The Effect of Reflux Ratio on Tray Distillation Efficiency

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

1. 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.
2. 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.
3. 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.
4. 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).
5. 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.
6. 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%.
7. 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

1. Put the feed flow controller to AUTO, and give it a setpoint corresponding to ~120 cm³/min.
2. 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.
3. Put the reflux flow rate controller in AUTO and give it a setpoint of ~80 % of the total reflux case as a starting point.
4. 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.
5. 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.
6. 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).
7. 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.

1. Select the Method and Run Control window. On the icon-filled menu bar, select Method and pick the START.M method.
2. On the front panel of the GC, press the Front Inlet button. The pressure should be set to 4.5 psi.
3. 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.
4. 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

1. Turn off all three heaters, the bottoms pump and the feed pump.
2. Put all controllers except the reflux flow rate in MAN and set their outputs (OP) to 0%.
3. Put the reflux flow rate controller in MAN and set its OP to 20%.
4. Turn on the reflux pump override and leave the reflux pump on until the reflux drum is empty.
5. 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%.
6. Leave the cooling water on until all six tray temperatures are below 60 °C, and then turn it off.