Packed-bed bioreactors are necessary in those cases in which the bed must not be mixed during the fermentation. Such bioreactors will always suffer from axial temperature gradients. These can be minimized, although not eliminated, by selection of appropriate design and operating variables. The question as to what combination of design and operating conditions will lead to best performance is not simple, and is best answered with the use of a mathematical model.
The models that were presented in this chapter for the traditional packed-bed bioreactor and Zymotis packed-bed bioreactors could be much more powerful tools for use in the design process if they were improved. Some possible improvements include:
• introduction of a water balance. This would require a description of the effect of the temperature and the water content of the substrate on its water activity and a description of the effect of water activity on the growth kinetics of the microorganism. Note that the model of the intermittently-stirred bioreactor presented in Chap. 25 can be used to explore the water balance during packed-bed operation, simply by suppressing the mixing events;
• incorporation of the effect of growth of the microorganism into the inter-particle spaces on the pressure drop through the bed and the resulting effects on air flow and heat transfer phenomena;
• description of changes in the bed due to the shrinkage of substrate particles as dry matter is converted into CO2 by the microorganism.
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