DETERMINAÇÃO DE TEORES DE ÓLEOS CONTAMINANTES EM BIODIESEL OBTIDO DO ÓLEO RESIDUAL DO PROCESSAMENTO INDUSTRIAL DE TILÁPIA DO NILO POR TERMOGRAVIMETRIA QUASI-ISOTÉRMICA (TGA-qISO) E CALORIMETRIA EXPLORATÓRIA DIFERENCIAL (DSC)

Abstract

The process of filleting Nile tilapia (Oreochromis niloticus) generates around 70% of waste, which includes: head, viscera, skins and scales. A promising destination for this waste is the production of biofuel. In Brazil, biodiesel is used in diesel blends, which is currently sold at 12% (v/v) biodiesel/diesel (B12). Diesel-biodiesel blends can be adulterated by illegally adding lower-value vegetable oil, which can cause overheating and excessive acceleration of the engine, in addition to increasing fuel consumption and the emission of particulate matter, which results in economic losses for the consumer. and environmental damage. The present work aimed to determine the levels of contaminating oils in biodiesel obtained from residual oil from the industrial processing of Nile tilapia by Quasi-Isothermal Thermogravimetry (TGA-qISO) and Differential Scanning Calorimetry (DSC). Crude tilapia oil was obtained from cooking the tilapia carcass. The oil was refined and biodiesel was obtained by transesterification via alkaline catalysis, in a molar ratio of 1:6 (oil: methanol) and 0.5% (m/m) of potassium hydroxide as catalyst. The physicochemical parameters of the oil and biodiesel and their chemical composition were determined using the techniques Gas Chromatography with Flame Ionization Detector (GC-FID), Spectroscopy in the Infrared Region (FTIR) and Hydrogen Nuclear Magnetic Resonance (1H NMR). The thermal behavior was evaluated using Themogravimetry/Derivative Themogravimetry (TG/DTG) and DSC techniques. Tilapia oil (OT) and biodiesel (BD) mixtures were prepared in the mass proportions of OT: BD (5:95 m/m), OT: BD (10:90 m/m), OT: BD (15: 85 m/m), OT: BD (20:80 m/m), OT: BD (25:75 m/m) and OT: BD (30:70 m/m). These mixtures were used to construct the calibration curve for the TGA-qISO and DSC techniques. Three samples were prepared at concentrations 7.01 OT %, 16.66 OT % and 27.05 OT % to evaluate the efficiency of the TGA-qISO and DSC techniques to determine the oil content in a biodiesel/oil mixture. Tilapia oil had a low water content and acidity index, which made it possible to obtain biodiesel via alkaline catalysis. Biodiesel exhibited density and iodine content in accordance with the literature and the National Agency of Petroleum, Natural Gas and Biofuels (ANP), however, oxidative stability, water content and ester content were outside the limits of legislation. The major fatty acids in tilapia oil were myristic (4.10%), palmitic (37.63%), palmitoleic (8.99%), stearic (7.96%), oleic (4.56%) and linoleic. (21.03%). The FTIR and 1H NMR techniques confirmed the conversion of tilapia oil into biodiesel, with the appearance of the band at 1435 cm-1 and the peak between 3.66 and 3.62 ppm, respectively. The thermal stability of the oil was lower in a synthetic air atmosphere than in nitrogen, due to the oxidation of poly and monounsaturated fatty acids, while biodiesel presents similar thermal stability regardless of the atmosphere used. The crystallization study showed that biodiesel crystallizes and melts at lower temperature ranges than tilapia oil. The biodiesel/oil mixtures showed two stages of mass loss, the first between 100 and 200ºC, which was attributed to the decomposition of biodiesel and from 250 ºC to the decomposition of the oil. The TGA-qISO measurements presented analytical curves with a correlation coefficient (r) of 0.9999, while the DSC analyzes obtained r of -0.9727 and -0.9903. Analysis of variance (ANOVA) confirmed that there is no significant difference between the measurements performed by TGA-qISO and DSC. This result evidences that both techniques can be used to determine oil adulteration in biodiesel samples.