Chemical Engineering Laboratory Manual Absorber
EXPERIMENT 461L_2
GAS ABSORBER
OBJECTIVES:
- The primary objective of this experiment is to become familiar with the modeling and operation of a gas absorber.
- To be able to describe differences between the gas absorber model and the data acquired on a pilot scale absorber.
SAFETY CONSIDERATIONS:
1. Always wear hard hat and safety glasses/goggles while running the experiment.
2. Turn off the ethanol pump and vaporizer switch if the digital display on the analyzer shows value more than 1.65 mol%. The lower explosive limit of ethanol is 3.3 mol% and it is important to safely shutdown when it reaches half of the lower explosive limit value to avoid the potential of creating an explosive atmosphere in the system.
3. Keep flames away from ethanol and ethanol pump as ethanol is very flammable.
4. Do not change the calibration of the analyzer during the run as it will show erroneous readings and cannot detect the exact concentration.
5. Do not change the flowrate through the rotameter for the GasBoss, as this may change the calibration of the GasBoss analysis meter. The gas flowrate through the rotameter for the gas analyzer should always be around 2 on the rotameter (approximately 2.1 LPM); read from the bottom of the ball.
6. Do not turn ON the ethanol pump without nitrogen flow in the vaporizer. Nitrogen is an inert gas and is used as the carrier gas. It must be supplied to the vaporizer before the ethanol pump is turned ON to insure and inert atmosphere in the unit, and to sweep out the ethanol vapors, thus preventing a buildup of ethanol.
7. Always prime the ethanol pump before running it. If the pump is not primed, it can cause damage to the pump due to cavitation.
8. During the experiment the ethanol pump should be the last instrument to be turned ON and the first to be turned OFF.
START-UP PROCEDURE:
1. Turn ON the heaters at least a half hour before starting the experiment. Do this by pressing the “Vaporizer” Start button. At the same time, open (switch up) the green handled valve labeled “Sample Valve” (left most sample valve). This will allow air to flow into the analyzer.
2. Turn on the instrument system by pressing the “Temperature Display / Gas Analyzer / Delta P, Heated Lines / Computer” Start button (you may need to turn on the monitor power too). Turn on the computer (switch is about eye level on the back of the control panel; upper right hand corner as you look at the back).
3. Start the OPTO 22 hardware system (DAQ and Control hardware) by turning on the switch on the side of the control panel that is facing the wall (east side/window side). It is labeled “OPTO 22” power, and is near the back of the panel box. (Note, it is not the metal switch on the front of the control panel box).
4. On the computer, start the OPTO 22 control and data acquisition software:
a. Open PAC Control Basic 9.0 by following the path: START / PAC Control Basic 9.0.
b. In the PAC Control Basic program, OPEN the strategy file called MAPS Absorber. After the file opens, click the Debug icon (see Figure 1).
c. Answer OK, to the pop-up window asking if “Power Clear up Expected”.
d. Answer YES, to the pop-up window indicating “Control Engine’s Memory Has Been Cleared”.
e. Click Run Strategy (see Figure 2).
f. Minimize the PAC Control Basic [Vaporizer_Monitor] window.
g. Double click desktop icon called PAC Display Runtime Basic 9.0.
h. Close the Event Log Viewer pop-up window.
i. Click ON the Data Log, to record the run data to the data log file.
j. The OPTO 22 control program is now downloaded to the OPTO controller system. Data is displayed on the user interface window (“MAPS Absorber Display.UUI). Changes to the air or water flowrate setpoints can be made from this window.
Figure 1. PAC Control Basic “Vaporizer_Monitor” window, showing location of Debug and Config icon buttons.
Figure 2. PAC Control Basic “Vaporizer_Monitor” window, showing location of Run Strategy or Stop Strategy icon buttons.
5. Determine which tower to use, and if needed, change the packing material. Fill it to the required height according to the design and connect all the inlet and outlet lines to the column. If you need to remove packing in the tower, use the wet/dry shop vacuum located on the 2nd level. Place the removed packing in the storage bins; ensuring not to mix different sizes or types of packing.
6. Turn ON the sample compressor.
7. If instructed to do so, calibrate the ethanol analyzer by checking the zero (room air) and by preparing a span gas standard in a 25 liter gas sampling bag. Follow the calibration procedure according to the instructions given in the Appendix . After calibration, the sample compressor must be left on.
8. When the vaporizer temperature reaches close to 400 oF, check with the Instructor/TA to obtain the container of absolute denatured ethanol.
9. Continue to monitor the temperatures on the heaters (the main heater controller display; and channels 5 – 7). The wall temperature (thermocouple 6; at the midpoint) of the vaporizer should be at least 400o F before any ethanol is fed to the vaporizer.
10. Open the yellow ball valve at the bottom of the tower being used and the ball valve going into the green PVC level-control tank to allow water to drain to the floor drain.
11. Set the water flowrate to the column (a typical value could be 2 GPM) using either manual or automatic mode. To allow for automatic control the instrument air must be supplied to the I/P transducer (ensure the instrument air regulator near the top of the columns is set at no more than 25 psig).
12. Before starting the blower, slightly open the 3” bypass valve to avoid “dead-heading” the blower. Then, press the Start button of the east side (window side) of the panel, close to the computer monitor, to start the blower. The air flowrate can be controlled in PAC Display Runtime Basic by clicking the “Enter New Air Flowrate” button. (NOTE: keep bypass valve slightly open when controlling the air.)
13. Let the system achieve steady state. This would be about 10 minutes.
14. Open the valve of the Nitrogen cylinder and adjust the regulator until it reads 10 psi. Adjust the nitrogen flow to the bottom of the vaporizer to about 10 SCFH using the rotameter on the pump cart.
15. Make sure ethanol is in the feed bucket. Open cap and feed plastic inlet line to ethanol pump into the ethanol. Press the “Ethanol Pump” Start button on the control panel. Then, click the power button on the pump to start the pump.
16. Prime the ethanol pump using the prime button. Priming the pump will clear any air in the ethanol feed lines. Use the ethanol pump calibration curve to determine the setting on the pump. Check the ethanol flow rate.
17. Achieve the desired liquid ethanol flowrate by adjusting the flow in the rotameter using the up and down arrows on the pump.
18. Monitor the rotameter for the ethanol pump using the calibration guide. Verify again that the value from the pump calibration curve and the rotameter calibration curve agree.
19. Determine the ethanol concentration in the gas inlet to the column by opening the Bottom Valve (it is a red handle valve; be sure to close the Sample Valve, green handled valve) and running the sampling compressor for 2 minutes. The calibrated flowrate of approximately 2.1 LPM should be maintained. In no case should the ethanol concentration ever be more than 1.65 mole percent. In the event an ethanol concentration of more than 1.65 mole percent is measured, immediately shut down the ethanol pump, allow the column to clear of ethanol and troubleshoot the process. Seek help from the lab assistant, instructor, or instrument/equipment specialist.
20. Using PAC Display Runtime Basic, monitor the process variables. Be sure that the water level in the bottom of column does not get high enough to backflow into the air line. Ensure the column does not flood. Similarly, if the pressure gets too high, the water might overflow the drain tank.
21. Find the upper gas effluent concentration (gas discharge) of the column at this ethanol flow rate by opening the Top Valve (blue handled valve; other two sample valves should be shut). Again allow the sample stream to run through the analyzer for at least 2 minutes.
22. Adjust variables in column and see what effect they have on the ethanol concentrations in the column.
SHUT DOWN PROCEDURE:
- Turn off the ethanol pump.
- Turn off the ethanol vaporizer (heater) switch and the sample compressor.
- Let the nitrogen flow through the vaporizer for about 20 minutes more to carry away any ethanol vapors in the vaporizer. When the vaporizer is cooled, open the bottom drain valve to check for any accumulation of ethanol. If any ethanol is found, inform the instructor, TA, and instrument/equipment specialist. This is not normal and indicates a vproblem.
- Shut down the OPTO 22 control software by the following:
a) From the control screen, turn OFF the data log.
b) Close the Runtime Basic window (MAPS Absorber Display.UUI). Click YES when asked if you want to exit PAC Display Runtime.
c) In the Control Basic window [Vaporizer_Monitor]
§ Click the Stop Strategy button (see Figure 2.)
§ Click on the Config button (see Figure 1.) stops the program from running on the OPTO hardware system.
§ Exit the Control Basic window.
d) One the Desktop, open the folder Student Data File. Save the data file (has .dat file extension) under your team name or number to the Fall 2011 folder for archiving purposes. Also, email the file to yourself. Then, delete the data file from the Student Data File folder (note it will be recreated the next time another groups starts up the procedure). Don’t delete the Student Data folder.
e) Shutdown the computer.
f) Turn off the OPTO hardware power switch on the east side of the control panel.
- Turn off all of the instrument power switches, except for the blower, on the control panel. Close the container with ethanol and return to the instructor/TA.
- Let the blower be on for at least 15 minutes to let any ethanol vapors in the column vent. Switch off the blower and clean up any mess. Ensure you have turned off the water to the column and the instrument air line should be closed.
REPORT:
1. Include in your report any design / simulation you created for the laboratory.
2. Report on the operation and efficacy of separation achieved.
3. Compare operation vs design / simulation. Are they in agreement? Discuss briefly why or why not.
4. Comment on how this write-up or laboratory may be improved.
REFERENCES:
1. Warren L. McCabe, Julian C. Smith and Peter Harriott, Unit Operations of Chemical Engineering, Fifth Edition, McGraw-Hill Book Co. (1993).
2. C.J. Geankoplis, Transport Processes and Unit Operations, Third Edition, Prentice Hall PTR (1993).
APPENDIX:
Calibrating the GasBoss Gas Analyzer:
The first step in the calibration procedure of the Gas Analyzer involves the calculation of the volume of ethanol required to inject in the air-sampling bag, so that we know the percentage of ethanol-air mixture in the bag. The gas analyzer is calibrated in such a way that the highest concentration of ethanol–air mixture that it can read is 1.65 mol %; which is half the lower explosive limit of ethanol. Therefore the range of the gas analyzer is 0-1.65% by volume of ethanol-air mixture. The analyzer gives a corresponding linear D.C. output signal of 4-20 mA. The sample calculations below are shown for 1.5 mol% ethanol-air mixture used for calibration.
Sample Calculation:
For an air-sampling bag of 10 liters capacity the total volume of ethanol vapor required
would be 0.15 liter. This value was calculated by multiplying the mole fraction of ethanol
by the total volume of the bag that it can hold (0.015 x 10 = 0.15).
Then using standard conditions of temperature and pressure
P = 0.8947 atm (680 mmHg)
T = 298 K
R = 0.08206 ltr.atm/mol. K
V = 0.15 ltr
By using the formula (ideal gas equation of state):
PV=nRT
and substituting the above values, the moles of ethanol required was calculated as follows:
0.8947 x 0.15=n x 0.08206 x 298
n = 0.0055 moles
The molecular mass of ethanol = 46.07 g/mol.
Therefore the mass of ethanol required to add to the bag is:
Mass of ethanol = n x molecular mass of ethanol
= 0.0055moles EtOH x 46.07g EtOH/moles EtOH
= 0.25 g EtOH
By looking up the density of liquid ethanol at room temperature, the volume of ethanol to add to the bag can be calculated :
Density of ethanol = 0.798 g/mL
Volume EtOH= 0.25g EtOH /0.798g EtOH/mL EtOH
Volume = 0.313 mL EtOH
Hence to obtain 1.5 % ethanol air mixture we need to inject 0.31 mL of ethanol into a 10 L gas sampling bag and allow it to vaporize. Then fill the bag with air until it is completely full.
Calibration procedure:
- Switch on the gas analyzer and the sample compressor. Get an air sampling bag and add air until the bag is almost full, then close the valve on the bag. Using a pipette, inject the amount of ethanol required. Allow sufficient time for the ethanol to completely vaporize. Then fill to full with air, ensuring no vapors leave the bag.
- Open the bypass valve on the control panel and let the air pass through the analyzer with a flow rate of 2.1 LPM (see calibration chart). Allow the value on the digital meter to stabilize for 5 minutes and check the value, if the display is not zero, then have the instrument/equipment specialist (Ivan) show you how to move the “zero” screw (Figure 3) on the analyzer till the digital display meter shows zero (note: the analyzer measures % of the LEL of ethanol).
- Close the bypass valve and attach the plastic tubing from the air sampling bag to the ambient air (sample) port and open the valve of the bag and the green handled valve. Close the bypass valve on the control panel. Let the mixture in the bag flow at a constant flow rate of 2.1 LPM through the analyzer for 2 minutes, and span the number on the analyzer to 45.45% (or 40 if the analyzer will not go up to 50). Remove the bag from sampling tubing.