Video: Distillation Column Operation and Effects of Reflux Ratio

The Effect of Reflux Ratio on Tray Distillation Efficiency

Overview

Source: Kerry M. Dooley and Michael G. Benton, Department of Chemical Engineering, Louisiana State University, Baton Rouge, LA

Tray and packed columns are both commonly used for distillation, absorption, and stripping separation operations.¹^,² The goal of this experiment is to distill a mixture of alcohols (methanol, isopropanol) and water in a sieve tray column and examine how closely simple theories of distillation based on equilibrium assumptions are followed. Sieve trays provide maximum interfacial area between the liquid and vapor. A P&ID schematic of the sieve tray (each tray contains holes in a support plate) distillation system can be found in Appendix A.

In this demonstration, the Tray Distillation Unit (TDU) is started in total reflux mode. After a steady reflux drum level is attained, a switch to finite reflux mode is made by adjusting the bottoms, distillate and reflux flow rate controllers as necessary to maintain steady levels in the reflux drum and the reboiler, and to maintain a target reflux ratio R_D = L/D. Once steady state is achieved (takes at least 90 min), liquid samples will be taken from the reflux drum, reboiler and on each tray and analyzed in a gas chromatograph. A typical protocol is to investigate the effects of reflux ratio over a wide range. From the sample analyses, the tray efficiencies can be determined for all three components on all six trays assuming constant molar overflow (McCabe-Thiele method). The results can also be simulated using an equilibrium process simulator, if available. These two methods can also be used to determine the overall tray efficiency. Additionally, data reconciliation of the mass balances can be performed to determine if gross measurement errors exist. Any Separations or Unit Operations textbook covers the fundamentals of distillation including basic concepts such as reflux ratio, Murphree efficiencies and the McCabe-Thiele method and diagram.²

Procedure

The TDU is operated from a computer interface of a distributed control system. The interface is entirely graphical. The column contains 6 trays, a total condenser, and a partial reboiler (Appendix A). The mixture is contained in the feed reservoir, and the feed nominally consists of methanol (~50 wt.%), isopropanol (~30 wt.%), and water (~20 wt.%).

1. Starting Total Reflux Mode

Turn on the cooling water. If the reboiler liquid level is too low, add feed to it; if too high, use the

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Results

The appropriate response factor (RF_i) for each component, which is the ratio of the signal intensity to the quantity of analyte injected and is provided in the software, is used to determine the wt% of each sample.

Equation 3 (3)

Reflux ratio (R_D = L/D) has a tremendous effect on both column

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Application and Summary

Tray distillation columns are often of the sieve type, with small holes for the vapor flow and larger downcomers to route the liquid from tray to tray by gravity. More volatile components mostly exit in the distillate, although some of the top vapor is condensed and returned to the column as liquid reflux. It was shown that determining the Murphree tray efficiencies can be important in pinpointing problems on specific trays in distillation columns, such as low mass transfer rates, weeping or flooding. While higher vapor/

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References

Encyclopedia of Chemical Engineering Equipment. Distillation Columns. http://encyclopedia.che.engin.umich.edu/Pages/SeparationsChemical/DistillationColumns/DistillationColumns.html. Accessed 10/01/16.
W.L. McCabe, J.C. Smith, and P. Harriott, Unit Operations of Chemical Engineering, 7th Ed., McGraw-Hill, New York, 2005, Ch. 21 & 22, C.J. Geankoplis, Transport Processes and Unit Operations, 3^rd Ed., 1993, Ch. 12, or J.D.Seader, E.J. Henley, D.K. Roper, Separation Process Principles, 3^rd Ed., Wiley, 2010, Ch.6 & 7.
Processing & Refining Crude Oil. Chevron Inc. http://pascagoula.chevron.com/abouttherefinery/whatwedo/processingandrefining.aspx . Accessed 10/01/16.
A Simple Guide to Oil Refining. ExxonMobil Inc. http://www.exxonmobileurope.com/europe-english/files/simple_guide_to_oil_refining.pdf Accessed 10/14/16.
R. Katzen, P.W. Madson and G.D. Moon Jr, Ethanol distillation: the fundamentals, in The Alcohol Textbook 3rd ed., K. Jacques, T.P. Lyons, and D.R. Kelsall, eds. Nottingham University Press, Nottingham, UK, pp. 269-288 (1999).
History and Technological Progress: cryogenic Air Separation. The Linde Groups. https://www.linde-engineering.com/internet.global.lindeengineering.global/en/images/AS.B1EN%201113%20-%20%26AA_History_.layout19_4353.PDF. Accessed 10/01/16.

Tags

Reflux Ratio Tray Distillation Chemical Engineering Technique Compound Mixtures Separation Efficiency Distillation Column Liquid Feed Stream Vapor Stream Sieve Tray Column Methanol Isopropanol Water Re boiler Bottoms Boil Up Ratio Reflux Drum Distillate Stream

The TDU is operated from a computer interface of a distributed control system. The interface is entirely graphical. The column contains 6 trays, a total condenser, and a partial reboiler (Appendix A). The mixture is contained in the feed reservoir, and the feed nominally consists of methanol (~50 wt.%), isopropanol (~30 wt.%), and water (~20 wt.%).

1. Starting Total Reflux Mode

Turn on the cooling water. If the reboiler liquid level is too low, add feed to it; if too high, use the bottoms pump to return liquid to the feed tank.
Turn on the main reboiler heater from the interface and the strip heaters using the switch on the unit at top right. Put the reboiler temperature controller in manual (MAN), give it an output (OP) of at least 60%, then wait for the overhead vapor to condense and the reflux drum to start filling, as seen on the interface. The reflux preheater power will start up with the reboiler heater.
When the reflux drum level reaches 50%, put the reflux flow controller in AUTO, give it a setpoint (SP) of 20%, and turn on the reflux pump.
As soon as you measure a reflux flow on the controller, gradually decrease the setpoint in steps of a percent every few seconds until the reflux flow rate is 12 - 13% of span (span = the total range of the transmitter).
Put the reflux preheat controller in AUTO, and give it a setpoint of ~65 °C, or some other suitable value near the temperature of the top tray.
Wait for the reflux drum level to reach at least 50% before manually adjusting the reflux rate by changing the setpoint on the reflux flow controller as necessary to provide a constant reflux drum level of 25 - 75%.
When all flows, levels, temperatures and compositions are close to their setpoints and aren't changing significantly over 2 min, steady state has been achieved.

2. Transitioning to Finite Reflux Mode

Put the feed flow controller to AUTO, and give it a setpoint corresponding to ~120 cm³/min.
Turn on the feed pump and feed preheater. Put the feed preheat controller in AUTO and give it a setpoint of ~65 °C or other temperature near that of the feed tray temperature.
Put the reflux flow rate controller in AUTO and give it a setpoint of ~80 % of the total reflux case as a starting point.
Start withdrawing distillate product (adjust flow controller to >0 flow) to maintain a level of 25 - 75% in the reflux drum. Put the distillate flow controller in AUTO and adjust its setpoint unless other instructions specifically call for MANUAL control. Try 3 different reflux ratios around 1 (e.g., 0.8, 1.2, 1.6), but hold the boilup rate constant by keeping the heat duty constant at a value that gives distillate flow.
Start withdrawing bottoms product (adjust flow controller to >0 flow) to maintain a constant level of 60-80 % in the reboiler. Put the bottoms flow controller in AUTO, turn on the bottoms pump, and adjust its setpoint, unless other instructions specifically call for MANUAL control. Both products should flow back to the feed reservoir.
When all flows, levels, temperatures and compositions are close to their setpoints, steady state has been achieved. The flow controllers may need to be adjusted; recall, F = B + D (feed = bottoms + distillate on a molar basis, and the distillate will be mostly methanol).
Collect a few mL volume samples of the feed, bottoms and distillate at the same time (see Figure 3, Appendix A). To collect the feed, insert a pipette through the top port of the feed tank. Only one set of samples per steady state is needed. It is also possible to sample liquid from each tray through its septum port using curved needle syringes Do this as only one of the steady states.

3. Setting Up the Gas Chromatograph

The gas ghromatograph (GC) is operated through the vendor's software. The column is Porapak Q, 1/8" in diameter and 2 ft long.

Select the Method and Run Control window. On the icon-filled menu bar, select Method and pick the START.M method.
On the front panel of the GC, press the Front Inlet button. The pressure should be set to 4.5 psi.
Inject a 0.1 microliter sample (same volume each time) and press the Start button on the GC front panel. The software will show 'Run' in Progress / Data Acquisition, with the elapsed time counting up.
Results will appear in a report on the software screen after approximately 3 min in the form of a chart containing GC peaks. Scroll down the report to see the Area% results.

4. Shutting down the Distillation column

Turn off all three heaters, the bottoms pump and the feed pump.
Put all controllers except the reflux flow rate in MAN and set their outputs (OP) to 0%.
Put the reflux flow rate controller in MAN and set its OP to 20%.
Turn on the reflux pump override and leave the reflux pump on until the reflux drum is empty.
As soon as the reflux drum is empty, take off the reflux pump override, turn off the reflux pump, put the reflux flow rate controller in MAN, and set its OP to 0%.
Leave the cooling water on until all six tray temperatures are below 60 °C, and then turn it off.

Skip to...

0:07

Overview

0:58

Principles of Distillation

3:35

Distillation Column Operation: Adjusting the Reflux

6:20

Transitioning to Finite Reflux Mode

8:25

Results

9:52

Applications

10:54

Summary

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