As for any other continuous chemical reactor, the flow of materials from the inlet to the outlet of a CSSFB could potentially fall anywhere between the ideal plug-flow and perfect-mixing regimes. In plug flow, all of the particles have the same residence time within the bioreactor as they move along parallel paths at the same speed. On the other hand, if the particles are mixed parallel to the direction of flow, they may spend different lengths of time in the reactor. In other words, different particles may have different residence times. In a completely mixed reactor, the residence time distribution for the population of particles is wide, some particles may exit almost immediately after they enter, while some other may remain within the reactor for longer times.
Theoretically, in order for all fermented substrate particles exiting a CSSFB to have the maximum possible growth and product formation, each particle should spend the same amount of time in the bioreactor between when it is inoculated at the solids inlet and when it is harvested at the solids outlet. The importance of this can be seen in a simple example. Let us assume that it requires 24 h, measured from the time of inoculation, for the microorganism on a particular substrate particle to produce the maximum activity of a desired enzyme, and that the enzyme activity falls off after 24 h due to denaturation or degradation by proteases. In this case, any particles exiting a CSSFB with residence times lesser than or greater than 24 h will have an enzyme activity less than the maximum possible value. In such a case it would be desirable for the residence time distribution to be as narrow as possible, with a mean of 24 h.
This is the consideration of "uniformity amongst harvested substrate particles", which is desirable, but may be difficult or impossible to achieve in practice. Note that "true" continuous operation does guarantee a uniform product regardless of whether all substrate particles have the same residence time or not, but in this case the concept of uniformity is applied differently: if a CSSFB does manage to establish a steady state, then the exiting product will have a uniform composition, averaged over the population of exiting substrate particles. In other words, the proportions of "young" substrate particles and "old" substrate particles in the harvested product will remain constant over time for true continuous operation, regardless of the flow regime and residence time distribution of the particles. However, in terms of bioreactor productivity, the exiting of a mixture of younger and older particles is disadvantageous when compared to the exiting of a uniform population of "fully-fermented" particles. There is a further consideration: heterogeneity of the inlet raw material and the presence of non-ideal flow patterns, dead volumes, air channeling, and solids short-circuiting may all contribute to fluctuations in the quality of the product exiting a CSSFB. These issues have received very little attention in SSF.
The wideness of the residence time distribution depends on the direction and extent of mixing (Fig. 11.4). Mixing of fermenting solid particles perpendicular to their flow direction in a bioreactor would typically be desirable, especially if the bed were forcefully-aerated with the air flow being perpendicular to the solids flow direction. In the absence of vertical mixing in the bioreactor shown in Fig. 11.4, undesirable temperature and moisture gradients, similar to those noted for packed-bed bioreactors (Chap. 7), would arise along the direction of the air flow. If it were possible to mix the solids in this bioreactor vertically (i.e., perpendicular to the solids flow direction) without any horizontal movement of the solids (i.e., parallel to the solids flow direction), then such mixing would have no influence on the residence time of the solid particles. However, this is an ideal that is impossible to achieve in practice: Mixing perpendicularly to the solids flow direction will also cause some mixing parallel to the solids flow direction. Mixing parallel to the solids flow direction, often called flow dispersion, leads to a broadening of the residence time distribution pattern.
"Horizontal mixing" - mixing perpendicular to air flow but parallel to solids flow
"Vertical mixing" - mixing parallel to air flow but perpendicular to solids flow
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