An extended 1D dynamic model of biological methanation considering water production, variable height and maintenance heterogeneity

Numerical simulation of biological methanation remains challenging due to the strong coupling between mass transfer, hydrodynamics, and bioreaction in such gas-fed bioreactors. This work presents an extension of a 1D spatio-temporal gas–liquid model for bubble columns to the case of biological methanation. To this end, three major improvements are added to the previously published model: (i) the consideration of water production due to the biological methanation reaction, (ii) the implementation of an additional equation for the liquid height, (iii) the introduction of a variable maintenance model based on the spatial heterogeneity of the hydrogen mass transfer. The model was validated with experimental data from literature. Results indicate that water production acts as a dilution term, as revealed by simulations performed under both constant and variable height conditions. This led to a reduced biomass concentration while preserving methane production owing to a redistribution of hydrogen consumption between maintenance and growth. These intriguing results were confirmed by analytical solutions at steady-state. The variable maintenance model further allows connecting the decrease in performances through scale-up to increased local deviations between hydrogen mass transfer and cell demand. Also, a basic moving mesh method now extends the gas-liquid dynamic 1D model capabilities to any fed-batch (bio) reactor.