Title : Anisidine number as a determinant of aging of ester biofuels
Abstract:
One of the challenges of testing the quality of biofuels is to study their oxidative stability, especially with variable feedstocks used for transesterification. Test methods developed according to the standards are based on measuring the time elapsed until the appearance of secondary reaction products that increase conductivity - the Rancimat method, the time to achieve a decrease in the value of maximum pressure in the measuring chamber by 10% - the Petrooxy method, or the sum of insoluble filterable and adherent precipitates formed under the test conditions. These methods are useful for testing oxidation resistance for freshly prepared fuel, but they are not authoritative for assessing oxidation resistance for testing over long periods of their storage. In such a case, it is useful to analyze the anisidine or acid number over time, which are standardly used in stability studies of cooking oils. The formation of stable oxidation products, such as carbonyl compounds, is then determined. Unfortunately, when testing the acid number, i.e., oxidation products that are organic acids, esterification can occur which affects the underestimation of this parameter and the problem in interpreting this parameter. The anisidine number, as an indicator of the total amount of secondary oxidation products that do not undergo further oxidation under the conditions of the experiment, can be the best indicator of oxidation resistance especially in studies conducted over a long period of time.
The purpose of this study was to determine the feasibility of using non-normal anisidine number to study changes after oxidation of methyl esters of higher fatty acids. The work used two methods to accelerate aging: elevated temperature and pure oxygen exposure.
The largest substitutions of anisidine number with time are observed for FAME obtained from frying oil and refined oil. The smallest changes are for FAME obtained from unrefined oil. The intensity of the changes directly indicates the relationship between the processes the oils undergo before transesterification and the quality and resistance of the FAME. The more extensive the refining or thermal degradation, the worse the quality of the FAME and its resistance to accelerated aging.
The study indicated a direct dependence of the anisidine number on the progressive oxidation of the tested samples of FAME produced from unrefined oil, refined oil and frying oil. These changes are reflected in the change in IR spectra obtained for the samples tested. The high R2 coefficients of the dependence of anisidine number on transmittance at 3545 cm-1 and 3300 cm-1 indicate the applicability of spectrophotometry in the study of biodiesel aging.
