the presence or absence of baffles, but also by the design of the air inlet and
outlet.
Discontinuous agitation of rotating drum bioreactors has been used in a few
studies [152, 153]. In the work of de Reu et al. [152], when the temperature
within the bed exceeded a setpoint, a one-minute agitation regime was initiated,
with rotation alternately clockwise and counterclockwise at 4–6 rpm. In different fermentations adequate temperature control was achieved with mixing events
being initiated at intervals from as short as 9 min to as long as 142 min.
The only modeling work available for the rotating drum bioreactor is the
heat transfer model of Stuart [146]. The model treated the substrate bed, the
headspace gases, and the bioreactor wall as separate phases, and assumed that
each of these phases was thermally homogeneous, which is equivalent to assuming that the substrate bed and headspace gases are well-mixed and that the
thermal conductivity of the bioreactor wall is high. Note that models of SSF
bioreactors have to date rarely taken the bioreactor wall into account. It was
only by doing so that Stuart [146] noted that the drum wall will be hotter than
the headspace gases and therefore gives an indirect route for transfer of heat
between the substrate bed and the headspace. The model predicts that for small
scale bioreactors a significant proportion of the heat removal from the bed occurs by conduction from the bed to the bioreactor wall followed by convection
to the surrounding air.
In conclusion regarding rotating drum bioreactors, although they have the
potential to provide relatively gentle mixing, with less compaction and crushing
of substrate particles than is likely to occur with agitators embedded within the
substrate bed, their true potential is largely unexplored due to the lack of
quantitative characterization of the mixing and heat and mass transfer phenomena occurring within them.
5.5
Mixed Beds with Forced Aeration – Gas-Solid Fluidized Beds, Stirred Drums,
and the Rocking Drum
This group of bioreactors includes those in which air is blown forcefully
through the bed and in which the bed is agitated either intermittently or continuously. The combination of mixing and forced aeration can help in avoiding
the temperature and moisture gradients which occur in other bioreactor types.
Note that this greatly simplifies modeling work because the bioreactor can be
treated as a well-mixed system, eliminating the spatial variable (Fig. 9). There
are several designs in which mixing with forced aeration can be achieved –
stirred aerated beds, rocking drums, and gas-solid fluidized beds.
5.5.1
Gas Solid-Fluidized Beds
This bioreactor consists of a vertical chamber, with a perforated base plate
through which air or some other gas is blown upwards at a sufficient velocity to
fluidize the substrate particles (see Fig. 5). Note that not all solid substrates will
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D.A. Mitchell et al.
outlet.
Discontinuous agitation of rotating drum bioreactors has been used in a few
studies [152, 153]. In the work of de Reu et al. [152], when the temperature
within the bed exceeded a setpoint, a one-minute agitation regime was initiated,
with rotation alternately clockwise and counterclockwise at 4–6 rpm. In different fermentations adequate temperature control was achieved with mixing events
being initiated at intervals from as short as 9 min to as long as 142 min.
The only modeling work available for the rotating drum bioreactor is the
heat transfer model of Stuart [146]. The model treated the substrate bed, the
headspace gases, and the bioreactor wall as separate phases, and assumed that
each of these phases was thermally homogeneous, which is equivalent to assuming that the substrate bed and headspace gases are well-mixed and that the
thermal conductivity of the bioreactor wall is high. Note that models of SSF
bioreactors have to date rarely taken the bioreactor wall into account. It was
only by doing so that Stuart [146] noted that the drum wall will be hotter than
the headspace gases and therefore gives an indirect route for transfer of heat
between the substrate bed and the headspace. The model predicts that for small
scale bioreactors a significant proportion of the heat removal from the bed occurs by conduction from the bed to the bioreactor wall followed by convection
to the surrounding air.
In conclusion regarding rotating drum bioreactors, although they have the
potential to provide relatively gentle mixing, with less compaction and crushing
of substrate particles than is likely to occur with agitators embedded within the
substrate bed, their true potential is largely unexplored due to the lack of
quantitative characterization of the mixing and heat and mass transfer phenomena occurring within them.
5.5
Mixed Beds with Forced Aeration – Gas-Solid Fluidized Beds, Stirred Drums,
and the Rocking Drum
This group of bioreactors includes those in which air is blown forcefully
through the bed and in which the bed is agitated either intermittently or continuously. The combination of mixing and forced aeration can help in avoiding
the temperature and moisture gradients which occur in other bioreactor types.
Note that this greatly simplifies modeling work because the bioreactor can be
treated as a well-mixed system, eliminating the spatial variable (Fig. 9). There
are several designs in which mixing with forced aeration can be achieved –
stirred aerated beds, rocking drums, and gas-solid fluidized beds.
5.5.1
Gas Solid-Fluidized Beds
This bioreactor consists of a vertical chamber, with a perforated base plate
through which air or some other gas is blown upwards at a sufficient velocity to
fluidize the substrate particles (see Fig. 5). Note that not all solid substrates will
112
D.A. Mitchell et al.