Bioprocess Engineering of Phototrophic Marine Organisms 9.4 Limiting Factors in Photobioreactor Design and Operation 287
Part B | 9.4
9.4.5 Process Monitoring and Control
Photobioreactors require online process measurement
sensors common to most bioprocess equipment for cell
cultivation, including dissolved oxygen electrodes, pH
electrodes, temperature probes, gas and liquid flowmeters, and liquid level indicators. Photobioreactors also
require sensors for gas phase CO 2 concentration and
light intensity. The suggested arrangement of process
sensors is provided in Fig. 9.25.
Photosynthetic liquid suspension cultures produce
oxygen and so the dissolved O 2 concentration in the
liquid medium is usually near 100% of its saturation
value with respect to the oxygen partial pressure in
the aeration gas. However, if the culture is not adequately aerated, oxygen can build up in the liquid
medium to higher concentrations which may be toxic
to the cells, a phenomenon called oxygen toxicity.
Therefore, the dissolved oxygen concentration should
be monitored in all aerated vessels as well as the entrance and exit points to tubular sections of tubular
photobioreactors. The consumption of dissolved CO 2
by photosynthesis can shift the equilibrium of dissolved
organic carbon species by dissociating bicarbonate to
dissolved CO 2 and OH- ions, which raises the pH.
Therefore, the pH should be monitored at all points
common to dissolved oxygen concentration measurement. Several commercially available dissolved oxygen
(DO) and pH electrodes are suitable for these bioprocess measurements. The CO 2 ultimately consumed by
photosynthesis is delivered to the photobioreactor by
the aeration gas. Therefore, the total aeration gas flow
rate and the gas-phase concentration entering and exiting the photobioreactor should be monitored. Online
infra-red (IR) sensors are suitable for measurement of
gas-phase CO 2 concentration in the inlet and exhaust
aeration gas streams.
Online culture pH and gas-phase CO 2 concentration
measurements can also be used to control photobioreactor operation during culture growth. For example, if
CO 2 demand by actively growing culture is high, the pH
will rise. The pH can be lowered by increasing the CO 2
concentration in the aeration gas or by direct injection
of CO 2 to the culture. Both methods improve the CO 2
transfer rate and lower the culture pH by shifting the
dissolved inorganic carbon equilibrium back to bicarbonate. For process controller operation, the gas phase
CO 2 concentration serves as the manipulated variable
and the pH serves as the response variable.
Photobioreactors should also be equipped with
online irradiance sensors that measure the PAR
(400700 nm) light intensity in units of mol photons
m
2 s
1 . Irradiance sensors should be placed on the
illuminated vessel surface with the face of the sensor pointed toward the light source. Other irradiance
sensors should have the face of the sensor pointed to
the culture to measure the light intensity transmitted
through the culture vessel. If possible, several irradiance sensors should be laid out in a grid pattern
along the illuminated vessel surface to monitor temporal distributions of both incident and transmitted light
intensity. Irradiance sensors can also be used to control
semi-continuous processes. For example, if the transmitted light intensity through the culture falls below
a target value indicative of a certain cell density, then
a portion of the suspension culture is pumped out and
fresh medium is added to dilute the suspension culture
and raise the transmitted light intensity back up to the
setpoint value.
9.4.6 Illustration of a Photobioreactor
Design Problem
This section illustrates the design of an enclosed photobioreactor system using concepts given in Sects. 9.2–
9.4.
9.4.7 Design Needs Statement
It is necessary to design and scale up an aerated planar photobioreactor for cultivation of a phototrophic
algal cell suspension culture. The cell mass contains an
expensive antibiotic compound in the concentration of
25 mg per 1:0 g cell mass. The cultivation will be carried out as a batch process with an inoculum cell density of 0:10 g cells .L culture/
1 . To achieve the needed
antibiotic production rate of 0:5 kg day
1 , the target
biomass productivity is 20 kg cells d
1 , and a final cell
density of 2:0 g cells .L culture/
1 is desired to facilitate downstream processing. The intrinsic growth parameters for the organism, including biomass yield coefficients, are Monod parameters for light and limiting
nutrient at 25
ı C and pH 8:0, and a specific light attenuation coefficient for the cell suspension. These characteristic growth parameters are summarized in Table 9.7.
The cell suspension culture becomes photo-inhibited at
light intensities exceeding 300 mol photons m
2 s
1 .
To complete the photobioreactor design, specify the
required nutrient loading in the culture medium, the illumination system delivery, the vessel dimensions, and
the required CO 2 transfer rate necessary to avoid CO 2 -
limited growth.
Part B | 9.4
9.4.5 Process Monitoring and Control
Photobioreactors require online process measurement
sensors common to most bioprocess equipment for cell
cultivation, including dissolved oxygen electrodes, pH
electrodes, temperature probes, gas and liquid flowmeters, and liquid level indicators. Photobioreactors also
require sensors for gas phase CO 2 concentration and
light intensity. The suggested arrangement of process
sensors is provided in Fig. 9.25.
Photosynthetic liquid suspension cultures produce
oxygen and so the dissolved O 2 concentration in the
liquid medium is usually near 100% of its saturation
value with respect to the oxygen partial pressure in
the aeration gas. However, if the culture is not adequately aerated, oxygen can build up in the liquid
medium to higher concentrations which may be toxic
to the cells, a phenomenon called oxygen toxicity.
Therefore, the dissolved oxygen concentration should
be monitored in all aerated vessels as well as the entrance and exit points to tubular sections of tubular
photobioreactors. The consumption of dissolved CO 2
by photosynthesis can shift the equilibrium of dissolved
organic carbon species by dissociating bicarbonate to
dissolved CO 2 and OH- ions, which raises the pH.
Therefore, the pH should be monitored at all points
common to dissolved oxygen concentration measurement. Several commercially available dissolved oxygen
(DO) and pH electrodes are suitable for these bioprocess measurements. The CO 2 ultimately consumed by
photosynthesis is delivered to the photobioreactor by
the aeration gas. Therefore, the total aeration gas flow
rate and the gas-phase concentration entering and exiting the photobioreactor should be monitored. Online
infra-red (IR) sensors are suitable for measurement of
gas-phase CO 2 concentration in the inlet and exhaust
aeration gas streams.
Online culture pH and gas-phase CO 2 concentration
measurements can also be used to control photobioreactor operation during culture growth. For example, if
CO 2 demand by actively growing culture is high, the pH
will rise. The pH can be lowered by increasing the CO 2
concentration in the aeration gas or by direct injection
of CO 2 to the culture. Both methods improve the CO 2
transfer rate and lower the culture pH by shifting the
dissolved inorganic carbon equilibrium back to bicarbonate. For process controller operation, the gas phase
CO 2 concentration serves as the manipulated variable
and the pH serves as the response variable.
Photobioreactors should also be equipped with
online irradiance sensors that measure the PAR
(400700 nm) light intensity in units of mol photons
m
2 s
1 . Irradiance sensors should be placed on the
illuminated vessel surface with the face of the sensor pointed toward the light source. Other irradiance
sensors should have the face of the sensor pointed to
the culture to measure the light intensity transmitted
through the culture vessel. If possible, several irradiance sensors should be laid out in a grid pattern
along the illuminated vessel surface to monitor temporal distributions of both incident and transmitted light
intensity. Irradiance sensors can also be used to control
semi-continuous processes. For example, if the transmitted light intensity through the culture falls below
a target value indicative of a certain cell density, then
a portion of the suspension culture is pumped out and
fresh medium is added to dilute the suspension culture
and raise the transmitted light intensity back up to the
setpoint value.
9.4.6 Illustration of a Photobioreactor
Design Problem
This section illustrates the design of an enclosed photobioreactor system using concepts given in Sects. 9.2–
9.4.
9.4.7 Design Needs Statement
It is necessary to design and scale up an aerated planar photobioreactor for cultivation of a phototrophic
algal cell suspension culture. The cell mass contains an
expensive antibiotic compound in the concentration of
25 mg per 1:0 g cell mass. The cultivation will be carried out as a batch process with an inoculum cell density of 0:10 g cells .L culture/
1 . To achieve the needed
antibiotic production rate of 0:5 kg day
1 , the target
biomass productivity is 20 kg cells d
1 , and a final cell
density of 2:0 g cells .L culture/
1 is desired to facilitate downstream processing. The intrinsic growth parameters for the organism, including biomass yield coefficients, are Monod parameters for light and limiting
nutrient at 25
ı C and pH 8:0, and a specific light attenuation coefficient for the cell suspension. These characteristic growth parameters are summarized in Table 9.7.
The cell suspension culture becomes photo-inhibited at
light intensities exceeding 300 mol photons m
2 s
1 .
To complete the photobioreactor design, specify the
required nutrient loading in the culture medium, the illumination system delivery, the vessel dimensions, and
the required CO 2 transfer rate necessary to avoid CO 2 -
limited growth.