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

Abstract

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.