BioMix - Analysis of flow and mixing in biogas fermenters with different agitator geometries

The mixing of substrates in biogas digesters is very important for gas production and methane gas yield. However, the flow pattern and the resulting mixing can hardly be measured at present. With new sensors, attempts are being made to carry out measurements on real digesters for this purpose. To determine the flow characteristics, the correct positioning of the sensors in the fermenter is very important. The required positioning of the sensors in the fermenter depends mainly on the arrangement and geometry of the agitator. However, there are currently no analyses of which position on the fermenter is suitable for recording the flow conditions. The aim of the project is to define locations on the periphery of the fermenter for the optimal positioning of a sensor system, which can provide information about the mixing quality. These are to be defined as a function of the diameter ratio of the fermenter and the agitator, the type of agitator, the speed of the agitator and the dynamic viscosity of the fermenter contents.

For this purpose, numerical flow simulations are performed with different types of agitators. These include a submersible agitator, a long-axis agitator and a paddle agitator.

To validate the results of the flow simulations, a laboratory facility with a 2D PIV system will be built in the pilot plant. This enables the investigation of the flow pattern and the velocity differences when the speed of different agitators is changed.

A transparent model liquid containing particles is used in the laboratory system. A laser (particle image velocimetry) and a camera are used to record the particle motion occurring in different layers of the transparent model fermenter. In this way, the flow in the fermenter liquid can be measured with different agitators. The different model agitators were produced using 3D printing, among other methods.

Target:

• Positioning criterion for a sensor system to detect the mixing quality

Methods:

• Dimensional analysis of the mixing process in the biogas digester
• Design of a mixer test rig based on the dimensional analysis
• CFD simulation
• Procurement and setup of the trial facility for validation of the flow simulation
• Experimental work on the trial facility using 2D PIV system and data analysis