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 successive trays to determine the local

efficiency.

Operation procedure:

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 25% is a good

start. 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.

West column:

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

Calculation:

Eoverall =Ntheoretical

Nactual× 100%

The theoretical tray numbers can be obtained as follows:

Generate the equilibrium curve with relative volatility;

D C

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

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

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

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

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.

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+1yn

∗ − yn+1=

xn−1 − xnyn

∗ − 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:

Get the minimum reflux ratio;

Choose different reflux ratios to compare the relationship between the

reflux ratio and top/bottom product composition.

As the operation procedure for west column is a little complicated than

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

the east column. The procedure for week 2 is just explained separately.

Operation procedure of west column:

8. 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. However, from week 2 onwards, it is suggested to check

whether the feed tank located on the 3rd floor has enough material

before starting the experiment. Also, collecting the feed sample from

the feed tank and getting information on its composition upfront will

be useful in deciding the appropriate plate on which feed should be

introduced.

9. From week 2 onwards, feed has to be introduced in the column. For

this, it is important to measure the composition of the feed by drawing

off the sample from the feed tank on the 3rd floor and thereby identify

the appropriate feed plate. Once the feed plate is identified, turn on

the feed pump located on the 3rd floor and position the valves such

that feed is introduced. If feed is being introduced, the feed rotameter

will indicate a value corresponding to the flow rate of the feed.

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.

Drawing off Distillate Product

1. Check the levels in the two distillate tanks by checking the

sight-glass on the tanks. Usually they will be empty. In case, they

are full or nearly full, seek assistance from the Lab Assistant/TA.

2. Make sure the red manual valves at the bottom of both tanks are

closed (see A).

3. Select a tank for the distillate product.

4. Open the red manual valve at the top of the tank selected for

distillate.

5. Make sure the red manual valve at the top of the other tank is

closed (see A).

6. Monitor distillate tank level visually using sight-glass (see A).

7. Open the distillate control valve (Distillate/PID) to 100% and

slowly close the reflux control valve (Reflux/PID) until flow is

established in the distillate line, keeping an eye on the standpipe

level.

8. Control distillate flow rate using distillate (Distillate/PID) and

reflux (Reflux/PID) control valves.

Note: If both distillate tanks become full during operation, seek assistance

Drawing off Bottoms Product

1. Open and close the appropriate manual valves on the 3rd floor

2. Open the red bypass valve beside the reboiler (see F).

3. Open the manual valve above the bottoms rotameter.

Note: Ensure that the level of the material in the reboiler does not get

too low. The system will eventually turn itself off when the level in the

reboiler goes below 6 inches or so.

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.

2. Use the computer to turn feed pump off.

3. Use the computer to turn off the reboiler mixing pump

4. Close the manual valve above the bottoms rotameter.

5. Close all manual valves associated with feed (B), distillate (C), and

bottoms tanks (F).

6. Set the “Steam/PID” on the computer screen to 0%.

7. Wait for 1~2min after turning off the steam, then turn off the water

and air valves by manually turning them fully in the clockwise

direction on both floors.

8. With the “Reflux/PID” at 100%, use the reflux pump to drain the

standpipe. Turn pump off.

A B

C

D

Red valve on top of the distillate tank

Sight glass

Feed sampling port

A- distillate tank

B- Feed rotameter

C- Rotameter for distillate

D- The port to take reflux sample

E- Storage tank for bottom product and feed

F- Reboiler of the West column

G- Control panel overview of the West column

E F

G

Bottom product sampling port

Calculation:

Measure the feed composition. Mark the point the q line intercept with

the quasi equilibrium line. Connect this point and the top product

composition. From the slop of this line, the minimum reflux ratio can be

calculated.

slope =Rmin

Rmin + 1

Fig 5 shows the slope of the line to get minimum reflux ratio.

0 10

1slope

q line

xF

xB

xD

equilibrium line

quasi equilibrium line

yA

xA

Usually, the reflux ratio is chosen with a range of 1.2~1.5Rmin.

Fig 6 is the McCabe-Thiele diagram with a fixed reflux ratio.

Fig 5. An illustration of the way to get minimum reflux ratio

0 10

1

stripping line

rectifying line

q line

xF

xB

xD

equilibrium line

quasi equilibrium line

yA

xA

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.

Fig 6. An illustration of McCabe Thiele diagram with fixed reflux ratios

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.