Dynamic modeling and modification of ternary semicontinuous distillation without a middle vessel for improved controllability and energy performance

Ternary distillation conventionally requires two sequential columns. In a process intensification technique, the second column is eliminated by operating the first column in a cyclic manner, where the intermediate component is withdrawn periodically from a side stream. This process, called semicontinuous distillation (SCD) without a middle vessel, can lower the separation costs significantly. However, it exhibits controllability challenges due to the periodic recycling of the side stream. In this work, the process controllability is improved by adding a surge tank in the side stream recycle path. The modification increases significantly the process robustness without changing the operation recipe. Also, the modified SCD can lower maintenance costs as the manipulated variables experience milder oscillations. Moreover, it is shown to have a faster startup than the original design, yielding about 13 % energy saving per feed processed and processing about 25 % more feed during the startup compared with the original design. The case studies show the surge tank volume should be chosen based on the trade-off between attenuation of undesired disturbances and slow-down of desired control actions. Also presented in this article is detailed hybrid discrete-continuous dynamic modeling of the SCD and how the resulting model is implemented in the open-source software OpenModelica.