Upgrading Biogas at Low Pressure by Vacuum Swing Adsorption

Performance of four adsorbents (three zeolites and one molecular sieve carbon) for CO<sub>2</sub>–CH<sub>4</sub> separation from simulated biogas feed containing 42% CO<sub>2</sub> (balance methane) is compared in a simple four step dual bed vacuum swing adsorption (VSA) cycle. The VSA cycle consisted of four sequential steps in each column namely adsorption, cocurrent depressurization, evacuation and pressure equalization. Among the adsorbents tested, zeolite NaUSY appears to give the best results for producing pipeline grade methane (>98%) at recoveries exceeding 85%. The methane productivity obtained by the present process is in the range of 11–16 mol/h/kg, which is considerably higher than the literature reported values with zeolite and carbon based adsorbents. The high methane productivity of the present process is due to the absence of purge step during regeneration with methane rich adsorption product as well as due to absence of any blow down step in the VSA cycle. Another advantage of the present process is that operation for biogas up-gradation is at near atmosphere pressure, which is typically the pressure available for biogas generated from an anaerobic digester.