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Fuel 86 (2007) 323–327

Molar enthalpy of vaporization of ethanol–gasolinemixtures and their colloid state

Roman M. Balabin *, Rustem Z. Syunyaev, Sergey A. Karpov

Gubkin Russian State University of Oil and Gas, 119991 Moscow, Russian Federation

Received 15 June 2006; received in revised form 4 August 2006; accepted 8 August 2006Available online 8 September 2006

Abstract

Vapour pressure measurements are used to evaluate the enthalpy of vaporization of ethanol–gasoline mixtures. Partial molar values arealso derived. The dispersed structure of ethanol–gasoline fuel is studied for the first time using the method of correlation spectroscopy ofscattered light. A large range of dispersed particle sizes in different alcohol–gasoline systems is found. The dependence of the mean radiusof drops on ethanol content is determined. It is found that coalescence phenomenon occurs in the systems when extra ethanol is added.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Ethanol–gasoline fuel; Enthalpy of vaporization; Dispersed structure

1. Introduction

Nations today often face divergent challenges in the formof climate change, air pollution, energy production, con-sumption security, and shrinking oil supplies. In responseto these challenges, countries around the world havedeveloped programs to support the use of clean fuels,including ethanol [1,2].

The properties of gasoline have been altered in recentyears to reduce motor vehicle emissions of carbon monox-ide, photochemical smog precursors, and toxic organic airpollutants such as benzene. Changes have been made tosulphur, olefin, and aromatic contents, and to distillationproperties of gasoline.

Presently, there is an increasing interest in adding oxy-genated compounds to gasoline, because of their octane-enhancing and pollution-reducing capabilities. In the lastseveral years, many interesting works on ternary, quater-nary, or quinary systems that contain a synthetic reformate(hydrocarbon mixtures), an oxygenated compound (ethers

0016-2361/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.fuel.2006.08.008

* Corresponding author. Tel.: +7 926 592 7920; fax: +7 495 335 8639.E-mail address: balabin.r@gubkin.ru (R.M. Balabin).

or alcohols), and water, at approximately ambient temper-atures, have been published [3–19]. However, studies ofsystems that contain gasoline, an oxygenated compound,and water are not often found in the literature [20].

Among the oxygenated compounds, ethers and alcoholsare the most important. Currently, among ethers and alco-hols, ethanol has been receiving more attention [21,22].

Controversy has surrounded another major fuel change:the addition of methyl tert-butyl ether (MTBE) [23,24].Presently, there is no requirement that a specific oxygenatebe added to gasoline. However, the use of MTBE in gaso-line was phased out in California at the end of 2002 due inpart to concerns about surface water and groundwater con-tamination. Likewise, the United States Environment Pro-tection Agency (USEPA) intends to significantly reduce theuse of MTBE in gasoline nationwide [25]. These decisionswill lead to greater dependence on ethanol–gasoline blendsand gasoline formulations that do not contain oxygenatedcompounds.

One of the major difficulties encountered in the use ofalcohol–gasoline blends is their tendency to phase-separateon contact with small amounts of water.

Vapour pressure is one of the most important physicalproperties of gasoline mixtures because it defines the

324 R.M. Balabin et al. / Fuel 86 (2007) 323–327

volatility of the mixtures. Vapour pressure can be deter-mined by a variety of methods that are rather time-con-suming. The refinery industry utilizes the Reid methods[26] to determine vapour pressure, which is related to thegasoline performance characteristics and to its storagebehaviour [27].

Evaporative emissions occur by a wide variety of mech-anisms, including fuel spillage and vapour displacementduring refuelling, venting of fuel tank vapours as ambienttemperature changes, fuel evaporation from the enginecompartment of parked vehicles due to residual engineheat, liquid leaks in vehicle fuel systems, and so on [28].

There is concern about increased evaporative emissionsof volatile organic compounds (VOCs) when ethanol isblended with gasoline, because such blends tend to havehigher Reid vapour pressure (RVP) than equivalentMTBE-blended fuel [24].

The thermodynamic properties of ethanol–gasoline mix-tures have not been widely covered in the literature (withthe exception of heat capacity [1,29,30]), but these proper-ties are necessary as they serve as basic thermodynamicdata for the investigation of clean fuels. Enthalpy of vapor-ization is the most important thermodynamic constant thatdescribes liquid–gas equilibrium.

The polarity of ethanol and nonpolarity of gasolinehydrocarbons are the reasons for the possible colloid stateof ethanol–gasoline mixture. Particle size (the main param-eter of any colloid system) was never reported for ethanol-containing fuel.

In this paper, vapour pressure measurements of puregasoline and its mixtures at three different temperatures(20, 40, and 60 �C) using a MINIVAP VPS Vapour Pres-sure Tester are presented. Enthalpy of vaporization is cal-culated using the Clausius–Clapeyron equation. Partialmolar enthalpy of vaporization of ethanol and gasoline isalso evaluated.

The colloid state of ethanol–gasoline mixtures is alsoexamined by scattered light correlation spectroscopy.

2. Experimental section

2.1. Materials

Table 1 lists the properties of two typical unleadedgasoline (supplied by Yukos Oil Company), called A and

Table 1Parameters of gasoline used in this paper

Parameter Units Value

Gasoline A Gasoline B

Motor octane number (MON) – 83.0 76.0Research octane number (RON) – 92.0 80.0Density at 20 �C kg/m3 750 725Actual gums mg/100 cm3 4.8 4.9Sulphur content wt% 0.05 0.05Benzene content vol% 5.1 2.5Water content in added ethanol vol% 4.0 <0.5

B, used in the experiments. Two types of ethanol withdifferent water content – 4 vol% in line A (with gasolineA) and <0.5 vol% in line B (with gasoline B) – wereused.

Alcohol–gasoline mixtures were prepared in confor-mance with standard procedures without any special equip-ment.

Line A: 0, 1, 2, 3, 4, 5, and 10 vol% of ethanol in gaso-line A.Line B: 0, 2, 4, 6, 8, and 10 vol% of ethanol in gasolineB.

Samples of line A (with 4% of water in ethanol) showedinstability and phase-separation tendency. Samples of lineB were stable. It confirms that water content in the alco-hol–gasoline mixture is the most important parameter thatdefines system phase stability.

2.2. Methods

All measurements of vapour pressure were performedusing a MINIVAP VPS vapour pressure tester (GRAB-NER INSTRUMENTS GmbH). This instrument is usedto determine the vapour pressure of low-viscosity petro-leum products. Tests can be carried out at temperaturesranging from 20 �C to 60 �C and at a vapour-to-liquid ratioof 4:1, with the condition that the resulting pressures donot exceed 1000 kPa. The accuracy of the temperaturereadings is 0.1 �C. Repeatability of vapour pressure mea-surements was 0.5 kPa, reproducibility – 1.6 kPa.

The vacuum in MINIVAP VPS is achieved by a pumpthat is capable of achieving and maintaining a pressureof better than 0.1 kPa. The vapour-to-liquid ratio is deter-mined by the volume of the sample used.

All the samples were prepared in strict conformancewith the requirements of ASTM D-5191. In all measure-ments performed in this study, the corresponding val-ues were always within the range recommended by themanufacturer.

The study of the sizes of scattered objects in alcohol–gasoline mixtures was made using the method of correla-tion spectroscopy of scattered light. He–Ne laser was usedas a radiation source. The beam of the light scattered bythe sample (angle of scattering was 90�) was divided afterdiaphragm into two beams, which were directed to thetwo photoelectronic multipliers HAMAMATSU R6358P,working in the photon count regime. The signals formedby the multipliers were intensified by the same amplifiers.The signals from the amplifiers were directed to the loga-rithmic 32-bit correlator PHOTOCOR-FC, measuring thecorrelation function in the real-time scale. The obtainedcorrelation function was equivalent to the correlation func-tion of the light, scattered by the sample, because photo-electronic multipliers registered the light from the samescattered volume and the noises of the two multipliers werenot correlated.

R.M. Balabin et al. / Fuel 86 (2007) 323–327 325

It is known that for monodisperse scattering objects insolution:

C ¼ Dq2 ð1Þ

where C is the diffusion expansion, characterizing the widthof scattered light spectral contour; D is the diffusion coeffi-cient; q is the vector of scattering, which is given by

q ¼ 4pnk

sinh2

ð2Þ

where h is the angle of scattering, k is the radiation length,n is the refraction index of the environment.

The hydrodynamic radius R of spherical scattering par-ticles is determined from the Stokes–Einstein relation:

D ¼ kBT6pg � R ð3Þ

where g is the viscosity of the environment, T is the temper-ature, kB is Boltzmann’s constant.

If particles are polydispersed, R is a hydrodynamicradius corresponding to an average of the type [31]

R ¼P

iR6iP

iR5i

ð4Þ

3. Results and discussion

3.1. Vapour pressure

Measured vapour pressure is presented in Fig. 1 for eachof the ethanol-containing gasoline samples (line A and B).Both the graphs are extremal curves. The maximum ofvapour pressure is near 4 vol% for line A and 3 vol% forline B.

This result shows the nonlinear behaviour of polar eth-anol in nonpolar gasoline hydrocarbons mixture. Ethanol–

Fig. 1. Vapour pressure of ethanol–gasoline mixture samples of both lines(A and B) at 20 �C.

gasoline mixtures were not expected to approach idealbehaviour (so Raoult’s law cannot be used). This resultwas repeatedly reported in both experimental and theoret-ical works but theoretical background of this phenomenonneeds further exploration. The behaviour of this kind canregard to azeotrope [32] or can be explained as exceed ther-modynamic property (by activity coefficients) [27,28,33].We should note that structure of hydrocarbons formingazeotrope with ethanol in alcohol–gasoline mixture is stillunknown.

3.2. Enthalpy of vaporization

The enthalpy of vaporization was calculated using thewell-known Clausius–Clapeyron equation for the evapora-tion process:

d ln pdT¼ DH vap

RT 2ð5Þ

where p is the vapour pressure at the temperature T; DHvap

is the enthalpy of vaporization (supposed to be constant).The plot of the logarithm of ethanol–gasoline mixture

vapour pressure versus inverse thermodynamic tempera-ture is presented in Fig. 2. Sufficient coincidence with Eq.(5) is shown so enthalpy of vaporization can be assumedtemperature independent.

From the fitted linear expression, enthalpy of vaporiza-tion was evaluated for all samples in both lines.

3.3. Partial molar enthalpy of vaporization

Fig. 3 shows the enthalpy of vaporization DHvap versusthe molar ethanol part in alcohol–gasoline mixture. Toevaluate molar values, the molar weight of gasoline wasassumed to be 100 g/mol; thus, all the obtained numbersare approximate.

Fig. 2. Dependence of vapour pressure logarithm on temperature forsamples 2 vol% (line A) and 2 vol% (line B).

Fig. 3. Enthalpy of vaporization of ethanol–gasoline mixture versusethanol concentration.

Fig. 4. Mean radius of dispersed particles (alcohol–water drops) inethanol–gasoline mixture (line B) at 22 �C.

326 R.M. Balabin et al. / Fuel 86 (2007) 323–327

It is evident that the enthalpy of vaporization is a linearfunction of the ethanol (or gasoline) molar part; hence,partial molar values are constant. The values of DHvap forboth lines were fitted to the linear expressions by theleast-squares method. The results are

ðDH gasvapÞA � 37:3 kJ=mol

ðDH ethvapÞA � 45:9 kJ=mol

ðDH gasvapÞB � 35:4 kJ=mol

ðDH ethvapÞB � 47:9 kJ=mol

where DH vap is the partial molar enthalpy of vaporization.The values of partial molar enthalpy of vaporization of

ethanol obtained in ethanol–gasoline fuel are close to thevalues of ethanol DHvap reported in the literature (25 �C,101.325 kPa): 42.3 kJ/mol [34].

3.4. Particle size

The study of sizes of the scattering objects in line B wasmade using the method of correlation spectroscopy of scat-tered light.

The intensity of the signal from microdrops of alcohol(plus water) is low because of the little difference in therefraction index of gasoline hydrocarbons and ethanol.This leads to the great fallibility of defined colloid particles’sizes (Fig. 4). However, it does not make any real sensebecause the main idea of this part of the paper is the factthat ethanol–gasoline mixtures consist of a dispersed phaseand a dispersion medium.

The colloidal structures are assumed to be microspheres,although other structures are possible.

It is obvious that the size of the drops grows morequickly (R / x3) than expected when the number of parti-

cles is constant (R / x1/3). We suppose that coalescenceoccurs in the system when ethanol is added but other expla-

nations are possible. We expect that our future works willgive us exact answer.

4. Conclusions

One of the most important thermodynamic properties ofethanol–gasoline system – enthalpy of vaporization – wasstudied. Values of partial molar enthalpy of vaporizationwere reported for the first time. They are found to be con-stant. These values can help normalize alcohol–gasolineblended fuel volatility and make this fuel more competitivethan ordinary gasoline.

The dispersed structure of ethanol–gasoline mixture wasshown. The most important parameter of any colloid struc-ture, i.e., the radius of the dispersed particles, was reported.Its value is hundreds of nanometres and it dramaticallydepends on the concentration of ethanol. These data canhelp prevent phase-separation in ethanol–gasoline fuelcaused mainly by the coalescence of alcohol–water dropsand precipitation.

Changing the colloid structure of ethanol–gasoline sys-tems can be an effective way to put in order the qualitycoefficients of the fuel.

Acknowledgements

Balabin Roman is grateful to ITERA InternationalGroup of companies for a nominal scholarship. The authorsacknowledge the Yukos Oil Company for supplying gaso-line and the corresponding data.

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