Study of spray dryer

5/22/2018 STUDY OF SPRAY
DRYER
Fahim Shahriar Sakib
ID: 1502043
GROUP NO: 03(A2)
PARTNERS’ ID: 1502041
1502042
1502044
1502045
Oindrila Hossain
LECTURER
DEPARTMENT OF CHEMICAL ENGINEERING
BUET

i
ACKNOWLEDGEMENT
The experiment reported here has been carried out at the Unit Operations Laboratory of the
Department of Chemical Engineering, Bangladesh University of Engineering and Technology as
a part of the curriculum of the course CHE 302. I am highly indebted to my course instructor
Oindrila Hossain, Lecturer at Department of Chemical Engineering, BUET for her constructive
suggestions, sincere co-operation, continuous guidance and constant supervision during the
experiment. I am very grateful to my group partners without whom it would be impossible to carry
out the experiment and preparing the report. I also express my gratitude to the laboratory assistants
who helped in different stages of the experiment.
May 21, 2018 Fahim Shahriar Sakib

ii
SUMMARY
The objective of this experiment is to study spray dryers and spray drying operation. In industry,
spray drying is used to separate liquid from solid. In spray dryer, a liquid solution is dispersed into
a stream of hot gas in the form of mist of fine droplets. Moisture is rapidly vaporized from droplets
and leaves residue particles of dry solid. These are then separated from the gas stream by separator.
In this experiment, dry milk was used for dry solid. The milk solution was used as liquid feed and
the performance of spray dryer is observed. For this experiment, mixed flow spray dryer was used
which contains a two-flow atomizer. 1000 gm milk solution was taken of which 50 gm was dry
milk powder. Total process is continued for 35 minutes and 10.1 gm milk was recovered after the
process. After calculation, percent recovery of milk was 20.2% and drying rate was 27.14 gm/min.
The reason behind low percentage recovery of dry milk are discussed in discussion section.

iii
Table of Contents
Topics Page No.
Introduction 1
Theory 2
Experimental Section 5
Results and Discussion 8
Conclusion 11
References 12
Nomenclature 13
Appendix 14
List of Figures
Figure Name Page No.
Figure 01: Two fluid nozzles (Internal contact and external contact) 2
Figure 02: Schematic illustrations of particles in spray drying process 3
Figure 03: Experimental setup of spray drying operation of milk liquid solution 6
Figure 04: Process Flow Diagram (PFD) of spray drying 15
Figure 05: Pressure nozzle atomizer 16
Figure 06: Centrifugal atomizer 17
Figure 07: Different types of spray dryer 18
Figure 08: Bag filter 22
Figure 09: Wet scrubber 23
Figure 10: Electrostatic precipitator 23

iv
List of Tables
Table Name Page No.
Table 01: Data for weight of dry milk and water, pressure of compressed air,
initial and final temperature of inlet hot air and outlet air, operation time,
weight of empty bottle and bottle with recovered milk.
8
weight of empty bottle and bottle with recovered milk of group 04(A2).
8
weight of empty bottle and bottle with recovered milk of group 05(A2).
9
Table 04: Data for recovered milk, time, percentage of milk recovery and
drying rate for different groups.
9

1
INTRODUCTION
Drying is the process of separating liquid from any wet medium. There are many processes for
drying. One of them is spray drying. Spray drying is the transformation of feed from a liquid form
into dried form by spraying feed into hot drying medium. The feed can either be a solution,
suspension or paste. The resulting product conforms to powders, granules or agglomerates. If the
liquid is a flammable solvent like ethanol then hot nitrogen is used instead of air. This method is
also used of drying of many heat sensitive materials such as foods and pharmaceuticals. Because,
some ion can be oxidized in presence of air and that will be harmful for our health.
The atomizer is the main thing in spray drying. The atomizer increases the surface area of the
liquid solution by creating fine droplets. The mist is sprayed into a chamber of air heated to
temperature above the vaporization temperature of the solution’s solvent. The contact between
mist and hot air makes the vaporization of water. The flow of liquid and gas may be co-current,
counter-current or the combination of both (mixed current). In this experiment, mixed current
spray dryer was used. The liquid drying rate depends upon the solution flow rate, flow rate of the
air, size of the droplets, temperature of the air and solution and amount of solvent in the solution.
Cyclone separator is used to separate dried particles from vaporized solvent. The air rotates
helically in the cyclone separator. The dried particle is forced to go to the bottom and the air is
expelled to the atmosphere pressure goes to a scrubber. The dried product can be in the form of
powders, granules or agglomerates depending upon the physical and chemical properties of the
feed, the dryer design and desired product properties.

2
THEORY
Any spray drying system has its major parts like feed delivery and atomization system, a drying
chamber, a hot gas production and delivery system, a solid gas separation system and a product
discharge system. There are some main parts of a common spray dryer. They are air blower, heater,
drying chamber, atomizer, compressor and cyclone separator system.
There are three types of atomizer. Two fluid nozzles, pressure nozzle atomizer and centrifugal
atomizer. In this experiment, two fluid nozzles atomizer was used. Spray is created by contacting
two fluids, the feed and a compressed gas. The atomization energy is provided by the compressed
gas, usually air. The contact can be internal or external. It is used in low production rate drying.
Because, it ensures the quality of particles size. The other types of atomizer are discussed in
appendix section.
Figure 01: Two fluid nozzles (Internal contact and external contact)
There are mainly four steps involved in the spray drying.
• Atomization of a liquid feed into fine droplets - Atomization is the process of breaking up
the bulk liquid into millions of individual droplets forming a spray. For atomization
process, pressure energy is needed. In this experiment, two fluid nozzles were used. The
liquid feed entered into the atomizer. Then the liquid stream comes in contact with
compressed air. Thus, fine droplets were created. The mist was sprayed into the dryer
chamber from the spray nozzle.

3
• Mixing of spray droplets into a heated gas stream – In the drying chamber, the mist comes
into contact with a heated air stream. In this experiment, mixed current flow was used.
There are also two types of flow. Co-current and counter-current. These are discussed in
appendix section. After mixing, the heated air vaporizes the solvent.
• Drying of spray by volatile evaporation – A constant rate phase ensures moisture
evaporates rapidly from the surface of the particle. After leaving the atomizer, the mist
mixes with a heated stream in the drying chamber. In drying solutions and emulsions, the
drying particle reaches temperature higher than that of wet bulb temperature as drying
produces. Initially the liquid evaporates from the droplet surface, the relatively dry surface
may form a tough shell through which liquid from the interior of the shell must diffuse in
order to escape. This diffusion is much slower process than the transfer of heat through the
droplet shell to the interior, so the liquid tends to evaporate in place. So, the droplet swells,
making the shell thinner and diffusion faster. If the shell is relatively inelastic and
impermeable than the internal evaporation is enough to cause rupture to the shell, either
producing fragments or making a new hollow bulb beside the original one. The drying rate
depends upon the temperature, humidity, feed rate, flow rate of drying gas and the size of
droplets.
Figure 02: Schematic illustrations of particles in spray drying process

4
• Separating and collecting dried powder from the gas stream – A cyclone separator is used
to separate dried powder from the gas stream. Without going directly to the center of the
cyclone separator, the dried particles fall at the bottom of the separator in a helical path
being adjacent to the cyclone body. The dried particles are forced to the bottom of the
cyclone separator. The gas goes to scrubber.
In the drying chamber both heat and mass transfer occur. The initial vaporization of the water in
the mist accounts for the majority of the heat transfer. The amount of heat necessary to increase
the solution temperature to the air inlet temperature needs to be a high enough temperature to
vaporize the solvent. Once vaporization occurs, the vaporized solvent is then mixed with the drying
air. If the drying air becomes saturated with solvent, no more liquid in the mist can be vaporized.
To get more dried solution, some things should be followed. Increasing of dry air inlet temperature
will allow more solvent vaporized in the air. Increasing the dry air flow rate will increase the
amount of air present in the chamber and allow the more mass transfer. Lowering feed rate into
the drying chamber will limit the amount of water that needs to be dried.

5
EXPERIMENTAL SECTION
Apparatus:
1. Spray dryer – [It contains drying chamber, an atomizer with compressor, air blower and
air heater. The liquid solution was dried with it. Normal air was supplied
from air blower and heated in air heater. Atomizer dispersed solution into
gas in form of mist.]
2. Cyclone separator – [It recovered dried solid particles from the gas stream. The
separation was done by rotating air.]
3. Feed tank – [Feed was supplied from feed tank]
4. Pressure gauge – [This gauge meter was used to check the pressure of compressed air of
the inlet of atomizer]
5. Temperature gauge – [This gauge meter was used to check the temperature of hot air.]
6. Milk collector bottle – [The recovered milk was accumulated in bottle]
7. Beaker – [The milk solution was made in beaker.]
8. Weighing scale – [Milk solution and recovered powder milk was weighed by this device.]
9. Stopwatch – [Total operation’s time was checked by stopwatch.]
10. Thermometer – [Outlet air temperature was measured by thermometer.]

6
Experimental Setup:
Feed Milk
Container
Air Blower
Thermometer
Chamber
Thermometer
Cyclone
Separator
Drying Chamber
Atomizer
Air
Air + Water
Vapor
Air
Heater
Jar
Compressor
Pressure
Gauge meter
Hot Air
Figure 03: Experimental setup of spray drying operation of milk liquid solution.

7
Experimental Procedure:
1. Some milk powder was taken in a big beaker and the weight of milk powder was taken by
weighing. The amount of milk powder was 50 gm.
2. Water was added to prepare a 5% (weight) milk solution. The powder was dissolved
thoroughly and was strained thorough a fine cloth.
3. The spray dryer was started by turning the hot air blown on. The heater was adjusted and
noted the temperature of hot air.
4. The dryer was allowed running for 10-15 minutes.
5. The dryer was run with prepared milk solution for a fixed atomized pressure.
6. At the end, the dryer was run with distilled water for 10-15 minutes.

8
RESULTS AND DISCUSSION
In Table 01, there are some data which was observed in experiment. These are sample of dry milk
powder, amount of added water, pressure of compressed air, initial and final temperature of inlet
hot air and outlet air, time of spray drying operation, weight of empty bottle and bottle with
recovered milk.
Weight
of milk
powder,
Mp
(gm)
Weight
of
water,
Mw
(gm)
Pressure of
compressed
air, P
(psi)
Inlet hot air
temperature
(0
C)
Outlet air
temperature
(0
C)
Time,
T
(min)
Weight of
empty
beaker, Mb
(gm)
Weight
of beaker
+ milk,
Mt
(gm)Initial Final Initial Final
50 950 20 150 150 76 62 35 561.5 571.6
Table 01: Data for weight of dry milk and water, pressure of compressed air, initial and final temperature
of inlet hot air and outlet air, operation time, weight of empty bottle and bottle with recovered milk.
There are also two groups’ experimental observed data which are shown below. (Table 02 and
Table 03)
Weight
of milk
powder,
Mp
(gm)
Weight
of
water,
Mw
(gm)
Pressure of
compressed
air, P
(psi)
Inlet hot air
temperature
(0
C)
Outlet air
temperature
(0
C)
Time,
T
(min)
Weight of
empty
beaker, Mb
(gm)
Weight
of beaker
+ milk,
Mt
50 950 20 153 153 78 64 35 561.5 577.8
of inlet hot air and outlet air, operation time, weight of empty bottle and bottle with recovered milk of
group 04(A2).

9
Table 02 indicates the experimental data of group 04(A2) and Table 03 indicates the experimental
data of group 05(A2). The total operation was run by 35 min and 10.1 gm milk was recovered
while the total operation time and recovered milk was 35 min, 16.3 gm and 56 min, 20.2 gm by
group 04(A2) and group 05(A2) respectively. These are shown in Table 04.
Weight
of milk
powder,
Mp
(gm)
Weight
of
water,
Mw
(gm)
Pressure of
compressed
air, P
(psi)
Inlet hot air
temperature
(0
C)
Outlet air
temperature
(0
C)
Time,
T
(min)
Weight of
empty
beaker, Mb
(gm)
Weight
of beaker
+ milk,
Mt
50 950 20 150 150 75 71 56 561.0 581.2
of inlet hot air and outlet air, operation time, weight of empty bottle and bottle with recovered milk of
group 05(A2).
Group Recovered milk,
Mm
(gm)
Time,
T
(min)
Percentage of
milk recovery (%)
Drying rate,
Rd
(gm/min)
03(A2) 10.10 35 20.20 27.14
04(A2) 16.30 35 32.60 27.14
05(A2) 20.20 56 40.40 16.96
Table 04: Data for recovered milk, time, percentage of milk recovery and drying rate for different groups.
Sample calculation are shown in appendix. The all results indicate that there were some losses
within the system. Here are discussed below.
The supplied milk powder already had some moisture content. So, the actual weight of sample dry
milk was not found. This caused some loss in the weight of milk powder. In drying operation,
some milk might accumulate in the apparatus. A considerable amount of milk was seen stucking
on the separator wall. It affected on the amount of recovered milk. Smooth surface might reduce

10
this loss. Atomization of milk solution was not perfect enough so that little drops of milk remained
unchanged in the top of dryer chamber.
The feed had been entered in the dryer at a quite high flow rate. This was done to dry the milk
quickly. If it was done slowly, result would be more efficient. This is seen in Table 04. The drying
operation was done more slowly by group 05(A2) then other two groups. The result is more
efficient than other two groups.
The pressure of the atomizing air and the working pressure of spray dryer could not be
maintained to be perfectly constant. If the atomizing pressure was higher then the feed would have
been broken into finer particles. More finer particles would help to get more recovered milk.
Some milk powder went out with outlet air in the cyclone separator. Recycle of the exhaust stream
could help to get more efficient result. It is seen that outlet air temperature was lower than inlet
temperature. It was for high moisture in milk powder. It required heat from hot air to drive the
moisture off and thus the temperature of air at the outlet decreased in a good amount.
The total operation was done by a simple spray dryer. In industries, large number of production
are ensured. So, the operating factors like selection of dryer, atomizer, separator, flow conditions
of the drying gas etc. may vary. The percentage of recovery milk can be improved by modification
of existing system. By using bag filter, wet scrubber and electrostatic precipitator, the result could
be improved. These are discussed in appendix section.
It is seen that group 05(A2) has more efficient result. Their outlet air temperature at initial and
final was 75 0
C and 71 0
C respectively. Temperature difference was lower than other groups. Thus
it differs a significant change in result. Another factor is slow feed rate. Group 05(A2) ran
operation more slowly than other groups.
There are some advantages of spray dryer which are given below.
Spray dryer is applicable to both heat-sensitive and heat-resistant material etc. It provides hygienic
drying conditions and the operation is continuous and easy which is adaptable to full automation.
In the whole spray drying process powder quality remains constant.
Types of spray dryer, it’s application and uses all are discussed broadly in appendix.

11
CONCLUSION
Spray drying is a process of removing the moisture from particles by hot air. The quality of spray
dried product is high. Because, evaporative cooling is used in the process. It ensures the protection
of particles. There are some factors which influence the performance of spray dryer like inlet air
flow rate, temperature, moisture content, amount of solid materials, density and viscosity of the
processed object. Performance also depends on type of atomizer, size and diameter. By this
experiment, something about spray dryer are learnt like spray drying technique, working principle,
operating variables, types of material handle in spray dryer, advantage and disadvantage of spray
dryer. The parts and their function are also learnt by this experiment. It will help to understand and
make a large installation of spray dryer in industries.

12
REFERENCES
1. Foust, A.S.L.A. Wenzel, C.W. Clump, L. Maus, and Anderson, L.B. (1980). Principles of
Unit Operations, 2nd
edition, New York: John Wiley & Sons
2. Coulson. J.M. and Richardson, J.F. (1999). Coulson & Richardson’s Chemical
Engineering, 6th
edition, Vol.1,2, Elsevier Butterworth Heinemann, Amsterdam
3. McCabe, W.L., Smith, J.C., and Harriott, P. (2005). Unit Operations of Chemical
Engineering, 7th
edition, McGraw-Hill, Singapore
4. Smith, J.M., Van Ness, H.C., and Abbott, M.M. (2005). Introduction to Chemical
Engineering Thermodynamics, 7th
edition. Boston: McGraw Hill
5. Masters, K. (1985). Spray Drying Handbook, 4th
edition, George Godwin, London
6. Spray drying (www.wikipedia.org) – [17 may, 2018 at 6:30 p.m.]
7. Spray nozzle (www.wikipedia.org) – [18 may, 2018 at 4:45 p.m.]

13
NOMENCLATURE
Symbol Name Unit
P Pressure psi
T Time min
Mp Weight of milk powder gm
Mw Weight of water gm
Mb Weight of bottle gm
Mt Weight of bottle + recovery milk gm
Mm Weight of recovery milk gm
Rd Drying rate gm/min

14
APPENDIX
Sample Calculation:
Sample dry milk, Mp = 50 gm
Water added, Mw = 950 gm
Weight of empty bottle, Mb = 561.5 gm
Weight of recovered milk + bottle, Mt = 571.6 gm
Recovered milk, Mm = 571.6 – 561.5 = 10.1 gm
Time needed, T = 35 min
Percentage of recovery milk = (10.1 / 50) × 100 = 20.2 %
Drying rate, Rd = 950 / 35 = 27.14 gm/min

15
Process Flow Diagram (PFD):
Air
Air
Compressor
Heater
Blower
Milk solution
Powder = 50 gm
Water 950 gm
Spray
Dryer
Compressed air
pressure = 20 psi
Air + water vapor
+ milk powder
Air + water
vapor
Cyclone
Separator
Hot air temperature
= 153 gm
Figure 04: Process Flow Diagram (PFD) of spray drying

16
Types of Atomizer:
There are three types of atomizer.
1. Two fluid nozzles.
2. Pressure nozzle atomizer.
3. Centrifugal atomizer
Two fluid nozzles were discussed in theory. The others will be discussed here.
Pressure nozzle atomizer: A spray is created by forcing the fluid through orifice. The energy
required to overcome the pressure drop is supplied by the spray dryer feed pump. The narrowest
particle size distribution is possible with this technique. Spraying pressure depends on feed
characteristics and desired particle size and can range from 300 to 3000 psi. It is the most energy
efficient. It is used for gaining fine particles, when it’s very important. This type of atomization
requires a positive displacement, high pressure feed pump, such as a plunger pump or a
piston/diaphragm pump.
Figure 05: Pressure nozzle atomizer
Centrifugal atomizer: These atomizers may be used to spray fluids that cannot be made
homogeneous enough to pass through a nozzle. The spray is created by passing the fluid across or
through a rotating wheel or disk. The energy required for atomization is supplied by the atomizer
motor. They produce an extremely uniform droplet size and do not require a high-pressure feed or
impart an axial velocity to the sprayed dryers. These are usually, the most resistant to wear and
requires periodic changing of wheel inserts.

17
Figure 06: Centrifugal atomizer
Types of Spray dryer:
On the basis of the type of flow, there are three types.
1. Co-current: In the co-current flow dryer, the spray is directed into the hot air entering the
dryer and both pass through the chamber in the same direction. Spray evaporation is rapid,
and the temperature of the drying air is quickly reduced by the vaporization of water. The
product does not suffer from heat degradation since once the moisture content reaches the
target level, the temperature of the particle does not increase greatly because the
surrounding air is now much cooler.

18
Figure 07: Different types of spray dryer.
2. Counter-current: In this dryer design, the spray and the air are introduced at opposite
ends of the dryer, with the atomizer positioned at the top and the air entering at the bottom.
A counter-current dryer offers more rapid evaporation. It is higher energy efficiency than
a co-current design. Because the driest particles are in contact with hottest air, this design
is not suitable for heat-sensitive products. Counter-current dryers normally use nozzles for
atomization because the energy of the spray can be directed against the air movement.
3. Mixed current: It is the combination of both co-current and counter-current flow. In a
mixed flow dryer, the air enters at the top and the atomizer is located at the bottom. Like
the counter-current design, a mixed flow dryer exposes the driest particles to the hottest
air, so this design is not used with heat-sensitive products.

19
Application and Uses of Spray Dryer:
There are a lot of applications of spray dryer which are shown below.
Non-pharmaceutical applications: Chemical industry, Ceramic materials, Detergents, soaps and
surface-active agents, pesticides, herbicides, fungicides and insecticides, dyestuffs, pigments,
fertilizers, mineral floatation concentrates, inorganic chemicals, organic chemicals, spray
concentration (purification), milk products, egg products, food and plant products, fruits,
vegetables, carbohydrates and similar products, slaughterhouse products, fish products and many
others.
Industrial applications: Spray dryers are used with an enormous range of products, because of
their wide range of utility and convenient product form. A partial list would include coffee, milk,
detergents, dyestuffs, pesticides, polymers, tile slips, blood plasma, enzymes, penicillin, starch,
metal concentrates, kaolin, alumina and iron oxide furnace fume washing.
Spray drying is applicable to both heat-sensitive and heat-resistive materials. It is applicable to
heat sensitive products such as milk powders and other foods and pharmaceutical because of the
short contact time in the dryer hot zone. One other class of products that particularly applicable
to spray dryers is solid slurries remains fluid at very low moister content.
The chief advantages of spray dryers are the very short drying time, which permits drying of
highly heat sensitive materials and the production of solid or hollow spherical particles. The
desired consistency, bulk density, appearance and flow properties of some products, such as,
foods or synthetic detergents, may be difficult or impossible to obtain in any other type of dryer.

20
Advantages and Disadvantages of Spray Dryer:
There are a lot of advantages of spray dryer. These are given below.
• Adaptable to fully automated control system that allows continuous monitoring and
recording of very large number of process variables simultaneously.
• Able to operate in applications that range from aseptic pharmaceutical processing
to ceramic powder production.
• It can be designed to virtually any capacity required. (Feed rates range from a few pounds
per hour to over 100 tons per hour).
• It can be used with both heat-resistant and heat sensitive products.
• Offers high precision control over Particle size, Bulk density, organic volatile impurities
and residual solvents.
• Powder quality remains constant during the entire run of the dryer.
• Nearly spherical particles can be produced, uniform in size and frequently hollow, thus
reducing the bulk density of the product.
But, it has also some disadvantages. These are also given below.
• The equipment requires a large amount of space.
• The equipment is very bulky and with the ancillary equipment is expensive.
• The capital cost of the equipment is high per unit annual weight of product, particularly
for low capacities.
• The overall thermal efficiency is low, as the large volumes of heated air pass through the
chamber without contacting a particle, thus not contributing directly to the drying.
• The recovery of dusty product from the exit gases may be a troublesome problem or
involve the use of expensive equipment.
• The spray dryer is not readily usable for toxic products, unless special precautions
are taken.
• All impurities in the liquor remain in the product.

21
Design Data Require for Spray Dryer:
The variables which make impact on the design of a spray dryer are given below.
• The size and installment of the drying chamber.
• Atomizer selection.
• Method of air introduction and the air disperse.
• Separation auxiliaries.
The selection of the variable is determined by the physical characteristics of the desired product.
They are given.
• The particle form of the final spray dried powder.
• The maximum temperature to which dried particles can be subjected.
• The required particle size, as directed by the powder specification.
• The number of products that is needed to produce.
The selection of the atomizer for a given spray drying operation depends upon the nature of the
feed and the particle size distribution required in the final dried product. The choice of the
atomizer influences the design of the entire spray chamber. Nozzles spray axially in the spray
chamber and require a vertically long, relatively thin tower. Disk atomizers spray radial. So, the
spray tower must have a large diameter but can be shorter. The cone-angle at the bottom of the
tower is usually 60-70 degrees and thus most of the tower height where a disk atomizer is used
will be occupied by this cone.
The other design variables are the drying rate, the time exposure if the droplets to the drying
atmosphere, thermal efficiency, method of separation etc. The drying rate depends upon the
temperature, humidity and flow rate of the drying gas, the size of droplets produced by the atomizer
and the properties of the products being dried. The design of the drying chamber is dependent on
the consideration of direction and degree of atomization, air flow pattern, desired products
characteristics and the reaction time for drying, air flow rates and discharge requirements.

22
Modifications of Spray Dryer:
The percentage recovery of the product can be improved by some modifications of the existing
system. These are described below.
Bag Filter: The design of the drying chamber is dependent on the consideration of direction and
degree of atomization, air flow pattern, desired products characteristics and the reaction time for
drying, air flow rates and discharge requirements. A compressed air jet frees the powder from the
bag and drops into a hopper and out of the system. The main features of bag filter are its relatively
low fan energy consumption and its usefulness for low density products. It is used downstream of
cyclone for emission control when no liquid effluent is desired.
Figure 08: Bag filter
Wet Scrubber: In a wet scrubber, the polluted gas stream is brought into contact with the
scrubbing liquid, by spraying it with the liquid, by forcing it through a pool of liquid, or by some
other contact method, so as to remove the pollutants. In wet scrubber, high efficiencies possible.
It is relatively low maintenance. Higher level of instrumentation usually required for density and
level control. It requires recycle or treatment of liquid effluent.

23
Figure 09: Wet scrubber
Electrostatic Precipitator: An electrostatic precipitator (ESP) is a particulate collection
device that removes particles from a flowing gas (such as air) using the force of an induced
electrostatic charge. Electrostatic precipitators are highly efficient filtration devices that minimally
impede the flow of gases through the device and can easily remove fine particulate matter such as
dust and smoke from the air stream.
Figure 10: Electrostatic precipitator

24
Operation and Maintenance:
The formation of a spray and then connecting the spray with air is characteristic feature of spray
drying. In the spray drying process the dryable liquid slurry is pumped through a nozzle which
sprays the feed fine droplets. The droplets are adjacent to a steam of hot air flowing either co
currently or counter-currently or complex mixture of two paths. As the atomized droplets fall, the
moisture evaporates to the hot gas and leaves the solid materials as particles. These small particles
are swept with the gas into cyclone separator in which the particles are separated. Thus, the
operation of spray drying consists of the following steps.
• Atomization (rotary or nozzle).
• Spray gaseous medium into feed.
• Evaporation.
• Product recovery.
Regular checking of the atomizer, air fans and powder collector will be maintained as they
controls the greater part of balanced mechanical work. Filters are required to check the atomizer.
Bag filter installation tend to require costly maintenance of losses of expensive dried
products are to be prevented. Checking of instruments must be frequent where close
temperature control is required.
All the usual automatic flame safety devices must be included on direct and indirect fuel-fired
heaters and complete interlocking of safety and operating controls for start-up and shut down
procedures, particularly in the case of the latter to guard against accidental failure of any part of
the system.

Study of spray dryer

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