Chemical Engineering June 2012 - 37

Spray Drying
Parameters
Department Editor: Scott Jenkins
GEA Niro
mong the most widely used technologies
in the chemical process industries
(CPI), spray drying involves dispersing
a liquid or slurry in a hot gas to produce
a dry powder product. A wide range of
pumpable solutions, suspensions and emulsions
can be used as spray-drying feeds.
Spray drying allows processors to generate
powders with precisely defi ned properties.
By controlling process parameters
- including the characteristics of the liquid
feed, the method of atomization, the confi
guration of the dryer and others - chemical
makers can control the shape, fl ow
properties and porosity of the solid particles
produced. Here, the major considerations
of spray drying are outlined.
A
Atomization
Several methods for atomizing the liquid
feed in a spray-drying system are available
(Figure 1). For most atomization equipment,
the liquid leaves the atomizing head as a
thin liquid fi lm. The fi lm fragments upon
leaving the atomizing head, and droplets
form immediately, driven by the surface
tension of the liquid. Because of this,
droplet formation depends heavily on the
rheological properties of the liquid and its
interaction with the heated drying medium
just outside the atomizing device.
Rotary. In centrifugal (rotary) atomization,
the most common method used in spray
dryers, a rotating disc or wheel breaks the
liquid stream into droplets. A liquid mist is
formed horizontally from the atomizer wheel.
Centrifugal atomizers rotate in the range of
5,000 to 25,000 rpm. The size of the droplets
produced is roughly inversely proportional
to the peripheral speed of the wheel,
or disc, which typically have diameters in the
range of 5 to 50 cm. The use of variablespeed
drives can make the control of the
droplet size straightforward. The smallest
rotary atomizers handle 1-10 kg/h of liquid
feed in the laboratory, while the largest commercial
units, driven by 1,000-kW motors,
can handle more than 200 metric ton/h.
Nozzle. Atomization with a pressure
nozzle involves pressurizing a liquid using
a pump, and forcing it through the orifi ce or
a nozzle. Typical orifi ce sizes are 0.5-3.0
mm, which limits the capacity of the nozzle
to 750-1,000 kg/h of liquid feed, depending
on pressure, viscosity and solids content
of the feed. Larger pressure drops across
the orifi ce produce smaller droplets, so to
reduce particle size for a given feedrate, a
smaller orifi ce and higher pump pressure
must be provided to maintain the same
mass fl ow. Although simple, the pressure
nozzle is often diffi cult to maintain,
especially in multiple-nozzle systems. Most
of the diffi culty results from plugging, as
well as wear of the nozzle insert, which can
change the characteristics of the nozzle.
Two-fl uid pneumatic. In this type of atomization,
the feed interacts with a second fl uid
(usually compressed air) through
a two-fl uid nozzle to accomplish
the atomization. Particle size is
controlled by adjusting the ratio of
the compressed air to the feed. Twofl
uid pneumatic atomization is used
primarily in smaller drying systems.
Sonic atomization. For applications
requiring fi ne droplets at low
fl owrates, sonic atomization can be
used. In this technique, the feed liquid
is passed over a surface that is
vibrated at ultrasonic frequencies. It
is employed in small-capacity dryers
when a highly uniform particle-size
distribution is required.
(a)
(b)
FIGURE 1. Rotary atomizers (a) produce a liquid mist
horizontally from the atomizer wheel. Atomization by
nozzle (b) often leads to a narrower
particle-size distribution
Dryer confi guration
To allow the liquid to evaporate,
the fl ow patterns of the droplets
and the gas through the dryer
must provide enough contact
time. Therefore, the size and
geometry of the spray-drying
chamber and gas disperser
become important parameters.
In many spray-drying systems,
the atomizer is installed at the
roof of a large-diameter drying
chamber, and the heated gas
is introduced through a roofmounted
air or gas disperser
around the atomizer. This creates
a co-current fl ow of gas
and droplets and particles. The chamber's
height must allow particles suffi cient retention
time to dry. Larger particle sizes require
larger-diameter drying chambers.
The residence time should be selected
based on the experience of the product's
known drying characteristics and on the
desired particle size. This allows direct
calculations of the drying chamber volume.
Another confi guration involves installing
the pressure nozzle at the bottom of the
chamber, so that the spray shoots upward
from the bottom. This confi guration is used
in cases where the product is a coarse powder
and the production rate is lower.
Collecting dried solids
Coarse powders are most easily collected
directly from the bottom of the drying chamber.
For fi ne powders, cyclones or bag fi lters
become the primary collecting points. The
particles must be separated from the drying
media, which is cooler (due to evaporation)
and more humid than before drying.
Gas fl ow
Heating the drying gas that fl ows through
the spray dryer may be accomplished by
direct combustion of natural gas, by indirect
heating with shell-and-tube heat exchangers,
or by electric heaters (used in small
spray dryers). Most gas dispersers are
confi gured with the help of computational
fl uid dynamics (CFD) analysis to defi ne
FIGURE 7. This spray drying
system is equipped with a heat
recovery unit
GEA Niro
airfl ow patterns and temperature distributions
within the drying chamber. For most
applications, the gas disperser has adjustable
guide vanes that allow for fi ne-tuning.
Industrial radial fans are used to move the
gas through the system. Sizing of system
components can be based on gas fl ow.
Evaporation rate
Inside a spray dryer, the evaporation rate
is directly proportional to the temperature
difference from input to outlet multiplied by
the mass fl ow. Values for the outlet temperature
are usually determined experimentally,
since they depend on the material's
equilibrium isotherm, and true equilibrium
is never actually reached. The inlet temperature
is also determined experimentally,
and should be as high as possible without
risking product degradation.
Safety
In spray-drying operations, safety procedures
related to dust explosions must be
considered carefully, including determination
of dust explosion pressure rise (Kst), the
maximum dust explosion pressure (Pmax),
minimum ignition energy (MIE), minimum
ignition temperature (MIT), and minimum
auto-ignition temperature (MAIT).
Editor's note: The material in this " Facts at Your Fingertips "
was adapted from the article cited here: .
Moller, J.T. and Fredsted, S., A primer on spray drying.
Chem. Eng., November 2009, pp. 34-40.
Chemical Engineering June 2012

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