A Scalable Functional Nanogel of Polyacrylamide-Piperazine-Copper Oxide Nanoparticles for Enhanced Biogas...

Efficient upgrading of biogas to bio-compressed natural gas (bio-CNG) quality requires the selective removal of carbon dioxide (CO₂) and hydrogen sulfide (H₂S) without compromising methane recovery or process economics. In this research, polyacrylamide-based nanogels were developed and evaluated as low-cost sorbents for biogas purification under continuous flow conditions. Dynamic sorption experiments were conducted using raw biogas, and the purification performance was assessed in terms of CO₂ and H₂S removal efficiency, methane enrichment, sorption capacity, selectivity and separation thermodynamics. The optimized nanogel achieved up to 95% CO₂ removal, increasing the methane (CH₄) concentration to more than 98% in the purified gas stream. Equilibrium analysis revealed high CH₄/CO₂ selectivity (32.75), accompanied by favorable equilibrium constants (32.74) and negative Gibbs free energy changes (-8640 J/mol), confirming the spontaneous nature of CO₂ sorption. The CO₂ removal capacity from raw biogas increased proportionally with gas flow rate, while equilibrium CO₂ concentrations at the outlet decreased markedly for improved nanogel formulations, indicating strong sorbent-gas affinity. Breakthrough curve analysis demonstrated delayed CO₂ breakthrough and sustained sorption performance. Adsorption isotherm modeling showed that the Freundlich model provided a superior fit compared to the Langmuir model, highlighting the heterogeneous and flexible nature of the nanogel adsorption sites and a predominantly physisorption-controlled mechanism. Overall, the results establish polyacrylamide nanogels as promising, regenerable, and energy-efficient sorbents for scalable biogas upgrading and bio-CNG production.