Introduction

The success of a fermentation depends upon the existence of defined environmental conditions for biomass and product formation. To achieve this goal it is important to understand what is happening to a fermentation process and how to control it to obtain optimal operating conditions. Thus, temperature, pH, degree of agitation, oxygen concentration in the medium and other factors may have to be kept constant during the process. The provision of such conditions requires careful monitoring (data acquisition and analysis) of the fermentation so that any deviation from the specified optimum might be corrected by a control system. Criteria which are monitored frequently are listed in Table 8.1, along with the control processes with which they are associated. As well as aiding the maintenance of constant conditions, the monitoring of a process may provide information on the progress of the fermentation. Such information may indicate the optimum time to harvest or that the fermentation is progressing abnormally which may be indicative of contamination or strain degeneration. Thus, monitoring equipment produces information indicating fermentation progress as well as being linked to a suitable control system.

In initial studies the number of functions which are to be controlled may be restricted in order to gain more knowledge about a particular fermentation. Thus, the pH may be measured and recorded but not maintained at a specified pH or the dissolved oxygen concentration may be determined but no attempt will be made to prevent oxygen depletion.

Also, it is important to consider the need for a sensor and its associated control system to interface with a computer (to be discussed in a later section). This chapter will consider the general types of control systems which are available, specific monitoring and control systems and the role of computers. More information on intrumentation and control has been written by Flynn (1983,1984), Armiger (1985), Bull (1985), Rolf and Lim (1985), Bailey and Ollis (1986), Kristiansen (1987), Montague et al. (1988), Dusseljee and Feijen (1990), Atkinson and Mavituna (1991) and Royce (1993).

It is apparent from Table 8.1 that a considerable number of process variables may need to be monitored during a fermentation. Methods for measuring these variables, the sensors or other equipment available and possible control procedures are outlined below.

There are three main classes of sensor:

1. Sensors which penetrate into the interior of the fermenter, e.g. pH electrodes, dissolved-oxygen electrodes.

2. Sensors which operate on samples which are continuously withdrawn from the fermenter, e.g. exhaust-gas analysers.

3. Sensors which do not come into contact with the fermentation broth or gases, e.g. tachometers, load cells.

It is also possible to characterize a sensor in relation to its application for process control:

1. In-line sensor. The sensor is an integrated part of the fermentation equipment and the measured value obtained from it is used directly for process control.

2. On-line sensor. Although the sensor is an integral part of the fermentation equipment, the measured value cannot be used directly for control.

Table 8.1. Process sensors and their possible control functions

Category

Sensor

Possible control function

Physical

Temperature

Heat/cool

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