Multistep process for biogas upgrading

In this work, we proposed a multistep membrane process for the upgrading of a mixture containing 60% of methane and 40% of carbon dioxide, considering three case studies that allow to cover a wide range of CO2 permeability (from 5 to 180 Barrer) and CO2/CH4 selectivity (from 30 to 200). The aim is to investigate to what extend the variation in steps number positively affects the separation performance as well as reduces the total membrane area and recycled flow rate required to obtain a methane stream at a purity of 98%. The multistep paths necessary to achieve the final target have been defined by using performance maps of methane purity versus its recovery. A higher number of steps leads to a significant reduction of membrane area (e.g., from 580 to 180 m2) and promotes the methane recovery, allowing to recover a purer carbon dioxide stream in the permeate (e.g., from 87.4 to 97.9%). Fixing the steps number, the higher CO2 permeability allows to save membrane area but provides more methane losses on the permeate side. Differently, an increment of selectivity reflects in higher methane recovery (i.e., carbon dioxide purity on the permeate side) but lower retentate concentration.