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Submitted to: Journal of the American Oil Chemists' Society
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 7/15/2008 Publication Date: 8/9/2008 Citation: Dunn, R.O. 2008. Crystallization behavior of fatty acid methyl esters. Journal of the American Oil Chemists' Society. 85:961-972. Interpretive Summary: Biodiesel is an alternative fuel derived from vegetable oil or animal fat. Biodiesel from the most common agricultural sources has flow properties that are susceptible to start up and operability problems in cold weather. This work investigates melting and crystallization properties of two high-melting point compounds mixed with a low-melting point solvent material (methyl oleate). These materials were selected because they are commonly found in biodiesel from soybean oil and because they could be obtained in very pure form. Data from analysis of pure compounds and binary solute-solvent mixtures were employed to determine activity coefficients for both solutes. Parameters from these analyses were then employed in a thermodynamic model based on freezing point depression theory to predict temperatures where crystallization in ternary mixtures (both solutes in methyl oleate solvent) initiates. Results showed that crystallization temperature (cloud point) of biodiesel may be predicted by freezing point depression theory if activity coefficients of the mixture compounds are known. Behavior in real solutions was found to have lower crystallization temperatures than calculated for ideal mixtures. Results from this study will assist fuel producers, distributors and terminal operators in predicting cold flow behavior of biodiesel during cold weather. Results will also directly benefit scientists and engineers studying effects of low temperatures on biodiesel. Technical Abstract: Biodiesel made from vegetable oils or animal fats has many attractive characteristics as an alternative fuel for compression-ignition (diesel) engines. However, biodiesel from the most common agricultural sources has flow properties that are susceptible to start up and operability problems during cold weather. Biodiesel from soybean oil is generally a mixture of long-chain fatty acid alkyl esters composed of 0.15-0.20 mass fraction saturated esters (high melting point [MP]) mixed with unsaturated esters (very low MP). This work investigates melting and crystallization properties of two saturated fatty acid methyl esters (FAME) commonly found in biodiesel from soybean oil. Differential scanning calorimetry (DSC) heating and cooling scans curves were analyzed for both solutes and solvent (methyl oleate) in pure form. Crystallization behavior in ternary FAME mixtures was calculated by application of thermodynamics based on ideal solution and freezing point depression (FPD) theory. Freezing point depression theory required determination of activity coefficients for both solutes in binary mixtures with methyl oleate from analysis of DSC cooling curves. Eutectic points were predicted by both models. Calc. crystallization onset temperatures were more accurately predicted by FPD theory than for ideal solutions when compared with measured data from DSC cooling curve analyses of ternary FAME mixtures. This work showed that crystallization onset temperature (cloud point) of biodiesel may be predicted by freezing point depression theory if activity coefficients of the components are known. |
