Molar enthalpy of vaporization of ethanol–gasoline mixtures and their colloid state

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.

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 octane-enhancing 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 or alcohols), and water, at approximately ambient temperatures, have been published [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [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 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 ethanol-containing 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.