Separation of product gas in supercritical CO₂ enhanced coalbed methane systems: Design, simulation, and performance validation of Laval nozzle

Under the pressures of an energy crisis and environmental pollution, supercritical CO₂ enhanced coalbed methane technology is gaining attention for its resource utilization and CO₂ sequestration benefits. However, separating high-pressure CO₂ and CH₄ mixtures in product gas hampers widespread application. This study introduces a method using a Laval nozzle device for effective separation and establishes a numerical model incorporating governing equations for vapor and liquid phases alongside phase equilibrium equations. Numerical simulations reveal that the molar fraction of CO₂ in the produced liquid phase can exceed 0.98, with higher inlet pressures and lower temperatures enhancing condensation effects, yielding a CO₂ mass fraction near 0.15. N₂ introduction reduces CO₂ partial pressure, inhibiting condensation. The latent heat released during condensation affects Mach number distribution and increases irreversible thermal entropy production. Additionally, a vortex-generating droplet receiver is designed to further separate vapor and liquid phases based on density differences. This integrated design with the supercritical CO₂ enhanced coalbed methane system facilitates CO₂ recycling, ensures stable CH₄-rich mixture production, mitigates secondary pollution, and enhances energy utilization efficiency. Overall, this innovative approach provides a new pathway for processing product gas in supercritical CO₂ enhanced coalbed methane systems, supporting clean energy development and carbon reduction initiatives.