Presented
at ESCAPE'12 Symposium,
Optimal Number of Stages in
Distillation with respect to Controllability
Marius
S. Govatsmark, and Sigurd
Skogestad
Department
of Chemical Engineering, NTNU
N-7491
Abstract
The
central question to be examined in this paper is if it is optimal to have a
large or small number of stages in a distillation column with respect to
controllability when the objective is to have dual composition control. With
multivariable controllers and without considering model uncertainty few stages
shows somewhat better controllability than many stages because (i) the available manipulated variables have larger effect
on the outputs (allowing larger changes in manipulated variables for few
stages) and (ii) it is not necessary to reject the disturbances as fast as for
many stages. However, in reality there will always be model uncertainty, and
with uncertainty included the conclusion is reversed: it is better to have many
stages. The reason is that with more stages the system is less interactive and
thus less sensitive to uncertainty. Physically, with many stages a pinch zone
develops around the feed stage, which tends to decouple the two column ends
from each other.