Since Nagel et al. (2001a) also investigated wall cooling, a simulation was done with a value of J of 30. Growth is much better (Fig. 22.5(a)) because this strategy manages to control the temperature of the solids, which does not exceed 35.7°C (Fig. 22.5(b)). This leads to water temperatures as low as -0.5°C (requiring the addition of antifreeze to the cooling water), and to wall temperatures as low as 16.9°C. Such low cooling-water temperatures are not impossible to obtain in the laboratory, but might be too expensive at large scale. Note that these predictions agree in general terms with those of Nagel et al. (2001a), who had to control the wall temperature at values as low as 18°C in order to keep the bed temperature around 35°C.
Simulations were done for larger scale bioreactors. For these simulations the coefficients for heat transfer from the gas and the solid phase to the bioreactor wall and from the bioreactor wall to the water in the water jacket are chosen as 200 W m-2 °C-1 in order to give an overall coefficient for heat transfer from the bed to the water in the water jacket (calculated on the basis of the law of resistances in series) of the order of magnitude of 100 W m-2 °C-1, a value determined experimentally for a water-jacketed industrial solids mixer adapted as an SSF bioreactor (Nagel et al. 2001a) (see Sect. 19.5.2).
Figure 22.6 shows simulations for a bed of 1 m diameter and 1 m height, which contains an initial substrate loading of 177 kg. When the temperature in the water jacket is held constant (J=0) growth is poor (Fig 22.6(a)), despite the higher heat transfer coefficients at the wall, because undesirably high solids temperatures are still reached, peaking at 44.9°C (Fig 22.6(b)). On the other hand, with a value of J of 2, growth is good (Fig. 22.6(d)) because the solids temperature does not exceed 40.4°C (Fig 22.6(e)). In this case, the minimum temperature of the cooling water is 24.2°C, which is quite reasonable and may be possible to achieve without refrigeration.
Figure 22.7 shows simulations for a bed of 2 m diameter and 2 m height, which contains an initial substrate loading of 1417 kg. In Figs 22.7(a) to (c) the temperature in the water jacket is held constant (J=0), but in order to promote evaporation, the water activity of the inlet air is set at 0.5. However, undesirably high temperatures of 46.1°C are reached (Fig 22.7(b)). Note that water is added at 23 and 35 h.
In Figs 22.7(d) to (f) near-saturated air is used (awgin=0.99) along with a value of J of 3. The temperature does not exceed 42.0°C (Fig 22.7(e)). In this case the minimum temperature of the cooling water is 14.1°C. Reasonable temperature control might also be achieved by using higher aeration rates. In this case the best strategy will depend on the comparative operating costs of higher aeration rates versus refrigeration of a cooling jacket.
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