Distillation Procedure (draft- final version posted by
3/1/08)
Pre-Lab:
1. Prepare for the Pre-Lab session by: reading Seader & Henley (chapter 7, sections
2 & 3) and these procedures, studying the detail photos and Excel sheets on the
Lab Manuals web page, and previewing the P&I diagram for the larger distillation
column.
2. In the Pre-Lab session you will:
i) learn how to use the refractometer
ii) learn how to use the data acquisition software
Lab Day:
1. Hand in your completed exercise for the large scale column.
2. Be prepared to answer questions about the distillation column and procedures for
the experiment.
3. Turn on the refractometer and the circulating bath. The temperature on the bath
should be set at 25 deg C.
4. Check with the TA or instructor to make sure the still is fully charged with the dry
methanol/ethanol mixture.
5. Start the data acquisition. Turn on the thermocouple reader for measuring delta T
across the condenser.
6. Make sure that the pinch valve on the fill/drain line for the reboiler is fully closed.
7. Make sure the vent valve at the top of the column is open
8. Make sure the product takeoff valve (stopcock between the condenser and the
product cooler) is closed.
9. Make sure that sample syringes are firmly in place on the sample ports.
10. Make sure that the product receiver/decanter is filled with 20 ml of dry methanol.
11. Make sure that the tubing running from the receiver/decanter to the reboiler is
firmly connected to the barbed fittings on each end and is fully clamped in the
peristaltic pump.
12. Make sure the cooling water is flowing through the condenser with the inlet
pressure at 20 psig and the rotameter set at approximately 80% max flow.
13. Plug the boiler heater into the control rheostat and set to “50”. Turn on the power.
14. Turn the auxiliary heater rheostat and set to “50”. The auxiliary heater supplies
14 watts of power to the heating ribbon wrapped around the column in order to
minimize heat losses.
15. With the insulation cover off of the column, allow the system to come to an
approximate steady state as determined by the temperature profile.
16. As the column comes to steady state, up the rheostat setting to “51” and allow to
come to steady state.
17. Carefully keep increasing the power in this way until flooding is observed.
Flooding will be seen as a rapid accumulation of liquid on some of the trays as
well as an accompanying rise in tray temperatures.
18. When flooding occurs, turn off the power to the boiler allow the trays to drain.
19. Stop the data acquisition and save the file.
20. Restart the data acquisition and the power to the boiler. Set the boiler power to
“47”, or 190 Watts.
21. Cover the column with the insulated cover.
22. Allow the column to come to steady state as indicated by both the column
temperature profile and the delta T reading across the condenser- the later will
take a little longer to stabilize. Note: you must check the flow rate frequently to
make sure it does not vary.
23. Note the approximate height of the liquid in the downcomers.
24. When the column appears to be at steady state, flush the sample syringes by
pulling about 1/10 ml of sample into the syringe and immediately pumping it back
into the column. Start at the top of the column and repeat rapidly, moving down
the column to the bottom syringe.
25. Wait 2-3 minutes for to make sure the column has returned to steady state.
26. Record column tray temperatures and the column delta T, cooling water flow rate
and bottoms temperature.
27. Repeat the sampling procedure, this time leaving 0.10-0.15 ml of sample in each
syringe. Note: you must do this quickly, moving from top to bottom in order to
get representative samples from each tray.
28. Shut down power to the boiler.
29. Stop the data acquisition and save the file.
30. When the column calms down, carefully remove the sample syringes from the
column and inject each sample into the appropriately numbered tube in the sample
tray and close the cover.
31. Save these samples for analysis on the refractometer.
32. Replace the syringes on the sample ports, making sure they are firmly mounted.
33. Turn the boiler back on and restart the data acquisition system.
34. Turn on the reflux timer (for now leave the product takeoff valve closed). Use the
stopwatch to calculate the reflux ratio. It should be between R = 5 and R = 6.
35. When the column is back to approximately steady state, open the product takeoff
valve.
36. Open the pinch valve on the boiler inlet and start the pump by moving the switch
to the left. This will start the pump in the ‘reverse’, counterclockwise mode and
pump the contents from the decanter into the reboiler.
37. Adjust the pump speed such that the liquid level in the decanter stays constant at
20.0 ml. You will need to check frequently and adjust to keep the level constant.
The pump calibration curve may be consulted to determine the steady state
distillate (product) rate. Be sure to record this in your data record at the end of the
run.
38. Observe the temperature profile as you begin taking off product. The
temperatures will rise as the column moves to a new steady state.
39. Analyze the samples from the infinite reflux run as time allows.
40. When the column temperatures level off, repeat steps 26-32 for sampling and data
recording.
41. Shut down the column and analyze the samples for the partial reflux run.
42. Turn off the pump.
43. Make sure that the auxiliary heater is turned off and that the boiler rheostat is
turned off and unplugged.
44. Turn off the reflux timer and close the product takeoff valve.
45. Securely close the pinch valve on the boiler drain tube.
46. Close the valve at the bottom of the decanter.
47. Turn off the circulating bath and refractometer light.
48. Place a Kim wipe between the crystal surfaces in the refractometer sample
chamber and close.
49. Wash any glassware and clean up the area.
50. Make sure that any chemical are returned to the storage area.
Analysis:
1. Calculate the molar and volumetric flooding boilup rate. You may assume
approximate linear mixing properties for the alcohols. At what % of flooding
velocity did you operate the column? Is this roughly in line with the
guidelines for a lab column given on p215 of the Seader & Henley text
(assume the bubble cap column is very similar- in reality the bubble cap
column can be effectively run at a slightly wider range of flows than can the
sieve tray Oldershaw column)? Discuss.
2. Describe your visual observations of flooding and discuss what caused these
observed effects.
3. Discuss the approach to steady state for the infinite reflux and partial reflux
runs. Explain why the temperature profiles change as they do during column
startup, and also during the change from infinite reflux to partial reflux.
4. Estimate the total pressure drop in the column from your observations of
liquid height in the downcomers and approximate liquid density. Is this
pressure difference enough to cause a significant error in determining bottoms
composition from the bottoms temperature using the T-xy plot? (The T-xy
plot given is at atmospheric pressure). Discuss.
5. For both the runs, do an energy balance on the column using the calibration
curve for the boiler rheostat and the flow rate and delta T data for the
condenser. Clearly present this data. Does the auxiliary heater approximately
balance out any heat losses? Discuss.
6. Use the McCabe-Thiele method to determine the theoretical number of trays
needed to accomplish the separation (remember to include the reboiler and
condenser as ideal stages). Use printouts of the “VLE” spreadsheet graphs to
do your analysis and clearly present in your report. Use drawing tools
(square, straight edge, French curve) to make your plots. Make your drawings
precise and legible. Use the expanded scale plot to step off trays at higher
concentrations. Note that the computer-drawn non-equilibrium line is by
polynomial curve fit and may fluctuate some in the expanded plot. You might
need to hand draw to smooth it. The non-equilibrium line should be Tray
Efficiency (TE) % of the distance between the operating line and the
equilibrium curve.
7. Compare the result of the McCabe-Thiele analysis in #6 to the actual number
of trays used to accomplish the separation and calculate the average tray
efficiency (use trays only for these calculations- the boiler and condenser will
be always be equilibrium stages even if the trays are operating below 100%
efficiency). Is the calculated tray efficiency roughly in line with the plots in
S&H (Figs. 7.31, 7.32, 7.33) for expected tray efficiency? Discuss.
8. Redo the McCabe-Thiele analysis, this time using a non-equilibrium line
based on the average tray efficiency (as in Fig 7.25 of the S&H text). Clearly
present in your report or appendix.
9. Do a McCabe-Thiele analysis for the partial reflux run. Use a nonequilibrium line based on the tray efficiency from your infinite reflux run and
assume constant molar overflow. Clearly present in your report or appendix.
10. Compare the tray and product concentrations predicted by the McCabe-Thiele
plot in the non-equilibrium reflux plot from # 8 to your data. Come up with a
creative way to illustrate this data and allow easy comparison between data
and prediction- clearly present in your report or appendix.
11. FULLY DISCUSS the results of # 8 and # 9. How do the profiles match up
near the bottom of the column? At the top? How are the results affected if a
small amount of heat is lost at each tray? How would the operating line
change. What would be the effect of some subcooling of the reflux stream as
it returns to the column? Could anything else affect the operating line?
12. DISCUSS ANY OTHER OBSERVATIONS AND RESULTS OF
SIGNIFICANCE.