Reactor design verified by CFD modelling

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Algiecel is developing a high-yield, closed photobioreactor (PBR) for carbon capture using microalgae and fitted in a standard shipping container. The business model involves making carbon capture cost-neutral (at least) and uncomplex for clients. In the PBR, algae flow through a system of horizontal pipes that are illuminated with LED lights and fed with (pure) carbon dioxide (CO2) from industrial point emissions. The flow distribution design has been verified by CFD modelling by FlowMotion. The flow in the system has the purpose of reducing the risk of fouling and of improving photosynthetically active radiation (PAR) by “mixing” the algae to optimize the distribution of light throughout the culture.

The algae used by Algiecel are photosynthetic, meaning that they convert light and CO2 into biomass and oxygen. The productivity of algae, and thus algae’s ability to take up CO2, is closely linked to the amount of light that can be delivered to the individual cells. The lighting in the Algiecel photobioreactor system is provided solely from LED lights, ensuring an optimal PAR and higher growth efficiency than would be achieved with natural sunlight.

A common growth restriction in photobioreactors is self-shadowing. As the cells proliferate, the light is increasingly blocked by the newly formed cells, reducing the light penetration through the culture. Algae located away from the light source are therefore not exposed and cannot photosynthesize while algae near the light source may be overexposed and utilize the light poorly. To avoid this, rigorous stirring of culture can reduce the time spent by algae in suboptimal light conditions, thus increasing the productivity.

The flow in the system is similarly important. The flow of the water leads the algae to and from the degasser, which removes the oxygen produced by photosynthesis. The flow also prevents the algae from settling on the surfaces of the reactor, creating fouling. In extreme situations fouling can cause a significant reduction of light entering the system, severely reducing productivity. If the flow is sufficiently high, algae are prevented from sticking to surfaces, thus avoiding the problem. However, if the flow is too high, the algae may experience shear stress to which some species of algae can be vulnerable. Algiecel has partnered with the plastic expert company Böhme, a company with experts in engineering and product development, which has carried out the necessary calculations and material selection to make the manifolds function. The manifolds are placed at each end of the system connecting all pipes in order to distribute the flow evenly throughout the system. The manifold design also creates some stirring of the substrate, increasing the light exposure. To examine the hydraulic performance of the photobioreactor, a computational fluid dynamic (CFD) analysis has been conducted by the company FlowMotion.

FlowMotion uses mathematical programming to predict the water movement in the tubes. The CFD analysis shows the flow rate of the algae culture in the system, illustrated as blue (slow) and red (fast). The model has verified that the system has no dead legs with critically slow flow, and that there are no critical shear stress zones either. Different viscosity values have been tested with the model, and functionality was maintained over the range of expected viscosity levels. 

The construction of the new prototype manifolds is well underway, and they will be tested in the pilot reactor in the summer of 2022. A lot of focus has also been on the cost of the material in terms of scalability as a low-CAPEX technological solution for carbon capture is key for success.

Algiecel is looking into a very exciting 2022 with the initiation of the pilot project at the Danish Technological Institute in Taastrup and the participation in the BioInnovation Institute´s Venture Lab accelerator, supported by the Novo Nordisk Foundation. The company is continuously improving the productivity of the pilot system with our partners to enable highly efficient CO2 reduction in future scaling projects. 

Team ALGIECEL          28.03.2022

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