Effect of long–term storage on the fatty-acid profile of biodiesel and its impact on key ultrasonic properties of biodiesels and blends

Biodiesel is a good alternative to Petroleum Diesel (PD). The storage of fuel is unavoidable for its future use. The objective of the study was to investigate ultrasonic parameters: viscosity, relaxation time, ultrasonic absorption and Gibb’s free energy for 2-year long-term stored biodiesels and blends with PD. The two biodiesels were Cotton Seed Oil Methyl Esters (CSOME) and Palm Stearin Methyl Esters (PSME). Five biodiesel blends with PD in 10, 20, 30, 40 and 50 volume per cent were studied at the room temperature of 301 K. The fatty-acid profiles were also investigated using GC–MS chromatographic analytical tool and the viscosity was determined using an Ostwald viscometer. Ultrasonic velocity was measured with an ultrasonic interferometer of fixed frequency 2 MHz. The results show that CSOME biodiesel contains 13.47 and 86.53 weight per cent of low molecular weight nonpolar and high molecular weight polar compounds, respectively, and PSME biodiesel with 38.66% and 61.33%, respectively. The measured parameters increased for both the biodiesel blends due to storage. The increase in CSOME blends is more than that in PSME blends. For CSOME blends, highest and lowest changes are 64.28% and 4.48% for 10% volume blend of viscosity and pure CSOME biodiesel of Gibb’s free energy, respectively. In PSME blends, they are 31.25% for 10% volume blend of viscosity and zero for 50% volume blend of Gibb’s free energy. The highest difference occurred for 50% volume blends between old CSOME and PSME blends. From the overall results, it is concluded that variations of the measured parameters strongly depend on the fatty-acid profile of biodiesels due to storage.