Luiz F. L. Luz

The Scale-up Challenge for SSF Bioreactors

Having now seen the various types of bioreactors used in SSF processes (Chap. 3) and the transport phenomena that occur within them (Chap. 4), we now return to the question of how the limitations on the efficiency of the transport phenomena within the bioreactor make it almost impossible to operate large-scale bioreactors in such a manner that the conditions within the substrate bed are maintained throughout the process at the optimum values for growth and product formation. Is it really difficult to design an efficiently operating large-scale SSF bioreactor? In the case of SLF, there are examples of successfully operating bioreactors of hundreds of thousands of liters. Why cannot we do the same for SSF processes? Or can we? The answer is that the challenges in operating a bioreactor of several hundreds of thousands of liters are typically more difficult to overcome in SSF than in SLF, and it is no simple matter to develop efficient large-scale SSF bioreactors. This difficulty, often referred to as “the scale-up problem”, is discussed in the following sections.

Estimation of Transfer Coefficients for SSF Bioreactors

relatively little attention within SSF bioreactors. At the moment the available information is not sufficient to allow the proposal of general correlations and therefore it will be necessary to determine the coefficients experimentally for each particular bioreactor. In the absence of experimentally determined correlations, correlations for non-SSF systems can be used, although it must be realized that doing this may bring inaccuracies in the model predictions.

Appropriate Levels of Complexity for Modeling SSF Bioreactors

While attention should certainly be given to furthering the development of fully-mechanistic models, at the moment fast-solving models are sufficiently accurate to be useful tools in the design of bioreactors and the optimization of their operation. The rest of the book concentrates on fast-solving models. Chapters 14 to 17 describe approaches to establishing and modeling the growth kinetics in a manner appropriate for incorporation into fast-solving models. Chapters 18 to 20 show how the heat and mass transfer phenomena within bioreactors can be described at an appropriate level of detail for a fast-solving model. Chapters 22 to 25 then present several fast-solving models and show how they can be used to give insights into optimal design and operation. We are confident that readers, with relatively little effort, can adapt these models to their own systems, and obtain useful results from doing so.

Group IVb: Intermittently-Mixed Forcefully-Aerated Bioreactors

Intermittently-mixed, forcefully-aerated bioreactors appear to have some potential, judging by the fact that several processes involving bioreactors that operate in this mode have been demonstrated at a reasonably large scale. They appear to offer some benefits in control of the conditions within the bed, while minimizing the deleterious effects that continuous mixing can have, at least for fungal processes.

A Model of an Intermittently-Mixed Forcefully-Aerated Bioreactor

True packed-bed operation can only be used in those cases where the bed does not dry out to levels that cause water limitations on growth, because water can only be uniformly distributed within a bed of solids while the solids are being agitated. If the organism tolerates some mixing, then the intermittently-mixed mode of operation can be used, in which the bioreactor operates as a packed-bed during the majority of the fermentation period and undergoes infrequent mixing events, during which water can be added to the bed (Chap. 10). In fact, once the intermittentlymixed mode of operation is selected, the use of dry air to promote evaporative cooling is potentially available as an operating strategy. The current chapter presents a model that can be used to investigate the operation of such bioreactors.

Modeling of Heat and Mass Transfer in SSF Bioreactors

This chapter has identified the forms of various terms that may appear within the balance/transport sub-model of a bioreactor model. Several of these will appear in energy and mass balances in the mathematical models of bioreactors presented in Chaps. 22 to 25. These equations contain various parameters that it will be necessary to determine before the model can be solved. Chapters 19 and 20 describe how these and other necessary parameters can be determined.

Basics of Heat and Mass Transfer in Solid-State Fermentation Bioreactors

Macroscale heat and mass transfer phenomena play important roles in determining the performance of SSF bioreactors. Therefore, in order for a mathematical model to describe bioreactor performance reasonably, it must describe these phenomena. The current chapter gives a qualitative overview of the various macroscale heat and mass transfer processes that occur within SSF bioreactors. These processes will be treated quantitatively in Chap. 18, where the various mathematical expressions that are used to describe them within bioreactor models will be presented. Note that, in a particular SSF bioreactor, some of the heat and mass transfer mechanisms presented in this chapter may not be present, and the relative importance of the various mechanisms that are present may differ from bioreactor to bioreactor. Details specific to each bioreactor type will be covered in Chaps. 6 to 11 and 21 to 24. The current chapter focuses on the period of high heat generation, when it is necessary to remove energy from the bed, although early in the fermentation it might be necessary to transfer energy to the bed to maintain the temperature high enough to initiate growth.