Course Number: ChE-302

Course Title: Chemical Engineering Laboratory-II

Experiment Number: 05

Name of the Experiment: Study of Spray Drying

Submitted by:

Mahe Rukh

Student Number: 1202036

Section: A2 Group Number: 02

Department of Chemical Engineering

Partners’ Student Numbers: 1202037

1202038

1202039

1202040

Date of Performance:

31/10/15

Date of Submission:

7 /11/15

Submitted to:

Dr. Md. Tanvir Sowgath

Assistant Professor

Department of Chemical Engineering

BUET

Summary The purpose of this experiment is to study spray dryers and spray drying procedure. Spray drying

is one of the best drying approaches to convert the fluid materials into solid or semi-solid

particles in a single step. This experiment helped us to understand spray drying technique, the

mechanism of spray drying, working principle of spray dryer, operating variables of spray

drying, types of material handle in spray dryer and advantages and disadvantages of spray dryer

over other types of dryer. We also studied different parts of a spray dryer and their functions.

This process is rapid and labor cost is low. Spray dryers employ atomizer or spray nozzle to

break the bulk of liquid or slurry into controlled drop size. This method of drying is applied in

food, pharmaceuticals and other industries for its unique features.

In this experiment, a solution of 1000.1 g liquor milk with 50.1g milk powder was spray dried

and after the drying process 13g milk powder was recovered. Hence, 26 % milk powder was

recovered and the drying rate was 38g/min. the recovery is not satisfactory. Probable reasons for

low recovery has been explained and modification techniques are also discussed in this report.

Introduction Spray drying, as the name suggests is a process of drying utilizing spray mechanism. This

process is a largely used industrial operation found in various industries. It is engaged in

producing powders from liquid feedstocks in a single step. The liquid feedstock which can be a

solution, emulsion or suspension is sprayed through a nozzle in a drying chamber where hot air

is flown simultaneously. It can be used for equally heat-resistant and heat sensitive products. In

the spray dryer heated gas mixes with an atomized liquid stream to accomplish evaporation

easily. And dry powder is produced with controlled average particle size at last.

The unit operation of spray dryer includes following steps:

Atomization of liquid stream- by atomization we mean separating something into fine

particles. It is the process of breaking bulk liquids into droplets. Primary functions of

atomization are gaining high surface to mass ratio resulting in high evaporation rates,

production of particles of desired shape, size and density.

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Heating of gas/air- Blower is set on the top of the dryer which draws air. Drawn air is

heated using heating coils.

Mixing of gas/air and atomized stream- This should be done for adequate residence time

for achieving high heat and mass transfer.

Separating and collecting the dried powder from the gas stream.

The unique feature of a spray dryer is the surface area per unit weight generated by atomization

of the liquid feed which enables easy and higher evaporation rate.  To see a real life exam, let’s

look at milk. The ability to keep cold large quantities of a product like milk is limited. Moreover,

milk has 80-90% water, by spray drying it, one can reduce its bulk weight.  Spray drying 100kg

of milk can possibly reduce that weight to 12kg, which offers savings in shipping expenses.

In this experiment, 5% solution of milk and water is fed to the spray dryer through atomizer. The

atomized stream then comes in contact with compressed air and creates mist. The mist comes

into contact with hot air in drying chamber. The temperature of fine droplets increases

instantaneously and solvent vaporizes. Increase in temperature of solvent and subsequent

vaporization counts for the heat transfer in the process. Once solvent vaporizes, it passes into dry

air which can be accounted as mass transfer. As a result, it can be said that spray drying includes

both heat and mass transfer processes. The humid air passes to the atmosphere and dry air and

powder mixture enters cyclone separator from which powder is recovered. In cyclone separator

centrifugal action is used to separate powder from the mixture.

The flow of air to the dryer can be co-current or counter-current to the atomizer direction. In the

latter case, the residence time of the particles in the chamber is greater than the former case. In

our experiment air flew in the counter current direction to the atomizer. Spray drying process

operates in one step which makes it very popular these days as in food industries rapid drying is

required. This process is versatile and particle size can be controlled too. By changing inlet

temperature, hot air flow rate, fluid volume, fluid velocity the quality of final product can be

controlled. This feature made spray drying very widespread in pharmaceutical industries too. But

in this process high maintenance is required while dealing with nozzles used. Nozzles are prone

to clogging and abrasion can also occur at the opening of nozzles. Rotary disc atomizers if used,

there is chance of internal corrosion. Finally, powder may stick to internal chamber contributing

to cleaning cost and low recovery.

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Experimental work Apparatus

Spray Dryer

o Drying Chamber

o Air blower

o Atomizer

o Air heater

Feed tank

Temperature gauge

Pressure gauge

Beaker

Milk-water solution

Stop watch

Thermometer

Cyclone separator

Dry air

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Experimental Set up

Figure-01: Schematic diagram of experimental setup of a spray drying system

Figure 02: Two fluid nozzle

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Experimental Procedure

About 50g milk powder was taken in a beaker by weighing

950gm of water was added to the beaker and mixed to prepare 5% solution

The spray dryer was started by by turning the hot air blower on. The heater was adjusted

according to requirements.

The operating atomizer pressure was set to 20 psi.

The milk powder was separated in a cyclone separator and was collected in a bottle.

Weight of recovered powder was measured and by comparing to initial weight percentage

of recovery was calculated.

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Figure-03: Cyclone Separator

Process Block Diagram

Air

Water Hot air

Air

Milk Milk Compressed Powder Solution (5%) air

Air Exhaust air Milk Powder

Product Milk Powder

Figure-04: Process block diagram of a spray drying system

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Mixer

Spray

Dryer

Heater

Blower

Cyclone

Separator

Compressor

Process Flow Diagram

Blower

Feed container Air

Water

Milk Heater

Feed Mixing unit solution

Hot air Flow controller

Spray dryer

Exhaust air Compressed air

Pressure controller

Dried milk & air Air

CycloneSeparator Recovered milk powder Compressor

Figure-05: Process flow diagram of a spray drying system

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Observed Data Weight measured

Weight of milk powder taken, w1 = 50 g

Weight of water, w2 = 950 g

So, wt % of prepared milk solution = 5%

Weight of the container, wc = 561.6 g

Weight of container + recovered milk powder, wc+m = 574.6 g

Temperature measured

Initial inlet temperature of air,TIi = 150°C

Initial outlet temperature of air, TIo = 80°C

Final inlet temperature of air, TFi = 150°C

Final outlet temperature of air, TFo = 66.5°C

Pressure measured (P)

Working pressure of atomizing air = 20 psia

Time measured

Required time for drying the solution, t = 25 minutes

Calculation

Data calculation:

Weight of liquor milk = W1 + W2

= (50.1+950.) g

= 1000.1 g

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Weight of milk powder recovered, = (Weight of beaker with recovered milk-Weight of empty

beaker)

Wp= Wc+m-Wc

= (574.6-561.6) g

=13 g

Percentage of milk powder recovered = (Weight of milk powder recovered)/(Weight of milk

powder initially taken)×100 %

= (13/50.1)×100 %

= 26 %

Weight of water W2= 950 g

Total time required for drying, t = 25.0 min

= (25×60) sec

= 1500 sec

Drying rate = (Weight of water, Mw)/( Total time required for drying ,t)

= 950/25

= 38 g min-1

Result Percentage recovery of milk powder = 26 %

The drying rate = 38 g/min

Discussions The percentage of recovery was 26% and drying rate is 38g/min. from the values we can easily

tell that the recovery of powder is not satisfactory. The possible reasons of this low recovery are

enlisted below:

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Some dried milk may accumulate in the apparatus

The flow of feed was controlled manually which resulted in low recovery

Dry milk powder is assumed to pass to the atmosphere with exhaust air which resulted in

loss.

In spray dryer heat transfer mode is convection and so it shows low thermal efficiency.

Heat is lost in the discharged air.

A good amount of particles had been attached within the walls of spray dryer and also at

cyclone separator.

Feed may have entered the dryer at high flow rate which reduced contact period between

droplets and dry air and thus affected evaporation process.

The pressure of the atomizing air and the working pressure of spray dryer should have

been maintained at constant value. If the atomizing air pressure were higher, the feed

would have been broken into finer particles, resulting in efficient drying.

Modifications To make drying more efficient in spray drying process the following measures can be taken.

Such as-

Allowing lengthier contact time.

Using bigger separator.

Lower flow rate feed.

Increasing atomizing pressure.

Ensuring better insulation.

Using a dryer with smoother wall, and using smooth pipes with fewer bends.

Replacing spray nozzle with rotary atomizer can increase contact area between

drying air and feed ensuring more mass transfer.

If flowing area of drying agent can be increased, more vaporization will occur.

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Some instrument can be used for better result. Such as-

Bag filter

Figure-06: Bag Filter

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Bag filters work very simply; air is pulled through the filter drawn by the exhaust fan powder

collects on the bags while air is allowed to pass through the filter medium. A compressed air

jet frees the powder from the bag and drops into a hopper and out of the system. It reduces

particle degradation and it is useful for low density powders.

Wet scrubber:

Figure-07: Wet Scrubber

Dust collectors that use liquid are commonly known as wet scrubbers. In these systems, the

scrubbing liquid (usually water) comes into contact with a gas stream containing dust

particles. The greater the contact of the gas and liquid streams, the higher the dust removal

efficiency. It ensures high efficiency and low maintenance cost. Higher level of

instrumentation is usually required for density and level control.

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Electrostatic precipitators (ESP)

An ESP is an emission control unit largely used in various industries e.g. power/electric, cement,

chemical, metal, paper. in many industrial plants, particular matter created in the industrial

process is carried as dust in the hot exhaust gases. These gases pass through an electrostatic

precipitator which removes most of the dust. Cleaned gas then gets out of the precipitator and

passes to the atmosphere. Precipitators usually accumulate 99.9% or more of the dust from the

gas stream.

Precipitators function electrostatically by charging the dust particles of the gas stream. The

charged particles are then attracted to and deposited on plates or other collection devices. When

enough dust has been collected, the collectors are shaken to remove the dust. The dust is then

removed by a conveyor system for discarding or reusing.

References Books

1.Foust et al: Principles of Unit Operations, 2nd ed, John Wiley & sons, New York (1994)

2.Masters, K.: Spray Drying Handbook, 4th ed., George Godwin, London, (1985)

3. McCabe, W.L., Smith, J.C. Harriot, P.: Unit Operation of Chemical Engineering, 6th ed.,

Mcgraw-Hill, Singapure, (2001)

Websites

https://www.academia.edu/3192734/

Spray_Drying_as_an_Appropriate_Technology_for_the_Food_and_Pharmaceutical_Indu

stries

http://www.eurotherm.com/industries/life-sciences/applications/spray-drying/

https://www.neundorfer.com/knowledge_base/electrostatic_precipitators.aspx

https://www.corrosionpedia.com/definition/120/atomization

http://www.fortitechpremixes.com/formulation/what-is-spray-drying/

http://www.spraydrysys.com/spray-dryers/spray-dryers.htm

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Nomenclature

Symbol Significance Unit

T Temperature ⁰C

W Weight g

t Time second

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Marking Scheme: Normal ReportName: Mahe RukhStudent number: 1202036

Section and marks allocated Marks

Summary (1)

Introduction (1)

Experimental Work (1.5)

Observed Data (1)

Calculated Data (1)

Sample Calculation (1)

Graphs (1)

Results and Discussion (1)

References and Nomenclature (0.5)

Writing Quality and Style (1)

Total (10)

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