Modelling and simulation of a trickling filter bioreactor for ex-situ hydrogenotrophic methanation

A mathematical modelling approach for a hydrogenotrophic methanation process in a trickling filter bioreactor (TFB) is formulated and simulated. As other works have suggested, the proposed model partitions the physical space in an arbitrary number of vertical levels and uses the first Fickian diffusion law along its vertical axis and along the thickness of the biofilm layer attached to the inert bed material. The biological hydrogenotrophy reaction is modelled using Monod kinetics. According to the ideal gas law, to calculate gas flows among levels, the model approach considers the fixed amount of moles of gas in each TFB level. Simulations of the proposed model were compared to experimental results of ex-situ hydrogenotrophic methanation in a TFB. Model performance against the experimental results of the real reactor fitted remarkably well the effluent proportions within an average 2% error band in the reported best experimental case. It even surpassed real productivity by 20% on average, considering an ideal scenario in which the model formulation assumes that hydrogenotrophic methanation by archaea is the sole biological transformation. A qualitative analysis of important model parameters was crucial for fitting simulation results to real experimental data. Particularly, influent raw biogas proportions near the ideal stoichiometry for hydrogenotrophic methanation is detrimental to purity and productivity of the desired biomethane effluent. The numerical effort needed for simulations was remarkably lower than expected, given the large-sized models the approach may produce. Simulation results provided insight into possible model modifications for further discussion.