ANAEROBIC DIGESTER CFD analysis : TO ENSURE OPTIMAL MIXING
Anaerobic digestion refers to the breakdown of biodegradable materials facilitated by various bacteria and archaea in an oxygen-free environment. The resulting biogas primarily consists of methane (50–60%) and carbon dioxide (30–40%), generated through the anaerobic digestion process. Several critical factors influence the efficiency of anaerobic digestion, including the characteristics of the substrate, feeding strategies, hydraulic retention time, pH levels, temperature, and mixing dynamics. Among these factors, mixing is particularly vital for enhancing the anaerobic digestion process and maximizing biogas production. Effective mixing within the digester promotes biological, chemical, and physical uniformity, preventing the creation of dead zones that hinder substrate conversion. Insufficient mixing can lead to reduced digester performance, while excessive mixing may disrupt the digestion process due to high shear forces. Therefore, it is essential to thoroughly understand and examine the optimal mixing techniques in anaerobic digesters.
OBJECTIVE OF CFD ANALYSIS
A gas nozzle system has been developed by our customer, which injects biogas from the base of the digester following a specific operational sequence. The nozzles, arranged circumferentially, will function in a sequential manner, ensuring that the sludge remains in a uniform state. A 3D transient viscous turbulent multiphase CFD analysis has been conducted for the bio digester tank. The operational sequence of 20 gas nozzles is implemented through a user-defined function (UDF). Eulerian multiphase modeling is utilized to simulate the transport of sludge constituents. The mixing process is meticulously examined during the operational sequence to assess the effectiveness of the gas nozzle sequence.
CFD MESH
The CFD mesh is composed of poly-hex core elements, ensuring adequate boundary layer resolution within the gas nozzle ducting system. The total mesh count is 18 million, achieving optimal quality metrics in accordance with established CFD practices.
CFD SOLVER SETUP
The flow within the anaerobic digester is represented as a three-dimensional, time-dependent, viscous turbulent multiphase system. The turbulence is characterized using the K-Omega turbulence model. To simulate the transport of sludge components, an Eulerian multiphase modeling approach is utilized. The operational sequence for 20 gas nozzles is executed via a user-defined function (UDF).
CFD analysis results : Homogeneity in the sludge
The sludge is a mixture of Napier grass and press mud as the main constituents in a water medium. Napier particles constitute about 15 % of the sludge, with an average particle size of 2 mm. Biogas stored in the top dome portion of the digester is recirculated through a compressor connected to the nozzle unit, in a pre-determined Nozzle operating sequence to achieve agitation. A consistent volume fraction of Napier grass particles indicates that the mixing process is optimal.
CFD analysis results
The enclosed graph indicates that throughout the nozzle operating sequence cycle, the volume fraction of Napier grass particles at a height of 5.5 meters from the ground has remained stable. This consistent pattern is also observed at various horizontal planes up to the maximum sludge height. Therefore, the agitation generated by the nozzle's operating sequence and the duration of gas firing is adequate in terms of operational and energy efficiency.
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