Spray Drying of Foods by Prof. Arun S. Mujumdar National University - - PowerPoint PPT Presentation

Spray Drying of Foods

by

• Prof. Arun S. Mujumdar

National 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

Definition

Definition

• What is spray drying?

Hot air Liquid feed Droplets

Moisture Heat Solid formation

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 droplets
• 2. Heating of hot drying medium
• 3. 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

P r oduct Feed co ncent r a t i on % R esi dua l - m

• i s

t ur e % D r yi ng- t em per at ur e ( 0C ) S pr ay dr yer desi gn I nl et O ut l et C

• f f ee

30- 55

• 2. 0- 4. 5

180- 250 80- 115 O C L; C C F; P N N ; S S ; C Y ; M S E gg 20- 24 3- 4. 5 180- 200 80- 90 O C L; C C F; C A / P N N ; S S ; C Y / B F E nzym e 20- 40

• 2. 0- 5. 0

100- 180 50- 100 O C L; C C F, C A / P N N , S S ; B F/ C Y +W C S ki mm i l k 47- 52

• 3. 5- 4. 0

175- 240 75- 95 O C L; C C F; C A / P N N ; S S / M S C Y / B F S pi r ul i na 10- 15

• 5. 0- 7. 0

150- 220 90- 100 O C L/ S C C L; C C F; C A ; S S ; B F/ C Y +W C M al t odext r i n

• 2. 5- 6. 0
• 2. 5- 6. 0

150- 300 90- 100 O C L; C C F/ M F; P N N / C A ; S S ; B F/ C Y +W C S

• ya

pr ot ei n 12- 17

• 2. 0- 5. 0

175- 250 85- 100 O C L; C C F; P N ; S S ; B F Tea ext r act 30- 40

• 2. 5- 5. 0

180- 250 90- 110 O C L; C C F; P N ; S S ; C Y / C Y +W C Tom at o past e 26- 48

• 3. 0- 3. 5

140- 160 75- 85 O C L; C C F; P N / C A ; S S / M S ; C Y / B F

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

O per at i on/ com put at i on par am et er s SD SD +VFB SD +I FB SD +I FB+VFB ( M SD ) Spr ay dr yi ng I nl et ai r t em per at ur e ( 0C ) 200 230 230 260 Ai r r at e ( kg/ h) 31500 31500 31500 31500 Spr ay r at e ( kg/ h) 2290 3510 4250 5540 Sol i d cont ent ( % ) 48 48 48 48 M

• i st ur e ( %

D B)

• 108. 3
• 108. 3
• 108. 3
• 108. 3

R esi dual m

• i st ur e ( %

)

• 3. 5

6 9 9 O ut l et t em per at ur e ( 0C ) 98 73 65 65 Evapor at i on r at e ( kg/ h) 1150 1790 2010 2620 Ener gy consum pt i on ( G J)

• 7. 6
• 8. 86
• 8. 9
• 9. 95

Energy consum pt i on/ kg pow der ( kJ/ kg) 6667 4949 3971 3428 VFB I FB I FB Ai r r at e ( kg/ h) 4290 6750 11500 Ai r t em per at ur e ( 0C ) 100 115 120 Evapor at i on r at e ( kg/ h) 45 125 165 R esi dual m

• i st ur e ( %

)

• 3. 5
• 3. 5
• 3. 5

Ener gy consum pt i on ( G J)

• 0. 48
• 0. 82
• 1. 11

O ver al l dr yi ng per f or m ance Tot al ener gy consum pt i on ( G J) 9

• 9. 34
• 9. 72
• 11. 1

Ener gy consum

• p. / kg pow

der ( M J/ kg)

• 6. 67
• 5. 35
• 4. 34
• 4. 01

Pow der di am et er ( m i cr on) 50- 150 50- 200 50- 500 50- 500 Fl ow abi l i t y poor Fr eef l ow

Fr eef l ow

Fr ee- f l ow Bul k densi t y ( kg/ m

3) ( 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

Modeling of Spray Drying

• 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

Modeling of Spray Drying

• Part 1: Reduction of particle­wall deposition

Web­like deposition (gelatin) Deposition at the conical wall (sucrose­maltodextrin) Dripping problem (sucrose­ maltodextrin)

Modeling of Spray Drying

• Part 1: Reduction of particle­wall deposition

– Experiments to determine deposition fluxes

Modeling of Spray Drying

• 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 0.025 100 120 140 160 180 Inlet temperature, °C Deposition flux, g m-2 s-1

SS TF

0.01 0.015 0.02 0.025 0.03

100 120 140 160 180 Inlet temperature, °C

Deposition flux, g m

• 2 s-1

SS TF

Modeling of Spray Drying

• 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

Modeling of Spray Drying

• 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

Modeling of Spray Drying

• 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

Modeling of Spray Drying

• Part 2: Evaluation of droplet drying models

Tracked particle moisture as it moves around

Modeling of Spray Drying

• Part 2: Evaluation of droplet drying models

– Findings

REA CDC modified

Evaporation rate from particles, kg s‐1

Modeling of Spray Drying

• Part 2: Evaluation of droplet drying models

– Deviation: Different response to initial moisture

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0.5 1 1.5 2 2.5 3 3.5 Time, s Particle moisture, %wt 80 % wt moisture 60 %wt moisture

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 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

REA CDC

Modeling of Spray Drying

• Part 3: CFD analysis of airflow stability

– Cotton tuft visualization – Hotwire measurments

Hot wire Protective sheathe

Modeling of Spray Drying

• Part 3: CFD analysis of airflow stability

Radial direction Circumference direction Inlet Outlet Axial direction 0.7 m 0.6 m

(into paper)

X Z Y

Modeling of Spray Drying

• Part 3: CFD analysis of airflow stability

– Findings: Jet feedback mechanism

Deflection to conical wall Upward recirculation at opposite side

Modeling of Spray Drying

• 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)

Modeling of Spray Drying

• Part 4: New deposition model

– Big challenge as rigidity changes – Proposed a Viscoelastic approach

120 ˚C inlet Amorphous glass 190 ˚C inlet Amorphous rubbery

Modeling of Spray Drying

• Part 4: New deposition model

– Viscoelastic contact modelling

t d d E ε η ε σ + =

Stress Storage coefficient Strain Loss coefficient Stra in rate

Modeling of Spray Drying

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

Modeling of Spray Drying

• Part 4: New deposition model

– Findings

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 15 17 19 21 23 25 27 29

T ‐ Tg, °C Restitution factor

0.2 m/s 0.5 m/s 1.0 m/s 1.5 m/s

Modeling of Spray Drying

• Part 4: New deposition model

– Viscoelastic contact modelling – Superposition technique

Storage modulus Loss modulus

247 . 1

) ( 228 . 1

T

A E ω = ′

056 . 1

) ( 235

T

A E ω = ′ ′

( )

T

A E ω ′

( )

T

A E ω ′ ′

( )

T

A E ω ′

( )

T

A E ω ′ ′

Modeling of Spray Drying Some more CFD modelling Work

Modeling of Spray Drying

Various tested geometries modeled by CFD

Example Specifications Remarks Different geometry Conical, hour‐glass, lantern, cylinder‐

• n‐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

Modeling of Spray Drying

H1=820mm H2=870mm H3=70mm H4=100mm D1=935mm D3=74mm D4=170mm D5=136mm Cylinder‐on‐cone Injection position At the center and H4 away from the top ceiling Conical Chamber H0=1690mm D1=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=820mm H2=870mm D1=935mm D2=400mm D3=74mm Inlet size is same as that in Case K Injection position At the center and H4 away from the top ceiling Lantern chamber H1=820mm H2=870mm D1=400mm D2=935mm D3=74mm Inlet size is same as that in Case K Injectio n position At the center and H4 away from the top ceiling

Modeling of Spray Drying

Cylinder­on­cone Conical chamber

Novel spray dryer geometry tests

Modeling of Spray Drying

Novel 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 Drying

Overall 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 (10‐3 kgH2O/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 Drying

Horizontal spray dryers

Modeling of Spray Drying

Horizontal spray dryers

Modeling of Spray Drying

Horizontal spray drying – Streamline patterns Recirculation zone resulting in particle remoisten or

• verheated

Modeling of Spray Drying

Better performance can be observed in Case G and H

More particles exit from

• utlet

Horizontal spray drying – Particle trajectories

Modeling of Spray Drying

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

Thank you for your attention