Your company produces an aqueous solution of 20 mol% ethanol that requires further purification before it can be sold on the market. The company plans to utilize an eight-stage sieve-tray distillation column to further purify the ethanol. The company can sell the distillate if its composition is more than 75 mol% ethanol and plans to reuse the bottoms stream if its composition is less than 10 mol% ethanol. Before operating column, however, the company has asked you characterize experimentally the column performance and utilize Aspen Plus® simulation software to assess the feasibility of the separation and to provide some recommendations. Specifically, the company wants to investigate the effect of reflux ratio on stream purity and ethanol recovery. For a fixed feed flow rate and reboiler duty, you are to recommend an appropriate reflux ratio given the company’s technical constraints and economic considerations.
DISTILLATION OF ETHANOL (DIST) OBJECTIVES Your company produces an aqueous solution of 20 mol% ethanol that requires further purification before it can be sold on the market. The company plans to utilize an eight-stage sieve-tray distillation column to further purify the ethanol. The company can sell the distillate if its composition is more than 75 mol% ethanol and plans to reuse the bottoms stream if its composition is less than 10 mol% ethanol. Before operating column, however, the company has asked you characterize experimentally the column performance and utilize Aspen Plus ® simulation software to assess the feasibility of the separation and to provide some recommendations. Specifically, the company wants to investigate the effect of reflux ratio on stream purity and ethanol recovery. For a fixed feed flow rate and reboiler duty, you are to recommend an appropriate reflux ratio given the company’s technical constraints and economic considerations. The company has requested you complete the following tasks: i. At total reflux, measure the overheads flow rate as a function of reboiler power and determine the column flooding limit. ii. Measure tray, overheads, and reboiler compositions at total reflux and 75 % of flooding, then utilize the McCabe-Thiele method to calculate the overall column efficiency and the individual Murphree tray efficiencies. iii. Characterize experimentally the column performance at a feed rate F = 2 L/hr, reflux ratio R = 5, and 75 % of flooding while feeding to the bottom of the column. iv. Model the column performance with Aspen software at the conditions specified in (iii) and discuss agreement between the experiment and model. DIST-1
Transcript
DISTILLATION OF ETHANOL (DIST)
OBJECTIVES
Your company produces an aqueous solution of 20 mol% ethanol that requires further purification before it can be sold on the market. The company plans to utilize an eight-stage sieve-tray distillation column to further purify the ethanol. The company can sell the distillate if its composition is more than 75 mol% ethanol and plans to reuse the bottoms stream if its composition is less than 10 mol% ethanol. Before operating column, however, the company has asked you characterize experimentally the column performance and utilize Aspen Plus®
simulation software to assess the feasibility of the separation and to provide some recommendations. Specifically, the company wants to investigate the effect of reflux ratio on stream purity and ethanol recovery. For a fixed feed flow rate and reboiler duty, you are to recommend an appropriate reflux ratio given the company’s technical constraints and economic considerations.
The company has requested you complete the following tasks: i. At total reflux, measure the overheads flow rate as a function of reboiler power and
determine the column flooding limit.ii. Measure tray, overheads, and reboiler compositions at total reflux and 75 % of flooding,
then utilize the McCabe-Thiele method to calculate the overall column efficiency and the individual Murphree tray efficiencies.
iii. Characterize experimentally the column performance at a feed rate F = 2 L/hr, reflux ratio R = 5, and 75 % of flooding while feeding to the bottom of the column.
iv. Model the column performance with Aspen software at the conditions specified in (iii) and discuss agreement between the experiment and model.
v. Utilize the Aspen model to recommend the appropriate R to obtain a distillate composition xD ≥ 0.75 and a bottoms composition xB ≤ 0.10 at fixed F and Qreb.
BACKGROUND & THEORY
A significant amount of literature is dedicated to the theoretical treatment of heat and mass transfer, thermodynamics, and hydraulics within distillation columns. These subjects are covered at length in Chapter 21 of Unit Operations of Chemical Engineering by McCabe et al.[1] and in Chapter 7 of Separation Process Principles by Seader et al.[2]. The following section provides a brief overview of column flooding, the McCabe-Thiele graphical method, Murphree tray efficiency, and vapor-liquid equilibrium (VLE) data for ethanol/water mixtures.
Column floodingFigure 1 (from McCabe et al. p 704 [1]) is a schematic cutaway illustrating the normal
operation of a sieve plate. Liquid from the stage above flows through the downcomer onto the stage below. As the liquid flows across the stage towards the overflow weir, it contacts the vapor from the stage below rising through the holes in the sieve plate. The resulting frothy mixture
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Figure 1: Cutaway depicting normal operation of a sieve tray. From McCabe et al. p 704 [1].
creates intimate contact between the liquid and vapor streams (and lots of area for mass transfer) before the liquid flows over the weir and onto the next stage. In an ideal case, the liquid and vapor phases are well mixed on a stage and no concentration gradients exist across the stage.
A pressure gradient between stage n+1 and stage n must exist to drive vapor flow through the perforated tray and through the liquid. Similarly, the liquid flows through the downcomer and across the tray due to a gradient in liquid head. Thus, the vapor velocity must exceed a minimum velocity or else liquid will flow down through the holes and prevent vapor flow through them. In this condition, called “weeping”, the vapor and liquid have limited contact and plate efficiency is decreased. On the other hand, if the vapor velocity is too high, then liquid is entrained in the vapor and transported to the stage above. Additionally, the pressure drop across a stage can be so great that it drives liquid up the downcomer and onto the stage above. In this conditions, generally called “flooding”, the plate efficiency drops and the pressure drop becomes very large. Correlations to predict the flooding limit of sieve-tray columns can be found in McCabe et al. p 705 [1]. “Normal” operation falls between weeping and flooding where the vapor velocity is high enough to create a frothy mixture but not high enough to cause entrainment.
In principle, measurement of column efficiencies and pressure drops can identify the flooding limit. Because the column is glass, you can observe the physical behavior on the trays and confirm flooding visually.
McCabe-Thiele methodBinary-component distillation can be analyzed using the McCabe-Thiele method to
predict theoretical column behavior. In this graphical method, equations of the operation lines in the rectifying section (portion of the column above the feed) and stripping section (portion of the column below the feed) are obtained by performing material balances over a control volume encompassing the relevant column section. Stages are numbered from top to bottom, with n and m denoting the tray number in the rectifying and stripping sections, respectively.
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Assuming constant molal overflow throughout the column, a material balance on the more volatile component (ethanol) in the rectifying section relates the vapor concentration entering stage n from stage n+1, yn+1, to the liquid concentration leaving stage n, xn:
yn+1=R
R+1xn+
xD
R+1 (1)where xD is the mole fraction of ethanol in the distillate, R=L/D is the reflux ratio, L is the
molar flow rate of liquid in the rectifying section, and D is the molar flow rate of distillate.Similarly, a material balance on ethanol in the stripping section relates the vapor
concentration entering stage m, ym+1, to the liquid concentration leaving stage m, xm:
ym+1=V B+1
V B
xm−xB
V B (2)where xD is the mole fraction of ethanol in distillate, VB = Ṽ/B is the boil-up ratio, Ṽ is the
molar flow rate of vapor in the stripping section, and B is the molar flow rate of the bottoms product.
The equation of the feed line is obtained by performing a material balance on the feed plate:
y=− q1−q
x+ 11−q
xF (3)
where y is the vapor ethanol mole fraction, x is the liquid ethanol mole fraction, xF is the mole fraction of ethanol in the feed, and q is moles of liquid flow in the stripping section that result from the introduction of each mole of feed. The value of q depends on the phase and temperature of the feed.
Starting from the condenser (or from reboiler), one can “step stages” graphically and calculate the composition of vapor and liquid on each plate using Eqs. (1)-(3) and equilibrium data. The calculations progress until the concentration of the liquid matches with that of the bottom product. This way one can calculate the number of equilibrium stages needed theoretically (Nth). Comparison of Nth with the actual number of column stages, Nact, needed to effect the same separation defines the overall column efficiency:
ηov=N th
N actual (3)The overall column efficiency, therefore, compares the actual column performance to that of a column where the liquid and vapor leaving each stage are at equilibrium. In practice, this rarely happens, necessitating the use of Murphree tray efficiencies outlined below.
Murphree tray efficiencyThe Murphree tray efficiency, ηM, is defined by
ηM=yn− yn+1
yn¿− yn+1 (4)
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where yn is the composition of vapor leaving stage n, yn+1 is the composition of vapor entering
stage n, and yn¿
is the composition of vapor in equilibrium with the liquid leaving stage n. The Murphree tray efficiency is, therefore, the change in vapor composition on stage n relative to the change that would occur if the liquid and vapor leaving stage n were in equilibrium. This provides a useful measure of how much mass transfer occurs on a stage.
The Murphree tray efficiencies can be used to characterize actual column behavior. In the McCabe-Thiele graphical method, stage stepping can be performed without reaching the equilibrium line. Instead, each step from the operating line only partially approaches the equilibrium line, as described in McCabe et al. p 715. As a result, effecting a particular separation necessitates more stages than if it the column were 100 % efficient.
Although Seader et al. outline some empirical and semi-theoretical models for predicting tray efficiency (ref. [2] pp 297-82), analysis of liquid or vapor samples from each tray during column operation enables direct calculation of tray efficiency according to Eq. (4). In practice, liquid samples are easier to collect and analyze. Calculation of ηM for each stage from Eq. (4) requires a relationship between the vapor compositions, y, and the liquid compositions, x, on each stage. The operating line provides this relationship. Note that the calculation is significantly simpler when the column operates at total reflux.
VLE dataMcCabe-Thiele analysis and Murphree tray efficiencies both require VLE data describing
the distribution of ethanol and water between the liquid and vapor phases at various mixture compositions. Reliable values of y and x for constructing an equilibrium curve are located in Table VI of Cornell et al.[3]. The Aspen software also contains a built-in module for calculating binary VLE data for non-ideal mixtures utilizing various property methods such as UNIQUAC or NRTL [4]. You should know that both of these property methods were developed by Prof. Prausnitz here at U.C. Berkeley in the Chemical Engineering department.
EXPERIMENTAL APPARATUS
The experimental apparatus for the distillation experiment consists of the distillation column with its associated tanks, computer, and control console as well as the gas-chromatography (GC) setup for measuring the composition of ethanol/water samples.
Distillation column and apparatusFigure 2 shows a schematic of the distillation-column apparatus in S1 Gilman. The
equipment comprises a 50-mm diameter sieve-plate column made up of two glass sections and each containing four sieve plates. The column sections are separated by a central feed section and are arranged vertically for counter-current vapor-liquid flow. Also installed within the framework are a reboiler, two 5-liter feed tanks, a peristaltic feed pump, a condenser, bottom and top product receivers, a decanter, and a reflux valve.
The reboiler, situated at the base of the column, contains a flameproof immersion-heating element. The column and the reboiler are insulated to minimize heat losses. In continuous
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Figure 2: Schematic diagram of distillation apparatus. Solid lines correspond to flow
and dashed lines correspond to vacuum. Note that the vacuum lines are not connected in S1 Gilman.
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operation, the valve (V1) from the reboiler is open and bottom product flows from the reboiler through the bottom product cooler to the bottom product receiver. It is possible to preheat thefeed by directing the feed through a heat exchanger where heat is transferred from product leaving the reboiler at the boiling point. A level sensor inside the reboiler protects the heating element from overheating due to low operating level and a sight glass allows the level in the reboiler to be observed.
Vapor from the top of the column passes to a water-cooled coil-in-shell condenser. The condenser incorporates a pressure relief valve to protect the system in the event of a blocked vent and/or cooling-water failure. Cooling water enters the condenser at a regulated rate through a variable area flowmeter (FI1) and the flowrate is controlled by diaphragm valve V5.
Condensate is collected in a glass decanter (phase separator). With valve V10 open, condensate from the condenser passes directly through the decanter to the inlet of the reflux-ratio control valve, which is 3-way solenoid actuated. Depending on the setting of the reflux timers, condensate is directed by the reflux valve either back to the top of the column or to the top product receiver (see reflux ratio control on page 11). When directed to the column, the reflux passes through a U-seal where a valve (V3) can be used for measuring boil-up rate or for draining the U-seal. The contents of the top product tank can be drained into the reboiler for re-use via valve V12.
Temperatures within the system are monitored by fourteen thermocouples located at various positions. Thermocouples T1 to T8 are located in the column and measure the temperature of the liquid on each sieve plate.
The total pressure drop across the column is indicated on a U-tube manometer via taps in the column fitted with isolating valves V6 and V7.
Liquid compositions are obtained by using a gas chromatograph (see pages 14-16). Samples of feed, bottoms, and distillate can be withdrawn from the feed tank, valve V2, and valve V3, respectively. There are 10-mL vials available on the counter north of DIST1 for storing the liquid samples.
The glass column incorporates a total of eight sieve plates in two sections each containing four plates as shown in Figure 3. Each plate contains a central support rod (E) and incorporates a weir (F) and a downcomer (G) to create a liquid seal between successive stages. The U-tube (H) achieves the liquid seal on the final plate in each section.
A pair of spouts is incorporated in the glass-column wall at each sieve plate (see Figure 3). One spout incorporates a temperature sensor to measure the liquid temperature. The second spout incorporates a septum seal that allows a sample of liquid or vapor to be drawn by inserting a hypodermic syringe through the seal. The septum heals when the syringe is withdrawn preventing leakage. If the septum leaks, please notify the GSI immediately.
Feed mixture from either of the feed tanks is pumped by the feed pump to the base, center, or top of the distillation column. Feed-stage selection is achieved by adjusting valves VA, VB, or VC. Note that the bottom feed point directs the feed mixture directly to the reboiler.
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Figure 3: Schematic diagrams of the sieve plates in the column. The leftmost diagram shows an entire section consisting of four plates, the middle diagram depicts liquid and vapor movement through three plates, and the rightmost diagram shows a cutaway of a single stage including the sample ports and thermocouple sensors.
Gas Chromatography Column
Ethanol/water liquid-mixture compositions are determined by using a gas chromatograph (GC). Gas chromatography, illustrated in Figure 4, involves a sample being vaporized and injected onto the head of the chromatographic column. The sample is transported through the column by the flow of an inert, gaseous mobile phase. The column itself contains a liquid stationary phase, which is adsorbed onto the surface of an inert solid. There are two GC units in S1 Gilman to be used while running the DIST1 and DIST2 experiments.
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Figure 4: Schematic diagram of a gas chromatograph.
The carrier gas within the GC must be chemically inert. Commonly used gases include nitrogen, helium, argon, and carbon dioxide. The choice of carrier gas is often dependent upon the type of detector, which is used. In this case, the carrier gas is helium. The carrier-gas system also contains a molecular sieve to remove water and other impurities.
For optimum column efficiency, the sample should not be too large, and should be introduced onto the column as a "plug" of vapor; slow injection of large samples causes band broadening and loss of resolution. The most common injection method is where a micro-syringe is used to inject sample through a rubber septum into a flash vaporizer port at the head of the column. The vaporized sample enters the column oven at a temperature of about 141°C. For these columns, you need only to inject 1-L samples. Too large of a sample will overload the detector and lead to poor results.
As shown schematically in Figure 4, the raw result from the GC is simply two peaks that correspond to ethanol and water. The PEAK software provided on the computers calculates the area under each peak. Through a calibration, the relative peak areas are converted into mole fractions to obtain composition measurements. Figure H-1 in Appendix H shows a calibration relating mole fraction ethanol to peak-area fraction of ethanol for the GC units in S1 Gilman and a sample size of 1 L. This calibration is good for ethanol mole fractions between 0.10 and 0.90. You are to use this calibration to convert your raw area fractions into compositions. This calibration should be confirmed by running at least one known ethanol/water sample (i.e., 25 or 50 mol% ethanol). Detailed operating procedures for the GC column are listed in Appendix H.
SIMULATION
As part of your objectives you are asked to simulate the distillation column with AspenOne® V8.6 (Aspen) process-simulation software. Aspen is capable of solving the necessary coupled mass and energy balances to predict the operation of numerous chemical-engineering unit operations such as flash drums, heat exchangers, reactors, pumps, columns, etc. To simulate distillation of the ethanol/water mixture, Aspen must also make VLE calculations
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utilizing appropriate property methods to calculate the requisite activities and fugacities in the liquid and vapor phases.
In this particular case, simulation of the column with the Radfrac module is sufficient to complete the objectives requested by the company. Note that you do not need to add in extra units for the condenser, reboiler, or feed pump. For step-by-step instructions on constructing an Aspen simulation of the column, see the Aspen tutorial uploaded to the bCourses website. The simulation requires a number of inputs that are either directly measured in the experiment or can be estimated from the experiment. The goal of the simulation is to predict the column outputs (distillate and bottoms flow rate and purity) given the controllable inputs (feed flow rate and conditions, number of stages, reboiler duty, reflux ratio). The only inputs in the Aspen simulation not directly controllable are the Murphree tray efficiencies. However, you estimate these based on column measurements at total reflux.
AspenOne® V8.6 software is available on the computers in the Chevron Computing Facility in Tan Hall. Additionally, discs are available for installation on your personal computer. The discs must remain in lab at all times; ask your GSI for a copy to borrow during lab. Installation instructions are located on the course website.
SAFETY
Review the Material Safety Data Sheets (MSDS) for ethanol and water. Although these chemicals are common, it is NOT advisable to breathe or drink any quantity of the ethanol-water mixture. It is also NOT advisable for the skin to be in contact with the mixture. In the event of a spill or leak, DO NOT attempt to block or collect the holt liquid or vapor. Turn off the power to the heater and immediately notify the GSI.
Although the column is designed to operate at one atmosphere, the student must always be aware of the column pressure. Elevated pressure is an indication of high condenser temperature and is accompanied by substantial venting of ethanol.
There are two primary safety issues while taking liquid samples. The liquid in the column – especially in the reboiler – is HOT. At close to 90 °C, there is a risk of scalding. Students should wear proper PPE (gloves, goggles, long pants, closed-toed shoes) at all times. When taking samples from each stage, the syringe plunger takes some strength to pull back. However, pulling back too hard can result in the plunger coming out of the syringe and scalding liquid spraying out. Additionally, take care not to pierce yourself with the sharp syringe.
PRE-LAB EXERCISES
1. Review the safety information described above. Go online and try to find some resources about typical scalding times for various temperatures. Can the reboiler liquid scald you?
2. Construct an equilibrium y-x curve for ethanol/water mixture using either Aspen software and an appropriate property method or the data given in ref. [3]. How do the various methods compare?
3. Identify the azeotrope in your y-x diagram. What is the maximum composition of ethanol
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theoretically obtainable in the distillate stream?
4. Why must the standardized samples of ethanol/water mixtures located with the GC units not be opened for long periods of time?
5. Identify which valves need to be open/closed to operate at (i) total reflux and (ii) continuous operation with feed to the bottom position.
6. Assume that the distillation column is initially operating at steady state. If you increase the feed flow rate and all other valve settings remain the same, what will happen to the temperature and composition of the bottoms? What change(s) must be made to return to the original bottoms composition and steady-state operation at this higher feed flow rate? The candidates for change include power to the heater and the flow rates of bottoms, distillate, or reflux.
7. Assume that the distillation column is initially operating at steady state. If you then increase the power to the heater and all other valve settings remain the same, what happens to the distillate composition? Are there other important changes to the operating condition? What change(s) must be made to return to the original distillate composition and a steady-state operation at this higher power? The candidates for change include flow rates of the bottoms, distillate, reflux, and the feed.
8. Assume that the distillation column is initially operating at steady state. If you decrease the reflux flow rate and increase the distillate flow rate by the same amount, so that the accumulator level remains constant, and all other valves settings remain the same, what happens to the distillate composition? What change(s) must be made to return to the original distillate composition and a steady-state operation at these new flow rates? The candidates for change include power to the heater and the flow rates of bottoms or feed.
PROCEDURE
A. Column startup
During startup, your goal is to begin column operation at total reflux in a safe, controlled manner. Before beginning this step, be sure that you are familiar with the valve locations and overall column setup.
1. Make sure you have at least one full feed tank with sufficient ethanol/water mixture. The feed-mixture composition should be 20 mol% ethanol. Verify the composition using gas chromatography.
2. Check that all the proper valves are open/closed to operate at total reflux. The feed valves VA-C should be closed, the bottoms valve V1 should be closed, etc.
3. Turn on the computer software and power up the control console and establish communication between the two. The passwords for the computers are listed in Appendix F.
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4. Ensure the reflux-ratio controller is set to total reflux (i.e., power is off).
5. Open valve V5 to set the flow rate of cooling water to the condenser to about 3 L/min. Important: Periodically check the rotameter to confirm the flow rate has not dropped significantly
6. Turn on the reboiler power to 1.2 kW (dial reading ~6.0). Once froth and bubbling are visible on trays, turn down the reboiler power to 0.75 kW (dial reading ~3.3).
7. Track the process temperatures during startup to determine when the column has reached thermal steady state. This typically takes at least 30 min. When you are confident of steady operation, move on to the determination of the column flooding limit.
8. At the end of startup each lab period, draw liquid samples from the reboiler and measure its composition with the GC to ensure composition is as expected.
B. Determination of column flooding limit
The objective of this portion is to determine the flooding limit of the column at total
reflux by varying reboiler power Q̇R
and, therefore, boilup rate V̄
, and measuring column pressure drop ΔP, overheads flow rate L, and tray hydraulics.
1. With column operating at steady state and total reflux (likely from Part A), record the reboiler power and begin performing measurements outlined below.
2. Note the hydraulic behavior (degree of foaming) on the stages with following descriptors: ‘none’, ‘gentle localized’, ‘violent localized’, ‘foaming gently over whole tray’, ‘foaming violently over whole tray’, or ‘liquid flooding in column’. Note it is possible that not all trays will exhibit the same behavior.
3. Measure the column pressure drop with the manometer. If the tray observations indicate liquid flooding, do NOT open the manometer. Zero manometer before each reading. To take reading, open V6 then V7. To isolate manometer, close V7 then V6.
4. Measure overheads flow rate L by a timed-volume collection with a 100-mL graduated cylinder. Open diversion valve V3 and close the green valve to the column to begin flow to the graduated cylinder. Collect enough volume to ensure accurate volume measurement, but do not divert the overheads for too long because it disturbs column operation. Typically 1-2 min is sufficient.
5. By observing the tray temperatures, allow the column to return to thermal steady state before repeating Steps 2-5 again to obtain duplicate measurements at each reboiler power.
6. Repeat Steps 2-6 for varying reboiler powers between ~0.4-1.0 kW. Allow adequate time for the column to reach each new steady state (usually >15 min). Once flooding is observed, only take one measurement and return the reboiler power to 0.75 kW or whichever power you had planned next. If the column operates at flooding conditions for too long, it will take a long time to recover for the next steady state.
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C. Measurement of efficiencies at 75% flooding
To calculate the overall column efficiency, ηov, and individual-tray Murphree vapor efficiencies, ηM, at 75% of flooding, you must measure the composition of liquid samples from each tray, the overheads, and bottoms during total-reflux operation. Tray temperatures should also be recorded.
1. Begin (or continue) column operation at total reflux with the reboiler duty determined in Part B to be 75% of the flooding limit (likely near 0.75 kW). In either case, wait the appropriate amount of time for the column to reach thermal steady state, as indicated by the tray temperatures.
2. Utilize the data-acquisition software in Table mode to record all thermocouple temperatures. This enables you to later collect a temperature profile in the column.
3. Dispense and collect a liquid sample from the bottom of the reboiler using valve V2. Careful – the liquid is HOT! Samples should be collected into the small glass vials provided on the counter to the north of DIST1. Save and label the sample for GC analysis.
4. Beginning at Tray 8 and sequentially ascending the column, collect a liquid sample from each tray’s sample port (see Fig. 3) utilizing the long-needled syringe provided near the vials. DO NOT use the microliter syringes located at the GC units. When possible, guide the syringe tip to the liquid by looking through the glass wall to locate where the tip is on the tray. Save and label each sample for GC analysis.
5. Divert the overheads product to collection vial using valve V3 and the green valve as in Step 4 of Part B. You need only collect enough for a sample; you do not need to perform a flow-rate measurement.
6. After column returns to thermal steady state, repeat Steps 2-5 to obtain duplicate measurements.
D. Steady-state column operation
The objective is to operate the column in continuous mode producing distillate and bottoms products at a fixed reboiler power (likely 0.75 kW) and a feed rate of 2 L/hr. You measure experimentally xD, xB, and L; D and B are calculated from these results.
1. Begin (or continue) column operation at total reflux with the reboiler duty determined in Part B to be 75% of the flooding limit (probably 0.75 kW). In either case, wait the appropriate amount of time for the column to reach thermal steady state, as indicated by the tray temperatures.
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2. Open the top of the peristaltic pump, place the plastic tubing inside it, and close it. Ask you GSI if you have questions. Next, open the bottoms feed valve, VC, and turn the feed pump on at 2 L/hr, commencing feed to the column. Utilize the calibration curve in Figure G-1 to choose the appropriate dial reading for the feed pump. Inspect the feed line to confirm that liquid is indeed flowing to the column. Quickly go to Step 3.
3. On the control console, change the reflux ratio to 5:1, initiating flow of distillate into the top-product receiver. See Figure A-2 and Appendix B for instructions on setting the reflux timer on the console.
4. Note the height of liquid in the level indicator on the front of the reboiler. Open gradually the needle valve V1 at the reboiler exit and attempt to use V1 to attain a steady liquid level in the reboiler. Frequent level checking and valve fine tuning are necessary during the entire operation. Ideally, one teammate would serve as a dedicated reboiler-level controller.
5. Observe the tray temperatures and reboiler liquid level to judge when the column has reached steady state.
6. At steady state, dispense and collect a liquid sample from the bottom of the reboiler for GC analysis.
7. Utilize either the data-acquisition software or pen and paper to record all tray temperatures. This will be useful in analyzing and checking your data.
8. Measure the overheads flow rate L by a timed-volume collection. Open diversion valve V3 and close the green valve to the column. Transfer a small amount of sampled liquid to a vial for GC analysis of the distillate composition. Again, do not collect overheads for longer than 1-2 min to avoid significant disruption to the steady state.
ANALYSIS & DESIGN
The following calculations and exercises form an outline of the analysis to be completed on the experimental and simulation results. You can and should go beyond these calculations to answer other questions relevant to the objectives. Keep in mind you may not be able to include all of these results in the oral report. Select those results that you feel are most important to tell the story.
1. Plot the measured overheads flow rate, L, as a function of reboiler power. Does the trend follow your expectations qualitatively and quantitatively? What are some possible reasons for disagreement?
2. Construct a McCabe-Thiele diagram for your column operation at total reflux and step off stages between the measured bottoms and distillate compositions assuming 100 % efficiency. Given the actual stages in the column, what is your column’s overall efficiency?
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3. Next, utilize the tray samples collected in Part C to construct a McCabe-Thiele diagram describing the actual column performance. What are the Murphree tray efficiencies for each of your trays? How do these tray efficiencies compare to the overall column efficiency?
4. Perform a mass balance around the distillation column with the data from Part D. Be sure to take into account the error in each measurement. You have enough data for redundancy. That is, you have measured more flow rates and compositions than necessary. It is possible that both the component and overall balances agree, but it is unlikely. Why not? Which measurements have the most uncertainty?
5. Simulate in Aspen the column behavior under continuous operation at the conditions listed in the procedure. Follow the instructions in the Aspen tutorial to initialize and run a simulation. The goal here is to compare the Aspen-predicted column flow rates, compositions, temperatures, etc. to those measured in the experiment. Which values agree best? Which agree the worst? Does changing any inputs to the Aspen simulation result in better agreement between experiment and simulation?
6. Next, utilize the “sensitivity analysis” function in the Aspen simulation to predict the behavior of the column at varying reflux ratio with all else held constant. From the simulation data, identify the effect of reflux ratio on the column performance, specifically, stream compositions and the ethanol recovery. Reason physically why the trends look the way they do.
7. Consider how you would have completed the same analyses from Questions 5 and 6 using the McCabe-Thiele graphical method. What steps would you have to take? Would multiple iterations be necessary?
8. Based on the trends elucidated in Question 6, recommend an ideal reflux ratio for column operation that enables the company to meet their target composition specifications while maximizing the ethanol recovery. Identify tradeoffs associated with changing reflux ratio.
REFERENCES
1. McCabe W.L, Smith J.C., Harriott P. Unit Operations of Chemical Engineering, 7th ed.; McGraw-Hill: New York, 2005.
2. Seader J.D., Henley E.J., Roper D.K. Separation Process Principles, 3rd ed.; Wiley: New York, 2011.
3. Cornell L.W., Montonna R.E. Ind. Eng. Chem. 1933, 25, 1331.4. Smith J.M., Van Ness H.C., Abbott M.M. Introduction to Chemical Engineering
Thermodynamics, 7th ed.; McGraw-Hill: New York, 2005; pp 447-9
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APPENDICES
Appendix A: Control ConsolesA control console is attached to the process unit by an umbilical cable. The individual
sections of the console with descriptions are shown in Figures A-1, A-2, A-3, and A-4.
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Figure A-1: Distillation-column main console. The various control panels within the main console control the reflux timer (Fig. A-2), the reboiler heater (Fig. A-3), the feed pump (Fig. A-3), and column/process temperature readings (Fig. A-4).
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Figure A-2: Power and reflux-ratio control panels.
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Figure A-3: Reboiler-heater and feed-pump control consoles.
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Figure A-4: Process and column temperature control consoles.
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Appendix B: Reflux-Ratio Control The bottom of Figure A-2 shows the reflux-ratio timer on the control console. This sets
the amount and frequency of condensate flow returning to the distillation column. With the timer switched off, all condensate is directed to the column (total reflux).
For example, if the reflux-ratio required is 2:1 and the total cycle time required is 21 seconds (arbitrary choice), then the reflux-ratio valve to the column directs condensate to the column for 14 s then to the top-product receiver for 7 s. This cycle is repeated continuously until different values are inserted to the controller or until the reflux control is switched off.
The time range and mode of the reflux-ratio timer can only be set when the electrical supply to the timer is switched off. Switch off the reflux-control switch on the control console (LED not illuminated). This switches off the power supply to the reflux-ratio timer. The reflux-ratio timer is now not controlling reflux flow (flow is continuous to the top of the column). Because the controller has an internal battery, the display is still illuminated, and the controller settings may be adjusted.
1. Press the SET button on the reflux-ratio timer. The timer should be set to Cycle mode (CY). If CY is not already set, use the button to cycle through the modes until CY is displayed. (Not ICY).
2. Press the SET key again. 3. Select the time range required for CY+ (time to the receiver vessel: distillate). 4. Press the SET key again. 5. Repeat the time range setting for CY- (time to the column: reflux). 6. Press SET to end time adjustment. To begin controller operation, switch on power to the
reflux-ratio timer using the reflux-control on/off switch.
Appendix C: Measuring Column Pressure Drop
The overall pressure drop over the column can be measured using the manometer. Before taking a measurement, make sure to zero the manometer by relieving any pressure with the pipette bulbs. Open V6 and V7 sequentially and take the pressure reading. When the reading is complete, close both valves (first V7 then V6). This procedure reduces the risk of contamination of the manometer water by ethanol. Also to prevent contamination, never open valves V7or V6 when flooding is occurring on the sieve plates (boil-up rate too high).
Appendix D: Collecting Liquid Samples
Samples for analysis can be taken from pertinent points in the system as follows: Feed liquid – from feed tank Liquid in reboiler – V2 (WARNING! Liquid at boiling point!) Condensate, reflux or top product – open V3
Samples can also be withdrawn from any of the sieve plates in the column by inserting the syringe with the long, metal tip through the septum at the end of the appropriate glass spout
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(see Figure 3). The septum heals when the syringe is withdrawn, preventing leakage. Handle the syringe with care because the column and the liquid samples are hot. Use a gas chromatograph and the calibration curve in Figure H-1 to determine the ethanol/water mixture concentration.
Appendix E: Temperature Measurement
As shown in Figure 2, there are fourteen thermocouples located throughout the equipment, designated as follows:
T1 = 1st (top) tray of distillation column T2 = 2nd tray “T3 = 3rd tray “T4 = 4th tray “T5 = 5th tray “T6 = 6th tray “T7 = 7th tray “T8 = 8th tray “T9 = Temp of liquid in reboiler T10 = Temp of vapor leaving the column above tray 1 T11 = Temp of cooling water entering condenser T12 = Temp of cooling water leaving condenser T13 = Temp of condensate and reflux/top product T14 = Temp of feed liquid from feed tank
Temperatures may be viewed on the display on the right panel of the control console (Fig. A-4). To display any temperature from T1 to T8, set the upper selector dial to the corresponding station designation. To display temperatures T9 to T14, turn the upper selector dial fully clockwise (to the furthest right setting) and then set the lower selector dial to the station designation required. The distillation-unit software described in Appendix F enables real-time visualization with a table and/or graph of any of the process temperatures. These data can also be exported to Excel.
Appendix F: Distillation-Unit Software
To run the software, first log onto the computer:DIST 1 DIST 2
(“Distillation” with all letter “i” replaced by number “1”)Next, open the Distillation Column icon on the desktop. The initial screen will load
displaying a mimic diagram of the distillation unit shown in Figure F-1. The toolbar at the top of the screen contains four buttons, which are used to navigate the software:
View Diagram – displays a mimic diagram of the apparatus, with sensor readings displayed in real time. Data values can be recorded by clicking the ‘GO’ button.
View Graph – displays a graph of selected recorded values. View Table – displays a table of recorded data.
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View Presentation – displays the presentation screens. Before operating the software, ensure that IFD: vCOM(3) m SE is displayed at the
bottom of the screen. If IFD: ERROR is displayed click red phone icon to start COM session, select port COM3, and click OK. Also check the USB connection between the IFD5 and the PC on the backside of the control console and confirm that the red and green LEDs are both illuminated. Save your data using a USB flash drive.
Figure F-1 shows the Mimic Diagram, which gives a pictorial representation of the equipment, with continuously updated display boxes for all the various sensor readings, calculated variables etc. directly in engineering units. To view the Mimic Diagram click the View Diagram icon from the main tool bar. Within the Graph and Table windows, you can record and plot thermocouple and other process data at specified intervals. The data can later be exported to Excel format for analysis.
Figure F-1: A screenshot of the Mimic Diagram of the distillation unit.
Appendix G: Feed-Pump Calibration
The peristaltic feed pump shown in Figure 2 delivers ethanol/water solution from the feed tanks to one of the three feed points of the column. Adjustment of the the dial on the feed-pump control console (illustrated in Figure A-3) controls the pump speed and, therefore, feed flow rate. The dial has 10 full turns; each full turn is marked in one-hundredth segments. Figure G-1 shows a calibration curve relating the feed flow rate [L/hr] to the dial reading on the feed-pump control console for Pumps 1 and 2 corresponding to DIST1 and DIST2, respectively. The calibration was produced by measuring the flow rate at 6-7 different dial readings between 0.5 and 3. Each point shown is the average of triplicate measurements at each reading. Error bars indicating the range of the three measurements are barely visible.
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Figure G-1: Volumetric feed flow rate [L/hr] versus dial reading for pumps 1 (red squares, red solid line) and 2 (blue circles, blue dotted line), corresponding to DIST 1 and 2, respectively. The markers denote the average of three measurements, and the error bars show the range, which is always <4% of the reading. The lines are linear-regression best-fit lines; the fit equations appear in the legend at the top-left corner.
Appendix H: Operation of the Gas Chromatograph
1. The GC is always in a hot stand-by mode (if not, ask the GSI to turn it on).
2. Turn on the helium carrier pressure to 30 psi. The needle valve for the carrier gas is located at the pressure regulator on the helium cylinder. Check that the green light for carrier gas is on (on the front panel of the GC cabinet).
3. When carrier-gas flow is established, lift the red cover on the GC and flip the detector switch to LOW current from its upright OFF position.
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Figure H-1: Calibration curve for ethanol/water mixture relating mole fraction of ethanol to peak area fraction of ethanol. Each point is the average of three measurements. The largest range of EtOH mole fraction for each peak area fraction is 0.003.
4. Turn on the computer attached to each GC unit using the following username and password:
Username: DistillationPassword: D1st1llat1on
5. Start the PEAK program from the icon on the desktop. Wait 30 min after initiation of PEAK software before injecting first sample. Do not inject sample unless the temperature on the front console of GC cabinet should read ~141 °C.
6. Prime your microliter syringe with the sample to be tested at least 3-5x before testing each sample. This prevents cross contamination of the syringe between samples of different composition.
7. Inject 1 μL of sample using the microliter syringe inserted into the sample-injection port and click the space bar or icon on the software to begin the sample analysis. Zero the sensor by clicking the “0” icon to the left of the graph.
8. Once separate outlet peaks are established, find the area under the peaks by clicking “View” “Results” from the top menu, calculate the ethanol fractional area, and use the calibration curve/polynomial fit in Figure H-1 to find a corresponding ethanol mole fraction.
9. Shut down the GC by turning the detector to the OFF position and exiting the PEAK software completely. Close the valve to the helium cylinder.
