modelling drying and particle formation in spray towers
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modelling drying and particle formation in spray towers. Christopher Handscomb Wednesday 9 th May 2007. outline. Introduction to spray drying Modelling approach Continuous phase gas flow Single particle drying Conclusions and further work. what is spray drying?. - PowerPoint PPT PresentationTRANSCRIPT
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vapour bubble formation
water removed by evaporation
‘blown shell’
modelling drying and particle formation in spray towers
Christopher Handscomb
Wednesday 9th May 2007
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Christopher Handscomb([email protected])
outline
• Introduction to spray drying
• Modelling approach
• Continuous phase gas flow
• Single particle drying
• Conclusions and further work
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Christopher Handscomb([email protected])
• An important technology in industry
• Used to produce, for example:– Pharmaceuticals– Food stuffs (e.g. milk powder and coffee)– Detergents
• Unique drying technology combining moisture removal and particle formation
what is spray drying?
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Christopher Handscomb([email protected])
what is spray drying?
Feed
Atomisation
rotary atomizer pressure nozzle
Spray-Air Contact
co-current mixedcounter-current
Spray EvaporationPowder
Separation
product discharge from chamber and
separation unit
total product discharge from separation unit
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Christopher Handscomb([email protected])
motivation
A computational model would…
• predict the effect of process conditions on final product properties
• guide the operator towards safe and efficient operating conditions
• facilitate the design of new plant based on physics, rather than correlations
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Christopher Handscomb([email protected])
modelling approach
Co
nti
nu
ou
s P
ha
se
(C
FD
)Sub Models
Particle Drying
Particle-Particle Interaction
Particle-Wall Interaction
• Adopt an Eulerian-Lagrangian framework
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Christopher Handscomb([email protected])
continuous phase
• Commercial CFD package – STAR CD – used to model the continuous phase– Well known in industry– Easy to test different geometries– Relatively simple to incorporate sophisticated
user defined sub models
• Test geometry developed representing a generic spray dryer
• Counter current dryer• Single spray nozzle• Height: 22m• Diameter: 4m• 118,807 cells in CFD mesh
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Christopher Handscomb([email protected])
continuous phase
• Can fairly easily produce plots of the flow field
z=0.5m
z= 4m
• Consider a single droplet
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Christopher Handscomb([email protected])
• Consider the drying sub-model
• Modelling assumptions:– Three component system:
A – solvent; B – solute; D – solid
– Spherical particles, 1D model– Small Biot number uniform particle temperature– Allow for a single centrally located bubble
single particle drying
Assumed ideal binary solution
time
drop
let t
empe
ratu
re
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Christopher Handscomb([email protected])
time
drop
let t
empe
ratu
resingle particle drying
wet bulb temperature
boiling temperature
Cheyne, A., Wilson, I., and Bridgewater, J. (2002).
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Christopher Handscomb([email protected])
single particle drying
• Spherical symmetry reduce to 1-D
• Solve for the moments of this equation
internal coordinates external coordinates
advection terms diffusion terms source term
• Population balance for solids
Cheyne, A., Wilson, I., and Bridgewater, J. (2002).
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Christopher Handscomb([email protected])
single particle drying
• Variable of interest is solids volume fraction
• Related to the moments of the population balance equation by:
• Obtained by solving the moment system:
assumed independent of internal coordinate (particle size)
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Christopher Handscomb([email protected])
evolution advection diffusion crystallization
single particle drying
• Volume averaged transport equations for the continuous phase
• Advection velocity calculated from volume conservation considerations
• Diffusion coefficient from measurements
Volume AveragesSuperficial
Intrinsic
Total
R(c)
S
z
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Christopher Handscomb([email protected])
single particle drying
• Population balance boundary conditions
• Solute boundary conditions
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Christopher Handscomb([email protected])
moving boundary
• Moving boundary handled through a standard coordinate transformation r z:
• This adds a ‘virtual flux’ to all equations
virtual flux
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Christopher Handscomb([email protected])
solution method
• Problem is a system of PDEs
and coupled ODEs
• Solved using Numerical Algorithms Group (NAG) library routines for convection-diffusion type equations
• Finite Volume approach with user-defined flux function
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Christopher Handscomb([email protected])
new drying model – example
• Model described so far can simulate
up to the point of shell formation
• e.g. Consider a system:– Initial 14wt% sodium sulphate solution – no solids– Crystallisation model from Rosenblatt et al. (1984):
‘Kinetics of Phase Transitions in the System Sodium Sulphate-Water’
– Droplet diameter = 1.78mm– Drying air T = 373K– Droplets initially well mixed
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Christopher Handscomb([email protected])
new drying model – example
0 20 40 60 80 100 1200
0.5
1
1.5
2
2.5
3
3.5Simulated vs. Experimental Mass of the Drying Droplet
Time/s
Dro
plet
Mas
s/m
g
Experiment
Model
• Compare with experimental data from Nesic and Vodnik (1990) Kinetics of Droplet Evaporation Chem. Eng. Sci.
Experimental data from Nesic and Vodnik (1990) Kinetics of Droplet Evaporation Chem. Eng. Sci.
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Christopher Handscomb([email protected])
new drying model – example
0 20 40 60 80 100 12020
30
40
50
60
70
80
90
100
110Simulated vs. Experimental Temperature of the Drying Droplet
Time/s
Dro
plet
Tem
pera
ture
/C
Experiment
Model
Experimental data from Nesic and Vodnik (1990) Kinetics of Droplet Evaporation Chem. Eng. Sci.
But the new model can give us much more…
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Christopher Handscomb([email protected])
new drying model – example
0 20 40 60 80 100 1200
0.5
1
1.5
2
2.5
3Simulated Evolution of the Solvent, Solute and Solids Masses
Time/s
Mas
s/m
g
Solvent Mass
Solute MassSolids Mass
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Christopher Handscomb([email protected])
new drying model – example
0 1 2 3 4 5 6 7 8
x 10-4
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1Simulated Continuous Phase Solvent Mass Fraction in a Drying Droplet
Radial Position/mm
Sol
ute
Mas
s F
ract
ion
[-]
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Christopher Handscomb([email protected])
new drying model – example
0 1 2 3 4 5 6 7 8
x 10-4
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7Simulated Solids Volume Fraction in a Drying Droplet
Radial Position/mm
Sol
ids
Vol
ume
Fra
ctio
n [-
]
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Christopher Handscomb([email protected])
new drying model – example
0 20 40 60 80 100 1200
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2x 10
-5Simulated Evolution of moments integrated over the drying droplet
Time/s
Inte
grat
ed N
orm
alis
ed M
omen
tsZeroth Moment
First MomentSecond Moment
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Christopher Handscomb([email protected])
conclusions…
• Introduction to spray drying and the associated modelling challenges
• Results of continuous phase simulation
• Overview of a new drying model
• Comparison with experiments for a ‘simple’ case…
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Christopher Handscomb([email protected])
…work not shown…
• Drying after shell formation
• Simulation of detergent droplets drying with experimental comparison
• Simplified drying models implemented in CFD code
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Christopher Handscomb([email protected])
…and further work
• Obtain data and validate model for high temperature drying
• Couple (simplified) model to CFD simulation
• Compare with existing drying models when used in CFD