Purification and upgrading of biogas by pressure swing adsorption on synthetic and natural zeolites

The cleaning (H2S removal) and upgrading (CO2/CH4 ratio adjustment for syngas synthesis) of biogas by pressure swing adsorption (PSA) with thermal desorption using two synthetic molecular sieves (5A and 13X) and a natural zeolite (Clinoptilolite) as adsorbent materials have been studied. Prior to use, zeolites were activated according to literature methods and supplier recommendations in order to dispose of adsorbed gases and soluble impurities which may compete with H2S and CO2 for the active sites of the zeolites. The behaviours of the zeolites have been compared by using three basic parameters: selectivity, capacity and regenerability. The experimental results indicate that Clinoptilolite is the best material choice because it is simultaneously suitable both for the purification and upgrading of biogas. For a CH4/CO2/H2S biogas molar composition of 59.95/39.95/0.10, the breakthrough capacity of Clinoptilolite at 1 cm/s biogas feeding velocity, 7 bar and 25 °C is 1.39 mgH2S/g Clinoptilolite, twice the adsorption capacity of the synthetic molecular sieves subjected to study. The high CO2 adsorption capacity observed (173.9 mg CO2/g Clinoptilolite) makes the adjusting of CH4/CO2 ratio in the biogas possible. Additionally, Clinoptilolite is completely regenerable and stable through several adsorption–desorption cycles which, together with its abundance and low cost, make this process highly attractive from an economical standpoint. Finally, this study demonstrates the importance of the selection of the optimal activation method of natural zeolites in order to offset variable composition and low purity often responsible for poorer separation performance compared to synthetic zeolites.