Info

Before 20 h 25 min the air was blown into the left end of the bioreactor, giving this temperature profile at 20 h 25 min

0.2 0.4 0.6 0.8 1.0 Fractional height within the bed

Fig. 24.7. Predicted axial temperature profiles at the time of peak heat production, for a fermentation in a packed-bed, in which there is no heat removal through the side walls and for which the direction of the air flow is reversed every five minutes. The inlet air temperature is 30°C. The bed height is 0.345 m and the superficial air velocity is 2.36 cm s"1. This figure predicts that the temperature at the ends of the beds never exceeds 40°C, however, there is essentially no cooling effect in the central regions of the bed, which reach temperatures of more than 50°C. Adapted from Ashley et al. (1999) with kind permission of Elsevier

0.2 0.4 0.6 0.8 1.0 Fractional height within the bed

Before 20 h 25 min the air was blown into the left end of the bioreactor, giving this temperature profile at 20 h 25 min

Since 20 h 25 min the air was blown into the right end of the bioreactor causing the temperature profile to change to this one by 20 h 30 min

Fig. 24.7. Predicted axial temperature profiles at the time of peak heat production, for a fermentation in a packed-bed, in which there is no heat removal through the side walls and for which the direction of the air flow is reversed every five minutes. The inlet air temperature is 30°C. The bed height is 0.345 m and the superficial air velocity is 2.36 cm s"1. This figure predicts that the temperature at the ends of the beds never exceeds 40°C, however, there is essentially no cooling effect in the central regions of the bed, which reach temperatures of more than 50°C. Adapted from Ashley et al. (1999) with kind permission of Elsevier the previous section, it is possible to give advice about the research and development program for scale-up.

After the preliminary kinetic investigation in Raimbault columns (Sect. 15.1), experiments should be done in a pilot-scale packed-bed. A reasonable scale would be of the order of 15 cm diameter and as much as 1 m height. The walls of the column should be insulated well, in order to mimic the situation in the large-scale bioreactor, in which radial heat removal will be relatively minor. The 1 m height will allow studies to be done at bed heights that might actually be used in large-scale bioreactors. As such, this pilot bioreactor will represent a vertical section of the full-scale bioreactor (Fig. 24.8). This enables a study of those phenomena that depend on bed height, such as axial temperature profiles and pressure drops, and biomass and product formation as functions of height, and how these are affected by the temperature and velocity of the inlet air.

The advantage of this approach is that you might identify limitations on performance that are not predicted by the mathematical model. For example, the pressure drop may be excessive in your particular system. It is better to identify such problems, and to modify the bioreactor to overcome them, in a pilot-scale bioreac-

tor than it is to build a full-scale bioreactor only to find that it does not work properly.

In the scale-up process, there will be a limit on bed height, in the sense that very tall beds will lead to unacceptably poor performance, due to axial temperature gradients or other considerations. Once this limit is reached, the capacity of the bioreactor can only be increased by making the bed wider. This "critical height" is not a constant, since it depends on the growth rate of the organism and the operating conditions, especially the superficial velocity of the air. An estimate of the "critical height" of a traditional packed-bed can be calculated for logistic growth kinetics as (Mitchell et al. 1999):

where the symbols have the meanings given in Table 24.1. Tout is the maximum temperature allowable in the bed while f is an estimate of dHsJdT.

Fig. 24.8. A relatively thin packed-bed can be used for pilot-scale investigations into packed bed design. If its sides are insulated, this will mimic the presence of "hot" substrate around an identical section within the bed of the commercial-scale bioreactor

Was this article helpful?

0 0

Post a comment