The vapor pressure (VP) of 87 grade gasoline was measured using an enhanced VP acquisition system over a temperature range of approximately 19.0℃ (292.2 K) and 69.0℃ (342.2 K). The empirical data were used to predict the thermodynamic entities the enthalpy of vaporization ( ΔH vap ) and the entropy of vaporization ( ΔS vap ) of gasoline. The results of this investigation yielded a ΔH vap value of 35.1 kJ/mol and ΔS vap of 102.5 J/mol·K. The value of ΔH vap was in excellent agreement with the findings of a prior study (Balabin et al ., 2007), which produced a ΔH vap values of 37.3 kJ/mol and 35.4 kJ/mol. The enthalpy and entropy of vaporization of n-heptane (37.2 kJ/mol and 100.1 J/mol·K) and n-octane (39.1 kJ/mol and 98.3 J/mol·K) were also determined after acquiring VP data for the two VOCs. The empirical results for n-heptane and n-octane were also in excellent agreement with the literature. These favorable comparisons strengthen the capacity of our system for acquiring the VP data for pure and volatile multi-component mixtures.
Automotive gasoline is a major product manufactured by the petroleum industry. The vapor pressure (VP) is a key physical property of automotive gasoline as well as aviation fuels. Gasoline is a petroleum fuel that is highly volatile. It is refined product of crude oil consisting of a mixture of hydrocarbons, additives, and blending agents. The VP is critically important for both automotive and aviation gasoline; this is due to these fuels being manufactured as liquids, but consumed in the vapor phase. Thus, high volatility is a prerequisite for a gasoline to ensure sufficient engine start-up, warm-up and acceleration under routine driving and flying conditions [
There are a number of methods for determining the VP and they are somewhat time- consuming. The Reid vapor pressure (RVP) is typically the measurement of choice for ascertaining the volatility of commercial gasoline in the refinery industry. The protocol and apparatus for acquiring the RVP are well described in ASTM D-323. The RVP is the absolute vapor pressure exerted by a mixture as determined at a temperature of 100˚F (37.8˚C/311.0K) and a vapor to liquid ratio of 4:1. It differs from the true vapor pressure (TVP) which is described as the pressure exerted by a vapor in equilibrium with its liquid phase at a specific temperature. Measuring the TVP is probably more conducive for determining the concentration of combustion contaminants emitted into the atmosphere since a maximum threshold has been imposed for the VP of gasoline in order to restrict air pollution [
The petroleum industry relies heavily on performing simulations in order to test and optimize processes prior to the manufacturing of gasoline [
There is a large database of enthalpy of vaporization (∆Hvap) values for pure volatile organic compounds (VOCs). This information can be readily found in reference books such as the CRC (Chemical Rubber Company) Handbook of Chemistry and Physics and Perry & Green Chemical Engineers Handbook. However, there is a lack of ∆Hvap data on complex mixtures such as gasoline and aviation fuel; this may be due to earlier studies not being well developed as suggested by Chapka et al. [
The intent of this study was to amass high quality TVP data of 87 grade gasoline over a series of temperatures, i.e. as a function of temperature. The VP is a crucial physical property that is closely associated with volatility. It can be used to approximate a number of thermodynamic entities. Hence, the gasoline VP data acquired in this work were employed to predict the enthalpy of vaporization (∆Hvap) and compute the entropy of vaporization (∆Svap). Although the Clausius-Clapeyron is a classic method for calculating ∆Hvap of pure liquids, it is well suited for this investigation. VP data were also acquired for n-heptane and n-octane in order to optimize the performance of this novel VP acquisition system and concomitantly serving as references data. This type of meticulous evaluation is necessary to ensure the reliability of the methodology and empirical results generated from this novel system. The VP data (water, ethanol, and toluene) and the computed thermodynamic data presented in an earlier paper are incorporated in this investigation due to coinciding data collection intervals [
The gasoline sample used in this work was purchased at a local Sunoco gas station, and had an octane rating of 87. The composition and information on the ingredients of the gasoline are listed on its material safety data sheet. The VOCs n-heptane (99% HPLC grade), n-octane, anhydrous ethanol (EtOH), toluene were purchased from the Aldrich Chemical Company and Fischer Scientific. These materials, gasoline and chemicals, were used without purification and handled using proper safety measures as specified from their Material Safety Data Sheet.
The VP data of gasoline along with n-heptane (99% HPLC grade), n-octane, distilled water, EtOH, and toluene was measured utilizing an enhanced VP acquisition system, which is an innovative modification of the “Boiling-Point Method” apparatus. In this procedure, liquid vapors are in equilibrium with its boiling liquid at a specific externally applied pressure. The method is well recognized in the literature [
The heating of the gasoline sample was carried out using a Büchi model B-490 water bath interfaced with a J-KEM Scientific digital temperature controller (DTC). The liquid reservoir was submerged in the water bath to equilibrate the sample at a predetermined set temperature in which the VP would be measured. The temperature of the sample was measured by the thermocouple sensor component (±0.5˚C) of the DTC, which was positioned in the middle of the sample. The DVR and DTC were interfaced to a desktop PC, which logged into an excel spreadsheet the temperature and pressure data in real-time every 20 seconds. The resulting VP data can be readily evaluated in the excel spreadsheet or exported for processing using the KaleidaGraph software package. A minimum of nine VP measurements were acquired at each specified temperature to ensure reproducibility of the data; this would correspond to a total minimum acquisition time of three minutes. There were between nine and 30 VP measurements acquired at each temperature interval throughout this investigation. The standard deviation in pressure (torr) and temperature (C) is shown in
In this investigation, the enhanced VP acquisition system was used to collect empirical VP data of Sunoco 87 grade gasoline, n-heptanes, and n-octane. Calibration of the system using distilled water, EtOH, and toluene have also been reported. The information also served as a reference indicator, and is displayed with all the results of this investigation in
Determining the vapor pressure of a pure liquid or mixture is rooted in understanding the existence of a liquid-vapor boundary. A dynamic equilibrium exists between the two phases, i.e. the liquid and vapor phases; this is an interface in which the pressures and temperatures of the two phases can co-exist. The Clapeyron equation is an exact expression for the slope of the liquid-vapor phase boundary. Equation (1) represents the equilibrium that occurs between the liquid-vapor phases, where the slope of the boundary is dP/dT:
In this equation, ΔHvap is the change in the enthalpy of vaporization and T is the absolute the temperature in Kelvin. The variable ΔVvap correspond to the change in molar volume of vaporization, where we assume the gas phase behaves as an ideal gas. If we infer that ΔVvap ≈ Vvap and use the ideal equation of state, we can derive Equation (2):
The variable P in Equation (2) is the vapor pressure and R is the gas constant (8.314 J/mol∙K). By separating the variables P and T, integration of Equation (2) yields the Clausius-Clapeyron equation; this well known phase equilibrium expression is repre- sented by Equation (3)
Compound | Temperature ˚C (Lit. value) | Pressure Torr. (Lit. value) | ΔHVap and ΔSVap |
---|---|---|---|
Water | 15.2 ± 0.1 | 12.5 ± 0.1 (12.95) | 43.3 kJ/mol |
21.0 ± 0.1 | 18.8 ± 0.2 (18.65) | 116.0 J/mol∙K | |
29.6 ± 0.1 | 31.6 ± 0.2 (34.86) | Slope = −5215.1 | |
39.8 ± 0.3 | 55.0 ± 0.3 (54.16) | R = 0.99989 | |
49.7 ± 0.1 | 91.5 ± 0.6 (91.14) | ||
59.5 ± 0.2 | 148.1 ± 0.8 (146.0) | ||
69.3 ± 0.1 | 230.9 ± 0.1 (226.7) | ||
79.3 ± 0.1 | 351.6 ± 2.3 (345.4) | ||
88.9 ± 0.3 | 520.1 ± 3.6 (504.2) | ||
94.7 ± 0.1 | 636.8 ± 1.9 (627.0) | ||
99.4 ± 0.1 | 766.1 ± 0.2 (743.85) | ||
EtOH | 18.0 ± 0.1 (19.0) | 39.2 ± 0.5 (40) | 42.0 kJ/mol |
33.9 ± 0.1 (34.9) | 96.2 ± 0.7 (100) | 119.5 J/mol∙K | |
62.0 ± 0.1 (63.5) | 387 ± 0.5 (400) | Slope = −5056.6 | |
78.1 ± 0.1 (88.4) | 761.6 ± 5.0 (760) | R = 0.99998 | |
Heptane | 21.9 ± 0.2 (22.3) | 39.8 ± 0.2 (40) | 37.2 kJ/mol |
41.2 ± 0.2 (41.8) | 98.7 ± 0.7 (100) | 100.1 J/mol∙K | |
76.6 ± 0.2 (78.0) | 395.9 ± 2.6 (400) | Slope = −4476.6 | |
91.5 ± 0.2 (98.4) | 754.8 ± 0.4 (760) | R = 0.9994 | |
Octane | 19.0 ± 0.1 (19.2) | 10.5 ± 0.1 (10) | 39.1 kJ/mol |
44.9 ± 0.03 (45.1) | 43.4 ± 0.9 (40) | 98.3 J/mol∙K | |
65.2 ± 0.3 (65.7) | 107.6 ± 1.3 (100) | Slope = −4698.5 | |
103.3 ± 0.4 (104.0) | 402.2 ± 2.1 (400) | R = 0.99935 | |
124.5 ± 0.01 (125.0) | 758.0 ± 2.3 (760) | ||
Toluene | 30.4 ± 0.3 (31.8) | 39.8 ± 0.3 (40) | 35.3 kJ/mol |
49.9 ± 1.8 (51.9) | 123.5 ± 1.5 (100) | 92.0 J/mol∙K | |
89.9 ± 0.5 (89.5) | 418.2 ± 4.5 (400) | Slope = −4247.5 | |
107.9 ± 1.5 (110.6) | 759.7 ± 0.1 (760) | R = 0.99548 | |
Gasoline | 18.9 ± 0.2 | 99.7 ± 0.5 | 35.1 kJ/mol |
30.5 ± 1.5 | 127.4 ± 2.2 | 102.5 J/mol∙K | |
39.7 ± 0.6 | 247.2 ± 1.7 | Slope = −4223.2 | |
51.9 ± 0.5 | 389.0 ± 2.9 | R = 0.99136 | |
60.8 ± 0.6 | 560.6 ± 7.6 | ||
69.1 ± 0.4 | 758.4 ± 0.2 |
The enthalpy of vaporization can be predicted from Equation (3) from a plot of the natural logarithm of pressure (lnP) versus the reciprocal absolute temperature (1/K). A straight line is generated with a slope equal to −ΔHvap/R and an intercept denoted by the constant C [
The enthalpy of vaporization (∆Hvap) and entropy of vaporization (∆Svap) of the reference liquids were determined prior to that of the 87 grade gasoline. This was completed in order to acquire the optimal VP data as well as ensure maximum performance of the VP acquisition system. Using the slope of the linear square regression fit of the water VP data, a value of 43.4 kJ∙mol−1 was evaluated as the enthalpy of vaporization (∆Hvap) of water; this deviated by approximately 1.0% from the literature value [
The VP measurements were obtained for Sunoco 87 grade gasoline. The data was subsequently used to predict the ΔHvap and ∆Svap of the fuel. These thermodynamic entities were predicted in the same manner as the water and VOCs references. A linear least square fit of the gasoline VP data resulted in a slope of −4223.3 as shown in
amassing VP data.
VP data was also amassed for n-heptane and n-octane in this study. Heptane was selected since it is used along with iso-octane to alter the octane rating of gasoline blends. In 1927, the octane rating was developed in order to control spontaneous combust of gasoline blends that produce a knocking sound in a standard engine. The straight n-heptanes caused severe knocking in an engine and assigned a rating of 0. In contrast, iso-octane (2,2,4-trimethylpentane) was assigned an octane rating of 100 since it did not cause knocking in engines [
The values for ∆Hvap and ∆Svap for n-heptane and n-octane were predicted and computed in the same manner as the Sunoco 87 grade gasoline. An 87 octane gasoline can be described as having the knock resistance as a mixture of 13% n-heptane and 87% iso-octane by volume (v/v) respectively. In
A linear square regression fit was also applied to literature VP data of iso-octane [
Vapor pressure data were obtained for 87 grade gasoline using the enhanced VP acquisition system described in this work. VP measurements were also acquired for n-hep- tane and n-octane, which served as reference and system calibration data. The VP data were also used to predict the enthalpy (∆Hvap) and entropy of vaporization (∆Svap) of these VOCs from their line fits using the Clausius-Clayperon equation and Troutons Law respectively. The VP data and the predicted as well as calculated thermodynamic results were in excellent agreement with the literature results. This work has reinforced the validity and practicality of the enhanced VP acquisition system as an efficient tool.
The authors acknowledge Howard University, Dr. Clarence Lee (Executive Director) of the Howard University LS-AMP (Louis Stokes Alliance for Minority Participation) Program, Marquia Whitlock (LS-AMP Program), Monique Yvette McClung (LS-AMP Program), NSF (Grant Number HRD-1000286), and NIH-NIGMS (Grant Number T34GM105660).
Abernathy, S.M. and Brown, K.R. (2016) Predicting the En- thalpy of Vaporization and Calculating the Entropy of Vaporization of 87 Octane Gaso- line Using Vapor Pressure. Open Access Li- brary Journal, 3: e2954. http://dx.doi.org/10.4236/oalib.1102954