Page 242 - Separation process engineering

0.6, L/D = 3.3, 0.99 recovery of A in the distillate, and 0.995 recovery of C in the bottoms.
Calculate the fractional recovery of B, the distillate and bottoms flow rates, the mole fractions
on every stage, the optimum feed stage, and the number of stages.

Note: This problem is challenging because the program is sensitive and fractional recoveries
change as the feed stage is changed.
H5. [VBA required] Including a bubble-point or dew-point calculation on every stage is not
substantially more work once the program for constant relative volatility has been developed.

a. Develop a program for a system with an LK, HK, and HNK (step off stages from the bottom up)
doing a bubble-point calculation on every stage.
b. Use this program to find the optimum feed stage, the total number of stages, distillate and
bottoms flow rates and compositions, and composition and temperature profiles for the
following problem. We wish to separate 100 kmol/h of a saturated liquid feed that is 30 mol%
n-butane, 30 mol% n-pentane, and 40 mol% n-hexane. Column pressure is 14.7 psia. The

fractional recovery of n-butane in the distillate is 0.995, and the fractional recovery of n-
pentane in the bottoms is 0.997. L/D = 8.0. Equation (2-30) and Table 2-3 can be used to
calculate K values.

Chapter 5 Appendix. Spreadsheet Calculations for Ternary Distillation with
Constant Relative Volatility

The spreadsheet results and the VBA program for ternary distillation calculations with constant relative
volatility (Section 5.2.) are shown in Figure 5-A1 and Table 5-A1. If you are not familiar with VBA look
at Appendix B of Chapter 4. The problem solved is to determine the number of stages and the optimum
feed stage for the distillation of 100 mol/h of a saturated liquid feed that is 30 mol% A, 20 mol% B, and
50 mol% C. L/D = 1, and the desired fractional recoveries are B in distillate = 0.99 and C in bottoms =
0.97. Component A = benzene, component B = toluene, and component C = cumene. The constant relative
volatilities with respect to toluene as the reference are α AB = 2.25, α = 1.0, and α = 0.21. By trial
BB
CB
and error, the optimum feed stage was determined to be the second stage from the top (the total condenser
is not counted as a stage).
The VBA program that goes with this spreadsheet and calculates the values of D, B, L/V, Lbar/Vbar and
everything below these values is given in Table 5-A1. If you want to understand how to use spreadsheets
with VBA, you need to work Problems 5.H1 to 5.H4. Note that the VBA program in Table 5-A1 includes
a number of bells and whistles (e.g., the tests for feed stage too high or reflux ratio too low or

convergence of mass balance based on guess of LNK recovery). These refinements would not normally be
included in a program developed to solve a single problem but are useful if a number of problems will be
solved. The use of a fractional number of stages to estimate bottoms mole fractions to determine the
fractional recovery of the LNK is included to prevent excessive oscillations in the answer when there are
an LK, a sandwich component, and an HK (see Problem 5.H4.).
Figure 5-A1. Spreadsheet results for ternary distillation with constant relative volatilities. Step off
stages from top of column