Problems in volumetric flow rate and liquid level control of a continuous stirred tank bioreactor with structured and unstructured kinetics

Abstract

All equipment and devices utilized in various chemical and biochemical technologies are subject to external disturbances. In continuous flow systems, usually operating under steady-state conditions, these disturbances result in departure from the steady state. The paper reports on the process consequences of perturbations of the volumetric flow rate in a continuous stirred tank microbiological reactor, and on the automatic control of the liquid level by means of continuous P and PI controllers. The dynamics of the uncontrolled and controlled bioreactor were compared based on two kinetic models: an unstructured Haldane model and an one-parameter structured model.The paper investigates the effects of sudden step and oscillatory disturbances. It was shown that, for oscillatory variations in the feed rate, the biomass washout phenomenon depends on the period of oscillation, and occurs for oscillations with a sufficiently long period. This unfavorable phenomenon can be eliminated by applying liquid level control. Additionally, it was demonstrated that the use of a structured model gives a more reliable assessment of the dynamics of uncontrolled and controlled bioreactors subjected to step forcing. If the bioreactor is subjected to sinusoidal forcing, then the type of kinetic model used, i.e. structured or unstructured, made no difference.