Novel Bioreactors for Culturing Marine Organisms 12.3 Airlift Bioreactors (ALBR) and Bubble Column Bioreactors (BCBR) 347
Part B | 12.3
helical flow promoters (ALBR C HFP). Now, in an earlier study Merchuk et al. [12.12] comparing BC and
ALR-type photobioreactors for Porphyridium growth, it
was concluded that the more ordered flow in the ALBR
leads to the exposure of the growing cells to light and
darkness in more homogenous cycles, resulting in the
superior reactor performance.
The HFP is a static device consisting of several fins
or baffles that causes fluid flow in a helical path along
the downcomer. It effectively modifies the fluid flow
path from a rectilinear trajectory along the axis to a helix. Although the HFP may be installed at any point
along the riser or the downcomer, a much preferred
location is the top of the downcomer – wherefrom
a helical flow is generated in the downcomer, which becomes a swirl at the bottom and a corkscrew-like path
in the riser. The helical fluid motion causes secondary
flow which leads to an enhanced radial mixing, and thus
more homogeneous distribution of light among liquid
elements and suspended particles. This increases the
likelihood of all fluid elements getting the same exposure to light. In this study, the HFPs were positioned at
the upper rim of the downcomer. As the ALBRs were illuminated through the external walls, aeration from the
bottom of the draft tube was preferred in order to minimize the quantity of bubbles in the downcomer, which
would cause loss of efficiency due to light scattering.
Installing the HFP at the downcomer inlet maximizes
the fluid dynamic effects (i. e., secondary flow) in the
photosynthesis zone of the reactor.
The reactors (I.D. 0:13 m, height 1:5 m) were made
of flexible polyethylene sleeves with a conical bottom.
For operation as an ALBR, an acrylic draft tube (external diameter 0:09 m, height 1:05 m) was inserted. For
the ALBR C HFP the same draft tube was used but with
the HFP installed. Selection of the draft tube diameter
was based on the fluid residence time in the riser as
time spent in the dark zone. The HFP comprises three
fins at 45
ı from the axis, placed at the upper 0:05 m
of the downcomer, with a width equal to the gap between draft tube and the external wall. Gas entered
the reactors through a sparger installed 0:03 m above
the bottom. The sparger was a ring of 0:05 m diameter made of a stainless steel tube of 0:005 m diameter,
with 15 holes (diameters 0:001 m) equidistantly positioned on its upper face. Now, the gas flow rate to the
reactor strongly influences the mixing of medium, the
distribution of cells, their nutrient availability, and CO 2
absorption. It was observed for the ALR C HFP that the
more ordered flow and the additional secondary circulation due to the HFP provide sufficient mixing and better
cell distribution at a lower gas flow rate, without too
many cells adhering to the reactor walls. Operability
at lower gas flow rates leads to lower consumption of
air and CO 2 resulting in lowered costs of air compression and CO 2 , thereby improving the efficiency of CO 2
uptake for algal photosynthesis – the basis for superior
performance of the ALR C HFP relative to the other reactors considered.
Rorrer and Cheney [12.38] have compared four
major photobioreactor configurations for microplantlet suspension cultures of photolithotrophic marine
macroalgae A. subulata, viz., bubble column, airlift (internal draft tube), externally illuminated stirred tank and
tubular recycle (helical array) photobioreactors in both
batch and medium perfusion modes of macronutrient
delivery. Of these four reactors considered, the ST-PBR
and tubular recycle PBR have been used earlier by Rorrer and Zhi [12.14] and Rorrer and Mullikin [12.13],
respectively, and have already been discussed at length
in Sect. 12.2. These four PBRs were compared on four
major parameters, viz., mixing and biomass suspension,
aeration, and gas exchange, light transfer and potential for shear damage. Both BC and ALR-type PBRs
have low shear damage potential and very good aeration capabilities, and while mixing and light transfer
are rated as adequate for BC, they range from good
to very good for ALR. The stirred tank PBR, on the
other hand, has high shear damage potential and poor
light transfer facilities but with regard to mixing and
aeration it is rated as excellent. Finally, for the tubular
recycle PBR, while light transfer is excellent and aeration adequate, mixing and shear damage are problem
areas. Certain essential common features of these PBR
systems are worth noting: the bioreactor control volume
being externally illuminated by artificial light source(s),
transparency of at least a part of the bioreactor vessel is
necessary – this requires the reactor to be made out of
glass/polycarbonate for BC/ALR and translucent silicone tubing for tubular PBR; also, the nonfriable nature
of the macroalgal cell and tissue precludes continuous
culture, because dilution of the culture suspension by
continuous addition of fresh medium will dilute out
these cell clumps. It may be noted that in microplantlet tissue cultures, since the biomass is compacted
into ball-like nonfriable multicellular tissue and not
dispersed in the liquid medium as single cells, light attenuation through the culture suspension is low relative
to microalgal suspension cultures at the same cell mass
density. Again, microplantlets are easily suspended but
also easily separated from the culture broth thus facilitating biomass harvesting.
Part B | 12.3
helical flow promoters (ALBR C HFP). Now, in an earlier study Merchuk et al. [12.12] comparing BC and
ALR-type photobioreactors for Porphyridium growth, it
was concluded that the more ordered flow in the ALBR
leads to the exposure of the growing cells to light and
darkness in more homogenous cycles, resulting in the
superior reactor performance.
The HFP is a static device consisting of several fins
or baffles that causes fluid flow in a helical path along
the downcomer. It effectively modifies the fluid flow
path from a rectilinear trajectory along the axis to a helix. Although the HFP may be installed at any point
along the riser or the downcomer, a much preferred
location is the top of the downcomer – wherefrom
a helical flow is generated in the downcomer, which becomes a swirl at the bottom and a corkscrew-like path
in the riser. The helical fluid motion causes secondary
flow which leads to an enhanced radial mixing, and thus
more homogeneous distribution of light among liquid
elements and suspended particles. This increases the
likelihood of all fluid elements getting the same exposure to light. In this study, the HFPs were positioned at
the upper rim of the downcomer. As the ALBRs were illuminated through the external walls, aeration from the
bottom of the draft tube was preferred in order to minimize the quantity of bubbles in the downcomer, which
would cause loss of efficiency due to light scattering.
Installing the HFP at the downcomer inlet maximizes
the fluid dynamic effects (i. e., secondary flow) in the
photosynthesis zone of the reactor.
The reactors (I.D. 0:13 m, height 1:5 m) were made
of flexible polyethylene sleeves with a conical bottom.
For operation as an ALBR, an acrylic draft tube (external diameter 0:09 m, height 1:05 m) was inserted. For
the ALBR C HFP the same draft tube was used but with
the HFP installed. Selection of the draft tube diameter
was based on the fluid residence time in the riser as
time spent in the dark zone. The HFP comprises three
fins at 45
ı from the axis, placed at the upper 0:05 m
of the downcomer, with a width equal to the gap between draft tube and the external wall. Gas entered
the reactors through a sparger installed 0:03 m above
the bottom. The sparger was a ring of 0:05 m diameter made of a stainless steel tube of 0:005 m diameter,
with 15 holes (diameters 0:001 m) equidistantly positioned on its upper face. Now, the gas flow rate to the
reactor strongly influences the mixing of medium, the
distribution of cells, their nutrient availability, and CO 2
absorption. It was observed for the ALR C HFP that the
more ordered flow and the additional secondary circulation due to the HFP provide sufficient mixing and better
cell distribution at a lower gas flow rate, without too
many cells adhering to the reactor walls. Operability
at lower gas flow rates leads to lower consumption of
air and CO 2 resulting in lowered costs of air compression and CO 2 , thereby improving the efficiency of CO 2
uptake for algal photosynthesis – the basis for superior
performance of the ALR C HFP relative to the other reactors considered.
Rorrer and Cheney [12.38] have compared four
major photobioreactor configurations for microplantlet suspension cultures of photolithotrophic marine
macroalgae A. subulata, viz., bubble column, airlift (internal draft tube), externally illuminated stirred tank and
tubular recycle (helical array) photobioreactors in both
batch and medium perfusion modes of macronutrient
delivery. Of these four reactors considered, the ST-PBR
and tubular recycle PBR have been used earlier by Rorrer and Zhi [12.14] and Rorrer and Mullikin [12.13],
respectively, and have already been discussed at length
in Sect. 12.2. These four PBRs were compared on four
major parameters, viz., mixing and biomass suspension,
aeration, and gas exchange, light transfer and potential for shear damage. Both BC and ALR-type PBRs
have low shear damage potential and very good aeration capabilities, and while mixing and light transfer
are rated as adequate for BC, they range from good
to very good for ALR. The stirred tank PBR, on the
other hand, has high shear damage potential and poor
light transfer facilities but with regard to mixing and
aeration it is rated as excellent. Finally, for the tubular
recycle PBR, while light transfer is excellent and aeration adequate, mixing and shear damage are problem
areas. Certain essential common features of these PBR
systems are worth noting: the bioreactor control volume
being externally illuminated by artificial light source(s),
transparency of at least a part of the bioreactor vessel is
necessary – this requires the reactor to be made out of
glass/polycarbonate for BC/ALR and translucent silicone tubing for tubular PBR; also, the nonfriable nature
of the macroalgal cell and tissue precludes continuous
culture, because dilution of the culture suspension by
continuous addition of fresh medium will dilute out
these cell clumps. It may be noted that in microplantlet tissue cultures, since the biomass is compacted
into ball-like nonfriable multicellular tissue and not
dispersed in the liquid medium as single cells, light attenuation through the culture suspension is low relative
to microalgal suspension cultures at the same cell mass
density. Again, microplantlets are easily suspended but
also easily separated from the culture broth thus facilitating biomass harvesting.