Experimental investigation and molecular dynamic simulation of thermophysical properties of biodiesel surrogates: The binary mixtures of n-hexadecane with ethyl hexanoate and ethyl heptanoate

https://doi.org/10.1016/j.molliq.2020.113980Get rights and content

Highlights

  • The binary mixtures of two FAEEs and n-dodecane might be used as model fuels.

  • Thermophysical properties of the selected systems were investigated.

  • Surface light scattering method was used for the determination of surface properties.

  • Molecular dynamic simulation method was used for the simulation of thermophysical properties.

  • Correlations were proposed for the investigated thermophysical properties.

Abstract

The thermophysical properties of biodiesel surrogates are of great importance for the development of the new and clean fuel substitutes or additives. Experimental investigation and molecular dynamic simulation of the related thermophysical properties such as the liquid density, kinematic viscosity and surface tension over a sufficient wide temperature range will facilitate the corresponding researches of selecting the proper fuel surrogates and designing the spray system of the internal combustion engine. Therefore, the present study investigated liquid surface tension and kinematic viscosity of the proposed physical biodiesel surrogates of n-hexadecane with ethyl hexanoate and ethyl heptanoate by the surface light scattering method at three mole fractions (0.25, 0.50 and 0.75) over the temperature range from (353.15 to 433.15) K. Additionally, the liquid density was also determined by a U-tube densimeter in the temperature range between (293.15 and 433.15) K. Meanwhile, three force fields including OPLS-AA, GROMOS and AMBER were selected and validated with n-hexadecane and ethyl heptanoate in a wide temperature range and the GROMOS force field was demonstrated to be the best one for the description of the three thermophysical properties of the two fluids. A further molecular dynamic simulation with GROMOS force field was applied to the proposed binary surrogates of n-hexadecane with ethyl hexanoate and ethyl heptanoate, and the results for the three properties agreed well with their experimental values over the entire temperature range.

Introduction

In light of the huge pollution released by the diesel-powered vehicles, clean and environmental friendly fuel substitutes or additives have drawn great concerns in recent years. Biodiesel is such kind of fuel, which is normally utilized as a drop-in replacement of fossil fuel or additives primarily due to its similar thermophysical properties but a large proportion of oxygen content in comparison with fossil fuel. By adding biodiesel into fossil diesel, the amount of pollution such as CO, NOx as well as fine particles could be reduced significantly. Biodiesel can be driven from many kinds of feedstock such as animal fat or plant oil by trans-esterification [1]. Most biodiesels are mixtures of several long chain fatty acid methyl or ethyl esters, and thousands of species could be generated from chemical reactions during their combustion, which makes the difficulty for the kinetic description of biodiesels [2]. Therefore, surrogates for different biodiesels are proposed by a combination of the short to middle chain fatty acid methyl or ethyl esters to represent either the fuel spray characteristics or their vapor-phase combustion [3]. The former one is known as physical surrogate which is related to the thermophysical properties of fuel, such as density, viscosity and surface tension etc. While the latter one is called chemical surrogate. As for the formulation of the physical surrogates, it is particularly crucial to match the thermophysical properties of the surrogate to real fuel, which is normally achieved by developing the binary or ternary mixtures containing short chain methyl or ethyl esters and n-alkanes. Since n-hexadecane has been utilized as the diesel surrogate and ethyl hexanoate, ethyl heptanoate could be adopted as biodiesel surrogates, their binary mixtures could be proposed to be the physical surrogate of the complex mixtures of diesel and biodiesel. A series of thermophysical properties could be generated by varying the composition of the mixtures, which could be selected to meet certain requirement of the target application.

Experimental way of obtaining the accurate thermophysical properties like the liquid density, kinematic viscosity and surface tension over sufficient wide temperature and pressure ranges is highly necessary. However, for most cases, performing an experiment is also expensive, time-consuming and even difficult especially at elevated temperature and pressure. In this case, molecular dynamic (MD) simulation becomes a crucial way to extend the limited experimental investigation to even wider ranges of temperature and pressure. While the experimental data could be served as reference data to validate or parameterize force fields in MD simulation. There are many force fields available in the literatures which are developed for charactering certain aspects of specified systems. The GROMOS force field, for example, was developed to model the properties of alkanes, proteins and some organic molecules [[4], [5]]. Other force fields, such as TraPPE-UA, OPLS, L-OPLS, OPLS-AA, Lipid14, MARTINI, etc. are frequently adopted for modeling the thermophysical properties of alkanes covering a sufficient wider temperature range [[6], [7], [8], [9], [10], [11], [12]]. Papavasileiou et al. [11] investigated the density, surface tension and viscosity of n-dodecane, n-octacosane and their mixture in the temperature range from 323.15 K to 573.15 K with TraPPE-UA, L-OPLS, Lipid14 and MARTINI by MD simulations, and the results illustrated that the TraPPE-UA reproduced the three properties accurately and MARTINI performed well for the surface tension and viscosity, but failed to capture the density. Zang et al. [8] performed an experimental investigation and molecular dynamic simulations for the density of methyl nonanoate, n-dodecane and their mixture with GROMOS-UA, CHARMM27, AMBER96 and OPLA-AA, and found that AMBER96 and OPLS-AA performed equally well for the modeling of the density at the temperature range from (293.15 to 463.15) K. Morrow et al. [10] studied the vapor-liquid equilibrium property of n-dodecane, n-hexadecane, n-octadecane, 1-dodecene and isooctane by MD simulation with long-range LJ interactions using the force fields of CHARMM, OPLS-AA and TraPPE-UA. They pointed out that TraPPE-UA developed for the modeling VLE behavior yielded most accurate results but this united-atom type potential might fail to represent the fluid structure at high pressure that could be correctly modeled by all-atom force fields such as OPLS-AA and CHARMM.

A literature review for thermophysical properties of n-hexadecane, ethyl hexanoate and ethyl heptanoate was provided in our previous work [[13], [14]]. Meanwhile, Table 1 also listed some literature studies applied to the alkanes or their mixtures by MD simulations. To the best of our knowledge, no study has been found on the validation of the force fields that are applicable to the pure substances of fatty acid methyl or ethyl esters and their mixtures with alkanes. Therefore, in the present work, the thermophysical properties including liquid density, surface tension, and kinematic viscosity of the proposed binary biodiesel surrogates containing n-hexadecane with ethyl hexanoate and ethyl heptanoate were investigated by an experimental way and MD simulation at the sufficient wide temperature range. The experimental thermophysical property data and the validated force fields obtained in the present work could be served as a sound basis to benefit the scientific research on the spray system in the internal combustion engine and to facilitate the application of biodiesel as clean fuels or fuel additives.

Section snippets

Materials

n-Hexadecane was purchased from Sigma-Aldrich. Ethyl hexanoate and ethyl heptanoate were provided by Aladdin Chemistry Co. Ltd. The specification of chemical samples is listed in Table 2.

Methods and procedures

Surface light scattering (SLS) method was used to simultaneously determine the liquid kinematic viscosity and surface tension of the proposed binary mixtures in the present work. This technique has been successfully applied to investigate the two properties of the similar systems containing fatty acid ethyl and

Force fields

A suitable force field applicable to the present systems is necessary to be determined firstly since the reliability and accuracy of the MD simulation for the specified systems strongly rely on the preciseness of the force fields. Our previous work [8] pointed out that OPLS-AA and AMBER could reproduce the liquid density of methyl nonanoate, n-dodecane and their binary mixtures with the deviations from experimental data within ±4% in the temperature range from (293 to 463) K. While the

Liquid density

The experimental liquid density data of the binary mixtures of n-hexadecane with ethyl hexanoate and ethyl heptanoate between (293.15 and 433.15) K close to saturation conditions were listed in Table 4.

The density data of the mixtures were furtherly correlated by a linear combination of the density of the corresponding pure substances scaled according to their mole fractions and an additional term of the reciprocal temperature as a correction. The full correlation equation was shown as follows,ρ

Pure substances

For the validation of the selected force fields which were applied in the present study to model the thermophysical properties of the alkanes and fatty acid ethyl esters, MD simulation was performed for the pure substances, n-hexadecane and ethyl heptanoate over a wider temperature range. As shown in Fig. 8(a) and (b), the density data of n-hexadecane and ethyl heptanoate from the MD simulation, density equations picked from literatures as well as the deviations of simulated values from the

Conclusion

In this work, the thermophysical properties of the proposed biodiesel surrogates of n-hexanoate with ethyl hexanoate and ethyl heptanoate were investigated with an experimental way and MD simulation over the temperature range between (353.15 and 433.15) K. On the basis of the experimental liquid density, surface tension and viscosity data, corresponding correlation equations were proposed. Three force fields were adopted in the MD simulation and the GROMOS force field was demonstrated to have

CRediT authorship contribution statement

Guanjia Zhao: Conceptualization, Methodology, Writing - review & editing. Zemin Yuan: Investigation, Writing - original draft, Writing - review & editing, Data curation. Xiaona Liu: Investigation, Data curation. Penglai Wang: Formal analysis, Software. Jianguo Yin: Formal analysis. Suxia Ma: Resources.

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

The authors are thankful for the financial support provided by National Natural Science Foundation of China (51976132, 51506140), Key Research and Development Plan of Shanxi (201803D121096), and Natural Science Foundation of Shanxi (201901D211048).

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