CHEG 200 Distillation Column for P2 to P5 Labs BEEF, Inc.
Continuous Distillation
(image from Sulzer Chemtech, Ltd.)
Introduction
Distillation is one of the most common techniques for liquid-liquid separation, and considered to be a classic chemical engineering unit operation.
Distillation works on the basis of vapor-liquid equilibrium. The operation takes advantage of volatility differences of the liquid components in a mixture at a given equilibrium temperature and pressure condition. By controlling the heat input, components with lower boiling points are strategically sent upwards as vapor, while the components with higher boiling points drop downwards as liquid. This equilibrium “stage” repeats over the length of the column so that the vapor-liquid separation occurs at multiple instances, leading to a large concentration difference of highly volatile material (termed light key) between the top and bottom of the column.
Even though distillation can be carried out in a batch system, like typically done in organic chemistry laboratory, distillation in a chemical engineering setting is often done continuously due to the large desired production rate.
This laboratory session exposes you to the chemical engineering principle of material balance in a distillation column, through which an isopropanol-water solution is fed at the feed port and separated into the distillate and bottoms streams. Specifically, the overall objective of the experiment is to validate the conservation of mass occurring around the overall system. In addition, you are to determine the isopropanol purity (i.e, its mass fraction) in the distillate stream.
Validation is conducted by calculating the percent relative imbalances (%RIB) of the all components as well as individual components, between all flows in and all flows out.
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Because your team should have sufficient time to conduct multiple runs, we would like you to determine how the %RIB and distillate isopropanol purity vary with different operating parameters (For example, reflux ratio or bottoms flow rate)
Preparation
Use the literature and/or internet resources, such as the Encyclopedia of Chemical Engineering Equipment, to become familiar with the process and process unit.
Use the literature and/or internet resources, to become familiar with the raw material involved.
Procedure
Equipment {prep work prior to meeting with your TA}
1. What is a distillation column? How does it work? 2. What is a condenser? How does it work? 3. What is a reboiler? How does it work? 4. What are the analytical instruments? How do they work?
Model {prep work prior to the laboratory session}
1. Review the conceptual diagram and complete the assumptions. 2. Review and complete the math model and understand its terminology. 3. Review and use the calibration data for refractive index and density of an isopropanol-water mixture. These data are provided in the distillation column template Excel “EZ Setup” file. 4. Complete the math algorithm and determine what is to be measured in the lab. 5. Prepare an experimental data sheet in your team’s laboratory notebook. 6. Ask instructor or lab assistant for clarification.
Safety {prep work prior to the laboratory session}
1. Obtain hazardous information for all chemical compounds. 2. List potential process hazards with this experiment
Procedure {laboratory session, under TA's supervision}
1. Operate the distillation column using a specific reflux ratio, like one or two. 2. Take a feed stream sample after the column reaches steady state (When does that occur?). 3. Take distillate and bottoms stream samples after the column reaches steady state. 4. Operate the distillation column with a different set of operating parameters. 5. Repeat steps 2 and 4 as time allows. 6. Shut down the computer, all equipment, all analytical instruments, and clean up area.
Data Acquisition {laboratory session, under TA's supervision}
1. Record the barometric pressure and room temperature several times during the lab period. 2. Record the temperature, pressure, and flow rate of the three process streams. 3. Record the streams’ refractive indices, after allowing the specimens to be cooled to room temperature.
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Analysis { before and after the lab session }
1. Review conceptual model and drafted material balances for the first Monday co-op. 2. Draft the “EZ Setup” mathematical model for the first Wednesday co-op session. 3. Draft the “EZ Setup” mathematical algorithm for the first Friday co-op session. 4. What are the relative imbalances of the total and component material balances? 5. How does the distillate isopropanol purity depend on the operating parameters? 6. What conclusions can be drawn about the imbalances and the isopropanol purity? Why?
To prepare for and complete this lab experiment, consult Worksheet “read” in the Excel “EZ Setup” file “distillation_packed.xls” at the Moodle CHEG 200 course. Also, read the information below about the “Material Balance Model for Labs P2 to P5”.
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Material Balance Model for Labs P2 to P5
See CinChE Manual, Pages 5-2 to 5-3
Material Balances with Mass Quantities (differential form)
A. Diagram of a System Boundary
1 ni + 1 m 1 , w1 m ni+1 , wni+1
2 ni + 2 m 2 , w2 m ni+2 , wni+2
ni nt
m ni , wni m nt , wnt
B. Total Mass Balance:
ni nt d(m) − = sys ∑m i ∑ m i (5.1d) i =1 i = ni+1 dt
C. Component Mass Balances:
ni nt nr d(m j ) − + υ = sys = ∑m i, j ∑ m i, j ∑M j l, j Rl for j 1, 2,, nc (5.2d) i =1 i = ni+1 l =1 dt
where m i, j = m i wi, j { composition equations }
The component mass balance is written “nc” times, once for each chemical component, while the composition equation is written for each component j in those process streams where the mass fraction of that component is not known and is to be found.
D. Mixture Equations: { By definition, a mixture is chemically inert; that is, no chemical reactions are occurring }
nc nc for those applicable m i = m i, j or wi, j = 1.0 ∑ ∑ process stream i’s j =1 j =1 standard form normalized form
The “standard form” of the mixture equation is written only for those process streams where each has at least one unknown component mass fraction. The “normalized form” of the mixture equation is often used as a check equation whenever you need to determine mass fractions.
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Distillation Conceptual Model Material Balances
column system boundary
condenser sat’d sat’d vapor liquid TD P R D D ṁ reflux distillate D sat’d liq sat’d liq wD,IP TF wD,WA PF F packed ṁ F feed column liquid wF,IP
wF,WA
sat’d liquid vapor TB P B B reboiler ṁ bottoms B sat’d liq wB,IP w B,WA
Chemical Components: Assumptions:
Stream F: isopropanol and water 1. continuous system Stream D: more isopropanol and less water 2. steady-state process Stream B: less isopropanol and more water 3. no chemical reactions
Column System Boundary
The material entering the system boundary is a liquid mixture of isopropanol and water at ambient conditions. The material leaving the system boundary is through two process streams—a distillate containing more isopropanol and less water and a bottoms containing less isopropanol and more water compared to the feed stream. The system is all of the liquid and gaseous materials within the system boundary. In this continuous system, the liquids into and out of the system are assumed to flow at a steady rate, and chemical reactions are not occurring.
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Use mass balances instead of mole balances, because fewer equations will be needed to convert volumetric quantities to mass quantities.
Total Mass Balance: { CinChE Eq. (5.1d) } units of g/min
mmmFDB−−=0
where mF= m F,, IP + m F WA
mmD= D,, IP + m D WA
mmB= B,, IP + m B WA
Isopropanol Mass Balance: { CinChE Eq. (5.2d) } units of g/min
mmmF,,, IP−−= D IP B IP 0
where mF,, IP= w F IP m F
mD,, IP= wm D IP D
mB,, IP= wm B IP B
Water Mass Balance: { CinChE Eq. (5.2d) } units of g/min
mmmF,,, WA−−= D WA B WA 0
where mF,, WA= w F WA m F
mD,, WA= wm D WA D
mB,, WA= wm B WA B
EZ Setup Material Balance Model in Excel File “distillation_packed.xls”
Start by writing the total and component mass balances, the equations for the relative imbalances, the composition equations, and the mixture equations, as provided in Worksheet “mBal_mod” or “eBal_mod”. Next, write equations to find the total mass flow rates from the volumetric flow rates for the isopropanol-water feed stream (F), the distillate stream (D), and, the bottoms stream (B). Finally, write equations for possible conversion of units, any physical constants, and the measured quantities.
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Energy Balance Model for Lab P5 Only
See CinChE Manual, Pages 7-7 and 7-5 to 7-6
Energy Balances with Molar Quantities ( heat-of-reaction method )
A. Differential Energy Balance:
ˆ ni nt nr d( nU ) ˆˆ− − ∆ ˆo ±±= sys ∑∑nHii nH ii ∑ R l H rxnl, Q WS (7.5d-a) i ==1i ni+= 11l dt
B. Mixture Enthalpy Equations:
nc ˆˆ ˆ Hi=∑ x ijjii, H[ TPPh,, i] +∆ H mixiii[ TPx,,] (7.7) j =1
ˆˆ=o +∆ ˆ⇐ oo o = Hj H j HTPPh j ii,,,, i TPPh j j j forj1, 2, , nc (7.8)
______
For the distillation column experiment, use the above equations but written in their mass forms, because the material balances were all written in terms of mass quantities. Note that no chemical reactions are occurring and no shaft work exists. ______
Energy Balances with Mass Quantities ( heat-of-reaction method )
A. Differential Energy Balance:
ˆ ni nt nr d( mU ) ˆˆˆ− − ∆o ±±= sys ∑∑mHii mH ii ∑ R l H rxnl, Q WS (7.5d-a) i ==1i ni+= 11l dt
B. Mixture Enthalpy Equations:
nc ˆˆ ˆ Hi=∑ w ijjii, H[ TPPh,, i] +∆ H mixiii[ TPx,,] (7.7) j =1
ˆˆ=o +∆ ˆ⇐ oo o = Hj H j HTPPh j ii,,,, i TPPh j j j forj1, 2, , nc (7.8)
______
For the distillation column experiment, the reference state for each chemical component is chosen to be the ambient temperature and pressure. The heat-of-mixing effect is assumed to be zero. Since Stream F is at ambient conditions, its mass enthalpy is automatically zero. ______
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Use heat-of-reaction form of the energy balance instead of heat-of-formation, because fewer terms will be needed in the mixture enthalpy equations.
Column Conceptual Model: Energy Balance
column system boundary
condenser sat’d sat’d vapor liquid TD P R D D ṁ reflux distillate D sat’d liq sat’d liq wD,IP TF wD,WA PF F packed ṁ F feed column liquid wF,IP
wF,WA
sat’d liquid vapor TB P B B reboiler ṁ bottoms B sat’d liq wB,IP
wB,WA QR
̇ Condenser Conceptual Model: Energy Balance Reboiler Conceptual Model: Energy Balance
system boundary T T sat’d CO CI vapor PCO CO CI PCI cooling water ṁ = ṁ ṁ liquid bottoms CO CI cool cool CI QR tap water out in water ̇ glass wall glass wall
TSO TSI
PSO SO SI PSI sat’d sat’d condensing ṁSO= ṁSI sat’d sat’d ṁSI vapor liquid steam water liquid vapor steam system boundary
Chemical Components: Assumptions:
Stream F: isopropanol and water 1. continuous system Stream D: more isopropanol and less water 2. steady-state process Stream B: less isopropanol and more water 3. no chemical reactions 4. no heat-of-mixing effect 5. is constant for all chemicals 𝑙𝑙𝑙𝑙𝑙𝑙 𝐶𝐶̂𝑃𝑃 Page 8 of 15
Column Energy Balance: { CinChE Eq. (7.5d-a) } units of kJ/min
ˆ ˆˆ mHFF+− Q R mH DD − mH BB −= Q C 0
ˆ but HF = 0 by the reference state for Stream F
Enthalpy of Stream F: { CinChE Eq. (7.7) and Eq. (7.8) } units of kJ/g
Assume no mixing effect and no pressure effect on the mixture enthalpy.
ˆˆ ˆ HF= wH F,, IP F IP+ w F , WA H F , WA ˆˆ=∆← HF,, IP HF IP T F,, P F liq Tref , P ref , liq ˆˆ=∆← HF,, WA HF WA T F,, P F liq Tref , P ref , liq
T ∆=ˆ F ˆliq = = = HF,, IP ∫ cP IP 0 for Tref T F and Pref P F Tref T ∆=ˆ F ˆliq = = = HF,, WA ∫ cP WA 0 for Tref T F and Pref P F Tref
Enthalpy of Stream D: { CinChE Eq. (7.7) and Eq. (7.8) } units of kJ/g
Assume no mixing effect and no pressure effect on the mixture enthalpy.
ˆˆ ˆ HD= wH D,, IP D IP+ w D , WA H D , WA ˆˆ=∆← HD,, IP HD IP T D,, P D liq Tref , P ref , liq ˆˆ=∆← HD,, WA HD WA T D,, P D liq Tref , P ref , liq
T ∆=ˆ D ˆˆliq =liq − ˆliq HD, IP ∫ cP ,, IP c P IP( TT D ref ) assuming a constant cP for isopropanol Tref T ∆=ˆ D ˆˆliq =liq − ˆliq HD, WA ∫ cP ,, WA c P WA( TT D ref ) assuming a constant cP for water Tref
Enthalpy of Stream B: { CinChE Eq. (7.7) and Eq. (7.8) } units of kJ/g
Assume no mixing effect and no pressure effect on the mixture enthalpy.
ˆˆ ˆ HB= wH B,, IP B IP+ w B , WA H B , WA ˆˆ=∆← HB,, IP HB IP T B,, P B liq Tref , P ref , liq ˆˆ=∆← HB,, WA HB WA T B,, P B liq Tref , P ref , liq
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T ∆=ˆ B ˆˆliq =liq − ˆliq HB, IP ∫ cP ,, IP c P IP( TT B ref ) assuming a constant cP for isopropanol Tref T ∆=ˆ B ˆˆliq =liq − ˆliq HB, WA ∫ cP ,, WA c P WA( TT B ref ) assuming a constant cP for water Tref
The condenser is a countercurrent heat exchanger where the vapor mixture from the top of the packed column is physically separated from the cooling water by a glass wall. An energy balance around just the condenser allows for the heat duty to be determined.
𝑸𝑸̇ 𝑪𝑪 Condenser Energy Balance: { CinChE Eq. (7.5d-a) } units of kJ/min
ˆˆ mHCI CI+− Q C m CO H CO =0
ˆ but HCI = 0 by the reference state for Stream CI
Substituting the Enthalpies for Stream CI and CO, gives:
ˆ liq QC= mH CO CO = mc CIˆ P, WA( T CO− T CI )
since mmCO= CI by the total material balance.
Enthalpy of Stream CI: { CinChE Eq. (7.7) and Eq. (7.8) } units of kJ/g
Assume pure water stream and no pressure effect on the mixture enthalpy.
ˆˆ HCI= wH CI,, WA CI WA but wCI, WA = 1.0 for tap water
ˆˆ HCI,, WA =∆ HCI WA[ T CI,, P CI liq ←= TCI ,, P CI liq] 0
T ∆ˆ =CI ˆˆliq =liq −= ˆliq HCI, WA ∫ cP ,, WA c P WA( TT CI CI ) 0 assuming a constant cP for water. TCI
Enthalpy of Stream CO: { CinChE Eq. (7.7) and Eq. (7.8) } units of kJ/g
Assume pure water stream and no pressure effect on the mixture enthalpy.
ˆˆ HCO= wH CO,, WA CO WA but wCO , WA = 1.0 for tap water
ˆˆ HCO,, WA=∆← H CO WA[ T CO, P CO , liq TCI ,, P CI liq]
T ∆=ˆ CO ˆˆliq = liq − ˆliq HCO, WA ∫ cP ,, WA c P WA( TT CO CI ) assuming a constant cP for water. TCI
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The reboiler is a countercurrent heat exchanger where the liquid mixture from the bottom of the packed column is physically separated from the steam by a glass wall. An energy balance around just the reboiler allows for the heat duty to be determined.
𝑸𝑸̇ 𝑹𝑹 Reboiler Energy Balance: { CinChE Eq. (7.5d-a) } units of kJ/min
ˆˆ mHSI SI− m SO H SO −= Q R 0
ˆ but HSI = 0 by the reference state for Stream SI
Substituting the Enthalpies for Stream SI and SO, gives:
ˆˆ QR =−=∆ mHSO SO m SO H vap[ P SI ]
since mmSO= SI by the total material balance.
Enthalpy of Stream SI: { CinChE Eq. (7.7) and Eq. (7.8) } units of kJ/g
Assume a pure steam stream and steam is available as a saturated vapor at PSI.
ˆˆ HSI= wH SI,, WA SI WA but wSI, WA = 1.0 for steam
ˆˆ HSI,, WA =∆ HSI WA[ T SI,, P SI sat vap ←= TSI ,, P SI sat vap] 0
Enthalpy of Stream SO: { CinChE Eq. (7.7) and Eq. (7.8) } units of kJ/g
Assume a pure steam stream and steam condenses to a saturated liquid at PSO =PSI.
ˆˆ HSO= wH SO,, WA SO WA but wSO, WA = 1.0 for steam ˆˆ HSO,, WA =∆← HSO WA[ T SO, P SO , sat liq TSI ,, P SI sat vap] but
ˆˆ HSO,, WA = −∆ HSO WA[ T SI,, P SI sat vap← TSO,, P SO sat liq] or ˆˆ HSO, WA = −∆ HPvap[ SI ] the heat of vaporization at PSI
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Appendix A: Packed Column Startup Procedure
Updated version from Nick Keramidas' Thesis by Diane S. Hall on 10/21/2016
NOTE: When opening gate valves, turn the handle counter-clockwise until fully open and then turn back clockwise ¼ turn to ensure valve does not become locked open.
1. Turn on steam generator. Make sure the main ball valve (red handle) on generator is closed. (A ball valve is closed when the straight handle is perpendicular to the pipe.)
2. Open the following valves: a. V1C, which allows fluid to flow from the column to the Bottoms pump. b. Check to ensure that both the Feed and Bottoms rotometers are fully open. c. V2D, which allows liquid to flow from the Feed tank. d. V2O, which allows fluid to flow to the Feed pump. e. V2I, which allows fluid to be pumped into the Bottoms holding tank. f. V2J, which allows the steam pressure transducer to receive steam. g. V2K, which allows the steam sight gauge to receive steam. h. V2L, which allows cooling water to flow to the condensers. i The condenser cooling water rotometer located on the upper column control panel set to the appropriate flow rate (~4 GPM, but no higher than 4.5 GPM). j. V6A, which allows cooling water to flow through the vapor trap condenser. k. V2R, which allows house air to flow to the air regulator l. The valve that corresponds to the feed location you choose.
3. On the front panel of the column, check to see if the air pressure is between 35 and 40 psi. To change air pressure, pull the air regulator knob out to unlock it, and set it to 40 psig (turn CW).
4. V2C, is used to bypasses the Feed rotometer which is not used for flow control. If you want the Feed flow rate to be seen on the rotometer, V2C must be closed.
5. Check to make sure that the Bottoms tank shut off valve V5B, Bottoms Tank Solenoid Bypass valve V5B, and the Feed Preheat valve V2P are closed.
6. Turn on the Packed Column Power Strip switch located on the left hand wall of the distillation bay. This switch will supply power to: a. The DAQ power strip mounted to the scaffolding left of the Field Point system which is found at the top of the yellow ladder between the two columns. b. The upper column power strip on the floor of the upper catwalk.
7. Check to make sure the red lights on the individual power strips are lit. If you need to, turn on each individual power strip as well.
8. Check to make sure the reflux coil is plugged into the upper power strip.
9. Check to ensure that the reflux control box on the upper column control panel is set to Computer.
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10. Turn on computer and begin running the Packed Column computer interface. a. On the desktop, locate the Packed Column Icon. Double click on the Icon to open the Packed Column computer interface. b. You will be asked for a location and name to save the data as, be sure to save the file in your network space and NOT on the local computer. c. When the interface is open, open the operate menu and click run. d. Check to be sure the reflux and steam controls are set to Manual in the computer interface.
11. In the computer interface click the thermocouples, thermowells, pressure transducers, pumps, and master output switch. a. Click each respective ON/OFF switch so that the light on the button is illuminated green. b. Set the flow rate of the pumps to zero. c. Set the Save Rate (suggested value = 120 sec.).
12. Once the steam generator has reached ~ 30 psig a. Open the main steam generator valve (red handle) which is located on the discharged line (dark red color) of the steam generator. b. Open the orange red valve and the red & white valve located next to the tray column. c. Open V2Q, which supplies steam to the packed column.
13. In the computer interface: a. Turn on the Save switch to begin saving data.
14. The temperature of the reboiler is controlled by changing the steam pressure. The steam pressure is controlled manually using the green handled manual steam valve V2F located on the right arm of the “T” junction which comes off from the main steam line to the Packed Column. a. An individual team member must become the operator of the manual steam valve. It is up to them to manually adjust the steam valve to maintain the reboiler at a safe pressure, (not to exceed 20 psig) and desired operating temperature. b. They must remain in constant monitoring of the manual pressure gauge which reads the pressure of the reboiler. (located near valve V2K). It is their job to make sure the pressure does not exceed 20 psig. c. Keep the reboiler steam pressure below 20 psi for safety.
WARNING: DO NOT LET THE REBOILER STEAM PRESSURE EXCEED 20 PSIG.
WARNING: THE STEAM COILS OF THE REBOILER MUST ALWAYS BE COVERED IN LIQUID. Adjustment of the Feed and Bottoms flow rate may need to be made to maintain a safe working liquid level covering the reboiler steam coils.
15. When running in Total Reflux mode a.. The Feed Pump is turned off. b. The Bottoms Pump is turned off. c. The Reflux ratio is at zero.
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16. To run in Partial Reflux mode a. On the computer interface input the desired flow rate for the Feed Pump and the Bottoms Pump into the appropriate input box by typing in the numerical value followed by pressing the ENTER key. b. Turn on the manual pump switches located on the lower column control panel. Be sure the appropriate valves are open to allow liquid to flow. NEVER RUN THE PUMPS DRY. THE PUMPS MUST HAVE LIQUID FLOWING. AIR IN THE PUMP CAN DESTROY THE PUMP. THE FEED PUMP CAN BE STARVED, GREATING A VACUUM, IF THE VALVES BETWEEN IT AND THE FEED TANK ARE CLOSED. c. Check to make sure you see liquid flowing. d. In the computer interface set the Reflux Ratio to computer control. e. Put in the desired Reflux Ratio in the input box on the computer interface. Most runs will be run with a Reflux Ratio of 1 (one). This will allow the Reflux paddle at the top of the column to be open the same amount of time it will be closed. 1. When the paddle is open, it allows distillate to be taken away from the column and stored in the Distillate storage tank, the clear glass vessel above the stainless steel Bottoms tank. 2. When the paddle is closed, the distillate is allowed to flow back down the column.
17. When taking liquid samples from the packed column liquid sample port, use the following procedure: a. Empty each liquid sample port into a container compatible with the chemical system in use. b. Wait approximately 5-10 minutes for each liquid sample port to refill with liquid. c. This will ensure that the liquid sample taken from each port is representative of the liquid at the location in the column.
18. Cool the liquid sample to room temperature before testing them on the Refractometer using an ice water bath.
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Appendix B: Packed Column Shutdown Procedure
Updated version from Nick Keramidas' Thesis by Diane S. Hall on 1/11/2016
NOTE: When closing gate valves, be sure that the valve is completely closed.
1. If the reflux timer is under computer control, switch it to manual in the computer interface.
2. In the computer interface: a. turn off the master output control switch. b. turn off the Save button to stop saving data.
3. Close the following steam valves: a. V2Q, b. the main steam generator valve. c. If the steam generator will no longer be used, turn it off.
4. Drain the contents of the Bottoms and Distillate tanks into the Feed tank a. Switch the bottoms and distillate tank solenoid valve switches, located on the lower column control panel to Open. b. Use the liquid level sight gauge on the Bottoms tank to ensure it is drained and visually inspect the glass Distillate tank to ensure it is drained as well. c. Be sure to close the solenoid valves when finished.
5. Shut off the air by closing V2R.
6. Turn off the manual pump switches located on the lower column control panel.
7. Shut off flow from the Feed tank by closing V2D.
8. When the upper column has cooled, close the cooling water rotometer, V2L.
9. Stop the computer interface by clicking the red stop sign located at the top of the computer interface.
10. Close the computer interface.
11. IMPORTANT: Turn off the Packed Column Power Strip switch.
12. DO NOT EMPTY THE CONTENTS OF THE REBOILER WHILE IT IS HOT.
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