Recent developments in modelling of industrial dryers · 2008. 10. 14. · Dryer Modelling in Industrial Practice Magdeburg - Slide 3 ©2002 AEA Technology plc Design models Four

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


“Scientific Approach”to Dryer Design

Material movementGas flow patterns

Heat transfer

Equipment Model

Drying KineticsDrying EquilibriaProduct QualitySolids Handling

Material Model

Overall System Model


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


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


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:


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)


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


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


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


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


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)


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=


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


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


Unsmoothed drying curves


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


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


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


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


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∞


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


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


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


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


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


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


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


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


Results before fitting to pilot plant data


Fitting CalculationsResults after fitting

Changes:dP(SM)330 300xins0.02 0.01Kf0.2 0.4


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


Case Study: Effect of Gas and Solid Flowrate


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


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


Model verification by LDA measurement


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


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


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


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

Documents

Recent developments in modelling of industrial dryers · 2008. 10. 14. · Dryer Modelling in Industrial Practice Magdeburg - Slide 3 ©2002 AEA Technology plc Design models Four