Testing various mixing rules for calculation of viscosity of petroleum blends

Seventeen mixing rules reported in the literature used for predicting kinematic viscosity of petroleum and its fractions were examined for accuracy by comparing the estimated values with the experimental viscosities of four crude oils (21.31, 15.93, 12.42 and 9.89°API gravity) and their blends with a diluent (diesel) at several proportion. Tested mixing rules were classified as pure mixing rules, mixing rules with a VBI parameter, and mixing rules with an additional parameter. The results indicated a general trend to fail as the crude oil API gravity decreased, although at high temperature of analysis the predictions improved. After calculating standard errors for all predictions, only four of these rules showed acceptable accuracy (Chevron, Walther, Einstein and Power law), nevertheless no rule was capable of estimating viscosity for all the crude oils, highlighting that predicting viscosity is a challenging task. This general result led a further analysis for testing the accuracy of mixing rules in predicting viscosity for light distillates (naphtha, diesel and vacuum gas oil) and their blends; basically the same results were found, although a fifth rule (Chririnos) showed good agreement with experimental values.