Catalytic CO2 methanation for biogas upgrading using a polytropic fixed bed reactor

The experiments of this study aim to determine the effect of distributed feeding of reactants throughout the many inlets of a polytropic fixed bed reactor. The effect of dosing either carbon dioxide or hydrogen, was analyzed on the Sabatier reaction (i.e., carbon dioxide methanation) using a Ni-Mn catalyst to carry out the biogas upgrading process. This work analyzes the influence of three feeding configurations (a conventional fixed bed, one with side distribution of biogas, and another with side distribution of hydrogen) and temperatures (350, 375, and 400 °C) for a wide gas hourly space velocity ( GHSV ) range from 30 × 10 3 (STP) mL g cat −1 h −1 to more than 200 × 10 3 (STP) mL g cat −1 h −1 . The molar ratios of reactants were always kept constant (H 2 :CH 4 :CO 2 = 12:7:3) simulating the hydrogenation of the CO 2 present in a biogas with a proportion of 70 v % of CH 4 and 30 v % of CO 2 . The empirical results highlight that side distribution of biogas yields improved results over those obtained in a conventional fixed bed reactor, or the one with side distribution of hydrogen. At temperatures of 375 and 400 °C, this feeding configuration brings higher conversions than the other two, while consistently shows greater selectivities to methane for all the conditions tested. As such, its optimal condition to conduct the process is extended to methane space-time yields ( STY ), for which the highest methane productions are obtained. In addition, the influence of contact time, or GHSV , was determined to be critical both on selectivities and flowrates of methane. It is shown that for a given conversion value, keeping constant all the other parameters, a longer contact time and lower temperature result in an improvement of selectivities to methane. On the other hand, it also affects STY values, where an optimum between the employed flows of reactants and reaction performances is reached at a value of 180 × 10 3 (STP) mL g cat −1 h −1 , independently of experimental conditions.