Constraints ◾  47
controller for a temperature point in the stripping section, and
the distillate or reflux is manipulated by another controller for
a temperature point in the rectification section.
6.2 Heat Transfer Constraints
The amount of boilup vapor produced from the reboiler is
constrained by the heat transfer surface in the reboiler and
the fouling in combination with the temperature of the heating medium compared to the boiling temperature. The amount
of condensation is constrained by the amount of heat transfer
surface in the condenser and fouling in combination with the
temperature of the cooling medium compared to the bubble
point of the overhead vapor stream at the column pressure.
6.3 Stripping Mass Transfer Constraint
There is also another constraint, or at least a point of diminishing return, based on the limitation of the rate of mass
transfer that can take place in a distillation column as shown
by Fair 1 and Kohl. 2 The stripping of volatile components in the
stripping section below the feed point in a column is driven
by the stripping factor of 1.0 or greater for the light key impurity, Equation 6.1. A stripping factor below 1.0 will be equilibrium pinched in the stripping section. A stripping factor above
about 2.0 will be mass transfer limited for stripping solute
from the liquid phase.
S = Vy/Lx
(6.1)
where:
S = stripping factor, or lambda (λ)
V = vapor flow rate