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Prediction of the Reid Vapor Pressure of Gasolines with MTBE and Other Oxygenates.

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Prediction of the Reid Vapor Pressure of
Gasolines with MTBE and Other Oxygenates
1. A. Furzer
E-mail: fune@furZer.ce. su.oz. au]
Department of Chemical Engineering
University of Sydney, NS W 2006
AUSTRALIA
A new method has been developed for the prediction of the Reid vapor pressure of
gasolines containing oxygenates such as MTBE, ethanol and methanol. This method
accountsfor the significant deviations in the liquid phase which dominate the sensitivity of
the Reid vapor pressure to small additions of oxygenates. Results obtained over a wide
composition range show that the Reid vapor pressure increases most rapidly with
methanol additions, to a lesser degree without MTBE, and least rapidly with ethanol.
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Introduction
The Reid vapor pressure is an important physical parameter that can be used to
compare the evaporative emissions from a variety of gasolines. The need to reduce
volatile organic components (VOC) in gasoline is becoming more important. Low
Reid vapor pressures correspond to low VOC evaporative emissions. The addition of
oxygenates to gasoline alters the Reid vapour pressure. This paper provides a new
method for the calculation of the Reid vapor pressure of gasolines containing
oxygenates. A review [ 1J has discussed in detail the oxygenate, methyl tertiary butyl
ether (MTBE), and its potential for production in Australia.
The Reid vapor pressure is defined by the ASTM Standard test: D323-82 [2], and
is an important physical parameter in determining the volatility of gasoline. It has
wide areas of application such as engine starting under cold conditions and the loss of
volatile organic components to the atmosphere. The ASTM test requires the
production of a small quantity of vapor at a standard ASTM temperature in a sealed
vessel under standardized conditions. These conditions approach a multicomponent
flash under isothermal conditions to provide the bubble point pressure. This bubble
point pressure is identified with the Reid vapor pressure. There are a number of
programs available to calculate the bubble point pressure [3] which require a list of
components in the gasoline. As a typical gasoline contains hundreds of components
some simplification of the problem is required. Most gasoline components can be
divided into groups that include the paraffinic, naphthenic and aromatic (PNA)
groups. A small number or a single component from each group can represent the
group in a gasoline. The problem can be reduced to selecting a single component from
the PNA groups to represent the gasoline for a bubble point pressure or Reid vapor
pressure estimation. The results of such a calculation show that more than 98% of the
sample remains as liquid for most gasolines. This is due to the small quantity of vapor
generated in the ASTM Reid vapor pressure test.
50
Prediction of the Reid Vapor Pressure of Gasolines with MTBE and Other Oxygenates
If it is assumed that only a single bubble of vapor is generated, then the bubble
point pressure of a small list of PNA components would approximate to the Reid
vapor pressure. This calculation of the bubble point pressure is considerably simpler
than the multicomponent flash calculation. It also has the advantage of slightly overestimating the Reid vapor pressure and is conservative in its application for engine
starting and control of volatile organic component emissions. Some earlier methods
of predicting the Reid vapor pressure use the ASTM distillation curve [4], or use a
small list of paraffins in a flash calculation [5].
MTBE and Oxygenates
The addition of oxygenates such as MTBE, methanol, ethanol, and other ethers and
alcohols causes a significant increase in the Reid vapor pressure. These molecules
differ in size, shape and structural groups from the PNA components found in
gasoline. The interactions between oxygenates and gasoline result in a positive
deviation from Raoult’s law; this can be expressed by the liquid phase activity
coefficients of the components exceeding a value of one for the gasoline mixture.
Previous multicomponent flash calculations [3] take into account these deviations by
use of the UNIFAC group conmbution method. The vapor phase under the conditions
of the ASTM test can be accurately described as ideal. The model that is proposed in
this paper to predict the Reid vapor pressure includes: (1) ideal vapor phase; (2) nonideal liquid phase described by the UNIFAC method; and (3) the bubble point
pressure of the gasoline mixture equals the Reid vapor pressure. Under these
conditions, the bubble point pressure (P) is given by,
where yi is the liquid phase activity coefficient of component (i) at the ASTM
reference temperature and bubble point pressure of the gasoline mixture; and P f is
the saturated vapor pressure of component (i) at the ASTM reference temperature and
using the Antoine data [3 and 61.
The Antoine equation used to calculate the saturated vapor pressure is given by:
where A,, 4 and A, are constants.
The components that model each PNA group in a gasoline are arbitrary and can be
changed if required. In this paper the model reference component for the paraffinic
group is 224 trimethylpentane; for the naphthenic group is cyclohexane, and for the
aromatic group is toluene. The mole ratio of these components will differ for different
blends of gasoline. Throughout this paper the mole ratio is kept constant at 1:1:1.
Predicted Reid Vapor Pressure
The predicted Reid vapor pressure has been calculated according to Equation (1) and
shown on Figure 1 for additions of MTBE, ethanol and methanol to gasoline. The
Reid vapor pressure of the gasoline without oxygenates is 14.670 kPa, which rises
51
I. A. Furzer
rapidly with the mole fraction of oxygenate in the gasoline. Methanol additions give
rise to the largest increase in the Reid vapor pressure. Figure 2 shows the same results
given in mass fraction units. There is a very rapid initial rise in the Reid vapor
pressure with small additions of methanol. At higher mass fractions, the Reid vapor
pressure reaches a maximum at 44.01 kPa. In contrast to the highly non-linear
phenomena with methanol, Figure 2 shows an almost linear rise in the Reid vapor
pressure for both MTBE and ethanol. Methanol has the greatest sensitivity to the
initial increase in Reid vapor pressure with MTBE, and ethanol shows increasing nonlinear rises in the Reid vapor pressure with oxygen mass fraction.
I
a
-r
.A
;
:
,,A-
d
loo00
'
0
I
I
I
I
I
0.05
0.1
0.15
0.2
0.25
x (mol frac) of oxygenate
Figure 1. Reid vapor pressure for methanol, methyl tertiary butyl ether, and ethanol
liquid compositions (molfrac).
The rise in the Reid vapor pressure is due mainly to the substantial increase in the
liquid-phase activity of methanol in the gasoline mixture. Table 1 shows the mole
fraction of methanol in the mixture, the Reid vapor pressure (P), and the liquid-phase
activity coefficients of methanol and the three model reference components (224TMP,
cyclohexane and toluene). The liquid-phase activity coefficient of methanol is in
excess of 15 at infinite dilution in this mixture, which accounts for the high sensitivity
of the Reid vapor pressure to small amounts of methanol.
52
Prediction of the Reid Vapor Pressure of Gasolines with MTBE and Other Oxygenates
I
I
I
I
>+-
t
4ooc0
+’
;r’*
-I
4,’
15c
I
lao00’
0
I
I
I
I
1
0.05
0.1
0.15
0.2
0.25
w (mass frac) of oxygenate
Figure 2. Reid vapor pressures for methanol, methyl tertiary butyl ether, and ethanol
liquid compositions (massfrac).
Table I . Reid vapor pressures and liquid-phase activity coeficients
for methanol (1) + 2,2,4 trimethylpentane(2) + cyclohexane(3) + toluene(4).
PlPa
0.0000
0.0164
0.0323
0.0476
0.0625
0.0769
0.0909
0.1045
0.1176
0.1304
0.1429
0.1549
0.1667
0.1781
0.1892
0.2000
0.2105
0.2208
0.2308
0.2405
0.2500
14670
21526
26642
30512
33472
35758
37538
38934
40033
40903
41592
42140
42576
42922
43195
43411
43579
43709
43809
43883
43936
b
15.36
13.49
11.99
10.77
9.75
8.89
8.17
7.55
7.01
6.55
6.14
5.78
5.46
5.17
4.92
4.69
4.48
4.29
4.12
3.96
3.82
1.05
1.05
1.05
1.06
1.07
1.08
1.09
1.10
1.11
1.12
1.14
1.15
1.17
1.19
1.20
1.22
1.24
1.26
1.28
1.30
1.32
1.02
1.03
1.03
1.04
1.05
1.06
1.07
1.08
1.10
1.11
1.12
1.14
1.15
1.17
1.18
1.20
1.21
1.23
1.24
1.26
1.28
1.19
1.19
1.19
1.19
1.19
1.19
1.20
1.20
1.21
1.21
1.22
1.23
1.24
1.25
1.25
1.26
1.27
1.28
1.29
1.30
1.31
53
I. A. Furzer
I
0
0.05
I
I
I
0.25
I
0.1
0.15
0.2
x (no1 frac) of Components
Figure 3. Liquid-phase activity coeflcients for methyl tertiary butyl ether, toluene,
2,2,4-trimethylpentane, and cyclohexane (MTBE in PNA Gasoline).
1.41
1.3
0
0.05
0.1
0.15
0.2
x (mol frac) of components
Figure 4. Liquid-phase activity coeficients for ethanol, toluene, 2,2,4trimethylpentane,cyclohexane (ethanol in PNA Gosoline).
54
0.25
Prediction of the Reid Vapor Pressure of Gasolines with MTBE and Other Oxygenates
The liquid-phase activity coefficients of MTBE and the 3 model reference
components are shown in Figure 3. The molecular interactions are not so severe, and
the activity coefficient of MTBE in dilute solutions in this mixture is approximately
1.30. Figure 4 shows a similar value for ethanol, and for the 3 model reference
components. The activity coefficient for ethanol is higher, approximately 1.70 in
dilute solutions. The significant deviations by methanol dominate the characteristics
of the mixture.
Conclusions
A new calculation method has been developed to calculate the Reid vapor pressure of
gasolines with oxygenates. The method is powerful and can be used for all
components that yield UNIFAC groups. The oxygenate components which have been
used to demonstrate the technique are methanol, MTBE and ethanol. These
components have major molecular interactions with PNA components in a gasoline,
resulting in major changes in the Reid vapor pressure. These changes could not be
effectively calculated if these interactions were ignored. A measure of the interactions
is the liquid-phase activity coefficient which can be estimated by the UNIFAC
method. Methanol has the largest interactions and the largest increase in the Reid
vapor pressure for the simulated gasoline used in these calculations. The method has
the potential to be an important tool for companies and government departments
whose aim is to reduce VOC emissions through Reid vapor pressure control.
References
1 . Furzer, LA. 1994. Australian Gasoline and MTBE. Chem. Eng. Aust., 19(4), 9-17.
2. ASTM Standard. 1985. Vapor Pressure of Petroleum Products (Reid Method). D323-82,
180, (volume 05.01).
3. Furzer, LA. 1986. Distillation f o r University Students, Dept. of Chem. Eng., Univ. of
Sydney, NSW, 2006, Australia, p.190.
4. Bardon, M.F., and Rao, V.K. 1984. Calculation of Gasoline Volatility. J. Inst. Energy,
September, 343-348.
5 . Vazquez-Espmagoza. J.J., Iglesias-Silva, G.A., Hlavinka, M.W., and Bullin, J. 1992. How
to Estimate RVP of Blends, Hydrocarbon Process., 71(8), 135-138.
6. Gmehling, J., Onken, U., and Ark, W. 1982. Vapor-Liquid Equilibrium Data Collection,
Dechema, p.487.
Received: 1 November 1994; Accepted after revision: 5 May 1995.
55
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