122 ◾  Appendix: Solutions to Chapter Exercises
Exercise 5.3
Either a scheme using a fixed reflux flow rate and an MRT
controller that manipulates the steam flow rate to the
reboiler, or a scheme using a steady steam flow rate to
the reboiler and an MRT controller that manipulates the
reflux flow rate.
Exercise 6.1
The maximum constrained value is 20,000 lb/h reflux
because it is 100% of the flow sensor output.

Exercise 6.2
Using Equation 6.1, S = (0.02) (39000) = 780
Equation 6.2, NTS = Ln[(1000/2)(1 − 1/780) + 1/780]/Ln 780
= 0.93 theoretical stages.
Exercise 6.3
Using Equation 6.1, S = (0.02) (39000) = 780
Equation 6.3, N OL = {Ln[(1000/2)(1 − 1/780) + 1/780]}/{1 −
1/780} = 6.2 liquid phase mass transfer units.
Exercise 6.4
Using Equation 6.9, N OG = Ln(200/0.1) = 7.6 gas phase transfer units.