"Control structure selection for Reactor, Separator and Recycle Processes"  

T. Larsson, M.S. Govatsmark, S. Skogestad and C.C.Yu

Ind.Eng.Chem.Res., 2003, vol. 42, 1225-1234 

Abstract. We consider control structure selection for a simple plant
with a liquid phase reactor, a distillation column and recycle of
unreacted reactants. Plants of this kind have been studied extensively
in the plantwide control literature. 
Our starting point is a clear definition of the operational
objectives, constraints and degrees of freedom.
Active constraints should be controlled to optimize economic performance.
This implies that reactor level should be kept at its maximum, which
rules out many of control structures
proposed in the literature from being economally attractive.
The main focus is on the selection of a suitable controlled variable for
the remanining unconstrained degree of freedom, where we use 
the concept of self-optimizing control, which is to search for a constant setpoint strategy
with an acceptable economic loss. We find that good controlled variables
is the recycle recycle ratio L/F. This applies for cases with a given feedrate as
well as for cases where the feedrate should be maximized. It also applies to both
 single-loop control and  multivariable model predictive control.
 In order to avoid the socalled ``snowball" effect, Luyben suggested
 as a generic rule to ``fix a flow in a liquid recycle loop''. However, 
 the rule seems to have little basis, as it leads
 to control structures that can only handle small feedrate changes.