Objective In batch transportation of methanol through existing long-distance diesel pipelines, stratification readily develops at the methanol-diesel interface during shutdowns or under low-disturbance conditions because of poor compatibility between the two fluids. On restart, interface slippage can lengthen the mixed-oil section. Therefore, reducing the volume of mixed oil at the methanol-diesel interface is critically important.

Methods A technical approach of using gasoline as a spacer fluid is proposed to improve interfacial phase stability. Bottle tests and transparent-tube experiments are conducted to analyze the effects of gasoline mass fraction, methanol and diesel proportions, gasoline composition, and pipeline inclination on the phase behavior of the methanol-gasoline-diesel ternary system. Based on the experimental results and adopted engineering safety margins, an engineering threshold of gasoline mass fraction for phase stability is established. Interface mixture volumes are calculated using a section-by-section method. The required spacer-fluid injection volume is determined by converting the gasoline mass fraction to a volume fraction, using Austin’s empirical formula and the actual inner diameters of sections of the Urumqi–Lanzhou Pipeline.

Results The gasoline mass fraction is the dominant factor affecting interfacial phase stability in the ternary system; the methanol and diesel proportions and pipeline inclination exert secondary influence, while gasoline composition has a lesser effect. Bottle tests show that, within the range of methanol and diesel proportions examined, the ternary system remains in a stable mixed state at gasoline mass fractions of 60％ and above. Accounting for component fluctuations, interface-tracking errors, shutdowns and restarts, and other operating condition variations, a gasoline mass fraction of 65％ was adopted as the engineering threshold for phase stability, corresponding to a gasoline volume fraction of 68％. Using a section-by-section method, the mixed-oil volumes at the methanol-gasoline and gasoline-diesel interfaces in the Urumqi–Lanzhou Pipeline were calculated as 704.93 m³ and 760.56 m³, respectively, yielding a required total gasoline spacer volume of 996.53 m³. This value is 1.86％ lower than the injection volume calculated using a uniform large pipe diameter (0.5448 m) and 21.16％ higher than that calculated using a uniform small pipe diameter (0.4938 m). These results indicate that using a single small uniform diameter for the entire line substantially underestimates the interface mixture volume, with greater diameter variability producing larger deviations.

Conclusion A methodology is proposed for calculating the required injection volume of gasoline spacer fluid for batch transportation of methanol in diesel pipelines, providing a reference for spacer fluid design and operational optimization.