International Journal of Hydrogen Energy 31 (2006) 21 – 28
www.elsevier.com/locate/ijhydene
Thermodynamic analysis of ethanol/water system with the
stoichiometric method
V. Mas, R. Kipreos, N. Amadeo, M. Laborde∗
Chemical Engineering Department, School of Engineering, University of Buenos Aires, Pabellón de Industrias, Ciudad Universitaria,
1428 Buenos Aires, Argentina
Available online 23 May 2005
Abstract
An analysis of the chemical equilibrium of ethanol/water system, using the stoichiometric method, has been performed.
Intermediate compounds and coke formation are analyzed.
Ethanol is completely converted to ethylene and/or acetaldehyde. Taking into account the equilibrium constant values of
formation and transformation reactions of ethylene and acetaldehyde, both compounds are intermediates in this system.
Due to the relevance of carbon monoxide if hydrogen is used as feed of a PEM-type fuel cell, CO concentration in the
equilibrium mixture was studied assuming two different scenarios: (a) CO as primary product and (b) CO2 as primary product.
These two scenarios lead to suggest different routes for reaching the equilibrium. Thus, the results obtained in this work might
help to interpret the experimental results far away from the equilibrium with the aim of elucidating the reaction mechanism.
The knowledge of this mechanism is essential in order to minimize the CO formation.
The thermodynamic feasibility of coke formation shown in a temperature vs. water/ethanol molar ratio has also been
analyzed. The results indicate that if moderate temperatures are used, a molar ratio higher than 3 is required to avoid coke
formation.
䉷 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.
Keywords: Hydrogen; Ethanol steam reforming; Chemical equilibrium
1. Introduction
A few years ago, the first thermodynamic analysis on
the ethanol/water system was carried out using the nonstoichiometric method [1]. Later, other thermodynamic studies of the same system were published [2–8] due to two
reasons: (a) the novel application of hydrogen as fuel and
(b) the increasing importance of bioethanol as a renewable
feedstock in hydrogen production. The non-stoichiometric
method was used in [1] since, at that time, the information
about the reaction scheme was scarce. Since then, a considerable amount of experimental papers about ethanol steam
reforming has been published [9–19], casting light on the
∗ Corresponding author. Tel./fax: 54 11 45763240/1
E-mail address: [email protected] (M. Laborde).
reaction system. It is known, nowadays, that CO concentration in the hydrogen flow is a key variable since Pt
electrode of the fuel cell is deactivated if the hydrogen
flow has as low as 20 ppm of CO. Then a purification
process is necessary in order to reduce CO produced in
the reformer. Another significant variable is the molar ratio
water/ethanol in the feed to the reformer. This ratio must be
as small as possible in order to close the energetic balance
of the entire device including the fuel cell. In addition,
the deactivation of the catalyst as a consequence of coke
formation can be reduced using a high molar ratio water/ethanol in the feed. In summary, the understanding of
ethanol steam reforming has increased significantly in the
last few years; nevertheless, many questions are still not answered to date. In order to contribute to the comprehension
of the ethanol/water system, the analysis of the chemical
0360-3199/$30.00 䉷 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.ijhydene.2005.04.004
22
V. Mas et al. / International Journal of Hydrogen Energy 31 (2006) 21 – 28
K2 =
Nomenclature
Kj
ni
nT
r
xj
X
equilibrium constant
mole number
total mole number
water/ethanol molar ratio
extent of reaction, [mole]
ethanol conversion defined as:
(n0C H OH -n0C H OH )/n0C H OH
2
5
2
2
5
nC2 H4 nH2 O
.
nC2 H5 OH nT
And the stoichiometric balance, taking into account that
n0C H OH = 1 is
2
5
nC2 H5 OH = 1 − x1 − x2 ,
nH2 = x1 ,
5
Subscripts
nH2 O = r + x2 ,
i
j
nC2 H4 O = x1 ,
reactant
reaction
Superscripts
nC2 H4 = x2 ,
0
nT = r + 1 + x1 + x2 .
initial
equilibrium of this system is carried out using the stoichiometric method.
In this work, a homogeneous system is assumed, except
when coke formation is analyzed; and it is considered to be
an ideal gas mixture at atmospheric pressure. Calculus were
performed considering n0C H OH = 1.
2
5
2. About the intermediate compounds
Previous experimental papers [9–13,19] pointed out that
working at very low contact times, the main products obtained were acetaldehyde and ethylene, according to the following reactions:
C2 H5 OH = C2 H4 O + H2 ,
(1)
C2 H5 OH = C2 H4 + H2 O.
(2)
Conversely, these products were not detected while working at moderate and high contact times. Iwasa [9] detected
reaction 1 on copper sites, while Mariño [10–12] and Mas
[13] observed these products on copper and nickel sites. The
dehydration of ethanol (reaction 2) occurs on acidic sites,
such as the acidic sites of alumina, which is a typical component of steam reforming catalysts. In Fig. 1, the equilibrium
constant of ethanol dehydrogenation, K1 , and ethanol dehydration, K2 , at different temperatures are shown. It can be
seen that both reactions are endothermic; K2 is larger than
K1 in the whole range of temperatures analyzed, although
K1 increases faster than K2 when temperature is increased.
The equilibrium of this system (reactions 1 and 2) is solved
using
K1 =
nC2 H4 OH nH2
,
nC2 H5 OH nT
Fig. 2(a) shows ethanol conversion (X) values vs. water/
ethanol molar ratio, at different temperatures. It can be seen
that the equilibrium conversion of ethanol is quite high since
at temperatures as low as 400 K and r = 0, conversion value
is 0.96 and at T > 500 K ethanol conversion is higher than
0.99 at all r values analyzed. It means that, whatever r value
is, for T > 500 K ethanol is not practically present in the
equilibrium mixture.
On the other hand, previous works have shown that in the
presence of a catalyst, ethanol is totally converted at temperatures as low as 523 K even in absence of water [10–13].
Figs. 2(b) and (c) show the extent of reactions 1 and 2
vs. temperature at different r values. The extent of reaction
1, x1 , increases with temperature, in the whole range, due
to the endothermicity of this reaction. Despite water not
being involved in reaction 1, the extent of this reaction is
influenced by r. This fact can be explained by the effect of r
on the total mole number. The total mole number increases
with r and, due to the stoichiometric, the equilibrium is
therefore shifted to products.
In the case of reaction 2, despite the fact that this reaction
is also endothermic, x2 first increases and decreases afterwards with temperature. This behavior is a consequence of
the competition between both reactions when the temperature increases. It must be noted that x2 increases with temperature at low temperatures and when x1 value is nearly
zero. The effect of water on reaction 2 is explained by the
stoichiometry of the reaction.
In summary, x2 is always higher than x1 , for all T and r
values analyzed in this work, and then ethanol, from a thermodynamic point of view, is preferentially consumed by the
dehydration reaction (reaction 2). It must be noted that the
free energy of acetaldehyde is lower than the free energy
of ethylene in the range of temperature studied. Nevertheless, the stability of water, the other product of reaction 2,
explains the preference of ethanol for this reaction. Only
for T > 1200 K and for high r values reaction 1 would be
favored.
V. Mas et al. / International Journal of Hydrogen Energy 31 (2006) 21 – 28
23
1.E+11
log K
1.E+07
1.E+03
Reaction 1
Reaction 2
Reaction 3
Reaction 4
Reaction 6
1.E-01
1.E-05
400
500
600
700
800
900
Temperature (K)
1000
1100
1200
Fig. 1. Equilibrium constants of reactions (1)–(6) at different temperatures.
0.5
1
r0
r1
r2
r3
r4
r5
r6
r7
r8
r9
r 10
0.4
423 K
0.3
473 K
0.96
x1
Conversion
0.98
523 K
0.2
573 K
623 K
0.94
0.1
0.92
0
0
(a)
2
4
6
8
Water to ethanol ratio (mole H2O/mole EtOH)
10
400
500
600
700
800
900
Temperature (K)
900
1000
1100
1200
(b)
1000
1100
1200
1
0.9
r0
r1
r2
r3
r4
r5
r6
r7
r8
r9
r 10
x2
0.8
0.7
0.6
0.5
400
(c)
500
600
700
800
Temperature (K)
Fig. 2. (a) Ethanol conversion vs. water/ethanol molar ratio at different temperatures. (b) Extent of reaction (1) vs. temperature at different
r values. (c) Extent of reaction (2) vs. temperature at different r values.
24
V. Mas et al. / International Journal of Hydrogen Energy 31 (2006) 21 – 28
3. About the transformation of intermediate
compounds
1.E+12
Acetaldehyde, by a decarboxylation reaction, gives
methane and carbon monoxide:
1.E+04
(3)
1.E+00
Reaction 7
Reaction 8
Reaction 9
Reaction 10
Reaction 11
1.E-04
1.E-08
This reaction was postulated by Cavallaro [15] using Rh
catalysts and by Mariño and Mas [10–13] working with
Cu/alumina and Cu/Ni/alumina catalysts.
Acetaldehyde, in presence of water can also be reformed:
C2 H4 O + H2 O = CH4 + CO2 + H2 .
log K
C2 H4 O = CH4 + CO.
1.E+08
(4)
This reaction is proposed by Comas [19] working
with Ni/alumina catalysts and Fatsikostas [20] using
Ni/La2 O3 /Al2 O3 catalysts.
The other intermediate compound, ethylene, can be reformed with one or two molecules of water, depending on
r value [15,19]:
C2 H4 + H2 O = CH4 + CO + H2 ,
(5)
C2 H4 + 2H2 O = CH4 + CO2 + 2H2 .
(6)
Equilibrium constant values of reactions (1)–(6) at different
temperatures, given in Fig. 1, indicate that acetaldehyde and
ethylene are intermediate compounds.
4. Analysis of chemical equilibrium using the
stoichiometric method
From a thermodynamic point of view, ethylene and acetaldehyde can be considered unstable compounds, then, in
order to examine the chemical equilibrium of the system
ethanol/water six compounds must be taken into account,
the four final compounds: CO, CO2 , H2 and CH4 and the
two reactants: C2 H5 OH and H2 O. These six compounds
have three atomic species: H, C and O; consequently, a system of three independent reactions is necessary. To decide
on which three reactions should be chosen, the following
relations between reactions (1) and (6) can be postulated:
(1) + (3) or (2) + (5) = C2 H5 OH
= CH4 + CO + H2 ,
(7)
(1) + (4) or (2) + (6) = C2 H5 OH + H2 O
= CH4 + CO2 + 2H2 .
(8)
Depending on r values, and which the intermediate compound is, two global reactions can be formulated (reactions
(7) and (8)) and one of them should be used in the analysis of the chemical equilibrium. Another two reactions must
be considered: the water gas shift (WGS) reaction and the
steam reforming of methane since the compounds involved
1.E-12
1.E-16
400
600
800
1000
1200
Temperature (K)
Fig. 3. Equilibrium constants of reactions (7)–(11) at different
temperatures.
in these reactions are present as final products in reactions
(7) and (8):
CO + H2 O = CO2 + H2 ,
(9)
CH4 + H2 O = CO + 3H2 .
(10)
In Fig. 3 equilibrium constant values of reactions (7)–(10) at
different temperatures are summarized. It can be observed
that K7 and K8 values are higher than K9 and K10 values in
all the ranges of temperatures. On the other hand, as it was
shown in Fig. 2(a), ethanol conversion is higher than 0.99 at
temperatures as low as 500 K. Then, it can be assumed that
reactions (7) and (8) are completely shifted to products under the conditions studied in this work and ethanol moles in
the equilibrium can be disregarded. Experimental evidence
supports this assumption [19,20]. Thus, the number of compounds in the equilibrium is reduced from six to five and
the number of reactions to be considered is reduced from
three to two. These two reactions are the WGS and steam
reforming of methane (reactions (9) and (10)).
Once the reactions have been defined, the initial composition of the mixture must be chosen. This choice is related
to reactions (7) and (8). If reaction (7) occurs (low r values),
the initial composition is CH4 :CO:H2 = 1:1:1; for moderate r values reaction (8) occurs and the initial composition
is: CH4 :CO2 :H2 = 1:1:2. These two scenarios will be considered; the most important difference between them is that
in the first scheme CO is a primary product while in the
second one the primary product is CO2 . This difference has
also been noted in many experimental works where some
authors consider CO2 [19,21] and some others CO [16], as
primary products.
Other situations could also be considered, such as the case
of high and very high r ratios; subsequently, CH4 could be
completely consumed by steam reforming, with one or two
moles of water obtaining CO and/or CO2 . Nevertheless, it
should be taken into account that the energetic balance of
the entire system (including the fuel cell) strongly depends
V. Mas et al. / International Journal of Hydrogen Energy 31 (2006) 21 – 28
on the amount of water used in the steam reforming (r value
in this work) and surplus water should be limited.
Scheme A (CO as primary product): Initial molar composition: CH4 :CO:H2 = 1:1:1. The reactions are:
CH4 + H2 O = CO + 3H2 ,
1.6
1.4
1.2
(9)
(10)
1
x9
CO + H2 O = CO2 + H2 ,
0.8
and the stoichiometric balance is
0.6
nCO = 1 − x9 + x10 ,
0.4
nH2 O = r − x9 − x10 ,
0.2
nCO2 = x9 ,
nH2 = 1 + x9 + 3x10 ,
r8
r9
r 10
600
800
1000
1200
1.2
1
0.8
0.6
x10
0.4
r1
r2
r3
r4
r5
r6
r7
r8
r9
r 10
0.2
0
-0.2
-0.4
-0.6
400
500
600
(b)
700
800
900
1000
1100
1200
Temperature (K)
Fig. 4. (a) Extent of reaction 9 vs. temperature at different r values.
(b) Extent of reaction 10 vs. temperature at different r values.
0.6
nH2 O = r − 1 − x9 − x10 ,
0.2
0
x9
nCO2 = 1 + x9 ,
-0.2
-0.4
-0.6
-0.8
The behavior of reaction (10) is equal to that of scheme
A, since nCH4 and nH2 O have the same initial values (Fig.
4(b)); WGS reaction behaves quite differently. In this case,
the extent of the reaction can be negative, depending on
temperature and r values (see Fig. 5), since, initially, nCO =0
and nCO2 = 1. The extent of reaction, as well as in scheme
r4
r7
Temperature (K)
0.4
nT = r + 3 + 2x10 .
r3
r6
400
nCO = 1 − x9 + x10 ,
nCH4 = 1 − x10 ,
r2
r5
(a)
nT = r + 3 + 2x10 .
nH2 = 2 + x9 + 3x10 ,
r1
0
nCH4 = 1 − x10 ,
In Fig. 4(a) and (b), the extent of reactions (9) and (10)
vs. temperature and at different r values are presented, respectively. It can be observed that methane steam reforming
is thermodynamically possible at T > 700 K for r = 10 and
at T > 850 K for r = 1. At T < 700 K the reverse reaction
(methanation) is thermodynamically feasible (see Fig. 2(b)).
Although the WGS reaction is exothermic, the extent of
reaction vs. temperatures presents a maximum. This behavior is owed to the influence of reaction (10), which suddenly
increases with temperature. WGS reaction is predominant
over methane steam reforming for T > 800 K and r > 5 in
agreement with Fishtik [5] who stated that WGS reaction
contribution is poor at low temperatures and low r values.
In all the ranges analyzed the extent of WGS reaction is
always positive (see Fig. 2(a)).
Scheme B (CO2 as primary product): Initial molar composition: CH4 :CO2 :H2 = 1:1:2. The reactions considered are
the same as in scheme A (reactions (9) and (10)) though in
this case the stoichiometric balance is
25
r1
r2
r3
r4
r5
r6
r7
r8
r9
r 10
700
800
-1
400
500
600
900
1000
1100
1200
Temperature (K)
Fig. 5. Effect of temperature and feed ratio on the extent of
reaction 9.
V. Mas et al. / International Journal of Hydrogen Energy 31 (2006) 21 – 28
A, is affected by methane steam reforming. At 1 < r < 3
and at all temperatures, x9 < 0, that is, inverse WGS can
occur. At r = 4, x9 < 0 except within a small range of T
between 800 and 900 K. At r > 5, x9 is positive for a range
of temperatures that increases with r. At high T values and
at almost all r considered, x9 < 0.
From the results obtained with both schemes, the following recommendation can be formulated: if CO is a primary
product, the catalyst used in the steam reforming of ethanol
should be active for WGS reaction in order to minimize CO
formation. On the contrary, if CO2 is a primary product and
ethanol steam reforming is carried out in conditions favoring inverse WGS reaction, the catalyst should be inactive
for WGS reaction (see Fig. 6).
Product distribution: In the equilibrium, product distribution is independent of the scheme used. Mole numbers of
H2 , CO and CH4 at equilibrium vs. temperature and at different r values are shown in Figs 7(a) to (c), respectively.
1200
1100
1000
Temperature (K)
26
800
reverse
wgs
700
600
500
400
0
2
4
6
8
10
Water to ethanol ratio (mole H2O/mole EtOH)
Fig. 6. Direct and inverse WGS regions.
2
6
5
r1
1.8
r1
r2
1.6
r2
1.4
r3
r3
4
r4
r6
r7
2
r5
nCO
3
r4
1.2
r5
nH2
direct
wgs
900
r8
1
r6
0.8
r7
0.6
r8
r9
r9
0.4
r 10
1
r 10
0.2
0
0
400
(a)
500
600
700
800
900
1000
1100
400
1200
500
600
(b)
Temperature (K)
700
800
900
1000
1100
1200
Temperature (K)
1.6
nCH4
1.4
r1
r2
r3
r4
1.2
r5
r6
1.0
r7
r8
r9
r 10
0.8
0.6
0.4
0.2
0.0
400
(c)
600
800
1000
1200
Temperature (K)
Fig. 7. (a) Moles of hydrogen per mole of ethanol fed in the equilibrium at different r values. (b) Moles of carbon monoxide per mole of
ethanol fed in the equilibrium at different r values. (c) Moles of methane per mole of ethanol fed in the equilibrium at different r values.
V. Mas et al. / International Journal of Hydrogen Energy 31 (2006) 21 – 28
2CO = CO2 + C,
(11)
CH4 = 2H2 + C,
(12)
CO + H2 = C + H2 O,
(13)
CO2 + 2 H2 = C + 2 H2 O.
(14)
Reaction (11), known as the Boudouard reaction, has the
lowest free energy of formation. Then, this reaction is chosen to perform the analysis. Anyway, whatever reaction is
used, the results obtained will be the same [22]. In Fig. 3,
K11 values vs. temperature are summarized; the exothermic
character of this reaction can be appreciated.
If coke formation is considered, the total number of compounds at the equilibrium is six and the number of equations
needed for the analysis is, therefore, three. Then, reaction 11
must be added to one of the two schemes studied formerly
(A or B). If scheme A is chosen, the equations system is
CO + H2 O = CO2 + H2 ,
CH4 + H2 O = CO + 3H2 ,
2CO = CO2 + C.
(9)
(10)
(11)
1.0
0.5
0.0
x11
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
400
r1
r2
r3
r4
500
600
700
800
900
1000
1100
1200
Temperature (K)
Fig. 8. Extent of reaction (11) vs. temperature at different r values.
1200
1100
this work
1000
Temperature (K)
The results obtained are similar to those reported previously
by other authors [1,2,19]. High temperatures and r values
favor hydrogen production; the tendency of methane is exactly the opposite of that of hydrogen. In order to minimize
CO formation, low temperatures and high r are suitable.
Coke formation: It is known that acetaldehyde and ethylene are promoters of coke formation [6]. In order to analyze coke formation from a thermodynamic point of view it
is assumed that carbon formed is elemental, in the graphitic
form, hence, free energy of carbon formation (Gf ) is 0 and
vapor pressure is 0 in the range of temperatures analyzed.
Possible reactions of coke formation are
27
No carbon
region
900
800
700
600
Carbon
region
500
García and Laborde
400
0
1
2
3
4
5
6
7
8
9
10
Water to ethanol ratio (mole H2O/mole EtOH)
Fig. 9. Range of conditions for carbon formation.
And the stoichiometric balance is
nCO = 1 − x9 + x10 − 2x11 ,
nH2 O = r − x9 − x10 ,
nCO2 = x9 + x11 ,
nH2 = 1 + x9 + 3x10 ,
nCH4 = 1 − x10 ,
nT = r + 3 + 2x10 − x11 ,
nC = x11 .
The extent of reaction (11) vs. temperature at different r
values, is presented in Fig. 8. It can be seen that the possibility of coke formation is higher if the r value is lower. For
r = 1 this possibility exists for this entire temperature range.
When r = 4 coke formation is possible only for T < 450 K.
The operation region where coke can be formed, compared to the region presented in the original work of García and Laborde [1], is shown in Fig. 9. It must be noted
that in [1] this region was estimated considering a homogeneous system. An objection to this procedure was made by
Vasudeva [2]. It is clearly observed that if r = 1 coke formation is likely to occur at all temperatures considered. In
order to work in the region free of coke formation, at r = 2,
the temperature must be higher than 900 K and at r = 3,
temperatures higher than 500 K are suitable. If the ethanol
steam reforming reactor is to work in the region where coke
formation is thermodynamically feasible, the use of acid
catalysts should be avoided.
28
V. Mas et al. / International Journal of Hydrogen Energy 31 (2006) 21 – 28
5. Conclusions
The thermodynamics analysis performed in this work
agrees with previous experimental results where acetaldehyde and ethylene behave as intermediate compounds in
ethanol steam reforming.
Ethanol is completely converted into ethylene and/or acetaldehyde; besides, the dehydration reaction is thermodynamically favored.
The two scenarios analyzed lead to suggest different
routes for reaching the equilibrium. Thus, the results obtained in this work might help to interpret the experimental
results far away from the equilibrium with the aim to elucidate the reaction mechanism. The knowledge of this mechanism is essential in order to minimize the CO formation.
The thermodynamic feasibility of coke formation shown
in a T vs. water ethanol molar ratio has been also analyzed.
The results indicate that if moderate temperatures are used, a
molar ratio higher than 3 is required to avoid coke formation.
Nevertheless, the energetic balance of the entire device
including the fuel cell is disfavored by high r values.
Then, the efforts must be addressed at developing an adequate catalyst that inhibits coke formation and CO production working at r values as low as possible.
Acknowledgements
Authors would like to acknowledge to the University of
Buenos Aires and to the ANPCYT for financial support.
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