An adsorptive reactor for biomethane purification and production

This work focuses on an innovative process for simultaneous biogas purification (through CO 2 adsorption) and catalytic conversion of CO 2 into CH 4 . With this aim, a column was filled with a CO 2 -selective adsorbent (K-promoted hydrotalcite) and a methanation catalyst (Ru/Al 2 O 3 ), both commercial materials. The operation consisted of continuous swinging between two stages until a cyclic steady state was achieved. In the CO 2 adsorption stage, the inlet was biogas (CO 2 plus CH 4 ), during which CO 2 was adsorbed. In the reactive regeneration stage, the inlet was H 2 , which reacted with the previously captured CO 2 , forming CH 4 (and H 2 O) through the Sabatier reaction. The process was tested at different pressures (1, 4, and 7 bar), temperatures (300 and 350 °C), adsorbent-to-catalyst mass ratios (1, 5, and 10), and bed configurations (layered and mixed), and its performance was assessed by several indicators, such as CH 4 productivity, and CH 4 purity, among others. The rise in pressure enhanced the system's overall performance for both temperatures and all bed compositions. It was concluded that an intermediate adsorbent-to-catalyst mass ratio (5) enhanced all performance indicators, followed by a mass ratio of 10 and then 1, as it provided the best balance between the amount of adsorbent in the bed and the quality of its regeneration (which benefited from the amount of H 2 O produced in the reaction). Working at higher temperatures (350 °C) was beneficial. The maximum CH 4 productivity and purity were obtained at 7 bar using an adsorbent-to-catalyst mass ratio of 5 at 350 °C, respectively, 0.526 mol CH4 ·kg cat+ads −1 ·h −1 and 82.1%. The mixed or layered bed performance was very similar (especially at 300 °C), although the mixed bed showed slightly enhanced performance indicators while reducing the temperature peaks inside the bed. Finally, a study aiming at purity enhancement was conducted using a mixed bed with an adsorbent-to-catalyst mass ratio of 5 and at 7 bar. The analysis yielded a maximum CH 4 purity of 90.3% obtained at 300 °C.