Design of stirred digester with optimization of energy and power consumption

Abstract

The goal of the present study was to design a new stirred digester according to laboratory experiments. For this purpose, amount of methane produced from co‐digestion of municipal waste, kitchen waste, and cow manure in different treatments was measured in laboratory conditions for 37°C. The substrate characteristics such as both total and volatile solids, pH, EC, amount of total carbon, nitrogen, phosphorus, potassium, and sulfate which can manage the methane production were measured. The results showed that the amounts of these characteristics were in suitable ranges for biogas production. The laboratory experiment results were then used to design an anaerobic digester for single‐family homes based on energy efficiency and mixing cost. As a result, the digester diameter, digester height, slurry height, baffle width, impeller diameter, blade width, blade length, and volume of designed stirred digester were determined as 57, 90, 60, 4.75, 25, 4, 10 cm, and 229 L, respectively. Furthermore, pitched blade turbine with six blades at 45° for speed of 300 rpm not only required a low power but also produced a high torque. Therefore, while the designed biogas plant consumes low energy, it has high mixing cost.Nowadays, the researchers in bio‐system engineering focus on the designing biogas plant according to the direct relation among the power, torque, and energy. The novelty of the article is not only an evaluation of main substrate characteristics on methane production, but also design a new digester with high methane content at suitable pressure based on the best relation among the equipment dimensions of digester determined using shape factors. Furthermore study of impeller types and rotation speeds for providing high torque in low power and reducing energy consumption indirectly, and uses rule of thumb for power consumption to determine the best impeller type and rotation speed. © 2016 American Institute of Chemical Engineers Environ Prog, 36: 104–110, 2017