Multi-objective optimization of the water scrubbing process for biogas upgrading

This study focuses on the techno-economic bi-objective optimization of two promising configurations, a single-flash and a double-flash scheme, of the water scrubbing process for upgrading biogas produced from anaerobic digestion of agricultural residues to biomethane. Bi-objective optimization allows to identify the trade-off between energy consumption and capital costs. The black-box optimization strategy is used: the NSGA-II evolutionary algorithm optimized the process design variables and, for each sampled solution, first the process is simulated with Aspen Plus®, then sizes and costs of the equipment units are computed with calibrated correlations. The resulting Pareto front shows that the single-flash scheme is superior in terms of costs and energy loss when high methane slip (> 0.5 %) is acceptable. Moreover, the Pareto front highlights that when moving from the most efficient to the cheapest solution, the energy consumption increases considerably, up to 27 %, while the capital cost decreases by 17 %. The doubleflash scheme results competitive only if the methane slip must be limited.