Vacuum distillation control

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Title: Vacuum distillation control
Author: Anderson, John Joseph
Abstract: 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 .
URI: http : / /hdl .handle .net /2346 /13006
Date: 1998-12


Anderson, John Joseph Vacuum distillation control. Doctoral dissertation, Texas Tech University. Available electronically from http : / /hdl .handle .net /2346 /13006 .

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