Viscosities of fatty acid esters: A study on various semi-empirical models

The viscosity information of fatty acid esters (FAEs) is crucial for the utilization of biodiesels. In order to accurately describe the viscosity of fluids, researchers have developed several semi-empirical models, but their different performance for FAEs has not been thoroughly discussed. This work focuses on a comprehensive comparison of several popular semi-empirical viscosity methods, including the absolute rate theory (AR), free-volume (FV), hard-sphere (HS), friction theory (FT), and entropy scaling (ES) models. Three equations of state (EoS), namely SRK, CPA, and PC-SAFT, are also selected, which represent the thermodynamic methods with varying levels of complexity and accuracy. Then, the density and viscosity of 14 saturated FAEs and 4 mixtures are calculated using the five viscosity models coupled with three EoS, and the influence of EoS performance on the viscosity results is analyzed. Special attention is paid to the viscosity models and the SRK EoS, to test the calculated viscosities with such a simple EoS. At last, considering both simplicity and accuracy, the AR and FV models can give the best overall performance for calculating the viscosity of FAE systems, and the FT model is also a potential approach.