Entrainer-Assisted Pressure-Swing Distillation for Separating the Minimum-Boiling Azeotrope Toluene/Pyridine: Design and Control

The separation of toluene/pyridine by distillation still remains an obstacle in industry because of not only the formation of a minimum-boiling azeotrope but also their relative volatilities, whose ratio is close to unity throughout the whole composition range. Extractive distillation is still not feasible because it is hard to find suitable entrainers. Heterogeneous azeotropic distillation might require high energy consumption to obtain both purified products. Conventional pressure-swing distillation is also not feasible. In this work, we apply entrainer-assisted pressure-swing distillation to separate toluene/pyridine (95 mol %/5 mol %) by introducing n-propanol as an entrainer. Based on the different compositions of the recycling stream sent back to the azeotropic column, a two-column sequence and a three-column sequence are established. Both sequences are partially heat-integrated. The steady-state designs of both sequences are optimized based on the total annual cost (TAC). The results reveal that the optimal two-column sequence has a 16.07% reduction of TAC and a 14.39% energy savings compared with the three-column sequence. Furthermore, the dynamic controllability of the two-column sequence is investigated by introducing ±10% disturbances in the feed flow rate and ±20% disturbances in the pyridine concentration. A conventional temperature control structure is implemented. As the fresh feed is quite dilute in pyridine, the purity of the pyridine product will be significantly influenced by even a small change in the recycling-stream flow rate. Therefore, the recycling-stream flow rate must be selected as a manipulated variable that is manipulated to control the pyridine product purity. The results reveal that robust control can be achieved.