Channeling is a potential problem in packed beds and the static phase of operation of intermittently-mixed beds. Channeling in intermittently-mixed beds will be discussed in Chap. 10. Channeling is problematic because air will flow preferentially through the cracks, such that in the regions of the bed where the particles are bound together, there will be no bulk flow, such that O2 transfer will be limited to diffusion and heat transfer will be limited to conduction (see Fig. 7.4).
One of the major causes of channeling in packed-beds is the shrinkage of particles due to the consumption of the solid material of the particle, combined with the fact that, in many fungal fermentations, the substrate bed is bound together by "inter-particle hyphal bridges". These two phenomena mean that, as the bed volume reduces, the particles will not simply settle downwards but rather the bed is drawn inwards, pulling away from the walls or cracking in the middle. For fungi that do not produce these hyphal bridges, the substrate particles remain free flowing as the bed shrinks and the bed does not pull away from the wall or develop cracks, but simply reduces in height (Weber et al. 2002).
For fungal fermentations in which the fungus does bind the particles together, shrinkage problems can be minimized by the use of "inert" hemp impregnated with nutrients (Weber et al. 1999). However, there is not necessarily free choice of substrates in SSF processes.
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