* Chemical & Process Engineering, University of Canterbury 1

SPRAY DRIER

ENCH 395 Chemical Engineering Laboratories*

Before you start

Please read this lab sheet

Complete the pre-lab preparation

Bring safety glasses, lab coat, cell phone and wear warm clothes.

OBJECTIVES

The aim of this experiment is to familiarise students with the spray-drying process

through analysis of:

the energy and mass balances

the thermal efficiency,

and to teach students the basic practical skills required to operate modern commercial

spray drying equipment.

INTRODUCTION

Spray drying is widely used in the food industry; examples are skim milk, fruit juices,

coffee, and vegetable extracts. The same process is frequently used in the chemical

industry; examples are washing powders (detergents, enzymatic powders) and dried

salts. For spray drying theory and practice refer to Masters (1972) and Masters (1991).

For operation of the Niro spray drier see Redstone (1999).

APPARATUS

The spray-drier ‘Mobile Minor™ 2000’ is located in the Particle Technology

Laboratory. It has been designed and constructed as a valuable research tool to handle

small quantities of product. The process involves the atomisation of the feed solution

into a spray of very small droplets using a rotary atomiser (30,000 rpm) powered by a

compressed air supply. These droplets are exposed to a flow of hot air. As these

droplets have a very large surface area, water evaporation takes place almost

instantaneously within the short residence time of the drier. The droplets are

transformed into dry powder particles, and then separated from the air using a

cyclone. The dried powder particles are collected in a glass sample collection jar

attached to the cyclone. The exhaust air is drawn out of the top of the cyclone,

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through the exhaust fan and ducting system, before being discharged to the

atmosphere outside the building.

System Interlocks

Heater

If the Chamber Outlet Process Gas Temperature exceeds 120 °C, the heater will automatically

turn off and an alarm light, ‘Chamber Outlet Temperature High’ will light red. The

temperature must drop below 120 °C and the alarm must be reset using the Alarm Reset

button on the Spray Drier Control Panel before the Heater can be restarted. The Heater cannot

be started without the Exhaust Fan running. The heater is interlocked with a pressure

detection system. If a leakage should occur in the system that causes a reduction in the air

drawn into the system through the filtering unit into the heater, the heater will shut off.

Atomiser

The only safety interlock for the atomiser is with the Main Switch located on the Spray Drier

Control Panel. The compressed air hose MUST be detached from the atomiser before the

atomising unit is removed from the chamber roof, (see shut-down procedure) and certainly

before the chamber roof is opened.

Feed Pump

The feed pump can be started once the Main Switch is on. Care must be taken to start the

drying process in the specific order indicated in the Start Up Procedure section of the

operating procedure, or flooding of the drying chamber could occur. This could lead to wall

deposits or blockages in the pipe-work.

PROCEDURE

Before You Start Up

1. Check that the air inlet filter is clean.

2. Check that the atomiser wheel rotates freely and the silencer is mounted on the rotary

atomiser.

3. Check that the atomiser is properly seated on the four bolts on the chamber roof and is

not screwed down or restricted in any way for easy removal.

4. Check the route/support of the feed and compressed air hoses from the supply to the

atomiser.

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5. Ensure that the feed system is set up to run water on start-up.

6. Weigh a clean and dry glass sample collection jar and screw under the cyclone. Ensure

the butterfly valve into the sample collection jar is open.

7. Ensure the compressed air is turned on, and the supply is pressurised to approximately 6

bar.

8. Do not open any doors or remove any covers while the equipment is in operation.

Start Up Procedure

1. Switch on the power at the isolation switch located on the wall to the left of the Control

Panel.

2. Switch power on by turning the Main Switch on the Pressurised Control Panel (PCP).

3. Check that the Chamber Roof is fully shut by pressing the Release Chamber Roof (V2)

and Chamber Roof Down () buttons at the same time.

4. Check any valves on the spray drier exhaust path to the fan are open.

5. Press the Exhaust Fan button on the control panel, then start the variable speed drive

(VSD) and adjust the frequency to 60 Hz (full speed).

6. Turn on the chamber observation light.

7. Set the inlet temperature controller (Chamber Inlet Process Gas Temperature E1) to the

desired temperature (200 °C) using and arrows.

8. Turn on the Heater. If any leaks occur in the system the heater will immediately cease

operation. The outlet temperature controller turns off the heater if the outlet temperature

exceeds 120 °C.

9. Start the rotary atomiser by turning the knob anticlockwise on the pneumatic control

panel for the spray drier slowly (approximately 0.5 bar at a time) to full speed at 6 bar.

The air pressure indicated on the pressure gauge (E73 Atomiser Air Supply Pressure),

located on the Spray Drier Pneumatic Panel, must be kept constant in order to obtain a

homogeneous atomisation. Decreasing speed results in increasing particle size and can

cause wet chamber deposits. If a deep buzzing sound is heard the atomiser must be

stopped immediately and the bearings must be checked.

10. Switch on the feed pump (Feed Pump P1) from the Spray Drier Control Panel. Check that

the power switch located on the back of the pump is also switched on. Ensure that the

pump is set up to operate with rotation in a clockwise (CW) direction. Insert the feed

supply tube into the pumping mechanism on the front face of the pump. Close the front

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panel. Set the pump to 5 rpm and start the pump until the water level reaches the top of

spray drier then stop the pump and wait until the outlet temperature reaches 85 °C.

11. When the outlet temperature reaches 85 °C push the pump start button again. This will

pump water to the atomiser. It is recommended that the pump speed be slowly ramped up

to give the required outlet temperature. For this experiment a water flow rate

corresponding to about 16 rpm is appropriate. An outlet temperature of not less than 85 °C

should be aimed for. This will prevent moisture condensing in the system.

12. Switch from water to the milk solution. When it reaches the drier, increase the pump

speed so that the total flowrate of water being fed to the drier remains the same (work out

the new speed required to keep the evaporative load constant before you do this step).

13. You can adjust the outlet temperature by changing the feed pump speed using the and

rpm buttons. Measure the diameter of the container and the change of height over time to

calculate the flow rate (kg h-1

) – you need to know the density of the milk solution.

14. Bang with the hammer, on the Hammering Point located on the cone of the drying

chamber, to dislodge any residues located on the inner chamber walls. This procedure

may need to be carried out regularly throughout the drying process, or just at the end of

the operation depending on the extent of residue build-up exhibited by the process. It is

important not to let residues build up, especially near the air inlet of the drying chamber as

smouldering could be initiated.

15. Examine the dried product frequently for specks of burnt, charred or discoloured product.

Stop the drier immediately if any burnt product is found, and clean the drier thoroughly.

16. If necessary exchange the sample collection jar with the damper closed. Weigh any

new jar used. Preheat the new glass sample collection jar to prevent condensation

occurring during the drying process.

EMERGENCY PROCEDURES

Emergency Shut Down other than in the case of fire or explosion (avoid using).

1. Turn off the Main Switch on the Spray Dryer Control Panel. Emergency shutdown may

result in damage to the heater elements and the atomiser.

2. Wait for approximately 3 minutes (if it is safe to do so) for the atomiser to stop and

visually check through the inspection window that the atomiser wheel has stopped

completely.

3. Wait till the plant has cooled down before opening the roof, as the surfaces may be hot.

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4. Do not touch the heater body as the surface temperature may increase drastically after

emergency shutdown.

5. Ensure the pressurised air supply to the atomiser is reduced to zero (the knob on the

Pneumatic Control Panel (PCP) is turned fully clockwise) before restarting the Spray

Dryer.

Emergency Shut Down in the Event of a FIRE, or smouldering, in the drier.

1. Switch off the Exhaust Fan to stop the flow of air into the drying chamber. This should

switch off the inlet air heater.

2. Switch the product feed to water, and increase the flow rate to douse the fire.

3. Ensure that the sample collection jar is isolated from the system, to prevent further

contamination of any collected product.

4. Continue dousing the chamber with water for a further 5 to 10 minutes after all signs of

smouldering have been eliminated.

5. Take care when re-opening the drying chamber after a fire or smouldering has occurred as

there is potential for re-ignition.

6. Ensure that the incident is thoroughly reported and the supervisor is contacted.

Emergency Shut Down in the Event of an EXPLOSION

Should an explosion occur in the drying chamber, evacuate the room immediately and turn off

the 175 Power Interlock switch by the door. Evacuate the building and follow the

departmental safety procedures regarding fires and explosions.

Emergency Shut Down when supplying other process equipment

1. Switch off the Exhaust Fan to stop the flow of air into the drying chamber. This should

switch off the inlet air heater.

2. Close any cyclone exhaust valve to prevent hot particles progressing into other processing

equipment where they may cause further damage.

Emergency Shut Down in the Event of a Fire Alarm, not in the powder lab building.

(To be written by the students on this page as part of the pre-lab.)

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MEASUREMENTS

Details of the data required are shown in Figure 1.

All calculations are based on air mass flow rate aM and solid mass flow rate sM .

aM mass flow rate of air kga s-1

sM mass flow rate of solids (dry basis) kgs s-1

iaT temperature of air at inlet °C

2aT temperature of air after evaporation (bottom of the chamber) °C

oaT temperature of air at outlet °C

sT temperature of solid particles °C

ah specific enthalpy for air kJ kga-1

sh specific enthalpy for solid particles kJ kgs-1

aH humidity of airflow kgwater kga-1

sH water content of ‘solid’ flow kgwater kgs-1

i.e. dry basis moisture.

Measure the ambient temperature and relative humidity (RH), near the filter, to obtain the air

humidity iaH at the inlet condition. The air humidity

iaH remains constant after heating the

air to iaT = 200 °C as there is no moisture added or removed during this heating process

(sensible heating).

At the exit of the exhaust pipe, measure the temperature oaT and RH to obtain the air humidity

oaH and measure the air speed to obtain the air volumetric flow rate from which you can

obtain the air mass flow rate aM .

Calculate the solid mass flow rate sM from the concentration and the solution feed rate

(obtained from measuring the change of the level with respect to the time and from the mass

of powder collected and time taken).

Calculate the humidity (water content) in the milk solution entering to the chamber isH .

iii ssss HhTM ,,,

ooo ssss HhTM ,,,

ooo aaaa HhTM ,,,

Figure 1. Drier data for calculation of heat and mass balances

2aT

iii aaaa HhTM ,,,

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ANALYSIS

1. Plot the process on a Mollier Chart.

2. Dry product balance

Compare the amount of dry milk powder collected in the jar at the end of the experiment with

the amount entered during the process. Discuss why/if there is any discrepancy.

3. Moisture balance

Compare the amount of water entering the process (taken from the milk solution) against the

increase of the humidity of the hot air stream. Discuss why/if there is any discrepancy.

Moisture entering the in feed iss HM

Moisture entering in the hot air stream iaa HM

Moisture leaving the drier in the product oss HM

Moisture leaving in the exhaust air oaa HM

Assume no product accumulation in the chamber: Input = output

ooii aassaass HMHMHMHM (1)

Assume 0osH (completely dry product)

ioi aaass HHMHM (2)

iss HM water evaporated (taken out) from the milk solution

io aaa HHM water gained (added to) the air stream

4. Energy balance

Compare the sum of the energy input against the sum of the energy output of the system.

Discuss why/if you there is any discrepancy.

Energy of air entering the drier iaahM

Energy of feed entering the drier (solids basis) isshM

Energy of exhaust air leaving the drier oaahM

Energy of product leaving the drier osshM

The specific enthalpy of the feed ish as it enters the atomiser is the sum of the enthalpies of

the dry solid and the water contents:

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iiii swssss TcHTch (3)

where: sc = specific heat of dry solid milk powder [kJ (kg K)-1

]

wc = specific heat of water [kJ (kg K)-1

]

isT = temperature of milk solution at inlet [°C]

The specific enthalpy of the solid product is oo sss Tch

The specific enthalpy of the air iah as it enters the drier and

oah as it exits to the atmosphere

can be determined from the Mollier Chart.

5 Efficiency calculations

Spray drier design is directed towards achieving desired-product properties at the highest

possible efficiency. Calculate the highest possible thermal and evaporative efficiencies

obtained from the experiment. Are these acceptable? If not, discuss why.

5.1 Thermal efficiency

The thermal efficiency of a spray drier depends upon the operating temperatures. Overall

thermal efficiency overall is defined as the fraction of total heat supplied to the drier used in

the evaporation process. It can be approximated to the relation:

01

21overall

TT

TTad

(4)

where T1 = air temperature entering into the drier (heated).

T2ad = air temperature after evaporation (adiabatic).

T0 = ambient temperature

5.2 Evaporative Efficiency

Is defined as the ratio of the actual evaporative capacity to the capacity obtained in the

ideal case of exhausting air at saturation:

saturation

ad

TT

TT

1

21eevaporativ (5)

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Where Tsaturation = the adiabatic saturation temperature corresponding to the inlet temperature

T1.

SHUT-DOWN AND CLEANING PROCEDURE

1. Change the atomiser feed from product to water. When the water reaches the drier, reduce

the pump speed so that the total quantity of water being fed to the drier remains the same

(ie. keep the evaporative load constant).

2. Switch off the heater then continually adjust the pump speed to retain a constant outlet

temperature. A decreasing temperature will result in absorption of excess water in the

powder still present in the chamber.

3. Run the feed pump for 5 to 15 minutes until an inlet temperature of approximately 115 °C

is attained.

4. Switch off the Feed Pump on the Spray Drier Control Panel. (The chamber inlet

temperature T1 must be below the alarm temperature of 120 °C.)

5. After approximately 3 minutes switch off the atomiser slowly (approximately 2 bar/min as

per the Start-Up Procedure) using the knob located on the Pneumatic Panel. Then turn off

the compressor control.

6. Allow at least 5 minutes to cool the plant down (until the temperature is around 60 °C).

7. Lift the front panel of the feed pump; unwind the side screws (if necessary) to release the

piping. Remove the tube from the pump to prevent kinking.

8. Replace the sample collection jar (located at the bottom of the cyclone) so that any

product brushed from the chamber walls during cleaning is kept separate from the

naturally collected sample.

9. When both the inlet and outlet temperatures are below 60 °C remove the atomiser.

10. Open the chamber roof by pressing the Release Chamber Roof (V2) button and the

Chamber Roof Up/Down arrow (V2) at the same time.

11. Turn the roof vertically upwards and insert the locking pin.

12. The remaining powder in the chamber can be swept down using the supplied broom while

the fan is still running to collect the particles in the cyclone (take care not to damage the

polished surface).

13. Switch off the Exhaust Fan.

14. If deposits are present, turn off the Main Switch on the Spray Drier Control Panel then use

a slightly damp cloth to wipe the interior surface.

15. You need the Main Switch on the Spray Drier Control Panel turned on to close the roof.

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16. Turn off the Main Switch on the Spray Drier Control Panel and the Isolating Power

Supply Switch located on the wall to the left of the Spray Drier Control Panel.

17. Fill in the instrument logbook and record operational events, particularly noting any

abnormal event or reading. Any abnormality should be reported immediately.

PRE-LAB PREPARATION

1 Complete your yellow safety sheet.

2 Why should you avoid instigating the “Emergency Shut Down other than in the case of

fire or explosion”? Why is needed in detail.

Pre-Lab Problem

One kg of milk solution containing 50% solids (by mass) at 15 °C was pumped at a flow rate

of 4.8 kg h-1

into a spray drier. At the end of the experiment 300 g dried milk powder was

collected in the jar. The inlet process temperature, Ti, was 200 °C, and the chamber

temperature, T2, was 100 °C.

Ambient conditions: 18 °C and 50% RH,

Exhaust exit conditions: 70 °C and 20% RH.

The exhaust pipe diameter is 50 mm where the air exits with a speed of 10 m s-1

.

Plot the following drying process on a Mollier chart showing the different air states:

ambient, start of drying, end of drying and exhaust state.

Perform the mass balance on the moisture and powder.

Perform an energy balance on the system.

Calculate the overall thermal efficiency and the evaporative efficiency.

REFERENCES

Masters, K. (1972). Spray drying: an introduction to principles, operational practice and

applications.London: L. Hill.

Masters, K. (1991). Spray drying handbook (5th ed.). Harlow: Longman Scientific and

Technical.

Redstone, J.E. (1999). Commissioning and characterisation of the Niro Mobile MinorTM

‘2000’ spray dryer. Final year project report CAPE, University of Canterbury.

(Revised 22 July 2013 by G. Thorn)

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APPENDIX - Tips

1. Mix the milk powder at 25 to 30% solids concentration. (See email)

2. Commercially the important temperature to control, to minimise degradation of the

product, is the inside roof surface. This can be controlled by adjusting the inlet air