Upgrading pipeline-quality natural gas to liquefied-quality via pressure swing adsorption using MIL-101(Cr) as adsorbent to remove CO2 and H2S from the gas

Abstract

We experimentally studied removal of acid gas from natural gas via a pressure swing adsorption (PSA) process that employed a metal–organic framework adsorbent, MIL-101(Cr) to upgrade raw-quality natural gas to liquefied-quality natural gas. We hydrothermally synthesized MIL-101(Cr) in the presence of acetic acid, and then we extrudated it using a 15% carboxymethyl cellulose (CMC) binder. The adsorption isotherms of CO2 and CH4 in the pressure range 0–3 MPa and H2S in the pressure range 0–0.1 MPa for both powder- and extrudate-form of the MIL-101(Cr) were measured. The CO2 and CH4 adsorption capacities at 3 MPa were found to be 26.31 and 8.89 mmol/g for powder-form and 19.26 mmol/g and 6.04 mmol/g for extrudate-form, respectively. The H2S adsorption capacities at 0.1 MPa were found to be 7.28 mmol/g for powder-form and 4.04 mmol/g for extrudate-form. Feasibility tests for upgrading 2 vol% CO2−98 vol% CH4 and 2% CO2−0.02% H2S−97.98% CH4 into liquefied-quality natural gas (CO2 < 50 ppm and H2S < 4 ppm) via PSA were performed under various operating conditions (e.g., feed gas pressure (Pfeed), amounts of purge gas (Qpurge), and feed linear velocities (Vlinear). In the case of 2 vol% CO2–98 vol% CH4, the extrudate-form exhibited 1.55 mmol CO2/g (i.e., 94% of the adsorption capacity at PCO2 = 0.12 MPa), under the condition of Vlinear = 0.127 m/s, Qpurge = 178.97 cm3/g, and Pfeed = 6 MPa. Upgrading 2% CO2 + 0.02% H2S + 97.98% CH4 via PSA was found to be difficult because of the strong adsorption of H2S to MIL-101 under the aforementioned conditions.