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CHE 4115 Chemical Processes Laboratory 2 Experiment 1

Batch Distillation

BACKGROUND

Distillation is one of the most commonly used unit operations in chemical

engineering. In general, a distillation operation may be designed as either a batch or a

continuous process and either with or without reflux. In this experiment, batch

distillation with total reflux will be studied.

OBJECTIVE

To evaluate the efficiency of a plate column for batch distillation.

PROCEDURE

1. Measure volume of each component (500 mL of water, 1000 mL of acetic acid).

Calculate mole fractions for the initial mixture. [Note: the specific gravity of

acetic acid = 1.049]

2. Mix together and charge into pot. Caution: get an instructor to help you with the

next step. Slip glass joints together until they align and seal. Close distillate

stopcock.

3. Turn on condenser cooling water to a moderate rate.

4. Turn on low heat. Gradually increase until slow boil occurs.

5. Adjust heat upward gradually until water vapor can be seen rising up into the

condenser. If too much heat is applied, the column will flood with liquid and

make a rattling noise. Turn off heat immediately until noise stops, then gradually

increase heat until there is vapor in the condenser but not flooding occurs.

CAUTION: An excessive amount of heat could cause the condenser to receive

more vapor than it can condense. If this happens, vapor may be released into the

room. Shut off the heat immediately and increase rate of condenser cooling

water.

6. When steady state occurs, take samples (approx. 50 mL each) of liquid and vapor

in stillpot, and of distillate product (close stopcock again). Measure both the

volume and the weight of each sample. Record temperatures in the stillpot and

the top of the distillation column. Measure the input and output temperatures and

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the flow rate of the cooling water. When taking a vapor sample, submerge the

receiving flask in an ice bath to ensure that the entire sample is condensed.

[Note: if the technique used to analyze sample composition is temperature

dependent (see Appendix), allow samples to cool to approximately room

temperature before analysis.]

7. Repeat step 6 a few minutes later: Collect all samples and apply all

measurements, as above.

ANALYSIS

1. Using the equilibrium data provided below, construct the y-x equilibrium curve

and the T-x-y plot of the system.

2. The given y-x equilibrium values were calculated using a constant value of

relative volatility in

A

A

Ax

xy

)1(1 −+=

α

α (11.8)

Plot 1/y versus 1/x to calculate that value from the slope or intercept of the plot.

3. Using the McCabe-Thiele graphical method, determine the theoretical number of

stages and the overall column efficiency, given that the column is equipped with

27 plates, and operated under nearly total reflux.

4. Using Equation (11.51) in the text, calculate another value of the number of

theoretical stages at total reflux. Compare the values and discuss any differences.

av

sA

B

dB

A

x

x

x

x

nαlog

log

1

=+ (11.51)

5. Calculate the water mole fraction (xs1) and total moles (S1) in the initial stillpot

mixture (from the given volumes of water and acetic acid used to prepare it).

6. Calculate the ratio of final to initial moles in the stillpot, S2/S1

a. from the measured values of xd av

, xs1, xs2

b. from the measured xs1, xs2 and assuming xd=k xs (since xs varies very little)

21

2211

SS

SxSxx SSav

d−

−= ∫

−=

1

22

1lnS

S

x

x sd

s

xx

dx

S

S

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Both methods should give the same result. Use it to derive S2, compare the result

to S2=S1-Db (Db: total moles of collected distillate), and discuss.

7. From the measured flow rate and temperature change of the cooling water,

calculate the heat removal in the condenser. Write a heat balance on the overall

system (column, stillpot, heater, and condenser) and use it to calculate the net heat

supplied to the system by the heater (i.e., heat supplied by the heater minus any

heat losses to the environment).

EQUILIBRIUM DATA

The following data has been calculated for the equilibrium mole fractions of water

in the liquid (xA) and vapor (yA) phases for the water (A)/acetic acid (B) binary system:

T (ºC) xA yA

118.3 0.000 0.000

110.6 0.188 0.306

107.8 0.308 0.447

105.2 0.450 0.597

104.3 0.520 0.658

103.5 0.582 0.711

102.8 0.675 0.780

102.1 0.726 0.824

101.5 0.795 0.867

100.8 0.879 0.919

100.5 0.913 0.941

100.2 0.958 0.971

100.0 1.000 1.000

REFERENCE

Coulson, J.M., and Richardson, J.F., Chemical Engineering, Vol. 2, 4th

Ed., Pergamon

(1991).

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APPARATUS

The glass distillation column is a versatile apparatus, which can be used in several

modes to determine useful data. The vacuum jacketed column has multiple plates with

disc bubble caps and liquid downcomers. Two still pots are available each with a

corresponding size electric mantle heater. On top, a condenser and flow splitter complete

the apparatus. All connections (joints) are ground glass and usually need no grease for

sealing, but must be handled in a careful way so that only vertical and no bending forces

are imposed on the joints. Since most liquids of interest are flammable and/or toxic,

safety precautions must be observed to prevent skin contact or emission to the room.

Figure 4.1. Distillation Unit and Gas Chromatograph

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Figure 4.2. Distillation Unit

1 - Condenser 5 - Heating mantel

2 - Distillate 6 - Still pot – vapor sample

3 - Distillation column 7 - Still pot –liquid sample

4 - Still pot 8 - Temperature readout

1

6

2

3

4

5

7

8

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Figure 4.3. Distillation Unit – Closer View

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APPENDICES

A. SAFETY PRECAUTIONS

Heater: Avoid spilling liquid on the heater. Always heat liquids slowly to prevent rapid

boiling and overflow.

Stillpot: The pot is under pressure. A sudden opening may permit hot liquid to flow out

rapidly, potentially causing a burn or a fire. Handle the connections with care.

Do not run the flask dry, keep at least 200 mL of liquid at all times. Do not force

glass materials. (See the instructor.)

Column: Any blow or bending motion will break the column and your experiment will be

over. Please use extreme care in handling the column. Rotate ground glass joints

slowly to free them. Lift slightly while rotating. Do not force them.

Condenser: This is delicate glass. When adjusting the condenser or any glassware, be

careful and concentrate fully on what you are doing.

Cooling Water: Be absolutely sure this is on at all times during the experiment.

Thermocouples: Be sure that they are set firmly in their connectors.

Sampling: (a) liquid from stillpot- open the stopcock slowly and have a cold flask in

position ready to catch liquid. Cover sample immediately after drawing it. (b)

Vapor from stillpot: this is under pressure, so open valve slowly and catch sample

with flask in ice bath. Either of these samples may be hot enough to burn your

skin. Cool all samples to room temperature before attempting analysis.

Liquid or Vapor Spills: Be sure the lab door is open. Wipe up the spill with sponge and

flush down a sink. Wash hands with lots of water. Avoid excessive inhalation of

vapors.

B. pH METER

A pH meter may be used to measure the molarity of an acetic acid solution

assuming that an equilibrium constant may be defined as Keq = [H+][Ac

-]/[HAc]. You

may assume Keq = 1.75 x 10-3

mol/L.

Note: the accuracy of this measurement is limited, especially at high

concentrations (low pH). That should not affect the ability to perform the calculations

required for the experiment. It may, however, affect the values of calculated parameters

and thus be worth “discussing” in the final report!

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Gas Chromatograph Operating Instructions

These are the steps that you should follow to start the GC properly. If this is your first

time, you may want to read the next section, “Choosing the GC Settings.”

1. Turn the helium cylinder valve so that the gas is fed to the GC.

2. Adjust the helium flow rate to approximately the desired value using the bubble

flow meter.

3. Turn on the power supply to the GC.

4. Set the oven, detector and injector temperatures.

5. Set the detector current and attenuation.

6. When all the settings have reached their set point values, adjust the helium flow

rate again (when the GC oven temperature changes, so does the flow rate). Turn

on the integrator. Wait for about 5 minutes before injecting a sample to let the

system stabilize.

7. Check that the integrator baseline does not drift, and then inject the sample.

8. When you are ready to turn the GC off, set the oven, detector and injector

temperatures and the detector current at their minimum. Let the GC cool down

for at least 20 minutes; if you turn it off when the oven temperature is still high,

the packing will melt and the column will have to be replaced.

9. Turn off the power supply and stop the helium flow rate.

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Choosing the GC Settings

Helium flowrate

The standard flow rates are 30 cc/min for 1/8” columns and 60 cc/min for 1/4”

columns. You can change these flow rates according to your needs. For example, if two

key components are overlapped you may want to use a lower flow rate, but keep in mind

that the peaks are not going to be as sharp as before. Conversely, if you want to reduce

the run time, you can increase the flow rate, although the peaks may overlap after doing

so.

Oven Temperature

You should try to use the oven temperature that the manufacturer of the column

recommends. If this does not yield satisfactory results, you may change it taking in mind

the following:

• The maximum oven temperature that the GC can handle is 400 ºC.

• All column packings have a maximum permissible temperature; never use an

oven temperature higher than that.

• Increasing the temperature reduces the elution time and produces sharper peaks

(and vice versa).

• It is not recommended to operate the oven at a temperature more than 10 ºC below

the boiling point of any of the components of the sample because the elution time

will become too long.

Injector and Detector Temperature

The injector and detector temperatures are usually chosen to be 5 to 15 ºC and 20

to 40 ºC above the oven temperature respectively.

Detector Current

Changing the detector current changes the sensibility of the apparatus. The

maximum current that can be used depends on the temperature and the carrier gas used.

You should check the GC manual to find the maximum allowed current for your

temperature and carrier gas.

When more sensitivity is needed, it is better to reduce the attenuation than to

increase the detector current; this will prolong the detector’s life.

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Attenuation

The attenuation has the same effect as the detector current. Using the lowest

attenuation without getting “noise” in the chromatogram will enable you to use the

smallest detector current and make it last longer.

Gas Chromatograph Parameters

Column Temperature → Tc = 170 °C

Injector Temperature → Ti = 180 °C

Detector Temperature → Td = 185 °C

Detector Current Setting → 85 mA

Attenuation → 64

Sample size → 2 µm

Volume and Mol Percentage versus GC Area Percentage for Acetic Acid/Water

Volume Percentage = 2

AA

3

AA X107666.3X3664.10831.1 ×−+

Mol Percentage = 2

AA

3

AA X103534.7X24127.05197.1 −×++