The transport of different products through a pipeline is widely used around the world as it ensures to the process operational and economic efficiency. However, it has as a side effect of creating an interfacial product mixture, which can lead to loss of quality due to contamination. Typically, the problem is analyzed considering constant flow rate over time while the interface travels along the pipeline length. Although this simplification is effective for several cases, transient operations are very common in practice. Therefore, this work analyzes the influence of transient operations on the mixture region length. A diffusive formulation is employed, based on an axial dispersion coefficient. A careful analysis of several correlations available for the axial dispersion coefficient is performed, showing the fundamental importance of considering its dependence to the Reynolds and Schmidt number. The mathematical model is discretized by the finite volume method with second order approximations on both time and space domains. The methodology is validated for constant flow rate and employed to transient operation situations, being compared to field data of real pipelines. The model offers excellent prediction of the mixture region length for operations with transient flow rates and pumping shut-down conditions of short duration. A higher uncertainty is obtained in the case of long lasting shut-downs if the interface stops on leaning regions of the pipeline.