Many different bioreactors have been used in SSF processes, and have been given different names by different authors. However, based on similarities in design and operation, SSF bioreactors can be divided into groups on the basis of how they are mixed and aerated (Fig. 3.2).
• Group I: Bioreactors in which the bed is static, or mixed only very infrequently (i.e., once or twice per day) and air is circulated around the bed, but not blown forcefully through it. These are often referred to as "tray bioreactors".
• Group II: Bioreactors in which the bed is static or mixed only very infrequently (i.e., once per day) and air is blown forcefully though the bed. These are typically referred to as "packed-bed bioreactors".
• Group III: Bioreactors in which the bed is continuously mixed or mixed intermittently with a frequency of minutes to hours, and air is circulated around the bed, but not blown forcefully through it. Two bioreactors that have this mode of operation, using different mechanisms to achieve the agitation, are "stirred-drum bioreactors" and "rotating drum bioreactors".
• Group IV: Bioreactors in which the bed is agitated and air is blown forcefully through the bed. This type of bioreactor can typically be operated in either of two modes, so it is useful to identify two subgroups. Group IVa bioreactors are mixed continuously while Group IVb bioreactors are mixed intermittently with intervals of minutes to hours between mixing events. Various designs fulfill these criteria, such as "gas-solid fluidized beds", the "rocking drum", and various "stirred-aerated bioreactors".
Note that this division is made on the basis of the manner in which the bioreac-tor is operated, and not on the outward appearance of the bioreactor. For example, there are bioreactors that are essentially identical with the "stirred drum", but in which the air is introduced within the substrate bed through the ends of the paddles. Such a bioreactor should then be classified as a "stirred-aerated bioreactor", although the bed will not be as efficiently aerated as when the bed receives an even aeration across its whole cross-section. Also note that the distinction is not always perfectly clear. It is an arbitrary decision as to what frequency of mixing is separates "static" and "agitated" operation. The advantage of grouping bioreactors on the basis of the manner in which they are operated is that principles derived on the basis of work with one member of a certain group of bioreactors can be applied to other bioreactors in the group.
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