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The features characterizing the transition from a diffusion process to the concentration gravitational convection in multicomponent gas mixtures containing hydrocarbon components are analyzed. It is demonstrated that this transition is possible due to the difference in interdiffusion coefficients of components, initial concentration of components, and the pressure of the experiment. A mathematical model for describing the transition from the diffusion process to the concentration gravitational convection in multicomponent gas mixtures is considered. The key feature indicating the transfer from the diffusion regime to the convective one is the curvature of an isoline of the heavy component of the gas mixture.

The possibility of intensifying the separation of isothermal ternary gas mixtures containing carbon dioxide was studied experimentally. It was shown that the preferential transfer of carbon dioxide requires the system to be initially diffusively unstable, and the interdiffusion coefficients of mixing components can either differ significantly from each other or be close in value. A linear analysis of the stability of isothermal ternary gas mixtures containing carbon dioxide for a flat vertical channel and a cylindrical channel of finite height was performed. Comparison of the results of the numerical study with experimental data showed qualitative coherency.

Mixing of carbon dioxide dissolved in a multicomponent gas mixture at different pressures was researched. It was found that the mechanical equilibrium of the ternary gas mixture 0.4163H2 (1) + 0.5837CO2 (2) − N2 (3) is violated at a pressure of p = 0.7 MPa and structured flows appear in the system. The pressure area (from 0.7 to 1.5 MPa) at which the conditions of priority transfer of components with the highest molecular weight in the mixture are realised in the system is fixed. To analyse the effect of pressure on the process of changing “diffusion–convection” modes, a mathematical model, which takes into account the kinetic features of multicomponent mixing, was applied. It was shown that the change in the modes of mass transfer is associated with a significant difference in the diffusion ability of the components. It is noted that the difference in the diffusion coefficients of components results in the nonlinearity of the concentration distribution, which leads to the inversion of the density gradient of the gas mixture, which is the cause of convective flows.