Multiproduct pipelines are used to transport different refined products, batching the products in continuous succession. The use of mechanical separators between products requires complex operation, therefore the products are transported without separators and a mixing volume is formed in the batch interfaces. This practice is common in the pipeline industry and different approaches for estimating the mixing volume can be found in the literature.

The available approaches for estimating the mixing volumes are validated and intended for steady state and long times of transfer, considering that the transmix has travelled the whole length of the pipeline.

In the scenario of short times of transfers, the transient estimation of the mixing volume along the pipeline can present significant differences in density, viscosity and length of the transmix. Also during loading and unloading operations of oil tankers pressure transients are generated, and therefore a transient estimation of the mixing volume is desired.

This work proposes a transient approach for the mixing volume estimation. The formulation is based on the drift-flux model with a dispersion coefficient for representing the convection and diffusion effects in the concentration in radial direction. The formal drift-flux model requires empirical correlations for the drag coefficient and for the apparent viscosity of the mixture, so the proposed approach is a simplification to be used in quasi-one dimensional models for control and leak detection purposes.