Lab Notes for Drying

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Lecture Notes and Lab Procedure for Drying Experiments

CHEN4860 Chemical Engineering Lab II

Prepared by: Dr. Timothy Placek

February 21, 2006

These materials are provided to highlight important concepts which will be employed in

the lab experiments dealing with drying and dryers. They are not intended to be completeor the soul source of information necessary to properly conduct and analyze the

laboratory experiment and data. Most of the material contained here is drawn from

Geankoplis Transport Processes and Separation Process Principles, 4/e.

Drying as a Process (Unit Operation)

Object: Removal of liquid (usually water) from solid material.

Batch drying: Wet material is inserted in drying equipment and removed after an

appropriate amount of time.

Continuous drying: Wet material is continuously introduced and dry material

withdrawn after a contacting period.

Methods:

1. Addition of heat: Heat is added to ambient air which then contacts the wetmaterial (the moist air is usually removed).

2. Vacuum drying: Evaporation is enhanced by lowering the pressure over the wetmaterial and heat may be added by direct contact with a metal tray holding thewet material or by radiation (IR).

3. Freeze drying: Low pressures and temperatures are employed to cause the waterto sublime from a solid state (ice).

Equipment: (see appropriate references)

1. Tray Dryer (as employed in our lab)


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2. Vacuum-Shelf Indirect Dryers (trays which operate below atmospheric)3. Continuous Tunnel Dryers (moving trays (trucks) or belts)

4.

Rotary Dryers (kilns)


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5. Drum Dryers (sludge drying, paper making)

6. Spray Dryers

7. Crop/Grain/Lumber Drying


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Chemical Engineering Principles: (review as necessary)

1. Phase Behavior of Water

2. Vapor Pressure of Water (steam tables, and equations)3. Humidity and Humidity Charts


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4. Humidity (kg WV/kg DA)

5. Saturation Humidity, Hs

6. Percentage Humidity, Hp

7. (Percentage) Relative Humidity, Hr

8. Dew Point Temperature

9. Humid Heat (Capacity), cs = kJ/kg DA . K, amount of energy to raise an amountof wet air 1 degree based on the number of kg DA

10.Humid Volume, vH , m3 occupied by wet gas/kg DA


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11.Total Enthalpy of air-water mixtures, Hy, kJ/kg DA

12.Adiabatic Saturation Temperature, Ts

13.Dry Bulb Temperature14.Wet Bulb Temperature

15.Use of Humidity Charts (wet bulb, dry bulb, adiabatic saturation lines, somecharts have enthalpy data as well.

Moisture Content of Materials Concepts


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Moisture Content

soliddrykg

watertotalkgXt =

=

Ws

WsW

Varies with time

Equilibrium Moisture

Content(see Fig 9.4-1) soliddrykg

eq)(atwatertotalkg*X =

=

Ws

WsW

Reflects the moisture

content held afterextended contact withair having humidity

H.

Temperature Effects

on Equilibrium

Moisture Content

Poorly understood (predictive models

inadequate, usually use empirical equations

(data).

Bound Water Hygroscopically bound water. Has a

physical/chemical association with the solid.

Extend Fig 9.4-1 to

100%

Unbound Water Moisture in excess of the bound water (held

primarily in voids)

Free Moisture Moisture in excess of X* This is the moisturethat can be removed

by drying.


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Procedure for Tray Drying Experiment

Equipment Description:

The drying apparatus located Wilmore 191-building consists of an electrically heated

tunnel dryer that is outfitted for on-line mass and temperature measurement. An insulatedtray is used to hold beds of glass beads that are saturated with water and placed on asupport that extends through the tunnel floor to an electronic balance. Thermocouples

protrude through one wall of the tray. These allow observation and analysis of the

temperature profile throughout the bed. Ambient (dry bulb) and wet bulb temperaturescan also be logged. Humidity and pressure data outside the dryer can be obtained from

the instruments on the wall behind the dryer and by use of the sling hygrometer (use both

and compare results). Thermocouple output is filtered and amplified before being fed to adata acquisition card on the adjacent PC. Mass data is acquired through an RS-232 serial

connection. Both mass and temperature data is collected and plotted in Excel by starting

the get data macro.

Experiment 1 Procedure:

1. Using the 100-200 micron beads, conduct a brief experiment to calculate bedporosity. There are several possible ways to do this.

2. Carefully saturate the bed with water and place in dryer. It is best to tare (zero)the balance before putting the tray of dry beads in the dryer. In order to get

accurate mass (weight) readings, it is important that the legs of the support stand

do not contact the edges of the holes through which they protrude.

3.

Close the dryer door, turn the dryer on, and start collecting data. You may leavethe experiment run all day in order to collect sufficient data to verify the entire

drying process. You should make arrangements with members of your group tostart the experiment in morning and continue taking data at various times during

the day. You may also decide to start in afternoon and run overnight.

4. Use both the sling hygrometer and the wall mounted panel meter to determine andrecord the humidity of the incoming (room) air. Use wet bulb data from inside the

dryer to check humidity of the warm air. Note: you can also use the sling

hygrometer as a check on the wet bulb temperature in the dryer by measuring at

the dryer outlet without any water in the tray (before making drying runs).

Experiment 2 Procedure:

1. Repeat the experiment using the 3 mm beads. For this run you will start theexperiment in a simulated 2nd falling rate regime. To do this, calculate how much

water you must add to the tray so that the 3 mm beads will be about 20%saturated. Add the water to the tray FIRST and add the beads on top. In this way


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the beads above the water will be dry and all drying will be due to evaporationfrom the air-water interface and diffusion through the beads above.

Minimum Report Requirements

1.

Groups should arrange to exchange data from the two experiments, that is, onegroup will run the 100-200 micron beads and the other group will run the 3 mmbeads.

2. Using mass and energy balances on the dryer tray, derive the equation you willuse to determine the convective heat transfer coefficient, h for the 100-200 micronbead run. Include in sample calculations section.

3. Determine the heat transfer coefficient (h) from gas to bed surface using yourequation and the constant rate data from the 100-200 micron bead data. Why

couldnt you use data from the 3 mm bead data?

4. Compare the calculated value of h to what you would expect from an empiricalequation from literature (i.e. Geankoplis or Faust.) Fully explain any possible

discrepancies between your experimental value of h and the predicted value,

assuming that the predicted value is correct. Separately, state any reasons youthink the predicted value might not be correct. Hint: think in terms of the

assumptions made, as well as propagation of error.

5. Compare the drying curve from the 100-200 micron bead run with what isdescribed in Geankoplis and Faust. In what ways are they similar? Different?Prepare the following plots:

a. mass (m) of tray in grams vs. time (t),b. rate of drying, dm/dt (grams/minute), on the y-axis vs. time in minutes on

the x-axis. (Use TableCurve 2-D or other software of your choice to

smooth and differentiate data.)

c. Plot bed temperature with time directly under this plot so that the timescale is the same and drying rate and temperature can be compared

directly. Discuss fully.

d. Percent saturation on y-axis, time in minutes on x-axis. Discuss fully.6. Using the steady state data from the 3 mm bead run, calculate the effective

diffusivity as follows:

Plot 2)1( S on the y-axis vs ( )txxH

CMA

e

A

TL

W

2

2

on the x-axis.

Use linear regression to find the slope, which should equal D eff(see derivation in

handout). Compare this result to what is given in the graph in the Whittaker

review article. Does your result match the experimental data given there? Why orwhy not?


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7. Include in your discussion section, a full description of what is happening in thedrying process by referencing both the drying curves, and the temperature data.

Additional References:

1. Bird, R.B., Stewart, WE, and Lightfoot, EN, Transport Phenomena, John Wiley &Sons, Inc., 1960

2. Kaviany, M., Principles of Heat Transfer in Porous Media, Springer-Verlag, NewYork. 1991

3. Ceaglske, NH and Hougen, OA, Drying granular solids, Indust. Eng. Chem. 29-7,805-812

4. Perrys Handbook


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Documents

Lab Notes for Drying