Investigation of Biodiesel Through Photopyroelectric and Dielectric-Constant Measurements as a Function of Temperature: Freezing/Melting Interval

Abstract

Biodiesel is a promising option for alternative fuels since it derives from natural and renewable materials; it is biodegradable and less polluting than fossil fuels. A gradual replacement of diesel by biodiesel has been adopted by many countries, making necessary the investigation of the physical properties of biodiesel and of its mixture in diesel. Photothermal techniques, specifically the photopyroelectric technique (PPE), have proved to be suitable in the characterization of biodiesel and of its precursor oils, as well as of the biodiesel/diesel mixtures. In this paper, we investigate thermal and electrical properties of animal fat-based biodiesel as a function of temperature, aiming to characterize the freezing/melting interval and the changes in the physical properties from the solid to the liquid phase. The samples were prepared using the transesterification method, by the ethylic route. Optical transmittance experiments were carried out in order to confirm the phase transition interval. Solid and liquid phases present distinct thermal diffusivities and conductivities, as well as dielectric constants. The PPE signal amplitude is governed by the changes in the thermal diffusivity/conductivity. As a consequence, the amplitude of the signal becomes like a step function, which is smoothed and sometimes delayed by the nucleation processes during cooling. A similar behavior is found in the dielectric constant data, which is higher in the liquid phase since the molecules have a higher degree of freedom. Both methods (PPE/dielectric constant) proved to be useful in the characterization of the freezing/melting interval, as well as to establish the distinction in the physical properties of solid and liquid phases. The methodology allowed a discussion of the cloud point and the pour point of the samples in the temperature variation interval.