In fact, negligible heat removal through the bioreactor wall is easily simulated by putting the proportional gain, J, equal to -1. This makes the cooling water temperature equal to the bed temperature. A simulation was done for the 2 m height by 2 m diameter bioreactor, using the conditions listed for run 9 in Table 22.3. Under these conditions growth is very poor (Fig. 22.9(a)) due to very high solids temperatures (Fig. 22.9(b)). Both water and temperature restrictions are significant (Fig. 22.9(c)), although the temperature restrictions have the greater effect. Evaporation, promoted by the use of 50% relative humidity air at the air inlet, removes almost 90% of the metabolic heat liberated by the microorganism (Fig. 22.9(d) and run 9 in Table 22.4). Quite reasonable performance can be obtained by increasing the air flow to the bioreactor (from 1 to 3 vvm) (see run 10 in Table 22.3) as shown by Figs 22.9(e), (f), (g), and (h). As with the previous simulation, evaporation is responsible for almost 90% of the overall heat removal, but note that the overall metabolic heat generation rate is almost double due to the better growth (Table 22.4).

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