Manual for continuous distillation

1. Week 1:

Objectives:

Run the column at total reflux.

When steady state is reached, take the sample from the top and

bottom of the column in order to determine the overall efficiency.

Based on the overall efficiency, draw the quasi equilibrium curve.

Compare the tray numbers you get from the quasi equilibrium line

with the actual tray numbers.

Take the samples from consecutive trays to determine the local

efficiency.

Operation procedure:

Preparation for Start-up:

Key purposes:

Get familiar with the system; locations of the valves and sample

taps

Check the liquid level in the reboiler

Check the composition in the reboiler and feed tank

Turn the Siemens system on

Check if the valves are in the correct position

1. Using the sight-glass on the reboiler to assess its level of the liquid

(in the range of around 70%).

2. Prepare to mix the contents of the reboiler.

a. Close the red gauge valve on the bottoms line.(see graph

below)

b. Open the green valve on the same line just after it exits

the reboiler.

Follow the line down to the bottom of the reboiler. Open the orange ball

valve and make sure that the blue ball valve on the sample port is closed

3. Ask the TA/Lab Assistant to turn on the computer system hooked

to the East column.

4. Using the computer control system, turn on the reboiler-mixing

pump on the 1st floor, as shown below to mix the contents of the

reboiler. Let mix for at least twenty minutes.

5. While you are waiting, check that the manual by-pass valves are

closed.

a. Reflux—red valve parallel to the Reflux control valve

b. Distillate—red valve parallel to the Distillate control

c. Bottoms—blue valve parallel to the Bottoms control

d. Water—blue valve parallel to the Water control valve

Figure 1. East Column Reboiler Pumping Line

Figure 2. East column second floor control valves

Orange valve at

bottom of

reboiler – open

while mixing

Blue ball valve on

line exiting

reboiler

Red gauge valve

on Bottoms line

Start up:

1. Ensure that the reboiler has enough material and then turn on the air

for the 1st floor by manually opening the air valve fully. The exact

position of the valve is A of the graph shown below. Air is needed to

operate pneumatic control valves.

2. Turn the water on for the 1st floor by manually opening the water

valve fully. The exact position of the valve is C in the picture below.

Water is needed for cooling purposes. Water valves by convention are

blue in color.

3. Kindly note the sequence of turning on the utilities, it is air first (no

cost) followed by water (somewhat expensive) followed by steam

(highly expensive). The sequence will be exactly reversed while

turning them off. The rationale is to cut-off the steam and save the

operating cost as much as possible.

4. Ask the Lab Assistant/TA to purge the steam lines using the steam

purge valve located on the 1st floor. While the lines are being purged,

the students should have fingers in their ears and should not be

directly facing the outlet of the purge line. The Lab Assistant/TA

should have ear plugs. The ear plugs should necessarily be discarded

after one use.

5. Repeat steps 1 and 2 on 2nd floor. i.e. turn on the air for the 2nd floor

by manually opening the air valve fully. The exact position of the

valve is B. Also, turn on the water for the 2nd floor by manually

opening the water valve fully. This valve is a large blue valve located

at the base of the water rotameter, which is D shown in the picture.

6. Then go to the Control Room. One of the computer systems is hooked

to the West Column. The system would have already been made on by

the Lab Assistant/TA. The computer screen will indicate values of

different process parameters like temperature, pressure and flow rates

as well as the positioning of different valves. Note that the values of

some of the parameters might not be correctly shown on the screen

due to some or the other reason and typically such values will be

grayed out. Set the “Steam/PID” on the screen. Usually 40% is a good

start for batch column. You will slowly see a rise in the temperature of

the liquid in the reboiler and the pressure. You should see the boiling

of the liquid in some time. Allow a stable level to be attained in the

sight glass. Set the “Reflux/PID” to a 100%. Now, it essentially

means that you are operating the column at total reflux.

In case, you see the column flooding meaning the sight glass getting

overfilled, turning on the “Reflux Pump” momentarily helps it to get

stable. When the “Reflux Pump” turns on, it will be indicated in green.

Reducing the % in the “Steam/PID” also helps to lower down the

level of the material in the sight glass.

7. After the level of the material in the stand pipe is at a fairly stable for

some time, check if the system has attained steady state or not. This

can be done by measuring the composition of one of the streams

(reflux is the easiest because it is located on the second floor) as a

function of time. When the composition changes by less than 5% with

time, the system can be considered in a steady state condition. The

reflux sample port is located on the second floor.

A B

A- Air supply on the first floor

B- Air supply on the second floor

C- Cooling water on the first floor

D- Cooling water on the second floor

Shutting Down

1. As mentioned before, the utilities will be turned off exactly in the

reverse order meaning steam first, then water and lastly air. Use the

computer system to close the Steam control valve. To close the steam

valve, click on the display box labeled “STEAM/PID” located above the

valve symbol on the reflux line. Click on the box to the right of “MAN.”

A small window will pop up. Change the opening of the valve to “0” and

D C

click “OK.”

2. Turn off the reboiler mixing pump (EBTP/PUMP):

a. To turn off the reboiler-mixing pump: click on “Bottoms/PUMP” or

the pump icon; if not yet in manual mode, click on “Manual”; click

“Stop” and close the window.

3. With the reflux control valve (Reflux/PID) fully open, use the reflux

pump (Reflux/PUMP) to drain the standpipe. Turn pump off.

a. To open the reflux valve: Click on the display box labeled

“Reflux/PID” located above the valve symbol on the reflux line. Click

on the box to the right of “MAN.” A small window will pop up.

Change the opening of the valve to “100” and click “OK.”

4. Open the red reflux valve (also serves as a manual bypass valve) (see

Figure 1).

5. Close manual valves associated with the distillate tank (Figure 3).

6. Turn off the water and air valves by manually turning them fully in the

clockwise direction on both floors.

Figure 3. Tanks on the first floor.

Calculation:

Overall efficiency can be calculated by comparing the actual number of

plates in the column with the number obtained from McCabe-Thiele

diagram when the column operates at total reflux. Local or point

efficiencies can be calculated by measuring the compositions of the liquid

streams corresponding to two successive plates. The local efficiency is

typically close to the overall efficiency.

Eoverall =Ntheoretical

Nactual× 100%

The theoretical tray numbers can be obtained as follows:

Generate the equilibrium curve with relative volatility;

Draw horizontal and vertical lines between the equilibrium line and

y=x from the top product composition to bottom product composition;

Tray numbers can be counted in the graph.

An example drawing of the procedure is shown in Fig 1.

From this graph,

Ntheoretical = 7 − 1 = 6

Assuming the actual tray number is 12,

Eoverall =Ntheoretical

Nactual× 100% =

6

12 − 1= 54.5%

0.0 0.2 0.4 0.6 0.8 1.00.0

0.2

0.4

0.6

0.8

1.0

7

6

4

3

2

xD

yA

xA

xB

equilibrium line1

Fig 1. An illustration of the way to get theoretical tray numbers

Given the analytical equation of equilibrium line, y = f(x), and the

overall efficiency of the column, Eoverall , we can get the analytical

expression of quasi equilibrium line as blow:

y = x + Eoverall(f(x) − x)

Compare the tray numbers getting between the quasi equilibrium line and

the y=x to the actual numbers to verify the validation of the

McCabe-Thiele diagram. The way to get quasi equilibrium line is shown

in Fig 2. In Fig 2, “b” represents the driving force at a certain tray if at

ideal case, “a” is the driving force in reality in the column we run.

0.0 0.2 0.4 0.6 0.8 1.00.0

0.2

0.4

0.6

0.8

1.0

equilibrium line

quasi equilibrium line

b

yA

xA

a

a/b=Eoverall

Fig 2. An illustration of the way to get quasi equilibrium line

0.0 0.2 0.4 0.6 0.8 1.00.0

0.2

0.4

0.6

0.8

1.0

xB

xD

equilibrium line

quasi equilibrium line

yA

xA

In Fig 3, the tray numbers getting from the quasi equilibrium line is 10

and the actual tray numbers is 11.

Error =11 − 10

11× 100% = 9.1%

By taking the sample from successive trays, the local efficiency can be

worked out as follows:

Elocal =yn − yn+1

yn∗ − yn+1

=xn−1 − xn

yn∗ − xn

Fig 4 is a graphical description.

Fig 3. An illustration of the way to count the number of trays

0 10

1

yn

yn+1

y*

n

xn-1

xn

equilibrium line

y=x

yA

xA

2. Week 2:

Run the column at different reflux ratios.

Study the effect of reflux ratio on the concentrations of top and

bottom product.

Operation procedure:

Fig 4. An illustration of the way to get local efficiency

From week 2 onwards, it is suggested to check if the reboiler has enough

material from the control panel before starting the experiment. If the

liquid level in the reboiler is less than 15 in, the liquid in the distillate

tank should be pumped back to the. To pump the distillate back, you need

to follow the procedure below. First open two valves located below the

distillate tank, which are shown in graph E. Then open the bypass valve

which is indicated on graph C. Finally open the pump below the reboiler

on graph F. When you pump the liquid back to the reboiler, make sure

someone checks the level in the distillate tank, as you may run the risk of

burning the pump if the distillate tank is pump to empty.

The general start up procedure followed is the same as the first week

during all the three weeks, so repeat procedure 1~7 at first for week 2 and

week 3. Open the valve beside the rotameter on B to change the distillate

flow rate in order to change the reflux ratio. The distillate will go through

the rotameter on the 2nd

floor down to the tank located on the 1st floor,

which is shown in graph E.

Notes: All composition measurements are done with a Perkin Elmer Gas

Chromatography instrument in the Lab. Get help from the lab assistant on

using it.

Shutting Down

Turn off the valve for distillate first. Then shut down the steam on the

control panel, which means set the PID of steam to 0%. Wait for 1~2min,

as after shutting off the stream, the vapor may still go to the top and that

means the water is still needed to condense the top product. Then close

the air and water valves on the first and second floor, just like the shutting

down procedure for the first week.

A B

Reflux sampling port

Reflux

rotameter

Distillate rotameter

C

D

Sight glass

Bottom product sampling port

Bypass valve

A- Reflux rotameter

B- Distillate rotameter

C- East column 1st floor reboiler

D- East column second floor control valves.

E

F

1

2

E- Distillate tank for east column

F- Control panel overview of the east column

3. Week 3:

Run the column at different reflux ratios.

If there is still time left, run the column with a reflux ratio smaller

than the minimum reflux ratio.

GC (gas chromatography)

For our GC system, the absolute value of the peak area will vary

a lot even if the same sample is injected for several times, which

can be observed with the error bar in the graph. For this graph,

four standard samples are tested and each concentration is

calibrated for three times. The absolute peak area and peak area

percentage is taken out to draw the curve.

In the graph, the black points are absolute value of the peak

area of ethanol, while the blue points are the peak area of

ethanol over the total area, which is a relative number. The error

bar for the absolute area is huge, especially for the mole fraction

around 0.6, but for the relative area, the error bar can be barely

seen, indicating a more reliable measurement for our GC

system.

0.00 0.25 0.50 0.75 1.00 1.250.0

0.5

1.0

1.5

2.0

absolute area

relative area

Mole fraction

Peak a

rea (

V*s

)Equation y = a + b*x

Weight Instrumental

Residual Sum of Squares

18.80314

Adj. R-Square 0.98832

Value Standard Error

MeanIntercept -316738.0310 42671.23728

Slope 2.11196E6 43147.6728

0

50

100

150 Peak a

rea p

erc

enta

ge(%

)

Equation y = a + b*x

Weight Instrumental

Residual Sum of Squares

6921.93153

Adj. R-Square 0.98842

Value Standard Error

MeanIntercept -21.25416 0.09766

Slope 116.4014 0.12339

For this method, there is not much calculation going on. The

students need to calibrate the curve with relative peak area as

the graph above. As the peak area percentage of a same sample

will not vary much for different runs, the students just need to

run each concentration once. After getting the calibration curve,

they can use this curve to get their mole fraction directly from

the relative peak area for the three-week experiment.