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Fig. 25.3. Results of the base case simulation, obtained using the values in Tables 25.1 and 25.2. (a) Volume-weighted biomass content. The curve with the solid circles represents the results of the simulation. The curve with the hollow circles represents the results of a simulation undertaken with identical values for all parameters and variables except that no mixing and water addition events were allowed. The dashed line represents growth with the specific growth rate constant equal to /uopt throughout the fermentation; (b) Biomass contents predicted at various fractional heights above the air inlet; (c) Temperatures of the solids at the various heights; (d) Water activities of the solids at the various heights; (e) Fractional specific growth rates based on temperature at the various heights; (f) Fractional specific growth rates based on water activity at the various heights. Key to graphs (b) to (f):

Fractional heights of (-) 0.025; (—▲—) 0.5; (—■—) 1.0. The arrows at the top of the graph denote the mixing events

Reasonable growth and temperature control are predicted for a lab scale biore-actor with a value of G of 0.1 kg-dry-air m-2 s-1 (which corresponds to a superficial velocity of 0.088 m s-1, calculated as 0.1 kg-dry-air m-2 s-1/(1.14 kg-dry-air m-3)). Figure 25.4 is plotted using the same axes as Fig. 25.3, to highlight the fact that the temperature remains closer to the optimum of 38°C (Fig. 25.4(c)) and the water activity remains higher (Fig. 25.4(d)). As a result, the temperature-limitation and water-limitation of growth are much less severe (Figs. 25.4(e) and 25.4(f)) and the growth profile is closer to the optimum profile (Fig. 25.4(a)). Due to the fact that the conditions are close to the optimum at all bed positions, there is no significant difference in the growth at different heights. There are two mixing events, one at 17.35 h and the other at 24.20 h.

25.3.2 Investigation of the Design and Operation of Intermittently-Mixed Forcefully-Aerated Bioreactors at Large Scale

To investigate operation at large scale, graphs could be plotted similar to those in the previous section for the operation of a laboratory-scale bioreactor. However, in the current section only graphs of the solids temperature and water activity will be plotted. Better performance can be judged on the basis of how closely the solids temperature is maintained to the optimum for growth of 38°C and how closely the solids water activity is maintained to the optimum for growth of 0.95. Table 25.3 shows the design and operating variables that were changed in these simulations.

Scaling up to a 1 m high bed while maintaining the air flux (G) constant at 0.1 kg-dry-air m-2 leads to poor performance with respect to the control of the temperature (Fig. 25.5(a)) and water activity of the solids (Fig. 25.5(b)), as might be expected. On the other hand, good control of the temperature (Fig. 25.5(c)) and water activity of the solids (Fig. 25.5(d)) is predicted when the bioreactor is scaled-up while maintaining the ratio of G/Z at 0.1 kg-dry-air m-2 s-1/0.3 m (i.e., 0.33 kg-dry-air m-2 s-1 for a 1 m high bed). In fact, almost identical performance is expected when the same scale-up strategy is used for a 2 m high bed (for which this strategy gives an air flux of 0.66 kg-dry-air m-2 s-1), as shown by Figs. 25.5(e) and 25.5(f).

Table 25.3. Design and operating variables changed in the various explorations of performance of a 1 m bioreactora

Figures

Z (m)

G (kg m-2 s-2)

a *

awgin

Tso & Tn (°C)

25.5(a) and (b)

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