WEEK 2 spray Drying and Dehydration Lecture 2_2021.pdf -...

Spray drying and drying of proteins
◼
Spray drying systems
◼
Drying of proteins (lactoferrin as
example)

SPRAY DRYING SYSTEMS

Spray Drying
• liquid (solution/suspension) drying method
• liquid phase- pumpable- containing soluble
solids, and/or insoluble in
suspensions/slurry
• Atomisation into droplets- large total
surface area of droplets (increased rate of
heat and mass transfer) – near
instantaneous evaporation

Spherical droplet surface versus droplet size
Total
Volume
(m
3
)
Diameter
of
droplets
No. of
droplets
Surface
area per
droplet
Total surface
of droplets
(m
2
)
Surface area
ratio
0.524 1 m 1 3.14 m 2 3.14 1
0.524 1 cm 1  10 6 3.14 cm
2
314.16 100
0.524 1 mm 1  10
9
3.14 mm
2
3141.59 1000
0.524
100  m 1  10
12 0.0314
mm
2
31415.93 10000
0.524 1  m 1  10 18 3.14  m 2 3141592.65 1000000

Advantages of spray drying
• Very short drying time (maximum moisture evaporation
within a second)
• Low particle temperature due to evaporative cooling
• Effective control of product properties and quality (almost
uniform spherical particles)
• Drying of heat sensitive foods, biological products, and
pharmaceuticals at atmospheric pressure and relatively low
temperatures
• Inert (N
2
)atmosphere can also be employed and non-water
solvent can be used
• High tonnage production in continuous operation
• Very high sanitation (
closed cycle and aseptic processing possible
)

Disadvantages
• Fails to obtain high bulk density products
• Not flexible (in general)
• High initial investment cost
• Product recovery and dust collection increase the
cost of drying
• Relatively low energy efficiency compared to
other water removal processes

Heat consumption (kJ/kg) for water evaporation
(
Filkova and Mujumdar, 1995
)
Membrane processes 140
Evaporator 1 stage 2600
Evaporator 2 stages 1300
Evaporator 6 stages 430
Evaporator 6 with
thermocompression
370
Evaporator 6, with mechanical
compression
220
Spray drying process Up to 6000

Stages of spray drying process
1. Atomisation
2. Spray-air mixing
3. Moisture evaporation
(drying)
4. Powder recovery

Atomisation
• most important operation
• influences:
– energy requirement to form the spray (type of
atomiser)
– size and size distribution of the atomised droplets
– trajectory and speed of droplets
– chamber design

Atomiser Types
• Rotary wheel (centrifugal)- most
common
• Pressure nozzle
• Pneumatic or twin fluid nozzle
• (Sonic..)
Atomised Drop Size and Size Distribution:
Single most important factor affecting quality of dry powder

Droplet size and atomisers
(
low viscosity, Newtonian fluids- Masters, 1991
)
Rotary atomiser (m/s) Droplet size (  m)
75-125 (m/s) 150-275
125- 150 (”) 75-150
150- 180 (”) 30-75
Pressure Nozzle
15-25 (atm) 150-350 (  m)
25- 50 (”) 50-150
>100 (”) 15-30
Twin fluid nozzle
1.5:1 (air:liquid) ratio 50-200
2.5:1 (”) 30-50
5:1 (”) 5-20

Wheel Atomiser
• Rotating wheel – uses centrifugal energy for atomisation -
disintegrates the solution sheet at the wheel edge into
droplets
• Spray angle  180 o
• broad cloud, horizontal trajectory
• requires large diameter chambers
• Angular speed 10000 to 50000 rpm (linear peripheral speed
100-200m/s)
• Radial or curved vanes
• Curved vane wheels used in dairy industries to produce high
bulk density powder (about 15% higher)- removal of air