Caracterização de glicerina bruta oriunda da produção de biodiesel e validação de método para determinação do teor de metanol por cromatografia à gás

Abstract

This work aimed to characterize crude glycerin from biodiesel production and to validate a method for determination of methanol in crude glycerin by means of gas chromatography. Firstly, ten crude glycerin samples were characterized by appearance and color tests and by determination of glycerol, chlorides, water, sulphated ash and fats and oils contents. The color was yellowish and brownish. The appearance from clear and free of impurities to cloudy with impurities was observed. The glycerol content ranged between 34 and 64 % (w/w), the chloride content found was 0.3 to 6.0 % (w/w), water content in the samples was in the range from 0.2 to 19.0 % (w/w), sulphated ash content measured was 3.5 and 8.0 % (w/w) content of oils and fats was 0.2 to 5.0 % (w/w). To develop the method for quantification of methanol, initially the EN14110:2003 standard for determination of methanol in biodiesel was tested, however the recovery assays were of 15.3 and 17.0 %, respectively, for low and high levels of fortification. Therefore, a method with mimicking matrix was developed where initially the headspace sampling variables (temperature, time and sample size) were optimized using a 33 factorial design. From the results of factorial design, the response surface was generated from the quadratic model found. The optimized condition was obtained with the equilibrium temperature of 80 °C and the equilibrium time of 12 minutes for an injection volume of 500 μL of vapor phase. This optimized condition was adopted for the construction of standard curves for low and high concentration yielding r2 of 0.97919 and 0.99931, respectively. The LD was 0.000301 % (w/w) and the LQ was 0.000362 % (w/w) , respectively. In this method, recovery tests for samples A2, B3, D1 were satisfactory and values obtained were between 80 and 102 %. The accuracy was assessed by comparing the results of the methanol content of B3 and D1 samples obtained by the

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methods of matrix mimicked and standard addition, relative errors of 1.6 and 0.1% being observed. The repeatability was 0.003 % (w/w), 0.051 % (w/w) and 0.262 % (w/w) for low, medium and high concentration of methanol, respectively. Nevertheless, intermediate precision was 0.006 % (w/w), 0.0058 %(m/m) and 0.297 % (w/w) for concentration ranges listed above. The estimated uncertainty of measurement (k = 2, and 95% probability) was 0.0046 % (w/w) in the range 0 to 0.5 % (w/w) and 0.12 % (w/w) for the range 0.50 % (w/w) to 10.00 % (w/w) with no dilution) and 0.15 % (m/m) for concentration above 10.00 % (w/w) and dilution. Robustness tests ensure that the method is able to be used under the conditions prescribed. Therefore, the method mimicking matrix was used for the determination of methanol in ten samples of crude glycerin and the values ranged from 0.0087 and 20.04 % (w/w). Among the ten samples studied, five crude glycerin samples showed methanol content above 0.1500 % (w/w), which is the limit set by the Brazilian Legislation (MAPA - Ministry of Agriculture, Livestock and Supply) for use in animal feed supplementation.