Case Studies of Control in SSF Bioreactors 2831 Control of the Bioreactors at PUC Chile

Two aseptic packed-bed bioreactors (with nominal capacities for 50 kg and 200 kg of fresh solids) with periodic agitation and forced air were built at Pontificia Universidad Católica (PUC) of Chile to scale up the production of gibberellic acid by the filamentous fungus Gibberella fujikuroi. This process is particularly difficult to control since G. fujikuroi is sensitive to temperatures above 36°C and, due to its slow growth, to contamination by other microorganisms. Due to its filamentous nature, its growth increases bed compactness, which creates a reduction in heat and mass transfer rates, but this filamentous nature also means that it is sensitive to mechanical stresses when the solid bed is agitated. In addition, bed overheating can easily occur during the growth period. Air channelling is also a major problem, causing heterogeneous growth and large temperature differences within the solid bed. Finally, the processing time is long, demanding a robust control system. Key points are outlined below. More information is given in Fernández et al. (1996) and Fernández (2001).

The measured process variables used in these bioreactors are shown in Table 28.1 and the manipulated variables in Table 28.2. Figure 28.1 shows schematically how the instrumentation and control devices presented above were put together to define the control loops used in both bioreactors. The control strategy shown in Fig. 28.1 is described in Table 28.3.

Table 28.1. Measured variables in the PUC SSF bioreactors

Variable

Instrument

Measurement

Inlet and outlet air temperature

Type K thermocouples

On-line

Relative humidity of inlet air

Vaisala HMP 122B

On-line

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