T3

0 10 20 30 40 50 0 10 20 30 40 50 60 Time (h) Time (h)

Fig. 23.4. Some of the outputs of the model (other outputs are shown in Fig. 23.5). A simulation was done with the case study model using the data in Table 23.1, with n = 10.

(a) Plots of (---) total solids and (—) total biomass; (b) Fractional specific growth rates on the basis of (---) temperature and (—--) water activity and (—) the resulting value of the specific growth rate constant, ¡X; (c) Water content of the solids: (—) actual water content of the solids, (---) water content that the solids would have to have to be in equilibrium with the gas phase; (d) Humidity of the gas phase: (—□—) actual humidity of the gas phase, (—•—) saturation humidity of the gas phase

The model can be used to explore the relative contributions of the various heat removal mechanisms (Figs. 23.5(b) and 23.5(d)). One of the output files gives the values of the four terms on the right hand side of the energy balance equation for the substrate bed (see the equation on the upper left of the bed within Fig. 23.2). Heat removal to the bioreactor wall is the major contributor under the conditions of the base case simulation, this holding for both n = 1 and n = 10. For n = 10, at the time of peak heat production (32 h) there is a metabolic heat generation rate of 37.9 W within the bed. Of this, 0.1 W is being removed by convection to the head-space, 0.7 W by evaporation, and 37.1 W by conduction to the bioreactor wall. The contribution of evaporation is low because of the low air flow rate. In fact, evaporation is so slow that the substrate does not lose enough water to make the addition of further water necessary, even during 100 h of operation.

Subsystem temperatures Heat production and removal n=1

n=10

Subsystem temperatures Heat production and removal

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