Constraints ◾  53
6.11 using the stripping factor for the heavy key impurity in
each stage, where NTS = 1:
N OG = NTS (Ln S)/(S − 1)
(6.11)
Distillation of the cyclohexane–n-heptane system does not get
into constrained mass transfer rate limited conditions, because
the relative volatility is not high enough. The stripping factor
for cyclohexane does not go above 1.75 in the stripping section, and the stripping factor of n-heptane does not go below
0.6 in the rectification section even at total reflux. In that case,
the use of theoretical stages for design is reasonable.
6.5 Summary
In this chapter, all of the process flow rates were considered to be constrained by zero flow and the maximum flow
allowable by the valve size and span of the flow measurement. The main column constraint because of flooding is
associated with the vapor traffic and pressure drop across
the trays or packing in the distillation column. The mass
transfer rate limit for stripping light key impurity from the
bottoms stream was presented. The mass transfer rate limit
for absorbing heavy key impurity from the overhead vapor
stream was also presented.
Exercises
6.1 If the reflux flow to a distillation column uses cascade
control, the span of the flow meter is 20,000 kg/h, and
the reflux valve will allow 25,000 kg/h of flow when it
is wide open, what is the reflux flow rate at the constraint limit?