Phase behavior of the base‐catalyzed transesterification of soybean oil

Abstract

Biodiesel is made by the transesterification of vegetable oils to form alkyl FA esters. High levels of conversion (>99%) are required to lower the total concentration of free and chemically bound glycerol to that allowed by the ASTM standard (0.240 wt%) for biodiesel. A polar dye was used to visualize the phase behaviors in methanolysis, ethanolysis, and butanolysis. The dye was more strongly colored in more polar phases. Methanolysis and ethanolysis reactions commenced as two phases (alcohol and oil), then formed emulsions, and ended as two phases as glycerol-rich phases separated. Ethanolysis was more easily initiated by mixing than was methanolysis. Ethanolysis also exhibited a much longer emulsion period and slower glycerol separation. The glycerol-rich phase was smaller in volume in ethanolysis than in methanolysis. Butanolysis remained one phase throughout, and no polar phase existed at any time. The results are consistent with the known phase compositions in these reactions. The concentrations of MG, DG, and TG in the products with time in stirred reactions were consistent with the observed phase behavior in the dye experiments.Paper no. J11376 in JAOCS 83, 1041 -1045 (December 2006. KEY WORDS:Base-catalyzed transesterification, biodiesel, ethanol, methanol, phase behavior.Biodiesel, a clean-burning, safe, and environmentally friendly transportation fuel, is being used to reduce emissions in many countries. Biodiesel is currently made by the transesterification of vegetable oils (TG) with methanol in the presence of basic catalysts to form FAME, which have significantly lower viscosities than the oils. Transesterification consists of three consecutive, reversible reactions: the TG is converted stepwise to DG, then MG, and finally glycerol, with methyl ester being formed in each step. Because the combustion of the glycerol moiety in MG, DG, TG, and glycerol can lead to the formation of acrolein, a photochemical smog ingredient, the ASTM standard for biodiesel limits the total glycerol moiety (G T ) in the fuel to 0.240 wt%, as determined by Equation 1, where G, MG, DG, and TG are the weight percentages of glycerol, MG, DG, and TG, respectively. G T = G + 0.26 (MG) + 0.15 (DG) + 0.1 (TG) [1]As a result, high conversion (>99% of the ester bonds) is required to achieve the glycerol limit set by the standard. However, the necessary conversion is not achievable in one single pass by current processes. In the biodiesel industry, biphasic base-catalyzed transesterification is the most common method for making biodiesel. Methanol and oils are immiscible, and vigorous stirring is required to promote the mass transfer between the oil and methanol phases. The catalyst, usually sodium methoxide, is exclusively in the polar methanol phase. During the transmethylation, the reaction mixture passes from a biphasic (methanol and oil) system to a biphasic (methyl ester-rich and glycerol-rich) system, probably via an emulsion. These phase transitions affect the kinetics and steady-state position of the reactions, which are c...

Dry washing in biodiesel purification: a comparative study of adsorbents

“…Both of these ions can cause the formation of insoluble soaps, which can be deposited in the motor and also promote catalytic polymerization reactions. [9][10][11] Reduction of the water-soluble contaminants is traditionally accomplished by water washing of the biodiesel. However, the water-wash method results in an aqueous effluent, which can then cause a detrimental environmental impact.…”

Monitoring of Biodiesel Transesterification Process Using Impedance Measurement

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First Generation Biodiesel

“…The transesterification reaction employing methanol commences as two immiscible phases as a result of the very low solubility of TAG in methanol (Boocock et al, 1996a;Boocock et al, 1996b;Zhou & Boocock, 2006a, which is about only 7.5 g of soybean oil soluble in 1 L of methanol at 30°C (Boocock et al, 1996b). 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 biodiesel industry currently uses sodium methoxide, because methoxide cannot form water upon reaction with alcohol such as with hydroxides, which influence the reaction and the quality of the production biodiesel (Zhou & Boocock, 2006a). Furthermore, base-catalyzed reactions are performed at generally lower temperatures, pressures, and reaction times and are less corrosive to industrial equipment than acid-catalyzed methods (Moser, 2009).…”