Gas-solid fluidized bed
Gas-solid fluidized bed
Fig. 3.2. Basic design features of the various SSF bioreactors, showing how they can be classified into four groups on the basis of how they are mixed and aerated. From Mitchell et al. (2000) with kind permission from Springer Science and Business Media
Group I bioreactors. These typically consist of a chamber containing a large number of individual trays, stacked one above the other with a gap in between (Fig. 3.2, upper left quadrant). Conditioned air (i.e., with control of humidity and temperature) is blown into the chamber and circulates around the trays. Agitation, if done, is very infrequent, and is typically done by hand. The trays themselves may be constructed of wood, bamboo, metal or plastic. They are typically open at the top and have perforated bottoms to increase the accessibility to O2, but there are other possibilities. For example, micro-perforated plastic bags containing substrate fall within this category.
Group II bioreactors. A typical packed-bed bioreactor consists of a column of cylindrical or rectangular cross section, oriented vertically, with a perforated base plate on the bottom which supports a bed of substrate (Fig. 3.2, lower left quadrant). Air is blown up through the base plate.
Group III bioreactors. These typically consist of a drum of cylindrical cross section lying horizontally (Fig. 3.2, upper right quadrant). The drum is partially filled with a bed of substrate, and air is blown through the headspace. In rotating drums, the whole drum rotates around its central axis to mix the bed. In stirred drums, the bioreactor body remains stationary and paddles or scrapers mounted on a shaft running along the central axis of the bioreactor rotate within the drum.
Mixed and forcefully aerated bioreactors. There are several types of designs that fall into this group (Fig. 3.2, lower right quadrant). They can be operated with continuous or discontinuous mixing.
• Stirred-bed bioreactors are similar to the static packed bed in that a bed of substrate sits on a perforated base plate and air is forcefully blown through the bed, but rather than being static, an agitator is inserted and provides continuous or intermittent mixing. Such stirred beds are typically aerated from the bottom, and have the agitator inserted from the top.
• Rocking-drum bioreactors consist of three concentric cylinders - an inner perforated cylinder, an outer perforated cylinder, and an outer solid cylinder. The substrate sits in the space between the two perforated cylinders. Air is blown through into the central cylinder, passes through the substrate bed and then into the space between the outer perforated cylinder and the outer solid cylinder, before leaving through the air outlet. The two outer cylinders rotate in relation to the inner cylinder, thereby mixing the substrate bed, although not very effectively.
• Air-solid fluidized beds (ASFBs). In this bioreactor air is blown upwards through a perforated base plate at sufficient velocity to fluidize the substrate bed, which then behaves as though it were a fluid.
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