Prediction of non-equilibrium condensation onset in a methane–carbon dioxide gas mixture flow through a supersonic separator nozzle and its operational parameters

Abstract

Amid rising energy demand and global efforts to reduce fossil fuel use, natural gas remains a critical energy source. A key challenge in its processing is the removal of carbon dioxide (CO${}_2$). Supersonic separators have emerged as promising passive devices for this purpose. However, most existing studies focus on limited operating conditions or rely on computationally expensive CFD models. This study presents and validates a numerical routine to predict the operational conditions under which phase change occurs in a supersonic separator nozzle. The approach combines the internally consistent homogeneous nucleation model with isentropic expansion and real-gas properties to compute nucleation rates. The working fluid is modeled as a binary mixture of methane and CO${}_2$, considering real-gas effects for mixture properties after using a state-of-the-art equation-of-state. The influence of the critical nucleation rate on non-equilibrium condensation was examined to assess condensation onset. Despite variations across several orders of magnitude, Wilson lines showed minimal divergence for the same stagnation conditions, indicating low sensitivity to this parameter in the evaluated conditions. Model validation against experimental data revealed small differences, supporting the method’s reliability in capturing condensation phenomena. Phase-change onset and stagnation properties were analyzed for different methane–CO${}_2$ mixtures. Increasing CO${}_2$ content narrows the stagnation fields, suggesting reduced operational flexibility at higher CO${}_2$ fractions—an important consideration for carbon capture applications. This methodology offers a practical tool for designing supersonic separators and evaluating new operating scenarios. It also identifies the Mach number range necessary to achieve phase change, supporting the development of efficient separation systems.