Variable Dead Time Compensation and Calculation of Salt-Acid Concentration Ratio Improves Evaporator Control

摘要

Control of the salt-to-acid concentration ratio in an evaporator has proved to be a difficult problem at the Oak Ridge Y-12 Plant. Rugged online measurement techniques had to be developed for the determination of salt and acid concentrations. A control scheme also had to be developed to cope with the large dead times present in a pilot-plant evaporator. To address the concentration measurement problem, density and conductivity measurements were interfaced to a microcomputer which was used to calculate salt and acid concentrations. Dead times which approached the process time constant in magnitude caused a standard PI controller to produce unacceptable undershoots of 25%. To address this problem, a dead time compensation scheme utilizing a first-order process model with variable dead time was implemented in the microcomputer. Control with dead time compensation dramatically improved the performance with the virtual elimination of undershoot. This paper discusses the development of both online measurements and variable dead time compensation used in the evaporator control. (ERA citation 12:023388)