Design, steady-state optimization, and cascade dynamic control of a continuous four-columns vacuum distillation process for coal tar-derived mixed phenols

Mixed phenolic homologues derived from coal tar, characterized by close boiling points and high thermal sensitivity, exhibit poor separation efficiency and thermal degradation in traditional atmospheric distillation processes. This study designs a continuous four-columns vacuum distillation process. The NRTL-HOC thermodynamic model is selected to rigorously describe the liquid-phase non-ideality and vapor-phase association of polar phenolic molecules. A separation sequence comprising a phenol column, an o-cresol column, an m-/p-cresol mixed-fraction column, and a xylenols column is constructed. The theoretical stages, reflux ratios, and feed locations are optimized by steady-state simulation to minimize energy consumption and total annual cost while satisfying purity and recovery constraints. Considering the strong coupling and large time delays, a temperature-concentration cascade control system with feedforward compensation is designed. Phenol, o-cresol, the m-/p-cresol mixed fraction, and xylenols achieved purities of 99.8%, 99.5%, 99.2%, and 99.0%, with recoveries of 99.1%, 98.7%, 98.2%, and 97.5%, respectively. The total reboiler duty of the four-columns system was 5686.7 kW. Under feed-flow disturbances of ±10% and feed-composition disturbances of ±5%, product purity was restored within 80 min, with a maximum deviation of no more than 0.15%. At 18.0 kPa, the optimized four-columns process achieved a maximum annual net economic margin of USD 1.585 million.