Modelling methane production in trickle bed reactors: A biokinetic approach

Biogas upgrading via ex-situ biomethanation is a promising way to generate high value gas products from anaerobic digestors. Thus, we developed and experimentally validated a mechanistic biokinetic model for ex-situ biomethanation in a trickle-bed reactor (TBR) that couples spatial discretisation with interphase mass transfer, axial dispersion, Monod kinetics with pH inhibition, and a solid retention time (SRT) growth constraint. Using gas-phase data at three temperatures, the model reproduces measured axial gas phase concentration profiles and outlet flows with two calibrated transport parameters (k_La and an effective axial dispersion). Two key findings emerge. First, bed performance appears to be mass-transfer-limited across operating points (as shown by Damköhler number) and nearly constant with flow, indicating reaction rates sit at the transfer ceiling wherever hydrogen is available. Second, scaling inlet flow increases methane production capacity up to 26.3 m³_CH4/m³_bed.day, but lowers conversion/purity, yielding hydrogen/methane mixtures (hythane) when desired. Axial dispersion materially shapes near-inlet profiles at low flow, but convection dominates at high flow. The modelling framework provides a practical design tool and guide for further experimental studies to guide the maturation of TBR biomethanation technology.