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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.
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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
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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.
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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
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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
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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
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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
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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
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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
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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
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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
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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
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standpipe. Turn pump off.
A B
C
D
Red valve on top of the distillate tank
Sight glass
Feed sampling port
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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
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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
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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
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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.
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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.