A BBREVIATIONS
Scheme 4.1: Proposed mechanism of calcium oxide catalyzed trans–esterification reaction
4.8 Analysis
Prior to the main analysis section, we describe herewith briefly the chemical mechanism of the process as a preamble. CaO catalyst being hydrophilic in nature preferably stays in the aqueous phase (or methanol).
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0 1 2 3 4 5
0 2 4
No. of Acoustic Cycles
Radius (R/Ro)
(A)
0 1 2 3 4 5
0 15 30
No. of Acoustic Cycles
Acoustic wave amplitude (bar)
(B)
0 1 2 3 4 5
0.004 0.001
No. of Acoustic Cycles
Microturbulene velocity (m/s)
(C)
Figure 4.8: Simulation results for radial motion of a 5 µm cavitation bubble (air) in methanol at 55oC. Time variation of (A) normalized bubble radius (R/Ro), (B) acoustic (or shock) waves emitted by the bubble, (C) micro–convection (or oscillatory liquid velocity) generated by the cavitation bubble.
As explained in mechanism of the process (scheme 4.1), calcium methoxide Ca(CH3O)2 formed on the surface of CaO is the actual catalyst for alcoholysis of triglyceride. As noted earlier, formation of calcium methoxide has been confirmed by XRD data of Fig. 4.3B.
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Sonication of freshly calcined CaO under reflux of methanol for 1 h shows growth of fresh peaks of calcium methoxide. Ca(CH3O)2 transfer one of its methoxy groups to the carbonyl carbon of the triglyceride forming an insoluble organic salt of Ca(CH3O)2 and triglyceride.
Alkoxy group of triglyceride is adsorbed by the calcium methoxide to give alkoxy calcium methoxide and a fatty acid methyl ester. Finally, alkoxy calcium methoxide react with excess methanol to regenerate the actual catalyst calcium methoxide and alcohol [49]. At the end of reaction, the catalyst precursor CaO is regenerated by separation of solid phase by filtration, washing with n–hexane, drying for 12 h at 100oC, and finally calcination at 900oC.
Concurrent analysis of simulations and experimental results helps us identify various facets of physical mechanism of the process. Some peculiar features of the transesterification process can be observed from the results of the Box–Behnken experimental design:
The least yield of 21.96% is seen in set 2 for the lowest temperature (45oC), highest molar ratio (18) and moderate catalyst (5 wt%).
Yield in similar range, i.e. 25.98% amd 24.25% is obtained in sets 5 and 6 for combinations of 60oC temperature, molar ratios of 6 and 18 (respectively), and catalyst concentration of 3 wt%. Interestingly, yield in similar range (27.85%) is obtained in set 9 for which the molar ratio is moderate (12), while temperature reduces to 45oC with catalyst concentration remaining the same at 3 wt%.
Comparison of yields of set 7 and 8 also gives interesting observation. The molar ratio reduces 3 times in set 7 as compared to set 8. Nonetheless, the yield of set 7 is approx. 2×
higher. This is completely contradictory to the expectation that yield would increase with molar ratio. This observation becomes even more remarkable as the temperature of both sets is 60oC and the catalyst concentration is the highest, i.e. 7 wt%.
Similar trend of reduction in yield with rise in molar ratio is also seen in sets 3 and 4, although in this case the yield reduces by ~ 30% with 3× rise in the molar ratio.
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The relative effect of temperature and catalyst can also be deduced from results of experimental sets 10 and 11. Lowest temperature of 45oC at the highest catalyst concentration of 7 wt% in set 11 gives almost similar yield as in set 10 for the highest temperature of 75oC and the lowest catalyst concentration of 3 wt%.
Comparing results of set 10 and 12 reveals that at the highest temperature of 75oC (which is well above the b.p. of methanol), doubling of catalyst concentration results in mere 30%
rise in the yield of transesterification.
The highest yield is obtained for sets 13 to 15 in which the temperature is just below the b.p. of methanol and both catalyst concentration and molar ratio are moderate.
Above results indicate that all three process parameters, viz. molar ratio, temperature and catalyst concentration have a significant influence on the transesterification yield.
Nonetheless, as noted earlier, the combination of optimum process parameters for the highest yield is temperature = 64oC, alcohol to oil molar ratio = 11.23 (approximated as ~ 11) and catalyst loading = 5.36 wt%, which are the middle or moderate values used in statistical design. We justify these values as follows:
1. Since the transesterification reaction occurs in the liquid phase, it is essential that both methanol and oil remain in liquid phase at the reaction temperature. The boiling point of methanol is 65oC. The optimum temperature obtained in our experiment is close to boiling point of methanol, for which methanol exists in liquid phase. At the highest temperature used in our experimental design, i.e. 75oC, the methanol in the reaction mixture vaporizes resulting in reduction in interfacial area for reaction.
2. The influence of molar ratio and catalyst concentration is inter–related. For very low catalyst concentrations (~ 3 wt%), the amount of methoxy ions generated are low giving smaller yields. Interfacial area between oil and methanol depends on extent of dispersion of these liquids in each other. Since the intensity of cavitation bubble motion is higher in
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methanol, one would expect higher dispersion and interfacial area for the highest molar ratio, and consequently higher yield. However, contrary to this expectation, we see reduction in yield at highest molar ratio. The probable cause leading to this effect is that catalyst particles being hydrophilic stay preferentially in methanol phase. The methoxy ions that induce the transesterification reaction are generated at the catalyst surface. As the volume fraction of methanol in the total reaction mixture increases with molar ratio, these ions have to diffuse through larger volume so as to reach the interface and react with triglyceride. On the other hand, if the molar ratio is too low, the volume fraction of oil in the reaction mixture is high and some catalyst particle may also stay in oil phase. The oil competitively wets the catalyst surface, thus reducing formation of methoxy ions. As a result of these competing phenomena, the highest yield is obtained at moderate alcohol to oil molar ratios.