Binary Distillation of Ethanol and Water:Improvements to Steady-State Operation and Measurement

October 15, 1999

Group 3

Abstract

Due to problems in past years with the operating controls of the Rothfus Lab distillation column, thefocus of this study is to try and correct some of the issues associated with flowrates, Labview and trayconcentration measurement methods. In order to evaluate the condition of the distillation column atsteady state, accurate measurements of feed, distillate, reflux, and bottoms flowrates need to be easilyobtained. Improvements to the operation of the column include implementing control of parameters suchas the feed and reflux flow through rotameters, which are manipulated to achieve a state of equilibrium inthe column. A volumetric calibration of the boiler was made as well as calibration curves for eachrotameter to verify flowrates. A control system was implemented for bottoms, feed, and refluxtemperature. Step response data was taken and PID values were determined from Cohen-Coon tuningalgorithms; these were programmed into Labview software. Due to software problems, only theproportional action of the controllers was functional. The boiler and reflux controls were effective;however, the feed control was extremely poor. Two methods were used to estimate column traycompositions: refractometry and T-xy diagram reading. A refractometer was used to examine therefractive index of samples taken from the distillation column trays at steady state, and thus manuallydetermine its composition. A T-xy diagram was used to correlate tray temperatures output Labview andideal temperatures calculated from Chemsep to real and theoretical tray compositions. There wasconsiderable discrepancy between the compositions determined by the refractometer and T-xy diagram.Based on previous studies, it may be suggested that the T-xy diagram is the better method to use indetermining tray compositions.

Perhaps you should mention if an azeotrope was present

Introduction

The purpose of this lab was to operate an eight stage binary distillation column of ethanol and water atsteady state. A number of issues are important in running a distillation column. Feed, distillate, reflux,and bottoms flowrates are important to know for characterizing holdup, adjusting heaters, and measuringoutput at different conditions. Composition of products and in each stage is similarly important,especially in determining column effectiveness. And finally, methods for automatically controlling thesteady state operation of the column are of great interest since they relieve much of the burden of manualadjustment. We improved the column setup by focusing on each of these three issues. We calibrated thereflux and feed rotameters, found temperatures for steady state operation, and implemented temperaturecontrol mechanisms. We also measured concentrations on each tray with a refractometer for comparisonwith values from a theoretical Tx-y diagram. Our system of ethanol-water was safe for laboratory use.The autoignition temperature of ethanol is 363 ºC; however, the column did not achieve temperatureshigh enough to cause self-combustion during our use of it. It should be noted that while watersignificantly decreases ethanol’s flammability, care should be taken to keep air out of the column sinceethanol is flammable at 13 ºC. We were careful not to introduce excess air to the column or expose it toany flame sources.

Theory

Binary DistillationBinary distillation is a unit operation commonly used to separate two-component mixtures. The basis forseparation is the difference in volatility of the two components. If the mixture is brought to a temperaturebetween the boiling points of the two components, two phases will be produced. Most of the morevolatile component will separate into the vapor phase while its less volatile counterpart will bepredominant in the liquid phase.

A Tx-y diagram illustrates the relationship between temperature and the corresponding vapor and liquidcompositions of the mixture at a given pressure. The relationship of water and ethanol in vapor-liquidequilibrium at 1 atm can be seen in Figure 1.

Tem

pera

ture

, F

196.00

204.00

212.00

T-X-Y Plot for ETHANOL and H2O

B Bubble Point

B

B

B

B

D Dew Point

D

D

D

D

An azeotrope occurs when the vapor and liquid phases of a mixture have the same composition. As canbe observed in Figure 1, an azeotrope exists at T = 172.5 ºF. At this point, further separation of themixture with distillation is impossible because the vapor and liquid both have a composition of 80 mol%ethanol and there is no driving force for mass transfer.

Relative volatility is used to characterize the tendency of one component to go into the vapor phase ascompared to another. It describes the degree of ease in separating the mixture; the greater the relativevolatility, the easier the separation. It is found by:

a ij = VPi / VPj (1)

where VPi and VPj are the vapor pressures of the two components i and j at a given temperature.

Distillation ColumnA distillation column is a countercurrent series of flash stages in a stack of several sieve-like trays. Areboiler at the bottom of the column operates at a temperature high enough to evaporate part of thebottoms back to the column. At the top is a condenser that cools the vapor stream to saturated liquid. Thecondensate is collected in a reservoir, some of which is refluxed back into the column. The amount ofcondensate sent back to the column is given by the reflux ratio:

R = L / D (2)

where L is the amount of liquid returned to the column and D is the amount of overhead distillate beingremoved. The vapor from the reboiler rises through the trays and mixes with the liquid coming down thecolumn from the condenser and the feed. Each of the individual trays acts like an equilibrium flash.Vapor and liquid of different compositions enter, mix, and reach equilibrium; the mixture then separatesinto two phases. The values of the exiting liquid and vapor compositions are predicted by the T-xydiagram, as mentioned earlier.

In reality, the equilibrium assumption does not always hold. The level of deviation from equilibrium isdescribed by the Murphee tray efficiency, which is defined as:

? = (yn – yn+1) / (yn* - yn+1) (3)

where yn = actual vapor composition leaving stage n, yn+1 = actual vapor composition leaving stage n+1(i.e. entering stage n), and yn* = the vapor composition in equilibrium with xn (actual liquid compositionleaving stage n).

Higher in the column, the vapor stream becomes richer in the more volatile component and leaner in theless volatile component. Descending through the column the reverse is true as the liquid stream is strippedof the more volatile component. The more volatile component tends to the top while the less volatile

RefractometerEach material has a specific ability to refract light. A measure called the refraction index (RI) is used todescribe the degree to which a particular substance bends light. In a two-component mixture, therefraction index generally lies somewhere between the refractive indices of each of the two components.For instance, in our water-ethanol system (where ethanol has a higher RI than water does), the refractiveindex increases as ethanol concentration rises.

By measuring the RI for several mixtures of known composition, one can prepare a calibration plot of RIversus composition and use the plot to determine the composition of an unknown binary mixture withexperimentally determined RI.

Equipment & Procedure

The Hampden model fractional distillation column has seven trays plus a partial reboiler (Figure H.1).The column also utilizes a total condenser, a feed preheater, and feed and reflux rotameters. The feedenters the column at tray number 3. Utilizing the LabVIEW software, control of the heaters and pumps inthe column can be achieved, and temperature data for each tray can be recorded.

Steady State OperationBefore the overall column data was taken and analyzed, the column was brought to steady state. A feedcomposition near 50 mole % ethanol was fed to the third tray. To bring the column to steady state, theboiler was first set to 0.015 amps filled with approximately 3 liters of water. As the bottoms temperatureneared 180° F, the boiler was lowered to 0.008 amps. The feed flow was subsequently turned to a lowrate of approximately 40 mL/min, and the feed preheater was also turned to 0.012 amps. Once the feedtemperature leveled off at 180° F, a flow of 20 mL/min was refluxed, and the reflux heater was turned to0.007 amps. These settings were maintained; this sequence allowed the column to reach steady state inapproximately 25 minutes.

Main Feed Rotameter CalibrationIn order to calibrate the feed rotameter used in this experiment, the inlet feed pipe was disconnected fromthe column and fitted with tubing to direct the flow. For this calibration, tap water was used (density ofethanol/water mixture and water not significantly different). After turning on the feed pump, therotameter was manually set and maintained at a specific reading. Meanwhile, the actual flowrate wasmeasured using a stopwatch and graduated cylinder. This procedure was performed at incrementalrotameter readings of 20 to 70. The following data was collected: rotameter reading, ml water drained,and time required. For this calibration, two trials were performed to produce the curve. To create thecalibration curve, volumetric flowrate (ml/s) is plotted against rotameter reading.

Reflux Rotameter CalibrationA similar procedure from above was followed for the calibration of the reflux rotameter. Since thedistillate chamber was already calibrated with volume markings along its axis, these markings were used

graduated cylinder and marked. With the column in steady state, these markings can then be used tomeasure the bottoms stream over time.

Control ImplementationThe column utilized three sources for heating streams: reboiler, feed preheater, and reflux heater. Thepower rating for each of these was initially adjusted manually within LabView by controlling theamperage to each. In order to better maintain steady state conditions, we developed a prototype controlsystem for boiler, feed, and reflux heaters.

Step response tests were performed on both the boiler heater and feed preheater. The column conditionwas 40 mL/s feed flow rate with about 3 liters of solution in the boiler and a 0.5 reflux ratio. We firstwaited for steady state with the boiler at 0.008 amps, feed preheater at 0.012 amps, and reflux heater at0.007 amps. We then incremented boiler power to 0.015 amps and noted the response in bottomstemperature over a period of about 7.5 minutes. We reset the boiler to 0.008 amps, set the feed preheaterto 0.010 amps, and waited for steady state. We subsequently increased the feed preheater to 0.015 ampsand noted the response in feed temperature for about 5 minutes.

Control implementation was performed within LabView software. Two VIs were modified: distillation.viand heaters.vi. To the former, we implemented an interface for each of the three heaters for switchingcontrol on/off and entering PID values. To the latter, we added PID subroutines to each heater outputloop which could be turned on or off. The subroutines used the controlled temperature as process variableand output a heater power value (amperage) in the range 0.000-0.016 amps. A value of 0.004 was addedto this output since the zero point of the heaters was at 0.004; the maximum heater power that the PIDcould specify was thus 0.020 amps. Setpoint range limits were also incorporated into the PIDsubroutines. These modifications were saved as two new files: distillation v2.vi and heaters v2.vi with thev2 to indicate the second version. As per the proposal, the labels for the output file were identified andare reported in Appendix H. The time increment problem in these programs was also corrected.

With the control programming in LabView, we attempted to bring the column to steady-state a number oftimes. The procedure was the same for each time: we first set the boiler temperature to 193 ºF undercontrol and waited until it began to approach the setpoint. At that point, we turned on the feed flow andset the feed temperature control to 166 ºF. When a steady distillate flowrate was achieved, reflux andreflux temperature control was initiated at 165 ºF. The temperature of the reflux was set slightly belowthat of the top column to achieve a saturated liquid.

RefractometerThe concentration on each tray can be measured using an Atago Hand Refractometer (see Figure H.2),which provides a measurement of the indices of refraction through three adjustable scale readings whichrange from 1.333 and 1.520. The Waters model gas chromatograph could also be used to measure thetray concentrations, but it was inoperable due to technical limitations.

Prior to performing any solution test with the refractometer, it must first be calibrated with pure water at

Results and Discussion

Flow CalibrationPrior to our proposal and in past labs, the feed flowrate was provided through Labview. The values giventhrough Labview compared with the actual feed observed during operation provided evidence of theinaccuracy of those measurements. The flow sensors used in correlation with Labview were not designedto measure flowrate with the magnitude as small as that used during distillation of an ethanol/watermixture. Initially, the flowrate was controlled by a relief valve located in the back of the distillationsetup. The orientation of the valve handle was noted and marked; in this position, steady state wasachieved. Due to the crudeness of this method and absence of functioning equipment, the current solutionto control and measure the feed flowrate was to install the rotameter already mounted to the column unit.A calibration curve specific to this rotameter for the feed is provided below.

Figure 2: Calibration Curve for Feed Rotameter. The fit of the data to the curve is good (R2=0.99). One datapoint is set to (0,0) such that for a rotameter reading of zero, there should be no flow.

Although conversions are provided with the equipment to adjust rotameter readings to specified units(mL/min), these additional calibration tests were performed to validate the given conversions. Thecomparison between experimental values and those conversions provided with the equipment is shown in

Main Feed Rotameter CalibrationCombined Trials

y = 1.3046xR2 = 0.9894

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0 1 2 3 4 5 6 7 8

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Reflux Rotameter CalibrationCombined Trials

y = 0.0052xR2 = 0.9617

y = 0.0061xR2 = 0.9946

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Equipment Specs

Linear(Experimental)

Linear(Equipment

Figure 3: Calibration Curve for Reflux Rotameter (Experimental and Equipment Specs). A slight deviationexists between the measured volumetric flowrate and the given conversions. Both curves are anchored at (0,0).

The divergence of the experimental calibration curve to the given one may be due to possible wear of theequipment. During the collection of data for the calibration, a stream of fluid was observed coming fromthe base of the rotameter. This leak could have easily caused a decrease in the measured volumetricflowrate, which is the discrepancy that has surfaced in figure 5.

Another problem confronted while trying to measure flowrate was a significant leak at the base of thecolumn between the bottom tray and the reboiler. The loose fitting between tray 7 and the reboiler allowsan excessive amount of fluid from the column to escape. Therefore, measurement of the bottoms flowratebecomes difficult and unreliable.

In an attempt to collect some data to assess the condition of the column (despite all the problems), thecolumn was run and brought to “steady state,” where conditions including flowrate were recorded.Equilibrium within the column was reached about 19 minutes following startup. At steady state and totalreflux, the feed was set at 2.61 ml/s (rotameter reading of 20), and the reflux at 0.21 ml/s (rotameter of40), according to the calibration graphs presented above. The resulting bottoms flowrate was measured tobe about 0.17 ml/s. Data at steady state, including temperature profiles, is presented in Appendix B. The

Control ImplementationThe results of the step response tests for the boiler and feed heater (Appendix C) were used to determinefirst-order parameters, which are listed in Table 1. Cohen-Coon tuning relations (Appendix E) were usedto determine PID values for these heaters. For the reflux heater, time considerations prevented a stepresponse test; however, PID values were approximated from those for the feed heater. Because the refluxheater is physically similar to the feed heater yet its flowrate is less, these PID values were made slightlyless than that for the feed heater. All of the PID values are listed in Table 2.

Table 1: First-order reaction curve parameters.time constant τ (sec) gain Kp (°F/amp) delay θ (sec)

Boiler step response 377 1600 27Feed preheater step response 62 1930 3

Table 2: PID parameters by the Cohen-Coon method.Kc (F/amp) τI (sec) τD (sec)

Boiler heater 0.012 64.5 9.7Feed heater 0.014 7.2 1.1Reflux heater (approximated*) 0.01 6.0 1.0*Reflux heater values not taken from step response data.

In the actual testing of the PID implementation, we found that the integral and derivative action inLabView would not work. This problem was the focus of much attention during our runs of the column.Its root, a relatively minor but obscure quirk of LabView, was only recently discovered and corrected.However, our steady-state data reflects that of pure proportional controllers. As a result, there are steady-state errors in each control loop as well as some low decay rates.

The boiler control was the most effective (Figures D.1 and D.2). The time to steady-state was short (6.7minutes), the fluctuation at steady state of the boiler temperature was very low (±0.1 ºF), and the gainerror was only 0.6 ºF. The feed control was ineffective with a proportional controller (Figures D.3 andD.4). The gain was too high and a large cyclic feed temperature pattern resulted (±25 ºF). When the gainwas decreased to observe effects, the cyclic pattern persisted and with greater gain errors. The refluxcontrol was reasonably effective (Figure D.5). Its gain error was 1.3 ºF and fluctuation at steady-statewas 0.7 ºF.

The feed flowrate varied throughout our runs on this column, and the initial PID values producedconsistent responses with no differences than those just described. Though exact numbers are not knownfor all of our runs because the feed rotameter was being installed and calibrated, the feed flowrate wasbetween 10 and 100 mL/min. Effects of flowrate changes on control systems were calculatedtheoretically rather than measured experimentally as initially proposed because of the rotameter issue andtime constraints. When modeled as stirred tanks, the heaters’ Cohen-Coon PID values are notsignificantly affected as shown in Appendix F. This confirms our observations that the control systemworked similarly despite flowrate differences. Our initial proposal suggested implementing an autotuning

occurred in the lower trays. The 0.5 °F temperature variation of the feed tray, tray 3, was the highest ofall the trays. This was probably due to irregular feed flow, which was visually observed as “spurts” fromthe feed line. The low variation in the temperatures of the other trays confirmed that the column was atsteady state.

Samples from each tray, the distillate, the bottoms, and the feed were taken at this steady state. Theircompositions were analyzed using a refractometer. We then estimated the concentrations at each stage bycomparing the refraction index of the samples to our calibration curve (Figure G.1). As soon as themixture was drawn into the syringe at ambient temperature, the vapor in the syringe began to condensate.This, of course, changed the compositions of the solutions, which gave us biased results. Furthermore,the refractometer was dependent on user accuracy since the refraction index must be read from amanually adjusted scale. Tray concentrations may also be determined by using tray temperatures fromTable A.1 in the Appendix to read data points from the Tx line of the T-xy diagram. Tray concentrationsestimated by the refractometer calibration curve and T-xy diagram are in Table 3.

Table 3. Estimated Ethanol Concentrations by TrayRefractometerTray

RI ConcentrationT-xy concentration, "Real"(Based on Labview output)

T-xy concentration, Ideal(Based on Chemsep temperatures)

Bottoms 1.3332 0 0.02 0.021 1.3335 0 0.025 0.092 1.3337 0 0.04 0.1253 1.341 0.2 0.12 0.144 1.342 0.22 0.08 0.155 1.341 0.2 0.125 0.166 1.35 0.42 0.13 0.17

Distillate 1.364 1 0.21 0.2

As can be seen in Table 3, there was a significant (30-60%) disparity between concentration valuesdetermined from the refraction index. Assuming that the T-xy diagram is an acceptable means todetermine distillation tray concentrations (Koch et al., 1998), the discrepancy between concentrationsbased on RI and those based on the T-xy diagram suggests that the refractometer is unsuitable formeasuring tray concentrations. However, this does not mean that the T-xy diagram is foolproof; it wasassumed that the overall column pressure was 1 atm, while in reality, there may have been a pressuregradient. We assumed that with a small temperature gradient, the pressure gradient from the top to thebottom of the column was also small, and the effect of pressure on the T-xy diagram was negligible.

Conclusions

The purpose of this exploration was to address flowrate control, Labview issues, and measurement of trayand product concentrations to provide calibrated reflux and feed rotameters, temperatures for steady state

Flowrate CalibrationFurther steps need to be taken in order to obtain more accurate flowrate readings for the column. Firstand foremost, the leaks that are present at the base of the reflux rotameter and the column should be fixed.Once this is done, additional data points for the reflux rotameter calibration should be collected, and theresulting curve compared to that given with equipment’s set specifications. In order to increase thesignificance of the curves derived in this study, further calibration trials should be performed for eachrotameter. One option to bypass the work outlined above is to replace the rotameters and implementappropriate flow sensors to be translated into Labview. This method of measuring would probably bemore effective and consistent than relying on manual adjustments of the rotameters.

ControlThere are a number of directions for future development of control of the column. First, since the integraland derivative action of the column have just now been implemented, the performance of the columnunder true PID control can be evaluated. Stability should be much greater under this control. Second,there might be a way to link the setpoint temperature of the reflux stream to the temperatures at the top ofthe column so that the reflux truly enters as a saturated liquid. This would involve a number of checks tofilter bad thermocouple data and to make sure the column is running at steady state versus start-up.

Finally, a manual autotuning procedure should used to compare to step response data. It would beperformed on a heater by first increasing the gain to a very high number and setting the derivative andintegral actions to zero; this would alternate the heater control variable between its two range extremes(0.004 and 0.020 amps) depending on the sign of the error. A limit cycle would occur, from which theultimate gain and period could be used in a Ziegler-Nichols tuning scheme for PID parameters.

Tray ConcentrationsTo obtain more accurate readings of the distillation tray concentrations, alternative methods to therefractometer should be used. A suitable method that is readily available in the laboratory is gaschromatography. Distillation tray concentrations determined via the gas chromatograph are generallywithin ±0.07 the values determined by a T-xy diagram (Koch et al., 1998). While there has been aconsiderable amount of exploration into composition and temperature relationships for ethylene glycoland water (Fox et all, 1997), it may be of interest to draw similar correlations for ethanol-water systems.

Did you consider measuring the efficiency of the column.This can be done by using the Macabe-Thiel method tocalculate the number of theoretical plates. This can thenbe compared to the actual number of plates to determinethe efficiency. I know this was not the purpose of theexperiment but it would be interesting to see what kind ofnumbers you get. Again, the refractometer method needsto be improved before this can be done.

All in all this report was easy to read and presented in anorganized format. I had difficulty fully understanding theLabView control software but that is not a big deal since I

References

1. ‘Operation Instructions for the Fractional Distillation Column’, Hampden Eng. Corp., 1993.2. Biegler L.T., Grossman I.E., & Westerburg A.W. Systematic Methods of Chemical Process Design.

New Jersey: Prentice Hall, 1993.3. Fox, Stephen, Altug Koymen, Kendra McCoy. “Separation of Ethylene Glycol and Water in a

Distillation Tower” Carnegie Mellon University, 1997.4. Geankopolis, Christine J. Transport Processes and Unit Operations. New Jersey: Prentice Hall,

1993.5. Koch, Eric, Jeff Linwood, Adam Madigan, Tobin McDaniel. “Distillation: Binary Mixture of

Ethanol-Water” Carnegie Mellon University, 1998.6. Prieve, D. Lecture Notes, Unit Operations, 06-202, Fall 1998.7. Reid, R.C., Prausnitz, J.M., & Sherwood, T.K. The Properties of Gases and Liquids, 2nd Edition.

New York: McGraw-Hill, 1987.8. Unit Operations Laboratory Reports. Fall, 1997, 1998.

Appendices

A: Steady-state operation of the column without control and R=0.5 .................................................... 13B: Steady-state operation of the column under control and total reflux ............................................... 15C: Step response curves ...................................................................................................................... 16D: Controlled column data.................................................................................................................. 17E: Cohen-Coon tuning ........................................................................................................................ 20F: Control modeling............................................................................................................................ 20G: Refractometer calibration curve ..................................................................................................... 22H: Data output key.............................................................................................................................. 22H: Equipment ..................................................................................................................................... 23

Appendix A: Steady-state operation of the column without control and R=0.5

Note: The theoretical predictions made for temperatures in the column were developed from ChemSepsoftware running a binary system of ethanol and water. The following specifications were input:Murphee efficiency of 0.8, 0.5 mol fraction EtOH in the feed, feed at 183.2 F, boiler temperature at 204.5F, reflux ratio of 0.5, and column pressure of 1.2 atm. The following models were used in thethermodynamic package: Gamma-Phi K model, Original UNIQUAC mode, Soave-RK Cubic EOS,Antoine Vapour pressure, Excess Enthalpy.

Table A.1: Temperature profile 1 hour 18 minutes after start-up.tray number temperature (F) tray number temperature (F)bottoms 204.5 4 189.3 (183.8)1 202.4 (188.4) 5 187.3 (183.1)2 196.1 (185.2) 6 184.8 (182.4)3 187.9 (184.2) 7 181.5 (181.7)feed 183.2Temperatures in parentheses are those predicted by ChemSep.

170

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actualtheoretical

Steady-State Operation of Column: Excerpt

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4050 4070 4090 4110 4130 4150 4170

time (s)

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boilerbottomstray 1tray 2tray 3feedtray 4tray 5tray 6top

Figure A.3: Column temperatures at steady state. Most temperatures showed less than 0.5 °F varation. Noise intray 3, the feed tray, was probably due to irregular feed flow.

Appendix B: Steady-state operation of the column under control and total reflux

Table B.1: Temperature Profile 18.8 minutes after start-up.tray number temperature (F) tray number temperature (F)bottoms 199.8 4 189.81 196.7 5 185.42 195.3 6 180.33 193.7 7 176.1feed 190.2

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tray 1tray 2tray 3

tray 4

tray 5

tray 6

tray 7

Figure B.2: Temperature profile at total reflux. Steady-state was difficult to reach at total reflux, as seen by thechanging temperatures in the column. The temperatures in the upper part of the column appear to be decreasing andbecoming closer together.

Appendix C: Step response curves

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=∆

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seK

G

KP

TT

eTTTT

sp

p

pboiler

if

t

if

f

F/ampss

°==θ=τ

160027377

pK

Figure C.1: Step response curve for boiler. The boiler power was increased 0.007 amps and bottoms temperaturewas monitored. The resulting response appears first-order.

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ss

°==θ=τ

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=∆

−

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s

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KP

TT

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Appendix D: Controlled column data

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Figure D.1: Control of boiler temperature. Boiler temperature was controlled using a proportional gain of 0.012amp/ºF and a setpoint of 193 ºF. Control was started at the beginning of start-up and it took 6.7 minutes to reach99% of the boiler steady-state value of 192.4 ºF.

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Figure D.3: Control of feed temperature. Feed temperature was controlled using a proportional gain of 0.014amp/ºF and a setpoint of 166 ºF. Control of the feed was started 4.6 minutes after that of the boiler, taking 1.3minutes to reach 99% of its steady-state value of 163.7 ºF.

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Figure D.5: Steady-state control of reflux temperature. Reflux was controlled using a proportional gain of 0.01amp/ºF and a setpoint of 165 ºF. The controlled stream had a temperature of 163.6±0.7 ºF. Heating was necessarybecause reflux was supercooled in the condenser and needed to be returned to saturation temperature.

Appendix E: Cohen-Coon tuning

Given a first-order process with a gain of K, a time constant of τ, and a delay of θ, PID parameters may becalculated by the Cohen-Coon approximation:

τθ+θ=τ

τθ+τθ+θ=τ

τθ+

θτ=

2114813632

4341

D

I

c KK

(4-6)

Appendix F: Control modeling

For each of the heaters in the distillation column (boiler, feed, and reflux), it is convenient to developmodels based on a continuously stirred tank with a heat input. The boiler is essentially a CST. Thoughthe feed and reflux heaters are more similar to heat exchangers, the CST model is used because of itssimplicity and absence of partial derivatives. These models assume no phase change. In reality, theheaters in the distillation column do cause phase change; however, for calculating general trends for usein PID parameters, these simple models are sufficient. The models also assume that the mass in the tankand the fluid flowrate through are constant. This is appropriate to a column running at steady-state.

For a continuously stirred tank with a mass flowrate of fluid m, mass holdup of M, inlet and outlettemperatures T0 and T, and heat input of Q, the following energy balance may be written:

dtdTMCQTTmC pp =+− )( 0 (7)

When this is rewritten and simplified in the Laplace domain, the following equation results:

)(1

)( sQs

KsT p

τ+= where

mM

mCK

pp =τ= ,1

(8)

The useful aspect of this modeling is the inverse proportionality of the process gain and time constant tom. As the flowrate of the fluid through the heater increases by a factor of x, the gain and time constantsdecrease by 1/x. Since control parameters in Cohen-Coon tuning are developed from these latter twoconstants, the effects of flowrate changes on PID tuning values are easily calculated.

Table F.1: Modeled effects of flowrate on PID constants.boiler feed

feedmL/min

KpF/amp

τsec

θsec

KcF/amp

τIsec

τDsec

KpF/amp

τsec

θsec

KcF/amp

τIsec

τDsec

10 6400 1508 27 0.0117 66.0 9.79 7720 248 3 0.0143 7.35 1.0920 3200 754 27 0.0117 65.5 9.75 3860 124 3 0.0143 7.31 1.0930 2133 503 27 0.0118 65.0 9.72 2573 83 3 0.0144 7.27 1.0840 1600 377 27 0.0118 64.5 9.69 1930 62 3 0.0144 7.24 1.0850 1280 302 27 0.0118 64.0 9.66 1544 50 3 0.0144 7.20 1.0860 1067 251 27 0.0119 63.6 9.63 1287 41 3 0.0145 7.17 1.0870 914 215 27 0.0119 63.2 9.60 1103 35 3 0.0145 7.13 1.0780 800 189 27 0.0119 62.7 9.57 965 31 3 0.0145 7.10 1.0790 711 168 27 0.0120 62.3 9.54 858 28 3 0.0146 7.06 1.07100 640 151 27 0.0120 61.9 9.51 772 25 3 0.0146 7.03 1.07

The table above shows that for flowrates of 10-100 mL/min through the boiler or feed, the PID constantsKc, τI, and τD vary little. The gains on both, which is the primary contributor to stability, vary a maximumof 3% on this range. Since this flowrate range is representative of typical flows that the boiler or feedmight experience, the model’s prediction is that the PID constants need not be changed based on thecolumn flowrate conditions.

Appendix G: Refractometer calibration curve

1.33

1.335

1.34

1.345

1.35

1.355

1.36

1.365

0 0.2 0.4 0.6 0.8 1 1.2

Volumetric Concentration

Ref

ract

ion

Inde

x

Figure G.1: Refractometer calibration curve.

Appendix H: Data output key

The tab-delimited data file that the distillation program produces does not include labels. The followingis a key to its columns. Unlabelled column numbers have not yet been identified or defined.

column # description 123456 boiler temperature (ºF)7 feed temperature8 reflux temperature9 distillate temperature

17 tray 2 temperature18 bottoms temperature19 top (tray 7) temperature20 feed heater temperature21 reflux heater temperature22232425

Appendix H: Equipment

Figure H.1. Hampden Distillation Column

DISTILLATION: ENERGY BALANCE ON THE

CONDENSER

SaKeithia Mason Mitesh Master

Meredith Welsh

ABSTRACT In this experiment we write an energy balance for an ethanol and water distillation column. We calculate the flow rate of the distillate produced by the condenser through batch distillation. Lab VIEW measures the inlet and outlet temperature of the cooling water. Using this data, we close the energy balance equation around the condenser. Lastly, we plot a T-xy diagram to assess flammability and to determine the reboiler explosion temperature.

TABLE OF CONTENTS INTRODUCTION 1 BACKGROUND: SAFETY DATA 1 ENERGY BALANCE CALCULATIONS 4 RESULTS 6 CONCLUSIONS 11 REFERENCES 13

INTRODUCTION Distillation is a common industrial method used for separation. In this lab, a simple batch distillation column of ethanol and water runs at steady state. The Hampden distillation column (reference Binary Distillation of Ethanol and Water) is comprised of a total condenser, a feed preheater, and feed and reflux rotameters. In this experiment, we allow the ethylene glycol and water mixture to distill through the column. The condenser produces distillate that flows into a collection vessel. Before measurements began, we stabilize the column at steady state. After distillate forms at a constant rate, we collect distillate and timed how long it takes for the distillate to reach a lesser volume. Further calculations are then needed to complete the energy balance. BACKGROUND: SAFETY DATA We want to assess the likeliness of an explosion of ethanol-water mixture in our reboiler because the reboiler uses a spark to heat the bottoms. If the bottoms level drops below the insert point of the spark, there is risk of an explosion. This analysis examines what vapor composition is necessary to cause an explosion. In the reboiler, there exists a vapor-liquid phase equilibrium dependant upon the temperature and pressure. For our analysis, we are assuming that the pressure is close to one atmosphere. Since this is a two-component system with differing vapor pressures, there will be differing compositions in the liquid and vapor mixtures. Our sensors can detect the temperature of the bottoms. We are assuming that the bottoms temperature does not change very much with vertical position, thereby allowing us to make the vapor temperature the same as the measured temperature in the bottoms. Once we know this along with T-xy data, we can feed the temperature data into the attached MathCAD module and see what the vapor composition of the ethanol will be. Embedded below is our MathCAD module. Feel free to enter different T-xy data and various temperatures by double clicking on the insert. Once we have this data, we can compare it to our flammability data give in Figure 1 and see if combustion is likely.

1

Note that this module only works for (near) constant pressure. Please be wary of this.Attempt to use Txy data that has plenty of values at the limits for a better fit.

Instructions:

1) Uncheck "automatic calculation" in the Math menu.

2) Enter tabular data for Txy-diagram here. If you need more or fewer rows use the "matrix" tool (View-Toolbars-Matrix).

3) All other parameters are entered next to the Txy diagram at the very end of this document. You can move there end now by pressing Ctrl-End.

xes

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

:= yes

0

0.711

0.868

0.926

0.955

0.972

0.982

0.989

0.994

0.997

1

:= Tes

387.1

324.2

292.2

272.2

257.6

246.5

237.39

229.8

223.19

217.3

212.1

:=

This MathCAD module allows us to enter either temperature and the liquid or vapor composition for the given component and returns the other two pieces of information. Click on the module within the Word file to experiment with collected data. Given the vapor mole fraction of the water-ethanol mixture, one can determine the water and ethyl alcohol volume percent. With this, one can now examine the graph below and see if flammable conditions exist.

2

Figure 1: Limits of Flammability of Ethyl Alcohol-Water Vapor-Air Mixture at 100ºC and Atmospheric

Pressure

With this information, we can assess the likeliness of ethanol vapor flammability.

3

ENERGY BALANCE CALCULATIONS

∆Q 1.12 103×=∆Q

∆Qcw60

:=∆Qcw 6.719 104×=

∆QcwVcw ∆H⋅ ρw⋅

MWw:=

ρw 998:=MWw 18.015:=

To find the total heat exchanged, we must also take the flowrate of the cooling water into account. To include this in our calculation, we need the molecular weight of water (MWw), in g/mol, and the density of water (ρ), in g/L. The resulting value, ∆Q is the total heat entering the cooling water per time, and is shown here in watts.

∆H 468.611=

∆H Cpw T To−( )⋅:=

Now we calculate the change in enthalpy, ∆H, in J/mol.

Cpw 66.983=

Cpw R AB2

To⋅ τ 1+( )⋅+C3

To2.⋅ τ

2τ+ 1+( )⋅+

D

τ To2⋅

+

⋅:=

τ 1.024=R 8.314:=τ

TTo

:=

D 8.59− 10 10−⋅:=C 2.452110 6−

⋅:=B 4.595 10 4−⋅:=A 7.7010:=

Next, we use the heat capacity of water at the operating temperature of the condenser to calculate how much energy enters the cooling water from the distillate. This correlation is based on the To and T values of the water, as well as the enthalpy coefficients specific to water, A, B, C, and D. Also necessary is R, given in J/mol*K.

Vcw 2.588:=

The flowrate of the cooling water is constant over time. For this run, in L/min, our flowrate is as follows:

T 301.973:=To 294.977:=

Here we base the calculations for our cooling water energy balance on a very narrow point in time. Because of this, we are able to assume that all of our values are constant over this range. The temperatures we use are the temperatures of the inlet (To) and outlet (T) cooling water streams. All temperatures are in K.

4

Cde 140.828=

Cde R AeBe2

To⋅ τ 1+( )⋅+Ce3

To2.⋅ τ

2τ+ 1+( )⋅+

De

τ To2⋅

+

⋅:=

Dw 8.59− 10 10−⋅:=De 3.280 10 10−

⋅:=τ 0.814=

Cw 2.452110 6−⋅:=Ce 2.004− 10 5−

⋅:=R 8.314:=τ

TTo

:=

Bw 4.595 10 4−⋅:=Be 5.113 10 2−

⋅:=

µmw .12:=µww .05:=

Aw 7.7010:=Ae 2.1530:=µme .88:=µwe .95:=

In order to calculate the amount of heat that leaves the distillate in the condenser, we first need to know the heat capacity of the mixture. Here we use a correlation to calculate Cp (values A, B, C, and D) with respect to T and To and the percent compositions by mass ( µme, µmw). R is given in J/mol*K, resulting in Cd in J/mol*K as well.

Vd .03848:=

Throughout the run, we also calculate the average volumetric flow rate of the distillate. This value (Vd) is in L/min.

5

RESULTS From the collected LabVIEW data, we plot the following graphs. We fitted trend lines and use the equations in the time dependent calculations.

Cooling Water Flowrate

y = 2E-06x + 2.5826

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

970 1170 1370 1570 1770 1970 2170 2370 2570 2770

Time (s)

Flow

rate

(L/m

in)

Series1 Linear (Series1)

Figure 2. Flow rate versus time. Figure 3. Temperature of distillate versus time.

Temperature of Distillate

y = 3E-07x2 + 0.0002x + 76.392

76.5

77

77.5

78

78.5

79

79.5

80

970 1170 1370 1570 1770 1970 2170 2370 2570 2770

Time (s)

Tem

pera

ture

of D

istil

late

(deg

rees

F)

Series1 Poly. (Series1)

6

Temperature of Tray 6

y = -2E-06x2 + 0.0139x + 180.43

190

192

194

196

198

200

202

204

206

208

970 1170 1370 1570 1770 1970 2170 2370 2570 2770

Time (s)

Tem

pera

ture

of T

ray

6(d

egre

es F

)

Series1 Poly. (Series1)

Figure 4.Temperature of tray 6 versus time.

Temperature of Outlet Cooling Water

y = 2.6887Ln(x) + 63.613

81

81.5

82

82.5

83

83.5

84

84.5

85

85.5

970 1170 1370 1570 1770 1970 2170 2370 2570 2770

Time (s)

Tem

pera

ture

of C

w(o

ut)

(deg

rees

F)

Series1 Log. (Series1)

Figure 5. Temperature of outlet cooling water versus time.

Temperature of Inlet Cooling Water

y = -1E-07x2 + 0.0007x + 70.413

70.4

70.6

70.8

71

71.2

71.4

71.6

71.8

970 1170 1370 1570 1770 1970 2170 2370 2570 2770

Time (s)

Tem

pera

ture

of C

w(in

)(d

egre

es F

)

Series1 Poly. (Series1)

Figure 6. Temperature of inlet cooling water versus time.

7

Below in Table 1, we calculated the distillate flow rate. These flow rates vary with time. However, they are not used in our calculations with respect to time dependence because the calculated flow rates are not taken with respect to time. The following table shows how variable the flow rate is. Table 1. Flow rate calculation of the distillate. Trial Volume (mL) Time (min.sec) Flow rate (mL/min) 1 250 5.34 45.2 2 200 5.14 38.31 3 250 6.56 36.08 4 150 4.23 34.33

8

∆Q 1.156 103×=∆Q

∆Qcw60

:=∆Qcw 6.934 104×=

∆QcwVcw ∆H⋅ ρw⋅

MWw:=

ρw 998:=MWw 18.015:=

Total heat entering per time:

∆H 483.628=

∆H Cpw T To−( )⋅:=

Enthalpy:

Cpw 66.986=

Cpw R AB2

To⋅ τ 1+( )⋅+C3

To2.⋅ τ

2τ+ 1+( )⋅+

D

τ To2⋅

+

⋅:=

τ 1.024=R 8.314:=τ

TTo

:=

D 8.59− 10 10−⋅:=C 2.452110 6−

⋅:=B 4.595 10 4−⋅:=A 7.7010:=

Heat capacity:

Vcw 2.588:=

The flowrate of the cooling water is constant over time.

T59

2.6887ln t( )⋅ 63.613+ 32−( )⋅ 273.15+:=

To59

1− 10 7−⋅ t2⋅ .0007t+ 70.413+ 32−( )⋅ 273.15+:=

t 2370:=

Now, by substituting the equations depicting time dependency into the equations in place of the constants we used previously, we are able to perform the entire calculation for any point in time, once the column is running. The only two variables that we use as time dependent are the cooling water inlet and outlet temperatures. Here, t must be entered in seconds.

9

Cde 140.985=

Cde R AeBe2

To⋅ τ 1+( )⋅+Ce3

To2.⋅ τ

2τ+ 1+( )⋅+

De

τ To2⋅

+

⋅:=

Dw 8.59− 10 10−⋅:=De 3.280 10 10−

⋅:=τ 0.813=

Cw 2.452110 6−⋅:=Ce 2.004− 10 5−

⋅:=R 8.314:=τ

TTo

:=

Bw 4.595 10 4−⋅:=Be 5.113 10 2−

⋅:=

µmw .12:=µww .05:=

Aw 7.7010:=Ae 2.1530:=µme .88:=µwe .95:=

Heat capacity:Note that for this calculation that we assume the composition of the distillate is not changing.

Vd .03848:=

Due to the nature of our measurements for the distillate flow rate, we are unable to calculate this with respect to time. Although we know that the flow changes with time, we perform this calculation with the average value.

T59

3 10 7−⋅ t2⋅ .00002t+ 76.392+ 32−( )⋅ 273.15+:=

To59

2− 10 6−⋅ t2⋅ .0139t+ 180.43+ 32−( ) 273.15+:=

t 2370:=

Again, here we use the equations depicting time dependency to calculate the heat leaving the distillate at any point in time.

10

∆Q 577.59=

∆Q∆Qd60

:=

∆Qd 3.466 104×=

∆Qd∆H Vd⋅ ρmix⋅

MWmix:=

ρmix 799.45=MWmix 42.703=

ρmix µwe ρe⋅ µww ρw⋅+:=MWmix µme MWe⋅ µmw MWw⋅+:=

ρe 789:=ρw 998:=MWw 18.015:=MWe 46.069:=

Total heat leaving per time:

∆H 4.811 104×=

∆H µme− Cde T To−( )⋅ ∆Hvape− ⋅ µmw− Cdw T To−( )⋅ ∆Hvapw− ⋅+:=

Enthalpy:

∆Hvapw 40683.1:=∆Hvape 38769.8:=

Heat of vaporization:

Cdw 67.571=

Cdw R AwBw2

To⋅ τ 1+( )⋅+Cw3

To2.⋅ τ

2τ+ 1+( )⋅+

Dw

τ To2⋅

+

⋅:=

CONCLUSIONS According to our energy balance, there is twice as much heat entering the cooling water as is actually leaving the distillate. Our results should actually be reversed; we are more likely loosing more heat from the distillate than enters the cooling water. After noting that our cooling water energy balance is heavily dependent upon the cooling water temperature change of only 7ºK, we realize that this has the potential for major error. One explanation for the energy balance is that the cooling water is actually colder than the column at ambient temperature. This means that the cooling water would be warming

11

even when no distillate is running through the condenser. If this is the case, then the temperature change of the cooling water appears artificially high, resulting in an apparent heat exchange that is higher than in reality. Our data should be verified in the future with more runs. We have provided an energy balance calculation module for future groups so that they can easily interpret their data. Also, future groups may want to investigate the time dependence of distillate flow rate and composition as these values also influence the validity of the energy balance.

12

13

REFERENCES Crawford, Chan, Guo, Shell, and Ip. Binary Distillation of Ethanol and Water: Improvements of Steady-State Operation and Measurement. Carnegie Mellon University. 1999. Geankoplis, Christie J., Transport Processes and unit Operations. Third Edition. 1993. Zabetakis, Michael G., Flammability Characteristics of Combustible Gases. NITS. 68

Installation of a Steam Heat Exchanger to the Rothfus Distillation Column Lab

Unit Operations Lab

Fall 2001: Team 7 Russell Ball

Lance Brown Chaitra Hakkal David Tucker

Abstract This report discusses the design and implementation of a steam heat exchanger in

the reboiler of the Rothfus lab distillation column. The overarching goal of the project was to improve safety and performance of the lab by replacing electric heaters with steam heat exchangers. Using pressurized steam as the heat source, the heat exchanger design goal was to heat up about 15 kg of water in less than 20 minutes. The heat exchanger design made use of both heat exchanger and distillation column design equations. We considered several different designs before deciding on the best choice of a coil within the reboiler. The new heat exchanger satisfied the requirements we designed it to perform. The startup heating time for the reboiler at a steam pressure of 25 psi was approximately 11 minutes.

Introduction Early in the semester our aspiration for this lab was to distill ethanol from a

water-ethanol mixture, but before this could happen we needed to repair the distillation column and replace two electrical heaters with a steam-powered heater in the distillation column reboiler. We did this because when the electric heaters failed, this could cause flammable fluids such as ethanol to ignite. In addition the electrical system had some problems with short-circuiting.

Our goals for this project were four-fold: (1) to replace the reboiler’s electric heater with steam, (2) to repair the electrical system and make the distillation column operational, (3) add a controller to the system, (4) and distill ethanol from water. We were able to achieve the first two goals, but did not complete the rest due to time constraints. After drafting and considering a few different designs, we settled on a heat exchange coil within the reboiler.

The Chemical Engineering Department of Carnegie Mellon University commissioned us to execute this work.

The background and theory section below discusses in greater detail the reasons for replacing electric heaters, preliminary designs in general terms, and the final design. The experimental section describes the equipment that went into our design and the procedure involved in the actual production process. Results and discussion summarizes the effectiveness of our improved design. Finally, recommendations outlines where future groups with the distillation column lab should focus their efforts.

Background and Theory Our initial design was to incorporate the external heat exchanger with the

distillation column. The large external heat exchanger would split into three parallel streams: one for the reboiler, one for the feed, and one for the reflux (see Figure A1). This design, however, would tie up a unit that is currently functioning as a stand-alone lab station, and may require more heat than the unit can provide. Additionally, replacing all three heat exchangers is a complicated task beyond reasonable time expectations of what we could accomplish in one semester. Our second design iteration was to use a new heat exchanger to perform the same function, but only on the reboiler. We could have made this option work, but it would be more expensive and more likely to break. The design required a pump to circulate the bottoms fluid through the external heat exchanger (see Figure A2). Since the bottoms fluid is near its boiling point, it could vaporize in the pump, causing the pump to fail. Since this design offers no advantage over an internal heat exchanger, we opted for the latter. Throughout this design phase we developed heat duty calculations. Because heat exchangers of various types are modeled similarly many of our calculations were adaptable to changes in design. For final heat duty calculations see Appendices B through E.

Because the unsteady state conditions are more demanding of a heat exchanger, we based most calculations on maximum capability rather than minimum capability. However, we confirmed that a steam pressure of 2 psi is adequately low to provide a

steady state heat duty for a variety of conditions. For example, in an ethanol-water system anything below about 14 psi will not boil the heavy component. The focus of our design effort then became how to incorporate an internal heat exchanger coil in the reboiler. Challenges included how to disassemble the column, developing a heat exchanger with high enough energy given the size restrictions, and how to incorporate a controller.

Electrical Heaters If not properly taken care of electrical heaters may fail and lead to a dangerous situation. This happens as current running through the electrical wires generates heat and degrades the wires?. If exposed wires are close enough, current can spark between them and ignite ethanol (the fuel) as it comes it in contact with oxygen from the air.

Heat Exchangers We had several design options in modeling the heat exchanger. Our initial plan called for a 1-2 shell and tube heat exchanger. Baffles in the heat exchanger forces the steam to flow across the tubes several times. The ethanol-water mixture in the tubes run parallel to each other as the steam runs perpendicular. Temperature difference is calculated by the log mean difference.

ciho

cohi

cihocohilm

TTTT

TTTTT

−−

−−−=∆

ln

)()( (Equation 1)

Correction factor FT are is used to correct the mean temperature difference for a 1-2 heat exchanger (Geankoplis, 270). So ∆Tm = FT∆Tlm, the overall heat transfer equation for the heat exchanger becomes Q = UiAo∆Tlm = UoAo∆Tlm. The preceding design assumes the heat exchanger is at steady-state. The design equation is a function of time Q(t) = UAhx∆T(t) (Appendix C2). ∆T(t) is the difference between the steam temperature and the water-ethanol mix.

Experimental The actual experiment that involved data acquisition consisted of starting up the

column from room temperature two times. The purpose of these trial runs was to test the effectiveness of the steam heat exchanger as a reboiler.

However, the main commitment of manpower was towards the building of the reboiler unit. This section will primarily outline the equipment and steps involved in the construction phase.

Equipment • Hampden distillation column as available to us in September including:

• Failed electric heating unit • Broken sight glass

• Short circuit and other inadequacies of electrical function • 3/8” stainless steel pipe, approximately 14 feet • Steam conduit from building steam supply to distillation lab: 25 feet Swagelok 1/2” Teflon-

lined stainless steel braid flex hose • Pressure regulator: bronze, 2-25 psi • Swagelok fittings to

• Connect steam conduit to wall and pressure regulator • Connect pressure regulator to coil connector piece • Pass the coil connector piece through the right reboiler opening • Connect the coil connector to the coil itself • Pass the other end of the coil through the left reboiler opening

• Steam trap • Oxalic acid

Procedure Before the design phase could even come to an end, we needed to see inside the reboiler so we would know what size the heat exchanger’s coils should be. We disassembled the column by loosening the condenser so we could remove the trays. After unscrewing all the bolts on the reboiler lid, we had to pry open the reboiler lid because the rubber seal had firmly held it down. The metal surface inside the reboiler was corroded. We mixed 250g of oxalic acid crystals with water and left it in the reboiler to soak for a few days to remove the corrosion. When we emptied the reboiler (and disposed of the acid by heavy dilution with water) the metal inside was clean and smooth with little agitation required. We designed the heat exchange coil based on the dimensions of the reboiler and the locations of the level sensors. The 6” diameter was designed such that the coil would rest in the main compartment of the reboiler rather than the narrower sunken region on the bottom, and the coil would fit between the two level sensor posts. Russell Ball, Matt, and Ken produced the coil in the machine shop by bending the 3/8” steel pipe tightly around a metal cylinder attached to a lathe.

To complete the installation, we fit one end of the coil through the reboiler hole, and then bent the other end so it could connect to the inlet piping. It was not possible to have one continuous piece of metal go in one side and out the other.

We had to assemble the column in the same position as it was when removed, so that the feed line would be inline with the column. Initially the column was offset by one bolt position, which prevented us from continuing. After correcting the column, we finished by connecting the pressure regulator and steam trap with Swagelok fittings.

The electrical system was shorted out so that turning on the main power caused the circuit breaker to immediately shut the system down. We removed a number of wires that were non-functional and replaced wires that were short-circuited. There was an exposed wire that was touching the back of the circuit breaker. After removing this wire, main power stayed on, but pumps were not interfaced yet. We only needed the feed pump in order to test the reboiler function, so we wired that pump directly to external 110V power. With the steam, the flow control system, and the feed pump all working, we began testing the reboiler. The first attempt was with the pressure regulator set lower

than the factory-set 15 psi, to around 13 psi. Two days later, we ran the same experiment with the maximum pressure setting for the pressure regulator, 25 psi. Stopwatches recorded the time from when we turned on the steam to when the reboiler thermocouple read near 2120F.

Results and Discussion The installed steam heater succeeded in heating the reboiler within the

predetermined time frame. At an approximate pressure of 25 psi, the steam heated the reboiler water from 59 ºF to 210 ºF in 11 minutes. This data corresponds well to the model we predicted that assumed saturated steam at 30 psi and heating time in the reboiler in of 7 minutes. The heat loss due to water vapor escaping the reboiler and heating up the distillation column can account for the extra heating time. We expected this type of deviation from our approximations, but we could not quantitatively account for such a complicated system. In an attempt to correct for this error, we used a low overall resistance value (U value) and used more pipe than our estimated length.

In a separate trial at a lower pressure, the heating time took 1 hour and 13 minutes. In this trial, the water temperature went from 59 ºF to a steady state temperature of 206 ºF. This steady state temperature corresponds to a pressure of approximately 13 psi. During these trials, the column leaked a small amount of water through cracks in the distillation column, which we fixed during the trials. Now, no visible leaks appear on the column. Although we successfully installed and operated the heat exchanger, we were not able to install any type of automated control system, which was one of the initial goals of our lab. Since the designs changed significantly from our original plans, we spent more time than we expected designing the heat exchanger.

Another large problem that hindered our progress was the unavailability of information and resources. As we talked with companies to gather information about available products, we found that information we requestedwe requested of outside sources was either slow to arrive or did not arrive at all. If we had more experience in dealing with industry, we could have made faster progress in finding and ordering the right parts and adapting to difficulties in communication.

We also lost time due to poor contingency planning. In some cases, our work depended on parts being available. When parts were not available, we were unable to be as productive as we would have liked to be.

Recommendations • Order a valve control actuator and install it on the pressure regulator. We

recommend an actuator with a 4-20 mA input that is compatible with the existing pressure regulator. (For more information on our recommended actuator, see ETI reference.)

• For steam temperature data and control, install thermocouples in the Swagelok fittings in the input and output streams of the heating coil.

• Rewire the feed, bottoms, and reflux pumps using wire with heat-resistant insulation.

• Order a new sight glass to replace the existing broken sight glass. • Install new column tray seals to replace the existing seals, which have been

warped by heat and time. • Create a permanent structural support for the pressure regulator. • We assumed water as the heavy component when sizing the pressure regulator.

Any experiments that use a component that has a significantly lower vapor pressure than water may need to resize the pressure regulator.

• Set up the computer interface and acquire more accurate data. Design a control system and apply it to the distillation column. This would include control apparatus and most likely a software component.

• Install a flow cell refractometer to measure real-time compositions of distillate stream. Incorporate into control system.

• Test the control system using various chemicals and compositions.

Conclusion The steam heat exchanger we designed works to our specifications. It meets physical constraints of size, duty, and usability. We expect it to be reasonably controllable with future modifications. It is relatively versatile with respect to future needs of the column. Stainless steel is resistant to chemical corrosion, and the range of heat duties is adaptable to various liquids.

The distillation column lab has many areas still that can be expanded and improved by future lab groups.

References ETI Electric Valve Actuators. Retrieved Dec. 17, 2001.

http://www.etisystems.com/valveactuator/index.html (talk to Dan when calling; apologize for not getting back to him for a year or more; tell him you have a valve you can send him so he can build the actuator for it)

Geankoplis, Christie J. Transport Processes and Unit Operations. Prentice Hall PTR,

1993. (design equations, steam condensing) Perry, Robert H. Perry’s Chemical Engineering Handbook. 7 ed. McGraw Hill, 1997. (U

value, Pvap’s & Cp’s of ethanol and water)

Appendix A

Figure A1: Preliminary design using heat exchanger lab as main heat source. Steam line is split into three heat exchangers. Temperature transmitters (TT) and pressure transmitters (PT) are fed into the computer, which commands the pressure controls (PC) for each stream.

3

4

5

6

7

Reboiler

feed

bottoms

distillate

cooling water

computer

pump

Heat exchanger

Cold water

Steam trap

Steam trap

Steam trap

steam

PC

Steam trap

PC

PC

PT

PT

PT

TT

TT

TT

1

2

Figure A2: Intermediate design with external heat exchanger. Proportional controls (temperature and pressure) are incorporated for only the reboiler.

7

Reboiler

bottom

steam

pump

Heat exchanger

PC

computer

Steam trap

PT

TT

Figure A3: Final design with internal heat exchanger. Proportional controls can be installed on the steam line. Refractometer transmitter can be installed to gather real time distillate composition data.

cooling water

distillate 1

2

3

4

5

6

7

Reboiler

feed

bottomSteam trap

steam

PC

computer

RT

PT

TT

refractometer

Appendix B: Design parameters Heat transfer coefficient Reflux ratio Feed flowrate Mass fraction of feed that is ethanol Water inlet temperature Diameter of condenser coil Turns of the condenser coil Diameter of the condenser tubing Mass flow of condensing water Mass flow of steam Boilup ratio Volume of the reboiler Steam pressure Steam temperature as a function of steam pressure

U400 BTU⋅

hr ft2⋅ R⋅:=

Rreflux 5:=

Feed 100gmmin

⋅:=

xfeedmEA 0.5:=

TinW 273 10+( ) K⋅:=

Dcoil 4 cm⋅:=

nturns 60:=

Dtube .25 in⋅:=

McondW 2.4L

min⋅ ρ W⋅:=

Fstm 10lb

min⋅:=

Rboilup 5:=

Volreb 16L:=

Pstm 30 psi⋅:=

Tstm Pstm( ) 394.475K=

Appendix C1: Temperature model in reboiler

Tw t( ) 298 K⋅rp

−

exp p− t⋅( )⋅rp

+:=

where p and r are simply constants of the system:

pU Ahx⋅

Volreb ρ W⋅ CpW⋅:= r

U Ahx⋅ Tstm⋅

Volreb ρ W⋅ CpW⋅:=

p 3.713 10 3−

× Hz= r 1.465Ks

=

0 100 200 300 400 500 600 700280

304

328

352

376

400387.818

298

Tw t( )

7200 t

Appendix C2: Heat exchange vs. time q hx t( ) U A hx⋅ ∆T t( )⋅:= where ∆T t( ) T stm Tw t( )−( ):=

0 100 200 300 400 5000

5

10

15

20

2523.983

3.746

q hx t( )

1000

5000 t

Appendix D: Calculation of condenser heat load Fcond Rreflux 1+( ) FnEA⋅:= <--assuming negligible W in vapor stream

Qreqdcond ∆HvapEA Fcond⋅:=

Qreqdcond 4.277kW=

Acond π Dcoil⋅ nturns⋅ π Dtube⋅( )⋅:= estimate ToutW 273.15 40+( ) K⋅:=

Qcond TinW ToutW−( ) CpW⋅ McondW⋅:=

Qcond 5.046− kW=

Appendix E: Steam pressure-temperature lookup charts

0 20 40 60 80 100 120 140360

380

400

420

440

T chart

PchartPa

psi⋅

0 20 40 60 80 100 120 1403.6 .10 4

3.7 .10 4

3.8 .10 4

3.9 .10 4

4 .10 4

4.1 .10 4

∆Hvap chart

PchartPa

psi⋅

Pressure in psi

Pressure in psi

kJ/mol

K

Appendix F: Experimental data Steam pressure Steady state temperature Startup time13 psi 206 degrees F 1 hr, 13 min 25 psi 212 degrees F (boiling point) 11 min

Batch Distillation: An Energy Balance

Scott Cunning Lauren Petruzzi

Jason Stieg

Carnegie Mellon University Unit Operations Lab

October 20, 2000

Abstract A complete energy balance was performed on the batch distillation column. It was determined that the difference between inlet and outlet power converges to zero as the distillation progresses. The difference in the energies can be accounted for by an accumulation term. This accumulation occurs in the heating of the column. It was determined that 85% of the heat loss occurs in the cooling water, and 15% is due to heat loss to the atmosphere.

Introduction Batch distillation is usually used when the initial feedstock to the distillation column is not available on a continuous basis. One reason for this could be due to the production of a by-product feedstock from a previous batch process. In batch distillation, liquid is charged to a heated kettle. There it is heated slowly to boil. The resulting vapors are withdrawn as soon as they form to a condenser, where the distillate is collected. The condenser at the top of the column aids in cooling the vapor stream to produce a saturated liquid. In the case of ethanol and water, the first portion of vapor condensed is richest in ethanol. As the process continues the more volatile component, ethanol, will dominant the vapor, while the liquid will be predominantly composed of the less volatile component, water. The purpose of this lab was to perform an energy balance on a six-tray batch distillation column of ethanol and water. Through this heat balance, heat losses were determined at various points throughout the apparatus. To perform this balance, the distillation column was modeled as two cylinders. The first cylinders included the six trays. It was separated into six sections, each one representing a tray with a uniform temperature. To account for the heat lost between the column (cylinder 1) and the surrounding air natural convection was assumed. From our column modeled as a vertical cylinder and a calculation of the Prandlt, Nusselt, and Grashof numbers, the heat transfer coefficient and therefore the rate of heat lost was determined. To simplify the calculations for the heat transfer to the air each section was assumed to be all glass with negligible internal resistance. The second cylinder was the overhead condenser in which the cooling water ran through to condense the distillate vapor into a saturated liquid. The boiler was assumed to be well insulated and the end effects on the cylinders were assumed negligible. To determine the pressure in the overhead cylinder a couple of assumptions were made. At a specific time, when the temperature was 203° F the compositions were assumed to be 90 mol% ethanol and 10 mol% water. Using this temperature and the dew point calculation,

Σ(yk/αk/n) = Pn°/P the pressure in the condenser was calculated. This pressure was then assumed to remain constant so that the other compositions could be back calculated. The compositions and the temperature changes allowed for the heat losses to be determined.

Procedure In order to collect distillate, it was necessary for the column to heat up. As vapor rose through the column, the trays were heated by the vapor’s condensing. Once a tray reached the dew point temperature, the vapor could proceed up the column to the cooling coil. The times at which the trays began heating and their equilibrium temperatures were recorded on LabView. At the cooling coil, condensate was collected on the coil as distillate. We measure the distillate flow rate manually with a stopwatch and volumetric flask. Results and Discussion The objective was to perform a complete energy balance over the entire column. Using LabView, data was taken from the column and formulated into tables. Distillate flow rate data was taken by hand. This data was used in calculations to determine the energy balance. Due to problems with the apparatus, it was only possible to take data during one lab session, yielding one trial. It was assumed that the input of heat to the system was soley due to the heater in the boiler. Also, it was assumed that heat left the system only through the cooling water and heat loss to the surrounding air. The calculation for the input power is represented by the following:

P= IV = (11 A)(220 V) = 2.42 kW It was assumed that the power input was constant for the entire distillation. The calculation for the heat loss in the cooling water is represented by the following:

Q = mCp(Tout-Tin)

Where m is the mass flow rate of water, the heat capacity, Cp, was assumed to be a constant of 4.184 KJ/Kg*K, and Tout and Tin represent the outlet and inlet water temperatures respectively. For the heat loss to the surrounding air the column was modeled as a vertical cylinder undergoing natural convection. The heat transfer coefficients were determined through the following equation.

h = 1.37(∆T/L)1/4

The relationship between power in and power out is shown in Figure 1.

ence in the power

he power leaving the distillation column begins low because the column has to allowed

t is important to note that the heat loss is dominated by heat loss through the cooling es

Energy Balance on Column

0

0.5

1

1.5

2

2.5

3

0 500 1000 1500 2000 2500 3000 3500

TIme (sec)

Pow

er k

W

Power OutPower In

Figure 1. Power in and Power out vs. Time Initially there is a large differcoming in and the power exiting. However, as the column heats up and the distillation process nears its end the power leaving the column approaches the power going into the column exponentially. Tto heat up first. As the six trays reach their dew points and the vapor moves its way up the column the power increases as well. Once all the trays are heated and distillate is being collected, the power gets closer to the value inputed into the system. Iwater. An average of 85% of the heat loss occurs through the cooling water. This leav15% to be accounted for by loss through natural convection of the column. To complete the balance the difference in input and output power must be accounted for.

Energy In -Energy Out

-0.5

0

0.5

1

1.5

2

2.5

3

0 500 1000 1500 2000 2500 3000 3500

Time (sec)

Pow

er k

W

Figure 2. Energy Balance Over Time The difference of energy in minus energy out is plotted versus time. This depicts the amount of energy lost through the column during the process. Initially a lot of energy is being lost because not enough power is being generated by the column. As the process nears completion the accumulation term decreases. Initially the distillation column starts at room temperature. It must be heated so that the vapor can reach the top of the column. Heat is therefore accumulated in the column. The total amount of heat accumulated during the process is equal to the area under the curve in Figure 2. The difference in energies decreases as time increases, showing that the column is no longer accumulating heat. The column has reached the same temperatures of the vapor inside, resulting in greater amounts of energy. An attempt was also made to complete an energy balance of the condenser at the top of the column. The balance was made on Q of the cooling water vs. the Q of the distillate. Both latent and sensible heat effects were taken into consideration for the calculation of the distillate. It was determined that the cooling water was absorbing 7 times more heat than the distillate was loosing. There are a number of reasons why these calculations may be off. To determine our heat of vaporization the distillate compositions had to be calculated. A number of assumption were made, such as constant pressure and initial compositions, so that the compositions could be determined through dew point calculations. Also, there could be errors in the readings from the thermocouples or a general problem with the apparatus. Conclusion In conclusion, we recognize that this is only a crude estimation of the heat balance on this distillation column. In the future we suggest that more than one trial be conducted so that replica data is available to back up calculations. Also, better assumptions should be made when calculating distillate compositions. Future groups may want to concentrate on determining the problem with the cooling water balance. For instance, why the cooling water is apparently absorbing so much energy from the system.

References Abbott, M.M., Smith J.M., Van Ness, H.C. Introduction to Chemical Engineering

Thermodynamics. McGraw Hill Company: New York, 1996. Beigler, Lorenz T., Grossman, Ignacio E., Westerberg, Arthur W. Systematic Methods of

Chemical Process Design. Prentice Hall:Upper Saddle River, 1997. Crawford, Chan, Guo, Shell, and Ip. Binary Distillation of Ethanol and Water. Group 3,

Fall 1999. Geankoplis, Christie J. Transport Processes and Unit Operations. Prentice

Hall:Englewood Cliffs, 1993.

Appendix A. - Nomeclature A = area Ak = Antoine coefficient Bk = Antoine coefficient Ck = Antoine coefficient Cp = heat capacity g = gravity h = heat transfer coefficent k = thermal conductivity L = length NGr = Grashof number NPr = Prandlt P = pressure P° = partial pressure q = heat-transfer rate T = temperature yk = composition of the distillate αk/n = P°k/ P°n

β = volumetric coefficient of expansion µ = viscosity ρ = density

Appendix B. - Calculations NGr = L3ρ2gβ∆T/µ2 NPr = Cpµ/k NNu = hL/k = a(NGrNPr)m h = 1.37(∆T/L)1/4 q = hA(Tw – Ta) Po

k(T) = exp(Ak – Bk/(Ck + T)) P = αavgPo

k(Tbub)/αk/n P= Po

n(Tdew)/Σ(yk/αk/n) A = 2πrL

Appendix C. – Graphs

igure C1. – Temperature of all Trays vs. Time

Temp vs Time all Trays

0

50

100

150

200

250

0 500 1000 1500 2000 2500 3000 3500

Time (s)

Tem

p (F

)

Tray 6TTray 5TTray 4TTray 3TTray 2TTray 1TTray 0T

F

Figure C2. – Temperature of the Cooling Water vs. Time

Tcw vs Time

0

20

40

60

80

100

120

0 500 1000 1500 2000 2500 3000 3500

Time (s)

Tem

p (F

)

Cooling OutCooling In

Tboiler vs. Time

Figure C3. – Temperature of the Boiler vs. Time

185

190

195

200

205

210

215

0 500 1000 1500 2000 2500 3000 3500

Time

Boi

ler T

emp

Boiler Temp

Energy Balance on Column

Figure 1. – Energy Balance on Distillation Column

0

0.5

1

1.5

2

2.5

0 500 1000 1500 2000 2500 3000 3500

TIme (sec)

Pow

er k

W

Power OutPower In

3

Figure 2. –Energy Difference vs. Time

-0.5

0

0.5

1

1.5

2

2.5

0 500 1000 1500 2000 2500 3000 3500

Time (sec)

Pow

er k

W

Energy In -Energy Out

3

Study of Distillation for Octanol, Water, and Ethanol for usewith a LLE Column

Estelle BurriYvonne Chen

Penny TanJohn Velicevich

Group 10

December 10, 1999

Abstract

The purpose of this lab was to determine the conditions necessary to distill a mixture of

octanol, water, and ethanol that comes from a LLE column. To do this we ran batch

distillation experiments at 1 atm using different feeds of unknown composition. The

approximate composition was a 30:25:45 ethanol:octanol:water. The GC results show

that there was no octanol in the distillate stream at a temperature of ~180 F and other

groups data showed that there was no ethanol in the bottoms. From this data we

recommend using the above conditions to combine the LLE and distillation column.

Background

Distillation is a common method for the separation of production streams in

industry. A distillation column acts like a series of equilibrium flash tanks stacked in a

column. The reboiler vaporizes part of the bottoms liquid stream and the vapor stream is

sent up through the column. The vapor stream is mixed with the liquid stream moving

down the column and allowed to reach equilibrium. The vapor stream then enters the

condenser where it is cooled to a liquid and sent to a holding tank. Part of the liquid can

then be sent back to the column to mix with the vapor phase.

If the stream is to remain in the liquid phase, another method of separation is

liquid liquid extraction. In this case one liquid phase stream containing the desired

component is contacted with an immiscible liquid. The key component will be

transferred to the other liquid, which can be easily separated by a different means.

IntroductionThe purpose of this lab is to separate a ternary mixture of ethanol, water and

octanol from the liquid liquid extraction column into two streams that can be returned to

the LLE column. The feed stream is approximately 30% ethanol, 25% 1-octanol, and 45%

water. The distillate phase from the distillation column was a mixture of ethanol and

water while the bottoms product was a two-phase mixture of water and octanol. We have

conducted our experiments using batch distillation. Our distillate products were analyzed

using a gas chromatograph (GC.) In what follows, we present our procedure, the results,

Collected the distillate

Run #2Used the bottoms product as feedRan the distillation columnCollected the distillate

Run # 3Used the bottoms product and added more ethanol and water mixtures as feed.Ran the distillation columnCollected the distillate.

Run #5Used what was already in the feed tank without adding anything.Ran the distillation columnCollected the distillate.

Results

Injection B Injection C Injection D Injection E Injection FT of distillate(degrees F) 171 171 172.5 172.5 172.5Water 0.708 0.778 0.209 0.4268 0.47Ethanol 2.788 2.419 1.155 2.86 2.83%water 20.3 24.3 15.3 13 14.2

Run number one Run number two

Injection G Injection H Injection I Injection JT of distillate(degrees F) 173.6 173.6 178 178Water 0.535 0.551 1.255 1.326Ethanol 2.9 2.92 2.2 2.126%water 15.6 15.9 36.3 38.4

Run number three Run number fiveTable 1.Results from the distillate. This shows that no octanol is present, and only a mixture of water and ethanol.

Discussion

Since the temperature of the distillate is slightly higher we expect the percentage of water

to increase slightly. Instead, the percentage of water decreased. However, the rest of the

data agreed with our expectations. In our analysis of the distillate stream, we see that the

ethanol and water mixture follows the Txy curve fairly well. Since there are impurities in

the system, the Txy curve should be treated only as an approximation. Our data for the

distillate stream is similar to that of the other groups’ data. We therefore assume that

since they have no ethanol in the GC analysis of their bottoms product, and we have

observed our bottoms product to split into a two-phase mixture, we can safely assume

that the amount of ethanol in the bottoms stream is negligible.

From the relative volatilities, we see that there is not going to be any octanol in

the distillate. The water is split into the distillate and bottoms stream, and since ethanol’s

relative volatility is higher than the light key, we can assume that all the ethanol will be

found in the distillate. If the feed composition were to change so that it contained a

higher percentage of water we would expect the flowrate for the distillate to decrease.

Since there is less ethanol in the system and the temperature would remain nearly the

same the flowrate will decrease. On the other hand if the amount of ethanol in the feed

increases compared to the amount water then the flowrate of distillate will increase.

Conclusions

Based on the results we feel that the system will be able to be connected to the

separate the two phase mixture of water and octanol before the octanol stream is sent

back to the LLE column.

Future work will be to determine the exact feed compositions that will be entering

the column from the LLE and to optimize the flowrates to be used in the LLE.

References

Biegler, L.T. et al, Systematic Methods of Chemical Process Design. PrenticeHall, NJ. 1997.

Brunsell, J, et al. “Liquid-Liquid Extraction Using the Force.” Group 6, 1999