Challenges in Spray Dryer Modeling - mdx2.plm.automation...
Transcript of Challenges in Spray Dryer Modeling - mdx2.plm.automation...
Challenges in Spray Dryer Modeling
Alfred Jongsma,
Tetra Pak CPS
March 2012
March 2012
CHALLENGES IN SPRAY DRYER MODELING
Content
Spray Drying
► Technology
Modeling
► Motivation
► Characteristics
Droplet Drying
► Modeling
► Parametric Study
Validation
Conclusions
Challenges in Spray Dryer Modeling
Spray Drying
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SPRAY DRYING
Technology Hot Air
Inlet
Powder
“Fines”
Return
Cyclone
Spray Spray
”Fines”
Fluid Bed
Chamber
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SPRAY DRYING
Technology
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SPRAY DRYING
Typical numbers
5 Ton/hr Whole Milk Dryer
► Volume 700m3
► Energy consumption 5 MW
► Efficiency 40-65%
8.5 m
20 m
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SPRAY DRYING
A ‘Small’ Dryer (1000 kg/hr)
Challenges in Spray Dryer Modeling
Characteristics of Spray Drying
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MODELING
Characteristic Spray Drying Process
Continuous Phase (Air)
► Confined Jet = Transient!
► Jet Reynolds Number 106
Dispersed Phase (Droplets / Particles)
► Dilute : no collisions (αp < 10-3)
► p / air >> 1 : only drag and gravity
Lagrangian approach favoured over 2 Fluid model:
► Easier closures
► Easier incorporation of additional Particle Models, e.g. drying
Challenges in Spray Dryer Modeling
Incorporation of Droplet Drying
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DROPLET DRYING
Inter-Phase Transfer
MILK
DROPLET
FILM FILM
0 R R
GAS GAS
HEAT MASS
TE
MP
ER
AT
UR
E
CO
NC
EN
TR
AT
ION
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DROPLET DRYING
Heat Transfer
MILK
DROPLET
FILM
0 R
GAS
HEAT
TE
MP
ER
AT
UR
E
Assumptions
► No internal gradient
Biot << 1
► Schiller Nauman
► Spherical Particles
► Ideal Shrinkage
► At Particle Surface
Tparticle = Tgas
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DROPLET DRYING
Mass Transfer
MILK
DROPLET
FILM
0 R
GAS
MASS
CO
NC
EN
TR
AT
ION
Assumptions
► Internal gradient
Biot > 1
► Transfer limited by internal gradient “CDRC”
► Spherical Particles
► Ideal Shrinkage
► At Particle Surface, Sorption Equilibrium:
Xparticle = f(Xgas)
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DROPLET DRYING
CDRC: Critical Drying Rate Curve
1.0
0.8
0.6
0.4
0.2
0 0 0.2 0.4 0.6 0.8 1.0
Xe
*
,
1
1log
w
bw
a
wp
Y
Y
M
MDShdf
dt
dm
n
XX
XXf
ec
e
Xc
f
Best fit n = 2
Yw,b Xw Yw *
X
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DROPLET DRYING
Implementation
Field functions
► Correction Mass Transfer Coefficient (Xc & n are constants)
► Sorption Isotherm
n
XX
XXf
ec
e
)1)(1(
0
www
we
Ckakaka
aCkmX
)(,2
2
Tp
pa
SatOH
OHw
CFD Model Spray Drying
Parametric Study
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PARAMETRIC STUDY
Simplified Geometry Air Inlet
► T = 200°C
► m = 2.9 kg/s
► Yw = 0.01
Skim milk
► T = 70°C
► m = 0.34 kg/s
► Xw = 0.5
► 1 Nozzle:
► v = 194 m/s
Air Outlet
► T = 75°C
► Yw = 0.064
15°
35°
Hollow
Cone
Base
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Parameter Base Case Variations
Air
► Velocity (Diameter) d=base small & large
► Velocity profile Normal Swirl
► Turbulence intensity Medium High & low
Droplets
► Inlet Velocity v = 194m/s 100 & 250
► Size PSD 44, 102 & 160 mm
Atomisation
► Cone angle 35° 25º & 45º
► Spray pattern Hollow cone Full cone
► Nozzle Position Medium High & low
PARAMETRIC STUDY
Parameter Variations
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PARAMETRIC STUDY
Results Base Case
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PARAMETRIC STUDY
Results Base Case
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PARAMETRIC STUDY
Results Base Case
Particle Tracks dp < 60mm & dp > 100mm
Observations:
► Small particles concentrate in Jet centre
► Large particles concentrate on Jet edges
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PARAMETRIC STUDY
Results Base Case
Particle Tracks: Temperature
Observations
► Initial stage: wet bulb temperature
► Highest temperatures near Jet centre
► Final stage: uniform temperature
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PARAMETRIC STUDY
Results Base Case
Particle Tracks dp < 60mm & dp > 100mm
Challenges in Spray Dryer Modeling
Validation
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VALIDATION
Drying in reality
Mechanism Depends on:
► Chemical composition
► Initial concentration
► Air temperature
► Mixing of droplets and air
► Trajectory
► Agglomeration
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VALIDATION
Drying, even more Real
Cone (Product) Cyclone (Fines)
Powder Morphology: External Structure
SEM Images “Regular” Whole Milk Powder
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VALIDATION
Drying, even more Real
Powder Morphology: Internal Structure
FIB SEM Images “Regular” Whole Milk Powder
Cone (Product) Cyclone (Fines)
Challenges in Spray Dryer Modeling
Conclusions
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CHALLENGES IN SPRAY DRYER MODELING
Conclusions
Spray Dryer Modeling
► Prohibitively Expensive to build Pilot Plants
► CFD Model Development
► Validation & Refinement Needed
Droplet Drying
► Simple approach already reveals important phenomena
► Real drying much more complicated
Planned refinements
► Account for Morphology
► Influence on Drag / Drying Rate etc.
► Collisions & Agglomeration