Approximate Methods for MulApproximate Methods for Multicomponent, Multistage Sepaticomponent, Multistage Sepa

rationsrations

Chapter9Chapter9

Exercises

Exercise9.1

A mixture of propionic and n-butyric acids, which can be assumed to form ideal solutions, is to be separated by distillation into a distillate containing 95 mol% propionic acid and a bottoms product containing 98 mol% n-butyric acid. Determine the type of condenser to be used and esti mate the distillation column operating pressure

Exercise9.2

For 15 minimum equilibrium stages at 250 psia, calculate and plot the percent recovery of C3 in the distillate as a function of distillate flow rate for the distillation of 1,000 lbmol/h of a feed containing 3% C2, 20% C3, 37% nC4, 35% Cs, and 5% nC6 by moles. Obtain K-values from Figures2. 8 and 2.9.

Exercise9.3

Calculate and plot the minimum external reflux ratio and the minimum number of equilibrium stages against per cent product purity for the separation by distillation of an equimolar bubble-point liquid feed of isobutane/n-butane at 100 psia. The distillate is to have the same iC4 purity as the bottoms is to have nC4 purity. Consider percent purities from 90% to 99.99%. Discuss the significance of the results.

Exercise9.4

Use the Fenske-Underwood-Gilliland shortcut method to determine the reflux ratio required to conduct the distillation operation indicated in Figure 9.25 if N/Nmin = 2.0, the average relative volatility = 1.11, and the feed is at the bubble-point temperature at column feed-stage pressure. Assume that external reflux equals internal reflux at the upper pinch zone. Assume a total condenser and a partial reboiler.

Exercise9.5

The hydrocarbon feed to a distillation column is a bub ble-point liquid at 300 psia with the mole fraction composi tion, C2 = 0.08, C, = 0.15, nC4 = 0.20, nCs = 0.27, nC6 = 0.20, and nC7 = 0.10.

(a) For a sharp separation between nC4 and nC5 , determine the column pressure and type of condenser if condenser outlet temperature is 120°F.

(b) At total reflux, determine the separation for eight theo retical stages overall, specifying 0.01 mole fraction nC4 in the bottoms product.

(c) Determine the minimum reflux ratio for the separation in part (b).

(d) Determine the number of theoretical stages at L/D ~ 1.5 times minimum using the Gilliland correlation. .

Exercise9.6

One hundred kilogram-moles per hour of a three component bubble-point mixture to be separated by distillation has the following composition:

Component Mole Fraction Relative k A 0.4 5 B 0.2 3 C 0.4 1 (a) For a distillate rate of 60 kmol/h, five theoretical stage, and total reflux, calc

ulate the distillate and bottomsi- tions by the Fenske equation. (b) Using the separation in part (a) for components B and C, determine the mini

mum reflux and minimum boilup ratio by the Underwood equation. (c) For an operating reflux ratio of 1.2 times the minimum, determine the numbe

r of theoretical stages and the feed-stage location.

Exercise9.7

One thousand kilogram-moles per hour of rich gas at 70°F with 25%, 15% C2, 25%, 20% C4, and 15% nC5 by moles is to be absorbed by 500 kmol/h of nC10 at 90°F in an absorber operating at 4 atm. Calculate by the Kremser group method the percent absorption of each component for: (a) Four theoretical stages, (b) Ten theoretical stages, and (c) Thirty theoretical stages. Use Figures 2.8 and 2.9 for lvalues.