Mixing of a model substrate in a scale-down laboratory digester and processing with a computational fluid dynamics model

Energy demand for mixing of biomass digesters is a crucial parameter in design and operation of biogas plants. Optimization of flow characteristics in the fermentation process is usually focused on the stirrers where their placement, shape and number, as well as their rotational speed and switching sequence are all important decision variables for overall energy efficiency planning. Digester stirring was analyzed in a cylindrical transparent physical digester model located in our laboratories in Ingolstadt. Real biomass was substituted with a chemical substrate based on cellulose exhibiting transparency, physico-chemical stability, ease of use and rheological behavior comparable to real biomass. Different mixing regimes were configured using propellers and paddle stirrers located in various positions. The motion of the liquid within the tank was investigated with optical and acoustic techniques in a noninvasive way. The software tool StarCCM + was used to develop a computational fluid dynamics (CFD) model to simulate the mixing process and the flow patterns within the digester. The results of the laboratory experiments were used to validate the CFD model.