High-Pressure Soybean Oil Biodiesel Density: Experimental Measurements, Correlation by Tait Equation, and Perturbed Chain SAFT (PC-SAFT) Modeling

Abstract

Biodiesel can easily become the crucial solution for environmental problems. The high production rate of soybean oil has been the subject of several research works to transform it into biodiesel. Knowledge of the thermodynamic properties of soybean oil biodiesel (SOB) such as densities and coefficients of expansivity and compressibility play an important role in the understanding of the intermolecular interactions between the different molecules, which in turn have an impact on fuel quality. The difficulty in measuring the thermodynamic properties of biodiesel is because they are complex structures and high-molecular-weight components. The experimental density (136 points) for SOB, as a pseudopure component, at several temperatures (298.15–393.15 K) and pressures up to 140 MPa is reported. An Anton Paar vibrating tube densimeter, calibrated with an uncertainty of ±0.7 kg m–3, was used to perform these measurements. To determine the chemical fatty acid methyl ester composition, SOB was analyzed by CHNS analysis, 1H NMR, 13C NMR, and gas chromatography–mass spectrometry and, then, the density experimental data were correlated by the Tait and perturbed chain-statistical associating fluid theory (PC-SAFT) equations of state (EoS). The experimental data were compared with correlated data, resulting in absolute average deviation (AAD = 0.01%), maximum deviation (MD = 0.03%), average deviation (Bias = −9.88 × 10–7%), and standard deviation (σ = 1.18 × 10–4 g cm–3) for the empirical Tait equation. Concerning PC-SAFT EoS, the density was reasonably correlated with AAD = 0.063%. On the other hand, isothermal compressibility, κ T , and isobaric thermal expansivity, α p , were derived from the Tait equation. The same behavior is observed for κ T and α p , consistent with the expected one. The isobaric thermal expansivity, α p , presents a crossing point at nearly 35 MPa, in agreement with what had been observed by other authors.

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