In this work mass transfer in liquid–liquid extraction is investigated with the two-dimensional high-order moment-conserving method of classes (2D-HMMC) (Buffo and Alopaeus, 2016). The solution of a realistic liquid–liquid test case, a counter-current rotating disc column (RDC) composed of three stages where the droplets exchange mass with the continuous phase, is studied. This detailed modelling approach is compared with two other possible approximated models. In the first all the droplets are assumed to have the same size and concentration, and in the second all the droplets are assumed to have the same concentration but different sizes. The results of this comparison show that the information regarding the two-dimensional droplet size-concentration distribution may be needed to properly evaluate the mass transfer rates and therefore the behaviour of the system for all the operating conditions investigated.