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Emission Control of VOC Components in Gasoline with Oxygenates A Comparative Study.

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Short Research Communication
Emission Control of VOC Components in
Gasoline with Oxygenates: A Comparative
Study
I. A. Furzer
Department of Chemical Engineering, University of Sydney, Sydney,
NSW 2006, AUSTRALIA
A new characterisation method for VOC emissions is proposed for gasolines
containing oxygenates such as methanol, ethanol and MTBE. The method is based
on determining the vapour composition of an oxygenated gasoline that contains 10%
(mol) oxygenate, and 10% (mol) is vaporised. A simulation of the method for a PNA
gasoline with oxygenates gave the characteristic vapour composition for methanol,
ethanol and MTBE as 0.536, 0.1 96 and 0243(mol frac) respectively. Major VOC
emissions may be expected in oxygenated gasolines containing methanol.
Introduction
Protection of the environment from volatile organic components (VOC) is receiving
more attention [l] and continues to affect the oil, chemical and petrochemical
indusmes. New developments in VOC emission control include the use of hollowfibre membranes [2]. The release of VOC compounds from gasoline can take place
during the run down from distillation columns, the filling of tanks, the operation of
pipelines, the filling of tank cars and drums, and the filling of automotive tanks.
Good qualitative measurements of the potential for VOC release to the atmosphere
are provided by the ASTM distillation method D86,1985 [3], with particular
* Authorfor correspondence (E-mail: ficrzer@furzer.ce.su.or.au).
245
I.A. Furzer
attention to the initial boiling point and small percentage recoveries (volume
fraction). The hue boiling point (TBP) curves also provide valuable qualitative
front-end information on the gasoline and can be expressed as volume or weight
fraction. In addition there is the ASTM Reid vapour pressure (D323, 1985) [4]that
provides a measure of the volatility, and its potential for VOC emissions at the
ASTM reference temperatureof 329.8K (100°F).
The ASTM distillation might be described as a multicomponentbatch distillation
at atmosphericpressure. The process simulation of the ASTM method would need to
assume that the ASTM equipment approached the characteristics of an ideal stage.
The ASTM Reid vapour pressure could also be described as a close approach to an
ideal stage. A prediction of the Reid vapour pressure for a gasoline containing
oxygenates has been published [SI.
A more quantitative approach to VOC emissions from gasoline would be a Dash
distillation of a simulated gasoline at atmospheric pressure. These procedures are
widely available to chemical engineers through either individual computer programs
or flowsheetingpackages. The process simulation developmentrequires that a small
number of pseudo components are used to characterise the gasoline. These are often
of the parafinic, naphthenic and aromatic
) "P (
groups in a blended gasoline.
Several procedures are available for selecting a component or a small number of
components from each PNA group. It might be expected that the ASTM D86 and
the TBP could be simulated through a simulated batch distillation using these pseudo
components. A simulated flash distillation will give a different characteristic, but
will be the flash temperature and the fraction of the feed vaporised (mole ratio) at
atmosphericpressure. This simulation will so require pseudo components.
If a simple PNA mixture is used to demonstrate the method, then the three
groups can be represented by 224 trimethylptane (TMP), cyclohexaneand toluene.
The pure component boiling points range from 353.9K for cyclohexane to 383.8K
for toluene. When an equimolar mixture of the three pseudo components is flashed,
the difference between the dew and bubble point temperatures is about 3K.
246
Emission control of VOC components in gasoline with oxygenates: A comparative study
Process Simulation
A process model of the flash distillation is based on the mass, equilibrium,
summation and enthalpy equations. For constant pressure flash distillation, a good
model can be achieved for the mass, equilibrium and summation equations.
Mass balances:
Fx,(i) = Vy(i) + Lx(i) i=1,2...n,
...(1)
Equilibria:
y(i) = K(i)x(i) i = 1,2...nc
...(2)
Summation:
1- C x ( i > =
o
...(3)
The fraction of the feed vaporised (VF) can be used to convert the equations to a
reduced form:
x,(i) = (V / F)y(i) + (1-(V / F ) ) x ( i ) i = 1,2...nc
...(4 )
Deviation functions can be written as:
f(i) = (V/F)y(i) + (1-(V/F))y(i)/K(i) i = l , 2...nc
...(5 )
...(6 )
There are (nc+l) equations and (%+1) unknowns, namely the vapour
compositions [y(i); I = 1, 2, ... n,] and the flash temperature. The solution of the
equations is often obtained by the use of a Newton-Raphson technique. The solution
requires K values:
K ( i ) = y(i)Pmt(i)
/ P i = 172...nc
...(7)
247
LA. Furzer
An important quantity is the liquid-phase activity cuefficient $i)
for
component (i). For PNA Compounds containing different structural groups, there
will be interaction between the molecules leading to y values being different from 1.
These deviations from ideal solution behaviour will affect the flash distillation
characteristics.
If oxygenates such as methanol, ethanol and MTBE are present in a blended
gasoline to improve the combustion properties, then there will also be a need to
control VOC emissions of these oxygenates if they are volatile in the gasoline blend.
The prediction of y values of oxygenates mixed with pseudo components can be
achieved by the W A C method.
Methanol
Figure 1 shows the results of a flash distillation of methanol mixed with an
equimolar mixture of 224 mmethylpentane (TMP), cyclohexane and toluene, over a
methanol composition range of 0.O00 to 0.200 mole fraction. A notable feature is
the remarkable drop in flash temperature at small fractions of the feed vaporised.
Pure methanol has a boiling point of 337.7K so a volatile front end could be
expected. The contours show the characteristics when there is zero methanol,
x = 0.OOO (mol frac). The temperature range is only about 3K. At higher methanol
compositions, e.g. x = 0.100 (mol frac), the temperature range is about 29K.
Figure 2 shows the vapour composition after the flash distillation for variations in
the fraction of the feed vaporised. At high methanol compositions, x = 0.200 (mol
frac) the vapour composition can reach y = 0.700 (mol frac). Figure 2 indicates that
these 'y vs. V/F' characteristics could be used as a measure to quantify methanol
emissions from a methanol-gasolineblend.
248
Emission conrrol of VOC components in gasoline with oxygenates: A comparative study
370
365
-
360
-
355
-
350
345
-
o.Oo0
__.- ...'
,,~D,MO
.'
;
,
.
-
0.7
O.*
r
-..
..:.'.
o.im
,
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,,'
0.150;'
,' X ~ m e l h a w l ~ . U x )
-
340335
330
-
,,'
04
0.6
0.8
F n a m Feed V.pounrcd (mol moo)
0.2
0'
0
Figure 1. Flash Distillation
Methanol-PNAGasoline Mixture
(P = 101.325 P a )
04
0.6
0.8
Fnnim Feed Vnpoumu( (mol d o )
0.2
I
Figure 2. Flash Distillation
Methanol-PNA Gasoline
Mixture (P = 101.325 P a )
EthanoYMTBE
Figures 3 and 4 are for ethanol in the PNA mixture and are similar to the methanol
results (Figures 1 and 2), however there is a dramatic reduction in the temperature
range of the contours. The vapour compositions reach y = 0.360 (mol frac) ethanol
when x = 0.200 (mol fiac) ethanol in the gasoline.
Figures 5 and 6 are for MTJ3E showing an intermediate range of temperatures
associated with the contours. The vapow compositions are also intermediate with
y = 0.460 (mol frac) MTBE when x = 0.200 (mol frac) MTBE in the gasoline.
249
LA. Furzer
0
0.2
0.4
0.6
0.8
1
Fnnim Fad Vapourkd (mol nrio)
Figure 3. Flash Distillation
Ethanol-PNA Gasoline Mixture
(P= 101.325 P a )
Figure 4. Flash Distillation
Ethanol-PNA Gasoline Mixture
(P = 101.325 P a )
-Y
c
--- 0
03
0.4
0.6
0.8
Pnaion F.ad Vlparwd (mol m o )
Figure 5. Flash Distillation
MTBE-PNA Gasoline Mixture
(P= 101.325 P a )
250
I
Figure 6. Flash Distillation
MTBE-PNA GasolineMixture
(P = 101.32 P a )
Emission control of VOC components in gasoline with oxygenates: A comparative study
New Characterization Method
The ‘y vs. V/F’ contours for the flash distillation of a gasoline at atmospheric
pressure provides a new method to assess the VOC emissions from a gasoline blend.
Volatile front ends are characterised by high vapour compositions of a particular
component. The characteristic curves show a decrease in the vapour compositions as
the V/F ratio is increased.
The initial vapour composition when V/F + 0 may be a useful indicator of VOC
emissions from a gasoline. If a standardised composition of a special component
such as an oxygenate was set to x = 0.100 (mol frac) and V/F was set to 0.100, then
the corresponding vapour cornposition could provide a good quantitative measure of
VOC emissions.
From Figures 2, 4 and 6, the vapour compositions at the standard composition
and V/F are:
Methanol 0.536
Ethanol 0.196
MTBE 0.243
We might conclude that the relative VOC emissions from the gasoline containing
oxygenates would be greatest for methanol additions using the standard
characterisation method of 0.100 (mol frac) oxygenate and 0.100 (mol frac)
vaporised. While these results apply to a simulated gasoline containing PNA
components, they would also extend to a real gasoline containing several hundred
components. One method of controlling front-end emissions would be the reduction
of light hydrocarbons, such as butane. If oxygenates were added to such a depleted
gasoline, then the VOC emissions could be expected to be large due to the high
vapour composition of oxygenates. The quantitative comparison method of VOC
emissions is required to establish a gasoline with a known standard oxygenate
composition, and to determine the vapour composition when a standard amount is
vaporised.
This method could be developed into a new ASTM characterisation
method for VOC emissions.
251
LA. Funer
Conclusions
The new characterisation method can be summarised thus: the VOC emissions from
the release of the fraction of the feed vaporised can be simulated by a simple flash
distillation. If the volatile component of concern is set to 0.100(mol frac) in the feed
and 0.100 fraction of the feed is vaporised, then the vapour composition provides a
quantitative means to characterise the expected VOC emissions.
References
1. Chadha, N. 1994. Develop Multimedia Pollution Prevention Strategies. Chem.
Eng. Prog., 90(1 l), 32-39.
2. Deng, S., Lui, T., Sourirajan, S., and Matsuura, T. 1995. A Study of VOC
Hydrocarbon Emission C by Polyetherimide Hollow Fibre Membranes.
J. Polymer Eng., 14,219-235.
3. ASTM D86. Distillation of Petroleum Products. 1985.
4. ASTM D323. Vapour Pressure of Petroleum Products (Reid Method). 1985.
5. Funer, I. A. 1995. Prediction of the Reid Vapour Pressure of Gasolines with
MTBE and other Oxygenates. Dev. Chem. Eng. Mineral Process., 3(1), 50-55.
Received: 23 November 1995; Accepted after revision: 20 February 1996.
252
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