Optimal membranes for biogas upgrade by removing CO₂: High permeance or high selectivity?

Biogas can be upgraded to biomethane through CO₂ removal as a sustainable replacement of natural gas. Membrane systems that are based on a two-step design have emerged as an attractive approach for the CO₂ removal. The objective of this work is to determine optimal membranes for this application by performing process simulation and techno-economic analysis. Practical membranes with the trade-off of CO₂ permeance and CO₂/CH₄ selectivity on the Robeson’s upper bound are considered, and the membranes in the two steps are varied independently. Optimal membranes should have high CO₂ permeance with sufficient CO₂/CH₄ selectivity (10–25) to achieve the lowest operating cost for the CO₂ removal.