Bioprocess Engineering of Phototrophic Marine Organisms 9.4 Limiting Factors in Photobioreactor Design and Operation 277
Part B | 9.4
the aeration section and so there is no net flow of the
culture out of the system. However, the tubular photobioreactor can be converted from batch operation to
continuous operation by adding a product line to the
re-circulation loop, as shown in Fig. 9.10. In the continuous recycle mode of operation, the liquid suspension
culture exiting the tubular section is divided into two
streams. The first stream, called the recycle line, is returned to the aeration section. The second stream, called
the product line, is withdrawn from the system at a constant volume flow rate. The recycle ratio is defined
as
R p D
v o
v p
;
(9.31)
where v o is the culture volumetric flow rate in the
recycle line and v p is the volumetric flow rate in the
product line. As described earlier in Sect. 9.3.2, to
maintain steady-state, constant cell density operation,
fresh liquid medium is added to the aeration section at
a volumetric flow rate equal to the volumetric flow rate
of culture removal (i. e., v p D v m ).
9.4 Limiting Factors in Photobioreactor Design and Operation
There are three major processes that limit photosynthetic biomass production in a photobioreactor. The
first process, macronutrient limitation, was discussed in
Sect. 9.3.2. The second process is the delivery of light
for photosynthesis. The third process is the delivery
of CO 2 for photosynthesis. Light delivery is traditionally seen as the limiting process to biomass production.
However, CO 2 delivery limitations can be significant
and are often overlooked in photobioreactor design and
operation. In this section, methods are presented to estimate the combined effects of light delivery and CO 2
delivery on biomass production in photobioreactors.
The modeling of light and CO 2 limitations in enclosed
photobioreactors is described in several excellent references [9.34–40]. Process scale-up, measurement, and
control issues in photobioreactor design and operation
are also described, as they follow from the presentation
of light and CO 2 delivery limitations.
9.4.1 Five Steps for Photobioreactor Design
Successful photobioreactor design depends on understanding light and CO 2 limitations. In this context,
photobioreactor design has five major steps:
1. Use biomass stoichiometry to calculate the limiting
nutrient concentration in the liquid medium necessary to achieve the desired final cell density in the
vessel (Sect. 9.2.1).
2. Select a batch or continuous biomass production
process. For relatively low rates of biomass production (e.g., 10100 kg d
1 ), consider a batch
process. For higher biomass production rates, consider a continuous process (Sect. 9.3.2).
3. Select the photobioreactor configuration and incident light intensity which provides sufficient light
delivery to the culture, and then compute the path
length for light transfer so that the mean light intensity I m I k (Sect. 9.4.2).
4. Based on step 3 above, design the aeration system
to provide a CO 2 transfer rate (CO 2 -TR) that is sufficient to avoid CO 2 -limited growth at the final cell
density in the culture vessel (Sect. 9.4.3).
5. Based on step 3, above, determine the cultivation
time (batch process) or the culture residence time
(continuous process) and the vessel volume required to satisfy the biomass production schedule.
Engineering analysis falls into two primary categories: design and performance analysis. Design starts
with a set of process performance targets, e.g., required biomass production rate or cell density, and
determines the photobioreactor design parameters necessary to achieve the production target. Specific design
parameters include nutrient concentration in the liquid medium, vessel size and configuration, illumination
system settings, and aeration system settings. In contrast, performance analysis usually begins with a photobioreactor system already in place that is operating
at a given set of medium, illumination, and aeration
conditions. Performance analysis estimates the biomass
production at this set of process conditions. Below, the
quantitative relationships needed for both process design and performance analysis are presented.
9.4.2 Light-Limited Growth
Light Attenuation
The specific growth rate of phototrophic organisms exhibits saturation growth kinetics with respect to incident light intensity. Therefore, the delivery of light to
the photobioreactor in part determines the biomass pro-
Part B | 9.4
the aeration section and so there is no net flow of the
culture out of the system. However, the tubular photobioreactor can be converted from batch operation to
continuous operation by adding a product line to the
re-circulation loop, as shown in Fig. 9.10. In the continuous recycle mode of operation, the liquid suspension
culture exiting the tubular section is divided into two
streams. The first stream, called the recycle line, is returned to the aeration section. The second stream, called
the product line, is withdrawn from the system at a constant volume flow rate. The recycle ratio is defined
as
R p D
v o
v p
;
(9.31)
where v o is the culture volumetric flow rate in the
recycle line and v p is the volumetric flow rate in the
product line. As described earlier in Sect. 9.3.2, to
maintain steady-state, constant cell density operation,
fresh liquid medium is added to the aeration section at
a volumetric flow rate equal to the volumetric flow rate
of culture removal (i. e., v p D v m ).
9.4 Limiting Factors in Photobioreactor Design and Operation
There are three major processes that limit photosynthetic biomass production in a photobioreactor. The
first process, macronutrient limitation, was discussed in
Sect. 9.3.2. The second process is the delivery of light
for photosynthesis. The third process is the delivery
of CO 2 for photosynthesis. Light delivery is traditionally seen as the limiting process to biomass production.
However, CO 2 delivery limitations can be significant
and are often overlooked in photobioreactor design and
operation. In this section, methods are presented to estimate the combined effects of light delivery and CO 2
delivery on biomass production in photobioreactors.
The modeling of light and CO 2 limitations in enclosed
photobioreactors is described in several excellent references [9.34–40]. Process scale-up, measurement, and
control issues in photobioreactor design and operation
are also described, as they follow from the presentation
of light and CO 2 delivery limitations.
9.4.1 Five Steps for Photobioreactor Design
Successful photobioreactor design depends on understanding light and CO 2 limitations. In this context,
photobioreactor design has five major steps:
1. Use biomass stoichiometry to calculate the limiting
nutrient concentration in the liquid medium necessary to achieve the desired final cell density in the
vessel (Sect. 9.2.1).
2. Select a batch or continuous biomass production
process. For relatively low rates of biomass production (e.g., 10100 kg d
1 ), consider a batch
process. For higher biomass production rates, consider a continuous process (Sect. 9.3.2).
3. Select the photobioreactor configuration and incident light intensity which provides sufficient light
delivery to the culture, and then compute the path
length for light transfer so that the mean light intensity I m I k (Sect. 9.4.2).
4. Based on step 3 above, design the aeration system
to provide a CO 2 transfer rate (CO 2 -TR) that is sufficient to avoid CO 2 -limited growth at the final cell
density in the culture vessel (Sect. 9.4.3).
5. Based on step 3, above, determine the cultivation
time (batch process) or the culture residence time
(continuous process) and the vessel volume required to satisfy the biomass production schedule.
Engineering analysis falls into two primary categories: design and performance analysis. Design starts
with a set of process performance targets, e.g., required biomass production rate or cell density, and
determines the photobioreactor design parameters necessary to achieve the production target. Specific design
parameters include nutrient concentration in the liquid medium, vessel size and configuration, illumination
system settings, and aeration system settings. In contrast, performance analysis usually begins with a photobioreactor system already in place that is operating
at a given set of medium, illumination, and aeration
conditions. Performance analysis estimates the biomass
production at this set of process conditions. Below, the
quantitative relationships needed for both process design and performance analysis are presented.
9.4.2 Light-Limited Growth
Light Attenuation
The specific growth rate of phototrophic organisms exhibits saturation growth kinetics with respect to incident light intensity. Therefore, the delivery of light to
the photobioreactor in part determines the biomass pro-