the relation between lowrate and oressureditrercnce.orf b) mean\ f perneabrl i$ est . n a f i \edbed ro by 6lmrion e.r.us.rgsuspensions f quartzand diatomaceous arth,Typical values of thc specific resistance of filier cakes, taken from the wo* olCArtIraN(]o),regiven n Table7.1. n theabsence f details fthe physical roperties fthe particles and of the conditions underwhich they had been formed, these vilues areapproximatealthoughthey do provide an indication of the ordersof masniruale.Tabh ?.1. TypicalValues f Specific esislhce, (roi

1.6 tora

3.5 l0 ' '

13 l0 ' {8 x I0 ' r

2,3 l0r77.2.3.Flow ofliquid through he clothExpcrimentalwo* on the flow of the liquid under streamlineconditjons(r0)has shownthat the flowrate is directly proportional to the pressuredifference. t is the rcsistanceofthe cloth plus inltial layers of deposited particles that is important since the latter, notonly form the true medirm, but also tend to block the poresofthe cloth thus increasins

cllciu'n cmbonarepHilnated)

Celadnousmagnesiun ydroxideCelatinoualuniniumnydroxideGelatinous eric lydroxidc

780l t 0170274780210780274?8027Q780214780210780270650


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Liquidltration 34

its resistarce. Cloths Inay have to be discaded becauseof high rcsistance well beforethey are mechanically wom. No true analysis of ihe buildup of rcsistance s possiblebecause he resistancewill depend on the way in which the pressure s developed andsmall vanaiions itr suppot geometrycan have an important niuence. lt is thereforeusualto combine the resistanceof the cloth with that of ihe fint few layers of parhcles andsuppose hat this correspondso a thicknessZ of cake as depositedat a later stage.Theresistance o flow though the cake aad cloth conbined is now considered.

7.2.4. Flow of ltrate through the cloth and cake combinedIfthe filter cloth and the hitial layers of cake are together equivaleni o a thickrcss a ofcake as depositedai a later stage n the Focess, and f -AP is the pressuredrcp aqossthe cakeand cloth combiDed, hen:

l dv (-aP)A& rLL ( l +L )which maybecompared ith equation .2.

dv A(-LP) A 2 ( - L P )

(7.15)

(:'7.16)

(7 t1)

(7.18)

Thus:

This equation nay be integratedbetween he limits, : 0, V : 0 and t : tr, v = q forconstant ate fi lfation, and t : t1, V : V1 and t : t, V = V for a subsequent onst ntpressureiltration.For the period of coirsldrt rdte ltrationl

dr

Vrt1

P\ -LP),r, (v.* to\

rp, ,, , fp.LAr(-Ap) YF A(-Ar). . , L A . . A l l - ^ P )

1) rtll)For a subsequent orrldr?tpressureltration:

| . L A A l r A P r- ' V ' - Y ' l + - ( V - V r ) ( t t t )2 r f p ul V - v t + z V t ) \ V V t ) l ' " \ V V ) - ' ' ' ' ' r - r t

t - t t r p u . , . , r p ' v t r p L" V V t - 2 A 2 \ - a P ) ' ' 4 ( - a P ) A L - A P )

,,,(,.T)t l \A+L )

(7.te)


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235Chemicalngineerifgro@sses

Thus there s a linear relationberween r-rr)/(y yr) and y_yr, as shown nFigurc?.2, and the slope s propodionato thc speciflc csistance, s in the caseofthe flow of the filtrate ttuough the filter cake alonegiven by cquarion7.14, althoughthelhe does not now go through the odgin.

V,U (cn3)

Figure7.2. A typicat ill.arion uneThe interccpt on the Q ti)/g - V) a\is shoutd enablc l,, rhe equivatent hicknessof the cloth, to be calculated although reproducible results are not obtained becausethis resistances ffitically dependent n lhe exact manner n which rhe operationscorlmenced.The time at which measxrementof y and I is commenceddoes not aflectthe slope olthe curve, only the intcrcept. It may be noted that a linear relation between/ and yi is no longerobtainedwhen hecloth esistances aDDreciable.

7.2.5.CompressibleFilter cakesNearly all iilter cakesarc comFessible to at least some extent althouqh in manv casesrl-edegree l compres. ibi l i i yr so.nalt loat hecakendy. tof prac;cdl u?oses. eregardedas ncompressible.The evidence or compressibilit is that the specificresistanccisa firnction ofthc pressurediflerenceacross hecakc. Compressibiliy miy be a reversibleor an irreversible process.Mosi filter cakesare inelastic andthe greater esistance fferedro lo$ arh,gh res.urei f ferencesscaured y hcmore ompacract


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Liquidllration236

any stage.h elastic ilter cakes he elasticity s attributable o compressionof the paniclesthemselves.his s essusual, lthough ome ormsofcarboncan ivedse o elasiic akes.As the filhate flows through he fi1tercake, t exertsa drag orceon thepariiclesand hisforce is iransmitted tuough successiveayers ofpaticles right up to the filter cloth. Tlenagnitude of dis force increasesprogressively tom the surface of the fl1ter cake to thefilter cloth since at any point it is equal o lhe summationofthe forces on all the paniclesup to thatpoint. f the cake s compressible,hen ts voidagewill decreaserogressivelyin thedirection fflow ofthe filrrate, ivingrise o a correspondingncreasen the ocalvalueofthe specific esistance,:, ofthe filtercake.Thestructuref thecake s,however,complex and may changeduring the courseofthe filtration process.f the feed suspensionis flocculated, he flocs may becomedeformedwiilin ihe cake,and this may give rise to achangen theefectivevalueofthe specific udace, . ln addition,hparticleshemselvesmay show a degree f comprcssibility.Whcnevcr ossiblc, xpcrimcntalmcasurcmentsshouldbe made o determinehow the specific esistance aries over the mnge ofconditionswhich wili be enployed n pncticc.It is usually ossibleo expresshevoidage.i: t a depth asa fuction ofthe dilTerencebtween he pressureat the ftee surfaceofthe cake Pr and the pressureP: ai that depth,that s d: as a functionof (4 - P.). Thc nonenclatuc s asdefincdn Figure7.3.

1.1|.vI lt v-tutv-"u*"f \ r "0 'n.

Figure.7.3 ow through conpEssible lter cake

For a compressibleake,equation .1 may be writtenas:l d v d l 1 / dP , \; dr 5(1 e)'s't \- di / (7.20)

whered. is now a functionofdepth: from the suface of the cake.In a compressiblecake, he volume u of cake depositedper unit area as a result ofiheflow ofunit volune offiltratewill not beconstant, utwill varyduring he iltration ycle.Ifthe particleshemselves rc not compressible,owever, he volume of panicles ( ')will be alnosr independentof the conditions under which the cake is formed assuminga dilute eed suspension.ny small variationsn D' arisebecausehe volumeof filtraterelahed n the cake s a functionof its voidage, lfiough he cffectwill be very small,


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237Chemicalngineeingroesses

excQt possibly for tlrc filtration ofvery highly concentrated uspensions. he increasencake hickrcss, dz rcsulting ftom rhe flow ofa volume of fithate dy is given by:dz=dvL (72r\\ r e . l A

By comparisonwith equation?.6, it may seer that:. . - t - , . \ 1 . 2 2 )

SubstitutingAom equation7.21 into equarion7.20 givesj .t d v e l r l - e l A t t d p \

A dt 5(l - e,\252 1,, t1 \ dV )_ , d v r ' 4 r , d p , \r t r u s : - l | , r 1 2 td r 5 r i - ? - ) S r r / u \ d y /A, / d,P"\ (7.24)u.L'rz d,v /

. 5 ( l - e , ) S lw n e r e : r _ : - 1 7 . 2 5 ).;Comparingequations7.8 and ?.24 shows hat for an incomFessible ake:

rtrz = rfor, r. =.1At any instant in a constantpressure iltration, integratiotrof equatiotr7.24 tbrough thewhole depth of the cake gives:

I ' d v . . . A r l F I d p )I -- i -ov ' - l - 0.261J0 or l t r JP IAt any time t, dy/dr is approximatelyconstant hroughout the cake, unlessthe mte ofcha.nge f holdup of liquid withir the cake s compaEblewirh rhe fitrration Iate dyl&,such as is the case with very highly compressitrlecakes and concentaied sluries, andtberefore:

. dv A2 th t-dp, l- | t7.2ip t ) V J p lr. has been showtr to be a firnction of the Fessure difference (pr pz) although it isiDdependent fthe absolutevalue of the pressrlle.Experimentalstudies requently showthat the rclation beh{een r? and (Pr p:) is of the form:

r - r t P - p ) , t : ' . 2 l )wherer' is itrdependertof P. and 0 < r, < I


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Thusl

Thus:

where r// : (i - n/Xand:

:I( dP,)Liquidllralion238

('7.30)d VLn'r' (t - n')(- LP ),,'

A2 - LP)Vl,tr'r'\-LP)tr

dv A,(-aP)dt V LLr'l

d P(Pi:E/= (Pr - P)t Ir ' l - n '

I l - A P r l ' '- -""'1- Q 29)r ' l ndv

(7.31)where i is the mean resistancedefined by:

(7.32)Hernr.rns(rr)asstudiedheeffectofpressure n heporosity fa filter cakeandsuggestedthat, as the pressures increasedabove atnospheric, the porosity decreasesn proportionto someDower of the excGRAcris)has related the a;ticipated resistance o the physical properties of the feealslurry. VALLERoY nd Mer-or.rEv(i2) ave examined he resistanceof an incompressiblebed of sphericalparticles when measured n a permeability cel1, a vacuum filter, and acent ifuge, and emphasised he need for caution in applying laboratory data to units ofdiflerent geometry.TrLL Rand HuANa(r3) ive futher defails ofthe problem of developing a usabledesignrelationship for filter equipment. Studies by Turn and Smn-qro(ra), rr-renand Yls(ts)and RusHroNand HAMEED(Iohow thc diffculty h presenthg practical conditions in away which can be used anallically. It is very impofart to note that tests on sluriesmustbe made with equipment hai is geometricallysimilar to that proposed.This meansthat specificresistances very dimcuit to de6ne n practice,since t is determircd by thenature of {he filtering unit and the way in which the cake is initially formed and thenbuilt up.

7.3.FILTRA.TION PRACTICE7.3.1. he lter mediumThe uction of ihe fi1termedium s generallyo act as a suppo$ or the ilter cake,andth nitial laye6of cakeprovide he rue ilter. The ilter medium houldbemechanicallystrong, esistanto thecorrosive ctionofthe fluid, andoffer as ittle resistancespossible


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239Chemicalngneering roesses

to the flow of filtrate. Woven materials are commonly used, though graDularmaterialsandporoussolids are useful fo. filtration of corrosive iquids in barch;its. An imDortanilearuren rbeselection f a $o en matcnals $e eareoacake emo\ai.sinceLb;si a kevfactor ir the operationofmodem automaticunits. Esr_ms(r?) as discussedhe selectionof woven s)rntheticmaterials and Wrorrowsrlr3) that of non-woven materials.Furtherdetailsofsome more rcentmaterialsarc given in the litemtue(re) anda useftl summarvi sp r c s e n r e dn . o ' ,R ' . f , r d . . r l r , { ,

7.3.2.Blocking Filhation tIn the previous discussion t is assumed hat there is a well-defined boundary betweenthe filter cake aM the 6lter cloth. The iniriai stages n the buitd-up of the filter cake areimportant,however,because hesemay havea large effector the flow resistanceand mavseriousl)affecr hevseful ife of lhe clolb.The blocking of the poresof the fiiter mediumby particles s a complexphenomenon,partly becauscof the complicated natue of the suface structue of the r;ual t'Des off iher media. nd panly becausehe tines fmolemenrof rbepanicles . # *. f fde6ned.Ar rhe slafl of filrralion. he majrtrern whicb $e cake onns uill lie berueentwo extremes the penetatior ol ihe poresby particiesand the shieldingof the enh.y othe poresby the particles forming bddges. HEERTs(rr) considereda numberof idealisedcases n which suspensions f speciied pore size distdbutions were filtered on a clothwith a rcgular pore distribution. FiNt, it was assumed hai an individual particle wascapableon ifs own of blocking a single pore, theD, as filtration Droceeded. uccessiveporeswouldbe blocked. 0 $al rie apparenlatLre f Lle specrncesisonceof tbe illercake would depeM on the amountof sotids deposired.The pore and particle size distributions might, however,be such that more thar orcparticle couid entera particular pore. In this case, he rcsistanceof the pore inoeases nsragcs ssuccessi\c anicles re rdpped nlil $e porc s complercl]bloiked. n practice.however, it is much more likely that many of the poreswill never becomecompietlyblockedand d cdkeof rela'ivet) o$ resislance i form over rheenr) ro rbepanialtjblocked porc.One ofthe most important variablesaffectitrg the tendency or blockiry is the concen_hation ofparticles. The greater he conccntration,he smaller will be the averaqedistancebeiween}e particles. nd hcsmdllerwilt be }e tendeocyor Lbe artjcleo b-e rawn nro lbe srreanlines irected ouard. rle opeD ores. oslead, he paniclesn lbe concen_trated suspension end to distribute themselves airly evenly over the filter surfaceardform bridges.As a result, suspensions f high concenhationgenerallygive rise to cakesof lower rcsistance han ihose formed from dilute suspensiors.

8.3.3.Efiectof particlesedimentation n filtrationThre are two important effects due to particle sedimentationwhich may aflect therate of filtmtion. Fi$t, if the sedimentparticles are a1l settling at approximately the$rme rate, as is liequently the case n a concentraiedsuspensiontr which the Dadclesizedistr iburions nor very rde. a more apidburtd-up;fparl ictes i l l occuron an


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Liquidltration240

8.3.4.Delayedcake iltrationln ihe filhation of a slurry, the resistaflceof the 6lter cake progressively ncreasesandconsequertly, r a constantpressureoperation, he rate of filtation falls. If the build-upofsolids can tre ieduccd, the cffective cake hicknesswill be less aM the laie of flow offilhate will be increased.ltr practice, t is sometimes ossible o incoq'oratemoving blades n the 61terequipmentso that the thickness of the cake s limited to the clearance etween he filter medium andthe blades.Filtrate then flows through the cake at an apFoximately constant ate and hesolids arc rctained n suspension. hus he solids concentration n the feedvessel ncreasesuntil the particles are in permarent physical contact with on another. At this stage heboundarybetween he slurry and he cakebecomes 1l-defined,anda signincant resistaneeto the flow of liquid developswithin the slxrry itself with a consequent eduction in theflowrate of filtate.By the use of this tecbdque, a much higher mte of filtration can be achieved thanis possible in a filter openied in a conventional nanne.. In addition, the rcsulting cakeusually has a lower porosiry because he blades effectively break down the bridges orarcheswhich give rise to a structur n th filter cake, and the final cake s sjgnificantlydder as a result,If the scmpels are in the form of rotating blades, the outcome differs according towhether they are moving at low or at high speed.At low speeds, he cake thickness sreducd o the clearancedepth ach time the sqaper blade passes,although cake thenbuilds up again until the nextpassage f the scraper. fthe blade s operatedat high speed,there s little time for solids to build up betweetrsuccessive assages fthe blade and thecake cachesan approximately constart thickness.SinceparticlesteM to be swept acrossthe surfaceofthe cakc by the moving slury, they will be trapped n the cake only if fiedrag orce which the filtrate exerts on them is greatenough.As the thickness ofthe cakeinffeases hepressuregradientbecomesessand herc s a smaller orce retainingpalliclesilt the cakesurface.Thus the thicknessolthe cake tends to reach an eouilibdum value.which can be considerably ess than the clearancebetween he medium a the blades.Expeimental results for the etrect of stirrer speedon the rate of filtration of a 10pelcent by mass suspensionof clay are shown in Figure 7.4 taken from the work of TrLrRand Cswc@o), in which the fittrate volune collectedper unit cross-sectio of filter isplotted against ime, for severalstfrer speeds.8,3.6,Preliminary reatmentot slurriesbefore iltrationIf a slurrys diluteand hesolidparticlesettleeadilyn the luid, t maybedesirableoeffect a preliminary concentrationn a thickeneras disclssed n Chapter5. The thickenedsuspensions ihen fed from the ihickener to the filter and the quantiry of materiaj to behandled s therebyreduced.Theoretical reatmenthas shown hat the natureofthe fr1tercakehas a very pronouncedeffect on the rate of flow of f iltrate and that it is, in general,desirable hat ihe particlesforming the filtr cake should have as large a size as possible.More rapid filtration isthereforeobtained fa suitableagent s added o the slurry to causecoagulaiion. fthe solidmaterial is formed in a chemicalreactionby precipitation, the paticle size cangenerally


14/57

241Chemic.Engheer gprocesses

be conholled to a certainextent by the actualconditions of lormatior For example, heparticle siz of ihe resultant prcipitatemay be conholled by varying the iempemtureandco_Dcetrtahon,ndsometimes he pH, ofthe reactingsolutions.As indicatedby Gnacr{s),a flocculateduspensioni es nse o a moreporous akeal$ough ie compr;ssibijiiysgrearer.n manycases. ryslatchapemdybe atre,cd y addjDgrace,of mareriat hichrs selectlvely dsorbed n Darticularaces_ Iilter aids are extensiveiyused wherethe filter cake is relatively impermeable o the8owof fil-nare. beseare mareridts hicbpack o form bedsof rery higb voidages ndrneretorehey are capable f Lncreasjoge poro.iryof $e fiher cake l addcdLo heslury before iltration. Apart ftom economicconsideratjo$, there s an optimum quantityof.fiherajd whichsboutdbe added n anygivencr,e. lt hcreashepre.ence f rbe ileraid reduces e specific esrsrancefr}e filler cake. l also e.ults in rle formarion Iathicker cake.The actual quantib,usedwill rhereforedepenal r rhenaturc ofthe matedat.The use of filter aids is rormally restricted o operations n which the filbate is valuableatrd the cake is a wasieproducl In somecircumstances, owevet fhe filter aid must bereadily separable tom the rest of the filter cakeby physical or chemicalmeans.Filtercakes ncorporatingfi1ter aid arc usually very compressibleanalcare should thereforebetaken to ensure hat rhe good effect of the filrer aid is not destroyedby emptoying toohigh a filtration pressure.Kieselgxbr, which is a commonly used6lter aid, has a"voiaageof about0.85. Addition ofretatively small quanrities ncreaseshe voidage ofmost filtiercakes,and the resr ting porosiry normally lies berween hat of rhe filrer aid a.nd har ofthe filtr solids. Sometimes he fitrer medium is ,,precoaied"with filter aid, and a rhinlayer of the filter aid is removedwith the cakeat rhe enalol eachcycte.ln somecasesbe filnatjonLimc an be redLrced} diluring be.Uspensionn order oreduce he viscosity of the filtlate. This does, of course, ncrease he bulk to be filtered

the temperaturemay be advantageousn that the viscosity of the filhate is reduceal.8,3,7.Washingol the fittercakeWhen the wash iquid is miscibtewith rhe filrrare andhassimilar physicalproperties, herate of washingat the sameprssurediference wjll be about the sameas th; final mteof filhation lf the viscosify of the wash liquid is less, a somewhargrearerrate wil beobtained.Channelli4 somerimesoccurs,however,with the resutt thai much of the cakers rrcompletely washedandrhe fluid passes referenrially kough the channels,which aregradualy enlargedby its continuedpassage. his doesnot occ; during filtration becausechannelsare self-sealingby virtue of depositionof solids ftom the slurry. Channellingis most narked with comprcssible6lter cakesand can be minimised by using a smalleipressureiffereDceor wasbingban or 6lE-ation.. W_ashingmaye regardedas taking place n two stages.Fint, fiIrrare is displaced romthe filte. cake by wash iquid dming the period o displacenent ,ashtug alli in this wayup to 90 per cent of the filtrare may be removed.Duftrg the secondltage, (tiff$ionil,dsr,hg, solvent diffuses into the wash liquid trom the Iess accessible-voids"and thefollowing relation applies:/ \ o l u m eo f w a s b l i q u i d p a s s e d L ^ ^ - ^ , ^ - , . . , ^ _ / D i t i a to D c e n r J d t i o nf s o l u r e\ cake$ickness / - " "-*" ' '"8 \ coo;;;;oo ur pani.utuJU-.,/

(7.3s)


15/57

Liquidnlhaiion42Although an immiscible liquid is seldom used for washing, air is often used ro effectpartial drying of the filter cake. The rate of iow of air must nomally be determtuedexperimentally.

7.4. FILTRATION EQUIPMENT7.4.1.Filter selectionThe most suitable ilter for anygivenoperation s theone which will fulfrl thercquirementsat minimum overall cost. Since he cost of the equipmert s closely elated to the filteringarca, t is nomally desimble to obiain a high overaii rate of filtration. This irvolves theuse of relatively high pressuresalthough the maximum pressuresare often limited bymechadcal design considerations.Although a higher throughput ftom a given filteingsurface is obtained from a continuous ilter than &om a batch operated iltef it maysometimesbe necessaryo use a batch filter, pariicularly if the filter cake has a highrcsistance, ince most continuous ilters operatundr reducedpressureard the maximumfiltration pressures therefore imited. Otherfeatureswhich are desirable n a filter includeease of dischargeof the filter cake in a convenientphysical fonn, and a method ofobserving he qualiry ofthe filtrate obtained rom eachsecttonofthe plart. These actorsare important in considering thc tlT,es of equipmentavailable. Thc most connnon rypesare filter Fesses, leaf filters, and continuous rotary filters. ln additio& there are filtersfor specialpurposes,such as bag filtrs, and the disc type offiiter which is used for theremoval of small quantitiesof soiids ilom a fluid.The most impodant factors in filter selection are the specific rcsistance of the filtercake, he quandryto be filtered, and the solids concentration.For free-filtering materials,a rctary vacuum ilter is generallythe most satisfactorysince t has a very high capacityfor its size and does not require any signmcant manual attentior If the cake has tobe washed, the rctary drum is to be prefeded to the rotary leal lf a high degree ofwashing s required, however, t is usually desirable o repulp the fi1ter cake and to filter

For large-scale iltration, there are tlree principal caseswhere a rotary vacuum filterwill not be used. Firstly, if the speciic resistance s high, a positive prcssure ilter willbe required, and a filter pressmay well be suitable,particularly if the solid conrelrt snot so high that frequelI dismantling of the press s necessary.Secondly, when efrcientwashing is requirc4 a leaf filter is effective, becausevery thin cakes can be preparedand the risk of chamelling during washing is rcduced to a ntuimum. Finally, whereonly very small quantities of solids are presert ir the liquid, an edge filter nay bemployed.Wlilst it may be possibie to predict qualitatively the effect of the physical propertiesof thefluid and the solid on the filtration characteristics f a suspension,t is necessarynall cases o carry out a test on a sample before the large-scaleplant can be designed.A simple vacuum filter with a filtcr area of 0-0065 m': is used to obtain laboratorydata, as illustated in Figure 7.5. The infonnation o filiration rates and specific resis-tance obtained in this way can be dtecily applied to industrial fi1tersprcvided dueaccount is taken of the compressibiliry of the filtr cake. It ca lot be stessed too


16/57

243Chemi@l ngineeringrocesses

Slurry.level

A C_ A T : ) \ COtr integration:

C Ca = e-^lwhere: C is the voiume concentrationof solids in suspensionn the filter,Co s the value of C at the surfaceof the filter,1 is the depth of the filter and

). is the filter coeficiert.

Figure .5. Labohbry esl ilt.strongly thal data ftom any laboratory test cell must not be used without practicalexperience lr the design of industrial u ts where the geometry of the flow chamelis very difTerent.The laying down ofthe cake niuences the struchue o a very a marked

A "compressibility permeability" rest cetl hasbendevelopedby RurHo and cRAcBG)for testing the behaviourof slurries under va ous condirionsof filtration.

7.4.2.Bed FiltersBedilters provide an exampleof the applicationofthe pilrciples of deep beclltrationin which the particlespenetratento the intersticesof the filter bed where iey aretrappedfollowing impingementon the sxrfacesof fhe material of the bed.For the purification of water supplies and for wasi,ewater treafinentwhere the solidcontent s about 10 g/mr or less,as roted by CLr,Asev(23)ranularbed filters have arselvrcplacedbe formerver) slow sand ilters.Tbe bedsare lormed iom eronJarmareriajof gnio s;7e .6-1.2mm in beds .6-1.8m deep. he ery f inepafl ic les tsct iG areremovedby mechanicalaction although he particlesfnally adhereas a result of surfaceelect c forcesor adsoplion.ds lr\r74r poinlsout. Ttis opemrion d5beeoanatysed )Iursex 'r) *ho proposes}e lolloumg equarjon:

('7.36)

(7.37)


17/57


18/57

245ChemicalEngineeg Proesses

aignre .6, A ldgnlrsrpies with2 n by 1.5n platesand rfit the press.Although the production of a thin filter cake results n a high averagemte of filhation, it is necessary o disman{le he pressmore oftea and a greater ime isthereforcspent on this operation.For a fiItration ca.rriedout entirely at constantpressure,a rcarrangement quation7. 9 gives:

t rpx rpLv 2A,(-LP) ,4(_aP)

: B t V * B zwhere Bt and 82 are constants,Thus he ime offiItration, is givenby:

t :B rv2+B2v

w:W is a maximumwhendwldy = 0.

(7.39\(7.40)

('7.4r'The time of dismandingand assemblirg the Fess, say //, is substantia.lly dependentolth thicknessof cakeFoduced. The total time of a cycle itr which a volume y of filtrateis collected s then (t + r/) and the ovelall rate of filhation is given by:

BrV2 BzV*t l


19/57

Liq idtltrarion 46

Figure7.7. Eateard lare pless. -inleipaqa B-fd portsc-f tfate 'ld. D- aire F-pat*

Diffsertiatingr with rqct to y andequdingo zro:B1v2+B2v+{ v (2B tV Bz ) :0

o f , l ' : B t V 2

":l(;)(7. 42)(1 .43 )

If the ei dance f the iltermediums neglected,= t1y' and hetime uringwhichfiltrdion s carried ut s e


20/57

247ChemilEngineeringroesses

It is $own in Exanple .5,whidt appedsder on thech4ter, hd, providedhedoth rej$arce is vsy low, dopting a filtrdion time equd to the do/vntime illgivethe rnadmum hrorghput.Where he doth risance is appreciable,hen hetermBr(r//Br)ur bomesdgnificat ad a longr iltrdion ime is dFirablet mavbesen n Figure .8,which s b6ed on datiaion Exanple .5. thd ndtherof ihesevdues epresentshe minimum o$ condition ower'er,(@t for the unique tudionwhere - {codof $utdown)/rcod uringiltring). nda dec;don as o be mde6 to whdhrcod oJ hroghput s theoveriding onsidrdion.n practice,perdingschedulesreprobblyhedomindingdu.e dthough ignificdrta,r'ingsaybemad;by operding theminimum o$condition.

4000300020001000

0

1 5

0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0Filt6iionime, (ks)Figure7.8. Opiirnigion of plde at irane pr6 (data ro.n Exande ?.t en

\\6shingTwo methods of washing may be employed, ..simpt,' washine and ,,through,, or"thorough"washing.With simplewashing, he wash iquid is fed in rhrough he samecha.nnel s the slulry although, as its velociry nearthe poinr of enrry is high, erosionofthe cake akes place.The channelswhich are thus formed gmdually enlaxgeand wevenwashing is usually obtained. Simple washingmay be usedonly when the fiame is notcompletelyull.In thorough washing,the wash iquid is introduced hrough a separate hannelbehindthe lter cloth on altemateplates, known as washing plates shown in Figure 7.9, andows through the whole thickness of the cake, rst in the oppositedirection and therin the samedirectionas the ltrate. The areaduringwashing s one-halfol thatduringItration and, n addition, the wash iquid has o ow througl tw:ce the thickness,so thatthe mte of washing should threforebe about one-quarterofthe nal rate of ltiation.The wash liquid is usualiy discharged hough the samechannel as the ltrate thoushsometimessepdraleurlet s pro\ ided. venwilh iorough ashing ome hannel l i igoccurs and several r ets are often provided so that the liquid is well distributed. lf thecake s apFeciably compressible,he minimum pressureshouldbe usedduring washing,

EI6 1 0

i o.2oei o.rsIE6E: 0.05

0

.9TS= x33E3


21/57

t 6 l6 |i : IIielre 7.9. Thoroughwashing

and in no case should the final filtration pressurebe excecded.Aftcl washing, the cakemay be made easier o handle by removins excess iquid with compressed n.For case n identification,mall buttonsare embossed n the sidesof the plalesandftames, one on the non-washingplates, wo on th ftames and hree on the washingplaiesasshown n Figure7.10.

Example .1A sluny s ilteredn aplate nd tanepressontaining2 frmes, ach .3m squarend25nnrih;ck.During he iNt 180 tle pressufeiflefeDceor iltrations slowly tised o the iDaLalueof400 kN/m: and.dxdng hisperiod,he raie of filtration s maiDtaiDedorstant-After lhe nitialperlod, iliration s cariedoutat constantressurend he cakes e compLetelyomed i a turt}er900 s. Thecakes e theDwashed ith apressueditTerencef 275 kN/nr for 600 s \\slng haroush,4rrirg (S.c r fi t an,l rni. rLtsenr si.llon i.1.ll. Whats thevolume l filn'ate ollectedpe.cycle d hownuch washwaters rsedA sample f theslu.ryhadpreviouslyeenested ith a eaffihd of0.05n':filteringsu.faceusilga vacuun iving ressureifferercef71.3kN/m']. hevolume fnltrate ollectedn thennt 300s,was250cm: and, fter tudher 00s,anadditjonal50 mrwas olLected.i maybeassumedhat hecake s incompressiblend hat hecloth esistances thesamen the eafas n


22/57

249ChemrcalEngineeg Processes

ligure 7.10. plaresand rades

SolutionIn the leaffilter, filtration is at coflsranrpresslre from the start.

a t - \ a P ) A 2r lus: , tz - r . r , lu" ,In the filter press,a volume yr offilhate is obiained undr constantmteand filtration is then cmied ou at constmr pressure.vi+lu=tfff,,

(1iomequation .18)condjtionsn time n,

(from quation .17)hus:


23/57


24/57

251ChemielEnglieeinqPrc@sses

The recessed plate filter pressThe recessed]?e ofpress is similar o the plateand rame lpe except hat the use offrames is obviated by recessing he ribbed surface of the plates so rhat rhe individualfilter chambersarc fomed betweensuccessive lates.In this type ofpress thercfore thethicknessofthe cakecannot be varied and it is equal to h{ice the deothofthe recessonindividual plates.

Fi8ure?.11. A recessedhanberplate,2m squreThe feed channelshown n Figure 7.11 usually differs from thai emptoyedon the ptateand ftame press.A11 he chambersare cotrnectedby meansof a comparatively largehole in the cenhe of each of the platesand the clotls are secured n Dositionby meansofscrc$ed unions. lurr ie.contai tr ingelat;vclyarge.ol idpanicl is.1ry r.uaityb.halrdled-in iis O?e ofpress without fear ofblocking ihe feed channcls.As describedbyCIcRRy0s), evelopmentsn filter presses avebeer t;wards fhe fabricationoflarger units,madepossibleby mechanisationand the use of newer lighter materialsof constluction.The plates of wood used in earlier times were limitcd in size becauseof limitations ofpressuesand largecast-ion platesprcsenteddifrculty in halrdling. Large platesarenowfrequently nade of rubber mouldings or of pob?ropylene alrhoughdistortion may be aproblem, particularly if the temperatures high.The secondarea of advances in mechanisationwhich enables he opening andclosillg1obe done automatically by a ram driven hydraulically or by an electric motor. platetransportations efTected y fitting triggers to two endtess hainsoperating he plates,andlabour costshave consequentlyben reducedvery considenbly. tmproved designshave


25/57

(b)(c)(d)(el(f)

Liquid [ration252

givenbdterdfajnage hidr h6 led o improved a$ing.Mud *rorter imecydesarenowobtaj edand he cakes re hjnner,moreuniform,anddrier.These vantees har'eb@n athermore eadily bta;ned ith reced pldeswhere hecloth s $bjeded olesswear.Advantages of the filter press

(a) BecaJsef its basjc mpliclty he ilterpres is vssatileard ma/ be usd for awide argeof materids nde' arying pratjngonditionsf cake hicknssdpresureI\,4dtqance o$ s or.I provid6 a Iage fi tering areaon a snd | fl oor spe and oir'additiondGociatedunitsarenedd.Mo$ ointsereexternd rd ekages #ly ddected.Highpf#rre opqations us.tdlyposible.It is equdlys.ritable hdhs thecd(eor the iquid s themajnprodud.

Disadvantages of the filter press(a) lt is ntermjiiet n operatlonndcontinud isrna,tlings apt o cause ighw@ron the cloths(b) Deqite he mprovernentsertjondrs/ioudy,t is faifly heavy n abour.

Example .2A dufrycontining00 gof whjting,f dendty 000 g/rn3,rm3of wats,a1d,s titteredin apldedd franepr6 whi6 td(6 900 to disnartle,ear,ad reasnbte. f the akesincompresjblerd has voidagef 0.4,whd s heoptimlmhickns of cekeora flltrdio'1presre a P of 1000 N/m'z?he endtyf hewhitings3000 g/m3.f hecd(eswdd d500 N/rn' d thetotd olumeofdl watssndoyd s25p6 cet of hdof heilirde, owstheopi rnunrhickns of ihe cd(edfectd? The ej Sfice ol the ilts mdi ummaybenegtddand heviscodtyl wats s 1mNsh'?.n aneeedmel,a prsre differencef 165 N/m,producedlow of water f 0.02cnf/s through catimdrecube f fillr cd(e

SolutionThe bai c fl trdion equdion naybe writtgl as:

1 d v ( - a P )A d/ tplwhse I is ddind 6 the specific ej stnce f lhe cd(e

Thedurryconleins 00kg whiting/mr f waterVolumeof 00kgwhiting: (100/3000) 0.0333m3Voluneol cake:0.0333/( - 0.4) 0.0556 m3.

(qudion .2)


26/57

253Chemical ngineennsbesses

(825, 10iqx io-a, ois69y - y

Volumef liquid nc*e= (0-0556 04) = 0.02 nf .Volumef filtrde: (1 0.02):0.978 n3.Thus volun|eof cd


27/57

Liquidilhaton254

That s:

For dR/dl- = 0, then:

The volune of $ash water = Y/4.Hence: washins ime ,u = 0/4)/(2.66 x t0 'A, lv )

'"=(ffi)(ff):'"'.,'.'The iltrationimet wasshown arliero be: / = t,/i.180 x t0-6= ?.25 105t,

Thus: total ycleime= r,(2.90 x 105 7.25 105) 900= 1.015 to6r,+ 900

TheFte ofcake productions t}enlL: - = Rr . 0 2 5 . t 0 6 L r + 9 0 0

1.025 1064'?+ 00- 2.050 t06I, :0

"' - lJ# ,tr andL:2s 6 r0I n : ze.6mmThus: Frame hickness 59.2 60 mm

7-4.5.Pressureeaf ltersPressureeaf fiiters are designed or final dischfige ofsolids in either a dry or wet state,under totally enclosedorditions, with fully automaticoperation.Each type of pressure eaf filter features a pressulevessel n which are located oneor more lilter elementsot leavesof circular or rectangularcofftuction. Tte filter mediamay be in the fonn of a synthetic fibrc or other fabrics, or metallic mesh.SuDDorts ndinlermedidleraiDage embers re n oarsemeshwilh all componenrseld ogerher yedge bhding. Leaf outlets are connectedndividually to an ourlet manifold which Easseslhrougb e watlof lhe pressurees.cl .The material io be fitered is fed into the vesselunder pressure,and separation akesplacewith ihe solids being depositedon the leaf surface,and the liquid passing hroughthe drainage systemard out of the filter. Cycle times are detemind by pressure,cakecapacityor batch quantity. Wlere particularly fiIIe solids must be rcmoved, a layer ofprecoatmaterial may be depositedon the leavesprior to filtration, using diatomaceousearth, Perlite, or other suitableprecoatmaterials.Cake washing, or recoveryof mother iquor or for removal of solubles,may becariedout beforc dischfige of the solids as a slu.ry or a dry cake.Presswe eaffilters arc supplied n a wide mnge ofsize and matrials of construction.Orc q,pical design is the "Veti-jet" unit wirh a verrical tank alrd vertical leaf fiIter, asshown in Figure 7.12, with rectangr ar leavesmounted ndividually but connected o acommonoutlet manifold. For sluice cleaning either a stationary or oscillaiingjet system


28/57

255Chemical ngineeing rocesses

Figu 7.12. .,Venijet pHsure leafilte.

usiry high efrciency spray nozzles is fitted so as to give complete cake removal. Forrcovery of dry solids, vibmtion of the leavesallows automatic dischaEe of the solialsthrougb bonomdrschargeori pro\ dedwiri a quirk opeoi_ogoor.Ir the "Auto jet" design,circular leavesare mormtedor a hodzontal shaftwhich servesas he filbate outlet manifold. The leavesarerotated during the cleaningcycle although, nadditioD,extra ow speedcontinuous otationduring operationensures niform cakebuild-up in difrcult applications.The Ieavesare of metallic or plastics construction coveretlwith fabdc or wire cloth for direct or precoatoperation,and rotation ofthe leavesaludnscleaning romoresasremcient5luicedischarge i(b njnrmum powercoosumprion.A;an altemafi\e. he lea\,esma] be rolatedoler knile blades hich remove ie;ake in adry state.Units of this type are used for haMling foodstufs and also for the processinqof mineruls trdeftluents.For the handliry of edible oils, molten sulphur,efruerrs and foodstufs, a Filrra-Maticunit is used n which either the burdle is retracted rom the shell as a unit, or the filter tankis retracted eaviry the frlter leavesand filter cover in position. Suchunits are availablem cylindrical, conical or trough shell coniguratiors, and cleaniry may be either wet ordry, nunual or automatic. n the latter case, or dry discharge,vibBtion systemsareusedand for wet removal spray ets mouded in an ovrheadmanifold sweepthe entire leafsurlace n an oscillating motion. In this design, he heavyduty leavescoverealwith clothoI screnare all interchangeable nd,whetherrcund or rectmgular, are all the samesizeto give uniform precoat,cake build-up and filtlatior In horizontal tray pressure ilten,used in batch processesand interrnittent flows, the trays are momted horizontally with


29/57

Liquidittration56

connections o a vertical fiitrate ma fold at the rear, and such udts are ideally suitedwhere cakewashing andpositivecakedrying are required. n many cases, he accumulaiedcakemay be sluiced otr without removing trays from the filter and a special ecovery cafis provided wherc heel filtration is requircd in which a thh layer of cake is 1ft semi-permanently n contact with the filter medium to improve the clarily of the filhate. Thissystem s used n variouschrmcaiionprocessesnd s ideal or handlinghigh flows ofliquidswith a low solidsconcentration.n mosi dsignshe ubesaremounted erticallyfiom a tube sheet at the top of the iank and cleaning s provided with a self-containedintemal "air-pump" backwash, hus a\roiding he use of lalge volumes of sluichg liquid orseparate umpsto provide fast and complete cmoval ofthe filter cdke. The heary gaugepedorated ube cores are coveredwith a seanlesscloth sleve sealedai either end by aclampirlg device. As an altemative, heavygaugewire is wound around the centre corc,with cortrolled spacing o give reliable filtration and easy cake release.Tubular elementunir.01 f i . ry?e rea\i i l .ble 1 .t inoaro. izcc p ro 40 n?.Carlridge iltersOre particular design of pressure ilter is the lter cartridge. typified by the Metafiltcrwhich employs a filter bed depositedon a basc of rings mountcdon a fluted rod, ad isexteffively used for claril,ing liquids containing small quantiiiesofvery fine suspcndedsolids. The rings are accumtelypressed tom sheetmetal of very uniform thicknessandare made n a large nunber of conosion csistant nctals, boughstainless teelsareusually employed. The standarddngs are 22 mm in cxtemal diamcter, 16 nln in inremaldiameter nd 0.8 mm thick, and are scalloped n ore side,as shown n Figure7.13,sothat he edgesof the discsare separatedy a distance f 0.025 0.25 mm accordingo

litsure 7 ll. Rinls for mcrahltcrStelbMe6)


30/57

257Chemicalngineenngrocesses

Fjgure?.14. MhRIrer?ack SteltaMera)

Tq:nements The pack is fomed by mounting the rings, aI the sameway up, on thedrainag od and tightening them togetherby a nut at one enalagainsta boss at the otheras sho$,n n Figure 7.14. The packsare mounted n the body ofthe filter which operatesunder either positive or reducedpressure.- The bed is lormed by feeding a dilute suspeffion of material, to the filter usually al'onnof kjeselgubr.wbich is strainedby .he packs o torm a bed ,U""r : -. rf,i'.1.Nesetgubr s a\artablen a number f gradec nd onnsa bedot- oose rncrure hichis _capable f trapping particles much smaller than the channels.During fittration, thesolids build up mainly on the surfaceand do not genemllypenetratemoie than 0.5 rnminto the bed. The filtrate passes ehveen he discs ind leaves hrough the fluted drainagero4 and operation s continuduntil the resistancebecomes oo l;gh. The filter is th-encleanedby back-flushing,which causes he filter cake to cmck antpeel away. In somecasesthe cleanitg may be incomplete as a result of channelling. If tor any reasonthe spacesbetweenthe dngs becomeblocked, the rings may be quickly removed andwashed.,The.\4etafil(ers $idery Jred lor nlleringdomesricwater.beer.orgatric olvents Ddorrs.. Ine irrralronharactencl icst ch)_l ikcmaterialsanoRenbc improred y rhecontinuousnE-oductioDf a smdlt quariib ol 6trer aid ro rhe sturrl as ;, .n,"i, ii.nner.Un ue orherband.wben hesuspeldedolid s relalively oar.e, heMel.afiller rl loperatesuccessfullyas a stuainer,without the useofa filter bed.


31/57

Liquidillrstion 58

TheMetafifter is very robustand s economical r usebecausehere s no filter cloth andthe bed is easily replacedand hence abour chargesarc low. Mono pumpsor diaphmgmpumpsare most conrmonly used for feeding the filter.

Example7.3The relaiionbetweenlow and head or a certair slury punp may be representedpprcximrelyty a straight irc, the haximum low ar zeroheadbeins0.0015nl /s md tte maximum eadat^.o flow760m or iqurd.Using his pump o feeda panicular lurry o a pressureeaffilter:

(a) How longwill it take oproduce mr offlhare?(b) Whatwill be the pressurecrossle flter alier tiris time?A saopleof the slurry wasfltered at a consrantate of 0.00015mr/s rbrousha leaf fittercove.edwiih a similar ilter cloth but of one enth he areaof the ull-scateunit, and after625 stle pressurcros tle ilter was360 n of liquid.Aftr a firrtner480s rhepressure as600 m ofliquid.

SolutionFor constaDt ate filtmtion througl the filter leaf:

v , L4, - a ' ' tcu reouar ion. t7 'L TPTAi a constantateof0.00015n3/s, then,when he ime:625 s:

y = 0.094mr, (-a") = 1530kN/in,and,a = 1105si y :0.166 1nrand(-Ap) :5890 kN/n,

Substituting}esevaluesnto equaiion .17givs:(0.094)? LA|L x 0.094: lA7/rpx) x3530 x 625

or: 0.0088 0.09441lu = 2.21 x 106A1/rpn (tand: (0.166) 'z+A/u x0.166= ( ,4, / rpu)x 5s90x l t05or i 0.0276 0.166LA/v= 6.51x 106 zlr t tv ( i tEquations i) and (ii) rnay be solved siftultaneously to sive:

L,\ / D= 0.01s4andA'z 4tt : 4.64 l}-eAs the tull-size plut is l0 times that of the leaf filre , tien:

LAlr:0.154 at.ld '/rtr1) = 4.64 x l0 7


32/57

259Chemicalngineeringrccesses

lf thepumpdevelo?s 60 m (7460kN/m,)ar ze.o low andhaszeroheadar O = 0.0015mr/s,itsperfomancemaybe expresedas:_AP ='7460 ('t460/0.0015)AP : 7460 4.9? 1060GN/n )dv A,(_LP)dr = ;t^(v + LAlf (equation16)Substinrdtgor (-Ap) and he ilhatioDconsrantsiv

dv A'1 i.460 4.g1 to,dv &,at ru, ,v -ojS4, -sinceq=dYl&,then:d,v _ 4.6j x 10 1174604.9j ,t t06(dvdt)l

. . d r . 0 / + 0 1 5 4 ) -i . l . (y + 0.154)dy 3.46 I0-3 2.3tdv/dtTheLime.rootlecrmr is fien given y:' . . 1 t . - . - - - .| (v+0.1s4+2.31)dv: / (3_46 r0-r)drJ O J

r = 8 5 7 s. Thepressreat this ime s fomd by substitutiDgn equation . 7 with y = I m3dd I = 857s

l , + 0.154 I = 4.64 t0 7x85?(_ap)and: -Ap = 2902 N/m,


33/57

Ljquidlllralon 260

Tlble 7.2. Clasificalionof vacuum lte6

(-':) C D ECske Cake lillraredryness washing cldity

Horizonral linar tipling pan

Rotary drun+tiing disclareeRotary drm knife dischargeRotary drun rouer discllrgeRohry drm leli discharee

l 3 x x2 1 2 X X3 2 5 X X4 2 0 X Xl 2 0 0 x x ,4 1 5 X X5 8 0 X X5 8 0 X X5 8 0 X X5 8 0 X X X6 l 0 x x6 1 0 X7 I 2 X X8 l 5 X9300xx

5 8I 2t 23-42-32-3

8 98 98 97 88 97 8651L

887'1888765

9

5.1.8.9.t0.

NOTES

L For small batn production. Has very wide application, s very adaptableand cBn be automBted.2. Usually 2 ro 4 pans, for nediunr size balch pmduction. Ve.y wide application, very adaprable,can be3. For free-draining maleriah whcre vcry Coodwashing k EquiFd wilh slar? sep{alion between norherliouorandwsh liouors.f; fEenFining ;atenab wlEF very good wsline is requirod.wide ranse of ?s dnd size available. cdenuy suitable for nost slmi* in categoriesB ard C. Canusually be itted wilh vanous necl8dcal devic* to inprove the {lshing and drying.Rstjcted to very ftee draining nalerials tut reqliring washing.Resrlicted o very fiee-draining naleials not requiring wishing, but w]le the solids eD be retaiftd by

Allows use of high drunr speedard n capableof very high Row rdtes.Large thrcughputs for small floor space.sunable or alnost any clarincaLionnd aorhodline natenab whict blindnormal6hermcdia.

A. Higl solids oncentnLion,omally greaterhan20 er cenl,havinesolidswhicl a@ ree-drainingndf6t settling. givins dimculry in nechanial asilation andciving lish fil1ntlon ra1es.B. Rlpld cake fomatlor wilh reaonably fat serling solids which can be kpt ii suspension y nechanicalC. Lower solids concenrEtion widr solids giviry slow cake fonnation and thin nlter caks which can bedifrcult lo dischdg.D. Low solids concentnlion irith solids giving slow cake fonnation hd lilter cake hdving very poornechanical srrenerh.E. very low solids conceniration i.e. chnn@fon duty). or containine solids w]rjcl blind nomal fllter nedia.Fillralesually rcquncd.

9 = the highestposible perfonnance.I = very poor or neclicible perfommce.


34/57

LIQUIDFILTRATION

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35/57

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36/57

263ChemjlEnsineerinsrccesses

Rotarydrum tiltersBcauseof it's.versatilityand simpliciry, oneofthe most widely usedvacuum ilters is therotary drumflter and, frIter of this type was patentedn England n 1872by Wi i; an;JamesHart. The basicdesignvarieswirh alifferentmamrfaciren, alttrough"e""";i;i;drum t)e vacuumfilters may be divided inro two categoriesi

(a) Thosewhere vacuum is createdwithin comparLnentsormed on the periphery ofthe drum, and(b) Thosewherevacurun s applied to the whole of the interior of rhe drum.The most frequertly usedcotrtinuousdrum t},?e ilters fall into the fi$t category_ hesegive maximlm versatility, Iow costper unit area,and atsoattow a wide variition oftherespectiveime periodsdevoted o filtration, washingand drying.

, Essendally. mutti-con,panmenrrum )?e vrcuJn, iller consisLsf a drumroulinsaDoul bonzontal xjs.ananged o bat hedrun i. pdrtiauy ubmergedn the oueh intiwhich hemateridlo be 6tlered s ted.Thepefipbery f rb;drum is i\ ioed n,o.o'Inpu.r_meol\. acb t$hich is pro\ ided .rh a nrmber fdrain ines. hese as. hrousi beiEside frhedrumand erminater a ringofponsco\eredb) a rorary at\e. rhrouah hichvacurm s applied.Thesurface f thedflm is co\ered$ ith a frlrer abric,and6e drumis arrangedo-rotare t tow speed, suay in rhe ange0.0016_0.004 z (-0.r_O.Z:pmJo. up to 0.05 Hz (3 rym) fof very fice filterins mareriats., As.rbedrum roralec. acb ompannent rdergoesbe amecyctcofopcrations -odhedumtion of eachof these s determinedby tire drum speed, he submergence f&e drumancl he anangementof the valve. The nomal cycle of operationsco;sists of filtration,drying and discharge. t is also possible,however,to introduceother operations nto thebasiccycle, including:

(a) Separationof initial dirty filfate _ which may be anaalvanragef a rclatively openfilter fabric is used.(b) Washiry of the filter cake.(c) Mechanicaldewateringof the filrcr cakv.(d) Clothclanins.. Figure 7.18a shows a tlpical tayout of a rcrary alrum insralation and Figure 7.18bshows the sequenceof cake formation, washing and dewatering.A lu.g" .o:ary d*;vacuum llter is showr in Figure 7.19.ln order to achieve consistentperfonnanceol a continuousfilter, it is necessarv omaintain the filter medium in a ctean condirion. Wirh a drum typ"' ";;';i;'thi"quiresthe completeand continuousrcmoval of the filter cake iom the rl.,r- ,u.f";;-and he operathg condirionsare ofter influencedby the need o form a fuily di."Id;i;cake.Again. n order o achjeve rghcapaciry ndgoodclle uajhina analorarvine. t isvery f ien fsirableo operale irh\eryrhiocates. heretore.be aike is"f ,*S.; i . ;or most drurn O?e vacuum filteft must b aranged so as to ensue the completeandconluruoLrsemoval ferrremel) hin i l ter akes. hemosteffecri \e ar of acbievirsrh is . dererminedoa targc \renr y r,re bys;catamre frhe.ot ia, U.ion1*al"a- '


37/57


38/57

265Chemicalnsineeins rcesses

LIQIJID ILTBATION

Figurc7.19. Rolaryvacuundnm ilrer Ned in a zinc eaching lemrion_.The various discharyesystems hat aresuitable or drum type filters include the stuingdischarye echniquewhich is effective for an extemely wide mnge of materials.Essenltiaily tJrisor olves ormirg rbe iltercakeon aDopen ypeof conveyorwbich s in contacrwiththe filter medium n the filtration, washinganddrying zones.Consequently,he solidswhich are trappedby the fittr medium form a cakeon top of the .,open' conveyo..nro-the. discharye point ol1 the dnrm the conveyot transports he filtei cake to a dischaxgeroll. at whjchpoinLhe cake s djslodged. be cotr\elor thetrpasseslrouei an alsl;smeclFnism odovera rerum olt $hich guidesr back ntoconraowit rfrenfrerOrinraithe conmencementof the cycle and ust above he level of feed iquor in the filter tough.,ln,rhestringdiscbargeyslem. heconveyor onsjsrs t a numberof endJesstri_oiswtrcb are spaced t a pirchof approximatety 2 mm over rle width of rhe 6lterdrum.t be stnngspacingma). howerer.be n ge range _25mm depetrdingn (bemecbanicalpropertiesof the solids.The advantages f the string dischargesystemare:(a) Thin and sticky fifter cakesdowr to about 1.5 rfin of materialssuch as clay maybe effectivly discharged.(b) The filter cloth is alrnost ree ofmechanical wear andtear so rhat thir and alelicatecloths rnay be used, and these cmr be selected almost solely for their filtrationp]opefies. Such cloths arc usually less prcne ro plugging than the strongeratralthicker cloths requied for other discharEe ystems.rc) The cloth can be adached o rle drum in a simplemannerso rhal Etringandsubsequetrtcplaceme canbe carriedour quickly. Normatly, the cloth is liselv


39/57

Liquiditrrarion66wrappedaround he drum and t is secured o thedrun at the edgesand, once acrossthe drum, at the overlap, by a siDlple caulking system. The use of wire winding,clampingbals, and the neccssity f securhg he cloth at evcry panelwhich isessentialwith other dischargesystcms, s avoided.(d) The use of compressedir, to loosen ake tom the drumsurface,s avoided ndconsequentlyhere s no possibilityof blowingback nto tlc filter cakemoisturewhichhaspreviously ccn emoved nder acuxm.This s a possibilitywith knifeJi .ch-rge rl ter. te-ar g \ i r b oq-orck.(e) If required,he patholfte discharge trings anbe altered o that he filter cakeis conveyedby the strings to a convenienrpolnt lor feeding a continuousdryer-extudr,or odrerprcccssing quipment.

A rypicalstringdischarge echanisns shown n Figure7-20.

Figurr I20. Stringdlschargeechannnr n a lilter handllne ilicage lThe knife dischargesystem ncorporatesa kdfe which is aranged so that the surfaceofthe drum unson or ncar o thc knife edge.Thecake s dislodged'rom heclotheirherby its own weight,with fiick and healy cakes,or by applyingcompressedir to theunderside fthe filier cloih.Thcblow-back ir caneitherbe adnriltcd t low pressureora longperiod, r at highpressurcnd Dstantaneouslyy means fa mcchanicalblow-ofTtinlei'. With suitable olids 1 s possibleo operate itb thcknife spacedtom thedrun

tr:'j


40/57


41/57

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42/57

269Chemrcalngrneeringrocesses

compressionor wash blanket which limits still firther any terdency for the air or washliquor to "channel". Washblanketsalso avoid dishr$ance ofthe cake whictr might occurwhen high pressue spraysare direciedon to rhe cake, anatfrey alow dre wasnTiqu;iobe_appliedmuch nearer o fhe poinr whercthe cake"-erges from th" slurry C;;;.."J;rol l


43/57

Liquidfiration 70

may aiso be used to preveni solids settlement,although this rcsults in higher operatingcosts hatr$iih a mecban;calgirarion yslem.Top-feed d.um frlters are used or the treatmentofvery fast settling solids that cafiloteasily be kspt in suspension.A drum filter may be prefemble, rather thar a horizontalpan or similar filter, becauseof its low cost, simpticity and reliability, and becauseofthe rclative easewith which it can be fitted with appropriateaccessory eatures.A topfeed dlum 61ter ncorporatesmulti-compartmeDts, rainlines and a rotaxyvalve identicalto those n a conventionaldrum filter. The feedslrlrry is introducedat, orjust beforc, topdead centle,.ard the cake is discharged tom 90" to 180' from the feed point. Ilte feedslulry can be distributed across he alflm by spmys or a weir box, or it may be containedwithin a 3-sided head-boxwhich is sealed agfist the iroving dium at each end analacross he back on the ascending ilter surface-For rnaterialswhich do not caketogethersumciently to be self-supporting otr the drum surface,a hopper-ttpe top feed filter may


44/57

271ChemilEngineeringrocesses

be usedwhich is similarto a conventionalop fecd filter except hat the walls extendabove he dum suface aroundall sidesof eachcompatment and thesesuppori the fil;cdkc.Be(ause .rgcrpanictescntenore rapidt) rte;e aredep",i"a *". i[.iri., .:",f,ano mauef anrctesbrmrhcoJrer onjorc l the i l lerc" l ,e.A cake f relat;vel) ighporosity is therebyobrainedand high fihration raresare achieved.Modemplantutilises nunswiih surfacesf60 100m2ascomparealith the20 m2ofthe older cast iron drums. Constructionnaterials suchas stainlessstcel, titanium, epoxvresin. nd pi6ricsdtt g.vemuch mp-o\ed ono.ion e.,srarce" "*y;r; . , ' ; ; ;ence nger r ,e. he reptacerncnll the knifcs)stem ) 5ome ormofbcl l hasqivenbettcr alc dischargerd perm,led heL.eof rhuue- i t ieirg ".dir, ." ,h ,. " ;" t . ; ;brcs. fhe b,-hpror dcs somc rppod tor rnecakeand m"re'riatry,.ir,. ,1,; jf.;;;icomp'essedir for l i f t i rrg f i rhccake.Drying aD c i jnpro\ed ] roLalJyo\erinc heh|er u th d lood lmpro\emenrsa\eaisobeermdden Lechniquesor'reducingcakecmchng,...Recent developments n rotary filters include equipmenrmarketd by Dorf-Oliver,illustratedr Figure7.22,which combincs acuum iitrationwith p."r.*" nlt;;;;$hicb f i l rcrcale.mo;srurcs reduced ) 20-.00 percenr.A conoined it ,ering Joryrng ldnlba\bcerdevelopedn qhichd co , iDr.ouretr. arherikca bedsori-nenconsffrction, passesound the undersideofthe drum filter and the frlter cake s depositedon the belt to which it adheres.The beti leavesthe filter iowards the top and is thenc&ried through a cabinei dryer. tt then rcturns to the undersideof the drum filter afterthe cake las been removedby agitation. The tnetai bett, which assists he drying of rhemare' i r l .by irtue i_rr goodhcaFconducrirgrcDenc..s tonred rr" f"opi *i i " f , "r.arneonrougnhedrycfoDd stal on\cyor. i lb I rcl) dividedn"rcf ialslheLotdtossor solrd iom rle bcl l . "5 I nle as I or 2 percenlExample7.4A slury containing0 percent y nasssotids o benlteredn a rorary rum itter2 m diamererand 2 'n longwhich. or$rallyoperales itt 40 percentof its surface;mersed i" ,t " ,t"ry _iu ' l d e rr r e 5 c - r e o f l -\ m . A , d b o r a r o De c r o c r m p e o f r h e .r y s r r g a e a r t r e r o i a r e az u u r a n o o \ e r e dt ha . n r . r . t . ho r h d L o n r h e d r u rr . d u . e d0 0 c r n t i t l b r e r l r h efirst60_s nd 40 cm3n rhenext60 s,whenhe eafwasunaei r absoluteressu."r iZ W.:.T h e b u l k d e N r D_ r h e d Da k e $ s < 0 0 k g n a l J L t , e d e n . i Df r h ( f i t r m , e $ a .0 0 0 k S m . .Ine rlrmum hrckne\si .dle which outd . ,edd.tyerovedror rhe o,h$d\ 5 mm.At whatspeed houldhedrum otareor maximumhroughput ndwhat s this throushournterms fthemalsoftheslunt, ed o theuir pe.unit ime?Solution

, , t 0.02 m,,( aP): ( lot.3 t7) : 843 kN/m? r 84.300 /m?

Y = 0.0003mrY = 0.00044 r


45/57

Liquidillration 72

Thesevalues ar snbstituted nto the constut pressnreilkarion equation:, 2 t 4 V 2 r L p t A zv'+- (equar iotr.18)far

which enables he filtration constantso be determinedas:. L/D =2.19 x l0 I md r t = 3.48 l0r .

lor rhe rory nrrer equarron .t8 sppliesas $" *h"h;pe-'ion is ar connor re,sure Themaximum throughput will be attainedwhen th cake thicknss s a minirnun, rhat is 5 mm or0.005m.Area of filterins surface (2n x 2) = 4r r'|,z.Bulk volumeofcake deposiled (42 x 0.005) 0.063 mr/revolutionIf the rate of filtrate production= u kC/s, hen the volune flow is 0.001 u mr /sFor a 40 per cent slurry: .s/(s + u) : 0.4, and the nals of solids : 0.66u.

Thus:voiune ofsolids deposited: (0.66Dl1500) 4.4 x 10 au mr/slfone rwolution takes s, then:4.4 x l0 4ur:0.061and the mas of filtate producedper revolutioD= I43 kg.Rateof producrion f filrrate= 0.001u mr/s - y/r

Thus: v7 :1 x 10 6uzt2= | x ro-6043) '?0.02 m6

Substituling = 0.141mr and he constantsnto equarion .18,gives:(0.141)'z+22.19 10-3 0.141 2 x 84, 00 (4 ') , r / (3.48 1010)

from which r = 26-95s,which s equal o tine of slbmergencdrevoluiion.TtDs: tine for I revolutio (26.9/0.4)= 67.3 smd: speed (l/67i) = q . IjZ

u = (t43/67.3) 2.11 dss: (0.66 2.l ) kgls

and: rnass f sluny (1.66 2.l ) = 3.5ks/s

Exan1ple .5A platedd frme presswith a filtration Ne of 2.2 tnz is opeEtedwith a pressur rop of413 kN/n'z dd widr a doxnlimeof2l-6 lis (6n). In a testwith a small eaffitter 0.05n: in area,


46/57

TTf.q yg ". ."'""1* ditrerenceof70 kN/rn,, 0.00025m3 offiltrate was obiained n 300 s anda toLal f 0.00040m' in 600 s. t-s,imdreheophrum fi rarion I me tor muirum rhrouuhDutr.i rFeopemnng on iluriig firBr on is r|0 ks rnLJ hecosrofs shurdoun r t00 , w;al is fteoptimum filtration time for mininum cost?

r0')(t

SolutionSubstituting= 0.00025 r at r = 300s in eqnarion.39 ives:

(300/0.00025)0.0002jr/(2 \ 0.0s, jo x lori+ rrrrl(0.05 70xo r : l . 2 x 1 0 6 : 7 . t 4 x 1 0 6 1 4 r + 2 . 8 6 x l 0 - a r r . I ,Snbstituting = 0.00040mr at r = 400s in equalion .39gives:

1.0 106 11.42 t0-6rLLr 2.a6 x rc atpLSolving eqxations i) ard (ii) simultaneouslygivesl

rpr:7 x 10,,Ns/m4and pt = 4.6 x toe N/smjThus,for the plareud ftme filter:

u ' = r 7 - o 7 , = 1 t 0 t 2 / \ 2 ' 2 2 " 4 t 3 ' l 0 r 1 7 5 l 0 s m o. t u Lo - q ; E c , = 4 6 t 0 " / t 2 2 4 l l l 0 r r s 0 o l 0 s m '

Substituiing for V from equation7_43 into equation7.4i, the filaarion time for naximnmt = t' + Bz( /Bt)4.5= 21.6 t0r+ 5.06 10rft21.6l0l /0.75 x t01fr:2.216 x 104 or ?4Lb (6.2h)

v = tel .6 x to\)/ \1. j5 l01l0r=0.l l l mland he meanaleoffilhation is:

v / (t + t') = 0.11t (2.26 >< 04 21.6 tol = 2.54 l0 6ni/s

273Chemicalngineeringrocesses

c:0.01r + 100 /cyclec = (00lr + 100)/yJ/mJ

ftomequation.41 ives:

(ii)

Sutstituting for Ic = (o.otr , + o.otBrv too)/y


47/57

Difierentiatingmd puttjngdcldy = 0:Y: 000/0.01trf5 r

dd liom equation .41, he optimumiltrafion ime fo. ninimum cost s:

Substituting or Ar and Br:r = (100/0.01) B,(r00/0.0181.5)

I = 104 5.06 r0r(104/1.75 106)05= 103 x loas or 10.3 s (2.86I t

Example 7.6A sluny, cortlinins 0.2 kg ofsolid per kilosrm of water is fed to a .orary drum filter 0.6 m longand 0.6 m dimeter. Tne drum rota s at one revolution in 360 s dd 20 per cent of the filieringsurface s in contact with the slurry at oy instmt. If nlhare is producedat the rate of 0. 125 kg/sand the cake has a voidaseof0.5, what ictness ofcake is producedwhen nltering wilh apressurediffereDceof 65 kN/m'1?The densib. ofr}e solids is 3000 ks/mr.The rotary filte. breaks doM ud tie opemtion has to be cffiied out lemlordily in a plate mdframepresswith frmes 0.3 m sque. The press akes 120 s ro disnantle and 120 s to reassenbleanq in addition, 120 s is required o renove the cake fton eeh fEme. IffihratioD is to te caniedout at tle same overall raie d before. wilh an operatingpressureditrercDceof 175kN/mr, what isthe minimum nunrberof fimes that needs o be used dd whar is rhe lhickress of each? t maybe assuned tha the cakesare incompressibleand that th resistuce of rhe nller medium may be

SolutionDrum filter

Area of filteringsurface: (0.6x 0.6u) = 0.362 n,Rateof iltration = 0.125kCA

: (0 125/1000) 1.25x l0r mrls of filrrateI ks or 10 r mr water s 4sociatedwith 0.2 ks of sotids 0.2/(3 x l0r) : 6.67 x l0 I mr of

Since the cake porosiq, s 0.5, 6.67 x 10-5mr of wate. is }eld in the filrer cake dd(10-3 6.67 x 10-5)= 9-33x l0r mr appears s ilr rate, er kg ofroralwater n the slury.Volurneof cake delositedby uDit volumeof filtrate, : (6.6?x l0 5 x 2)/(9.33 x 10 4) :0.143.Volumetric ateof delositionof solids: (l 25 x l0 a x0 i43) = l.?9 x t0-5 6r/s. One re-volution takes360 s. Thereforc hegivn pieceofnltering surface s immersed o. (360 x 0.2) = 72 sThe bulk volune of cake deposited er revolntion: (1.79x t0 5 x 360)=644 x t0-3 n3.Thickness f cakelroduced: (6.44x 10 r)/(0.36r) = 5.7 x l0 r m or 5.7 nm


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49/57

Liquid llEtion 276

Hncs (0.030+ 5.8 0.041d)"= (orr'00640-on'E)= 5.4Thusa minimumof qt@ mu$ be used.Thedzsof frdneswhichwill giveo(adly he quiredde cf illrdion when x ae used eEVn y: ' 0.030 25.8b' 3.n4b- o.@o

qt b2 o.146b o.oo465 0ad: ,:0.073+ /(0.005329 0.00465)

: 0.047 r 0.m9mThus,6 franes of thickns eiths 47 rnm or 90mm will givee(actly he rquird filtrdion rdeiintsmdiate dzesEve higher dsThus dV f6ne th;ckn6 betwen47 dd 9g mm will be sdi$adory. ln prdice,50 mm(2 in) frdnes wouldprobdly be used.

Rotary disc fiftersIn essencqhe disc ilter operalesn a manner imilar o a bottom-fed drum iltr. Theprincipddifference o be notedare he compa'tments hidr are ormdon both acsofverticd discs The6e omprise ght or moresegmnts,@h of whidl is connecledo thehorizontd dt of themahine.he filtrdeanddr aredrarn hroughhe iltermedium,into he drd nage ydrnof thesegments,rd findly throughpass4esn therotay $dtto a vdve locded d oneor bothends.Thediscsarearrarged n he *rdt abod 0.3 m apart s lown in Figure7.23, s-lliingin economy f eae, andconsequentlyfa greder iltrationareacnbe accommodddin a given loor spee than s posiblewith a drum ype ilter.A disc iltr ar ofapproximatdy0 nf can be ahis/ed with dght discq eadl approximddy .5m indiarnds. n thesame ee 6 a2.5 m dia_nder 2.5m drum ilterwhidr wouldhar'ea filtrdionaea of 20 m2.BecaJsef ihe arge rs andconseque-tlyhe dge liquoranddr flowq two rotay vdvs, oneai edr erd of the mdn grdt, are fequentlyitted.Cake ischages usrdly achier'eddnga knifeor wire,anddishargemaybeasidedby blow-backir.Thecakedls through erticdopeningsn the ilter rough.ln generd, isc iltershar'eheadvat4es hat,notonly s the codperunit iltrdiona tow and arge iltrdion ars can be accommoddedn a sndl floor $ace, but hetroughcar be dividd nto two sectionso hat he diffsert d urriscanbe handled t thesameime n thesarnemachi e-Thedsadva{ruE of these nits s thd h,r'y i tr d othsareneb@y, with agred gldglcy o "blind", nd d(ewd ng s virtudly mposibledueto the easewith whidr depostdsolidscan be didurbed rom the vsticd faces,In contra* o a drum ype ilter, n whidr dl pats of the ilter mediumake hesarnepdh through lre durry, eachpat of a disc segmentd(es a differentpdh, depe'dingon ts rdid di$ancerom hehub.Consequa{ly,f homogeneousonditionsannot emdntdnedn the lltef rough,hen ryuner'enakesmaybe ormed, dtuation hidt


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277chemicl ngineeing ro@sses

Figure7.23. Rotarydiscnlter

favous prefereDtialair flow anal esults n a widely va.iablemoisturecontentacrosseachsegment.Disc filteN also have the limitation ihat, becauseevery segmentmust be fullysubmergedn slu.ry prior to theapplicationofvacuum, very little variition in submergencelevel canbe accorrunodated.Precoat filtercAll the filters described previousty have been in the category of cake filters for thevall rcly on the solids present n the feed slury haviry propertieswhich are suirable orthem to act as the actual filter medium and to form cakeswhich are relatively easilydischarged.Applications arise where exrremely hin fittff cakes,pefiaps onty u-auctiooof a millimefe thick, have ro be discharged,where exrremely-good fittr; irh.iq, isessential or wherc a particularly blinding substances prcsent, and filtmtion throu;h apermaneDrilter mediumalone. ucbas a frl(e, abric.may not bc salisfaclory. muchmoreefrcielrt perfomrance s achievedby utilising a bealof easily filtered material whichts precoated o to whal js otherwisea stardardalrum lre fiiter.ARer heprecoals esrabtisbedhcsolids o be remo\;d fromrhe iher teeddre appeJon the sirface ofthe prccoatedbed.This thin laye. of slime is removedby a knife whichis caused o advanceslowly towards the drum. The knife also removes-a hin iayer oithe Fecoated bed so that a rew surfaceof the filter medium is exposed.This pro;edureallows steady iltEtion rates to be achieved.A precoated rotary dru vacuunflter is the only filtsarion equipment from whichextremely hin filter cakescan bepositively and continuously emoved n a semi_drvstate.


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Liquidlhtion 278

The usual Fecoat materials, for most production processes,are conventional filteraids, such as diatomacouseadh and expandedperlite, alrhough ftorhed coat, calciumsulphateand other solids, which folm very pemeable filter cakes,may be usedwheresuchmaterialsare compatiblewith the materialsbeingprocessed.Woorl flour and fly-ashhave also beenused n someapplications,particularly in effuent h.catmentDlants.wheresomc onlaminar.onf L}e ' rercd iquor . ol no great onjequence.he srabt,shmenrof the precoaibed shouldpreferablybe caried out using a precoatslurry of low solidsconcentration,at high drum speedandwith low submergence,o that he bed s built up inthe forn of a thin layer with each cvolution, therebye$udng a u foim and compactedbed which $ ill nor easil)be djsturbed om fie drum.For tongperiods foperarion, hema\imum bed hicknessshouldbe used.This is often determinedby themechanicaldesignof the fiiter and by the cake fomarion properliesof rhe precoarmaterial. Normally rhebed thickness s 75 100 nm and, with mostmateriats his can be established n the drumin a period of about one hour. The t}?e analgrade of precoatmaterial used, n additionto processconsideralions,can iniuence the late at which the bd must be removedandthe cost ofprecoat vacuum 61teroperation.Gcnerally, for maximum economy a precoatfilter shouid operate at the highesrpossible submergmce,consisrentwirh rhe rcquireddegreeof cake washing and drying, when required,and with the lowest possibleknifeadvance rate. Someprccoat filters are capable of operationat up ro 70 per cent drumsubmergencehough this, particularly wiih rhe facility to accommodare100 nrn thickbeds,necessitates pecial design features o give adequateclearancebetween he drumand the trough while at the same ime giving acceptabte ngles for the dischargeknife.Krlifr advance ates arc 0.013 0.13 mn per rvolurion, and drum speedsare fiequentty0-002 0.03 Hz (0.1 2 rev/min). By cafeful conrrol of aU rhe variables,and by corectsizing of the equipment, t is possiblero obtain precoatbeds which last 60 240 houn.To obtain maximum economy n opemtion,a precoatry?evacuumfilter shoutdhave:

(a) Ea5y ariarionf f te kni leddvaDcear((b) Very accuratecontrol of the knife advance ue.(c) A very dgid knife assembly due to the need to advance at extiemely low ratesovef a face widih up to approximately6 m).(d) Rapid advanceand retractionof the knifc.Mechanical and hydlaulic sysrens for control of the knife are now used. The latterallows easy selection of the mte and dircction of knife movemnt and the system spaticuiarly suited to remote conhol or automaticoperation.

EXamDle . /A sludge s filtered n a platedd ftamepressned with 25 mm franes.For thefirct 600s rheslurry pmp runsat maximum apacity.During hjs prioddrepressurises o 415 kN/n: and25 per ceDtof the total filtmte is obtained. he fittration akesa turrher3600s to complete rconstmtpressurend900 s is required or flprying and esettingtrepress.It is found hat f theclothsareprecoaredith filteraid o a depth f 1.6mm. heclorh csisranceis reducedo 25percentof its fome. vahe. Wlat wjll be rhe ncreasen rheovenll ttrrouehDutof thepressf theprecoat anbe appliedn I 80s?


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279Chemicalnsineeinstocesses

SolutionCase 7

For constantEie filhationl

For constantpressnre fltration:

dV*r(, , \ (equanon.16)

' o = oto vo+'bv3 + bvo ato

t(v' - vi) + b(v - vi : a(t- to)ro=600s .- ro 3600 , yo=y /4v2vi+bV=6ood

_ t ^ _d(t i(v'- v"/t6)+ b(v v/4) _3600aThus:

Tbus:

zeoo"fv, +lw =b=+"=^(#.h=i^*Totalcycle ime= (900+ 4200)= 5 00 sFilhationaie= 7/5100= 0.000196yCase2

\ a V o at t , , , D t t V a l b1 / 4 9 ^ ^ \ h t r. l ; v ' - v i l + - l ; v - v , = a( l r , l/ + \ . . /

Thtrs:

%

3 _,"v t9.2002 4 o o - = ; t ,, , - i600 U ,, ll00


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Subsr i tut ingiveql Liquid ltralion 80

rsJau'( ' -" )1a(H*,r#,,).#(;"-.,rr=49 t21 t7 3t28 2048 t024 ta,200

/ t9.200\' - '=l , Jl784 1212o4B )= 2l7ECycleime: (180 900+ 825+ 2178) 4083

t ' 7 v \Filnat ion re= ; : : l=0.000214y\ d 4u . i /. (0.000214 .000196)y

7.8.9.10.1 1 .12.

REFERENCES1. lvES, . J.(ed.):te 9i srinc Bad oi FitrralionNoordhoff,eyder, 975).2. lvEs,K. J:Trans Ing.Chsn.E.4a(190) T94.A.rarcsn de*bed itirdion.3. SrrLL H K-: 'reC.erlcalErgrrcs No.3 4 rod 1976) 75.Dfleopasi o' ndJs.id itrdiol.4. R r". B. E..l,4oNT|o\ G. H. aE t',4oNro\Nd.E.: rd.Eng.Cfm 25 1933) 6ad i53.qJd6in_ filhdion.L Criticdadygsof fdtdio tho.y.t. Fundmsid aiomof ;nd;Fprssrre tilldion.5Rurr ,B.F: lnd.Eng.Chsn27(19s5)708ard806.$udt6inf i t r rd jon. I t .Deivdi ;noigersdnl id ionequdiffs V Naluol 'l-io 'lowthorqh trr eta 40 its aForlr@ In rfe f rrdion-eudid6 RurH. . F.dd KLV'E L.: t.ar6 AJ-l N.Chm E.g.33(j$7j?. Ana(te1door heli,ra methods

4d qudiorsol dch illrdol pf&l ce.o hendd ot continJoj t.rainR rH.B F: Ind Erg Chfl 38 9a6)56. Cdrda,ngntr.tiol lhea/ wirhp-*lieGRAC". . P: cl'en Ers. P.og. 19 19$) 3m. 367. d 42l. R6js;ce ai onpr*ibit.ry ot ftrsro n\G. E. H. and oc' an. F.J.:Clgn Erg.P.og. 7(195.) . Rssalceo kar or .c/tuAN p c: TE.s td. che Erg. 6 11938168 FlrddrflJ pindde o. irdusrid fi raioHEERrs, l',1.: hen.Eng. ri.6 (19t4 190 d 269. udi* in nlidiofr.VaLLr-Ro".. V a_d \4Aro\F", O.:A.t.Ch.E.Jt.{ 1960)82 CoTpais.n t theScifi.rsiSarcsol cd


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B1B2cc'

L

i

281ChemielEng eering roesses

NOTATIONCmssectiond ae of bed or nlrdion aea

Volume coMlration of solids in the f,tier'fotd mst of Rllniio. ler unit volufreValueofc at lter surfacevoidlge of bed or nller cakeLiquid fiadion in feed slurryLiquid fraciion in siurry in vesslVoidaeeat disrancez fron sufaceM6s fraclion of solids in sluryThiclmss of filter ake wirh same esislmcc d clorhThicknessof filter cake oi bedPressue ar downstHn faceof cakePressue al upstrean face of cakePressurear distancez from sufaceDrop in pEssule acrosscakePre$urc dsp acros clolhRateof cakprcductionSlecificHisranceofnlr{ cakeSpecific rsisranceof compressibte ake at disrancezliom suface (equaiion7.25)Funclions of r: independentof A PTime of dismrtling ilter pressTime ar begituing of opemtionMn velocily of flow @iculaiedover dre wnot reaVolumeof liquid Rowi.g in rine tvolumeof liquidpa$ing n time rlVolune of @te deposiiedby unii volme of fitlraleVolumeofsolidsdeposicdby uit votuneof nhrateMss rate ofptudrciion of nltnreOverall voluhetric mre of filr'aiionDislance frcm $rface of tiller cake

Volume fsolids delositedler ujt voluneofflte.

S Sygsn

ii N M , L , T

L 3 T._L-3

LL

ML-IT-?ML_rT-zML_ iT-2ML-IT 2ML-IT 2I]T IL-z

L- rTTTLT'IL3LrL3

;- 'L ] T ILL- rML_LT- 'M L TM L 3

5


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Liquid iltrafon 282PFOBLEMS

7.1. A slurry.conraining .2 kg oi solid^s of warer, fed to a rotxry drum nlrer,0.6 n in diameler d0.6 n long The drom rootes t one evolutionn 160 s and20 percentofthe filtcrinssuface( in conracrwitb the slurryat rny eivcn nstant fhltrle is nroducedt $e t uteof0.12i kg/s and hccak. hasa voidageol0.i. Nhat hicknesol cake s foned vhen filtenngat a presre difference f 65 kN/m/? Th densiry fthe solidh 3000kg/mr.The ohry filter hrerksdoqn and he oleraiionh.s lo be cdied out enporaily in a plareand rahe pre$with ranes0.3 n squarc. hc rcss rdkes 20s to dnnrntle and 120s to reasseinble,od, n addiion,120s sreqtriredo rcmove hccak non eachrane Ifnlradon is (. be caded our rt thesrmcovsall raleasbefore,wnn an opcrating $ure ditfeEnce i 275 N/nr?, whath the nrinnnnm umber f tilnes |hll nust be usedsd what s $e $icknessoi each? t na) be ssumed har he cakes c incompre$ible nd he aistance ofthc ilrer nedia nay beneglecred.7.2. A slniiy conraiiing 01] s oi whitins/mrolwarer. s filieredn a tlrtc and rameFess-which akes OOro dismurlc, clcanand c assenble. de liirer cde is incompresible ndhxs a voidagc f 0.4, whar s lheoptimum hickness i cake or r nhrdon pressuF f 1000 N/nr2lThe densittofthc whiting s 3000kg/nr.lfthe c,*e is washed 500 kN/n2 and&e toul volume fwash waterenlloyed is 25 pd ccntof tharoi rhenlrate. hoq is ihe opdnun rhicknessfcake rffectedlThe resisrancef the ilter mediummay bc neglecredaDd he riscosnyof water s I mN s/m'?,lnatrcxperi'nent, pre$ureoi 165kN/m2produced now of waloof0.02 cnr^ $o gh a cenrimerr eubeoi nlrerct'ke7.3, A platcand ranc p@sr. dle a lord or 8 nr ol nlraLe n l8OO and 1.3 nrr nr 3600s whennrmdonpas stopped Eslimate lhe washnrg ime if 3 Dr of wash watcr is uscd. The rcsistan.e of the cloth nay beneglected nda constant ressus used hroughoui.?,4. In rhe nhdon oi a sludle. he ni(ialpernrd efecredat a consantmte Ri& drc ccd Nnp fullcrpacny.unril rhepe$ure dirieFncet eaches1l]l] Nln2. The pEssuE s the

Chapter 7 Liquid Filtration

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Transcript of Chapter 7 Liquid Filtration