Objective Batch transportation of liquid ammonia via product oil pipelines can expand transportation routes, reduce costs, and enhance pipeline efficiency to address the low throughput of existing product oil pipelines. Currently, there is little research on the batch transportation of liquid ammonia via product oil pipelines. In addition, due to the mutual insolubility of liquid ammonia and product oil, the blending behavior differs from traditional product oil blending, posing challenges for predicting and controlling the liquid blending interface.

Methods By coupling the phase-field method with fluid control equations, a two-phase flow model for batch transportation of liquid ammonia and product oil was developed to examine how transportation sequence, pipeline flow rate, and pipeline dip angle affect the two-phase flow behavior during batch transportation.

Results In the horizontal pipeline, the sequence of transporting "product oil before liquid ammonia" would cause Rayleigh-Taylor (RT) instability at the interface due to density difference, resulting in a discontinuous phase distribution in the two-phase flow. Under this condition, the stability at the interface was improved at low flow rates, but the length of the blending section was long. However, in the transportation sequence of "liquid ammonia before product oil", the stability at the interface could be maintained even at high flow rates, resulting in a shorter blending section. In addition, in the up-dip pipeline, gravity opposed the flow, inhibiting the entrainment of product oil into liquid ammonia, thus maintaining a stable interface and a short blending section. In contrast, in the down-dip pipeline, gravity aligned with the flow, increasing RT instability and leading to interface fractures and a significant increase of the blending section.

Conclusion The results provide a theoretical basis for predicting and tracking the blending interface during the batch transportation of liquid ammonia in product oil pipelines.