On the controllability of continuous fermentation processes

Abstract

Different reactor con®guration for continuous fermenters are examined in terms of their controllability properties. This is done by examining the control input required for perfect disturbance rejection. It is shown that using an additional inlet stream (which does not contain any growth limiting substrate) as a control input leads to superior control performance than a single stream fermenter. Recycles can improve the controllability if the outlet stream is used as a control input. The controllability analysis is based on linearised process models and is illustrated with simulations of the nonlinear process models using PI controllers. IntroductionIn a process plant, some of the states are controlled by manipulating certain inputs. These states are known as the controlled outputs, and they are controlled for reasons of safety, the demand for constant product quality or because the plant economics are sensitive to changes in their values. The manipulated inputs are usually adjusted in a bounded range during plant operation. Once the plant is built, it is necessary to design a control system that satis®es the aims of control. If this is possible the process is said to be controllable. However, it is possible for a process to be uncontrollable. In such cases no control system (no matter how sophisticated it is) will be able to achieve the aims of control. Controllability should therefore be taken into account at early stages of the design procedure in order to avoid major problems related to the control of the process at later stages.An evaluation of controllability aims to answer the following questions. Which plant properties or design variables make the plant more or less controllable, and what corrective actions can be taken if it is found that the plant has bad controllability properties or is even uncontrollable?In a biochemical plant the performance of the downstream processes depends strongly on the performance of the fermentation. It is therefore important to keep variations of the fermentation's states small. In [1] various con®gurations of continuous bioreactors were examined for their controllability properties based on steady state arguments. It was found that a better control performance of a bioreactor can be expected if a second feed stream, which does not contain any growth limiting substrate, is used as the manipulated control input. In [2] comparison was made using the dilution rate or the input substrate concentration as the manipulated variable in order to keep the cell concentration constant. The substrate concentration of the inlet¯ow yielded better controllability properties than the dilution rate. This result was con®rmed by others ([3]). In [4] a high concentrated feed stream and a substrate recycle were added to a single continuous fermenter to improve the controllability. In [5] different con®gurations of a continuous bioreactor with cross¯ow ®ltration recycle were studied. All these results show that different fermenter con®gurations have different controllability properties. The object...

Analysis and Control of Nonlinear Process Systems

“…Various difficulties are reported in the literature, which include instability and controllability problems for both continuous and fed-batch fermenters ( [44], [43]). Various difficulties are reported in the literature, which include instability and controllability problems for both continuous and fed-batch fermenters ( [44], [43]).…”

Fuzzy servo controller for CSTB with substrate inhibition kinetics

“…Szederkényi et al [7] compared several controllers for a continuous bioreactor operating near optimal productivity. Different reactor configurations were examined in terms of their controllability properties [8]. Sliding mode control has been used in continuous fermentation processes to keep the culture in steady state [9][10][11].…”

Reachability of nonlinear fed‐batch fermentation processes

“…On the other hand, there are very few publications on the controllability analysis of fermentation processes. In Reference [9] different reactor configuration for continuous fermenters are examined in terms of their controllability properties using linearized process models. It is shown that using an additional inlet stream (which does not contain any growth limiting substrate) as a control input leads to superior control compared to a single stream fermenter.…”