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Repurposing existing natural gas pipelines is a promising solution for large-scale transportation of mixed hydrogen-methane gas. However, it remains debatable whether gravitational stratification can notably affect hydrogen partial pressure in the gas mixture. To address this issue, we combined molecular dynamics simulation with thermodynamic and diffusion theories. Our study systematically examined the equilibrium distribution of hydrogen-methane mixtures in gravity fields. We demonstrated that partial pressures of both gases decrease with altitude, with hydrogen showing slower decrease due to its smaller molar mass. As a result, the volume fraction of hydrogen is maximized at the top end of pipes. The stratification is more favorable at low temperature and large altitude drops, with notable gas stratification only occurring at extremely large drops in altitude, being generally negligible even at a drop of 1500 m. Furthermore, we showed that the diffusion time required to achieve the equilibrium distribution is proportional to gas pressure and the square of pipeline height. This requires approximately 300 years for a 1500 m pipeline at 1 bar. Therefore, temporary interruptions in pipeline gas transportation will not cause visible stratification. Our work clarifies the effect of gravity on hydrogen-methane gas mixtures and provides quantitative insights into assessing the stratification of gas mixtures in pipelines., Comment: 14 pages, 8 figures

Wax deposition in diesel transportation pipe at low temperatures is one of the harassing problems faced by the diesel transportation companies. In winter, the temperature is very low and the demand of diesel is increasing greatly in northern region of China. This work is focused on proposing a new method to transport diesel at low temperature, by analyzing the operating process of diesel transportation, studying the characteristics of the diesel wax deposition process in pipe by experimental measurements and ensuring the safety of diesel transportation at low temperature. The new diesel transportation method at low temperature built up some key parameters about the process of diesel entering to pipeline and achieved normal operation of the 0# diesel transportation system in winter. This achievement has been added into the operating regulations to promote the application of refined oil pipelines in the northern region.

Using BWRS state equation, as well as two specific heat correlations, the temperature drop due to Joule-Thomson effect in natural gas pipeline is calculated and the algorithm of this is proposed. As for the west-to-east gas transmission pipeline, comparing the result calculated using the algorithm above with practical value measured, we find that the algorithm developed is steady and has a high accuracy of maximum deviation less than 1.42℃, thus the algorithm can provide an accurate result for temperature drop calculation. KEYWORD: Gas pipeline; simulation; algorithm for temperature drop due to throttling; BWRS state equation; thermal physical parameters