Principles of Instability and Stability in Digital PID Control Strategies and Analysis for a Continuous Alcoholic Fermentation Tank Process Start-up

Some professionals in the industrial community are not as familiar with digital PID controllers as the analog one. So the art work of this present paper is to show properly conditions and aspects to reach stability in the process control, as also to avoid instability, through the use of digital PID controllers, with the objective of help the industrial community. The discussed control strategy used to reach stability is to manipulate variables that are directly proportional to their controlled variables. To validate and to put the exposed principles into practice, it was done two simulations of a continuous fermentation tank process start-up with the yeast strain S. Cerevisiae NRRL-Y-132, in which the substrate feed stream contains different types of sugar derived from cane bagasse hydrolysate. These tests were carried out through the resolution of conservation laws, more specifically mass and energy balances, in which the fluid temperature and level inside the tank were the controlled variables. The results showed that if the exposed proceedings are followed, the system stabilize without difficulties.