Optimal operation of Petlyuk distillation: steady-state behavior

The ``Petlyukʹʹ or ``dividing-wallʹʹ or ``fully thermally coupledʹʹ distillation column is an interesting alternative to the conven- tional cascaded binary columns for separation of multi-component mixtures. However, the industrial use has been limited, and diculties in operation have been reported as one reason. With three product compositions controlled, the system has two degrees of freedom left for on-line optimization. We show that the steady-state optimal solution surface is quite narrow, and depends strongly on disturbances and design parameters. Thus it seems dicult to achieve the potential energy savings compared to con- ventional approaches without a good control strategy. We discuss candidate variables which may be used as feedback variables in order to keep the column operation close to optimal in a ``self-optimizingʹʹ control scheme.