Part B | 9.4
288 Part B Tools and Methods in Marine Biotechnology
Photobioreactor Process Design
The photobioreactor design process has five steps.
Step 1. Determine the initial concentration of limiting
nutrient (C N;i ) necessary to achieve the target cell density (C x;f ) of 2 g cell L
1 . Using (9.20) and the Y X=N
value provided in Table 9.7, C N;i is
C N;i D
.C x;f C x;i /
Y X=N
D
.2:0 0:1/ g cell l
1
224 g cell mol 1 N 1
1000 mmol
1 mol
D 8:48 mmol N L
1
:
Steps 2 and 3. Determine the light path length L.
An aerated planar photobioreactor in the batch cultivation mode of operation must be used. Also, the vessel
must be artificially illuminated since it will be located
indoors. The light path width for the planar vessel
configuration, the incident light intensity to the vessel
surface, and the mode of light delivery (one or twosided illumination) must be specified. With respect to
the mode of light delivery, the two-sided illumination
system improves light delivery and biomass productivity and hence reduces vessel size, but has a higher
operating cost than the one-sided illumination system
since the electricity to run the illumination system is
a utility cost. First, the mean light intensity (I m ) at the
final cell density in the process of 2 g cell L
1 is set to
I k for the culture of 50 mol photon m
2 s
1 so that
I m I k . Second, to avoid photoinhibition of cells residing near the wall of the vessel, the incident light
intensity I o is set to 150 mol photon m
2 s
1 , which
is one half the observed photoinhibition threshold of
300 mol photon m
2 s
1 . Finally, the path length L is
backed out from (9.33) for both ˛ D 1 (one-sided illumination) and for ˛ D 2 (two-sided illumination)
I m D
˛I o
k c C x L
1 e
kcC x ;f L
D
˛ 150 mol m
2 s
1
0:2 l .g cm/ 1 2:0 g l 1 L
1 exp
0:2l cm g
1 2:0 g l
1
L
50 mol m
2 s
1
:
The required path length L is calculated using the
root-solving function on any scientific calculator or
spreadsheet program (e.g., Solver on Microsoft Excel).
For ˛ D 1, L is 7:05 cm, and for ˛ D 2, L is 15 cm.
Therefore, a path length of L D 15 cm with two-sided
illumination is selected to make the final vessel size
practical.
Step 4. Determine the volumetric CO 2 transfer rate
(CO 2 -TR) necessary to avoid CO 2 -limited growth. According to (9.42), the CO 2 demand increases as cell
density C x increases and
0 increases. The maximum
CO 2 -TR required for the aeration process is determined
at peak CO 2 demand. As the cultivation proceeds, C x increases but
0 decreases because I m , which determines
0 , decreases as C x increases. The magnitude of C x increase is higher than the magnitude of decrease, and
so the design is based on C x;f . At final cell density C x;f
of 2:0 g cell L
1 , I m is 50 mol photon m
2 s
1 , and
0
is 0:1 h
1 based on Model 1 in Table 9.4. Therefore,
the maximum CO 2 -TR at C x;f is determined by (9.46)
in the form of
Peak CO 2 TR D k L aC A
D
0 C x;f
Y X=CO2
D
0:1 h
1 2:0 g l
1 1000 mmol mol
1
33:8 g mmol 1 CO 2
D 5:92 mmol CO 2 .L h/
1
:
Now, the aeration system itself can be further specified.
First, consider that ambient air serves as the aeration gas, which contains 350 ppm CO 2 (0:00035 atm
CO 2 ). Using a Henry’s law coefficient of H D
0:0339 L atm mmol
1 CO 2 (Table 9.3), the required volumetric mass transfer coefficient k L a to achieve this
CO 2 -TR is
k L a D CO 2 TR
H
P A
D 5:92 mmol CO 2 l h
1
0:0339 l atm mmol
1
0:00035 atm
D 573 h
1
:
A k L a value of 573:1 hr
1 is too large to accomplish
within a bubble-column bioreactor. However, if the partial pressure of CO 2 in the aeration gas is increased to
3500 ppm (0:0035 atm CO 2 ), i. e.,
k L a D CO 2 TR
H
P A
D 5:92 mmol CO 2 l h
1
0:0339 l atm mmol
1
0:0035 atm
D 57:3 h
1
;
288 Part B Tools and Methods in Marine Biotechnology
Photobioreactor Process Design
The photobioreactor design process has five steps.
Step 1. Determine the initial concentration of limiting
nutrient (C N;i ) necessary to achieve the target cell density (C x;f ) of 2 g cell L
1 . Using (9.20) and the Y X=N
value provided in Table 9.7, C N;i is
C N;i D
.C x;f C x;i /
Y X=N
D
.2:0 0:1/ g cell l
1
224 g cell mol 1 N 1
1000 mmol
1 mol
D 8:48 mmol N L
1
:
Steps 2 and 3. Determine the light path length L.
An aerated planar photobioreactor in the batch cultivation mode of operation must be used. Also, the vessel
must be artificially illuminated since it will be located
indoors. The light path width for the planar vessel
configuration, the incident light intensity to the vessel
surface, and the mode of light delivery (one or twosided illumination) must be specified. With respect to
the mode of light delivery, the two-sided illumination
system improves light delivery and biomass productivity and hence reduces vessel size, but has a higher
operating cost than the one-sided illumination system
since the electricity to run the illumination system is
a utility cost. First, the mean light intensity (I m ) at the
final cell density in the process of 2 g cell L
1 is set to
I k for the culture of 50 mol photon m
2 s
1 so that
I m I k . Second, to avoid photoinhibition of cells residing near the wall of the vessel, the incident light
intensity I o is set to 150 mol photon m
2 s
1 , which
is one half the observed photoinhibition threshold of
300 mol photon m
2 s
1 . Finally, the path length L is
backed out from (9.33) for both ˛ D 1 (one-sided illumination) and for ˛ D 2 (two-sided illumination)
I m D
˛I o
k c C x L
1 e
kcC x ;f L
D
˛ 150 mol m
2 s
1
0:2 l .g cm/ 1 2:0 g l 1 L
1 exp
0:2l cm g
1 2:0 g l
1
L
50 mol m
2 s
1
:
The required path length L is calculated using the
root-solving function on any scientific calculator or
spreadsheet program (e.g., Solver on Microsoft Excel).
For ˛ D 1, L is 7:05 cm, and for ˛ D 2, L is 15 cm.
Therefore, a path length of L D 15 cm with two-sided
illumination is selected to make the final vessel size
practical.
Step 4. Determine the volumetric CO 2 transfer rate
(CO 2 -TR) necessary to avoid CO 2 -limited growth. According to (9.42), the CO 2 demand increases as cell
density C x increases and
0 increases. The maximum
CO 2 -TR required for the aeration process is determined
at peak CO 2 demand. As the cultivation proceeds, C x increases but
0 decreases because I m , which determines
0 , decreases as C x increases. The magnitude of C x increase is higher than the magnitude of decrease, and
so the design is based on C x;f . At final cell density C x;f
of 2:0 g cell L
1 , I m is 50 mol photon m
2 s
1 , and
0
is 0:1 h
1 based on Model 1 in Table 9.4. Therefore,
the maximum CO 2 -TR at C x;f is determined by (9.46)
in the form of
Peak CO 2 TR D k L aC A
D
0 C x;f
Y X=CO2
D
0:1 h
1 2:0 g l
1 1000 mmol mol
1
33:8 g mmol 1 CO 2
D 5:92 mmol CO 2 .L h/
1
:
Now, the aeration system itself can be further specified.
First, consider that ambient air serves as the aeration gas, which contains 350 ppm CO 2 (0:00035 atm
CO 2 ). Using a Henry’s law coefficient of H D
0:0339 L atm mmol
1 CO 2 (Table 9.3), the required volumetric mass transfer coefficient k L a to achieve this
CO 2 -TR is
k L a D CO 2 TR
H
P A
D 5:92 mmol CO 2 l h
1
0:0339 l atm mmol
1
0:00035 atm
D 573 h
1
:
A k L a value of 573:1 hr
1 is too large to accomplish
within a bubble-column bioreactor. However, if the partial pressure of CO 2 in the aeration gas is increased to
3500 ppm (0:0035 atm CO 2 ), i. e.,
k L a D CO 2 TR
H
P A
D 5:92 mmol CO 2 l h
1
0:0339 l atm mmol
1
0:0035 atm
D 57:3 h
1
;