Thermodynamics of biofuels: Excess enthalpies for binary mixtures involving ethyl 1,1-dimethylethyl ether and hydrocarbons at different temperatures using a new flow calorimeter

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Abstract

Excess molar enthalpies for the binary systems: (ethyl 1,1-dimethylethyl ether + heptane); (ethyl 1,1-dimethylethyl ether + cyclohexane); (ethyl 1,1-dimethylethyl ether + toluene); (cyclohexane + toluene), and (toluene + heptane) have been measured at T = (298.15 and 313.15) K using a new isothermal flow calorimeter developed in the laboratory. The technique was previously checked by measuring test systems. The experimental results have been correlated with the Redlich–Kister polynomial equation. The mixing effects observed and the influence of the temperature are discussed.

Introduction

The transport sector accounts for more than 30% of final energy consumption in the European Community and it is expanding, a trend which is bound to increase, along with carbon dioxide emissions. Greater use of biofuels for transport forms a part of the package of measures needed to comply with the Kyoto Protocol. There is a wide range of biomass that could be used to produce biofuels, deriving from agricultural and forestry products. Promotion of the production and use of biofuels could contribute to a reduction in energy import dependency and in emissions of greenhouse gases. In addition, biofuels, in pure form or as a blend, may in principle be used in existing motor vehicles and use the current motor vehicle fuel distribution system. The blending of biofuel with fossil fuels could facilitate a potential cost reduction in the distribution system in the Community.

In this context, ethyl 1,1-dimethylethyl ether, better known as ethyl tert-butyl ether or ETBE, is synthesized from isobutylene and ethanol and the use of ethanol of vegetable origin in its manufacture process, increases the interest of ETBE or bio-ETBE as oxygenated additive.

Recently, a series of measurements of ternary and binary mixtures containing ethyl tert-butyl ether ETBE have been started, and the first results concerning (vapor + liquid) equilibrium data have been already published [1], [2], [3], [4]. The heat capacity of ETBE as a function of temperature and pressure has also been measured [5]. In this paper, excess molar enthalpies for five binary mixtures at T = (298.15 and 313.15) K are reported and a new flow calorimeter is presented.

Section snippets

Materials

Cyclohexane, heptane, and toluene were purchased from Fluka Chemie AG and were of the highest purity available, chromatography quality reagents (of the series puriss. p.a.) with a purity >0.995 (by gas chromatography, GC) for heptane and cyclohexane and >0.997 (by gas chromatography, GC) for toluene. Their purities were also checked by gas chromatography and all were found to be >0.997.

Ethyl 1,1-dimethylethyl ether was supplied by La Coruña Refinery of REPSOL-YPF, it was purified by

Apparatus

Excess enthalpies have been measured with a quasi-isothermal flow calorimeter, developed in our laboratory. A scheme of the equipment is shown in figure 1.

Two precision isocratic pumps with dual floating pistons in series (Agilent 1100 normally used in HPLC chromatography) deliver the liquids through the tubes at programmable constant flow rates into the mixing coil placed on the bottom of the cell.

The cell is immersed in a water bath, (Hart Scientific, model 7041) thermostated at the same

Results

The calorimeter has been tested by measuring the enthalpy of mixing of the system (cyclohexane + hexane), which has been recommended as a test system for excess enthalpy studies and there are values at atmospheric pressure available in the literature [7], [8], [9]. The experimental results of the excess molar enthalpies as a function of the composition are given in table 2. The comparison between our results and those used as references are shown in table 3 and also graphically in figure 2.

The

Discussion

Four of the five systems measured give positive excess enthalpies, this endothermic behaviour is always greater at T = 298.15 K than at T = 313.15 K. This behaviour is shown in figure 3 where the experimental values of the excess molar enthalpies as a function of the composition are represented.

The highest effect is observed when cyclohexane and toluene are mixed, the maximum excess molar enthalpy is HmE=625J·mol-1 for equimolar composition at T = 298.15 K, and it decreases to HmE=595J·mol-1 at T = 313.15 

Acknowledgment

Support for this work came from the Spanish Ministry of Education Project ENE2006-133 and from the Junta de Castilla y León reference GR152.

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