ChE 4013 PACKED TOWER EXPERIMENT Objectives: To generate a Strigle-Leva-Eckert data chart for 5/8” Pall rings and measure gas absorption characteristics of a packed column. Chemicals: Air, aqueous sodium hydroxide solution. Theory: A packed column (or tower) takes liquid in the top of the column while a gas (air in our case) coming in the bottom flows upward in a countercurrent type arrangement. The liquid flows down over a packing which disperses the liquid as the gas flows up, and the intent is to amplify the interfacial area between the gas and liquid. A simplified schematic is given in Figure 1. Packed columns are used in a number of different processes where components need to pass from one phase to the other. In a stripping operation, a volatile component in the liquid moves to the gas phase, while in an absorption process, a gaseous component moves from the vapor phase to the liquid. A packed tower can be used for either type of process. The packing is a critically important component. An efficient packing will disperse the liquid effectively and help to facilitate a large interfacial area between the gas flowing up and the liquid flowing down. A large interfacial area facilitates a large capacity for transport of mass from one phase to the other. We are particularly interested in the hydraulics (pressure drop and flooding phenomena) of this system. You will also be studying absorption of CO2 from air using the packed column. Read “Flooding and Loading” beginning in the second column on page 14-41 of Perry’s, 7th edition [Ref. 1]. Note that there is a mistake in equation 14-151. The (G/L) 0.5 should be in the numerator, not denominator of that equation. The correct equation is given as the abscissa in Figure 14-48. Also note that Figure 1. Simple Packed Column the label on the ordinate of Figure 14-48 is wrong. The correct ordinate is equation 14-150, or, the ordinate should simply read “Cs” (not CsFp0.50.5). Read up to the section “Pressure Drop” in the first column of page 14-42. Make sure you understand the equations, units, and Figure 14-48. The use of this generalized pressure drop correlation chart has several limitations, detailed in References 2 and 3. When experimental data are combined with literature values for the packing factor to create a Strigle-Leva-Eckert data chart, the points do not always fall on the lines of the chart. Sometimes they systematically fall off the lines. The suggestion of these references is to combine (interpolate between) the experimental data and the chart lines using a chart specific to the packing and fluid. No such chart exists for our packing, so we will be creating one.
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ChE 4013 We will measure four main variables: 1) liquid flow rate, 2) air flow rate, 3) column pressure drop, and 4) CO2 inlet and outlet concentrations. We will also record the column temperature which will be used to look up properties of the two main components, sodium hydroxide solution and air. The liquid flow rate given in gpm (gallons per minute) will be converted to the superficial mass rate per square foot of column cross-sectional area, L, in lbm/(sec-ft2) and the air flow rate given in SCFM (standard cubic feet per minute) will be converted first to actual cubic feet per minute and then to the superficial mass rate per square foot of column cross-sectional area, G, in lbm/(sec-ft2). Knowing the densities of air and sodium hydroxide solution, FLG, which is the abscissa of Figure 14-48, can be calculated. We will also calculate the value of the ordinate from the column pressure drop and the literature value for the packing factor and plot the point for each run on the graph. If the point does not fall on the existing lines, we will calculate the value of the packing factor which makes our experimental points fall on the graph lines. Packing factors are tabulated in the literature for common packings. In lieu of data for a particular packing, a common practice is to compute Fp as Ap/3 where Ap is the total surface area of the packing per unit volume of bed and is the void space in the packing. For gas absorption, the efficiency of the process can be expressed in terms of the overall height of gas-phase transfer units, HOG. The smaller the HOG, the more efficient the absorption process will be. For dilute systems, HOG is defined as: H OG
where
K y' a
G K y' a
(1)
is the overall mass transfer coefficient (moles/volume/time).
NOG, the number of transfer units, is a function only of compositions and depends on the operating conditions, e.g., gas and liquid flow rates, temperature, pressure, etc. It is defined as N OG
y A0 y A1 y A y *A lm
(2)
* where y A y A lm is the log mean (average) driving force for absorption. Note that yA0 is the gas phase mole fraction of component A at the entrance (bottom) of the column, yA1 is the mole * fraction of A in the exiting gas, and y A is the mole fraction of A in equilibrium with the bulk
liquid. The overall height of gas-phase transfer units is then calculated from NOG: H OG
z N OG
(3)
where z is the height of packing.
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ChE 4013 When sodium hydroxide solution is the liquid phase, a chemical reaction occurs between the CO 2 and the hydroxide, which greatly enhances the absorption of CO2. Chemical reaction in the liquid phase reduces the equilibrium partial pressure of a solute over the solution, which greatly increases the driving force for mass transfer. If the reaction is considered to be instantaneous and irreversible (as is the case here), the equilibrium partial pressure will be reduced to essentially zero, maximizing the driving force for absorption. In this case, NOG reduces to: y A0 y A1
N OG ln
(4)
Experimental: A dilute solution of sodium hydroxide (~0.1 M) is circulated via pump and a tank maintains a liquid inventory. The pump is set up to circulate the liquid continuously through the tank so that the pump exhaust pressure does not rise too high when the liquid flow rate to the column is small. As demand for column liquid is increased, the circulation rate through the tank will subside. Similarly, to keep air pressure from building on the exhaust side of the air pumps, two control valves are arranged to be complementary. As the valve supplying air to the column opens, a second valve venting the air (currently going to the Plexiglas tray column) opens, and vice-versa. Two air pumps in parallel are required to provide enough air. A pressure relief valve protects the equipment. A distributor at the top of the column is used to initially break the liquid stream into many smaller streams that flow down from the top of the column. The distributor uses a cup that has a number of holes in the bottom that let liquid drain out and four larger holes with stacks that let air out the top. As liquid flows into the center of the cup, a liquid level builds up inside until the height of the liquid in the cup provides enough head to force flow out the holes at a rate that balances the liquid flow rate coming into the cup. Adventurous students may get up on the stairway/stand and observe the operation of this device. At the bottom of the column, several things are happening. A riser (gray and about 2" in diameter) with a cap and side slots allows air in. The cap and side slots divert the air toward the column walls. Otherwise, air would tend to focus more into a jet that would concentrate in the center of the column, pushing the liquid coming down the column more toward the outside, thus attenuating the overall gas/liquid interfacial area. Also at the bottom, liquid draining off the packing accumulates in the bottom, occupying 6-8" of the column. A capacitance-type level detector senses this level and reports the value to the Honeywell control system. When facing the column, the control valve to the left of and below the column is used to control the liquid level in a typical feedback loop. If a liquid level was not properly maintained in the bottom of the column, supply air would tend to go out the drain port with the liquid and get into the intake of the pump. This is an operational difficulty that should be avoided as much as possible. Finally, the bottom flange also contains a tap that connects into a differential pressure transducer which measures the column pressure drop. The other side of the pressure gauge runs to the top of the column. Students should also observe the devices and connections in the bottom of the column under operating conditions.
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ChE 4013 Familiarize yourself with the column. Photographs with critical components marked are attached in the Appendix. Trace the lines running from the air compressors to the air riser in the bottom of the column. The air flowrate is measured by an orifice meter in the PVC pipe. Trace the liquid lines beginning at the lower left tap in the bottom of the column running down through the liquid level control valve, into the pump, through the silver metal turbine meter, and up to the top. Some valving around the bottom of the tank can be used to drain and circulate liquid through the tank. Find the differential pressure transducer and trace the lines into the top and bottom of the column. Review the column in person carefully. For CO2 measurement there are two electronic boxes, a CO2 scrubber, and a control on the column control page to switch the CO 2 system on and off. A valve allows you to direct the inlet or the outlet gas flow to one of the detectors. You can use this to determine the offset between the two detectors. Another method that will give you a different value of the offset is to run air through the column with no liquid and compare the inlet and outlet concentrations, but this is not the preferred method. Procedure: 1. Column preparation: Empty the sodium hydroxide solution from the air line by opening the air line drain valve below the column, draining liquid, and closing the valve. Return solution to the supply tank. The riser seal leaks, but its repair requires disassembling the column. Empty this line every two hours during your experiment with the air control valve closed. Measure the zero offset for the column pressure drop. Check that the manual valves in the liquid return from the column to the pump are open. 2. Check the pH of the solution in the tank with pH paper to make sure that the sodium hydroxide concentration is about 0.1 M. A fresh NaOH solution may be made by mixing 400 g of NaOH pellets with 100 liters, but do this only on instructions from the faculty instructor. 3. Power setup: Check the emergency shutoff switch and the two compressor electrical boxes on the front panel. The emergency shutoff switch should be pulled out, and the two boxes should have the "Auto" switches set on. Bring up the "Packed Column" display in one instance of Experion (see the Honeywell operating instructions if necessary). 4. Unlock and enable the pump: The pump is normally left locked out and tagged out at the end of each lab. The tag will say who has the key. Unlock the three valves isolating the pump. Open the inlet valve to the column completely and the other two halfway. Change the pump control box from “Off” to “Auto”. The liquid flowrate controller on the Honeywell system is locked out with a Red Tag, too. You will not be able to change the OP for the sodium hydroxide control until the Red Tag is removed. If you try to change the OP, the following error message will appear:
. To clear the Red Tag so you can run the equipment, you must log in as manager (mngr). Double click on the NaOH control box. On the Main tab, find the section labeled Safety interlock, Red
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ChE 4013 tag, and Operator tag. Uncheck the box next to Red tag. You should now be able to change the OP for the NaOH controller from the main display page. 5. Control valve preparation: a. Put the liquid flow loop in MAN and set the controller output to 25% (OP = 25%). b. Put the level control loop into MAN, and set the controller output to zero (OP = 0%). c. Put the air flow loop into MAN, and set the controller output to 25% (OP =25%). 6. Power up: Click on the empty buttons for the pump, both compressors, and the CO2 Absorber System to turn them on. They will be filled with a √ when they have been turned on (the √ may take a few moments to appear) and the pumps will crank up. Watch for liquid coming out of the liquid distributor cup at the top of the column, and in a minute or two, liquid will start flowing down the column. If no liquid is flowing, consider increasing OP to 30 or 35%. As the liquid works its way down the packing, it will begin to drip off the packing and into the bottom of the column. 7. Establish liquid level control: When liquid is dripping from the packing and the liquid level in the bottom of the column is about 4" or more (there is likely at least that much left from the last time the column was run), set the level controller output to 25% (OP = 25%), then switch the level controller to AUTO mode. The trick here is to give the level controller a reasonable starting point. You would prefer not to overfill the bottom as the liquid will go back into the air riser, but you also do not want to drain the bottom dry because as air will get into the intake side of the pump. Thus, you do not want the level controller to open before there is a liquid supply draining from the packing. The level controller will generally take over without complications and bring the level to a stable value. If you drain the bottom of the column dry and air gets in the intake of the pump, the liquid supply rate to the column will drop and it might even quit flowing completely. To fix this, proceed as follows: 1. Put the level control valve into MAN mode and close the valve (OP = 0%). This will stop air from flowing into the intake side of the pump, and liquid from the supply tank will replace the air in short order. 2. Put the liquid supply valve into MAN mode and set OP = 25% if that is not already done. 3. Wait for the pump to expel the air and watch for liquid to appear at the top of the column. 4. Continue to wait as liquid travels down the column and begins to collect in the bottom. 5. When about 4" of liquid has collected in the bottom of the column, set OP = 25% on the level controller then switch the level control valve to AUTO mode. The controller should bring the level to a stable value in a few minutes.
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ChE 4013 If you overflow the bottom and liquid goes into the air supply riser, usually, you do not need to do anything but wait for a minute and see that the level controller gets control and brings the level down. If you are operating under high liquid flow rates, cut the liquid flow rate as quickly as possible, but if you are just starting up the column, you should probably just be patient. Once the upset has subsided and level control is established, close the air control valve. Then, go into the lab and open the air line drain valve at the bottom of the column to drain out the liquid that went into the air riser. Close the air line drain valve and open the air control valve. If you experience any problems that you do not understand, and the liquid level is rising in the column such that it has completely covered the air riser and is still rising, shut off the pump and both air compressors with the pushbuttons on the control page, or in an emergency, go into the lab and hit the emergency shutoff switch. 8. Establish liquid and vapor flowrate control: Once the column is started up, liquid is flowing down the packing, and the liquid level is stable in the bottom of the column, set the liquid flow rate to 1 gal/min (SP = 1.0) and switch the liquid supply flow controller to AUTO mode. Set the desired air flow rate (SP = 5) and put the air flow control loop into AUTO mode. 9. Packing Factor Experiments: Note what the pressure drop is at 1 gpm liquid and 5 scfm air, and adjust both the liquid and gas flowrates until the pressure drop is 0.1 inches of sodium hydroxide solution per foot of packing. Record the liquid flow rate, air flow rate, column pressure drop, column temperature, and CO2 inlet and outlet concentrations (this is one experiment). Repeat the experiment near 10 SCFM and 0.25 inches per foot of packing. Generate at least 6 data points at 0.50 inches per foot of packing and 15 to 20 SCFM and another 6 data points at 1.0 inches per foot of packing and 25 to 30 SCFM. Do not change the liquid rate more than 1 gal/min in any single change. After making a liquid rate change, wait for the level in the bottom of the column to stabilize before changing it again. Remember to drain the liquid from the air riser every two hours or so during the lab. 10. Absorption Characteristics Experiments: collect data for at least five different liquid flowrates at a constant gas flowrate. Do not change the liquid rate more than 1 gal/min in any single change. After making a liquid rate change, wait for the level in the bottom of the column to stabilize before changing it again. Remember to drain the liquid from the air riser every two hours or so during the lab. 11. Shut down the column. a. Bring the liquid flow rate to 1 gal/min in steps of no more than 1 gal/min, waiting for the level in the bottom to stabilize between changes. b. Put the liquid supply control valve in MAN mode. c. Set OP = 25% on the liquid flow control valve. d. Wait for the bottom level to stabilize. e. Turn off both compressors and the CO2 detectors and immediately change the level controller to MAN mode. Set OP = 0% on the liquid flow control valve to close it.
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ChE 4013 f. When most of the liquid has drained from the packing or if the liquid level falls to 4" or less, close the level control valve (OP = 0%) and turn off the pump. Close the remaining control valves and the valves isolating the pump. Lock out and tag out the valves isolating the pump. Set the electrical control for the pump to “Off”. Return the key to the person named on the tag. Red tag the NaOH control on the Honeywell system. 12. Check that the concentration of the supply tank is still about 0.1 M. Data: The inside diameter of the glass column is 150 mm = 5.91". The packing is 5/8" steel Pall rings, and the length of the packed bed is approximately 42” (students should measure it). Prelab Report Calculations: 1. The equipment is capable of producing more than 30 SCFM and higher liquid flowrates. For what other physical reason might we not want to try for 1.5” pressure drop per foot of packing? 2. Assuming the height of the packing in the column is 42”, calculate the pressure drops required to reach 1.0, 0.5, and 0.10 inches per foot of packing. ' 3. V L From the given data, calculate H , N , and K y a . Plot the data point (using the OG
OG
experimental FLG and Cs calculated using literature packing factor) on Figure 14-48 from Perry’s. If this point (using the literature packing factor and experimental FLV) does not fall on the experimental pressure drop line, determine the value of the packing factor that will make the point fall on the right pressure drop line using the same experimental FLV. Include an error estimate for each calculated value. Show hand calculations. VG
= 17.5 + 0.1 scfm, = 4.2 + 0.1 gpm, h = 1.75 + 0.03 inches, T = 74.5 + 0.1ºF, yA0 = 470 + 1 ppm, and yA1 = 275 + 2 ppm. When the outlet gas is routed to the inlet detector, yA1(0) = 280+ 1 ppm. 4. Fill a 1 liter (or larger) beaker with 5/8" Pall rings. Do not use a graduated cylinder because the rings are too large to pack randomly in the narrower device. Count the number of pieces and weigh the packing material. Measure an individual piece with a ruler or micrometer and make the calculations of Table 1 on the next page: Compare these numbers you have calculated with properties in Perry’s handbook, 5th Edition (see Appendix) and with data from the Sulzer website. Did you do your measurements and calculations correctly? Prelab Quiz Your prelab quiz will be to individually demonstrate that you can successfully get the sodium hydroxide solution flowing and the level controller in auto with the flowrate and level both stable.
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ChE 4013 Table 1. Physical measurements of the packing Your calculations pieces of packing / liter
from Perry’s 5th ------
pieces of packing / ft3 weight of packing / liter (g/liter)
------
weight of packing / ft3 (lbm/ft3) surface area (in and out) of a single piece (in2)
------
surface area (in and out) of a single piece (ft2)
------
surface area in one ft3 of packing (ft2/ft3)
1.
2. 3.
Suggested Calculations and Report Requirements: Make a copy of Figure 14-48 in Perry’s and place a point on the graph for each of your data points, assuming that the literature packing factor is correct. Use a different shape for each pressure drop. If the experimental data do not fall on the right pressure drop lines, calculate the value of the packing factor that makes the experimental points best agree with the graph. Are there any trends in discrepancies with liquid and gas flow rates? If so, explain. Compare your best fit packing factor to the literature values and to that computed from Fp = Ap/3. Calculate HOG and NOG for each set of conditions. Also calculate the mass transfer ' coefficient K y a for each condition. Calculate the uncertainties for your values. Are you able to see any trend in
K y' a
values with liquid flow rates?
References: 1. Perry’s Chemical Engineers’ Handbook, 7th Edition, Robert H. Perry and Don W. Green, Ed., McGraw-Hill, 1997. 2. Henry. Z. Kister and David R. Gill, Chemical Engineering Progress, February 1991, p. 32 – 42. 3. Henry Z. Kister, Distillation Design, McGraw-Hill, 1992, p. 492 – 506. 4. Perry’s Chemical Engineers’ Handbook, 5th Edition, Robert H. Perry and Cecil H. Chilton, Ed., McGraw-Hill, 1973, p. 18-24.
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ChE 4013 Appendix: Perry’s Data and Equipment Pictures
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ChE 4013
From Reference 4, p. 18-24.
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Sodium hydroxide solution tank
ChE 4013 Air flowrate control valves
Sodium hydroxide solution pump
Air compressors Figure 2. Air and sodium hydroxide solution feed equipment Emergency cutoff switch
Compressor switches
Level detector Air riser
Pump switches CO2 detectors
Level control valve
Drain valves
Figure 3. Control panel and bottom of column equipment
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ChE 4013 Air outlet
Thermocouple
Distributer cup
Differential pressure cell Liquid flowrate control valve Figure 4. Equipment near the middle and top of the column
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