Transesterification is the reaction of triglycerides (or other esters) with alco­hols to produce alkyl esters (biodiesel) and glycerol, typically in the presence of acid and base catalysts (see Figure 6.10). Triglycerides are one of the three types of biomass obtained in 350 types of crops such as soybean, cotton seed, rapeseed, and algae, among others, as well as in fatty acids present in a vari­ety of fresh and waste cooking oils. The basic mechanism of transesterifica­tion of triglycerides is described in Figure 6.10. Methanol is most commonly used because of its low cost, although 2-propanol gives better biodiesel and ethanol is preferred in Brazil because of its easy and inexpensive availability. Alkyl esters or biodiesel are also called fatty acid methyl esters (FAME) and they can be directly used in diesel engines.

For the purpose of this chapter, it is the transesterification of waste cook­ing or frying oil that is of interest. Worldwide there is significant production of waste cooking and frying vegetable oils. The process of transestrification allows the conversion of these waste oils into useful biodiesel. The literature

O

FIGURE 6.10

Overall and intermediate reactions for transesterification of triglyceride and alcohol to pro­duce alkylester (biodiesel) and glycerol. (After Huber et al., 2006. Synthesis of transportation fuels from biomass: Chemistry, catalysis and engineering, Chem. Rev., ACS.

reported [99-117] for the conversion of waste oils into biodiesel indicates that the efficiency of this conversion process is lower than the one for fresh vegetable oil. This is due to the presence of water and free fatty acid in the waste oils that create a soap film and reduce the effectiveness of the trans­esterification process. Hossain and Mekhled [111] showed that the trans­esterification of waste canola (often called oilseed rape), which is the second largest oilseed crop in the world, results in the biodiesel yield of about 49.5% when transesterification is carried out for two hours with 1:1 molar ratio
of methanol and waste oil, and 0.5% sodium hydroxide catalyst. Normally, methanol/waste oil ratio used is about 6:1 or even 9:1 and this often results in nearly 100% conversion to biodiesel. The study showed that NaOH is a better catalyst than KOH. Numerous other studies [112-116] with frying oils and a mixture of fresh and used frying oils showed similar results. Al-Zuhair [114] used lipase immobilized on ceramic beads and entrapped in a sol-gel matrix for the production of biodiesel from waste cooking oil. The study showed that the immobilized lipase on ceramic beads was more capable of transesterifying waste cooking oil with high water content to biodiesel than lipase in free or entrapped in sol-gel matrix forms.

Studies [99, 117] have also been carried out to examine the effectiveness of supercritical alcohols for transesterification of vegetable oil in the absence of catalyst. These studies have produced some promising results. Demirbas, Ozturk, and Demirbas [99] and Demirbas and Kara [117] examined biodiesel production from vegetable oils via catalytic and noncatalytic transesterification using supercritical alcohols. The raw vegetable oils (as well as other 350 differ­ent types of crop oils) have high viscosity and low volatility; they do not burn completely and form deposits in the fuel injector of diesel engines. Vegetable oil viscosity (which is 11-17 times that of biodiesel) can be reduced by (a) dilu­tion, (b) microemulsion, (c) thermal decomposition, (d) catalytic cracking, and (e) transesterification with alcohols, preferably methanol and ethanol.

Demirbas et al. [99, 117] showed the transesterification of vegetable oils by supercritical alcohols to be a very effective process. Their results for hazelnut kernel oil under sub — and supercritical conditions for methanol showed a sharp increase in yield of methyl ester near the critical temperature of meth­anol. A further increase in temperature beyond critical temperature did not significantly affect the yield of methyl ester. The results of Demirbas et al. also indicate the molar ratio of waste to alcohol to be at least 1 to 9 to get significant yield of methyl ester. Within all the lower alcohols (e. g., methanol, ethanol, propanol, etc.) tested, methanol was found to be the best extracting agent. Afify et al. [108], among others, examined transesterification of algae (crop waste) to biodiesel and they found two-solvent systems to work more effectively. More work for the conversion of algae to diesel oil via improved transesterification process is needed.