This document provides information about an experiment using a laboratory-scale spray dryer. The objective is to evaluate the spray dryer's performance and drying characteristics. The document outlines the preliminary work, operation, calculations, and cleaning procedures for the experiment. It also includes background information on drying, definitions of key terms, and an overview of spray drying technology and its applications.

1. METU Chem. Eng. Dept.
Ch.E. 420 Chem. Eng. Lab III
EXPERIMENT 7 - SPRAY DRYER
OBJECTIVE
The objective of this experiment is to evaluate the performance and drying
characteristics of a laboratory scale spray dryer.
PRELIMINARY WORK
a) On the basic concepts
1. Read the attached information on the basics of drying and psychometry
and answer the questions below.
2. Find the relative humidity of air with
Td = 50°C, Tw = 30°C
Td = 60°C, Tw = 38°C
Td = 100°C, Tw = 55°C
where Td and Tw are the dry- and wet-bulb temperatures.
3. Find the humid volume and humid enthalpy of air for the conditions given in
part 2. What additional information do you have to supply for the enthalpy
values?
4. What are the parameters affecting the equilibrium moisture in the drying
operations?
5. What are the advantages and the disadvantages of spray drying?
b) On the experiment
1. The equipment to be used in this experiment is the Lab-Plant Model SD-04
spray dryer located in room number 105 on the first floor of the A Block,
The Unit Operations Laboratory building. Study the equipment carefully
with the help of Spray Dryer Manual which can be obtained from your
supervisor and familiarize yourself with its operation.
2. Ask for instructions on the choice, availability, or the advanced preparation
that needs to be done for the feed solution or slurry that will be dried.
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2. METU Chem. Eng. Dept.
Ch.E. 420 Chem. Eng. Lab III
Experiment 7
Spray Drier
3. Prepare a data sheet.
OPERATION
(See Spray Dryer Manual p.3,9, Fig.2, Fig.5)
1. Connect the spray drier to the electrical main supply. Main connection
indicator light illuminates (Fig.5 (c)).
2. To feed the pump feed tube (Fig.2 (10)) to the peristaltic pump head (Fig.2
(9)) first, remove the clear plastic and metal covers from the pump head.
Press the main on/off switch (Fig.5 (d)) followed by the pump switch (Fig.5 (j))
and with the pump rotor turning very slowly, push the tubing between the
rollers (Fig.2(12)) until it is correctly positioned between the rollers and the
pump head inner wall. Switch off the pump. (Take care not to trap your
fingers.)
3. Wait for a few seconds to allow inlet (Fig.5 (a1)) and outlet (Fig.5 (a2))
controller/indicator to settle at approximately ambient temperature.
4. Fill the sample bottle (Fig.2 (11)) with distilled water. Place pump feed tube
into sample bottle.
5. Switch on first the blower (Fig.5 (e)) and then the heater (Fig.5 (g)).
6. Switch on compressor (Fig.5 (h)) and open compressor pressure setting
(Fig.5 (i)) to full.
7. Start taking inlet and outlet air temperatures as a function of time until the
required inlet air temperature (set temperature) is reached and uniform outlet
air temperature is attained.
8. Switch on pump to feed distilled water. Suitable spraying rate, i.e., the rate at
which the sprayed liquid cannot be seen to be hitting the chamber walls, is
achieved by means of pump speed control (Fig.5 (k)). Flow rate is determined
using the scaled sample bottle as mL/s or mL/min. Feed distilled water until
steady state condition is reached (uniform outlet temperature is obtained) and
variation in airflow is not too much. Continue taking inlet and outlet air
temperature data while feeding the distilled water.
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3. METU Chem. Eng. Dept.
Ch.E. 420 Chem. Eng. Lab III
Experiment 7
Spray Drier
9. Start pumping the feed solution prepared before. Dried powder should be
observed spiraling down the cyclone (Fig.2 (7)) into the sample collection
bottle (Fig.2 (8)). Take inlet and outlet air temperatures.
10.Measure, by means of an anemometer, the air velocity at the air exit.
11.Record, using a psychrometer, the wet- and dry-bulb temperatures of the inlet
and outlet air. (Ask your supervisor for instructions to use anemometer and
psychometer.)
12.After terminaton of sending the feed solution, feed the dryer again with
distilled water, now to clean pump feed tube completely.
13.Weigh the solid material collected in the sample collection bottle.
STOPPING THE OPERATION AND CLEANING
1. Switch off the pump.
2. Switch off the heater, compressor, and blower, in the given order.
3. Switch off the mains connection.
4. Remove the product collection tube (Fig.2 (6)) from the cyclone.
5. Remove the sample bottle from the cyclone.
6. Open the bottom clamp which connects the main chamber and cyclone.
Disconnect the cyclone.
7. Open the top clamp and disconnect the main chamber.
8. Clean all these parts by washing with water.
9. Pump feed tube should be removed from the peristaltic pump head
following the same but opposite procedure as in (2) in operation part.
CALCULATIONS
1. Plot inlet and outlet air temperatures as a function of operating time.
2. Make a material balance for water.
3. Find the percent recovery of the solid based on total solid input.
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4. METU Chem. Eng. Dept.
Ch.E. 420 Chem. Eng. Lab III
Experiment 7
Spray Drier
4. Make an energy balance during the steady state operation. Determine the
energy input by the hot air and all the energy outputs.
5. Define an appropriate thermal efficiency for this spray drying apparatus
and calculate it.
REFERENCES
1. Perry, R.H., Green, D.W., Maloney, J.O., “Perry’s Chemical Engineers’
Handbook”, 6th ed., pp.8-70, 20-51 to 20-58, McGraw-Hill: New York,
1984.
2. Foust, A.S., Wenzel, L.A., Clump, C.W., Maus, L., Anderson, L.B.,
“Principles of Unit Operations”, pp.340-349, Wiley: N.Y., 1960.
3. Treyball, R.E., “Mass Transfer Operations”, 3rd ed., pp.655-716, McGraw-
Hill: Kogakusha, Tokyo, 1980.
4. Mc Cabe, W.L., Smith, J.C., Harriot, P., “Unit Operations of Chemical
Engineering”, 5th ed., pp. 767-806, McGraw-Hill Kogakusha: Tokyo, 1993.
5. Masters, K., “Spray Drying Handbook”, 3rd ed., pp. 1-52, 74-107, George
Godwin: London, 1979.
SHORT NOTES ON DRYING AND SPRAY DRYING
The term drying refers generally to the removal of moisture from a substance. A
solution can be dried by spraying it in fine droplets into a hot, dry gas, which
results in evaporation of the liquid, but evaporation of the solution by boiling in the
absence of a gas to carry away the moisture is not ordinarily considered a drying
operation.
Equilibrium
The moisture contained in a wet solid or liquid solution exerts a vapor pressure to
an extent depending upon the nature of the moisture, the nature of the solid, and
the temperature. If then a wet solid is exposed to a continuous supply of fresh
gas containing a fixed partial pressure of the vapor p, the solid will either lose
moisture by evaporation or gain moisture from the gas until the vapor pressure of
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5. METU Chem. Eng. Dept.
Ch.E. 420 Chem. Eng. Lab III
Experiment 7
Spray Drier
the moisture of the solid equals p. The solid and the gas are then in equilibrium,
and the moisture content of the solid is termed its equilibrium-moisture content at
the prevailing conditions.
Definitions
Types of Moisture
Moisture content, dry basis:
This is expressed as kg moisture/kg dry solid = X
Percentage moisture, dry basis = 100 X
Moisture content, wet basis:
The moisture content of a solid or solution is usually described in terms of weight
percent moisture, and unless otherwise qualified this is ordinarily understood to
be expressed on the wet basis, i.e.,
(kg moisture/kg wet solid) = [kg moisture/(kg dry solid + kg moisture)] = X/(1 + X)
Percentage moisture, wet basis: 100 X/(1 + X)
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Rela
tive
Hum
idity
of
Gas
Equilibrium
moisture curve
1.0
A
Bound
moisture
Unbound
moisture
Equilibrium
Moisture
Free
moisture
X* X
Moisture content, kg moisture / kg dry solid
0.0
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6. METU Chem. Eng. Dept.
Ch.E. 420 Chem. Eng. Lab III
Experiment 7
Spray Drier
Equilibrium moisture X*:
This is the moisture content of a substance when at equilibrium with a given
partial pressure of the vapor.
Bound moisture:
This refers to the moisture contained by a substance, which exerts an equilibrium
vapor pressure less than that of the pure liquid at the same temperature.
Unbound moisture:
This refers to the moisture contained by a substance, which exerts an equilibrium
vapor pressure equal to that of the pure liquid at the same temperature.
Free moisture:
Free moisture is that moisture contained by a substance in excess of the
equilibrium moisture: X – X*. Only free moisture can be evaporated, and the free-moisture
content of a solid depends upon the vapor concentration in the gas.
SPRAY DRYERS
Solutions, slurries, and pastes can be dried by spraying them as fine droplets into
a stream of hot gas in a spray drier.
The liquid to be dried is atomized and introduced into the large drying chamber,
where the droplets are dispersed into a stream of heated air. The particles of
liquid evaporate rapidly and dry before they can be carried to the sides of the
chamber, and the bulk of the dried powder which results falls to the conical
bottom of the chamber to be removed by a stream of air to the dust collector. The
principal portion of the exit gas is also led to a dust collector before being
discharged. Many other arrangements are possible, involving both parallel and
counterflow of gas and spray. Arrangements and detailed designs vary
considerably, depending upon the manufacturer.
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7. METU Chem. Eng. Dept.
Ch.E. 420 Chem. Eng. Lab III
Experiment 7
Spray Drier
Spray dryers are used for a wide variety of products, including such diverse
materials as organic and inorganic chemicals, pharmaceuticals, food products
such as milk, eggs, and soluble coffee, as well as soap and detergent products.
In order to obtain rapid drying, atomization of the feed must provide small
particles of surface/weight ratio, whose diameter is usually in the range 10 to 60
mm. For this purpose, spray nozzles or rapidly rotating disks can be used. Spray
nozzles are of two major types: pressure nozzles, in which the liquid is pumped at
high pressure and with a rapid circular motion through a small orifice, and two
fluid nozzles, in which a gas such as air or steam at relatively low pressures is
used to tear the liquid into droplets. Nozzles are relatively inflexible in their
operating characteristics and do not permit even moderate variation in liquid-flow
rates without large changes in droplet size. They are also subject to rapid erosion
and wear.
The drying gas, either a flue gas or air, enter at the highest practical temperature,
80 to 760°C, limited only by the heat sensitivity of the product. Since the contact
time for product and gas is so short, relatively high temperatures are feasible.
The short time of drying requires an effective gas-spray mixing, and attempts to
improve this mixing account in part for the large number of designs of spray
chambers. Cool air is sometimes admitted at the drying-chamber walls in order to
prevent sticking of the product to the sides. The effluent gas may convey all the
dried product out of the dryer or only the fines, but in either case the gas must be
passed through some type of dust collector such as cyclones or bag filters, and
these are sometimes followed by wet scrubbers for the last traces of dust.
The drops of liquid reach their terminal velocity in the gas stream quickly, within
inches of the atomizing device. Evaporation takes place from the surface of the
drops, and with many products solid material may accumulate as an impervious
shell at the surface. Since heat is nevertheless rapidly being transmitted to the
particles from the hot gas, the entrapped liquid portion of the drop vaporizes and
expands the still-plastic wall of the drop to 3 to 10 times the original size,
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8. METU Chem. Eng. Dept.
Ch.E. 420 Chem. Eng. Lab III
Experiment 7
Spray Drier
eventually exploding a small blowhole in the wall and escaping, to leave a hollow,
dried shell of solid as the product. In other cases, the central liquid core diffuses
through the shell to the outside, and the reduced internal pressure causes an
implosion. In any event, the dried product is frequently in the form of small hollow
beads of low bulk density. Some control over the bulk density is usually possible
through control of the particle size during atomization or through the temperature
of the drying gas (increased gas temperature causes decreased product bulk
density by more extensive expansion of the drop contents).
Spray drying offers the advantage of extremely rapid drying for heat-sensitive
products, a product particle size and density which are controllable within limits,
and relatively low operating costs, especially in large-capacity dryers.
Humidity
When the air-water system is the subject, the concentration is expressed as
humidity, which is defined as the mass of water vapor contained per unit mass of
air.
Dew Point
The temperature to which the vapor-gas mixture must be cooled to become
saturated (at constant humidity) Td
Wet Bulb Temperature
Steady state temperature attained by a wet-bulb thermometer exposed to rapidly
moving stream of gas-vapor mixture. The bulb of the wet-bulb thermometer is
coated with the same liquid that forms the vapor. This temperature together with
the temperature of the gaseous mixture is used in the determination of humidity.
Humid Volume
Volume occupied by a unit mole (mass) of gas plus the humidity it contains.
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9. METU Chem. Eng. Dept.
Ch.E. 420 Chem. Eng. Lab III
Experiment 7
Spray Drier
3
V T Y m
3
0.0224 (1 ) (molar)
273
0.0224 ( 1 ) (mass)
273
h
H
b a
gr mole
= +
V T m
M M gr
-
= + H
Y moles of water vapor
one mole of air
mass of water vapor
one g
ram of air
=
H =
Humid Heat
Heat capacity of gas together with its humidity.
C C YC J
gr mole K
% % %
C C C J
gr K
H
H H YH C T T Y C T T
H H H C T T C
0 0 0
b a
0
b a
ˆ ˆ ˆ
Humid Enthalpy
( ) ( )
b a
ˆ ˆ ˆ ˆ ( ) ˆ
b
p p p
p p p
V b a p p
( ) ˆ a éë T -T +l ùû
V b a p p
o
o
l
= +
-
= +
= + = - + éë - + ùû
= + = - +
% % % % % %
H H 0 0
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10. METU Chem. Eng. Dept.
Ch.E. 420 Chem. Eng. Lab III
Experiment 7
Spray Drier
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11. METU Chem. Eng. Dept.
Ch.E. 420 Chem. Eng. Lab III
Experiment 7
Spray Drier
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