Binary Distillation University of Illinois at Chicago
(Image from "Fractional Distillation.")
Binary Batch Distillation
Final Lab Report
Unit Operations Lab 2
19 February, 2011
Group 4
Tien Diep
Kevin Estacio
Sebastian Iskra
Zack Labaschin
Kevin Thompson
Felix Velazquez
1. Summary
The purpose of this lab is to run a batch binary distillation in order to separate methanol and water. By heating the mixture to near its boiling point, the more volatile component will vaporize first and travel up the distillation column. Data will be collected from the column at various trays and compared with a calibration curve in order to determine weight fraction. Once the weight fractions are determined, the tray efficiency can then be calculated. This was tested with a 10 wt% methanol solution.
For this experiment the distillation column featured six bubble cap type trays and was set up to operate at total reflux. At each stage there were two sample ports, one for collecting vapor product and the other for liquid, with a single liquid sample port at the top just below the condenser for drawing off a sample of reflux. Temperature probes were placed all over the apparatus and connected to a digital thermometer unit used to monitor the temperature at each tray, in the condenser, where the reflux was collecting, and the reboiler. The water pipes feeding the condenser featured mechanical temperature gauges for monitoring the change in temperature of the cooling water as it flowed through the condenser. Each sample taken from the column was then put into a refractometer and analyzed against a calibration curve to determine its composition.
The methanol concentration increased moving up the column, which makes sense as the concentration should be increasing moving up the column as the methanol boils sooner than the water; it is thus the lighter component and should be recovered in the distillate. The liquid concentration consequently decreased up the column due to a high relative volatility (McCabe, 578). The vapor and liquid fractions of methanol per stage do not add up to 1 which means that the species mass balance is not satisfied. This indicates a problem in the experiment, possibly with the design of the apparatus. The diameter of the column is most likely too small causing entrainment of the liquid. Another source of this error is that the methanol may have escaped the system through the top of the column thus creating false results.
2. Results
The distillation module required that operators set up a calibration curve in order to determine weight fractions at each tray via index of refraction (Figure 2.2). Different wt% methanol/water solutions were placed on the cooled refractometer at intervals of 10 wt% methanol. The purpose of this calibration curve is to compare it to the provided index of refraction curve of the system (Figure 2.1) and to find out how to analyze the data collected from the different stages of the distillation column after it has reached steady state.
A 32 Liter sample of 10wt% methanol was prepared in a side tank and then added to the boiler pot of the column. The column was then heated to steady state (approx. 2 hours). Steady state was distinguished by a lack of increase in temperature. Once reached, the compositions of the system at each stage of the column along with the reflux was collected and analyzed in the refractometer. The results for the gas composition at each stage showed that the methanol weight fraction increases as gas at each stage of the column moving upward. At the same time, the weight fraction of methanol in liquid decreased at each stage moving up the column (Bird, 698-702). The total weight fraction of methanol did not add to one though, often being over or less than 1. The final vapor weight percent exiting the column was found to be 79.51 wt% methanol. For the liquid portion, there was a general trend for the weight fraction of the methanol to decrease going from the top to the bottom of the column, but this was not the case for the liquid reflux stage (Figure 2.4). The weight percent of the liquid leaving the column above the reflux stage was found to be 1 wt% methanol, but the reflux stage was found to be 28.64 wt%.
Figure 2.1: Index of Refractions for the Methanol/Water System at 22 °C
Figure 2.2: Calibration Index of Refraction for Methanol/Water System at 34 ˚F
Figure 2.3: Index of Refraction vs Vapor Stage of Distillation
Figure 2.4: Index of Refraction vs Liquid Stage of Distillation
3. Discussion
In this experiment, a 10 wt% methanol water solution was prepared to gather the VLE data of the distillation system to determine the batch column efficiency for the system at total reflux. The 10% wt. methanol solution took about 2.5 hours to reach an equilibrium temperature across the 6-stages in the distillation column. The compositions of vapor and liquid samples collected at various stages of the column were determined using the calibration curve. The calibration curve was plotted by measuring the refractive indices of methanol and water solutions of known compositions (0-100 wt% methanol in increments of ten). The feed was assumed to be saturated vapor. The liquid that gathers on the trays was then reheated via convection of upward flowing vapor to allow for more of the volatile component to become a vapor and for water to flow downward as liquid. This allowed for a high purity in both the top and bottom of the distillation column and the component with the higher volatility was easier to vaporize.
The data indicates that the higher stages contained more methanol vapor, which is expected as methanol is the more volatile component of the methanol-water system. The index of refractions of the liquid and vapor compositions were analyzed at each stage of the distillation system in the refractometer and a corresponding index of refraction was recorded. In Figure 2.3, stage 0 in the index of refraction was 1.3431 and 1.3321 at stage 6. Since the calibration curve is a parabola, the higher the index of refraction, the closer to 50% wt for each component. As the stage height increases, the lower index of refraction is expected to increase the weight fraction of methanol in gas and decrease in liquid. Therefore the index of refraction is much higher at stage 0, where there is a higher concentration of both methanol and water, than is at stage 6, which is at the top of column.
As a result, to achieve a higher percentage of separation, more stages are needed or the vapor collected should be fed to another column for purification. The experimental errors were due to the fact that the concentration of the binary mixture was taken as an approximation. The temperature kept fluctuating from time to time, so the steady-state temperature might not have been at steady-state when taking stage readings for both liquid and gas.
4. Conclusions
A 10 wt% methanol solution was to be distilled from water and analyzed for purity. Solutions of differing concentrations of methanol were analyzed with a refractometer and a calibration curve, shown in Figure 2.2, was generated. This calibration curve was used to classify the composition of methanol in the samples analyzed. Once the column reached steady state samples were drawn from each stage of the column in the vapor and liquid phases. Each sample was then analyzed using the refractometer and compared against the calibration curve.
The final vapor weight percent exiting the column was found to be 79.51 wt% methanol. For the liquid portion, there was a general trend for the weight fraction of the methanol to decrease going from the top to the bottom of the column. The weight percent of the liquid leaving the column above the reflux stage was found to be 1 wt% methanol, but the reflux stage was found to be 28.64 wt%. The methanol concentration increased moving up the column in the gaseous state as it is the lighter component (Wankat, 26).
The composition of the product was not equal to 100% methanol. Methanol present in the column from previous experiments could be a source of impurity that may have oxidized to formaldehyde. Even though there were impurities present while conducting the experiment, the separation of two components was observed applying the concept of binary distillation.
5. References
"Binary Batch Distillation."University of Illinois at Chicago - UIC. Web. 14 Feb. 2011. <http://www.uic.edu/depts/chme/UnitOps/che382-2005f-frame.html>.
Bird, R. Byron, Warren E. Stewart, and Edwin N. Lightfoot.Transport Phenomena. New York: J. Wiley, 2007. Print.
"Fractional Distillation."Engineering. Web. 14 Feb. 2011. <http://engineering.wikia.com/wiki/Fractional_distillation>
.
McCabe, Warren L., Julian C. Smith, and Peter Harriott.Unit Operations of Chemical Engineering.Boston: McGraw-Hill, 2005. 576-78. Print.
Wankat, Phillip C.Separation Process Engineering. Upper Saddle River, NJ: Prentice Hall, 2007. Print.
6. Appendix I: Data Tabulation/Graphs
Gas Weight FractionStage / Index of Refraction / Weight Fraction / Efficiency
0 / 1.3431 / 0.51859 / -0.059913762
1 / 1.3405 / 0.54175 / -0.116102287
2 / 1.3391 / 0.573386 / -0.205783243
3 / 1.3379 / 0.602508 / 2.42089838
4 / 1.3345 / 0.7 / 0.259461356
5 / 1.3321 / 0.7951 / 3.085329478
Table I.1
Liquid Weight FractionStage / Index of Refraction / Weight Fraction / Efficiency
1 / 1.3419 / 0.45825 / 0.15444719
2 / 1.3432 / 0.5138 / -0.112569933
3 / 1.3404 / 0.45607 / -0.304611073
4 / 1.3375 / 0.387298 / 1.163148916
5 / 1.3331 / 0.248897 / 0.261854623
6 / 1.3305 / 0.1 / -0.309287681
Reflux / 1.3341 / 0.286356 / 0.478263469
Table I.2
7. Appendix II: Error Analysis
Component / Uncertainty Expected / Description100 mL Graduated Cylinder / ± 0.5mL / The 100mL graduated cylinder measures liquid in increments of 1mL. The lines can be accurately read to within ± 0.5mL
Digital Temperature Monitor / ± 0.1 oC / The temperature monitor measures in increments of 0.1 oC and is accurate to ± 0.1 oC.
Refractometer / ±0.0001 / The refractometer measures the refractive index of a mixture in increments of 0.0001 and values can be read accurately to the nearest 0.0001.
Table II.1
Error existed all throughout this lab. The preceding table shows the uncertainties of equipment used in the experiment. This uncertainty comes from the approximated error that comes from the reading of the instrumentation as these readings are fairly objective.
There was also error in the running of the operation of the experiment. The calibration curve itself was an error as it is a polynomial regression of data points. This regression was done over as many data points as possible in order to minimize this error, but this regression is only an estimate and not exact. Also, when mixing the solutions due to the uncertainty of the equipment more error was added into the polynomial regression. A polynomial regression was chosen due to the shape of the graph being a parabola as well as the polynomial regression being the best fit with a correlation factor of 0.94.
During column operation, when taking samples of low stage liquid or gas, the column sputtered and leaked, making contact with the operator. Not only is contamination of stage composition possible, but also the operator is put at risk by making undesired contact with methanol.
One improvement that can be made would be to obtain a different refractometer or different cooling system for the current refractometer, as the constant fluctuating coolant temperature of the system may have contributed to error in this lab’s results. Potential error in results may have occurred at the refractor station, as the refractor surface must be cleaned after every test, and the potential for accidental spills or contamination is high. Also, it would be more accurate to use fresh methanol as opposed to methanol that has been sitting in a drum for a long time since it’s possible that the old methanol oxidized over time.
8. Appendix III: Sample Calculations
The quadratic equation is obtained from plotting the index of refraction versus the weight percent of methanol:
-5*10-06x2 + 0.0001x + 1.331
For vapor composition:
-5*10-06x2 + 0.0001x + 1.331= 1.3431 (stage 1)
x = 51.85wt% vapor
mol% ratio: Molecular weight of MethanolMolecular weight of Methanol and water = = 0.6403
mol% methanol = 0.6403 * 0.5185 = 0.3319 mol% methanol
For liquid composition:
-5*10-06x2 + 0.0001x + 1.331= 1.3419 (stage 1)
x = 45.82 wt% liquid
mol% ratio = 0.6403
mol% methanol =0.6403 * 0.4582 = 0.2933 mol% methanol
For Murphee efficiency:
= 0.5189-0.54171-0.51850.5185-0.5417= - 0.05991
9. Appendix IV: Individual Team Contributions
Name: Kevin Sean ThompsonTime (HOURS) / Description
Operator (Both Lab Days) / 8.00 / Operator both days
Pre- Lab Editing / 0.50 / Final compilation
Final-Lab Editing / 1.00 / Final compilation
Summary / 1.00 / Wrote section
Introduction / 0 / ---
Literature Review/Theory / 0 / ---
Apparatus / 0 / ---
Materials and Supplies / 0 / ---
Procedure / 0 / ---
Anticipated Results / 1.50 / ---
Results / 0.50 / Reformatting
Discussion / 0 / ---
Conclusion / 0.50 / Editing grammar and content
References / 0.50 / Found and formatter all references
Data Tabulation/Graphs / 0 / ---
Error Analysis / 1.00 / Wrote section
Sample Calculations / 0 / ---
Job Safety Analysis / 0 / ---
Power Point Presentation / 2.00 / Wrote powerpoint
Total / 16.50
Name: Zachary Daniel Labaschin
Time (HOURS) / Description
Operator (Both Lab Days) / 8.00 / Operator both days
Pre- Lab Editing / 0 / ---
Final-Lab Editing / 0 / ---
Summary / 0.50 / Proof read
Introduction / 0 / ---
Literature Review/Theory / 4.00 / Wrote section
Apparatus / 0 / ---
Materials and Supplies / 0 / ---
Procedure / 0 / ---
Anticipated Results / 0 / ---
Results / 0 / ---
Discussion / 1.00 / Restyled emphasis
Conclusion / 0.50 / Proof read
References / 0 / ---
Data Tabulation/Graphs / 3.00 / Did calculations
Error Analysis / 0 / ---
Sample Calculations / 0 / ---
Job Safety Analysis / 0 / ---
Power Point Presentation / 0.50 / Proof reading
Total / 17.50
Name: Kevin Manuel Estacio
Time (HOURS) / Description