The fact that ideal two-product distillation columns may have multiple
steady states and unstable operating points has only recently been re
cognized. This paper discusses the dynamics and control implications.
It is shown that columns operating with reflux and boilup as independe
nt variables become unstable if the internal flows are sufficiently la
rge. An open-loop unstable operating point may be stabilized by use of
one-point control, i.e., feedback control of a column composition or
temperature. If the control is not sufficiently tight, the column may
go into a stable limit cycle. Finally, it is shown that with distillat
e flow and boilup as independent variables the operating points may be
come unstable if the level control is not sufficiently tight. The colu
mn may also in this case go into a stable limit cycle.