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Fig. 24.10. (a) Predictions of the mathematical model about the axial temperature profile at several horizontal positions within the Zymotis packed-bed at the time of peak heat production at 23 h. Note that in the upper regions of the bed the temperature is high from the central plane to 12.7 cm from the central plane (7.3 cm from the plate), only reducing significantly at positions close to the plate (which is at x=20 cm). (b) Comparison of the central axial temperature profile in a wide traditional packed-bed and the central plane temperature profile of the Zymotis bioreactor, at 20 h, in the case in which L = 0.03 cm. In both cases the bioreactor is 1.0 m high, the superficial air velocity is 5 cm s-1, the inlet air temperature is 38°C and the specific growth rate constant (pcpt) is 0.236 h-1

Regarding bioreactor dimensions, the maximum practical value for the front-to-back depth will depend on the size of heat transfer plates that can be constructed. Given that more plates can be added to extend the width of the bioreactor, theoretically there is no limitation on the width. One advantage of the Zymotis bioreactor is that, with the flattening out of the temperature profile, larger heights are theoretically possible than for traditional bioreactors, at least based on temperature considerations, although pressure drop may become problematic at large heights.

In the upper regions of the bed much of the waste metabolic heat is removed by conduction to the heat transfer plates, as evidenced by the flattening out of the temperature profile (Fig. 24.10(b)); in these regions only a relatively minor proportion is removed by axial convection and evaporation. Note that the predicted profiles are similar to the experimental temperature profiles that Saucedo-Castaneda et al. (1990) measured in a water-jacketed packed-bed bioreactor of 6 cm diameter (Fig. 7.5). The position at which the axial temperature profile flattens out in the Zymotis bioreactor depends on the heat production rate and on the various operating parameters, including the superficial air velocity. In fact, due to the acceleration and deceleration in the growth rate during the various phases of a fermentation, and the corresponding changes in the rate of production of waste metabolic heat, the extent of the "flat zone" will fluctuate during the fermentation.

Effect of a cooling water control scheme and of the gap between the plates. In these simulations only data about overall predicted performance is presented; no information is given about the gradients within the bioreactor. However, clearly the best performance must correspond to those operating conditions that minimize the temperature deviations from the optimum temperature, in both space and time.

The spacing between the cooling plates makes a large difference to the predicted performance, with all other parameters and operating variables being held constant (Fig. 24.11). Performance worsens rapidly as the size of the gap increases from 1 to 10 cm. That is, t90 increases rapidly over this range. Further increases in gap size worsen performance even further.

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