[90]. This bioreactor has a capacity for approximately 20 kg of yeast biomass.
This system can potentially relieve product inhibition problems since the
ethanol is continuously stripped by the circulating gas.
The high gas flow rates required for fluidization means that temperature control of the bed is not difficult. High convective cooling rates occur, and if
evaporation is required to boost cooling rates, the evaporated water can be replaced by spraying water onto the bed. As a result, models of gas-solid fluidized
beds have ignored the energy balance [89, 90]. Rather, they have concentrated
on the intraparticle diffusion phenomena, as mentioned in Sect. 4.1.1.
5.5.2
Stirred Aerated Beds
Stirred aerated beds typically have an appearance similar to packed beds, with
air passing upwards through a perforated plate or screen which supports the
substrate bed. The difference is that an agitator is embedded within the
substrate bed (see Figs 1 and 5). This agitator can be operated either continuously or intermittently. Agitator design and operation becomes an important consideration in agitated beds. These bioreactors may sometimes be
referred to as horizontal stirred beds or vertical stirred beds depending on
whether the breadth or height dimension is larger.
Stirred beds have been used successfully on a large scale. A 50-l bioreactor
with a planetary mixing device and a 50 kg capacity bioreactor in which the bed
is held within a cylindrical basket which is rotated past stationary agitator blades
have been designed for aseptic operation, making them suitable for processes involving slow growing organisms such as Gibberella fujikori [155–157]. Much
larger bioreactors have been developed for non-aseptic aeration. The 1 tonne-capacity bioreactor of Durand and Chereau [2] consists of a steel box 2 m long,
0.8 m wide, and 2.3 m high (see Fig. 1). A 1 m thick substrate bed sits on a
perforated base plate through which pre-humidified air is supplied. Three screw
agitators are mounted across the breadth of the reactor on a trolley above the
substrate bed, with the screws extending down into the substrate bed. The trolley moves backwards and forwards along the length of the bioreactor, such that
each location in the bed is intermittently mixed. This bioreactor has been used
successfully for protein enrichment and for the production of enzymes and biopesticides [158]. A larger version of this bioreactor was built by Xue et al. [159]
and used for the production of microbial protein from sugar beet pulp by
Aspergillus tamarii. The reactor was 17.6 m long, 3.6 m wide, and 2.0 m high. The
bioreactor had a capacity of 25 tons of moist substrate. Little information was
given on performance, although during the rapid growth phase with inlet air
conditions of 88% relative humidity and a temperature of 33 °C, the outlet air
was measured as 33 °C and 100% relative humidity, which suggests that adequate
temperature control within the bed was achieved.
A variation of the stirred bed design involves a horizontal stirred drum, filled
to two-thirds depth with substrate, with introduction of air through a perforated central shaft embedded in the substrate bed and upon which mixer
blades are mounted (Fig. 10). This bioreactor has been successfully used for
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This system can potentially relieve product inhibition problems since the
ethanol is continuously stripped by the circulating gas.
The high gas flow rates required for fluidization means that temperature control of the bed is not difficult. High convective cooling rates occur, and if
evaporation is required to boost cooling rates, the evaporated water can be replaced by spraying water onto the bed. As a result, models of gas-solid fluidized
beds have ignored the energy balance [89, 90]. Rather, they have concentrated
on the intraparticle diffusion phenomena, as mentioned in Sect. 4.1.1.
5.5.2
Stirred Aerated Beds
Stirred aerated beds typically have an appearance similar to packed beds, with
air passing upwards through a perforated plate or screen which supports the
substrate bed. The difference is that an agitator is embedded within the
substrate bed (see Figs 1 and 5). This agitator can be operated either continuously or intermittently. Agitator design and operation becomes an important consideration in agitated beds. These bioreactors may sometimes be
referred to as horizontal stirred beds or vertical stirred beds depending on
whether the breadth or height dimension is larger.
Stirred beds have been used successfully on a large scale. A 50-l bioreactor
with a planetary mixing device and a 50 kg capacity bioreactor in which the bed
is held within a cylindrical basket which is rotated past stationary agitator blades
have been designed for aseptic operation, making them suitable for processes involving slow growing organisms such as Gibberella fujikori [155–157]. Much
larger bioreactors have been developed for non-aseptic aeration. The 1 tonne-capacity bioreactor of Durand and Chereau [2] consists of a steel box 2 m long,
0.8 m wide, and 2.3 m high (see Fig. 1). A 1 m thick substrate bed sits on a
perforated base plate through which pre-humidified air is supplied. Three screw
agitators are mounted across the breadth of the reactor on a trolley above the
substrate bed, with the screws extending down into the substrate bed. The trolley moves backwards and forwards along the length of the bioreactor, such that
each location in the bed is intermittently mixed. This bioreactor has been used
successfully for protein enrichment and for the production of enzymes and biopesticides [158]. A larger version of this bioreactor was built by Xue et al. [159]
and used for the production of microbial protein from sugar beet pulp by
Aspergillus tamarii. The reactor was 17.6 m long, 3.6 m wide, and 2.0 m high. The
bioreactor had a capacity of 25 tons of moist substrate. Little information was
given on performance, although during the rapid growth phase with inlet air
conditions of 88% relative humidity and a temperature of 33 °C, the outlet air
was measured as 33 °C and 100% relative humidity, which suggests that adequate
temperature control within the bed was achieved.
A variation of the stirred bed design involves a horizontal stirred drum, filled
to two-thirds depth with substrate, with introduction of air through a perforated central shaft embedded in the substrate bed and upon which mixer
blades are mounted (Fig. 10). This bioreactor has been successfully used for
114
D.A. Mitchell et al.