Introduction

Biodiesels have been more and more attractive as one of the potential alternatives of fossil diesel because of its components friendly to the natural environment and less toxic gases emission during combustion process. Biodiesels are mainly composed of fatty acid methyl esters (FAMEs) and fatty acid ethyl esters (FAEEs). Ethyl myristate (Figure 1) is a member of FAEEs and produced from myristic acid which accounts for large amounts of total fatty acids in coconut and palm kernel oils (15–23?wt% and 15–17?wt%, respectively). Therefore, ethyl myristate is one of the main components of biodiesels.[1]Ethyl myristate is a member of fatty acid ethyl esters and produced from myristic acid that accounts for large amounts of total fatty acids in palm kernel and coconut oils.This material has been considered by many researchers.[2]

Chemical-physical properties of ethyl myristate

The chemical-physical properties of ethyl myristate is shown in Table 1.[2]

Synthesis of Ethyl Myristate

In this study, laboratory-scale equipment for continuous synthesis of ethyl myristate was built and the effects of the solvent water content and fluid flowrate on the reaction rate were systematically investigated.The enzymatic esterification of myristic acid with ethanol catalyzed by an immobilized lipase from Mucor Miehei has been performed under supercritical conditions.After previous experiments in supercritical carbon dioxide showing a high stability of this enzyme and leading to a kinetic description of the reaction in a stirred reactor, the reaction was realized in a continuous mode through a fixed-bed reactor. The effects of water concentration and fluid flowrate on the reaction rate are investigated. An increase of carbon dioxide moisture content from 0 to 0.25% (mass) causes an increase in the conversion whereas, beyond this value, the lipase activity is irreversibly altered. Production rate surprisingly increases with fluid flowrate when reactants flowrates are maintained constant, but decreases when reactant concentrations are kept constant, confirming a reaction mechanism including inhibition by ethanol.

During these experiments, operating parameters are maintained constant (P=12.5 MPa,T=323K) and ”water saturated” CO₂ is used so that the supercritical carbon dioxide water concentration is nearly 0.22% by wt. At first, the substrate mixture flowrate is kept constant (myristic acid:4.38 mmol/h,ethanol: 103 mmol/h) and supercritical carbon dioxide flowrate is varied from 0.1 to 0.4 kg/h. Surprisingly, the conversion rate increases with increasing CO₂ flowrate, although the mean residence time decreases in the process.This increase in the rate of conversion could be explained by substrates dilution. When the CO₂ flowrate increases from 0.1 to 0.4 kg/h, molar concentrations of myristic acid and ethanol vary from 14.6 and 343 mM to 6.25 and147 mM, respectively. Since ethanol has been proven to be an inhibitor for the reaction, it is probable that a higher dilution has a global positive effect on the converslon rate.During a second step, concentrations of myristic acid (31.3 mM) and ethanol (735.7 mM) are kept constant. The conversion rate.decreases exponentially when supercritical carbon dioxide flowrate increases as can be expected from the decrease in the mean residence time,confirming the precedent conclusions regarding conversion rate dependence on substrate concentration.[3]

Speed of sound and thermal diffusivity of ethyl myristate

In recent years, many researches have been carried out on the measurements and predictions of thermophysical properties of ethyl myristate, such as critical properties and viscosity. There are very scarce reports on the measurement of speed of sound and thermal diffusivity for ethyl myristate. Ndiaye et al. reported on the speed of sound in ethyl myristate in the temperature at T?=?293.15 to 403.15?K and under pressure from atmospheric pressure to 100?MPa. Dzida et al. measured the speed of sound in ethyl myristate within the temperatures from 293 to 318?K and at pressures from 0.1 to 101?MPa. There is no report on the thermal diffusivity measurement of ethyl myristate by now. Freitas et al. measured the speed of sound in ethyl myristate at atmospheric pressure from 293.15 to 343.15?K. Aissa et al. determined the speed of sound in ethyl myristate in the temperature ranging from 288.15 to 343.15?K at atmospheric pressure. However, the speed of sound and the thermal diffusivity are important properties for fuels. The speed of sound plays an important role in the injection of engines. Besides, it can be used to model some the thermodynamic properties, such as compressibility and heat capacity. The thermal diffusivity represents the capacity of temperature propagation in a substance and is commonly used to calculate the temperature distribution of fuels in the engines.

In this work, the speed of sound and thermal diffusivity of ethyl myristate were measured by the light scattering method. The investigated temperature and pressure is ranged from 303.15?K to 560.15?K and up to 10?MPa. The relative expanded uncertainties of the speed of sound and thermal diffusivity were estimated to be less than 1.3% and 2.6% (k=2), respectively. The correlations for the speed of sound and thermal diffusivity as function of temperature and pressure were presented based on the experimental data. The absolute average deviations (AADs) between the experimental data and the calculated results are 0.04% for the speed of sound at atmospheric pressure, 0.07% for the speed of sound at p?>?0.1?MPa and 0.28% for the thermal diffusivity, respectively.[1]

Effect of temperature on thermophysical properties of ethyl myristate

The dual-beam laser thermal lens spectroscopy is used for accurate measurement of thermophysical parameters of ethyl myristate. First, to check the accuracy of our optical setup, thermal lens characteristic time, thermal diffusivity and thermal conductivity of carbon disulfide and benzene were measured at 298 K. There was a reasonable agreement between the measured values and the reference values. Then, the effect of temperature changes on thermal diffusivity and thermal conductivity values of ethyl myristate was studied. The results showed that with increasing temperature from 285 to 358 K, thermal diffusivity decreases from 8.75 to 7.45×10^?8 m²/s, and thermal conductivity from 0.155 to 0.1432 W/mK. It was also shown that by changing in pump laser energy in the range of 0.06–0.12 joules, both thermal diffusivity and thermal conductivity of ethyl myristate are almost constant.[2]

References

[1] Zhang Y , Chen J , Zhan T ,et al.Speed of sound and thermal diffusivity of ethyl myristate[J].The Journal of Chemical Thermodynamics, 2019, 140:105899.DOI:10.1016/j.jct.2019.105899.

[2] Mohebbifar M R .Study of the effect of temperature on thermophysical properties of ethyl myristate by dual-beam thermal lens technique[J].Optik: Zeitschrift fur Licht- und Elektronenoptik: = Journal for Light-and Electronoptic, 2021(247-):247.DOI:10.1016/j.ijleo.2021.168000.

[3] Dumont T D , Barth D , Perrut M .Continuous Synthesis of Ethyl Myristate by Enzymatic Reaction in Supercritical Carbon Dioxide[J].Journal of Supercritical Fluids The, 1993, 6(2):85-89.DOI:10.1016/0896-8446(93)90022-P.