they allow better control over the conditions within the bed than can be
achieved in tray bioreactors. However, the small spacing of only 5 cm between
heat transfer plates which is required for optimal performance means that
loading and unloading operations will be more difficult than in the case where
there are no internal heat transfer plates. Despite the significant amount of
attention that packed-bed bioreactors have already received, more attention is
required with respect to pressure drops, the change in bed structure during the
fermentation, and flow patterns of air through the bed.
5.4
Mixed Beds Without Forced Aeration – Rotating Drums, Stirred Drums
and Screw Bioreactors
These bioreactors are horizontal or inclined cylinders. Their main feature is
that air is not forcefully blown through the bed itself. Rather it is blown through
the headspace above the bed, and gas exchange between the headspace and bed
is promoted by the mixing of the bed (see Fig. 5). In rotating drum bioreactors,
mixing within the substrate bed is caused by rotation of the bed, and may be
promoted by the use of internal lifters. In stirred drum bioreactors, the drum
itself remains static and the mixing action is caused by paddles or scrapers
mounted on a shaft running through the central axis of the drum. Screw bioreactors have been used for continuous ethanol production in SSF, in which
there is no need to blow air through the bioreactor. In this bioreactor a screw
with a diameter equal to the internal diameter of the drum is operated intermittently to push the substrate from one end to the other. However, after some
interest in the 1980s, the screw bioreactor has not received further attention [81,
82]. Only rotating drums have received modeling attention and the discussion
below is limited to this design.
Heat removal from the substrate bed in rotating drums occurs by two main
routes – transfer directly to the headspace air by convection and evaporation
and transfer through the bioreactor wall by conduction followed by convective
cooling of the bioreactor wall (Fig. 8) [146]. The mixing in rotating drum
bioreactors allows the use of dry air to promote evaporation, because water can
be replenished by spraying a fine mist of water onto the bed as it is being mixed
[147]. Note that convective cooling of the drum wall occurs to both the surrounding air and the headspace air [146]. Although transfer to the headspace air
is a minor contributor to overall heat removal, the resulting increase in
headspace air temperature does increase the driving force for evaporation from
the bed to the headspace. Unfortunately no work has been done to obtain
experimental values for the various heat and mass transfer coefficients which
are important in rotating drum bioreactors.
Transfer between the bed and headspace will be affected significantly by the
mixing patterns of substrate particles within the bed and of air within the headspace. Note that transverse mixing considerations affect the substrate loadings
that can be used in rotating drum bioreactors. In non-SSF applications optimal
fill fractions, which maximize the amount of material within the drum but still
allow good mixing, range from 17% to 22% of the drum volume [148]. In SSF
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D.A. Mitchell et al.
achieved in tray bioreactors. However, the small spacing of only 5 cm between
heat transfer plates which is required for optimal performance means that
loading and unloading operations will be more difficult than in the case where
there are no internal heat transfer plates. Despite the significant amount of
attention that packed-bed bioreactors have already received, more attention is
required with respect to pressure drops, the change in bed structure during the
fermentation, and flow patterns of air through the bed.
5.4
Mixed Beds Without Forced Aeration – Rotating Drums, Stirred Drums
and Screw Bioreactors
These bioreactors are horizontal or inclined cylinders. Their main feature is
that air is not forcefully blown through the bed itself. Rather it is blown through
the headspace above the bed, and gas exchange between the headspace and bed
is promoted by the mixing of the bed (see Fig. 5). In rotating drum bioreactors,
mixing within the substrate bed is caused by rotation of the bed, and may be
promoted by the use of internal lifters. In stirred drum bioreactors, the drum
itself remains static and the mixing action is caused by paddles or scrapers
mounted on a shaft running through the central axis of the drum. Screw bioreactors have been used for continuous ethanol production in SSF, in which
there is no need to blow air through the bioreactor. In this bioreactor a screw
with a diameter equal to the internal diameter of the drum is operated intermittently to push the substrate from one end to the other. However, after some
interest in the 1980s, the screw bioreactor has not received further attention [81,
82]. Only rotating drums have received modeling attention and the discussion
below is limited to this design.
Heat removal from the substrate bed in rotating drums occurs by two main
routes – transfer directly to the headspace air by convection and evaporation
and transfer through the bioreactor wall by conduction followed by convective
cooling of the bioreactor wall (Fig. 8) [146]. The mixing in rotating drum
bioreactors allows the use of dry air to promote evaporation, because water can
be replenished by spraying a fine mist of water onto the bed as it is being mixed
[147]. Note that convective cooling of the drum wall occurs to both the surrounding air and the headspace air [146]. Although transfer to the headspace air
is a minor contributor to overall heat removal, the resulting increase in
headspace air temperature does increase the driving force for evaporation from
the bed to the headspace. Unfortunately no work has been done to obtain
experimental values for the various heat and mass transfer coefficients which
are important in rotating drum bioreactors.
Transfer between the bed and headspace will be affected significantly by the
mixing patterns of substrate particles within the bed and of air within the headspace. Note that transverse mixing considerations affect the substrate loadings
that can be used in rotating drum bioreactors. In non-SSF applications optimal
fill fractions, which maximize the amount of material within the drum but still
allow good mixing, range from 17% to 22% of the drum volume [148]. In SSF
110
D.A. Mitchell et al.