Understanding the dynamic behaviour of semicontinuous distillation

Semicontinuous, ternary distillation was first envisioned using a single distillation column and a middle vessel (MV) in the seminal work by Phimister and Seider. In the “sequential design methodology” introduced by Pascall and Adams for semicontinuous distillation, firstly, a steady state side-stream column that only meets the distillate and bottoms product purity specification is designed. Then by recycling the side-stream to the MV, the system is forced to move away from the steady state (initial state). Through repetitive input actions the system becomes periodic. The objective of the current study is to understand the effect of changing the above initial state on the nature of the periodic orbit; specifically, its period. The initial state is changed by varying the values of the internal column recycle rates and also the external recycle rate. The periods of the converged periodic orbits are remarkably lower (almost 18 %) when compared to the base case design. Results revealed that the sequential design methodology is inherently sub-optimal. In conclusion, a rigorous design procedure that searches the space of initial states to reach an economically optimal periodic orbit is essential.