Seite - 202 - in Advanced Chemical Kinetics

4.Validationof theexperimentalultrasonicmethod In previous studies, the value of the propagation speed in soybean oil in a range of 20–50�C was determined [20]. For the practical application presentedwithin this paper, all reactions wereperformedat 40�C.At this temperature, soybeanoil has avelocityof 1418.3ms�1,with expandeduncertainty (p = 0.95)Uexp = 5.2m s �1 [21]. As soon as the oil starts to reactwith methanol and the catalyst, variation on the speed of soundwill indicate that something is happeningwithin themedium.Despite it is easy tomeasure speedof sound, it isnot trivial to relate thisvariationwithanything that isgoingon in the reaction.Thechemicalkinetics isnot directly assessed, unless somemethodological study is conducted. Thatwas the case, insofar we conducted an experimental method validation. The idea was to compare the speed of soundmeasured throughout the transesterification processwith a quantification of the reac- tion stoichiometric situationatdifferentmoments. Theworldwideaccepted referencemethod for determination of ester content is based on gas chromatography (GC), according to the standardEN14103.However, thismethod,besidesbeing time-consuming, it isnot applicable in the process line anddemands expensive equipment, supplies, and specific technical train- ing.Thus, less costlymethodshaveemergedasanalternative fordetermining theconversion, as is thecasewith1HNMR.Despite it isnotacheaptechnique, it ismuchless time-consuming than theGCanalysis [13, 14, 21,22, 30–32]. In establishingparameters for the reactions thatwill be analysed, an isothermal batch reactor (T = 40�C) is chosen. The validation experimentwas restricted to two concentrations for the catalyst (0.2%and1.5%w/w)andtwomechanicalstirringrotationalspeeds(200and520rpm). Thereaction timewasset toa limitof40min. There are several studies that propose equations that take into account thenumberof hydro- genspresent in themoleculesconsumed(TAG) inrelation to thenumberofhydrogenspresent in the formedmolecule (FAME). Figures 3 and 4disclose the 1HNMR spectra for the pure soybeanoil andthebiodieselmade fromthisoil, respectively. The formation ofmethyl ester (methylic biodiesel) can be noticed by the appearance of the signal of the methylic hydrogen from the methoxyl group at 3.7 ppm (chemical shift represented per B in Figure 4), while occurs the disappearance of themethylene hydrogens fromglycerol in the triacylglycerol from 4.0 to 4.4 ppm (chemical shift represented per B in Figure 3). Eq. (6) presents a method described in the literature [14] used to determine the conversionofTAG. XTAG %ð Þ¼100 � 2A13A2 (6) Here,XTAG (%) is the amount of TAG that has been converted into biodiesel,A1 andA2 are areas of themethylic hydrogens (δ = 3.7 ppm) ofmethoxyl group, themethyl ester and the glycerol methylene hydrogens (δ = 4–4.4 ppm), respectively. Calculation of the 1H NMR conversionwas compared to the referencemethod (GC) for the two reactionswith 200 rpm (seeFigure5). Advanced Chemical Kinetics202