Optimal design of catalytic reactors for direct biogas methanation through thermodynamic analysis and 2-D reactor modeling

Catalytic methanation upgrades biogas by reacting it with hydrogen. Thermodynamic analysis of a two-step adiabatic process explored temperature control (below 550 °C for catalyst stability). Reactant staging proved insufficient for hot spot management. Product recycling in the first reactor effectively controlled temperature and produced synthetic natural gas (SNG) approaching grid quality. A two-dimensional model of cooled multi-tubular reactors revealed significant radial thermal gradients. A once-through configuration exceeded the temperature limit despite cooling. Reactant staging failed to simultaneously control temperature and achieve targeted conversion. Conversely, product recycling successfully addressed both constraints. Two configurations were proposed: stoichiometric hydrogen (STOIC) and hydrogen-deficient (H-DEF). Optimized reactor designs, employing a 0.30–0.35 recirculation ratio, were developed for both. The STOIC configuration required 18 parallel tubes for the first reactor and 33 for the second. The H-DEF unit utilized 22 tubes in both reactors. These findings highlight product recycling as a viable strategy for efficient and controlled biogas upgrading via catalytic methanation in multi-tubular reactors.