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Fuel 107 (2013) 4751

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Vapor pressure measurements of ethanolisooctane and

1-butanolisooctane systems using a new ebulliometer
Rama Oktavian, Vika Amidelsi, Agung Rasmito, Gede Wibawa
Department of Chemical Engineering, Faculty of Industrial Technology, Sepuluh Nopember Institute of Technology (ITS), Campus ITS Sukolilo, Surabaya 60111, Indonesia

h i g h l i g h t s

" A new simple ebulliometer has been developed in this work.

" The ebulliometer is reliable to measure vapor pressure of alcoholsisooctane mixture.
" The experimental data were well correlated with Wilson, NRTL, and UNIQUAC models.


Article history: A new, simpler ebulliometer has been developed to accurately measure the vapor pressure of ethanol
Received 8 November 2011 isooctane and 1-butanolisooctane systems at various mixture compositions. The reliability of the exper-
Received in revised form 31 January 2013 imental apparatus was validated by comparing the experimental data with published data for pure iso-
Accepted 2 February 2013
octane and ethanol and ethanolisooctane system. Our data were in good agreement with the published
Available online 22 February 2013
data, with a deviation of less than 1.9%. The vapor pressure of an ethanolisooctane mixture was initially
elevated by the addition of ethanol until the ethanol concentration reached 0.3 mass fractions, then
dropped at concentrations above 0.7 mass fractions. For a 1-butanolisooctane system, the vapor pres-
sure decreased with increasing 1-butanol concentrations. The experimental data were correlated using
Isooctane the Wilson, Non-Random Two-Liquid (NRTL), and Universal Quasi-Chemical (UNIQUAC) activity coef-
Vapor pressure cient models with an average absolute deviation in the vapor pressures of 3.5%, 3.3%, and 6.1%,
2013 Elsevier Ltd. All rights reserved.

1. Introduction physical properties of a gasoline mixture because it refers to the

volatility of the gasoline and can affect the evaporative emission
Fossil fuels, which remain the primary energy resources around of the fuel mixture. Therefore, accurate experimental data are
the world, cause signicant increases in the atmospheric CO2 con- important in designing fuel mixtures with specic vapor pressures.
centration and have an effect on global warming [1,2]. Fossil fuels The vapor pressures of gasoline mixtures with four alcohols at
combustion contributes 73% of CO2 production [3]. Thus, nding a 37.8 C (100 F) of various compositions were measured by Pum-
solution to greenhouse gas emissions is a major research focus for phrey et al. [6]. Sun et al. [7] measured the bubble-point vapor
solutions to the global warming problem. pressures for mixtures of an endothermic hydrocarbon fuel with
Recently, the addition of oxygenates, such as simple alcohols, ethanol. The experimental results indicated that the addition of
that are primarily produced from renewable resources has received ethanol had a critical effect on the vapor pressure of the fuel. An
tremendous attention because these compounds have an ability to ethanol-fuel blend with a low ethanol composition can have an in-
reduce pollution and enhance the octane number of gasoline. Nich- creased volatility.
olas Otto [4] rst used ethanol in internal combustion engines. Eth- The vapor pressures for both pure substances and mixtures are
anolgasoline mixtures have been successfully used in various traditionally measured using an ebulliometer, which was rst pro-
machines and vehicles that typically run on gasoline [5]. However, posed by Cotrell [8] to determine the boiling points of pure sub-
the mixture can have a higher vapor pressure than the base gaso- stances. Rogalski and Malanowski [9] developed a new
line which raises concern regarding the evaporative emission from ebulliometer in 1980 using Cotrells principle for determining va-
the fuel mixture. Vapor pressure is one of the most important porliquid equilibrium data. However, in these ebulliometers anal-
yses of both vapor and liquid-phase compositions are necessary
Corresponding author. Tel.: +62 31 5946240; fax: +62 31 5999282. due to a difference between the liquid-phase composition at
E-mail address: (G. Wibawa). equilibrium and the feed composition. Therefore, Li et al. [10]

0016-2361/$ - see front matter 2013 Elsevier Ltd. All rights reserved.
48 R. Oktavian et al. / Fuel 107 (2013) 4751

developed an inclined ebulliometer that no longer required liquid simpler ebulliometer by removing the inclined portion from the
and vapor-phase composition analysis to measure the bubble point previous ebulliometer. In addition, a temperature detector was at-
for a chloroformethanolbenzene system. Li et al. [10] also noted tached at the liquidvapor interface to ensure the accuracy of equi-
that they adopted the angle of inclination of 30 for their work to librium temperature measurements.
ensure the accuracy of the equilibrium temperature measurement. The mixtures with varying isooctane and ethanol, isooctane and
In this work, a new, simpler ebulliometer is proposed based on 1-butanol compositions were prepared by directly weighing the
the work of Li et al. [10] by removing the inclination and enlarging constituent component on an Ohaus balance with a precision
the volume of the cell. We can ensure that the equilibrium and the of 0.0001 g. The temperature was measured using an RTD Pt
feed compositions do not change signicantly during the measure- 100 sensor connected to a Shimaden SD 15 temperature display
ments due to the small proportion of vapor formed during equilib- with a measurement accuracy of 0.1 K. The vapor pressures at
rium by enlarging the volume of the cell. Therefore, the present various temperatures and compositions were measured using a
apparatus can be used to accurately measure the vapor pressure mercury manometer with an accuracy of 0.1 mm Hg.
of a gasoline mixture. We used isooctane in this work to represent
gasoline because it is the main component in gasoline.
2.3. Experimental procedure
2. Experimental
A sample containing approximately 220 cm3 of a liquid mixture
of known composition was charged in the ebulliometer cell. First,
2.1. Materials
the system was created under vacuum conditions to remove air
and impurities from the equilibrium cell. Then, the equilibrium cell
All materials used in this work (isooctane, ethanol, and 1-buta-
was heated to reach the desired equilibrium temperature. The tem-
nol) were supplied by Merck, Darmstadt, Germany. These materi-
perature was controlled by a PID controller (Shimaden SR 64). After
als were used without further purication. Table 1 lists the
the ebulliometer reached a constant pressure at a desired equilib-
properties of all materials used.
rium temperature, the pressure was recorded as the vapor pressure
at a desired temperature. The experimental procedure was re-
2.2. Experimental apparatus peated at different temperatures and compositions.

A new, simpler ebulliometer based on the principle of the quasi-

static method is shown in Fig. 1b. The ebulliometer has a 5.5-cm
3. Results and discussion
cell diameter, 14-cm height, and a 332.5-cm3 volume and is con-
nected with a vacuum pump. The ebulliometer cell was turned
3.1. Reliability test of the apparatus
on via moderate heating, which was controlled using a tempera-
ture controller to maintain equilibrium at the desired temperature.
The proposed ebulliometer was tested for the reliability by
The quasi-static method is based on the overall concentration
comparing the experimental data for the vapor pressures of both
instead of the equilibrium composition of the liquid phase as de-
pure liquids and mixtures as measured by this apparatus with
scribed by Li et al. [10] and shown in the inclined region in
the published data for pure ethanol, isooctane, and an ethanoliso-
Fig. 1a. The enlargement of the cell volume as shown in Fig. 1b al-
octane mixture. The Antoine equation [11] for pure isooctane and
lows for an increase in the amount of substance being tested so
the Wagner equation [12] for pure ethanol were used to compare
that the composition of the uid hardly changes during the vapor
the experimental data. The Antoine and Wagner constants were
pressure measurement. This can be calculated using the simple
obtained from Poling et al. [13]. For the ethanolisooctane mixture,
mass balance in Eq. (1) as follows:
the experimental data were compared with the experimental data
zi F xi L yi V 1 from Hull et al. [14].
A comparison of the experimental data obtained in this work
In which, zi is the mole fraction of component i in the initial with the published data is presented in Tables 2 and 3 and Figs. 2
mixture injected into the ebulliometer, F the total moles of mixture and 3 along with their deviations. The results indicate that the data
injected into the ebulliometer, xi the mole fraction of component i were in good agreement with the published data, with a deviation
in the liquid phase at equilibrium, L the number of moles in the li- of less than 1.9% calculated as an average absolute deviation (AAD)
quid phase at equilibrium, yi the mole fraction of component i in in the vapor pressures as follows:
the vapor phase at equilibrium, V the number of moles in the vapor
phase at equilibrium. n  
1X 
Plit  Pexp  100%
We assume, with the same value of V, the same mole fraction AAD 2
for component i in the vapor phase (yi), and the same feed compo- n i1  Pexp 

sition (zi), that a smaller ebulliometer means that the amount of

mixture injected into the ebulliometer (F) must be smaller, and sig- in which Pexp is the vapor pressure obtained in the experiment
nicant change can occur in the mole fraction of component i in li- while P lit is the vapor pressure calculated using both the Antoine
quid phase at equilibrium (xi). In our work, we developed a new, [11] and Wagner [12] equations and n is the number of data.

Table 1
Properties of materials used in this work.

Property 1-butanol CH3(CH2)3OH Isooctane C8H18 Ethanol C2H5OH

Molecular weight (g/mol) 74.12 114.23 46.07
Purity >99.5% >99.5% 99.9%
Density (d 20 C/20 C) 0.8090.812 0.6910.696 0.7900.793
Boiling range (C) 116119 98100 78
R. Oktavian et al. / Fuel 107 (2013) 4751 49

1. Condenser tube joint

2. Vacuum jact
3. Inner casing
4. Thermometer well 1. Ebulliometer cell
5. Feed inlet 2. Thermocouple
6. Stirrer 3. Condenser
7. Boiler 4. Vacuum unit
8. Condenser 5. Mercury manometer
9. Vacuum pump 6. Heating system controller
10. Mercury manometer 7. Magnetic stirrer
(a) (b)
Fig. 1. Comparison of the ebulliometer apparatus: (a) Ebulliometer developed by Li et al. [10]; (b) new, simpler ebulliometer developed in this work.

Table 2
Pexp Isooctane
Comparison of experimentally measured isooctane vapor pressures (Pexp) with the 33
literature data (Plit). Pexp Ethanol
30 PLit
T (K) Plit (kPa) Pexp (kPa) AAD (%)
300.35 7.31 7.33 0.34 27
302.75 8.17 8.13 0.51
305.95 9.46 9.47 0.08 24
P (kPa)

308.25 10.48 10.80 2.94

310.15 11.39 11.73 2.89 21
313.15 12.97 13.07 0.76
316.25 14.77 14.80 0.18 18
318.95 16.51 16.40 0.68
Overall AAD 1.05 15


Table 3
Comparison of experimentally measured ethanol vapor pressure (Pexp) with the
300 303 306 309 312 315 318 321 324 327
literature data (Plit).
T (K)
301.35 8.86 8.93 0.86 Fig. 2. Vapor pressure data for isooctane and ethanol.
302.55 9.50 9.60 1.09
308.45 13.24 13.60 2.62
314.35 18.19 18.80 3.23
concentrations above 0.7 mass fractions. For the 1-butanolisooc-
318.95 23.07 23.46 1.66
321.15 25.78 26.26 1.86 tane, the vapor pressure of the mixture decreased with increasing
323.65 29.17 29.73 1.89 1-butanol concentrations.
325.85 32.46 33.06 1.83
Overall AAD 1.88 3.3. Correlation with the activity coefcient models

The experimental data were correlated using the Wilson [15],

3.2. Vapor pressure measurement Non-Random Two-Liquid (NRTL) [16], and Universal Quasi-Chem-
ical (UNIQUAC) [17] activity coefcient models. The best t corre-
The vapor pressures at various temperatures and compositions lation of the experimental data for ethanolisooctane and 1-
for the ethanolisooctane and the 1-butanolisooctane systems butanolisooctane mixtures was presented by Wilson activity
obtained in this work are presented in Tables 4 and 5 and Fig. 4 coefcient model as shown in Fig. 4. Comparisons of the experi-
in which w represents the mass fraction of the ethanol and 1-buta- mental data with the three activity coefcient models are pre-
nol in the mixtures. For ethanolisooctane mixture, the vapor pres- sented in Fig. 5, which depicts one of the mass fractions
sure was initially elevated by the addition of ethanol until the observed in our work with the parameters obtained from the cor-
ethanol concentration reached 0.3 mass fractions. Then, the vapor relation of all experimental data in this work.
pressure stabilized as the ethanol concentration increased from 0.3 The average absolute deviations between the experimental
to 0.6 mass fractions. Finally, the vapor pressure dropped at vapor pressures and the calculated vapor pressures with the
50 R. Oktavian et al. / Fuel 107 (2013) 4751

Table 5
20 Bubble point vapor pressure data for 1-butanol (1)isooctane (2) mixture.

18 T (K) P (kPa) T (K) P (kPa) T (K) P (kPa) T (K) P (kPa)

w1 = 0.10 w1 = 0.20 w1 = 0.30 w1 = 0.40
16 302.75 7.73 300.95 7.33 306.55 8.80 305.75 8.53
306.15 9.20 307.05 9.87 308.15 10.40 308.15 9.60
14 308.15 10.67 308.15 11.33 308.95 10.27 309.05 9.65
309.45 10.80 309.35 11.47 316.45 14.40 313.05 11.73
316.65 14.53 316.05 15.20 320.05 16.27 316.75 14.00
319.35 15.60 322.35 19.33 321.65 17.47 322.35 17.47
P (kPa)

Ref [14] w1 = 0.50 w1 = 0.60 w1 = 0.70 w1 = 0.80

10 304.05 8.53 301.95 6.27 301.25 5.07 302.15 4.13
Exp Data at T = 308.15 K 315.05 14.00 307.05 8.13 308.15 7.60 308.15 6.27
8 322.75 19.47 308.15 8.93 308.85 7.47 310.95 6.80
325.45 21.33 308.75 8.80 311.15 8.13 315.25 8.40
6 Overall AAD = 1.23% 328.15 23.60 312.35 10.27 319.65 12.13 318.95 9.20
316.15 12.67 326.45 16.00 322.25 10.53
4 w1 = 0.90
305.85 3.20
308.15 4.53
308.35 4.00
320.95 7.47
0 0.2 0.4 0.6 0.8 1 323.45 8.13
326.65 9.20
ethanol mol fraction

Fig. 3. Vapor pressure data for the ethanolisooctane mixture.

Table 4
Bubble point vapor pressure data for ethanol (1)isooctane (2) mixture. 18
w1 = 0.10 w1 = 0.20 w1 = 0.30 w1 = 0.40 15
305.65 16.13 302.95 15.47 306.75 19.20 302.15 14.13
308.15 18.80 307.05 19.20 308.15 20.27 302.95 14.53
P (kPa)

311.95 21.60 308.15 20.00 313.45 27.06 303.65 15.20

314.45 24.00 310.15 22.40 316.15 30.80 304.95 15.87
316.35 26.26 311.75 23.73 319.45 35.46 308.15 20.00 9
w1 = 0.50 w1 = 0.60 w1 = 0.70 w1 = 0.80
303.15 15.33 304.65 15.20 308.15 19.00 302.55 12.87
305.45 16.67 305.85 15.73 310.75 20.73 306.65 15.67
308.15 20.00 308.15 19.20 312.85 22.46 308.15 18.73 Pexp ethanol-isooctane at T = 308.15 K
311.65 22.53 310.95 20.93 313.55 23.40 310.85 19.67 3 Pexp 1-butanol-isooctane at T = 308.15 K
312.15 23.06 313.05 23.20 315.15 24.73 314.45 22.73 Wilson eq.
w1 = 0.90
306.95 15.87
0.0 0.2 0.4 0.6 0.8 1.0
308.15 16.67
309.25 17.33 w
310.35 17.73
313.15 20.40 Fig. 4. Vapor pressures of the ethanolisooctane and 1-butanolisooctane

interaction parameter pairs of the systems tted to the experimen- 24

tal data are presented in Table 6. The binary interaction parameters
for the mixtures studied in this work were obtained by minimizing
the average absolute deviation in the vapor pressures dened in 20 Pexp ethanol(1)-isooctane(2) at w1 = 0.5
Eq. (2) as the objective function. Pexp 1-butanol(1)-isooctane(2) at w1 = 0.7
The experimental data were well correlated with the Wilson, Wilson eq.
NRTL eq.
NRTL, and UNIQUAC activity coefcient models with average abso- 16
P (kPa)

lute deviations in the vapor pressures of 3.5%, 3.3%, and 6.1%,

4. Conclusion 10

The new, simpler ebulliometer proposed in this work is a reli-
able tool to accurately measure the vapor pressure of both a pure 6
liquid and a mixture. This experimental apparatus was used to
measure the vapor pressure of an ethanolisooctane and a 1-buta- 300 303 306 309 312 315 318 321 324 327
nolisooctane mixture with deviations of less than 1.9%. The
T (K)
experimental data were well correlated with the Wilson, NRTL,
and UNIQUAC activity coefcient models with average absolute Fig. 5. Vapor pressure data for the ethanolisooctane and l-butanolisooctane
deviations in the vapor pressures of 3.5%, 3.3%, and 6.1%, mixtures: measured and correlated with three activity coefcient models.
R. Oktavian et al. / Fuel 107 (2013) 4751 51

Table 6
Summary of models comparison and models parameter calculated for the studied mixtures.

Mixture Wilson NRTL UNIQUAC

a12 a21 a b12 b21 u12 u21
Ethanol(1)isooctane(2) 1630 231 0.29 8475 1231 364 1470
Overall AAD (%) 3.48 3.29 6.09
1-butanol(1)isooctane(2) 1952 163 0.18 8537 1314 214 662
Overall AAD (%) 4.19 4.74 4.82

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