Multi-objective Optimization of Pneumatic Mixing Systems for Anaerobic Digesters: A Hybrid Technique of Statistical Modeling and Numerical Simulations

Abstract

Development a suitable mixing system for biogas plants is one of the most important factors to degrade organic matters in digesters. Therefore, pneumatic and mechanical mixing systems were integrated with their optimum conditions to produce a suitable strategy for mixing slurry. Pneumatic mixing parameters were investigated using computational fluid dynamics and image processing techniques. The mixing characteristics and biogas compounds were modeled according to biogas injection parameters, and integrated into the desirability function to determine the optimum conditions. The models indicated that for lower injection repetition in different injection positions, a higher mass flow rate was needed to improve the mixing characteristics and CH4 content. Optimization of these parameters showed that the best amounts of injection mass rate, injection pressure and injection repetition were 0.02 g/s, 2.84 bar and 3 times, respectively, and their optimum ranges were 0.015–0.028 g/s, 1.5‒4.7 bar and 1‒4 times. However, the best injection was injection through mixer and injection in floor of digester for 2 or 3 times in 24 h. From the optimum ranges, some amounts were investigated in the digesters using the computational fluid dynamics, and the determined mixing characteristics were in agreement with findings of the image processing method.