Phase distributions of alcohol, glycerol, and catalyst in the transesterification of soybean oil

Abstract

Two-phase base-catalyzed transesterification of vegetable oils is the most common method for making biodiesel. The reaction starts as separate oil and alcohol phases. At the end of the reaction, the mixture, if allowed to settle, consists of an upper ester-rich layer and a lower glycerol-rich layer. The compositions of these layers from the methanolysis and ethanolysis of soybean oil were measured. Synthetic mixtures and actual reaction mixtures were used either to represent or generate steadystate reaction mixtures resulting from the initial condition of 6:1 alcohol/oil molar ratio and catalyst concentration (1.0 wt% sodium methoxide or 1.26 wt% sodium ethoxide). At 23°C, for methanolysis, 42.0% of the alcohol, 2.3% of the glycerol, and 5.9% of the catalyst were in the ester-rich phase at steady state. In ethanolysis, 75.4% of the ethanol, 19.3% of the glycerol, and 7.5% of the catalyst were in the ester-rich phase. The volume of the glycerol-rich phase decreased from methanolysis to ethanolysis to propanolysis; butanolysis remained monophasic throughout. The results explain some of the general kinetic behavior observed in transesterifications and provide useful information for alcohol recovery and product purification.Two-phase base-catalyzed transesterification of vegetable oils is the most common method for making biodiesel. The reaction, as normally practiced, commences as two phases. These are an upper methanol phase, in which the catalyst is dissolved, and a lower vegetable oil phase. Stirring initiates the reaction, which transforms to another two-phase system comprising an ester-rich phase and a glycerol-rich phase. When stirring is stopped, the glycerol-rich phase settles to the bottom. Since the production of biodiesel standard fuel requires extremely high conversion and efficient isolation of the ester from the glycerol by-product, it is particularly important to characterize the steady-state compositions of the final phases.Many studies (1-3) have shown that the transmethylation reaction decelerates prematurely. It was previously concluded that this was due to the destruction of catalyst through soap formation. Feuge and Gros (4) reported that for the ethanolysis of peanut oil, more than 50% of the catalyst was destroyed in the first 15-20 min at 50°C. Boocock et al. (5) measured the catalyst (NaOH) concentration for base-catalyzed methanolysis of soybean oil (SBO) at 23°C and found that 67-83% of the catalyst was "depleted" in about the same time. This was at first attributed to soap formation by the irreversible attack of hydroxide ion on ester groups. When the reaction mixture is acidified, this soap is converted into FA. However, if all of the catalyst is consumed (1.0 wt% sodium hydroxide based on the weight of oil) by this reaction, then after acidification the FFA content in the transesterified product will be 7.0%, which is equivalent to an acid number of 14 based on oleic acid. Such high values are not usually observed.In a later study, Zhou and Boocock studied the phase behavior of two-phas...

Biodiesel production, properties, and feedstocks

“…Although butanolysis proceeds at a faster initial rate than methanolysis, the final conversion to products after 1 h reaction (114°C and 60°C reaction temperatures, respectively) is 96 wt.% versus 98 wt.% for methanolysis (Schwab et al 1987). In addition, after 1 h (at 23°C), 14.4 wt.% of bound glycerol (TAG + DAG + MAG) remained, whereas only 11.7 and 7.2 wt.% remained in the cases of methanolysis and ethanolysis, respectively (Zhou and Boocock 2006b). In summary, the butanolysis reaction is monophasic throughout, which results in a faster initial rate of reaction but may yield lower overall conversion to butyl esters in comparison to methyl or ethyl esters.…”

“…1. The rate at which FAME are produced during the transesterification reaction is thus controlled by mass-transfer limitations, which results in a lag time before conversion to FAME begins (Boocock et al 1998;Zhou and Boocock 2006b;Doell et al 2008). Once DAG and MAG intermediates are formed in sufficient quantity during the transesterification reaction, they serve as surfactants that improve mass transfer of TAG into the methanol phase.…”

Cinética Da Etanólise Do Óleo De Girassol

“…Em geral, a primeira fase é mais rápida na reação de etanólise em relação à reação de metanólise, devido à diferença na solubilidade entre os álcoois. A segunda fase é rápida e é controlada pela cinética da reação, enquanto que a última fase é dominada pelo equilíbrio químico (Zhou, 2006).…”

First Generation Biodiesel

“…Sufficient magnitude Stirring can make TAG transport into small drops which contact the methanol phase more effectively, and then convert into FAME and glycerin (Moser, 2009). The rate at which FAME are produced during the transesterification reaction is thus controlled by mass-transfer limitations, which results in a lag time before conversion to FAME begins (Boocock et al, 1998;Doell et al, 2008;Zhou & Boocock, 2006b). This condition is more obvious when the reaction is catalyzed by solid catalysts.…”