Elsevier

Fluid Phase Equilibria

Volume 540, 15 July 2021, 113046
Fluid Phase Equilibria

Surface tensions of biodiesel blends with pentanol and octanol isomers at different conditions: measurement and new correlation

https://doi.org/10.1016/j.fluid.2021.113046Get rights and content

Highlights

  • Experimental surface tensions of liquid mixtures of biodiesel + pentanol, and + octanol isomers at different temperatures.

  • A new equation to correlate the surface tension of biodiesel mixtures as a function of the temperature and mole fraction.

  • The new equation correlates the surface tension better than commonly used equations.

Abstract

Surface tension is an important biodiesel property, its calculation and prediction are fundamental because high values of surface tension significantly difficult proper atomization, and it can cause a poor engine performance. Surface tensions have been measured for binary liquid blends of pentan-1-ol, pentan-2-ol, 2-methylbutan-1-ol, octan-1-ol, octan-2-ol and 2-ethylhexan-1-ol with biodiesel using a Du Noüy ring tensiometer from (288.15 to 338.15) K at 0.1 MPa over the whole composition range. The measured surface tension data for the pure components shows a good agreement for the six alcohols when it is compared with reported values in the literature, showing an average absolute percentage deviation of 0.93%. Surface tension deviations show positive deviations from the average mole fraction of the pure components and they are correlated using the Redlich-Kister equation over the whole concentration range. Surface entropy and surface enthalpy have been estimated from experimental surface tension values of biodiesel mixtures. A new equation has been developed to correlate the surface tension of biodiesel + alcohols mixtures considering the effects of both temperature and mole fraction. This equation is based upon a quadratic mixing rule for the Gibbs free energy. Our model demonstrates to correlate the surface tension of biodiesel mixtures with an average absolute percentage deviation of 0.4%. The correlative capability of the proposed model is compared with Li et al., Fu et al., Myers and Scott, Wang-Chen, Hoke and Patton, and Jouyban et al. equations.

Introduction

Biodiesel is an alternative or complementary fuel to conventional diesel for compression ignition internal combustion engines. Biodiesel can be produced from vegetable or animal oil by the transesterification reaction to form Fatty Acid Methyl Esters (FAME) [1], [2], [3], [4], [5]. Biodiesel from animal oil contains mainly saturated FAME, C16:0 and C18:0 (higher than 40%), and monounsaturated FAME, C18:1 (between 30-40 %), among others [6]. The high content of saturated fatty acids increases the cetane number (more than 50 for lard and tallow), favoring a better ignition quality [7]. On the other hand, the biodiesel composition from vegetal oil preferably contains unsaturated FAME, C18:1 and C18:2 (higher than 70%) [8]. Regardless of the production source, biodiesel has to meet the standards established by EN14214 in Europe and ASTM 6751 in the United States to be used.

Biodiesel has advantages over the conventional diesel: it is biodegradable; non-toxic; sulfur free; it decreases the acid rain [9], [10], [11] it has higher lubricity than diesel favoring the engine's life [10]; and it has a higher flash point contributing to the storage and transport security [10]. However, biodiesel has some disadvantages with respect to conventional diesel such as: lower calorific value, increasing fuel consumption; and low oxidation stability, shortening it is storage time [10,11]. Also, biodiesel has a higher density, viscosity and surface tension comparing to those of diesel. These differences affect the volume of fuel supplied, the combustion efficiency, and increase the energy needed to transport the fuel through the fuel pump, transfer valve and fuel filter [12]. Nevertheless, high values in surface tension make the drop formation difficult, decreasing the quality of the atomization and the engine performance. Proper atomization helps the fuel-air mixing and reduces the emission of toxic contaminants [13].

Surface tension of biodiesel can be reduced using alcohols as additives. Methanol and ethanol are normally used to improve the thermodynamic and transport properties of biodiesel. Nonetheless, these alcohols show some disadvantages as limited miscibility, low calorific value, low octane number, and low lubricity [14], [15], [16]. Recently, long chain alcohols as pentanol and octanol isomers have emerged as an alternative to compensate the disadvantages of short-chain alcohols. Likewise, pentanol and octanol isomers decrease the biodiesel surface tension.

The knowledge of new experimental data of surface tension of biodiesel + pentanol, and + octanol isomers blends is essential for the design of fuel injection system (injection pump, high pressure pipe, injection nozzle, and feed pump) and the atomization study of the fuel in a combustion chamber. Unfortunately, experimental surface tension values of biodiesel + pentanol, and + octanol isomers blends have not been reported in the literature.

Having experimental values of surface tension is fundamental for the development of new predictive models as a function of temperature and alcohol concentration. In literature, there are some models reported to correlate the surface tension of liquid mixtures [17], [18], [19], [20], [21], [22], [23].

In the present work, experimental surface tensions of liquid mixtures of biodiesel blended with pentan-1-ol, pentan-2-ol, 2-methylbutan-1-ol, octan-1-ol, octan-2-ol and 2-ethylhexan-1-ol are measured in the temperature range of (288.15 to 338.15) K and pressure of 0.1 MPa over the whole composition range. Values of the surface tension deviation of these mixtures have been correlated using the Redlich-Kister equation [24]. Furthermore, we develop a new equation to correlate the surface tension of biodiesel mixtures as a function of the temperature and mole fraction. This equation is based upon a quadratic mixing rule for the Gibbs free energy [25]. In addition, we have tested the correlative capability of our equation with proposed models in the literature by Li et al. [17], Fu et al. [18], Myers and Scott [19], Wang-Chen [20], Hoke and Patton [22], and Jouyban et al [23].

Section snippets

Chemicals

Pentan-1-ol (99.8% in mass fraction, Sigma-Aldrich), pentan-2-ol (99.7% in mass fraction, Merck), 2-methylbutan-1-ol (99.9% in mass fraction, Merck), octan-1-ol (99.0% in mass fraction, Sigma-Aldrich), octan-2-ol (99.5% in mass fraction, Sigma-Aldrich), and 2-ethylhexan-1-ol (99.1% in mass fraction, Sigma-Aldrich), are acquired. Table 1 shows the purity, purification method, analysis method, and the CAS registry number of the reagents mentioned. The mixtures are prepared with

Literature Models

There are several models reported to correlate the surface tension of liquid mixtures [[17], [18], [19], [20],22,23]. Li et al. [17] develop an equation to predict the surface tension of liquid mixtures based on Wilson's model to represent Gibbs free energy as,σm=(x1σ1+x2σ2)x1x2RTx2+x1Λ21(Λ21Λ)[11Λ21]where σm is the surface tension of the mixture in mN∙m−1; R is the gas constant (8.314 J·mol−1·K−1); T is the temperature in Kelvin; Λ21Λ and Λ21 are fitting parameters at each temperature; xi

Results and discussion

In this study, we have modeled and measured surface tensions of liquid mixtures of biodiesel blended with pentan-1-ol, pentan-2-ol, 2-methylbutan-1-ol, octan-1-ol, octan-2-ol, and 2-ethylhexan-1-ol from (288.15 to 338.15) K over the whole composition range and at atmospheric pressure. The fatty acid methyl esters composition of biodiesel has been determined by GC, as shown in Table 2. Also, characterization of the biodiesel has determined in accordance with the methods established in the EN

Conclusions

This paper reports surface tensions of biodiesel blended with pentan-1-ol, pentan-2-ol, 2-methylbutan-1-ol, octan-1-ol, octan-2-ol, and 2-ethylhexan-1-ol from (288.15 to 338.15) K at 0.1 MPa. The surface tensions determined in this work for the pure alcohols agree with the literature data within an AAPD of 0.93%. Surface tension deviations show positive deviations from an average mole fraction of the pure components for the six systems investigated in this work. Positive deviations, Δσ,

Supporting information

Experimental densities of pure components (octan-1-ol, octan-2-ol, and 2-ethylhexan-1-ol) and biodiesel mixtures from (288.15 to 338.15) K with corresponding literature sources given in references [51], [52], [53], [54], [55], [56], [57], [58], [59], [60], [61], [62], [63], [64], [65], [66], [67], [68], [69], [70].

CRediT authorship contribution statement

Leidy T. Vargas-Ibáñez: Formal analysis, Investigation, Methodology. José J. Cano-Gómez: Conceptualization, Formal analysis, Writing - original draft, Writing - review & editing. Iván A. Santos-López: Writing - review & editing. Gustavo A. Iglesias-Silva: Supervision. José de los S. López-Lázaro: Software, Supervision. Mónica M. Alcalá-Rodríguez: Data curation. Carolina Villarreal-Mendoza: Data curation. Carolina Armendáriz-Ovalle: Data curation.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the Consejo Nacional de Ciencia and Tecnología (CONACyT) [CB-2016-285320].

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