DEVELOPMENT OF IMPROVED PID CONTROLLER FOR SINGLE-EFFECT EVAPORATOR

Abstract

Kraft pulping is the pulping process that is mostly used in paper mills. This is because the chemicals used in cooking are being recovered, and environmental pollution is minimized, as the chemicals are not discarded into the environment, but rather recycled. These chemicals are recovered by burning the black liquor that leaves the digester in a reactor called a recovery boiler. Feeding the black liquor directly from the digester to the recovery boiler will cause a water/smelt explosion. This explosion happens if the percentage of water in the liquor is high, the solid content of the black liquor leaving the digester is usually between 13-18%. To avoid such disaster the concentration of the black liquor has to be increased, which should in the range of 60-70%, this is done in a single or multiple effect evaporator. In this paper, a single effect evaporator control has been designed using a Proportional Integral Derivative (PID) controller. During the first design a simple PID controller with a feedback control was used. Then taking into consideration the load disturbance in the process, a feed forward control has then been introduced into the controller design. Subsequently the concept of boiling point rise (BPR) to measure concentration has then been used in the controller design of a single effect evaporator.