Abstract: 　　Objective Utilizing existing product oil pipelines for batch transportation of methanol not only improves transportation efficiency and reduces costs, but also effectively addresses challenges in hydrogen energy storage and transportation. The mixing characteristics of methanol and product oil under stable flow conditions can be generally predicted in current researches. However, the understanding of their hydraulic and mixing behaviors under complex transient flow remains insufficient, posing significant challenges to the safe design and risk assessment of methanol-product oil batch transportation pipelines. Methods The numerical model for methanol-product oil batch transportation in pipelines was established, by coupling the hydrodynamic and mixing calculation methods.
　　The accuracy of the model was comprehensively validated through both dynamic and static evaluations. The pressure and mixing characteristics of methanol-product oil batch transportation under pipeline shutdown, water hammer, and leakage conditions were analyzed through simulation. Results Under pipeline shutdown conditions, when the methanol preceded gasoline in upward pipe sections or the gasoline preceded methanol in downward pipe sections, the mixing volume of methanol and gasoline increases significantly compared to other situations, due to the intensified natural convection driven by the density difference between methanol and gasoline. In these cases, the mixing volume can reach approximately 20% of the pre-shutdown value within 24 hours. Under water hammer conditions, the pressure waves generated by valve-closure-induced water hammer have a greater impact on the pipeline system than those from pump-shutdown-induced water hammer, and the pressure difference between the two different water hammer types is more pronounced in horizontal pipelines. Both types of water hammer pressure waves induce a slight increase in mixing volume as they pass through the methanol-gasoline interface. Under leakage conditions, when the leakage flow rate reaches a critical threshold (0.1 m/s), both the pipeline pressure and mixing volume downstream of the leakage point decrease. The pressure loss was more significant near the pipeline inlet under high flow velocities, and the mixing reduction became more evident near the middle and end of the pipeline with low flow velocities. Conclusion The results provide practical guidance for the design of safe operational strategies and the optimizing mixing control for the methanol-product oil batch transportation systems. (10 Figures, 3 Tables, 22 References)