Kinetics of hydrolysis and methyl esterification for biodiesel production in two-step supercritical methanol process

Abstract

For high-quality biodiesel fuel production from oils/fats, the catalyst-free two-step supercritical methanol process has been developed in a previous work, which consists of hydrolysis of triglycerides to fatty acids in subcritical water and subsequent methyl esterification of fatty acids to their methyl esters in supercritical methanol. In this paper, therefore, kinetics in hydrolysis and subsequent methyl esterification was studied to elucidate reaction mechanism. As a result, fatty acid was found to act as acid catalyst, and simple mathematical models were proposed in which regression curves can fit well with experimental results. Fatty acid was, thus, concluded to play an important role in the two-step supercritical methanol process.

Transesterification in Supercritical Conditions

“…The chemical kinetics of supercritical transesterification is divided into three regions, that of the slow (<280 ºC), transition (280 -330 ºC) and fast (>330 ºC) regions, and usually follows the first-order rate law with respect to the triglyceride concentration alone (He et al, 2007a;Minami & Saka, 2006). Here, the reaction mechanism is merged into one overall step and the concentrations of all intermediates (mono-and diglycerides) are ignored.…”

“…However, the required reaction time to nearly complete transesterification conversion at a 42:1 alcohol to oil molar ratio is then significantly longer at more than 90 min (Minami & Saka, 2006). This prolonged reaction time might cause a decline in the production efficiency obtained by supercritical transesterification, but it could be shortened by the use of assisting methods, as discussed in Sections 3.2 -3.4.…”

“…In 2005, carbon dioxide and propane were introduced as co-solvents to obtain milder operating parameters for the supercritical transesterification with methanol Han et al, 2005). Then, the two-step supercritical process (Minami & Saka, 2006) was demonstrated to reduce those operating parameters. In the following years, various catalysts were employed to assist the supercritical transesterification to achieve the maximum alkyl esters content but at milder operating conditions (Demirbas, 2007;Yin et al, 2008b).…”

“…In the following years, various catalysts were employed to assist the supercritical transesterification to achieve the maximum alkyl esters content but at milder operating conditions (Demirbas, 2007;Yin et al, 2008b). The continuous production of biodiesel in supercritical methanol was reported in 2006 (Bunyakiat et al, 2006) (Minami & Saka, 2006) and 2007 (He et al, 2007b). Therefore, the research focus on the reduction of the elevated operating conditions and continuous process has been ongoing since 2005.…”

“…The two-step process reduces the optimal operating parameters successfully since the hydrolysis and esterification reactions reach complete conversion at a lower temperature than the transesterification reaction does (Minami & Saka, 2006). Nonetheless, the two-step process is more complicated than the single-step process.…”