For mass cultivation of microalgae can be used different types of photobioreactors. Vertical ones are compact and simple to use.
Currently, there are several vertical reactor configurations:
-Air-lift type with suction tube;
-Air-lift type with internal recirculation concentric tubes (the main drawback is that the swirl formed by the bubbles is smaller than the diameter of the photobioreactor, so that the walls thereof are coated with adhered microalgae, preventing thus an optimal light transfer).
-Submersible reactor, enabling economic control of the culture temperature (the main problem is the structure cost and complexity of use. In addition, it must be controlled gases as oxygen, which is accumulated because the container is leakproof).
-The present invention has been developed to overcome the limitations previously described them and to provide a better performing photobioreactor.
This novel photobioreactor combines the mechanism of a bubble column with the air-lift type for higher biomass production than that obtained for both systems separately.
In this sense, it has been developed a vertical cylindrical photobioreactor (rigid or flexible), with a flat or a conical bottom, made of transparent material for the optimum light transmission. The photobioreactor can be closed by a transparent lid to prevent foreign substances (dust, insects, etc...) to introduce into it, although the lid does not seal the container, so gases can leave. It also has, at least, one internal recirculation pipe located at the bottom that allows combining the air-lift and bubble column effects.
For the optimal microalgae growth, it is necessary to provide light, air and CO2 to control the culture pH and not to reach values that threaten the microalgae survival.
The injection of air and CO2 is carried out at the bottom of the photobioreactor. The amount of CO2 injected is controlled by a pH probe with internal temperature compensation.
The photobioreactor also has a heat exchanger to maintain the optimum temperature for the microalgae growth.
Possible system configurations
The first possible configuration is a modification of the conventional air-lift wherein the cultivation and gas mixture, instead of being introduced by the top, it is introduced through a drilling recirculation pipe located at the bottom. Thus, two aspects are improved:
• The use of CO2 supplied: the gas rises to the top of the photobioreactor and have more time to be dissolved in the culture.
• The culture mixing: gradients (temperature, pH and concentration of nutrients) are lower and the light transfer to the culture is better.
An alternative configuration is to place several drilling recirculation pipes (see Figure 1), thereby achieving greater and more uniform mixing, thus solving the problems of the air-lift type (for example, getting dirty the walls of the photobioreactor).
Another possible configuration is to drill the recirculation pipe (see Figure 2 (a)), thus introducing bubbles rising forming a swirl. The drillings should be sized so that the entrance of bubbles in the medium is homogeneous through the pipe and avoid cell death of the culture incorporated back into the photobioreactor due to stress caused by an incorrect hydrodynamic diameter of the drilling. The number of perforations must be enough so the output of the gas flowing therethrough is uniform. In this way, it achieves a more uniform mixing and the bubbles are distributed throughout the diameter of the photobioreactor. Therefore, this technology improves:
• The microalgae exchange between areas of light and dark;
• The light transfer to the culture;
• The control of temperature, pH and nutrients concentration gradients;
• The use of CO2 (increases the residence time of CO2 in the culture medium);
• Keep clear microalgae on the walls of the photobioreactor when the culture reaches high concentrations.
Another alternative configuration is to place several recirculation pipes with multiple drillings (see Figure 2 (b)), thereby achieving the effect of the stirring produced is more uniform along the photobioreactor.
Figure 1: Photobioreactor with three drillings recirculation pipes located at the bottom (including the detail at the bottom).
(a) Photobioreactor which comprises a recirculation pipe with multiple drillings equally spaced.
(b) Photobioreactor with two recirculation pipes with multiple drillings distributed along the photobioreactor.