Multi-factorial central composite and complete mixing model for optimizing industrial biogas valorization

There is an increasing interest to use CO₂ selective membrane to purify methane from natural or biogas because of the ease of access to this membrane configuration. The use of this membrane, however imposed an additional strain on the system because there was H₂S contamination in the enriched product stream and requirement of high CO₂/CH₄ selectivity. To address this issue, an empirically validated complete mixing model integrated with a multi-factorial central composite design was applied to optimize the membrane process in methane enrichment from biogas containing H₂S. The optimization resulted in 100% methane purity and 91.51% recovery using a methane selective membrane at 837.43 CH₄/CO₂ selectivity, 35.00 bar feed pressure, and 0.643 stage cut, without H₂S contamination in the product stream. For comparison with CO₂ selective membrane, methane purity of 91.74% and 90.26% recovery were achieved using CO₂/CH₄ selectivity of 1086.87, 31.82 bar feed pressure, and 0.310 stage cut. Methane selective membrane was performed better than CO₂ selective membrane on four fronts, namely lower selectivity, higher CH₄ recovery, higher CO₂ recovery, higher CH₄ purity, and the product stream was H₂S free. CO₂ selective membrane was superior in other aspects, namely higher CO₂ purity, lower stage cut and feed pressure.