Manual for continuous distillation

Transcription

1 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 consecutive trays to determine the local efficiency.

2 Operation procedure: Preparation for Start-up: Key purposes: Get familiar with the system; locations of the valves and sample taps Check the liquid level in the reboiler Check the composition in the reboiler and feed tank Turn the Siemens system on Check if the valves are in the correct position 1. Using the sight-glass on the reboiler to assess its level of the liquid (in the range of around 70%). 2. Prepare to mix the contents of the reboiler. a. Close the red gauge valve on the bottoms line.(see graph below) b. Open the green valve on the same line just after it exits the reboiler. Follow the line down to the bottom of the reboiler. Open the orange ball

3 valve and make sure that the blue ball valve on the sample port is closed 3. Ask the TA/Lab Assistant to turn on the computer system hooked to the East column. 4. Using the computer control system, turn on the reboiler-mixing pump on the 1st floor, as shown below to mix the contents of the reboiler. Let mix for at least twenty minutes. 5. While you are waiting, check that the manual by-pass valves are closed. a. Reflux red valve parallel to the Reflux control valve b. Distillate red valve parallel to the Distillate control c. Bottoms blue valve parallel to the Bottoms control d. Water blue valve parallel to the Water control valve

4 Red gauge valve on Bottoms line Blue ball valve on line exiting reboiler Orange valve at bottom of reboiler open while mixing Figure 1. East Column Reboiler Pumping Line Figure 2. East column second floor control valves

5 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

6 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 40% is a good start for batch column. 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

7 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. A B

8 C D 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 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. Use the computer system to close the Steam control valve. To close the steam valve, click on the display box labeled STEAM/PID located above the valve symbol on the reflux line. Click on the box to the right of MAN. A small window will pop up. Change the opening of the valve to 0 and

9 click OK. 2. Turn off the reboiler mixing pump (EBTP/PUMP): a. To turn off the reboiler-mixing pump: click on Bottoms/PUMP or the pump icon; if not yet in manual mode, click on Manual ; click Stop and close the window. 3. With the reflux control valve (Reflux/PID) fully open, use the reflux pump (Reflux/PUMP) to drain the standpipe. Turn pump off. a. To open the reflux valve: Click on the display box labeled Reflux/PID located above the valve symbol on the reflux line. Click on the box to the right of MAN. A small window will pop up. Change the opening of the valve to 100 and click OK. 4. Open the red reflux valve (also serves as a manual bypass valve) (see Figure 1). 5. Close manual valves associated with the distillate tank (Figure 3). 6. Turn off the water and air valves by manually turning them fully in the clockwise direction on both floors.

10 Figure 3. Tanks on the first floor. Calculation: Overall efficiency can be calculated by comparing the actual number of plates in the column with the number obtained from McCabe-Thiele diagram when the column operates at total reflux. Local or point efficiencies can be calculated by measuring the compositions of the liquid streams corresponding to two successive plates. The local efficiency is typically close to the overall efficiency. E overall = N theoretical N actual 100%

11 The theoretical tray numbers can be obtained as follows: Generate the equilibrium curve with relative volatility; 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, N theoretical = 7 1 = 6 Assuming the actual tray number is 12, E overall = N theoretical N actual 100% = = 54.5% equilibrium line y A x B x A Fig 1. An illustration of the way to get theoretical tray numbers x D

12 Given the analytical equation of equilibrium line, y = f(x), and the overall efficiency of the column, E overall, we can get the analytical expression of quasi equilibrium line as blow: y = x + E overall (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. In Fig 2, b represents the driving force at a certain tray if at ideal case, a is the driving force in reality in the column we run equilibrium line b a y A a/b=e overall quasi equilibrium line x A Fig 2. An illustration of the way to get quasi equilibrium line

13 equilibrium line 0.6 y A quasi equilibrium line x B x A Fig 3. An illustration of the way to count the number of trays x D In Fig 3, the tray numbers getting from the quasi equilibrium line is 10 and the actual tray numbers is 11. Error = % = 9.1% By taking the sample from successive trays, the local efficiency can be worked out as follows: Fig 4 is a graphical description. E local = y n y n+1 y n y n+1 = x n 1 x n y n x n

14 1 equilibrium line y * n y A y n y n+1 y=x 0 0 x n x n-1 1 x A Fig 4. An illustration of the way to get local efficiency 2. Week 2: Run the column at different reflux ratios. Study the effect of reflux ratio on the concentrations of top and bottom product. Operation procedure:

15 From week 2 onwards, it is suggested to check if the reboiler has enough material from the control panel before starting the experiment. If the liquid level in the reboiler is less than 15 in, the liquid in the distillate tank should be pumped back to the. To pump the distillate back, you need to follow the procedure below. First open two valves located below the distillate tank, which are shown in graph E. Then open the bypass valve which is indicated on graph C. Finally open the pump below the reboiler on graph F. When you pump the liquid back to the reboiler, make sure someone checks the level in the distillate tank, as you may run the risk of burning the pump if the distillate tank is pump to empty. 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. Open the valve beside the rotameter on B to change the distillate flow rate in order to change the reflux ratio. The distillate will go through the rotameter on the 2 nd floor down to the tank located on the 1 st floor, which is shown in graph E. 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.

16 Shutting Down Turn off the valve for distillate first. Then shut down the steam on the control panel, which means set the PID of steam to 0%. Wait for 1~2min, as after shutting off the stream, the vapor may still go to the top and that means the water is still needed to condense the top product. Then close the air and water valves on the first and second floor, just like the shutting down procedure for the first week. Reflux rotameter Reflux sampling port A Distillate rotameter B

17 Sight glass Bypass valve Bottom product sampling port C D

18 2 1 E F A- Reflux rotameter B- Distillate rotameter C- East column 1 st floor reboiler D- East column second floor control valves.

19 E- Distillate tank for east column F- Control panel overview of the east column 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. GC (gas chromatography)

20 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.

21 Peak area (V*s) Peak area percentage(%) Equation y = a + b*x Weight Instrumental Residual Sum of Squares Adj. R-Square Mean Value absolute area relative area Standard Error Intercept Slope E Equation y = a + b*x Weight Instrumental Residual Sum of Squares Adj. R-Square Slope Mean Mole fraction Value Standard Error Intercept 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.