Drying of Wet Solid- KERONE

Kerone are manufacturing and design the various type of drying systems in Mumbai, India.
Drying is a mass transfer process consisting of the removal of water or another solvent by evaporation from a solid, semi-solid or liquid. This process is often used as a final production step before selling or packaging products.

1. Complete Engineering Solutions…
Drying of Wet Solid

2. Who are we…
Conceptualized
and shaped in
1976
One of the most
admired and
valuable
company for
customer
satisfaction.
We are pioneer
in application
and
implementation
engineering
40+years
experience in
engineering
excellence.
Immense
expertise in
providing
engineering
solutions.
Witnessed
growth in
revenue and
team strength
YoY

3. ISO 9001:2008 | ISO 9001:2015 | OHSAS 18001 | EMS 14001
Recognized and Rated by
CRISIL
CRISIL Verified
Member of A.M.P.E.R.E.
(Europe)
ASCB(E) Certification for
Best practice
Member of AIMCAL Member of IHEA
Strategic Partners of
EmitechItaly
IRQAO Certified For
Quality
We are accredited by…

4. Why We…

5. Drying-Introduction …
•Drying–isanoperationinwhichtheliquid,generallywater,
presentinawetsolidisremovedbyvaporizationtogeta
relativelyliquid-freesolidproduct.
•Drying–removalofmoisturefromawetsolid,asolutionora
gastomakeitdryisoftennecessaryinindustries.
•Examples:SugarProduction–finalstage
•Washedandcentrifugedsugarcrystalsaredriedtogeta
finishedproductforpackaging.
•Dryingofleatherundercontrolledconditioninleather
processing.
•Soapbarsaredriedtoreducemoisturecontent.
•Dryingofasoliddoesnotdemandorensurecompleteremoval
ofthemoisture.
•Sometimesitisdesirabletoretainalittlemoistureinthesolid
afterdrying.
•Howdoesfiltration,settling,centrifugationvaryfromdrying?
•Thefirstthreeprocessarebasedonmechanicalseparationand
oftenneeddryingasfinalstep.
•Dryingisbasicallygovernedbyheatandmasstransferprinciples.
•Whenawetsolidisheatedtoanappropriatetemperature,moisture
vaporizesatornearthesolidsurface.
•Theheatrequiredissuppliedbyahotgas.
•Asthemoistureisvaporizedfromthesurfacemoremoistureis
transportedfrominsidethesolidtoitssurface.
•Moisturecanmovewithinthesolidbyvariousmechanisms–
dependinguponsolidtypeandstateofaggregation.

6. Drying Process…
Types of solids for drying
•Mechanismofmoisturetransportindifferentsolids:
•Transportbycapillaryforces
•Liquiddiffusion
•Pressureinducedtransport
•Vapourdiffusion
•Thedominantmechanismdependsonnatureofsolid,itsporestructure
anddryingrate
1
•CRYSTALLINE
(SUGAR, SALT
ETC)
2
•GRANULAR
3
•POWDERS
4
•BEADS
5
•SHEETS
6
•FILTER-CAKES

7. Stages of drying of moist solid…
The solid is
uniformly wet
The upper region
is partly dry
a b
The upper region
of the solid is dry
c
•The resistance to moisture diffusion increases from (a) to (b) to (c) and the drying rate decreases
Vertical arrows indicate direction of diffusion.

8. Transport resistance in drying
process…
Resistance to convective heat transfer from
bulk gas to solid surface.
Resistance to liquid or vapourtransport inside
solid
1
Convective mass transfer of vapourfrom
solid surface to bulk gas
2
3
Conduction resistance due to heating of solid at
low temperature operations
4
Three major Transport resistance play important
role in drying
Case Hardening …
In certain solids like soap bar
If drying is rapid, the outer surfaces lose moisture very
quickly, become hard and does not allow moisture to
pass through (impervious) to gas.
Under such condition, drying almost stops although
sufficient moisture is still there inside.
Such solids are dried in a controlled rate by adjusting
temperature and humidity.
3
1
2

9. Drying Phase Equilibria…
•Dryingequilibria–relationbetweenthemoisturecontentofasolidandthe
humidityoftheambientairatequilibrium.
•Ifawetsolidisexposedtoacontinuoussupplyoffreshgaswithpartialpressure
pA,
•Thesolidwilleitherlosemoisturebyevaporationorgainmoisturefromthegas,
•UntilthePAsatvapourpressureofmoistureinsolidequalsthepartialpressureof
gas(PAsat=pA)
•WhenPAsatofmoistureinsolidisequaltopAofmoistureingas,
•Thesolidandthegasaretheninequilibrium.
•Themoisturecontentofthesolidistermedasitsequilibrium-moisturecontentat
theprevailingconditions.
•Equilibriummoistureinasolidcanbedeterminedexperimentallybykeepingthe
solidsuspendedinagasatconstanthumidityandtemperatureforasufficiently
longtime.
•Theliquidcontentofdriedsolidvariesfromproducttoproduct.
•Occasionally,theproductcontainsnoliquidandiscalledbone-dry.
Equilibrium moisture isotherms
•ConsiderthepointMoncurve6forwheat
•At(pt.M)relativehumidityisRH=1andthemoisturecontentisXM
•FormoisturecontentequaltoormorethanXM
•Theequilibriumvap.pr.exertedbythesolidisequaltothevapour
pressureofwateratT(RH=1).
•AtmoisturecontentlessthanXMsolidexertsvap.Pr.Lessthan
thatofpurewater.
•AllmoistureinthesolidaboveXMisunboundmoisture
•AllmoisturebelowXMisboundmoisture

10. Important Definitions…
Moisture Content
Moisturepresentinthesolid(kg
moisture/kgdrysolid).
Bound Moisture
Theamountofmoisturethatexertsa
vapourpressurelessthanthevapour
pressureofwateratthegivenT.
Unbound Moisture
Amountofmoistureinawetsolidin
excessoftheboundmoisture
Equilibrium Moisture
Moisturecontentinasolidthatcan
remaininequilibriumwiththedrying
mediumofagivenrelativehumidityata
givenT.
Free Moisture
Moistureinawetsolidinexcessofthe
equilibriummoistureiscalledfree
moisture

11. Important Definitions…
•AsolidwithinitialmoisturecontentXiisdriedincontactwithgas
ofrelativehumidityRH.
•ThemoistureinsolidexertsavapourpressurePAsatequalto
thatofpurewatertillamoistureXbisreached.
•(Xi-Xb)istheunboundmoisture.
•AsmoisturedecreasesbelowXb,thevapourpressurealso
decreases.
•AsmoistureinsolidreachesX*,thevapourpressureinsolid
becomesequaltothepartialpressureofwatervapouringas.
i.e.itreachestheequilibriummoisturecontent.Corresponding
relativehumidityisR*H
•Xbis–boundmoisture
•Xi-X*-thefreemoisturecontent.

12. Drying Rate Curve…
•Timerequiredfordryingofamoistsolidtoafinalmoisturebecan
determinedfromrateofdryingunderagivenconditions.
•DryingrateofasolidisafunctionofT,Y’,Q,NRe,NScofdrying
gas.
•Dryingratehastobedeterminedexperimentally.
•Themoistsolidistakenonapanwhichiskeptsuspendedina
dryingchamber.
•Dryinggasispassedatagivenflowrate
•Changeinmassofsolidwithtimeisrecorded.
•Massofbonedrysolidisdeterminedseparately

13. Drying Rate Curve cnt…
•Ws=massofbonedrysolid
•a=dryingareai.e.exposedareatogas
•X=moisturecontentatanytimet,
•dX/dtisslopeofX-tplot.Itisnegativequantity.
Rateofdryingis AplotofNvsX(moisturecontent)is
calleddryingratecurve

14. Drying Rate Curve cnt…
•Xi-D:heatingofsolidandreleaseofmoisture
•D-E:Moistureconc.fallslinearlyinthissection.
•TheslopedX/dtisconstant
•Thedryingrateremainsconstantinthissection.
•Thecorrespondingtimeisconstantrateperiod.
•Primarilyunboundmoistureisreleasedduringthisperiod
•SectionsEF&FGshownon-lineardropofmoisturecontent
•Slopeofcurvedecreasesinmagnitudewithincreasingtime
•Dryingrateprogressivelydiminishes
•Bothsectionsarecalledfallingrateperiod.
•AtpointF,slopemaybediscontinuous.
•EquilibriummoisturecontentX*isreachedatG
•AfterG,curvebecomesflat,dryingstopshere.

15. Drying Rate Curve cnt…
•SectionPQ:solidgraduallyisheatedandrateof
dryingincreases.
•OversectionQR:dryingrateremainsconstantatNc.
•MoisturecontentatR:iscriticalmoisturecontentXc
(thepointwhereconstantrateperiodend)
•SectionRSiscalledfirstfallingrateperiod.
•SectionSTissecondfallingrateperiod.
Dryingratecurvecorrespondingto
t-Xcurveisshown…

16. Mechanism of moisture transport…
•Moisturetransportmechanismanddryingisdifferent
overdifferentrangesoftime.
•Atconstantrateperiod:Solidsurfaceisuniformly
moistduetorapidtransportofmoisturefrominside
solidtoitssurface.
•Virtuallynoresistancetoliquidtransportinthesolid.
•Atendofconstantrateperiod:drypatchesappearon
surfacebecausecapillaryforcesarenolongerableto
transportenoughmoisturetokeepentiresurfacemoist.
•Firstfallingperiod:dryingrateisgovernedbycombined
resistances
•Resistancetoliquidorvapourtransportinsidesolid.
•Resistancetoconvectivemasstransferofvapourfrom
surfacetobulkofdryinggas.
•Secondfallingperiod:moisturecontentofsolidbecomes
quitelow.
•Internaldiffusionofmoistureessentiallycontrolstherateof
drying.

17. Critical Moisture Content (CMC)…
•CMCofwetsolidisnotaconstantquantity.
•Itdependsonfactorslike
•Porestructureandparticlesizeofsolid.
•Bedthicknessofwetsolid.
•Dryingrate.
•CMC:isviewedastransitionofthenatureandofmasstransfer
resistancetowardsdrying.
•Forsmallsolidparticles:intra-particleresistancetomoisture
transportisless.
•Constantrateofdryingcontinuesforlongerperiod.
•CMCmaybelessthanthatoflargersolidparticlesofsamematerial.
•Solidswithmoreandlargepores&smallbedthicknessCMCis
small.
•Morehotair(largerdryingrate):CMCislargerduetoresistanceto
internalmoisturediffusioninsolids.

18. Classification of Dryers…

19. Tray Dryer…
•Directheatbatchdryerissimpleinconstruction.
•Theycontaintrays,trucks,shelfetc.
•Thewetmaterialisspreadevenlyonthetrays
whicharestackedoneovertheother.
•Dryinggas(air)isgenerallyheatedbysteam
coils.
•Theblowerorfanforcesthehotgastoflowover
thetrays

20. Rotary Dryer…
•Rotarydriersareworkhorseofchemicaldryer
•Mostwidelyusedcontinuousdryersinprocessindustries.
•Suitableforrelativelyfreeflowing,non-sticky,granularmaterials.
•Almostalltypesofcrystalsaftercrystallizationandwashingaredriedby
thisdrier.
•Arotarydrierconsistsofaslowlyrotatingslightlyinclinedcylindricalshell.
•Itisfedwithmoistsolidattheupperend.
•Thematerialflowsalongtherotatingshell.
•Getsdriedandleavesthedryeratthelowerend.
•Supplyofheatmaybedonedirectlyorindirectly.
•Indirectheatissupplied–itisdirectheatrotarydryer
•Inindirectheatsupply–itisindirectheatrotarydryer.•Hotgasmayflow
directlyparallelofcountercurrentlytothefeed.
•Inindirectdryers:hetissuppliedthroughshellwallbyahotfluegas
flowingoutside.
Drying of
table salt
Ammonium
sulphate
Sodium
sulphate
Drying of
sand
Drying of
minerals
Drying of
organic
solids
Drying of
polymer

21. Direct Heat Rotary Dryer …

22. Flash Dryer…
•TheflashdryerisalsocalledPneumaticconveyer.
•Itconsistofaverticalconveyerpipeintowhichthewetsolidisfednearthe
bottom.
•Hotairentersthedryeratapointbelowthesolidfeedingpointat20to30m/s.
•Velocityshouldbehighenoughtoconveythesolidthroughdryer.
•Smallresidencetime:1to10s(generally3s)
•Dryingrateisveryhigh–thatwhyitiscalledaflashdryer.
•Solidtemperaturedoesnotrisemuchbecauseofshortresidencetime.
•Solidtemperatureoftenremainsatwetbulbtemperatureofdryinggas.
•Suitableforremovingheatsensitivematerial.
•Oftenusedforremovingsurfacemoisture

23. Belt or Conveyer Dryer…

24. Indirect Heat Dryers: Drum Dryer…
•Adrumdryerconsistsoneormoresteamheatedrotatingdrums.
•Thefeed,asolutionoraslurryformsathinlayersonthedrum.
•Thefilmthicknesscanbecontrolledbyadjustingthegapbetweenthedrums.
•Thefeedgetsdriedincontactwiththeheateddrums
•Acycloneisatthetopseparatesthesolidfromthegas.
•Inletgastemperatureisveryhigh.
•Agas,(air)maybeblownoverthesurfaceforquickremovalofmoisture
•Thedryproductisscrappedoffasflakesbyaknife.
•Speedofrotation,1-24rpm
•Dryingrate-10-30Kg/m2.h
•Drumdryerissuitableformaterialsthatistoothickforspraydryingandtoo
thinforarotarydryer.

25. Calculation of drying time from drying rate data
•Thedryingtimecanbecalculatedfromdryingratedatafromlab
experiments
•Thedryingconditions–velocity,T,Y’ofhotgas(air),geometryofdrying
solidshouldbesame.
•Labexperimentsaregenerallydoneatconstantdryingconditions.
•Intialmoisturecontentofsolid=Xi
•Ithastobedriedtofinalmoisturecontent=Xf
•Massofbonedrygas=Ws
•Dryingarea=‘a’
•DryingrateNisconstantatNctillXi=Xc
•X≥Xc,N=Nc=constant
•ButNdependsonXwhen,X≤Xc,N=N(X)
•Sointegrationhastobedonefortwointervals..

26. Calculation of drying time from drying rate data
•Thesecondintegralmaybeevaluatedgraphicallyornumericallyif
thetabulardataNvsXisavailable.
•IfNdecreaseswithXasalinearfunction,inthefallingrateperiod,
•N=pX+q
Drying rate N=0, at equilibrium X=X*
Substituting these results in below
equation

27. Mechanism of Batch Drying …
CrossCirculationDrying

28. Mechanism of Batch Drying …
•Constantrateperiod:Inthisperiod,surfaceevaporationof
unboundmoistureoccurs.
•Therateofdryingisestablishedby
•abalanceofheatrequirementsforevaporationandthe
rateatwhichheatreachesthesurface.
•Therateatwhichmoistureevaporationisgivenby
Nc=KY(Ys-Y)(1)
•Considerasectionofamaterialdryinginastreamofgas.
•Thesolidofthickness=zSisplacedonatray
•TraythicknesszM.
•Thewholesolidisimmersedinastreamofhotgas.
•Hotgastemperature,TG,
•humidity,YG(massmoisture/massofdrygas)
•Gasmassflux,G,mass/time.Area.
•Gastemperature,Ts.
•Dryingsurfacereceivesheatfrom
•qC,byconvectionfromhotgasstream
•qK,byconductionthroughthesolid
•qR,bydirectradiationfromahotsurfacei.e,fromatraylyingabovethe
trayofanalysis.Allunitsare,energy/area.time.
•Theheatarrivingfromthesesourcestothe,surface,isremovedby
evaporatingmoisture.
•Then,thesurfacetemperatureTS,remainsconstant.
•Theentiremechanismresemblesthewetbulbtemperatureprocess,
complicatedbyadditionalheatsources.
•Therateofevaporationandsurfacetemperaturecanbeobtainedby
aheatbalance,
•If,qisthetotalheatarrivingatthesurface,then,
q=qC+qR+qK(2)
•Consideringthelatentheatofvaporization,λs

29. •Thefluxofevaporation,Ncandthefluxofheatflowarerelatedby
•Theheatreceivedatthesurfacebyconvectioniscontrolledbythe
appropriateconvection,heattransfercoefficient,hc,
•Heatreceivedbyradiationcanbeestimatedbyusualmeansand
canalsobeexpressedasaheattransfercoefficient,hR
•Heattransferbyconductiontosolidsurface
•Convectionatthetraybottomwithh.tr.coeff.,hc
•Conductionthroughthetraywallofthickness,zM
•Conductionthroughthesolidlayerofthickness,zS
•Heatreceivedbyconductionandconvectionthroughthesolid
canbecomputedbyheattransferthroughaseriesof
resistances
qk=Uk(TG–Ts)(3)
•Localheattransfercoefficienth=k/z
•UK=overallheattransfercoefficient

30. q =qc + qR+ qK
q= hc(TG-Ts) + hR(TR-Ts) + UK (TG-Ts)
q= (hc+ UK )(TG-Ts) + hR(TR-Ts)
N = KY (Ys-Y)
•KY = gas mass transfer coefficient KY
•Combining all equations and substituting of q,
•Weget:
hc/KY’ = cHor cS
•Theequationhastwounknowns,TsandY’
•Itcanbesolvedusingsaturationhumiditytemperaturerelation
forwatergraphicallybytrialanderrormethod.
•Iftheconductionthroughsolidandradiationeffectsare
neglected,theaboveequationreducestowetbulbequation.
•Thenthesurfacetemperatureisthewet-bulbtemperatureof
thegas.
•Thedryingsurfacewillalsobeatthewet-bulbtemperatureif
thesolidisdriedfromallsurfacesintheabsenceofradiation.
•Forflowofgasparalleltoasurfaceandconfinedbetween
parallelplates,asbetweenthetraysofatraydryers,
•KY’(Y’s-Y’)istheevaporationmasstransferrate
•KY’-humiditytransfercoefficient(m.tr.Coeff.)
•Thesurfacetemperaturemustbeknowntousethisrelation.
•ByconsideringtheLHSoftheequation,
•Dividethenumeratoranddenominator,byh

31. •The hcand Kyare described by heat –mass transfer
analogy
•de= equivalent diameter of the airflow space
•With properties of air at 95 C, this equation becomes
•For drying of sands in trays
•The correlations and the final equations permit direct
estimates of the rate of drying during the constant rate
period
•They are not considered as complete substitutes for
experimental measurements.
•They are of great value when combined with the limited
experimental data.

32. Trusted Partner of following consultants…

33. Our Clients…

34. KERONE
Serving Across Borders…

35. Locate-Us
UNIT I
B/10,Marudhar Industrial Estate,
GoddevFatakroad, Bhayander(E),
Mumbai-401105
Phone : [phone]/14
UNIT II
Plot No. B-47, Addl.
MIDC Anandnagar, Ambernath(East),
Dist. Thane-421506
Phone : [phone]/44/45/46
EMAIL
[email protected] | [email protected] |
[email protected]
WEBSITE
www.kerone.com | www.kerone.net |
www.keroneindia.com