8/10/2019 Lecture Note SprayDryer PPT
1/25
Desi nofDr ers
Ashok Kumar Dasmahapatra
De artment of Chemical En ineerin Indian Institute of Technolo Guwahati
September 16, 2013
8/10/2019 Lecture Note SprayDryer PPT
2/25
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
8/10/2019 Lecture Note SprayDryer PPT
3/25
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 .
8/10/2019 Lecture Note SprayDryer PPT
4/25
Types of Moistures: Definitions
,
p/p
umidit
elative
Gas
FromTreybal,3rd Edn.,pp.661
SolidMoistureContent,X
8/10/2019 Lecture Note SprayDryer PPT
5/25
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.
8/10/2019 Lecture Note SprayDryer PPT
6/25
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)
8/10/2019 Lecture Note SprayDryer PPT
7/25
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)
8/10/2019 Lecture Note SprayDryer PPT
8/25
Desi n of S ra Dr er
8/10/2019 Lecture Note SprayDryer PPT
9/25
Spray Dryer
Removalofmoistureisdoneb convertin the
solutiontosprayandexposingittohotgasstream
Atomizationofthefeedintosmalldropletsincreasesthe
surfaceareforheatandmasstransfer
Usedinfoodanddairyindustry,pharmaceuticalindustry,
deter ents i mentsetc.
Characteristicsofdriedproductsuchasparticlesize,density,morphology,moisturecontentcanbecontrolled
8/10/2019 Lecture Note SprayDryer PPT
10/25
Spray Dryer Schematic
8/10/2019 Lecture Note SprayDryer PPT
11/25
Spray Drying: Stages
FEED SPRAY-AIR
(slurry)ATOMIZATION
CONTACT
EVAPORATION
SEPARATION
8/10/2019 Lecture Note SprayDryer PPT
12/25
Spray Dryer Layouts
Cocurrent Counter
current Mixed
commonlyemployedarrangement
8/10/2019 Lecture Note SprayDryer PPT
13/25
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
8/10/2019 Lecture Note SprayDryer PPT
14/25
Rotary Disc Atomizer
AtomizingDisc
AtomizerAssembly
ro ype Pattern
8/10/2019 Lecture Note SprayDryer PPT
15/25
Drying Chamber Shape
RotaryAtomizer Nozzle
arge
ratio
SmallL/Dratio
S S
8/10/2019 Lecture Note SprayDryer PPT
16/25
Steps to Design Spray Dryer
Material(moisture)andenergybalancetofindoutunknown
parameters
Designofatomizer(speed)
Powerconsumptionbyatomizer
Designofspraychamber(heightanddiameter)
Calculateresidencetime
Calculatevolumeofspraychamber
Calculateheightandangleofconicalbottom
D i O t t
8/10/2019 Lecture Note SprayDryer PPT
17/25
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
8/10/2019 Lecture Note SprayDryer PPT
18/25
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
8/10/2019 Lecture Note SprayDryer PPT
19/25
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
8/10/2019 Lecture Note SprayDryer PPT
20/25
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
8/10/2019 Lecture Note SprayDryer PPT
21/25
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
8/10/2019 Lecture Note SprayDryer PPT
22/25
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
8/10/2019 Lecture Note SprayDryer PPT
23/25
Spray Chamber Dimensions
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)
Spray Chamber Dimensions
8/10/2019 Lecture Note SprayDryer PPT
24/25
Spray Chamber Dimensions
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.
Spray Chamber Dimensions
8/10/2019 Lecture Note SprayDryer PPT
25/25
Spray Chamber Dimensions
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