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Spray Drying of Foodsby
Prof.Arun
S.
Mujumdar
National University of Singapore
InternationalWorkshoponDryingofFoodand
Biomaterials
Bangkok June
6
-7,
2011
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Definition
a special process
which is used totransform the feedfrom a liquid state into
a dried particulateform (Powder orParticles) by spraying
the feed into a hotdrying medium.
Definition
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DefinitionDefinition
Whatisspraydrying?
Hot airLiquidfeed
Droplets
Moisture
Heat
Solidformation
POWDER
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Continuous and easy to control process
Applicable to both heatsensitive and heat
resistant materials Applicable to corrosive, abrasive, toxic and
explosive materials
Satisfies aseptic/hygienic drying conditions Different product types: granules, agglomerates,
powders etc can be produced
Different sizes and different capacities
TheAdvantagesofSprayDrying
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Figure Typical spray dryer layout
A conventional spray drying process consists of the following four stages:
1. Atomization of feed into droplets
2. Heating of hot drying medium
3. Sprayair contact and drying of droplets
4. Product recovery and final air treatment
ComponentsofSprayDryingSystem
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Advantages:
Handles large feed rates with single
wheel or disk
Suited for abrasive feeds with proper
design
Has negligible clogging tendency
Change of wheel rotary speed to control
the particle size distributionMore flexible capacity (but with changes
powder properties)
Limitations :
Higher energy consumption compared topressure nozzles
More expensive
Broad spray pattern requires large drying
chamber diameter
Typesofatomizers:Rotaryatomizer
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Advantages:
* Simple, compact and cheap* No moving parts
* Low energy consumption
Limitations:
* Low capacity (feed rate for single nozzle)
* High tendency to clog
* Erosion can change spray characteristics
Typesofatomizers:Pressurenozzle
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Advantages:
* Simple, compact and cheap* No moving parts
* Handle the feedstocks with highviscosity
* Produce products with very small size particle
Limitations:
* High energy consumption
* Low capacity (feed rate)
* High tendency to clog
Typesofatomizers:Pneumaticnozzle
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PowderCollectors
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System A:
It maintains the outlet temperature by adjusting the feed
rate. It is particularly suitable for centrifugal spray dryers.
This control system usually has another control loop, i.e.,controlling the inlet temperature by regulating air heater.
System B:It maintains the outlet temperature by regulating the air
heater and keeping the constant spray rate. This system
can be particularly used for nozzle spray dryers, because
varying spray rate will result in change of the droplet size
distribution for pressure or pneumatic nozzle.
Controlsystems
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SelectionTreeforSprayDryingSystem
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Some
Examples
of
Spray
Drying
Systems
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SomeBasicSprayDryingProcessesusedin
FoodProduction
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SprayDryingofSkimMilk
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MicrographofspraydriedSkimMilk
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SprayDryingofTomatoJuice
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SprayDryingofCoffee
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DevelopingTrends
in
Spray
Drying
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Advantages:
* No fire and explosion hazards
* No oxidative damage
* Ability to operate at vacuum and high operating pressureconditions
* Ease of recovery of latent heat supplied for evaporation
* Better quality product under certain conditions
* Closed system operation to minimize air pollution
Limitations:
* Higher product temperature
* Higher capital costs compared to hot air drying
* Possibility of air infiltration making heat recovery from
exhaust steam difficult by compression or condensation
SuperheatedSteamSprayDrying
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A schematicflowchart of the
conventional spray
freeze drying
SprayFreezeDrying
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ModelingofSprayDrying
Part1: Reductionofparticle-walldeposition
Part2: Evaluationofdropletdryingmodels
Part3: CFDanalysisofairflowstability
Part4: Newparticle-walldepositionmodel
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ModelingofSprayDrying
Part1: Reductionofparticle-walldeposition
Web-likedeposition
(gelatin)
Depositionat
the
conical
wall
(sucrose-maltodextrin)
Dripping
problem
(sucrose-
maltodextrin)
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ModelingofSprayDrying
Part1: Reductionofparticle-walldeposition Experimentstodeterminedepositionfluxes
0.14m2
0.14m2
0.15m2
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ModelingofSprayDrying
Part1: Reductionofparticle-walldeposition Depositionstrengthtester
Airsparger
Adjustabledisperser
angle
Clipstohold
the
plate
Quickcoupling
tocompressed
airline
d l f
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ModelingofSprayDrying
Part2: Evaluationofdropletdryingmodels
Evaluated: ReactionEngineeringApproach (REA) vs
Characteristic
Drying
Curve
(CDC)
Comparedwithsingledropletdata(Adhikari etal.)
Hot drying air
Glass filament
Droplet
M d li f S D i
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ModelingofSprayDrying
Part2: Evaluationofdropletdryingmodels
Axisymmetric model(FLUENT)
Steadystate
Euler-Lagrangian Turbulence:RNGk-e Includedmoisturetransport UDF
(C
language)
for
models
Coupled(2nd orderaccuracy)
Airinlet
Outlet
1.75m
0.50m
0.70m
M d li f S D i
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ModelingofSprayDrying
Part2: Evaluationofdropletdryingmodels
Tracked particle
moisture as it
moves around
M d li f S D i
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ModelingofSprayDrying
Part2: Evaluationofdropletdryingmodels Findings
REA CDC modified
Evaporation rate from
particles, kg s1
M d li f S D i
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ModelingofSprayDrying
Part2: Evaluationofdropletdryingmodels
Deviation: Differentresponsetoinitialmoisture
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 0.5 1 1.5 2 2.5 3 3.5
Time, s
Particlem
oisture,
%wt
80 % wt moisture
60 %wt moisture
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 0.5 1 1.5 2 2.5 3 3.5
Time, s
Particle
moisture,
%wt
90 % wt moisture
80 % wt moisture
70 % wt moisture
50 % wt moisture
REACDC
Modeling of Spray Drying
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ModelingofSprayDrying
Part3: CFDanalysisofairflowstability
Cottontuftvisualization Hotwiremeasurments
Hot wire
Protective sheathe
Modeling of Spray Drying
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ModelingofSprayDrying
Part3: CFDanalysisofairflowstability
Radial direction
Circumference
direction
Inlet
Outlet
Axial
direction
0.7 m
0.6 m
(into paper)
X
Z
Y
Modeling of Spray Drying
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ModelingofSprayDrying
Part3: CFDanalysisofairflowstability Findings: Jetfeedbackmechanism
Deflection to
conical wall
Upward
recirculation
at oppositeside
Modeling of Spray Drying
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ModelingofSprayDrying
Part3: CFDanalysisofairflowstability Findings: Effectofexpansionratio
20.16 s
50.88 s 100.32 s
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0.5
1.0
1.5
2.0
Axial velocity (m s1)
20.16 s
Modeling of Spray Drying
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ModelingofSprayDrying
Part3: CFDanalysisofairflowstability Findings: Effectofexpansionratio
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0.5
1.0
1.5
2.0
5.28s 30.72s
Axial velocity (m s1)
Modeling of Spray Drying
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ModelingofSprayDrying
Part4: Newdepositionmodel Bigchallengeasrigiditychanges ProposedaViscoelastic approach
120 C inlet
Amorphous glass
190 C inlet
Amorphous rubbery
Modeling of Spray Drying
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ModelingofSprayDrying
Part4: Newdepositionmodel Viscoelastic contactmodelling
td
dE
+=
Stress
Storage
coefficient
Strain Loss
coefficient
Strain
rate
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ModelingofSprayDrying
Strongreboundandescape
(diameter:100m,initialvelocity:0.5ms-1,T-Tg:23
C)
Modeling of Spray Drying
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ModelingofSprayDrying
Part4: Newdepositionmodel Findings
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
11.1
1.2
15 17 19 21 23 25 27 29
T Tg,C
R
estitution
factor
0.2 m/s
0.5 m/s
1.0 m/s
1.5
m/s
Modeling of Spray Drying
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ModelingofSprayDrying
Part4: Newdepositionmodel Viscoelastic contactmodelling Superposition technique
Storage modulus Loss modulus
247.1)(228.1 TAE = 056.1)(235 TAE =
( )T
AE ( )TAE ( )TAE ( )TAE
Modeling of Spray Drying
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ModelingofSprayDrying
SomemoreCFDmodelling Work
Modeling of Spray Drying
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ModelingofSprayDrying
Various tested geometries modeled by CFD
Example Specifications Remarks
Different geometry Conical, hourglass,lantern, cylinder
oncone
New idealimitedexperience
Horizontal SDZ New development
Coffee spray dryer two nozzles
installed
Industrial scale
Conventional spray
dryer with rotary
disc
Cylinderoncone
geometry. Rotary
disc atomizer
Conventional
concept first try
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ModelingofSprayDrying
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g p y y g
Cylinder-on-cone
Conicalchamber
Novel
spray
dryer
geometry
tests
ModelingofSprayDrying
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g p y y gNovelspraydryergeometrytests
The possibility of changing the spray chamber geometry was investigated for
better utilization of dryer volume and to obtain higher volumetric heat and
mass transfer performance compared to the traditional cocurrent cylinderon
cone configuration.
The predicted results show that hourglass geometry is a special case and the
cylinderoncone is not an optimal geometry.
The predicted overall drying performance of different geometry designs
show that pure conical geometry may present a better average volumetric
evaporation intensity.
Limitation: no experimental data to compare
The predicted results are useful for the spray dryer vendors or users who are
interested in developing new designs of spray dryers.
ModelingofSprayDrying
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g p y y gOverallheatandmasstransfercharacteristicsofthefourchambers
Case A Case B Case C Case D
Volume of chamber (m3) 0.779 0.501 0.623 0.623
Evaporation rate (103 kg/s) 0.959 0.951 0.9227 0.955
Net Heattransfer rate (W) 2270 2236.88 2165.1 2285
Heat loss from wall (W) 2487.56 2067.67 2300.96 2038.76
Average volumetric
evaporation intensity qm (103kgH2O/s.m
3)
1.23 1.91 1.48 1.53
Average volumetric heat
transfer intensity qh(W/m3)
5463.27 8591.9 7168.6 6940.2
ModelingofSprayDrying
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g p y y gHorizontalspraydryers
ModelingofSprayDrying
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Horizontalspraydryers
ModelingofSprayDrying
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Horizontalspraydrying Streamlinepatterns
Recirculationzoneresultinginparticleremoistenor
overheated
ModelingofSprayDrying
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Betterperformance
can
be
observed
in
Case
G
and
H
More
particlesexitfrom
outlet
Horizontalspraydrying Particletrajectories
ModelingofSprayDrying
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Coffeespray
drying
Deposit conditions:
Topconewall:1(Matched)
Cylinderwall:1293(Not
Matched)*
Fouroutlets:340(Matched)
ConicalWall:329(Matched)
Otherwalls:37(Matched)
*Due
to
18
hammers
shocking
Temperature contours in the drying chamber
ClosingRemarks
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Spray dryers, both conventional and innovative,will continue to find increasing applications invarious industries.
Some of the common features of innovations areidentified. There is need for further R&D andevaluation of new concepts.
Spray drying is an important operation forindustries that deserves multidisciplinary R&Dpreferably with close industryacademiainteraction
ClosingRemarks(Continued)
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In the future, the mathematical model of spraydrying will include not only the transport
phenomena but also product quality predictions.
In the meantime, it is necessary to test andvalidate new concepts of drying in the laboratory
and if successful then on a pilot scale.
Numerous papers dealing with mathematicalmodels for conventional and modified spray
dryers appear regularly in Drying Technology
An International Journal
Thank you for your attention
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Please e-mail for further information:
[email protected]: http://serve.me.nus.edu.sg/arun/
Thank you very much!
Thank you for your attention