Vacuum distillation control
Anderson, John Joseph
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A detailed study of vacuum distillation column control implementations was performed with special emphasis placed on control configuration selection. Two vacuum separations were studied; toluene from xylene and ethylbenzene from styrene. Rigorous, dynamic simulations were developed for these two systems that incorporated varying tray-to-tray pressure drops and coupled, dynamic material and energy balances for each tray of the column. These columns were benchmarked against published data. For single-ended composition control, manipulating the reflux flowrate provided the best control of the overhead impurity for setpoint changes as well as for feed composition disturbance rejection. Bottom impurity control was best handled by ratioing the vapor boilup rate and the bottoms flowrate (boilup ratio, V/B). For dual-ended control, the [L,V] and [L,V/B] configurations provided better control of both product streams when the column has a reflux ratios near 1. In addition, the [L/D,V] and [L,V] configurations both provide good product impurity control especially when the bottom product stream is more valuable. These two configurations also performed best as the column's reflux ratio increased. Advanced control techniques such as decoupling and feedforward compensation were studied and decoupling was found to improved control performance on both product streams. Feedforward compensation improved configurations were ratio control was implemented (reflux ratio or boilup ratio) or when the process has slow dynamics. In addition, Dynamic Matrix Control (DMC) was applied to both the xylene/toluene columns and the styrene/ethylbenzene column. A [2x2] DMC controller was compared with decentralized PI controllers on several control configurations. For setpoint tracking, DMC improved control responses by decoupling control action on both ends of the column. For unmeasured feed composition disturbances, DMC did not have the control performance of PI as DMC lacked feedforward compensation for disturbances. In addition, DMC considered both product compositions as having equal importance. Finally, the minimum move suppression factors allowed by the DMC package were used which limited DMC performance for unmeasured disturbances. DMC does provide benefits on the styrene/ethylbenzene column by allowing the styrene composition to have a higher control priority. As a result, DMC performed comparably to PI for feed disturbances.