Bioprocess Engineering of Phototrophic Marine Organisms 9.6 Notation 291
Part B | 9.6
controllable. Under controlled conditions for light and
CO 2 delivery, cell densities exceeding 5 g dry cells L
1
of culture are possible. If 1) the achievable cell density is high, 2) the product is extremely valuable (>
$1000 kg
1 ), and 3) required production slate of the target compound in the biomass is small (e.g., less than
1 kg d
1 ), as is the case with many high-value specialty
chemicals or pharmaceutical compounds, then photobioreactor culture volumes for process-scale units will
most likely be under 20 000 L. Consequently, for process biotechnology applications, photobioreactor development is likely to move toward internally illuminated,
stirred-tank photobioreactors, or batteries of externally
illuminated bubble-column/airlift photobioreactors.
Second, environmental and renewable energy applications will drive large-scale photobioreactor development using natural illumination. The future development of photobioreactors will also focus on environmental or renewable energy applications, including
CO 2 mitigation, waste water cleanup, and hydrogen gas
production, and the production of biofuels. In the future, CO 2 emissions from industrial processes may be
capped in the attempt to reduce the release of greenhouse gases to the atmosphere. One way to reduce
CO 2 emissions is to capture the CO 2 and convert it
to something else. Phototrophic marine organisms are
ideal for this purpose because they convert CO 2 into
biomass and lipids which can be processed into biofuels. However, there are several challenges to photobioreactor design for biological CO 2 mitigation. Stack
gases from combustion processes typically contain at
least 10% CO 2 by volume. In the simplest process,
the stack gas is bubbled directly into an open pond
or tank culture. However, this process is not efficient
because only a fraction of the CO 2 in the stack gas
is transferred to the liquid culture. In contrast, tubular
photobioreactors may be more effective for CO 2 capture, but the residence time of the culture in the tubular
section must be designed to completely consume the
CO 2 .
Whether the process application for cultivation of
phototrophic organisms in enclosed photobioreactors is
for high-value compounds at a small scale or CO 2 mitigation at large scale, this chapter has shown simple
but effective approaches for the successful design of the
cultivation system that incorporates nutrient consumption, light transfer, and CO 2 delivery into the process
analysis.
9.6 Notation
a
Interfacial area of aeration gas bubbles in liquid
suspension per unit culture volume, m
2 m
3
A i
Total interfacial area of aeration gas bubbles in
liquid suspension, m
2
C A
Concentration of dissolved CO 2 in culture
medium, mol L
1
C A
Concentration of dissolved CO 2 in equilibrium
with the CO 2 partial pressure in the aeration
gas, mol L
1
C AL
Concentration of dissolved CO 2 exiting tubular
section or entering aeration tank (tubular photobioreactor), mol L
1
C AO
Concentration of dissolved CO 2 exiting aeration tank or entering tubular section (tubular
photobioreactor), mol L
1
C A;T Total dissolved carbon concentration, mol L
1
C N
Concentration of dissolved nutrient in liquid
medium, mol L
1
C N;i Initial concentration of dissolved nutrient in
batch bioreactor liquid medium, mol L
1
C N;o Concentration of dissolved nutrient in inlet flow
stream of well-mixed continuous flow bioreactor, mol L
1
C x
Cell density in liquid suspension culture,
g cells L
1 culture
C x;c
Cell density where growth kinetics become CO 2
transfer rate limited at a given set of process
conditions, g cells L
1 culture
C x;f
Final cell density at nutrient depletion,
g cells L
1 culture
C x;i
Initial cell density in batch bioreactor,
g cells L
1 culture
C x;o Cell density of inlet flow stream of well-mixed
continuous flow bioreactor, mol L
1
d
Inner diameter of tubing, cm or m
D
Dilution rate for well mixed continuous flow
bioreactor, s
1 or h
1
D CO2 Diffusion coefficient for CO 2 dissolved in seawater, m
2 s
1
D O2
Diffusion coefficient for O 2 dissolved in seawater, m
2 s
1
Part B | 9.6
controllable. Under controlled conditions for light and
CO 2 delivery, cell densities exceeding 5 g dry cells L
1
of culture are possible. If 1) the achievable cell density is high, 2) the product is extremely valuable (>
$1000 kg
1 ), and 3) required production slate of the target compound in the biomass is small (e.g., less than
1 kg d
1 ), as is the case with many high-value specialty
chemicals or pharmaceutical compounds, then photobioreactor culture volumes for process-scale units will
most likely be under 20 000 L. Consequently, for process biotechnology applications, photobioreactor development is likely to move toward internally illuminated,
stirred-tank photobioreactors, or batteries of externally
illuminated bubble-column/airlift photobioreactors.
Second, environmental and renewable energy applications will drive large-scale photobioreactor development using natural illumination. The future development of photobioreactors will also focus on environmental or renewable energy applications, including
CO 2 mitigation, waste water cleanup, and hydrogen gas
production, and the production of biofuels. In the future, CO 2 emissions from industrial processes may be
capped in the attempt to reduce the release of greenhouse gases to the atmosphere. One way to reduce
CO 2 emissions is to capture the CO 2 and convert it
to something else. Phototrophic marine organisms are
ideal for this purpose because they convert CO 2 into
biomass and lipids which can be processed into biofuels. However, there are several challenges to photobioreactor design for biological CO 2 mitigation. Stack
gases from combustion processes typically contain at
least 10% CO 2 by volume. In the simplest process,
the stack gas is bubbled directly into an open pond
or tank culture. However, this process is not efficient
because only a fraction of the CO 2 in the stack gas
is transferred to the liquid culture. In contrast, tubular
photobioreactors may be more effective for CO 2 capture, but the residence time of the culture in the tubular
section must be designed to completely consume the
CO 2 .
Whether the process application for cultivation of
phototrophic organisms in enclosed photobioreactors is
for high-value compounds at a small scale or CO 2 mitigation at large scale, this chapter has shown simple
but effective approaches for the successful design of the
cultivation system that incorporates nutrient consumption, light transfer, and CO 2 delivery into the process
analysis.
9.6 Notation
a
Interfacial area of aeration gas bubbles in liquid
suspension per unit culture volume, m
2 m
3
A i
Total interfacial area of aeration gas bubbles in
liquid suspension, m
2
C A
Concentration of dissolved CO 2 in culture
medium, mol L
1
C A
Concentration of dissolved CO 2 in equilibrium
with the CO 2 partial pressure in the aeration
gas, mol L
1
C AL
Concentration of dissolved CO 2 exiting tubular
section or entering aeration tank (tubular photobioreactor), mol L
1
C AO
Concentration of dissolved CO 2 exiting aeration tank or entering tubular section (tubular
photobioreactor), mol L
1
C A;T Total dissolved carbon concentration, mol L
1
C N
Concentration of dissolved nutrient in liquid
medium, mol L
1
C N;i Initial concentration of dissolved nutrient in
batch bioreactor liquid medium, mol L
1
C N;o Concentration of dissolved nutrient in inlet flow
stream of well-mixed continuous flow bioreactor, mol L
1
C x
Cell density in liquid suspension culture,
g cells L
1 culture
C x;c
Cell density where growth kinetics become CO 2
transfer rate limited at a given set of process
conditions, g cells L
1 culture
C x;f
Final cell density at nutrient depletion,
g cells L
1 culture
C x;i
Initial cell density in batch bioreactor,
g cells L
1 culture
C x;o Cell density of inlet flow stream of well-mixed
continuous flow bioreactor, mol L
1
d
Inner diameter of tubing, cm or m
D
Dilution rate for well mixed continuous flow
bioreactor, s
1 or h
1
D CO2 Diffusion coefficient for CO 2 dissolved in seawater, m
2 s
1
D O2
Diffusion coefficient for O 2 dissolved in seawater, m
2 s
1