Process Design for Reduced Disturbance Sensitivity of Integrated Plants

The disturbance sensitivity of an integrated plant is to a large extent caused by interactions between the process units, imposed by material and energy recycle flows. In this paper we show how a plug flow (delay) tank integrated in the recycle path can be used to modify these interactions such as to reduce the disturbance sensitivity in a given frequency range. The principle idea is to modify the physical feedback present in the process such that it provides disturbance damping, rather than amplification, in the frequency range where feedback controllers can not be made effective. As we show, the required capacity for a given level of disturbance attenuation with the proposed method can be severalfolds smaller than that required by a traditional cascaded buffer system. The results are illustrated by application to a reactor-separator system with recycle of unconverted material.