recent developments in modelling of industrial dryers · 2008. 10. 14. · dryer modelling in...
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Dryer Modelling in Industrial PracticeMagdeburg - Slide 1©2002 AEA Technology plc
Recent developments in modelling of industrial dryers
Ian C KempProcess Manual Product Manager, Hyprotech,
AEA Technology plc, Harwell, UK
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Dryer Modelling in Industrial PracticeMagdeburg - Slide 2©2002 AEA Technology plc
“Scientific Approach”to Dryer Design
Material movementGas flow patterns
Heat transfer
Equipment Model
Drying KineticsDrying EquilibriaProduct QualitySolids Handling
Material Model
Overall System Model
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Dryer Modelling in Industrial PracticeMagdeburg - Slide 3©2002 AEA Technology plc
Design models
Four levels of design:Heat and mass balanceScoping (approximate) design
Psychrometric charts to give hot air flow ratesHeat transfer area for contact dryers
Scaling methods (integral model)Based on experimental batch drying curves
Detailed (full) designIncremental models - stepwise integrationSpecialised models/techniques, e.g. CFD
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Dryer Modelling in Industrial PracticeMagdeburg - Slide 4©2002 AEA Technology plc
Detailed calculations
What do we want to use the models for?
Three types of calculation:Design mode - design new dryer from basic spec, physical properties from databanksPerformance mode - for existing dryer, find effect of changing operating conditionsScale-up - from laboratory-scale or pilot plant experimental data to new full size dryer
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Dryer Modelling in Industrial PracticeMagdeburg - Slide 5©2002 AEA Technology plc
Heat and mass balance(continuous dryer)
Dryer
Dry gas
Wet solids
Wet gas
Dry solids
Heat losses
Indirect heating
WS,XI,TSI,ISIQin
Qwl
(Conduction, radiation, RF/MW)
WS,XO,TSO,ISO
WG,YO,TGO,IGOWG,YI,TGI,IGI
W Y Y W X XG O I S I O( ) ( )− = −Mass balance:
W I W I Q W I W I QG GI S SI in G GO S SO wl+ + = + +Heat balance:
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Dryer Modelling in Industrial PracticeMagdeburg - Slide 6©2002 AEA Technology plc
Scoping Calculations
Design modeThroughput WS and inlet/outlet moistures XI, XO are known, need to calculate size of dryer requiredSelect heat source temperature T, humidity YIContinuous convective dryers: find outlet humidity, required air flow and hence cross-sectional areaContinuous contact dryers: find evaporation rate and required heat transfer surface area, hence dimensionsBatch dryers: find dimensions of dryer to physically contain batch, estimate required drying time
Performance modeSize of existing dryer is known, deduce maximum drying duty (find WS or XI or XO given the other two)
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Dryer Modelling in Industrial PracticeMagdeburg - Slide 7©2002 AEA Technology plc
Use of Psychrometric Chart
Mollier Chart for Air/Water at 101.325 kPa
0
20
40
60
80
100
120
140
160
Ent
halp
y (k
J/kg
)
0 20 40 60 80 100
Gas humidity (g/kg)
0
20
40
60
80
100
120
140
Gas
Tem
pera
ture
(C)
180 200 220 240 260 280 300 320 340 360 380 400 420
Boiling PtTriple PtSat. LineRel HumidAdiabat SatSpot Point
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Dryer Modelling in Industrial PracticeMagdeburg - Slide 8©2002 AEA Technology plc
Drying Models
Scoping design - initial approximate sizing
Scaling/Integral
Layer dryersFluidised beds (simple models)
CFD
Spray drying,complex flows
Incremental
Local conditions
Flash dryers
Rotary dryers
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Dryer Modelling in Industrial PracticeMagdeburg - Slide 9©2002 AEA Technology plc
Drying Kinetics
Moisture Loss from Solid as a Function of Time
Measure•Periodic Weighing•Continuous Weighing•Humidity Difference
Model•Mass Transfer from Surface•Internal Mass Transfer•Characteristic Drying Curve•Receding Evaporative Front•Diffusion
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Dryer Modelling in Industrial PracticeMagdeburg - Slide 10©2002 AEA Technology plc
Drying Kinetics
Induction / Unhindered (Constant Rate) PeriodsDrying rate depends on external conditionsFairly easy to calculate and scale (by ∆T or ∆p)
Falling Rate (Hindered Drying) PeriodMulti-phase moisture transport by: diffusion, convection, capillary action, adsorption etc.Drying rate depends on many parameters which cannot be measured easily, e.g. internal pore structureHence difficult or impossible to calculate rigorously from first principles, though models exist e.g. WhitakerShould always be measured by experiment
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Dryer Modelling in Industrial PracticeMagdeburg - Slide 11©2002 AEA Technology plc
Mass Transfer
Drying KineticsThe Characteristic Drying Curve Concept
Wet Particle
Evaporation
Drying rate per unitexposed surface =
Mass Transfer Humidity f, Relative Coefficient x Difference x Drying Rate
(Configuration (Driving (Materialdependent) Force) dependent)
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Dryer Modelling in Industrial PracticeMagdeburg - Slide 12©2002 AEA Technology plc
Drying rate curves for reference case
φ =−
−
X XX X
eq
cr eq
00.10.20.30.40.50.60.70.80.9
1
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Advanced model
CDC
NN cr
f=
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Dryer Modelling in Industrial PracticeMagdeburg - Slide 13©2002 AEA Technology plc
Drying curves
(kg/kg)
0
0 .1
0 .2
0 .3
0 .4
0 .5
0 .6
0 .7
0 5 0 1 0 0 1 5 0 2 0 0Time (s)
X Advanced model
CDC
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Dryer Modelling in Industrial PracticeMagdeburg - Slide 14©2002 AEA Technology plc
Processing of kinetics data
Moisture - time curveUsually OK; can smooth using cubic splinePeriodic sampling may give few, scattered points
Humidity - time and drying rate - time curvesInvariably more jagged than moisture curveCan be smoothed, but retain raw data
Rate - moisture (Krischer) curveAlso tends to have fluctuations, especially at low rateGreat care needed if drying times are back-calculated
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Dryer Modelling in Industrial PracticeMagdeburg - Slide 15©2002 AEA Technology plc
Unsmoothed drying curves
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Dryer Modelling in Industrial PracticeMagdeburg - Slide 16©2002 AEA Technology plc
Integral (Scaling) Model
Considers the dryer as a whole
is the mean outlet moisture contentE(t) is a residence time function; for batch dryers,
all particles have same residence time τX(t) is the drying curve function, found by scaling
an experimental drying curve
XO
X E t X t dtO =∞
∫ ( ) ( )0
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Dryer Modelling in Industrial PracticeMagdeburg - Slide 17©2002 AEA Technology plc
Drying Curves forWell Mixed Fluid Bed
Batch Drying Curve • Design Curve
0 400 800 1200 1600 2000 2400 2800Time, seconds
Moi
stur
e co
nten
t, kg
/kg
0.3 0.28 0.26 0.24 0.22 0.2 0.18 0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02
0
IBBDC IITBDC
0 100 200 300 400Mean residence time, seconds
Out
let m
oist
ure
cont
ent,
kg/k
g
0.3 0.28 0.26 0.24 0.22 0.2 0.18 0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02
0
t1 =220Z =7.85t2 =1727
XI=0.3
XO=0.0753
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Dryer Modelling in Industrial PracticeMagdeburg - Slide 18©2002 AEA Technology plc
Scaling factors for X(t)
Normalisation factor Z from pilot- to full-scaleFactors involved in scale-up (modify time axis):
Gas temperature TGI or bed temperature TBGas velocity UG - as mass velocity (flux) GBed depth z - as bed weight per unit area mB/AB
e.g. Plug-flow and batch units:
Type A:
Type B:
)( )() )((Z
m A G T T
m A G T TB GI w b
B GI w b
= =−
−∆∆ττ
2
1
2 1 1
1 2 2
/
/
)()(Z
T T
T TGI w b
GI w b
= =−
−∆∆ττ
2
1
1
2
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Dryer Modelling in Industrial PracticeMagdeburg - Slide 19©2002 AEA Technology plc
Recent analysis of fluid bed scaling rules
Original rules based on experimental resultsWhy Types A and B, and how about transition?Recent rigorous derivation gives:
Type A; exponential term tends to zeroType B; (1-e-f.NTU.z)=fkYφaz/G, so
( ) ( ) ( ){ }( ) ( ) ( ){ }
( )( )2
1
22221
11112
1
2
//////
wbGI
wbGI
BwbGIB
BwbGIB
TTTT
GAmTTGAmGAmTTGAm
Z−−
=−−
=∆∆
=ττ
( ) ( ) ( )( ) ( ) ( ) ZeTTGAm
eTTGAmXX
zNTUfwbGIB
zNTUfwbGIB =
∆∆
=−−−−
=∆∆∆∆
−
−
1
2
2..
221
1..
112
21
12
1/1/
ττ
ττ
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Dryer Modelling in Industrial PracticeMagdeburg - Slide 20©2002 AEA Technology plc
Effect of NTU
Mollier Chart for Air/Water at 101.3 kPa
0
20
4060
80
100
120
140
160
180
200220
240
Ent
halp
y (k
J/kg
)
0 20 40 60 80 100
Gas humidity (g/kg)
0
20
40
60
80
100
120
140
160
180
Gas
Tem
pera
ture
(C)
260 280 300 320 340 360 380 400 420 440 460 480
00.2
0.5
1
2510∞
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Dryer Modelling in Industrial PracticeMagdeburg - Slide 21©2002 AEA Technology plc
Drying curves for fluidised bed
0
0.05
0.1
0.15
0.2
0.25
0.3
0 100 200 300 400 500 600 700 800Time
Moi
stur
e co
nten
t
0.00E+00
1.00E-04
2.00E-04
3.00E-04
4.00E-04
5.00E-04
6.00E-04
7.00E-04
8.00E-04
9.00E-04
0 100 200 300 400 500 600 700 800Time
Dry
ing
rate
0.00E+00
1.00E-04
2.00E-04
3.00E-04
4.00E-04
5.00E-04
6.00E-04
7.00E-04
8.00E-04
9.00E-04
0 0.05 0.1 0.15 0.2 0.25 0.3Moisture content
Dry
ing
rate
Falling-rate dryingBut external conditions (G, z) affect drying strongly -Type ANTU>10, drying is controlled by heat content of inlet air except in very final stages
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Dryer Modelling in Industrial PracticeMagdeburg - Slide 22©2002 AEA Technology plc
Temperatures in bed
0
20
40
60
80
100
120
0 100 200 300 400 500 600 700 800
Tem
pera
ture
Gas - Bottom Layer 1Gas - Lower Layer 2Gas - Upper Layers 3/4 Particles
Time
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Dryer Modelling in Industrial PracticeMagdeburg - Slide 23©2002 AEA Technology plc
Integral (scaling) model - summary
Basic concept: scaling an experimental batch drying curve to new conditions / throughputSimilar to scoping method, but allows for falling-rate drying kinetics and heat transferImplicitly assumes the characteristic drying curve concept (CDC) appliesEffective for many layer and batch dryersWill have problems if heat transfer or kinetics change on scale-up or are not limiting factor
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Dryer Modelling in Industrial PracticeMagdeburg - Slide 24©2002 AEA Technology plc
Incremental Model
Stepwise integration along duct, drum or bed
dzz
W Y T UG G G, , ,
W X T US S S, , ,
Gas
Solids
dQWl
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Dryer Modelling in Industrial PracticeMagdeburg - Slide 25©2002 AEA Technology plc
Incremental Model
Over a small increment of time, dtEquations involved: Gives:
Heat transfer to particle surface QPMass transfer from particle dX/dtMass balance on moisture X, Y Heat balance on particle TSHeat balance over increment TGParticle transport and velocity UP, z Local gas properties, e.g. density UG
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Dryer Modelling in Industrial PracticeMagdeburg - Slide 26©2002 AEA Technology plc
Incremental ModelDetailed Equations
Heat transfer to solid
Drying
Function f obtained by drying kinetics test or model
Heat balance on solid
− = = =dXdt
N function X Y T T h a fNS G S cr( , , , , , )
ha T TdXdt
C C XdX dT
dtS G S ev PS PLS( )− = − + + +⎛⎝⎜
⎞⎠⎟
⎛⎝⎜
⎞⎠⎟
λ2
Q hA T T m NS S G S S ev cr= − =( ) λ
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Dryer Modelling in Industrial PracticeMagdeburg - Slide 27©2002 AEA Technology plc
Incremental ModelDetailed Equations
Material transport
Mass balance
Heat balance (ignoring second order terms)
dz U dtS
=
− =W dX W dYS G
( )( )( ) ( )( )( )
W C C X dT C T dX
W C C Y dT C T dY dQ
S PS PL S PL S
G PG PY G PY G Wl
+ +
+ + + + + =λ 0 0
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Dryer Modelling in Industrial PracticeMagdeburg - Slide 28©2002 AEA Technology plc
Particle Motion in a Vertical Flash Dryer
m dUdt
C U U A m g m f UDP
PDS
GG P xs P P
P P = − − −ρ2 2
22
( )
Force Balance:
Acceleration Drag Weight WallForce Force Term FrictionfP = solids-wall friction factor Kf = fPUP
Acceleration for spherical particle/agglomerate:
dUdt
3C U4d a
g f U2D
P D G R2
P PW
P P2
= − −ρρ
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Dryer Modelling in Industrial PracticeMagdeburg - Slide 29©2002 AEA Technology plc
Results before fitting to pilot plant data
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Dryer Modelling in Industrial PracticeMagdeburg - Slide 30©2002 AEA Technology plc
Fitting CalculationsResults after fitting
Changes:dP(SM)330 300xins0.02 0.01Kf0.2 0.4
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Dryer Modelling in Industrial PracticeMagdeburg - Slide 31©2002 AEA Technology plc
Scale-up Method
Obtain results from existing dryerPilot plant or current operating unit
Test against model in Fitting/Pilot modeCompare theory with actual results
Adjust model parametersFit model to experimental results
Design new full-scale plantUsing optimised model as above
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Dryer Modelling in Industrial PracticeMagdeburg - Slide 32©2002 AEA Technology plc
Case Study: Effect of Gas and Solid Flowrate
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Dryer Modelling in Industrial PracticeMagdeburg - Slide 33©2002 AEA Technology plc
Drying in layers
One-dimensional vertical incremental model for layer dryersGives temperature, humidity and moisture profiles through bede.g. deep-layer grain dryerCan be extended to 2-D or 3-D grid by combining with horizontal increments e.g. thick-layer band dryer
TopBoundary
BottomBoundary
Layer 1Layer 2
Layer N
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Dryer Modelling in Industrial PracticeMagdeburg - Slide 34©2002 AEA Technology plc
Computational Fluid Dynamics (CFD)
Rigorous three-dimensional modelSolves Navier-Stokes equationsFine mesh grid in body-fitted coordinates
Requires extensive computing e.g. CFXTested on industrial dryers e.g. by SPS
Verification by observations of flow patterns Small-scale and industrial spray dryersGas flow patterns and particle trackingFeedpoint of pneumatic conveying dryersStill limitations and unknowns for internal transport
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Dryer Modelling in Industrial PracticeMagdeburg - Slide 35©2002 AEA Technology plc
Model verification by LDA measurement
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Dryer Modelling in Industrial PracticeMagdeburg - Slide 36©2002 AEA Technology plc
Spray dryer simulation with CFD
Particle trajectories with swirl- longer residence times
- more effective use of chamber
- lower final moisture content
Particle trajectories with no swirl- short particle residence times
- high final moisture content
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Dryer Modelling in Industrial PracticeMagdeburg - Slide 37©2002 AEA Technology plc
Conclusions
Drying has often been a graveyard for “pure” theorySimple heat and mass balances are very usefulIntegral model is effective for fluidised beds and for most layer and contact dryersOne-dimensional incremental model works well for pneumatic conveying and rotary dryersCFD useful in spray dryers and swirling flowsModels are generally more reliable for scale-up than for design from published data onlyExperimental characterisation of a new material in lab or pilot-plant is essential
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Dryer Modelling in Industrial PracticeMagdeburg - Slide 38©2002 AEA Technology plc
Typical solids process flowsheet
Particle Formatione.g. Crystallization
Solid-Liquid Separatione.g. Filtration
Solids Dryinge.g. Fluidised Bed
Post-processinge.g. Comminution
Gas Cleaninge.g. Bag Filter
Slurry handlingand Pumping
Wet Solids Handlinge.g. Screw Feeder
Effluent ProcessingWastewater treatment
Solution Preparatione.g. Solvent Extraction
FEED
PRODUCT
Solution
Slurry Wet cake
Filtrate
Dry solids
Particles, VOC’s
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Dryer Modelling in Industrial PracticeMagdeburg - Slide 39©2002 AEA Technology plc
Overall Process Concept
Chemical DevelopmentMolecular deconstructionSelect synthesis strategy
Isolation developmentEvaluate potential
isolation routes
CrystallizationEstablishes:Polymorph
PurityAYieldCSDHabit
Defines:SLS task
SLSEstablishes:
PurityBMay alter:
YieldCSD
Defines:Drying task
DryingRemoves:Solvent(s)
May alter: CSDAgglomeration
LumpingBreakageDefines:
Micronisation
MicroniseDelumping
CSD reductionProvides
consistency
Bulk product
NCEfrom
discovery
“Scientific Approach” to Dryer DesignDesign modelsDetailed calculationsHeat and mass balance(continuous dryer)Scoping CalculationsUse of Psychrometric ChartDrying ModelsDrying KineticsDrying KineticsMass TransferDrying rate curves for reference caseDrying curvesProcessing of kinetics dataUnsmoothed drying curvesIntegral (Scaling) ModelDrying Curves forWell Mixed Fluid BedScaling factors for X(t)Recent analysis of fluid bed scaling rulesEffect of NTUDrying curves for fluidised bedTemperatures in bedIntegral (scaling) model - summaryIncremental ModelIncremental ModelIncremental ModelDetailed EquationsParticle Motion in a Vertical Flash DryerResults before fitting to pilot plant dataFitting CalculationsResults after fittingScale-up MethodCase Study: Effect of Gas and Solid FlowrateDrying in layersComputational Fluid Dynamics (CFD)Model verification by LDA measurementSpray dryer simulation with CFDConclusionsTypical solids process flowsheetOverall Process Concept