Correlation and Prediction of Waste Cooking Oil and Biodiesel Viscosities

The study of oil and biodiesel viscosity for a wide range of temperatures and pressures is crucial for predicting the behavior in injection and combustion of fuel systems in diesel engines as well as for the optimization of these systems. In this work, the viscosity of waste cooking oil (WCO) and biodiesel (WCB) was measured within the global temperature range 298 to 373 K at atmospheric pressure. A rotational Brookfield viscometer was used for the viscosity measurements from which the rheological behavior of the oil and biodiesel was examined. The acquired data were then correlated using the Arrhenius, Vogel-Fulcher-Tammann, and Mauro equations. Among these, the latter two equations proved to offer the most accurate description of experimental data, yielding an average relative deviation (AARD) of 1.4%, compared to 4.6% and 3.6% obtained from Arrheniuśs equation for WCO and WCB, respectively. Additionally, various predictive methods including the Zong fragment-based approach, the group contribution method proposed by Ceriani, and recent methods derived from the Lewis-Squires equation were used to predict the experimental viscosity data, yielding reliable results. Specifically, for oils the Ceriani method resulted in an overall average relative deviation (OARD) of 8%, while for biodiesel the Lewis-Squires led to an OARD of 3%. This work describes in detail the sources of literature dedicated to the viscosity of used oils, with particular emphasis on used frying oils. A detail found in this study is that used oils from various sources, typically used in cooking, show a similar viscosity behavior with temperature, with only small variations between them. This is a fact rarely described in the literature.