On-Off and PI control of methane gas production of a pilot anaerobic digestion reactor

Abstract

A proposed feedback control system for methane flow control of a real pilot anaerobic digestion reactor fed with dairy waste is designed and analyzed using the modified Hill model, which has previously been adapted to the reactor. Conditions for safe operation of the reactor are found using steady-state responses of dynamic simulations, taking into account the upper limit of the volatile fatty acids (VFA) concentration recommended in the literature. The controllers used are standard process controllers, namely the on-off controller and the PI controller. Several PI controller tuning methods are evaluated using simulations. Two methods are favoured, namely the Skogestad method, which is an open loop method, and the Relaxed Ziegler-Nichols closed loop method. The two methods give approximately the same PI settings. Still, the Skogestad method is ranged first as it requires less tuning time, and because it is easier to change the PI settings at known changes in the process dynamics. Skogestad's method is successfully applied to a PI control system for the real reactor. Using simulations, the critical operating point to be used for safe controller tuning is identified.