spray drying of foods

8/10/2019 Spray Drying of Foods

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Spray Drying of Foodsby

Prof.Arun

S.

Mujumdar

National University of Singapore

InternationalWorkshoponDryingofFoodand

Biomaterials

Bangkok June

6

-7,

2011


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Definition

a special process

which is used totransform the feedfrom a liquid state into

a dried particulateform (Powder orParticles) by spraying

the feed into a hotdrying medium.

Definition


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DefinitionDefinition

Whatisspraydrying?

Hot airLiquidfeed

Droplets

Moisture

Heat

Solidformation

POWDER


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Continuous and easy to control process

Applicable to both heatsensitive and heat

resistant materials Applicable to corrosive, abrasive, toxic and

explosive materials

Satisfies aseptic/hygienic drying conditions Different product types: granules, agglomerates,

powders etc can be produced

Different sizes and different capacities

TheAdvantagesofSprayDrying


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Figure Typical spray dryer layout

A conventional spray drying process consists of the following four stages:

1. Atomization of feed into droplets

2. Heating of hot drying medium

3. Sprayair contact and drying of droplets

4. Product recovery and final air treatment

ComponentsofSprayDryingSystem


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Advantages:

Handles large feed rates with single

wheel or disk

Suited for abrasive feeds with proper

design

Has negligible clogging tendency

Change of wheel rotary speed to control

the particle size distributionMore flexible capacity (but with changes

powder properties)

Limitations :

Higher energy consumption compared topressure nozzles

More expensive

Broad spray pattern requires large drying

chamber diameter

Typesofatomizers:Rotaryatomizer


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Advantages:

* Simple, compact and cheap* No moving parts

* Low energy consumption

Limitations:

* Low capacity (feed rate for single nozzle)

* High tendency to clog

* Erosion can change spray characteristics

Typesofatomizers:Pressurenozzle


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Advantages:

* Simple, compact and cheap* No moving parts

* Handle the feedstocks with highviscosity

* Produce products with very small size particle

Limitations:

* High energy consumption

* Low capacity (feed rate)

* High tendency to clog

Typesofatomizers:Pneumaticnozzle


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PowderCollectors


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System A:

It maintains the outlet temperature by adjusting the feed

rate. It is particularly suitable for centrifugal spray dryers.

This control system usually has another control loop, i.e.,controlling the inlet temperature by regulating air heater.

System B:It maintains the outlet temperature by regulating the air

heater and keeping the constant spray rate. This system

can be particularly used for nozzle spray dryers, because

varying spray rate will result in change of the droplet size

distribution for pressure or pneumatic nozzle.

Controlsystems


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SelectionTreeforSprayDryingSystem


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Some

Examples

of

Spray

Drying

Systems


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SomeBasicSprayDryingProcessesusedin

FoodProduction


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SprayDryingofSkimMilk


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MicrographofspraydriedSkimMilk


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SprayDryingofTomatoJuice


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SprayDryingofCoffee


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DevelopingTrends

in

Spray

Drying


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Advantages:

* No fire and explosion hazards

* No oxidative damage

* Ability to operate at vacuum and high operating pressureconditions

* Ease of recovery of latent heat supplied for evaporation

* Better quality product under certain conditions

* Closed system operation to minimize air pollution

Limitations:

* Higher product temperature

* Higher capital costs compared to hot air drying

* Possibility of air infiltration making heat recovery from

exhaust steam difficult by compression or condensation

SuperheatedSteamSprayDrying


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A schematicflowchart of the

conventional spray

freeze drying

SprayFreezeDrying


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ModelingofSprayDrying

Part1: Reductionofparticle-walldeposition

Part2: Evaluationofdropletdryingmodels

Part3: CFDanalysisofairflowstability

Part4: Newparticle-walldepositionmodel


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Part1: Reductionofparticle-walldeposition

Web-likedeposition

(gelatin)

Depositionat

the

conical

wall

(sucrose-maltodextrin)

Dripping

problem

(sucrose-

maltodextrin)


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Part1: Reductionofparticle-walldeposition Experimentstodeterminedepositionfluxes

0.14m2

0.14m2

0.15m2


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Part1: Reductionofparticle-walldeposition Depositionstrengthtester

Airsparger

Adjustabledisperser

angle

Clipstohold

the

plate

Quickcoupling

tocompressed

airline

d l f


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Evaluated: ReactionEngineeringApproach (REA) vs

Characteristic

Drying

Curve

(CDC)

Comparedwithsingledropletdata(Adhikari etal.)

Hot drying air

Glass filament

Droplet

M d li f S D i


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Axisymmetric model(FLUENT)

Steadystate

Euler-Lagrangian Turbulence:RNGk-e Includedmoisturetransport UDF

(C

language)

for

models

Coupled(2nd orderaccuracy)

Airinlet

Outlet

1.75m

0.50m

0.70m

M d li f S D i


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Tracked particle

moisture as it

moves around

M d li f S D i


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Part2: Evaluationofdropletdryingmodels Findings

REA CDC modified

Evaporation rate from

particles, kg s1

M d li f S D i


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Deviation: Differentresponsetoinitialmoisture

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 0.5 1 1.5 2 2.5 3 3.5

Time, s

Particlem

oisture,

%wt

80 % wt moisture

60 %wt moisture

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.5 1 1.5 2 2.5 3 3.5

Time, s

Particle

moisture,

%wt

90 % wt moisture

80 % wt moisture

70 % wt moisture

50 % wt moisture

REACDC

Modeling of Spray Drying


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Cottontuftvisualization Hotwiremeasurments

Hot wire

Protective sheathe



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Radial direction

Circumference

direction

Inlet

Outlet

Axial

direction

0.7 m

0.6 m

(into paper)

X

Z

Y



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Part3: CFDanalysisofairflowstability Findings: Jetfeedbackmechanism

Deflection to

conical wall

Upward

recirculation

at oppositeside



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Part3: CFDanalysisofairflowstability Findings: Effectofexpansionratio

20.16 s

50.88 s 100.32 s

3.0

2.5

2.0

1.5

1.0

0.5

0.0

0.5

1.0

1.5

2.0

Axial velocity (m s1)

20.16 s



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Part3: CFDanalysisofairflowstability Findings: Effectofexpansionratio

3.0

2.5

2.0

1.5

1.0

0.5

0.0

0.5

1.0

1.5

2.0

5.28s 30.72s

Axial velocity (m s1)



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Part4: Newdepositionmodel Bigchallengeasrigiditychanges ProposedaViscoelastic approach

120 C inlet

Amorphous glass

190 C inlet

Amorphous rubbery



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Part4: Newdepositionmodel Viscoelastic contactmodelling

td

dE

+=

Stress

Storage

coefficient

Strain Loss

coefficient

Strain

rate



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Strongreboundandescape

(diameter:100m,initialvelocity:0.5ms-1,T-Tg:23

C)



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Part4: Newdepositionmodel Findings

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

11.1

1.2

15 17 19 21 23 25 27 29

T Tg,C

R

estitution

factor

0.2 m/s

0.5 m/s

1.0 m/s

1.5

m/s



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Part4: Newdepositionmodel Viscoelastic contactmodelling Superposition technique

Storage modulus Loss modulus

247.1)(228.1 TAE = 056.1)(235 TAE =

( )T

AE ( )TAE ( )TAE ( )TAE



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SomemoreCFDmodelling Work



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Various tested geometries modeled by CFD

Example Specifications Remarks

Different geometry Conical, hourglass,lantern, cylinder

oncone

New idealimitedexperience

Horizontal SDZ New development

Coffee spray dryer two nozzles

installed

Industrial scale

Conventional spray

dryer with rotary

disc

Cylinderoncone

geometry. Rotary

disc atomizer

Conventional

concept first try


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g p y y g

Cylinder-on-cone

Conicalchamber

Novel

spray

dryer

geometry

tests



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g p y y gNovelspraydryergeometrytests

The possibility of changing the spray chamber geometry was investigated for

better utilization of dryer volume and to obtain higher volumetric heat and

mass transfer performance compared to the traditional cocurrent cylinderon

cone configuration.

The predicted results show that hourglass geometry is a special case and the

cylinderoncone is not an optimal geometry.

The predicted overall drying performance of different geometry designs

show that pure conical geometry may present a better average volumetric

evaporation intensity.

Limitation: no experimental data to compare

The predicted results are useful for the spray dryer vendors or users who are

interested in developing new designs of spray dryers.



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g p y y gOverallheatandmasstransfercharacteristicsofthefourchambers

Case A Case B Case C Case D

Volume of chamber (m3) 0.779 0.501 0.623 0.623

Evaporation rate (103 kg/s) 0.959 0.951 0.9227 0.955

Net Heattransfer rate (W) 2270 2236.88 2165.1 2285

Heat loss from wall (W) 2487.56 2067.67 2300.96 2038.76

Average volumetric

evaporation intensity qm (103kgH2O/s.m

3)

1.23 1.91 1.48 1.53

Average volumetric heat

transfer intensity qh(W/m3)

5463.27 8591.9 7168.6 6940.2



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g p y y gHorizontalspraydryers



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Horizontalspraydryers



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Horizontalspraydrying Streamlinepatterns

Recirculationzoneresultinginparticleremoistenor

overheated



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Betterperformance

can

be

observed

in

Case

G

and

H

More

particlesexitfrom

outlet

Horizontalspraydrying Particletrajectories



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Coffeespray

drying

Deposit conditions:

Topconewall:1(Matched)

Cylinderwall:1293(Not

Matched)*

Fouroutlets:340(Matched)

ConicalWall:329(Matched)

Otherwalls:37(Matched)

*Due

to

18

hammers

shocking

Temperature contours in the drying chamber

ClosingRemarks


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Spray dryers, both conventional and innovative,will continue to find increasing applications invarious industries.

Some of the common features of innovations areidentified. There is need for further R&D andevaluation of new concepts.

Spray drying is an important operation forindustries that deserves multidisciplinary R&Dpreferably with close industryacademiainteraction

ClosingRemarks(Continued)


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In the future, the mathematical model of spraydrying will include not only the transport

phenomena but also product quality predictions.

In the meantime, it is necessary to test andvalidate new concepts of drying in the laboratory

and if successful then on a pilot scale.

Numerous papers dealing with mathematicalmodels for conventional and modified spray

dryers appear regularly in Drying Technology

An International Journal

Thank you for your attention


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Please e-mail for further information:

[email protected]: http://serve.me.nus.edu.sg/arun/

Thank you very much!

Thank you for your attention

spray drying of foods

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