Question

a. A mixture of 60 mol % n-proplycyclohexane and 40 mol % n-propylbenzene is distilled through a simple distillation apparatus; assume that no fractionation occurs during the distillation. The boiling temperature is found to be 157 °C (760 torr) as the first small amount of distillate is collected. The standard vapor pressures of n-propylcyclohexane and n-propylbenzene are known to be 769 torr and 725 torr, respectively, at 157.3 °C. Calculate the percentage of each of the two components in the first few drops of distillate.

b. A mixture of 50 mol % benzene and 50 mol % toluene is distilled under exactly the same conditions as in Part a. Using Figure 5.1, determine the distillation temperature and the percentage composition of the first few drops of distillate.

c. The normal boiling points of n-propylcyclohexane and n-propylbenzene are 156 °C and 159 °C, respectively. Compare the distillation results in Parts a and b. Which of the two mixtures would require the more efficient fractional distillation column for separation of the components? Why?

Answer 1

(a)

Vapour pressure of n-propylcyclohexane = 769 torr

Vapour pressure of n-propylbenzene = 725 torr

partial pressure of n-propylcyclohexane = 0.60 x 769 torr =461.4 torr

partial pressure of n-propylbenzene = 0.40 x 725 torr = 290 torr

P_total = 290+ 461.4 == 751.4 torr

mole fraction of n-propylcyclohexane = 461.4 torr / 751.4 torr x 100 == 61.4 %

mole frcation of n-propylbenzene = 290 torr / 751.4 torr x 100 == 38.6 %

For other part provide figure 5.10 or do same calculations like in part-a

(c)

Since the diffrence in boiling points of n-propylbenzene and n-propylcyclohexane is very low 3 degrees So, simple distillation cannot be used here. So fractionla distillation is necessary here. Where as the temp diffrence of benzene and tolune is about 30 degrees.

(b)

initial liquid composition is 50- 50 , so draw a line from 0.5 toluene to the liquid line. The point here is A .Now draw a straight line from point A on to vapor line. This is the first equilibrium stage. This point is B. Now check the concentration of point-B from the x-axis.As this is single stage distillation u can stop with here and note the temperature corresponding to B from y-axis

The compostion is 20% toluene and 80 % benzene.

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