Enrichment of low-grade methane gas from nitrogen mixture by VPSA with CO2 displacement process: Modeling and experiment

Abstract

To upgrade efficiently the low-concentration methane gas from nitrogen mixture, CO2 displacement was employed into the traditional vacuum pressure swing adsorption (VPSA) process to form the VPSA-CO2DIS process. Non-isothermal and non-isobaric mathematical model for the VPSA-CO2DIS process was developed, where ternary competitive adsorption equilibrium of CH4, N2 and CO2 was calculated using IAST-Sips formulation, and bi-pore diffusion model was used to describe adsorption kinetics. Experiments were carried out to enrich the low-grade methane gas using one-bed four-step VPSA-CO2DIS process with the home-made granular activated carbons (GACs), and results compared to process simulation predictions. Both experimental and simulated results confirmed that a high purity CH4 product gas (75.40% CH4) could be obtained continuously from 10% CH4 feed gas with 89.02% recovery. The combustion heat of the enriched CH4 product gas was estimated higher than the power consumption of this process, and carbon emissions penalty in this process was evaluated to be activated, so this VPSA-CO2DIS process was technically feasible.