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 1cm/s biogas feeding velocity, 7bar and 25°C is 1.39mgH₂S/g Clinoptilolite, twice the adsorption capacity of the synthetic molecular sieves subjected to study. The high CO₂ adsorption capacity observed (173.9mg 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.