Batch Azeotropic Distillation
You must wear goggles while running this experiment!
We need to evaluate the validity of our analytical prediction of the performance of a batch distillation of a binary mixture which forms an azeotrope. Our method is a follows:
(1) Find the azeotropic composition, temperature, and pure component vapor pressure data from handbook or Internet sources.
(2) Use a two parameter activity coefficient equation (van Laar, Margules, Wilson, etc.) with the data from (1) to determine the parameters. Equilibrium vapor and liquid compositions, x and y, can now be predicted.
(3) Obtain published data of equilibrium y versus x data for 1-propanol/water system under 1 atm from handbook or Internet source. Compare the published data with the predicted y versus x obtained in (2). How well do they agree, especially in the range of concentration below the azeotropic concentration?
(4) Use the Rayleigh model for a batch distillation (available in any Unit Operations book or Separation Processes book) to generate the plot of distillate composition y, versus fraction of liquid distilled both experimentally and theoretically (using published or predicted y-versus-x data). This “sharpness curve” is useful to determine where “cuts” need to be made to achieve desired product fractions. (Imagine that you are a moon shiner in the 15th century who wishes to produce rich product). You may use either your predicted y versus x or published y versus x for this purpose.
(5) Compare the experimental sharpness curve for batch distillations of mixtures containing 5 and 20 mol % propanol. Also, compare the experimental sharpness curve with theoretical sharpness curve.
Supplies Available: Othmer still (for equilibrium y versus x), boiling flask, heating mantle, consenser, vials, refractometer, electronic balance, thermometers, and batch distillation bottle. Be careful when assembling the Othmer still. Observe the structure of the still and understand why this device is indeed an equilibrium still. Check with the instructor before you start to run the still.
Data may be analyzed with Excel that are installed on every computer in ECS 114.
Key measurements: The composition of the binary mixture is analyzed with refractometer. Search published data of refractive index versus concentration of 1-proponal in water. Suggested source: CRC Handbook.
Questions that will Help You Design Experiment or Analyze Data:
Equilibrium Study (with Othmer Still)
(1) Can the van Laar equation be directly solved for parameters A and B based on the published azeotropic condition? The Marqules equation? The Wilson equation?
(2) If you wish to apply the model parameters obtained in (1) to the prediction of y versus x, what assumptions are necessary? Let us assume that the model parameters are weak functions of temperature, can you tell that further assumption of constant vapor-pressure ratio (of 1-propanol to water) might help the theoretical prediction? Why? Note that the equilibrium temperature varies with composition under 1 atm.
(3) Make plots of activity coefficient versus liquid composition and of equilibrium vapor composition versus liquid composition based on experimental observations and theoretical predictions.
(4) From the published data of refractive index versus concentration of 1-propanol in water, can you tell what problem could arise if the starting mixture in the batch still (or Othmer still) contain more than 70% by mol?
(5) How can you check if the Othmer still has reached “steady state” and “equilibrium”?
(6) Why do we choose a binary system that has a potential to form an azeotrope?
Batch Distillation
(7) (1) Write the material balance leading to Rayleigh equation. What are its assumptions? Should x and y in the equation be mol fraction or mass fraction? Which is more convenient?
(8) Compare the sharpness curves generated with different starting 1-propanol concentrations from
(a) Theoretical prediction using published y-versus-x data with the knowledge of initial x in the still.
(b) Theoretical prediction using y-versus-x data predicted with various models and model parameters from (1) and (2) above.
(c) Experimental data of measured y and cumulative mass of binary mixture distilled recorded during the course of batch distillation.
(9) Do we generate “waste” in this experiment? Why or why not? Does distillation “destroy” 1-propanol? Certainly not! Be sure to save the “waste” and reuse it in further experiments.
Published Vapor-Liquid Equilibrium Data at 1 atm (partial—you may make an effort to search more complete data)
Source 1
Y(mole frac) X Temperature (oC)
0.375 0.075 89.05
0.388 0.179 87.95
0.4317 0.4317 87.72 (azeotropic condition)
0.438 0.482 87.80
0.560 0.712 89.20
0.685 0.850 91.70
Source 2 The following data are published at Journal of Chemical and Engineering Data, vol. 41, No. 3 (1996) on Page 403 . Students doing this experiment are urged to find the original paper and study it for background of this experiment.
| x | y | T (oK) |
| 0 | 0 | 373.15 |
| 0.053 | 0.353 | 362.85 |
| 0.1 | 0.378 | 361.85 |
| 0.15 | 0.387 | 361.3 |
| 0.2 | 0.395 | 361.25 |
| 0.253 | 0.4 | 361.2 |
| 0.3 | 0.403 | 361.05 |
| 0.347 | 0.414 | 361 |
| 0.4 | 0.421 | 360.95 |
| 0.438 | 0.438 | 360.9 |
| 0.5 | 0.447 | 360.95 |
| 0.538 | 0.461 | 361 |
| 0.604 | 0.485 | 361.3 |
| 0.65 | 0.514 | 361.7 |
| 0.7 | 0.55 | 362.1 |
| 0.751 | 0.585 | 362.8 |
| 0.8 | 0.634 | 363.6 |
| 0.847 | 0.693 | 364.65 |
| 0.9 | 0.77 | 366.1 |
| 0.952 | 0.87 | 368 |
| 1 | 1 | 370.4 |
Source 3 You may also use ProII or HYSYS to find the equilibrium composition under 1 atm. Remember to choose appropriate liquid activity model. Note: Peng-Robinson Equation and SRK equations of state do no apply here!
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