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Fig. 25.5. Effect of the air flux on predicted performance of larger bioreactors. (a) Solids temperatures and (b) solids water activities at various heights within in a 1 m high bioreactor with an air flux of 0.1 kg-dry-air m-2 s-1; (c) Solids temperatures and (d) solids water activities at various heights within in a 1 m high bioreactor with an air flux of 0.33 kg-dry-air m-2 s-1; (e) Solids temperatures and (f) solids water activities at various heights within in a 2 m high bioreactor with an air flux of 0.66 kg-dry-air m-2 s-1. In all cases, the other parameters are as given in Tables 25.1, 25.2, and 25.3. The horizontal lines represent temperatures and water activities that give fractional specific growth rates of (---) 0.9 and (■■■■) 0.8. Key to graphs (a) to (f): Fractional heights of (-) 0.025; (—▲—) 0.5; (—■—) 1.0. The arrows at the top of each graph denote the mixing events

However, such aeration rates might not be feasible at large scale. The superficial velocity corresponding to an air flux of 0.33 kg-dry-air m-2 s-1 is 0.29 m s-1 while that corresponding to an air flux of 0.33 kg-dry-air m-2 s-1 is 0.58 m s-1. It may be overly costly to provide such high aeration rates. The simulations presented in Fig. 25.5 represent a search for an operating strategy that will allow good performance in a 1 m high bioreactor but at the lower air flux of 0.1 kg-dry-air m-2 s-1. They should be compared with the results presented in Figs. 25.5(a) and 25.5(b), which are for a 1 m high bioreactor with an air flux (G) of 0.1 kg-dry-air m-2 but otherwise operated identically to the laboratory-scale bioreactor:

• Increasing the outlet gas relative humidity set point (awg*) from 0.87 to 0.95 improves performance minimally (Figs. 25.6(a) and 25.6(b)). With the higher set point there are three mixing and water replenishment events instead of two.

• Maintaining the outlet gas relative humidity set point at 0.95 but decreasing the inlet gas relative humidity to 0.75 enables the bed temperature to be maintained at values that give a juPT of 0.8 or greater for most of the bed and for most of the time (Fig. 25.6(c)). However, the bottom of the bed dries out rapidly (Fig. 25.6(d)). Due to the faster drying, there are four mixing and water replenishment events.

• Maintaining the outlet gas relative humidity set point at 0.95, returning the inlet gas relative humidity to 0.99, but decreasing the initial bed temperature and the inlet gas temperature to 35°C leads to better performance. Note that juPT is equal to 0.9 at 35°C, so use of this gas temperature does not unduly slow growth. The bed conditions are predicted to remain at near optimal values for growth for the majority of the fermentation (Figs. 25.6(e) and 25.6(f)).

• Maintaining the conditions of the previous simulation but reducing the inlet gas temperature even further, to 33°C, leads to even better performance. Note that HPT has a value of 0.8 at 33°C, so growth is still acceptable. As shown by Figs. 25.6(g) and 25.6(h), the bed conditions are predicted to give values of nPT and HPW of at least 0.8 for almost the entire fermentation.

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