Revolver Airlift Photobioreactor

Designing a reactor for algae cultivation has to take several factors into account. These are a good light availability, sufficient gas exchange and homogenous mixing. Therefore, different reactor types like open pond systems, tubular and flat panel airlift reactors were developed.

Uniting all the advantages of these reactor types, we constructed the revolver airlift photo reactor short “RAPtor”.

RAPtor differs from commonly used reactors due to an innovative illumination concept. Six light tubes were embedded into the lid formed like a revolver drum. This enables an even light distribution inside the reactor which circumvents the disadvantages of external illumination.

The drawbacks of external illumination lie in the scattering of light passing through the reactor shell. In addition, the depth of light penetration is limited by the absorption of the cells. These problems are avoided in the RAPtor concept.

For the algae cultivation, airlift reactors are recommended as they ensure a high gas input. This gas input additionally provides the mixing of the culture which supersedes the use of a stirrer. As a result, no energy input by mechanical components occurs.

Thus, the reactor can be operated energy efficiently and the cells are not subject to additional mechanical stress. Since the only heat input is caused by the lamps the heat can be dissipated through the reactor shell and the gas outlet. Thus, there is no need for external cooling which highlights RAPtor as an power saving cultivation system.

In summary, RAPtor is an energy-saving photobioreactor which is optimally designed for the cultivation of microalgae. The arrangement of the lighting tubes guarantees optimal lighting. The airlift effect provides the necessary mixing and gas exchange.

Due to its simple and highly efficient design, the reactor can be replicated by anyone.

So don´t waste time build your own RAPtor.

Figure 1 – Simplified plan of the reactor structure.
riser: L=1250 mm r= 100 mm; 3 gas supply windows; 6x illumination tubes L=1250 mm, r= 24 mm;
downcomer: L=1400 mm, r= 210mm

Reactor set up

A preliminary technical drawing of the reactor was created. The dimensions of the reactor were chosen according to the empirical data of Prof. Dr. Takors (Biochemical engineering).
In order to create a sufficient gas exchange, the height of the liquid volume needs to exceed 1 m. With the aim of creating an airlift effect, the ratios of the area of the outer tube to the inner tube must be approximately 3 to 1.

Conferring this advice, we created a drawing of the reactor and used these dimensions to model the liquid flow of the reactor (Figure 2). Therefore, we conducted a computational fluid dynamics (CFD) simulation to verify that the reactor works as expected. The model proofed that the inner light tubes do not disturb the airlift circulation by causing severe turbulence (Model). In-silico data revealed a sufficient liquid flow despite the illumination tubes (Figure 3).

Figure 2 - Technical drawing of the Revolver Airlift Photobioreactor RAPtor.

Based on these data, construction of the reactor started. Filling the reactor with water approved the model. An airlift circulation was observable, and the light tubes were working. The reactor can be used for cultivation.
Chlorella vulgaris was cultivated in the RAPtor for 17 days. A 7-fold increase of the OD was achieved (Figure 4).

Figure 4 - Growth of Chlorella vulgaris in the RAPtor (A) compared to an aerated and illuminated 5 L bottle (B).
Figure 3 - Simulation of the liquid flow through the Revolver Airlift Photobioreactor. A simulation of the fluid dynamics was achieved via CFD simulation. Hereby, the presented liquid flow in y-direction confirms a typical airlift flow.