spray drying of foods - kmutt drying of foods.pdf · spray drying of foods by ... heating of hot...

Spray Drying of Foodsby

Prof. Arun S. MujumdarNational University of Singapore

International Workshop on Drying of Food and Biomaterials

Bangkok – June 67, 2011

CONTENTS• Definition of Spray Drying• Advantages and limitations of spray drying* Advantages* Limitations

• Classification of spray dryers• Components of spray dryer* Types of atomization* Flow patterns* Collection types* Control methods

• Examples of spray drying• Some typical spray drying processes • Developments in spray drying• Closures

CONTENTS (continued)

Definition

• a special process which is used to transform the feed from a liquid state into a dried particulate form (Powder or Particles) by spraying the feed into a hot drying medium.

Definition

DefinitionDefinition

• What is spray drying?

Hot airLiquid feed

Droplets

Moisture

Heat

Solidformation

POWDER

• Continuous and easy to control process• Applicable to both heat‐sensitive 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

The Advantages of Spray Drying

• High installation cost• Large air volumes at low product hold‐up implies gas cleaning costly

• Lower thermal efficiency• Heat degradation possibility in high‐temperature spray drying

The Limitations of Spray Drying

Figure Typical spray dryer layout

A conventional spray drying process consists of the following four stages:1. Atomization of feed into droplets2. Heating of hot drying medium3. Spray‐air contact and drying of droplets 4. Product recovery and final air treatment

Components of Spray Drying System

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 distribution•More flexible capacity (but with changes powder properties)•Limitations :•Higher energy consumption compared to pressure nozzles•More expensive•Broad spray pattern requires large drying chamber diameter

Types of atomizers: Rotary atomizer

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

Types of atomizers : Pressure nozzle

Advantages:* Simple, compact and cheap* No moving parts* Handle the feedstocks with high‐viscosity * Produce products with very small size particle

Limitations:* High energy consumption* Low capacity (feed rate)* High tendency to clog

Types of atomizers : Pneumatic nozzle

Co‐current flow Counter‐current flow Mixed‐current flow

Types of Spray Dryersflow patterns: Cocurrent flow

Powder Collectors

• 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.

Control systems

Selection Tree for Spray Drying System

Some Examples of Spray Drying Systems

17

Pr oduct Feed concent r at i on %

Resi dual - moi st ur e %

Dr yi ng-t emper at ur e ( 0C)

Spr ay dr yer desi gn

I nl et Out l et

Cof f ee 30- 55 2. 0- 4. 5 180-250

80- 115 OCL; CCF; PNN; SS; CY; MS

Egg 20- 24 3- 4. 5 180-200

80- 90 OCL; CCF; CA/ PNN; SS; CY/ BF

Enzyme 20- 40 2. 0- 5. 0 100-180

50- 100 OCL; CCF, CA/ PNN, SS; BF/ CY+WC

Ski mmi l k 47- 52 3. 5- 4. 0 175-240

75- 95 OCL; CCF; CA/ PNN; SS/ MS CY/ BF

Spi r ul i na 10- 15 5. 0- 7. 0 150-220

90- 100 OCL/ SCCL; CCF; CA; SS;BF/ CY+WC

Mal t odext r in

2. 5- 6. 0 2. 5- 6. 0 150-300

90- 100 OCL; CCF/ MF; PNN/ CA; SS; BF/ CY+WC

Soyapr ot ei n

12- 17 2. 0- 5. 0 175-250

85- 100 OCL; CCF; PN; SS; BF

Tea ext r act 30- 40 2. 5- 5. 0 180-250

90- 110 OCL; CCF; PN; SS; CY/ CY+WC

Tomat opast e

26- 48 3. 0- 3. 5 140-160

75- 85 OCL; CCF; PN/ CA; SS/ MS;CY/ BF

Spray Drying Applications in Food Technology

Some Basic Spray Drying Processes used in Food Production

Spray Drying of Skim Milk

Micrograph of spray dried Skim Milk

Spray Drying of Tomato Juice

Spray Drying of Coffee

Developing Trends in Spray Drying

Oper at i on/ comput at i on par amet er s SD SD+VFB SD+I FB SD+I FB+VFB ( MSD)Spr ay dr yi ng

I nl et ai r t emper at ur e ( 0C) 200 230 230 260Ai r r at e ( kg/ h) 31500 31500 31500 31500Spr ay r at e ( kg/ h) 2290 3510 4250 5540Sol i d cont ent ( %) 48 48 48 48Moi st ur e ( %DB) 108. 3 108. 3 108. 3 108. 3Resi dual moi st ur e ( %) 3. 5 6 9 9Out l et t emper at ur e ( 0C) 98 73 65 65Evapor at i on r at e ( kg/ h) 1150 1790 2010 2620Ener gy consumpt i on ( GJ) 7. 6 8. 86 8. 9 9. 95Ener gy consumpt i on/ kg powder( kJ/ kg)

6667 4949 3971 3428VFB I FB I FB

Ai r r at e ( kg/ h) 4290 6750 11500Ai r t emper at ur e ( 0C) 100 115 120Evapor at i on r at e ( kg/ h) 45 125 165Resi dual moi st ur e ( %) 3. 5 3. 5 3. 5Ener gy consumpt i on ( GJ) 0. 48 0. 82 1. 11

Over al l dr yi ng per f or manceTot al ener gy consumpt i on ( GJ) 9 9. 34 9. 72 11. 1Ener gy consump. / kg powder ( MJ/ kg) 6. 67 5. 35 4. 34 4. 01Powder di amet er ( mi cr on) 50- 150 50- 200 50- 500 50- 500Fl owabi l i t y poor Fr eef l ow Fr eef l ow Fr ee- f l owBul k densi t y ( kg/ m3) ( Appr ox. ) 600 480 450 450

Multistage Spray Drying System

Advantages :* No fire and explosion hazards* No oxidative damage* Ability to operate at vacuum and high operating pressure conditions* 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

Superheated Steam Spray Drying

A schematic flowchart of the conventional spray freeze drying

Spray Freeze Drying

• At present, Computational Fluid Dynamic (CFD) is popular in modeling of spray drying process with the computer developing.

Modeling of Spray Drying

CFD modelling and deposition study of spray dryers


• Part 1: Reduction of particlewall deposition

• Part 2: Evaluation of droplet drying models

• Part 3: CFD analysis of airflow stability

• Part 4: New particlewall deposition model


• Part 1: Reduction of particlewall deposition

Weblike deposition (gelatin)

Deposition at the conical wall (sucrosemaltodextrin)

Dripping problem (sucrosemaltodextrin)


• Part 1: Reduction of particlewall deposition– Experiments to determine deposition fluxes


• Part 1: Reduction of particlewall deposition– Experiments to determine deposition fluxes

0.14 m2

0.14 m2

0.15 m2

Modeling of Spray Drying• Part 1: Reduction of particlewall deposition– Findings

Middle plate Bottom plate

0.005

0.01

0.015

0.02

0.025

100 120 140 160 180Inlet temperature, °C

Dep

ositi

on fl

ux, g

m-2

s-1

SS

TF

0.01

0.015

0.02

0.025

0.03

100 120 140 160 180Inlet temperature, °C

Dep

ositi

on fl

ux, g

m-2

s-1

SS

TF


• Part 1: Reduction of particlewall deposition– Deposition strength tester

Air sparger

Adjustable disperser angle

Clips to hold the plate

Quick coupling to compressed

air line


• Part 2: Evaluation of droplet drying models– Evaluated: Reaction Engineering Approach (REA) vs

Characteristic Drying Curve (CDC)

– Compared with single droplet data (Adhikari et al.)

Hot drying air

Glass filament

Droplet


• Part 2: Evaluation of droplet drying models– Axisymmetric model (FLUENT)– Steady state– EulerLagrangian– Turbulence: RNG ke– Included moisture transport– UDF (C language) for models– Coupled (2nd order accuracy)

Air inlet

Outlet

1.75 m

0.50 m

0.70 m



Tracked particle moisture as it moves around


• Part 2: Evaluation of droplet drying models– Findings

REA CDC modified

Evaporation rate from particles, kg s‐1



– Deviation: Different response to initial moisture

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

Par

ticle

moi

stur

e, %

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, sPa

rticl

e m

oist

ure,

%w

t 90 % wt moisture

80 % wt moisture

70 % wt moisture

50 % wt moisture

REACDC



– Cotton tuft visualization– Hotwire measurments

Hot wire

Protective sheathe



Radial direction

Circumference direction

Inlet

Outlet

Axial direction

0.7 m

0.6 m

(into paper)X

Z

Y


• Part 3: CFD analysis of airflow stability– Findings: Jet feedback mechanism

Deflection to conical wall

Upward recirculation at opposite side


• Part 3: CFD analysis of airflow stability– Findings: Effect of expansion ratio

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 s‐1)

20.16 s

Modeling of Spray Drying• Part 3: CFD analysis of airflow stability– Findings: Effect of expansion ratio

3.0

2.5

2.0

1.5

1.0

0.5

0.0

‐ 0.5

‐ 1.0

‐ 1.5‐ 2.0

5.28 s 30.72 s

Axial velocity (m s‐1)


• Part 4: New deposition model– Big challenge as rigidity changes– Proposed a Viscoelastic approach

120 ˚C inletAmorphous glass

190 ˚C inletAmorphous rubbery


• Part 4: New deposition model– Viscoelastic contact modelling

tdd

Eε

ηεσ +=

StressStorage

coefficientStrain Loss

coefficient

Strain rate


Strong rebound and escape(diameter: 100 µm, initial velocity: 0.5 ms1, TTg: 23°C)


• Part 4: New deposition model– Findings

00.10.20.30.40.50.60.70.80.91

1.11.2

15 17 19 21 23 25 27 29

T ‐ Tg, °C

Res

titu

tion

factor 0.2 m/s

0.5 m/s

1.0 m/s

1.5 m/s


• Part 4: New deposition model– Viscoelastic contact modelling– Superposition technique

Storage modulus Loss modulus

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

( )TAE ω′ ( )TAE ω′′( )TAE ω′ ( )TAE ω′′


Some more CFD modelling Work

Modeling of Spray DryingVarious tested geometries modeled by CFD

Example Specifications Remarks

Different geometry Conical, hour‐glass, lantern, cylinder‐on‐cone

New idea‐limited experience

Horizontal SDZ New development

Coffee spray dryer two nozzles installed

Industrial scale

Conventional spray dryer with rotary disc

Cylinder‐on‐cone geometry. Rotary disc atomizer

Conventional concept – first try


H1=820mmH2=870mmH3=70mm H4=100mm D1=935mm D3=74mm D4=170mmD5=136mm

Cylinder‐on‐cone

Injection position

At the center and H4away from the top ceiling

Conical Chamber

H0=1690mmD1=935mm D3=74mm

Inlet size is same as that in Case K

Injection position

At the center and H4 away from the top ceiling

Hour‐glass Chamber

H1=820mmH2=870mmD1=935mm D2=400mmD3=74mm


Injection position

At the center and H4 away from the top ceiling

Lantern chamber

H1=820mmH2=870mmD1=400mm D2=935mmD3=74mm


Injection position

At the center and H4away from the top ceiling


Cylinderoncone

Conical chamber

Novel spray dryer geometry tests

Modeling of Spray DryingNovel spray dryer geometry tests

•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 co‐current cylinder‐on‐cone configuration.

• The predicted results show that hour‐glass geometry is a special case and the cylinder‐on‐cone 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.

Modeling of Spray DryingOverall heat and mass transfer characteristics of the four chambers

Case A Case B Case C Case D

Volume of chamber (m3) 0.779 0.501 0.623 0.623

Evaporation rate (10‐3 kg/s) 0.959 0.951 0.9227 0.955

Net Heat‐transfer 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 (10‐3kgH2O/s.m3)

1.23 1.91 1.48 1.53

Average volumetric heat‐transfer intensity qh (W/m3)

5463.27 8591.9 7168.6 6940.2

Modeling of Spray DryingHorizontal spray dryers

Modeling of Spray DryingHorizontal spray drying – Streamline patterns

Recirculation zone resulting in particle remoisten or overheated


Better performance can be observed in Case G and H

More particles exit from outlet

Horizontal spray drying – Particle trajectories

Modeling of Spray DryingCoffee spray drying

• Deposit conditions: Top cone wall: 1 (Matched)Cylinder wall: 1293(Not

Matched)*

Four outlets: 340(Matched)Conical Wall:329(Matched)Other walls: 37(Matched)* Due to 18 hammers shocking

Temperature contours in the drying chamber

• Spray dryers, both conventional and innovative, will continue to find increasing applications in various industries.

• Some of the common features of innovations are identified. There is need for further R&D and evaluation of new concepts.

• Spray drying is an important operation for industries that deserves multi‐disciplinary R&D preferably with close industry‐academia interaction

Closing Remarks

Closing Remarks (Continued…)

• In the future, the mathematical model of spray drying will include not only the transport phenomena but also product quality predictions. In the meantime, it is necessary to test and validate new concepts of drying in the laboratory and if successful then on a pilot scale.

• Numerous papers dealing with mathematical models for conventional and modified spray dryers appear regularly in Drying Technology‐‐‐‐‐‐An International Journal

Please e-mail for further information:[email protected]

Websites: http://serve.me.nus.edu.sg/arun/Thank you very much!

Thank you for your attention

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