Stable plantwide control of recycle systems

The interaction between design and control of chemical plants involving material recycles is addressed. Typical flowsheet and control structures are identified based on reaction stoichiometry and the physical properties of reactants and products. For the case of floating recycle, state multiplicity occurs associated with the instability of the operating points on the low-conversion branch. Fixing the reactor-inlet flow rate of each reactant is an effective means to control the mass balance, which makes the reactor behaviour similar with a stand-alone operation. When two or more reactants are recycled together, this would require composition measurements. In this case, designs achieving almost complete conversion of one reactant are suggested.