This document discusses pneumatic drying technology for food products. It describes the working principle of pneumatic dryers, which use high-velocity hot air to quickly dry materials through indirect heating and co-current flow. Key factors that influence drying speed such as temperature, moisture dispersion, and particle size are examined. Common types of pneumatic dryers like flash dryers, rotary dryers, and two-stage dryers are outlined. Applications for drying foods like wheat, starches, and meat are provided. Advantages of short residence times and disadvantages like required efficient exhaust systems are also summarized.

1. FOOD DEHYDRATION TECHNOLOGY
(FT-403)
PNEUMATIC DRYING OF FOODS
PRESENTED BY:
KRATIKA SINGHAM
INT.FOOD TECHNOLOGY

2. CONTENTS
 INTRODUCTION
 WORKING PRINCIPLE
 FACTORS GOVERNING DRYING SPEED
 MECHANICAL FORCES
 MECHANISM OF WATER EVAPORATION
 FACTORS AFFECTING RESIDENCE TIME
 BLOCK DIAGRAM
 CONSTRUCTION
 TYPES
 ADVANTAGES & DISADVANTAGES
 APPLICATIONS
 LIMITATIONS
 EXPECTED NEW DEVELOPMENTS
 REFERENCES

3. Introduction
• Also known as “flash drying”
• Continuous convective process
• Indirect heating or direct firing
• Co-current flow
• High convective heat & mass transfer coeffient
• High drying rates & product quality
• Air velocity> free fall velocity of largest product
• Granular, crystalline, paste, powder, cake, flakes, gels,
slurry
• Extension of fluidized bed drying
• Feeder- screw feeder, rotary valve, vibrating feeder,
mixer
• Processes- pre-drying, drying & dehydration

4. SPECIFICATIONS
 Dried product size range= (10- 500)µm
 Short residence time = (0.5-10)sec
 Dried product(m.c) = <=12%
 Throughput= few kg/hr to several 100s tons/hr
 Inlet air temp= (180-300˚c)
 Incoming feed moisture= <40%(wb)
 Bag filter pressure=(0.4- 0.6)Mpa

5. COMPONENTS
• Heater
• Wet material feeder
• Venturi
• Drying chamber
• Air Seperator(cyclone)
• Exhaust fan
• Dried product collector
• Dust seperator
ex-( bag filters, electrostatic precipitators, wet scrubbers & fabric filters)

6. BLOCK DIAGRAM OF THE PNEUMATIC DRYER SYSTEM

7. PNEUMATIC DRYER

8. MECHANICAL FORCES
Mechanical forces ( solid particle):
 Drag force (FD)
 Buoyancy force (FB )
 Weight force (FW ) of particle

9. MECHANISM OF WATER EVAPORATION
1. Convection heat transfer
2. Heat conduction
3. Mass transfer (granuels/ particles to surface)
4. Mass transfer(surface to drying air)

10. FACTORS GOVERNING THE SPEED OF DRYING
1. Moisture Dispersion
2. Temperature Differential
3. Agitation
4. Particle Structure
5. Particle Size

11. RESIDENCE TIME CAN BE ALTERED BY-
 Increasing the flash tube length
 Increasing holdup times in the product collection cyclone(s)
 Adding cyclones in series.

12. Cross-section Area of Pneumatic
Dryer

13. CALCULATIONS

14. HEAT TRANSFER
TOTAL HEAT REQUIREMENT
DRYING TIME

15. FLASH DRYING SYSYTEM WITH RECIRCULATION OF EXHAUST GASES

16. FLASH DRYING SYSTEM WITH VENTURI

17. SPIN FLASH DRYER
APPLICATIONS OF SPIN FLASH DRYER
 Extracting Wheat starch
 Soy protein
 Concentrated yeast

18. ROTARY FLASH DRYER
Applications : soy protein, gelatinized starch

19. TWO STAGE PNEUMATIC DRYER WITH VAPOUR UTILIZATION

20. ADVANTAGES OF PNEUMATIC DRYING
• Short contact time and parallel flow make possible to dry thermolabile
materials
• The dryer is easy to control.
• The low material content in the dryer enables equilibrium conditions to
be reached very quickly.
• Due to small no. of moving parts ,the maintenance cost is low.
• Handle high flow rates.

21. DISADVANTAGES
• High efficiency gas cleaning system is required
• For lumped materials difficult to disperse
• There is a risk of fire and explosion
• Dryer cannot be used for toxic materials
• Less suitable for removal of internal moisture .

22. APPLICATIONS
• Wheat sugar , gelatinized starch
• Powders like potato starch and corn starch
• Salt
• Flours
• Bread crumps

23. (a) Corn gluten (b) caesin ( c) soup powder (d) vegetable protein ( e) spent tea (f) meat residues (g) bone meal

24. PACKAGING MATERIALS
• Powder products – Aluminium foil laminates , lined paperboards
• Meat products- HDP or polyamide-polyethylene-polyester-polyethylene
laminated films.
• Fruits & veg pdt- paperboard folded cartons laminated with liner
• Flakes- foil laminated flexible pouches, friction lidded tins
• Cereals - cellophane plastic bag, LDP films

25. LIMITATIONS
 loss of power to the dryer will cause the product to fall out of suspension
and build up in the dryer base & feed throat.
 If product hardens under heat ,it will cause blockage and require
significant time to remove.
 Due to relatively high velocities, the particle size may be reduced
(undesired) to attrition & impact.
 High velocities also may contribute to pre- mature component wear if
system is not designated to inhibit it.

26. EXPECTED NEW DEVELOPMENTS IN
FLASH DRYERS
These developments are expected in the fields of: .
 Increased efficiency by heat recovery
 Applications of heat pumps in drying systems
 Efficient combined processes (hurdle technology)
 Development of models for overall systems (mechanical dewatering,
drying, heat recovery, powder collection, cooling stages)

27. REFERENCES
• Earle RL, unit operations in food processing, second edition, pergamon press, 1983, pg-85-89
• Ramaswamy H, Marcotte M, Food Processing Principles and Applications,
Taylor and francis, 2009, Pg- 233- 302
• Kemp, I.C. and Oakley, D.E., Simulation and scale-up of pneumatic conveying and cascading
rotary dryers, Drying Technology, 15(6–8),1699–1710, 1997
• Mindziul, Z. and Kmiec, A., Modelling gas–solid flow in a pneumatic-flash dryer, Drying
Technology, 15(6–8), 1711–1720, 1997
• Thorpe, G.R., Pneumatic Conveying Driers, Chemical Industry Development, Incorporating
CP&E, 1975, pp. 13–19
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