Study on H₂/CH₄/CO₂ Adsorbents screening and Competitive Adsorption Characteristics

Abstract The purification of gray hydrogen, such as by-product hydrogen from industrial processes, primarily faces challenges in separating H 2 , CH 4 , and CO 2 . Pressure Swing Adsorption is a extensively used for hydrogen purification. During the purification process, CO 2 and CH 4 are adsorbed, while H 2 t is collected as the product. However, the purification process generally has issues such as hydrogen easily being retained in the bed layer. To investigated the competitive adsorption behavior of H 2 , CH 4 , and CO 2 on adsorbents, this paper first focuses on commonly used adsorbents for hydrogen purification (porous carbon, zeolites (13X and 5A), and silica gel). Through static adsorption experiments, porous carbon was selected as the adsorbent with strong adsorption capacity for both CO 2 and CH 4 . Then, Monte Carlo were conducted to establish a graphite slit model. Comparing the gas adsorption amounts, adsorption heats, and adsorption equilibrium configurations under single and ternary adsorption, the influence of the adsorbent pore structure—micropores (0.73 nm) and mesopores (2.64 nm)—on the competitive adsorption characteristics of H 2 , CH 4 , and CO 2 was investigated. The study found that within the research range of 25-45°C and 0-10 bar, temperature had almost no effect on hydrogen adsorption. Hydrogen was widely present in cavities formed by adsorbents or other fluids. The adsorption amount of hydrogen in 2.64 nm pores was 32 times that in 0.73 nm pores (0.48 mol/kg vs. 0.015 mol/kg). CH 4 and CO 2 exhibited dominant surface adsorption driven by interactions with the pore walls, which adsorption amount in 0.73 nm pores was higher than that in 2.64 nm pores (about 1.36 times). Additionally, from the adsorption equilibrium configurations, it was observed that theoretically, 2.64 nm pores could accommodate up to six layers of CO 2 or CH 4 . However, only two layers of gas molecules adhered to the walls, and due to the wide pore size, the interaction between the gas molecules and the pore walls was reduced, leading to a decrease in the adsorption amounts of CH 4 and CO 2 . In summary, micropores around 0.73 nm are qualified for the separation of H 2 /CH 4 /CO 2 . Wider pores reduce the adsorption amounts of CH 4 and CO 2 while providing more space for hydrogen retention.