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

March 2012

SPRAY DRYING

Technology Hot Air

Inlet

Powder

“Fines”

Return

Cyclone

Spray Spray

”Fines”

Fluid Bed

Chamber

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SPRAY DRYING

Technology

March 2012

SPRAY DRYING

Typical numbers

5 Ton/hr Whole Milk Dryer

► Volume 700m3

► Energy consumption 5 MW

► Efficiency 40-65%

8.5 m

20 m

March 2012

SPRAY DRYING

A ‘Small’ Dryer (1000 kg/hr)

Challenges in Spray Dryer Modeling

Characteristics of Spray Drying

March 2012

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

March 2012

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

March 2012

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

March 2012

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)

March 2012

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

March 2012

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

March 2012

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

March 2012

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

March 2012

PARAMETRIC STUDY

Results Base Case

March 2012

PARAMETRIC STUDY

Results Base Case

March 2012

PARAMETRIC STUDY

Results Base Case

Particle Tracks dp < 60mm & dp > 100mm

Observations:

► Small particles concentrate in Jet centre

► Large particles concentrate on Jet edges

March 2012

PARAMETRIC STUDY

Results Base Case

Particle Tracks: Temperature

Observations

► Initial stage: wet bulb temperature

► Highest temperatures near Jet centre

► Final stage: uniform temperature

March 2012

PARAMETRIC STUDY

Results Base Case

Particle Tracks dp < 60mm & dp > 100mm

Challenges in Spray Dryer Modeling

Validation

March 2012

VALIDATION

Drying in reality

Mechanism Depends on:

► Chemical composition

► Initial concentration

► Air temperature

► Mixing of droplets and air

► Trajectory

► Agglomeration

March 2012

VALIDATION

Drying, even more Real

Cone (Product) Cyclone (Fines)

Powder Morphology: External Structure

SEM Images “Regular” Whole Milk Powder

March 2012

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

March 2012

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