Design and economics of a hybrid membrane–temperature swing adsorption process for upgrading biogas

Processing biogas from wastewater digesters allows recovery of valuable methane and reduction in green house gas emissions. A two-stage membrane process, coupled with a temperature-swing-adsorption (TSA) as pre-treatment, was designed to generate pipeline quality methane. To improve methane recovery and process energy efficiency, the non-product streams of the membrane process were recycled and the permeate of the first membrane stage was maintained at a given pressure as the driving force for second membrane stage. The membrane process design was optimized by minimizing the objective function; the overall processing cost. It was found the membrane approach excels the PSA for producing pipeline quality methane (97% purity) in terms of methane recovery, processing cost and lower emissions. A techno-economic analysis showed that the payback time for an operation processing 200 Nm3/h of biogas was 6.8 months.