and uniform light distribution, which implies enhanced performance on the
microalgae growth. In addition, they can be operated easily, their cell density is 5–6
times higher than that of open ponds, and their capacity can reach up to 25,000 L
and occupy a restricted area of about 10 m
2 (Raeesossadati et al. 2014; Jacob-Lopes
et al. 2015; Pawar 2016). However, in addition to the drawbacks related to overheating, the main critical issue in these systems is photo-inhibition, energy consumption, high costs, and dissolved oxygen (DO) accumulation (Huang et al.
2017).
As oxygen is a product of photosynthetic metabolism, its formation and solubilization in tubular photobioreactors indicate high inorganic carbon consumption
rates, reaching O 2 generation rates of up to 10 mg/L min, even with a very frequent
gas exchange (Chisti 2007). To prevent inhibition by O 2 accumulation, the DO
concentration in the culture medium should not exceed the maximum tolerable
value of 400% of the saturation level achieved in the presence of air. In a study by
Raso et al. (2012), O 2 concentration increased from 75 to 250%; air saturation
inhibited the growth of microalgae. To improve productivity in tubular photobioreactors, the oxygen level must be controlled or removed. However, optimal
control parameters have not yet been well established to improve productivity in
these systems.
With regard to operation, in a tubular photobioreactor, the airlift column circulates the broth with the culture medium to ensure light penetration through the
solar collector, in which place most of the photosynthesis occurs, with ensuing DO
accumulation. This, in turn, cannot be easily removed from the tubes
(Molina-Grima et al. 2001). Therefore, in theory, if the oxygen is not removed
within about one minute after accumulation, the inhibitory effect on the cells will
occur immediately (Huang et al. 2017). In this case, the collecting tubes should be
designed with restricted length for continuous DO removal, as well as the insertion
of degasser systems.
Despite the fact that tubular photobioreactors are currently the most suitable
configurations for application in oxygen generation, the main bottleneck of this type
of equipment is its configuration, especially characterized by the geometry of these
systems.
Due to the currently limited operational scale, the conventional configurations
meet the basic requirements of the photosynthetic process. However, for a potential
scale-up of the production process, operational failures must be overcome.
Parameters such as the ratio of height/diameter column (H/D) are fundamental to
build industrial photobioreactors. For this reason, hybrid photobioreactors compensate the drawbacks caused by limitation of S/V ratio and scale-up, since these
systems can be based on a proper H/D ratio, generating configurations of reactors
with heavy workloads in contrast to very long tubes or shallow ponds (Jacob-Lopes
et al. 2016). If these aspects are considered, photobioreactors could be a fundamental step forward for the consolidation of the industrial biological oxygen
generation.
284
I. Aguiar Severo et al.
microalgae growth. In addition, they can be operated easily, their cell density is 5–6
times higher than that of open ponds, and their capacity can reach up to 25,000 L
and occupy a restricted area of about 10 m
2 (Raeesossadati et al. 2014; Jacob-Lopes
et al. 2015; Pawar 2016). However, in addition to the drawbacks related to overheating, the main critical issue in these systems is photo-inhibition, energy consumption, high costs, and dissolved oxygen (DO) accumulation (Huang et al.
2017).
As oxygen is a product of photosynthetic metabolism, its formation and solubilization in tubular photobioreactors indicate high inorganic carbon consumption
rates, reaching O 2 generation rates of up to 10 mg/L min, even with a very frequent
gas exchange (Chisti 2007). To prevent inhibition by O 2 accumulation, the DO
concentration in the culture medium should not exceed the maximum tolerable
value of 400% of the saturation level achieved in the presence of air. In a study by
Raso et al. (2012), O 2 concentration increased from 75 to 250%; air saturation
inhibited the growth of microalgae. To improve productivity in tubular photobioreactors, the oxygen level must be controlled or removed. However, optimal
control parameters have not yet been well established to improve productivity in
these systems.
With regard to operation, in a tubular photobioreactor, the airlift column circulates the broth with the culture medium to ensure light penetration through the
solar collector, in which place most of the photosynthesis occurs, with ensuing DO
accumulation. This, in turn, cannot be easily removed from the tubes
(Molina-Grima et al. 2001). Therefore, in theory, if the oxygen is not removed
within about one minute after accumulation, the inhibitory effect on the cells will
occur immediately (Huang et al. 2017). In this case, the collecting tubes should be
designed with restricted length for continuous DO removal, as well as the insertion
of degasser systems.
Despite the fact that tubular photobioreactors are currently the most suitable
configurations for application in oxygen generation, the main bottleneck of this type
of equipment is its configuration, especially characterized by the geometry of these
systems.
Due to the currently limited operational scale, the conventional configurations
meet the basic requirements of the photosynthetic process. However, for a potential
scale-up of the production process, operational failures must be overcome.
Parameters such as the ratio of height/diameter column (H/D) are fundamental to
build industrial photobioreactors. For this reason, hybrid photobioreactors compensate the drawbacks caused by limitation of S/V ratio and scale-up, since these
systems can be based on a proper H/D ratio, generating configurations of reactors
with heavy workloads in contrast to very long tubes or shallow ponds (Jacob-Lopes
et al. 2016). If these aspects are considered, photobioreactors could be a fundamental step forward for the consolidation of the industrial biological oxygen
generation.
284
I. Aguiar Severo et al.