Lecture Note SprayDryer PPT

8/10/2019 Lecture Note SprayDryer PPT

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Desi nofDr ers

Ashok Kumar Dasmahapatra

De artment of Chemical En ineerin Indian Institute of Technolo Guwahati

September 16, 2013


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Drying

Removal of moisture from a substance by exposing it

to a gas stream

Moisture is often water

Gas is often air

Both heat and mass transfer take place during drying

Heat transfer from air (mostly hot air) to wet solid,

evaporates surface moisture

Mass transfer (transfer of internal moisture) to the


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Types of Moistures: Definitions

Moisturecontent,drybasis(X):kgmoisture/kgdrysolid.

Equilibriummoisture(X*):Moisturecontentofsolidinequilibrium

withagivenpartialpressureofvapor.

FreeMoisture(X X*):Moisturecontentinexcessofequilibrium

moisture.Onlythiscanbeevaporated.

UnboundMoisture:Exertsequilibriumvaporpressureequalto

ureli uid.

BoundMoisture:Exertsequilibriumvaporpressurelessthan .


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Types of Moistures: Definitions

,

p/p

umidit

elative

Gas

FromTreybal,3rd Edn.,pp.661

SolidMoistureContent,X


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Dryer Classification

MethodofOperation MethodofHeatSupply NatureofSubstance

Batch Continuous Direct Indirect Sticky,granular,

heatsensitive,

TrayDryer, TunnelDryer, To

prevent

contaminationTruckDryer,

ShelfDryer

RotaryDryer,

DrumDryer,

S ra Dr er

Oxidationetc.


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Dryer Design: Rules of Thumb

. ry ng mes:

Spraydryer:

few

seconds

Rotarydryer: Fewminutesto1hr

.

2.Drumdryers(forpastesandslurries):

Produceflakes13mmthickwithevaporationratesof1530kg/m2 hr.

Diameter:1.55.0ft;

Rotation rate: 210 r m.

3.Pneumaticconveyingdryers:

Particlesize:13mmdia,upto10mm(surfacedrying).

Airvelocity:1030m/sec.

Singlepassresidencetime:0.53.0sec,upto60secwithnormalrecycling

4.Fluidizedbeddryers:

Particlesize:fewtenthsofammdia,butupto4mmdia.

Gasvelocity:twicetheminimumfluidizationvelocity.

Dryingtime:

12

min

(continuous),

2 3

hrs

(batch,

especially

for

pharmaceutical

products).

(Ref:Walas,ChemicalProcessEquipment,2e)


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Dryer Design: Rules of Thumb

. o ary ryer:

Superficialair

velocities:

510

ft/sec,

sometimes

up

to

35

ft/sec

for

coarse

materials.

Residencetimes:590min.

.

Usually,an85%freecrosssectionistakenfordesignpurposes.

Incountercurrentflow,theexitgasis1020oC abovethesolid;inparallelflow,theo

.

Rotationspeeds(s):~4rpm

D(feet)*s(rpm)~1525.

6.Spraydryers: Forheatsensitivematerials.

Dryingtime:lessthan60sec.

Parallelflowofairandslurryismostcommon.

Atomizingnozzleshaveopenings0.0120.15inwithanoperatingpressuresof3004000

psi. Atomizingspraywheelsrotateatspeedsto20,000rpmwithperipheralspeedsof250600

ft/sec.

Withnozzles,thelengthtodiameterratioofthedryeris45;

Withspray

wheels,

the

ratio

is

0.51.0.

(Ref:Walas,ChemicalProcessEquipment,2e)


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Desi n of S ra Dr er


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Spray Dryer

Removalofmoistureisdoneb convertin the

solutiontosprayandexposingittohotgasstream

Atomizationofthefeedintosmalldropletsincreasesthe

surfaceareforheatandmasstransfer

Usedinfoodanddairyindustry,pharmaceuticalindustry,

deter ents i mentsetc.

Characteristicsofdriedproductsuchasparticlesize,density,morphology,moisturecontentcanbecontrolled


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Spray Dryer Schematic


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Spray Drying: Stages

FEED SPRAY-AIR

(slurry)ATOMIZATION

CONTACT

EVAPORATION

SEPARATION


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Spray Dryer Layouts

Cocurrent Counter

current Mixed

commonlyemployedarrangement


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Types of Atomizer

Atomizer Nozzle

neuma c

Nozzle

RotationRate:

500060000rpm

Pressure:

1060kPa

AirPressure:

13kPa

30120m

Highcapacity

ar c e ze:

180250m

Sim ledesi n

20250m

Canhandlethick,

Largedroplet

trajectory Viscosity

limitation viscous

feed

Expensive


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Rotary Disc Atomizer

AtomizingDisc

AtomizerAssembly

ro ype Pattern


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Drying Chamber Shape

RotaryAtomizer Nozzle

arge

ratio

SmallL/Dratio

S S


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Steps to Design Spray Dryer

Material(moisture)andenergybalancetofindoutunknown

parameters

Designofatomizer(speed)

Powerconsumptionbyatomizer

Designofspraychamber(heightanddiameter)

Calculateresidencetime

Calculatevolumeofspraychamber

Calculateheightandangleofconicalbottom

D i O t t


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Design Output

Rotarytype:RPM,Diameter

SprayChamberDiameter

pray am er e g t y n e r

BottomConeHeightandAngle ResidenceTime

ower onsumpt on

M t i l d E B l


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Material and Energy Balance

GS,Y1, GS,Y2,GAS GAS

DRYERG1, G1 G2, G2

SOLID SS,

X2,S

S,X

1, SOLID

TS2,HS2S1, S1

CocurrentOperation

Material(Moisture)Balance:ObtainGSandY2

EnthalpyBalance:

' '2 1 1 2S SS X X G Y Y

' '2 1 1 2S SS S G GS H H G H H

Assumingadiabatic

operation

of

dryer,

heat

loss

Q

=0

M t i l d E B l


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Material and Energy Balance

GS,Y1,GAS

SS,X1,SOLID

G1, G1TS1,HS1

DRYER

GS,Y2,

GASSOLID

SS,X2,

G2,

G2S2,

S2

At i D i


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Atomizer Design

Assume a reasonablemean droplet size,

Alternatively, based onmean particle size and

say 150 microns and disc

diameter, say 150 mmfeed rate, table below can

be used to obtain disc

Use the correlation below

to estimate disc RPM

3 0.53 0.21 0.391.62 10 (2 )avg S D N S r

N:speedofatomizer,rps

S

2r:discdiameterinm

Davg:averageparticlediameter,mm

(Ref.: Handbook of Industrial Drying, Edited by Arun Mujumdar, Taylor & Francis, 2006, Chapter-10. )

Power Consumed: Atomization


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Power Consumed: Atomization

The power required for atomization depends on solidslurr feed rate eri heral velocit of the atomizer and

the efficiency of the chamber, and is approximated as:

2vS

P S

sS

v

Feed rate

Peripheral velocity, 2v Nr Chamber efficiency

(Ref.: Handbook of Industrial Drying, Edited by Arun Mujumdar, Taylor & Francis, 2006, Chapter-10. )

Operating Air Velocity


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Operating Air Velocity

Assumeoperatingairvelocityastwice

,

s

2

Particle diameter:pd

18p p f

sv

:p :

f

Particle densityAir density

r v scos y

Check Reynolds number, ,Re p s fd v

N

It should be sufficiently low to apply Stokes law

Operating air velocity, 2op sv v

Spray Chamber Dimensions


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From material and energy balance, get air flow rate, (Kg/s)Get operational air velocity, (m/s)

'G

opv

Get density of air, (Kg/m3)

Cross sectional are of spray chamber:

'

o

GA

v

1 2' ' 12

s

Y YG G

Assuming circular cross-section,

Diameter:4

D

Weight fraction of water in feed

or cy n r ca sec on, s assume a e g , =

Weight fraction of solid in feedr

.

(residence time) X (volumetric flow rate of air)



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tot G r V q tVolume of spray chamber2

4

ccyl

D HV

Volume of cylindrical portion

cone tot cylV V V Volume of conical section

2

12cone

cone

VH Height of the conical section

(Diameter is same

as cylindrical section)c

tan 2 cD 2

coneH

Usually, an angle of 60o is used.Revise the volume calculation based on 60o angle.



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Modified calculation: if the angle is too small, a reasonable value could be 60o

2 tan 2

ccone

DH

Height of the conical section(Diameter is same

as cylindrical section)

2

c conecone

D HV

Volume of the conical section

tot cyl coneV V V Total volume of the spray chamber

totV

(Volume of the cylindrical

section does not change)

r

Gqes ence me

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Lecture Note SprayDryer PPT

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