Integrated membrane gas separation process for the valorisation of H2 and CO2 to biomethane

In this work, we focused on a membrane gas separation system aimed at enhancing the efficiency of a process for CO2 valorisation into synthetic methane production via hydrogenation of a biogas stream. This system is designed for separating and recycling unreacted CO2 and H2 downstream of a methanation reactor. The inlet stream of the membrane separation system consists of unconverted CO2 and H2, apart from the CH4 from biogas and that produced by CO2 conversion. The membrane separation is analysed by using performance maps based a 1D mathematical model, already developed and validated, considering the selectivity and permeance properties of a polyimide membrane. The outcomes of the membrane system, constituted by two steps operated at 20 bar, show that the proposed integrated process allows a quantitative CO2 conversion into methane. The integration of the multi-step membrane process leads to a final retentate stream suitable for direct injection into the gas grid with CH4 ≥ 97.5 % molar, and CO2 and H2 within the targets of Italian regulation, CH4 yields up to 0.987 confirmed a nearly complete CO2 valorisation into CH4 and a near-zero emissions process.