Page - 207 - in Advanced Chemical Kinetics

Looking atTable 1 and Figure 9, one can note that there is a large difference inCTAG andυ decaybetweenbothreactions,whatreflects in theconversionrate (XTAG).Theuncertaintybars calculated for the velocity are disclosed to determine the accuracy of the study [33]. The behaviourobserved in the reactionwith0.2%KOHand200 rpmpresents a slowdecay in the concentration ofTAG in the first 10minof reaction,which is expected for reactions inwhich mechanism exchange occurs. That region on the graphics of Figure 9 (first 10 minutes of reaction) is inwhich the rate of consumption and conversion rate are slow, and it occursdue to the lowmiscibility between alcohol and oil. Thus, as the methyl ester is produced, the chemical reaction starts to control the kinetics of the reaction and, therefore, a large jump between10and20minof reaction isobserved.After20minof reaction, it is clearlynoticeable that there is stability between the conversion values and propagation velocity. This result indicates that the ultrasound can determine the maximum point of conversion even in the presenceof secondarysubstances (by-products). Ontheotherhand,whenthereactionwith1.5%KOHand200rpmisonfocus,yet inFigure9, it isobservedanexpressiveconsumptionofTAGinthefirst10minofreaction.After that time, the stability in thevalues of the conversion aswell as in thepropagationvelocity is evident.Reac- tions likethat, inwhichthereisarapidconversion, theregioncontrolledbymasstransfercanbe consideredinsignificant.Here, it isobservedthat forreactionswith200rpmstirring, theincrease incatalystconcentrationnotonlyacceleratesthetransesterificationprocessbutalso increasesthe finalconversionaswell,andconsequentlydecreasestheremainingTAGinthereactionmedium. Letuscheck if thesamewilloccuranalysing theresultspresented inTable2andFigure10, in which the rotationspeedwas increased to520rpm. For the reactionwith0.2%ofKOHand520 rpmof stirring, it isnoticeable that there is a slow decrease in theTAGconcentration in the first 10min and stability of thevalues after 20min. On the other hand, the reaction with 1.5% of KOH and 520 rpm reaches the maximum conversion as fast as 5 min after the reaction had begun. In the sameway as observed in Figure 9, the mass transfer controls the start of the reaction for the reaction with 0.2% of catalyst, independently of the rotational speed employed.With the increase ofKOHconcen- trationby7.5 times, there is a300%decrease in reaction time for the reactionwith520 rpmof 0.2%and520rpm 1.5%and520rpm Time[min] XTAG [%] CTAG [molL 1] υ [ms 1] XTAG [%] CTAG [molL 1] υ [ms 1] 0 0 0.83 1426.8 0 0.83 1421.3 1 0.3 0.81 1342.9 52.8 0.65 1363.2 5 5.5 0.78 1353.0 85.9 0.12 1358.8 10 8.7 0.76 1354.0 87.7 0.10 1357.6 20 78.6 0.22 1357.3 88.7 0.09 1356.3 40 82.1 0.18 1360.0 89.1 0.09 1356.9 Table 2. Results for reactions at 520 rpm. Ultrasound as a Metrological Tool for Monitoring Transesterification Kinetics http://dx.doi.org/10.5772/intechopen.70501 207