Optimal mixer placement in industrial-size biogas fermenters

Renewable energy sources are crucial to react to the continuous increase of energy consumption and pollution by carbon dioxide emissions worldwide. Biofuels, where the energy content is derived from a biological source, are one of the alternatives to oil and natural gas. Still, some biofuels are controversial, like biodiesel, because they imply cultivating certain crops, and not others, what may have huge economic and social implications. For this very reason, one of the most interesting biofuels is biogas, a mixture of methane and carbon dioxide resulting from the anaerobic decomposition of such waste materials as domestic, industrial, and agricultural sewage.

The decomposition (fermentation) is carried out by methanogenic bacteria; these obligate anaerobes produce methane, the main component of biogas, which can be collected and used as an energy source for domestic processes, such as heating, cooking, and lighting. The production of biogas is carried out in special digesters that must be fed continously in order to have a smooth supply of gas. This implies mixing fresh sewage with partially fermented one, thus presenting a design challenge. In other words, to make the production of biogas more efficient regarding ecological stability and economic profitability, the fermentation process has to be optimized.

In chemical engineering, saying that a process has to be optimized most of the time implies that the shape and location of some piece of equipment must be made optimal to guarantee that you get the most from the way the incoming and outgoing products and their mixtures flow. In the case of a biogas plant, this piece of equipment is the mixer.

For a start, the simplest solution is not the optimal one: natural mixing is not sufficient in anaerobic digesters because of the high substrate viscosity. Hence, mechanical mixing is needed for providing contact of the active biomass with the fresh substrate, distributing fresh feed uniformly in the whole tank, avoiding builtup of toxic substances, and preventing temperature gradients and substrate layering inside the tank. An optimal mixer placement has to be determined before constructing the plant.

Mixers installed at the exterior of the biogas plant provide easier access during maintenance and repairs than submerged mixers but concerns of insufficient mixing deter many operators from using this technology. Recent studies have focused on different mixer types, the underlying model, the shape of the mixer and also the optimization of geometry parameters. However, most investigations have been made for small-scale reactors but results can not be scaled up easily, due to the non-linearity of fluid behaviour.

Now, a team of researchers presents ¹ a new approach where the focus is on minimizing the region which is poorly mixed (dead volume) in the tank, optimizing external mixer configurations across a wide range of rheological properties.

The scientists simulate the steady-state flow of sludge inside a real biogas fermenter driven by an external mixer. In order to make the flowfield more homogeneous, an optimization framework is wrapped around the computational fluid dynamics simulation.

One may assume that the best parameter for optimization is the goodness of the mixture, but it is almost impossible to define such a variable when your subject matter is sewage of changing composition. Thus, the problem needs to be formulated using a robust optimization approach taking the dry substance content as uncertainty into account. Dead volume is the next best option.

In practice the formation of dead volume zones brings along a huge risk. For example, if a dead volume zone forms in the shape of a column in the middle of the tank that means that the compound and pH-value in this zone will change over time compared to the well-mixed parts closer to the tank walls. Consequently, the breakdown of this pile can lead to a collapse of the ecosystem. Thus, a very costly draining of the fermenter is required, followed by a slow restart over a few weeks time.

The researchers define the dead volume zone as the region in which the velocity magnitude during mixing falls below a certain threshold. Different dry substance contents are then investigated to account for the varying rheological properties of different substrate compositions. The velocity thresholds are calculated for each dry substance content from the mixer-tank configuration of a real biogas reactor.

The optimized configurations reduce the dead volume zones significantly across all dry substance contents compared to the original configuration. These results can be particularly useful for plant manufacturers and operators for optimal mixer placement in industrial-size biogas fermenters.

Author: César Tomé López is a science writer and the editor of Mapping Ignorance

Disclaimer: Parts of this article may be copied verbatim or almost verbatim from the referenced research paper.