University College - Chemistry Chem 112A Lecture Notes - Lecture 40: Azeotrope, Fractional Distillation, Phase Diagram

1.[ Plot temperature (oC) vs. volume of distillate (ml) collected for the mixture with and without the fractionating column. Use the graph to determine the boiling ranges of the two compounds in the mixture and identify the compounds. Plot both sets of data on the same graph]- I know how to do that part and by the way the unknown compounds are acetone and 1-propanol. --Also plot on this graph an ideal distillation curve. Why are the real distillation curves different from the ideal? 2. Use a vapor/liquid diagram to explain the differences in the simple and fractional distillation plots. Describe what is happening in the fractionation column and explain why this results in a better separation with purer components isolated. 3. Why is it important to maintain a distilling rate of ~0.5 drop/s? Why not faster or slower? 4. What is an azeotrope? Sketch a distillation curve (as in Question 1) for a simple distillation of a mixture of toluene (70 ml) and water (30 ml). Explain the behavior of the curve.

1.[ Plot temperature (oC) vs. volume of distillate (ml) collected for the mixture with and without the fractionating column. Use the graph to determine the boiling ranges of the two compounds in the mixture and identify the compounds. Plot both sets of data on the same graph]- I know how to do that part and by the way the unknown compounds are acetone and 1-propanol. --Also plot on this graph an ideal distillation curve. Why are the real distillation curves different from the ideal? 2. Use a vapor/liquid diagram to explain the differences in the simple and fractional distillation plots. Describe what is happening in the fractionation column and explain why this results in a better separation with purer components isolated. 3. Why is it important to maintain a distilling rate of ~0.5 drop/s? Why not faster or slower? 4. What is an azeotrope? Sketch a distillation curve (as in Question 1) for a simple distillation of a mixture of toluene (70 ml) and water (30 ml). Explain the behavior of the curve.

An equimolar mixture of hexane and heptane is continuously produced in an oil refinery at a rate of 390lbmol/h. The alkane mixture is to be separated via continuous distillation with a design specification of 2% impurity allowable in each stream. The feed is a 2-phase mixture (75 mol% liquid and 25 mol% vapor). Assume that the liquid is ideal and that the column’s operating pressure is 1 atm. Assume a full condenser and a partial reboiler are used.

Do the following. You will find L08C and L08D to be helpful.

1.Use http://app.knovel.com/web/toc.v/cid:kpYHACVPEH to find the relative volatility a when the temperature is such as to give about 1 atm vapor pressure for hexane, and another value at the temperature giving about 1 atm vapor pressure for heptane.

2.Use the average of the relative volatilities found in 1 to calculate y versus x, choosing the component giving y > x. (Hint: see slide 18 in L05B.) Plot this using whatever method you like. Include the 45^o x=y line. You’ll need 4 copies of this.

3.Graphically determine the minimum number of theoretical trays N_min in the column that would meet the specifications.

4.Graphically determine the minimum reflux ratio R_min .

5.Graphically determine the minimum boilup ratio R_B,min . (This corresponds to the minimum reflux ratio determined in 4, but using the operating line for the stripping section of the column.)

6.Let us consider the economics of distillation. There are two costs – construction and operating. The operating cost is primarily for coolant for the condenser and heating fluid for the reboiler. At total reflux, we get the lowest construction cost and a low operating cost, but no product. Thus, the cost per unit of product is infinite. At minimum reflux, the operating cost is a minimum but the column is infinitely high, and so again the cost per unit of product is infinite. The economic optimum condition is somewhere in between.

Estimate the economic optimum reflux ratio using the Gilliland Correlation (Figure 9.20 on
p 306 of the text) by selecting (R - R_min)/(R+1) = 0.4. Calculate R using your R_min, construct a McCabe-Thiele diagram using this R, and graphically determine the number of theoretical trays in the column. Compare this with that predicted by the Gilliland Correlation. What is the optimal feed tray, counting from the top?

Submit all of your calculations and plots.