Industrial Drying Experiment

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Industrial Drying Experiment

Prepared by American Institute of Chemical Engineers, Mid-Michigan Local Section

Background:

An important process in the manufacture of dry goods, whether it is food you take home(sugars, powdered drink mixes, etc.), a medicine you take, or a product you may use (salts,detergents, concrete powder), is the removal of water. Sometimes water is not what is beingremoved as other solvents, including alcohols, oils, and ethers, are commonly used inindustrial applications. The general term describing the removal of water or another solventfrom a solid is known as drying.

Industrial drying is done at manufacturing facilities across the globe. While the techniquesfor doing this vary from problem to problem, the challenge for chemical and mechanicalengineers is to develop equipment for doing this on a large scale (millions to billions ofpounds per year) using the most cost effective method possible without sacrificing thequality of the product. Regardless of the products requirement, engineers rely uponfundamental scientific principles in all of their designs, and wherever possible, they definethe problem using mathematical descriptions of these tenets. While some methods are easierto practice, these fundamental principles determine the limits of what can be achieved for agiven design. One cannot violate these principles.

The two most important principles for drying are the principle ofconservation of massand the principle ofconservation of energy.

These rules can be stated in simple terms. Conservation of mass (weight) means that matteris conserved in physical processes. The material may change phase (solid to liquid, liquid togas, etc.) or may be converted into different chemical compounds as a result of a chemicalreaction, but the principle amount of material is held constant. The conservation of energyprinciple may be harder to understand. Energy exists in many forms (energy of motion,energy of potential, electrical energy, energy held within matter, heat, radiation), but despiteits many forms, energy cannot be created, it can only change form. For example, when one

boils water on an electric stove, heat travels from the electric coils through the pan and intothe water. In this process, energy is lost from one system and changes form to enter anothersystem, i.e. conversion of electrical energy into energy held within matter.

To dry a substance, two things are required.

1 Water must have a place to go other than the material with which it is mixed


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The amount of energy required to convert the water from a liquid into a gas is known as the

heat of vaporization. When a fluid is at its boiling point, total vaporization can occur.When the fluid temperature is below its boiling point, a small fraction of the fluid canvaporize. This is known as evaporation. However, there is a limit to how much water cango into the air when the temperature is below the boiling point. For example, have you everheard the weatherman talk about humidity, especially during the summer months? On somedays, it is easier for one to sweat than others. This is because the air can only hold so muchwater for a given temperature. When the air is saturated with water, it is said to be at itsdew point.

Relative humidityis a term used in weather forecasting. It measures how much water ispresent in the air relative to the amount at saturation. This differs from specif c humidity,which is the ratio of the mass of water vapor to the mass of dry air. A chart which iscommonly used to depict this information is referred to as the Psychrometric Chart (seeFigure 1).

i

1As one sees in Figure 1, the warmer the air is, the more water it can hold.

Psychrometric Chart

0.000

0.005

0.010

0.015

0.020

0.025

0.030

20 30 40 50 60 70 80 90 100 110

Temperature [=] F

MoistureContent[=]

lbwater/lbdryair

10%

20%

30%

40%

50%

60%70%80%90%Saturation

Figure 1: Simplified Psychrometric Chart


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process operation to do this, which in turn involves energy. An example of this is vacuumdrying.

For the classroom experiment, the type of industrial drier which is being used is known as afluidized drier (See Figure 2). With this equipment, air is drawn through the material fromthe bottom of the tube through use of a small vacuum cleaner. The flow of air can beregulated using the valve. As the air moves through the solids, the solids will lift upward andthen move about in circular patterns, a process known as fluidization. The level of solidswill expand and the solids will appear to behave strangely, as if they are liquid-like. This isdue to the air flow through the mixture creating bubbles of different sizes. The type of

motion which occurs depends upon the air flow rate, the type of particle, and the size of theparticle. However, the key to the drying process is to force the air to mix well with thesolids. Too much air flow will cause the solids to leave the system.

Water will be added to two materials, poms and sand. Students will measure out how muchwater is added. The solid material will be added to the drier. During the course of theexperiment, students will measure the change in temperature of the solids in the drier andrecord their observations about what is going on in the equipment at various points in time.

Experimental Procedure:

Assemble fluidized drier stations using reference drawing.

Experiment #1: Wetting and Drying of the Poms

1. Add small volume of water (1/2 teaspoon or so) to mixing bowl

2. Mix poms with water in bowl.3. Squeeze out excess water from poms.4. Add poms to fluidized drier.5. Insert temperature probe into base of drier.6. Start clock and record initial temperature.7. Start vacuum cleaner and adjust valve to regulate air flow. Flow should not be so high as

to cause poms to be sucked to top of drier. Movement should be fluidized.

Experiment #2: Wetting and Drying of Sand

1. Measure one cup of sand using measuring cup.2. Funnel sand into mixing bottle.3. Add 1/8 teaspoon of water to bottle.4. Shake sand with water so that it is well mixed.


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Figure 2: Fluidized Bed Drier for Industrial Application

Optional Demonstrations: Notes to Instructor


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Classroom Discussion Questions:

1) Why does the thermometer show a drop in temperature early on into the drying process?

2) Does the temperature of the air return to room temperature? If not, comment why youthink this occurs.

3) Was there a difference in the drying behavior of the poms versus sand? Explain.


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Prepared by American Institute of Chemical Engineers, Mid-Michigan Section 1

The Scientific Method


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Drying Experiment Temperature Data Sheet

Conservation of Mass

My problem is: What will happen to the water on the poms when they are fludized by air?

My Hypothesis is:

Conservation of Energy

My Problem is: What will happen to the air temperature in the bed during fluidization?

My Hypothesis is:

Time (min) Fluid Temperature (F) Time (min) Fluid Temperature (F)

10:00 05:00

09:50 04:5009:40 04:40

09:30 04:30

09:20 04:20

09:10 04:10

09:00 04:00

08:50 03:50

08:40 03:40

08:30 03:3008:20 03:20

08:10 03:10

08:00 03:00

07:50 02:50

07:40 02:40

07:30 02:30

07:20 02:20

07:10 02:10

07:00 02:00

06:50 01:50

06:40 01:40

06:30 01:30

06:20 01:20


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Discussion Questions for Industrial Drying Experiments

1. Does the thermometer show a change in temperature early in the drying process?Why do you think this is so?

2. Is the temperature at the start of the experiment the same at the end of the

experiment? Why do you think this is so?

3. Do you think there would be a difference in the behavior of the poms and the sand?

Explain.


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Psychrometric Chart Worksheet

1. If the outside temperature is 70F, and the weatherman says the relative humidity is

70%, what is the moisture content of the air (specific humidity)?

2. In your clothes dryer at home, I am flowing air past your hot clothes at 1 lb/min. The

relative humidity of the air above your clothes as you start your dryer is 90% at 91F.

When you remove the dry clothes at the end of the cycle, the relative humidity of the

air above your clothes is 10% at 70F. You run your dryer for 60 minutes. Howmuch water did you remove from your clothes?

3. It is a hot summer evening, the temperature is 80F. A hydrometer (an instrumentthat measures humidity) in your back yard shows that absolute humidity is 0.18 lb-

water per lb-dry air. What is the current relative humidity?

Bonus Question. As the sun sets, the temperature begins to drop. At what

temperature will you have an evening shower?


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Discussion Questions for Industrial Drying Experiments -

Answers

4. Does the thermometer show a change in temperature early in the drying process?

Why do you think this is so?

Energy is necessary to vaporize the water from the pom surface. This energy is takenfrom the flowing air. This results in a sudden decrease in temperature as the fluidizationstarts.

5. Is the temperature at the start of the experiment the same at the end of the

experiment? Why do you think this is so?

While the temperature drops suddenly, the temperature does not quite return to thestarting point. This is because water is still retain in the interior pore structure of the

pom, which is basically a sponge. This slow diffusion, but relatively constant process toremove the water from the pom interior results in a temperature off-set.

6. Do you think there would be a difference in the behavior of the poms and the sand?Explain.

The sand has not internal pore area. It is basically a marble, and not a sponge. There

should be very little if any temperature offset at the beginning and the end of the


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Psychrometric Chart Worksheet - Answers

1. If the outside temperature is 70F, and the weatherman says the relative humidity is

70%, what is the moisture content of the air (specific humidity)?

Answer: About 0.010 lb-water per lb-dry air

2. In your clothes dryer at home, I am flowing air past your hot clothes at 1 lb/min. Therelative humidity of the air above your clothes as you start your dryer is 90% at 91F.

When you remove the dry clothes at the end of the cycle, the relative humidity of the

air above your clothes is 10% at 70F. You run your dryer for 60 minutes. Howmuch water did you remove from your clothes?

Answer: (0.030 0.001 lb-water per lb-dry air) x 1 lb/min x 60 min = 1.74 lb H2O

3. It is a hot summer evening, the temperature is 80F. A hydrometer (an instrument

that measures humidity) in your back yard shows that absolute humidity is 0.018 lb-

water per lb-dry air. What is the current relative humidity?

Answer: About 80%.


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Psychrometric Chart

0.000

0.005

0.010

0.015

0.020

0.025

0.030

20 30 40 50 60 70 80 90 100 110

Temperature [=] F

MoistureContent[=]lbwater/lbd

ryair

10%

20%

30%

40%

50%

60%70%80%90%

Saturation

Documents

Industrial Drying Experiment