Conclusions on the Operation of Well Mixed Bioreactors

Based on the simulations presented in this chapter, it appears that, if we can in fact keep the bed well mixed, then continuously-mixed, forcefully-aerated bioreactors can be operated well at scales containing over a ton of dry substrate. In fact, if we increased the porosity and the solids-to-gas heat transfer coefficient in order to simulate an air-solid fluidized bed, then we would predict reasonable operation at even much larger scales. Of course, the model does not take into account any practical difficulties that there might be in operating air-solid fluidized beds at large scale.

The ability to provide a well mixed bed at large scale will depend on the effectiveness of the mixing method. Poor mixing will lead to poorer bioreactor performance than that predicted by the simulations done in this chapter. Further, the practicality of continuous mixing depends strongly on the susceptibility of the process organism to shear damage.

Fig. 22.9. Performance of a bioreactor 2 m high by 2 m diameter with negligible heat transfer through the wall. The aeration rate is 1 vvm for the left hand column (Run 9) and 3 vvm for the right hand column (Run 10). (a) and (e) Growth (—), compared to that which would be achieved with ¡j, = 0.236 h-1 throughout the fermentation (---); (b) and (f) Bed temperature. (c) and (g) Relative limitations of growth: (—) /uFT; (---) /uWT; (d) and (h) Rates of heat production and removal: (—) Metabolic heat production; (+) Sensible heat removal from the solids to the bioreactor wall; (□) Sensible heat removal from the solids to the gas phase; (•) Evaporative heat removal

Fig. 22.9. Performance of a bioreactor 2 m high by 2 m diameter with negligible heat transfer through the wall. The aeration rate is 1 vvm for the left hand column (Run 9) and 3 vvm for the right hand column (Run 10). (a) and (e) Growth (—), compared to that which would be achieved with ¡j, = 0.236 h-1 throughout the fermentation (---); (b) and (f) Bed temperature. (c) and (g) Relative limitations of growth: (—) /uFT; (---) /uWT; (d) and (h) Rates of heat production and removal: (—) Metabolic heat production; (+) Sensible heat removal from the solids to the bioreactor wall; (□) Sensible heat removal from the solids to the gas phase; (•) Evaporative heat removal

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