Molar Enthalpy Of Vaporization Of Ethanol–gasoline Mixtures And Their Colloid State

Fuel 86 (2007) 323–327 www.fuelfirst.com

Molar enthalpy of vaporization of ethanol–gasoline mixtures 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 2006 Available online 8 September 2006

Abstract Vapour pressure measurements are used to evaluate the enthalpy of vaporization of ethanol–gasoline mixtures. Partial molar values are also derived. The dispersed structure of ethanol–gasoline fuel is studied for the first time using the method of correlation spectroscopy of scattered light. A large range of dispersed particle sizes in different alcohol–gasoline systems is found. The dependence of the mean radius of drops on ethanol content is determined. It is found that coalescence phenomenon occurs in the systems when extra ethanol is added. Ó 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 form of climate change, air pollution, energy production, consumption security, and shrinking oil supplies. In response to these challenges, countries around the world have developed programs to support the use of clean fuels, including ethanol [1,2]. The properties of gasoline have been altered in recent years to reduce motor vehicle emissions of carbon monoxide, photochemical smog precursors, and toxic organic air pollutants such as benzene. Changes have been made to sulphur, olefin, and aromatic contents, and to distillation properties of gasoline. Presently, there is an increasing interest in adding oxygenated compounds to gasoline, because of their octaneenhancing and pollution-reducing capabilities. In the last several years, many interesting works on ternary, quaternary, or quinary systems that contain a synthetic reformate (hydrocarbon mixtures), an oxygenated compound (ethers

*

Corresponding author. Tel.: +7 926 592 7920; fax: +7 495 335 8639. E-mail address: [email protected] (R.M. Balabin).

0016-2361/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2006.08.008

or alcohols), and water, at approximately ambient temperatures, have been published [3–19]. However, studies of systems that contain gasoline, an oxygenated compound, and water are not often found in the literature [20]. Among the oxygenated compounds, ethers and alcohols are the most important. Currently, among ethers and alcohols, 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 oxygenate be added to gasoline. However, the use of MTBE in gasoline was phased out in California at the end of 2002 due in part to concerns about surface water and groundwater contamination. Likewise, the United States Environment Protection Agency (USEPA) intends to significantly reduce the use of MTBE in gasoline nationwide [25]. These decisions will lead to greater dependence on ethanol–gasoline blends and gasoline formulations that do not contain oxygenated compounds. One of the major difficulties encountered in the use of alcohol–gasoline blends is their tendency to phase-separate on contact with small amounts of water. Vapour pressure is one of the most important physical properties of gasoline mixtures because it defines the

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volatility of the mixtures. Vapour pressure can be determined by a variety of methods that are rather time-consuming. The refinery industry utilizes the Reid methods [26] to determine vapour pressure, which is related to the gasoline performance characteristics and to its storage behaviour [27]. Evaporative emissions occur by a wide variety of mechanisms, including fuel spillage and vapour displacement during refuelling, venting of fuel tank vapours as ambient temperature changes, fuel evaporation from the engine compartment of parked vehicles due to residual engine heat, liquid leaks in vehicle fuel systems, and so on [28]. There is concern about increased evaporative emissions of volatile organic compounds (VOCs) when ethanol is blended with gasoline, because such blends tend to have higher Reid vapour pressure (RVP) than equivalent MTBE-blended fuel [24]. The thermodynamic properties of ethanol–gasoline mixtures have not been widely covered in the literature (with the exception of heat capacity [1,29,30]), but these properties are necessary as they serve as basic thermodynamic data for the investigation of clean fuels. Enthalpy of vaporization is the most important thermodynamic constant that describes liquid–gas equilibrium. The polarity of ethanol and nonpolarity of gasoline hydrocarbons are the reasons for the possible colloid state of ethanol–gasoline mixture. Particle size (the main parameter of any colloid system) was never reported for ethanolcontaining fuel. In this paper, vapour pressure measurements of pure gasoline and its mixtures at three different temperatures (20, 40, and 60 °C) using a MINIVAP VPS Vapour Pressure Tester are presented. Enthalpy of vaporization is calculated using the Clausius–Clapeyron equation. Partial molar enthalpy of vaporization of ethanol and gasoline is also evaluated. The colloid state of ethanol–gasoline mixtures is also examined by scattered light correlation spectroscopy. 2. Experimental section 2.1. Materials Table 1 lists the properties of two typical unleaded gasoline (supplied by Yukos Oil Company), called A and Table 1 Parameters of gasoline used in this paper Parameter

Motor octane number (MON) Research octane number (RON) Density at 20 °C Actual gums Sulphur content Benzene content Water content in added ethanol

Units

– – kg/m3 mg/100 cm3 wt% vol% vol%

Value Gasoline A

Gasoline B

83.0 92.0 750 4.8 0.05 5.1 4.0

76.0 80.0 725 4.9 0.05 2.5 <0.5

B, used in the experiments. Two types of ethanol with different water content – 4 vol% in line A (with gasoline A) and <0.5 vol% in line B (with gasoline B) – were used. Alcohol–gasoline mixtures were prepared in conformance with standard procedures without any special equipment. Line A: 0, 1, 2, 3, 4, 5, and 10 vol% of ethanol in gasoline A. Line B: 0, 2, 4, 6, 8, and 10 vol% of ethanol in gasoline B. Samples of line A (with 4% of water in ethanol) showed instability and phase-separation tendency. Samples of line B were stable. It confirms that water content in the alcohol–gasoline mixture is the most important parameter that defines system phase stability. 2.2. Methods All measurements of vapour pressure were performed using a MINIVAP VPS vapour pressure tester (GRABNER INSTRUMENTS GmbH). This instrument is used to determine the vapour pressure of low-viscosity petroleum products. Tests can be carried out at temperatures ranging from 20 °C to 60 °C and at a vapour-to-liquid ratio of 4:1, with the condition that the resulting pressures do not exceed 1000 kPa. The accuracy of the temperature readings is 0.1 °C. Repeatability of vapour pressure measurements was 0.5 kPa, reproducibility – 1.6 kPa. The vacuum in MINIVAP VPS is achieved by a pump that is capable of achieving and maintaining a pressure of better than 0.1 kPa. The vapour-to-liquid ratio is determined by the volume of the sample used. All the samples were prepared in strict conformance with the requirements of ASTM D-5191. In all measurements performed in this study, the corresponding values were always within the range recommended by the manufacturer. The study of the sizes of scattered objects in alcohol– gasoline mixtures was made using the method of correlation spectroscopy of scattered light. He–Ne laser was used as a radiation source. The beam of the light scattered by the sample (angle of scattering was 90°) was divided after diaphragm into two beams, which were directed to the two photoelectronic multipliers HAMAMATSU R6358P, working in the photon count regime. The signals formed by the multipliers were intensified by the same amplifiers. The signals from the amplifiers were directed to the logarithmic 32-bit correlator PHOTOCOR-FC, measuring the correlation function in the real-time scale. The obtained correlation function was equivalent to the correlation function of the light, scattered by the sample, because photoelectronic multipliers registered the light from the same scattered volume and the noises of the two multipliers were not correlated.

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

It is known that for monodisperse scattering objects in solution: C ¼ Dq2

ð1Þ

where C is the diffusion expansion, characterizing the width of scattered light spectral contour; D is the diffusion coefficient; q is the vector of scattering, which is given by q ¼ 4p

n h sin k 2

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 particles is determined from the Stokes–Einstein relation: D¼

kBT 6pg  R

ð3Þ

where g is the viscosity of the environment, T is the temperature, kB is Boltzmann’s constant. If particles are polydispersed, R is a hydrodynamic radius corresponding to an average of the type [31] P 6 R R ¼ Pi i5 ð4Þ i Ri 3. Results and discussion 3.1. Vapour pressure

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gasoline mixtures were not expected to approach ideal behaviour (so Raoult’s law cannot be used). This result was repeatedly reported in both experimental and theoretical works but theoretical background of this phenomenon needs further exploration. The behaviour of this kind can regard to azeotrope [32] or can be explained as exceed thermodynamic property (by activity coefficients) [27,28,33]. We should note that structure of hydrocarbons forming azeotrope with ethanol in alcohol–gasoline mixture is still unknown. 3.2. Enthalpy of vaporization The enthalpy of vaporization was calculated using the well-known Clausius–Clapeyron equation for the evaporation process: d ln p DH vap ¼ dT RT 2

ð5Þ

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 temperature is presented in Fig. 2. Sufficient coincidence with Eq. (5) is shown so enthalpy of vaporization can be assumed temperature independent. From the fitted linear expression, enthalpy of vaporization was evaluated for all samples in both lines.

Measured vapour pressure is presented in Fig. 1 for each of the ethanol-containing gasoline samples (line A and B). Both the graphs are extremal curves. The maximum of vapour pressure is near 4 vol% for line A and 3 vol% for line B. This result shows the nonlinear behaviour of polar ethanol in nonpolar gasoline hydrocarbons mixture. Ethanol–

3.3. Partial molar enthalpy of vaporization

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

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

Fig. 3 shows the enthalpy of vaporization DHvap versus the molar ethanol part in alcohol–gasoline mixture. To evaluate molar values, the molar weight of gasoline was assumed to be 100 g/mol; thus, all the obtained numbers are approximate.

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

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

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

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

ðDH gas vap ÞA  37:3 kJ=mol

One of the most important thermodynamic properties of ethanol–gasoline system – enthalpy of vaporization – was studied. Values of partial molar enthalpy of vaporization were reported for the first time. They are found to be constant. These values can help normalize alcohol–gasoline blended fuel volatility and make this fuel more competitive than ordinary gasoline. The dispersed structure of ethanol–gasoline mixture was shown. The most important parameter of any colloid structure, i.e., the radius of the dispersed particles, was reported. Its value is hundreds of nanometres and it dramatically depends on the concentration of ethanol. These data can help prevent phase-separation in ethanol–gasoline fuel caused mainly by the coalescence of alcohol–water drops and precipitation. Changing the colloid structure of ethanol–gasoline systems can be an effective way to put in order the quality coefficients of the fuel.

ðDH eth vap ÞA  45:9 kJ=mol ðDH gas vap ÞB  35:4 kJ=mol ðDH eth vap ÞB  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 the values 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 was made using the method of correlation spectroscopy of scattered light. The intensity of the signal from microdrops of alcohol (plus water) is low because of the little difference in the refraction 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 sense because the main idea of this part of the paper is the fact that ethanol–gasoline mixtures consist of a dispersed phase and 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 more quickly (R / x3) than expected when the number of particles is constant (R / x1/3). We suppose that coalescence occurs in the system when ethanol is added but other expla-

4. Conclusions

Acknowledgements Balabin Roman is grateful to ITERA International Group of companies for a nominal scholarship. The authors acknowledge the Yukos Oil Company for supplying gasoline and the corresponding data. References [1] Nan Z, Tan ZC, Sun LX. Energy Fuels 2004;18:84. [2] Overend RP, Chornet E, editors. Proceedings of the fourth biomass conference of the Americas, 2. New York: Pergamon Press; 1999. p. 1725.

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