Experimental study on the variation law of mixed oil in methanol-diesel batch transportation pipelines

Objective Methanol is an ideal carrier for hydrogen energy, effectively addressing challenges across the “production-storage-transportation-refueling” spectrum. Utilizing the existing product oil pipeline infrastructure, batch transportation of methanol and product oil enables flexible long-distance hydrogen transportation while maximizing pipeline resource utilization and operational efficiency.

Methods A methanol-diesel static miscibility experimental device was set up to investigate the miscibility of methanol and diesel at pressures ranging from 0.5 MPa to 0.9 MPa. The variation patterns of stratification velocity and mixed oil volume in two batch transportation modes (methanol-then-diesel and diesel-then-methanol) were analyzed under different inclinations (5° to 15°). Additionally, a self-designed methanol-diesel batch transportation flow loop experimental device was utilized to explore the effects of flow rate and batching sequence on the volume of methanol-diesel mixed oil.

Results At room temperature, methanol and diesel were immiscible and stratified. As pressure increased, the rate of volume change of the mixed oil also increased, with pressure and oscillation in the pipeline generating small oil-in-methanol emulsion droplets. When the pipe inclination ranged from 5° to 15°, the volume of mixed oil increased linearly over time, with the volume for the “methanol-then-diesel” transportation mode being smaller than that for the “diesel-then-methanol” mode. As flow velocity increased, the volume of mixed oil decreased. At a flow velocity of 0.2 m/s (laminar flow), the mixed oil volumes for the “methanol-then-diesel” and “diesel-then-methanol” modes were 0.017 27 m³ and 0.019 63 m³, respectively. At a flow velocity of 0.5 m/s (turbulent flow), the volumes for these two modes were 0.014 13 m³ and 0.015 31 m³, respectively. Under the same batching sequence, compared with the laminar flow, the forward medium adhered to the pipe wall but was quickly flushed away by the backward medium during turbulent flow, resulting in no significant mixed oil tailing. Increased flow velocity of the mixed oil inhibited the axial diffusion of the backward medium in the pipeline, with convective diffusion driven by the radial flow velocity prevailing.

Conclusion The stratification behavior of methanol and diesel under varying pressures and inclinations was examined, alongside the effects of different flow states and transportation sequences on the volume of methanol-diesel mixed oil in the pipeline. To minimize mixed oil formation in the pipeline, turbulent flow should be maintained, with lower-density methanol as the forward medium, and pipe inclination should be minimized.