Optimizing the parameters of the steam-carbon dioxide conversion of methane by Gibbs energy minimization

Article abstract of DOI:10.1134/S0036024411020245

The thermodynamic equilibrium for the steam-carbon dioxide conversion of methane was studied by Gibbs energy minimization. The degree of coke formation, the content of methane and carbon dioxide in the synthesis gas, and the synthesis gas H₂/CO

Full text of DOI:10.1134/S0036024411020245

ISSN 0036ꢀ0244, Russian Journal of Physical Chemistry A, 2011, Vol. 85, No. 2, pp. 202–210. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © M.N. Mikhailov, D.V. Demidov, A.N. Loginova, L.M. Kustov, 2011, published in Zhurnal Fizicheskoi Khimii, 2011, Vol. 85, No. 2, pp. 251–259.
CHEMICAL THERMODYNAMICS
AND THERMOCHEMISTRY
Optimizing the Parameters of the Steam–Carbon Dioxide Conversion
of Methane by Gibbs Energy Minimization
M. N. Mikhailova, b, D. V. Demidovb, A. N. Loginovab, and L. M. Kustova
a Zelinskii Institute of Organic Chemistry, Russian Academy of Sciences, Moscow, 119991 Russia
b United Research and Development Center, Moscow, 119333 Russia
eꢀmail: mik@ioc.ac.ru
Received April 12, 2010
Abstract—The thermodynamic equilibrium for the steam–carbon dioxide conversion of methane was
studied by Gibbs energy minimization. The degree of coke formation, the content of methane and carbon
dioxide in the synthesis gas, and the synthesis gas H2/CO ratio were plotted as functions of the molar
ratios of CO2/CH4 and H2O/CH4 in the initial mixture at different temperatures and pressures. The
regions of the optimum CH4/CO2/H2O molar ratios for steam–carbon dioxide conversion were discovꢀ
ered, with no coke formation taking place in these regions. The optimized CH4/CO2/H2O molar fractions
characterized by the minimum content of methane and carbon dioxide in the synthesis gas were found for
each region.
Keywords: steam–carbon dioxide conversion of methane, Gibbs energy minimization.
DOI: 10.1134/S0036024411020245
INTRODUCTION
the complete deactivation of the catalyst. The formaꢀ
tion of carbon fibers upon methane decomposition
could also lead to degradation of the catalyst surface
and a worsening of heat exchange. A great deal of
attention has therefore been given in the literature to
enhancing the stability of coke formation for the cataꢀ
lysts of methane conversion [2–5].
The most promising method of methane converꢀ
sion is a combination of the processes of methane oxiꢀ
dation with the oxygen, steam, and carbon dioxide
conversion of methane. A combination of two or three
types of the oxidative conversion of methane could
lead to increased efficiency in raw material use and an
improvement in the heat balance, and allow us to corꢀ
rect the composition of the synthesis gas. Adding CO2
to a CH4 + H2O mixture enables us to optimize the
methanol synthesis and the Fischer–Tropsch process
[1]. The H2/CO = 2 ratio required for these syntheses
is achieved if CH4/CO2 = 3, in accordance with the
stoichiometry of the reaction
The optimum conditions for attaining steam–
carbon dioxide conversion can be chosen on the
basis of a thermodynamic analysis of the process.
Many works have appeared in the literature in recent
years on calculating the thermodynamic equilibrium
by minimizing the Gibbs energy for the steam
reforming of dimethyl ether [6]; the steam reforming
of methanol [7]; the autothermal steam conversion
of dimethyl ether [8], autothermal carbon dioxide,
and autothermal steam; and the steam reforming of
methane [9–11].
3CH4 + СO2 + 2Н2О
(1)
→
4CO + 8H2,
ΔH
= 57 kcal/mol.
Investigations of the thermodynamic equilibrium
The conditions for attaining a steam–carbon dioxꢀ for the autothermal carbon dioxide conversion of
ide conversion that yields synthesis gas suitable for use methane [9] showed that the optimum CH4/CO2/O2
in the Fischer–Tropsch synthesis must satisfy the folꢀ molar ratios for this process lie within the range
lowing requirements: (1) no coke formation; (2) the 1.0/0.8–1.0/0.1–0.2, and the temperature of the proꢀ
residual content of methane and carbon dioxide in cess must not be lower than 730°С. Analysis of the
synthesis gas must be lower than 7–8%; (3) the synꢀ thermodynamic equilibrium of the steam and autoꢀ
thesis gas H2/CO ratio should lie within 2.1–2.2. The thermal reforming of methane [11] suggests that durꢀ
absence of coke formation is of special value from the ing steam conversion, coke formation ceases only at a
perspective of maintaining the activity and stability of molar ratio of H2O/CH4 > 1.0 and temperatures above
the catalyst of steam–carbon dioxide conversion. The 900°С. In the processes of the autothermal carbon
deposition of carbon on the catalyst surface could dioxide and autothermal steam reforming of methane,
block the active sites and reduce catalytic activity up to coke formation can be suppressed by increasing the
202

OPTIMIZING THE PARAMETERS OF THE STEAM–CARBON DIOXIDE CONVERSION
203
temperature of the process and increasing the amount number of moles of each element in the system must
of steam, respectively [10]. For the process of autoꢀ
thermal steam reforming, either the O2/CH4 ratio
must be higher than 0.4 or the H2O/CH4 ratio must be
remain constant:
NS
higher than 1.2 at reaction temperatures above 700°С
.
,
(5)
aijN j − bi = 0 i = 1,..., NE
(
)
For the autothermal carbon dioxide conversion of
methane at the optimum CH4/CO2/O2 = 1.0/0.8–
1.0/0.1–0.2 ratios, the temperature of the reaction
must be above 800°С. The increase in pressure results
in increased coke formation.
∑
j =1
where bi is the number of moles of the
the system, aij is the number of atoms of the
ment in a molecule of component , and NE is the
i
th element in
i
th eleꢀ
There is unfortunately still nothing in the literature
as to determining the optimum conditions for attainꢀ
ing steam–carbon dioxide conversion. We therefore
used Gibbs energy minimization to study the effect of
temperature, pressure, and component composition
on coke formation, the quantity of impurities, and the
synthesis gas H2/CO ratio during steam–carbon dioxꢀ
ide conversion of methane.
j
total number of elements in the system. Using the
Lagrange multipliers, we obtain NS equations of the
form
NE
,
(6)
g j + λ aij = 0 j = 1,..., NS
(
)
i
∑
i =1
CALCULATION METHOD
one for each component in the system. Here, λi is a
Lagrangian coefficient. There is one more condition
for the total number of moles (N) in the system:
All calculations were performed using total Gibbs
energy minimization. Minimization can be used to
calculate the equilibrium composition of any reaction
system [12]. The equilibrium condition of a particular
system can be written as
NS
.
(7)
N j − N = 0
∑
j =1
NS
,
(2)
µ jdN j = 0
∑
j =1
The system of equations (5)–(7) can be solved
using the Newton–Raphson method for nonlinear
equations. Here, we obtain the following NS
equations:
+
NE + 1
where Nj is the number of moles of component
the system, NS is the total number of components,
and μj is the chemical potential of component
j in
j
,
which is determined for the following relation for
ideal gases:
NE
g j
,
(8)
Δln N j − Δln N − aijπi = −
∑
RT
i =1
⎛
⎜
⎝
⎞
⎟
⎠
∂g
∂N j
μ j ≡
= g j = g0j + RT ln N j
P,T, Ni
(3)
⎛ ⎞
P
NS
NS
− RT ln N + RT ln
.
⎜ ⎟
P
,
(9)
aijN jΔln N j = bi − aijN j
⎝ ⎠
0
∑
j =1
∑
j =1
Using the Lagrange multiplier, the condition for the
Gibbs energy minimum of the system can be rewritꢀ
ten as:
NS
NS
,
N jΔln N j − NΔln N = N − N j
∑
(10)
∑
j =1
NS
NS
j =1
⎡
g jdN j =
g0j + RT ln N j
∑
j =1
∑
j =1
⎢
⎣
(4)
where πi = –(λi/RT).
⎛ ⎞⎤
P
− RT ln N + RT ln
dN j = 0.
Equations (8)–(10) can be written in matrix form.
For the process of methane conversion, we thus obtain
three elements (C, O, and H) and six main gaseous
components (H2, O2, CO, CO2, H2O, and CH4) in the
⎜ ⎟
⎥
P
⎝ ⎠⎦
0
In expression (4), not all Nj are independent variꢀ
ables, since there is an additional condition that the system. The matrix equation takes the form
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
Vol. 85
No. 2
2011

204
MIKHAILOV et al.
−g1 RT
−g2 RT
−g3 RT
−g4 RT
−g5 RT
⎡
⎢
⎢
⎢
⎢
⎢
⎢
⎢
⎢
⎢
⎤
⎥
⎥
⎥
⎥
⎥
⎥
⎥
⎥
⎥
⎥
⎥
⎥
⎥
⎥
⎥
⎥
⎥
⎥
⎥
⎥
⎥
1
0
0
0
0
0
0
1
0
0
0
0
0
0
1
0
0
0
0
0
0
1
0
0
0
0
0
0
1
0
0
0
0
0
0
1
−a11 −a21 −a31 −1 Δln N
⎤ ⎡
⎡
⎢
⎢
⎢
⎢
⎢
⎢
⎢
⎢
⎢
⎢
⎤
⎥
⎥
1
⎥ ⎢
−a12 −a22 −a32 −1 Δln N2
⎥ ⎢
−g6 RT
−a13 −a23 −a33 −1 Δln N3⎥
⎥ ⎢
6
⎥ ⎢
−a14 −a24 −a34 −1 Δln N4
⎥
⎥
⎥
⎥
⎥
⎥
⎥
⎥
⎥
⎥ ⎢
b1 − a1jN j
⎢ ∑
−a15 −a25 −a35 −1 Δln N5
⎥ ⎢
⎥ ⎢
.
=
⎢
⎢
⎢
j =1
−a16 −a26 −a36 −1 Δln N6
6
⎥ ⎢
a11N1 a12N2 a13N3 a14N4 a15N5 a16N6
a21N1 a22N2 a23N3 a24N4 a25N5 a26N6
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
π1
π2
π3
b2 − a2jN j
⎥ ⎢
⎥ ⎢
⎥ ⎢
⎥ ⎢
⎥ ⎢
∑
⎢
j =1
⎢
⎢
⎢
⎢
⎢
⎢
⎢
⎢
6
a31N1 a32N2 a33N3 a34N4 a35N5 a36N6
⎢
b3 − a3jN j
∑
⎢
⎣
⎥
⎦
N1
N2
N3
N4
N5
N6
−N Δln N
⎦ ⎣
j =1
6
N −
N j
∑
j =1
⎢
⎣
⎥
⎦
NE
As can be seen, for a system consisting of three eleꢀ
ments and six components, we obtain a 10 10 matrix,
the dimension of the problem growing linearly as the
number of components increases. By making simple
transformations, we can reduce the dimension of the
g j
×
,
(14)
(15)
aijπ =
j = NG + 1,..., NS
(
)
i
∑
i =1
RT
NE NG
NG
⎛
⎞
⎜
⎜
⎝
⎟
⎟
⎠
∑∑a N π +
ij j i
∑
j =1
N j − N Δln N
system to (NE + 1)
×
(NE + 1):
i =1 j =1
NE NS
NS
⎛
⎞
NG
NG
⎜
⎜
⎝
⎟
⎟
⎠
akjN j Δln N
∑∑a a N π +
kj ij j i
∑
j =1
g j
= N − ∑N j + ∑
j =1
N j
,
i =1 j =1
RT
(11)
j =1
NS
NS
g j
where NG is the number of gaseous components, and
= bk − akjN j + akjN j
k = 1,..., NE ,
(
)
∑
∑
the solution vector takes the form (πi, …, πNE
,
RT
j =1
j =1
Δ
NNG + 1, …, NNS ln ). For the process of methane
Δ
,
Δ
N
NE NS
NS
⎛
⎞
conversion, in which the system comprises three eleꢀ
ments (C, O, and H), six main gaseous components
⎜
⎜
⎝
⎟
⎟
⎠
N j − N Δln N
∑∑a N π +
ij j i
∑
j =1
i =1 j=1
(
H2, O2, CO, CO2, H2O, and CH4) and one conꢀ
(12)
NS
NS
densed phase (C), the matrix size increases to
5 5.
×
g j
= N − ∑N j + ∑
j =1
N j
.
Equations (13)–(15) are used to calculate the equiꢀ
librium composition of the mixture during the steam–
carbon dioxide conversion of methane. The following
components are specified in the calculation: hydroꢀ
gen, oxygen, carbon monoxide, carbon dioxide, water,
methanol, hydrocarbons С1–С8, and solidꢀphase
deposits of carbon.
RT
j =1
The solution vector in this case is (πi, …, πNE
,
Δ
ln ). Now the dimension of the system depends
only on the number of elements, and for methane
conversion, the matrix size is reduced from 10 10
to
N
×
4
×
4.
For the process of methane conversion, the formaꢀ
tion of the condensed phase (carbon deposits) is posꢀ
sible. Equations (11)–(12) in this case acquire the
form
RESULTS AND DISCUSSION
The main gaseous substances that are formed in
steam–carbon dioxide conversion are H2, CO, CO2,
H2O, CH4, and solid deposits of carbon. In addition to
the reactions of the steam and carbon dioxide converꢀ
sion of methane, the steam–carbon dioxide reforming
of methane on Ni, Co, Rh, Ru, and Pt catalysts was
accompanied by a waterꢀshift reaction and various
processes of coke formation [13–16]. The transformaꢀ
NE NS
NS
NG
⎛
⎞
⎜
⎜
⎝
⎟
⎟
⎠
akjΔN j +
akjN j Δln N
∑∑a a N π +
kj ij j i
∑
j = NG +1
NG
∑
j =1
i =1 j =1
(13)
NS
g j
=bk − akjN j + akjN j
k = 1,..., NE ,
(
)
∑
∑
RT
j =1
j =1
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
Vol. 85
No. 2
2011

OPTIMIZING THE PARAMETERS OF THE STEAM–CARBON DIOXIDE CONVERSION
с, mole
205
N, mole
N
1.2
1.0
0.8
0.6
0.4
0.2
0
8
1, (20)
2, (21)
3, (22)
4, (23)
1
1
2
3
4
5
6
, H2
, CO
, CO2
, H2O
, CH4
, C
7
6
5
4
3
2
1
0
1
2
3
2
4
5
6
3
4
600
800
1000
1200
°
600
800
1000
1200
T, °C
T,
C
Fig. 1. Equilibrium composition of the mixture obtained
during the steam–carbon dioxide conversion of methane
Fig. 2. Contribution from different reactions to coke forꢀ
mation under a pressure of 20 atm.
(
CH : CO : H O = 3 : 1 : 2,
p
= 20 atm).
4
2
2
tions that occurred during the steam–carbon dioxide monoxide disproportionation (20) and methane
conversion of methane can be represented by the folꢀ pyrolysis yield the greatest quantity of carbon deposits.
lowing equations.
An appreciably smaller contribution is made by reacꢀ
tion (22), while the carbon formed by Eq. (23) disapꢀ
pears at temperatures above 600°С. An increase in
temperature of the process results in an increased
amount of coke obtained by methane pyrolysis (21),
while the contribution from reactions (20) and (22) to
coke formation declines. We can suppress coke formaꢀ
tion and minimize the number of impurities in the
synthesis gas by optimizing the CO2/CH4 and
H2O/CH4 ratios in the initial mixture. We therefore
calculated the dependences of the contents of coke,
methane, and carbon dioxide in the synthesis gas and
of the synthesis gas H2/CO ratio from the CO2/CH4
and H2O/CH4 molar ratios at different temperatures
(Figs. 3–6). The coke content is defined as the ratio
between the molar fraction of carbon in the coke and
the number of carbon moles in the system. The conꢀ
tents of methane and carbon dioxide in the synthesis
gas were calculated without considering the steam.
The points on the diagrams are denoted as the number
The steam reforming of methane:
(16)
(17)
CH4 + Н2О
↔
CO + 3H2,
CO2 + 4H2.
CH4 + 2Н2О
↔
The carbon dioxide reforming of methane:
CH4 + CO2 CO + H2.
The waterꢀshift reaction:
CO + Н2О CO2 + H2.
The coke formation reactions:
(18)
(19)
↔
↔
2CO
CH4
↔
CO2 + С,
C + 2H2,
(20)
(21)
(22)
(23)
↔
CO + Н2
↔
C + H2O,
CO2 + 2Н2
↔
C + 2H2O.
pair (
initial mixture, and
x
,
y
), where
x
is the CO2/CH4 molar ratio in the
is the H2O/CH4 molar ratio.
y
Figure 1 shows the equilibrium composition of the
mixture obtained in steam–carbon dioxide converꢀ
sion (1) under a pressure of 20 atm. The synthesis gas
H2/CO ratio required for the Fischer–Tropsch synꢀ
thesis (2.1–2.2) is attained at temperatures of 870–
890°С. Under these conditions, however, the ratio of
coke deposits is high (6.3–8.5%) and the synthesis gas
contains appreciable amounts of methane (12.2–
13.7%), while the CO2 content is less than 5%.
Figure 3 shows the dependences between the coke
content and CO2/CH4 and H2O/CH4 molar ratios
under a pressure of 20 atm at 700–1000°С. At 700°С
,
coke formation is observed over almost the entire
range of CO2/CH4 and H2O/CH4 molar ratios,
excluding a small region above the line (0.15,1.1;
1
0.75,1.5). An increase in temperature leads to a gradꢀ
ual reduction of the coke formation zone. For a temꢀ
The contributions from reactions (20)–(23) to the
coke formation process are demonstrated in Fig. 2. At
temperatures of 850–900°С, the reactions of carbon
1
Here and below, the coordinates of regions are specified in
brackets.
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
Vol. 85
No. 2
2011

206
MIKHAILOV et al.
31.60
27.65
23.70
19.75
15.80
11.85
7.90
(a)
(b)
1.4
1.2
1.0
0.8
0.6
1.4
1.2
1.0
0.8
0.6
3.95
1.00
0.2
0.3
0.4
0.5
0.6
0.7
0.2
0.3
0.4
0.5
0.6
0.7
(c)
(d)
1.0
0.8
0.6
1.0
0.8
0.6
0.2
0.3
0.4
0.5
0.6 0.7
CO2/CH4
0.2
0.3
0.4
0.5
0.6 0.7
CO2/CH4
Fig. 3. Coke content (%) as a function of the CO /CH and H O/CH molar ratios under a pressure of 20 atm and temperatures
2
4
2
4
of (a) 700, (b) 800, (c) 900, and (d) 1000°C
.
perature of 800°С, this zone is limited by line (0.15, 1.0; in a reduced content of methane and carbon dioxide in
0.75, 1.05); for 900°С, by line (0.15, 0.85; 0.75, 0.6);
and for 1000°С, by line (0.15, 0.8; 0.55, 0.55).
synthesis gas. Using the earlier obtained regions for
which the synthesis gas H2/CO ratio lies within the
range of 2.1–2.2 and no coke formation is detected,
we introduce restrictions for the content of methane
and carbon dioxide in the synthesis gas: at 800°С, the
CH4 and CO2 content is <15%; at 900°С, the CH4 and
CO2 content is <8%; and at 1000°С, the CH4 and CO2
content < 4%. Finally, we obtain the region with values
of CO2/CH4 and H2O/CH4 molar ratios in which
there is no coke formation, the synthesis gas H2/CO
ratio lies within the range of 2.1–2.2, and the synthesis
gas has the minimum amounts of methane and carbon
dioxide it its composition. The corresponding regions
are denoted in Fig. 6 by shaded rectangles and are charꢀ
acterized by the following coordinates: 800°С, (0.52,
1.22; 0.57, 1.18; 0.66, 1.32; 0.61, 1.4); 900°С, (0.35,
1.1; 0.4, 1.05; 0.49, 1.3; 0.45, 1.4); and 1000°С, (0.26,
0.95; 0.31, 0.9; 0.42, 1.23; 0.36, 1.38).
The dependences of synthesis gas H2/CO ratio on
the CO2/CH4 and H2O/CH4 molar ratios under a
pressure of 20 atm at 700–1000°С are shown in Fig. 4.
the module value of 2.1–2.2 is not attained for temꢀ
peratures of 700°С over the range of values of
CO2/CH4 and H2O/CH4 molar ratios. Moving on to
higher temperatures with allowance for the restrictions
imposed by the coke formation process, the regions of
CO2/CH4 and H2O/CH4 molar ratio values for which
the synthesis gas H2/CO ratio is 2.1–2.2, are restricted
by the following rectangles. For 800°С, the rectangle
is (0.46, 1.05; 0.52, 1.1; 0.65, 1.5; 0.74, 1.5); for
900°С, it is (0.25, 0.79; 0.3, 0.83; 0.48, 1.5; 0.55, 1.5);
and for 1000°С, it is (0.22, 0.75; 0.26, 0.71; 0.4, 1.5;
0.49, 1.5). As can be seen, the optimum CO2/CH4
molar ratio shifts toward lower values as the temperaꢀ
ture rises.
Similar dependences for the content of coke,
methane, and carbon dioxide in the synthesis gas and
synthesis gas H2/CO ratio on the CO2/CH4 and
H2O/CH4 molar ratios were obtained for a pressure of
5 atm as well. As the pressure falls, the region of coke
formation is diminished and restricted by the followꢀ
ing coordinates: at 800°С, (0.15, 1.0; 0.75, 0.75); at
900°С, (0.15, 0.9; 0.6, 0.5), and at 1000°С, (0.15,
Let us consider the possibility of minimizing the
content of methane and carbon dioxide in the syntheꢀ
sis gas. The corresponding dependences for CH4 and
CO2 content in the synthesis gas on the CO2/CH4 and
H2O/CH4 molar ratios are shown in Figs. 5 and 6. At
700°С in the region of no coke formation, the content
of methane and carbon dioxide is higher than 24 and
12%, respectively. An increase in temperature results 0.85; 0.55, 0.5).
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
Vol. 85
No. 2
2011

OPTIMIZING THE PARAMETERS OF THE STEAM–CARBON DIOXIDE CONVERSION
207
(a)
(b)
6.76
6.25
5.74
5.22
4.71
4.20
3.69
3.17
2.66
4.16
3.85
3.54
3.23
2.92
2.62
2.31
2.00
1.69
1.4
1.2
1.0
0.8
0.6
1.4
1.2
1.0
0.8
0.6
0.2 0.3 0.4 0.5 0.6 0.7
CO2/CH4
(c)
0.2 0.3 0.4 0.5 0.6 0.7
CO2/CH4
(d)
3.19
2.96
2.73
2.50
2.26
2.03
1.80
1.56
1.33
2.94
2.73
2.53
2.33
2.13
1.93
1.72
1.52
1.32
1.4
1.2
1.0
0.8
0.6
1.4
1.2
1.0
0.8
0.6
0.2 0.3 0.4 0.5 0.6 0.7
CO2/CH4
0.2 0.3 0.4 0.5 0.6 0.7
CO2/CH4
Fig. 4. Synthesis gas H /CO ratio as a function of the CO /CH and H O/CH molar ratios under a pressure of 20 atm and temꢀ
2
2
4
2
4
peratures of (a) 700, (b) 800, (c) 900, and (d) 1000°C
.
The dependences of the synthesis gas H2/CO ratio dioxide in the synthesis gas (at 800°С, the CH4 and
on the CO2/CH4 and H2O/CH4 molar ratios under CO2 content is <7%; at 900°С, the CH4 and CO2 conꢀ
pressures of 5 and 20 atm are close in terms of shape.
Just as under pressures of 20 and 5 atm at 700°С, the
values of module of 2.1–2.2 are not attained over the
region of values for CO2/CH4 and H2O/CH4 molar
ratios. The regions of values of CO2/CH4 and
H2O/CH4 molar ratios for which the synthesis gas
H2/CO ratio is 2.1–2.2 and there is no coke formation
are restricted by rectangles with coordinates (0.34,
0.91; 0.39, 0.89; 0.61, 1.5; 0.68, 1.5), (0.24, 0.81; 0.29,
0.78; 0.47, 1.5; 0.55, 1.5), and (0.24, 0.76; 0.28, 0.72;
0.44, 1.5; 0.5, 1.5) for 800, 900, and 1000°С, respecꢀ
tively.
tent is <4%, and at 1000°С, the CH4 and CO2 content
is < 2%) and we obtain the following regions of the optiꢀ
mal values of CO2/CH4 and H2O/CH4 molar ratios for a
pressure of 5 atm: 800°С, (0.42, 1.1; 0.47, 1.05;
0.53, 1.2; 0.49, 1.3); 900°С, (0.25, 0.85; 0.3, 0.8;
0.40, 1.1; 0.35, 1.18); and 1000°С, (0.26, 0.85; 0.32, 0.8;
0.31, 1.05; 0.36, 0.98).
Table 1. Optimized ranges of CH4/CO2/H2O molar ratios
for the steam–carbon dioxide conversion of methane
T, °C
p, atm
CH4/CO2/H2O
800
5
20
5
1/0.42–0.53/1.05–1.3
1/0.53–0.66/1.18–1.4
1/0.25–0.4/0.8–1.18
1/0.35–0.49/1.05–1.4
1/0.26–0.36/0.8–1.05
1/0.26–0.42/0.9–1.38
A drop in pressure from 20 to 5 atm leads to a conꢀ
siderable reduction in the methane content in the synꢀ
thesis gas by 2–6 times, depending on the temperaꢀ
ture. Reduced pressure helps produce a decrease in
carbon dioxide content as well, but this effect is
expressed less clearly. Just as under higher pressure, an
increase in temperature under a pressure of 5 atm
results in a reduced content of methane and carbon
900
20
5
1000
20
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
Vol. 85
No. 2
2011

208
MIKHAILOV et al.
(a)
(b)
40.90
25.20
23.49
21.77
20.06
18.35
16.64
14.93
13.21
11.50
1.4
1.2
1.0
0.8
0.6
1.4
38.76
36.61
34.47
1.2
32.33
30.18
28.04
25.89
1.0
23.75
0.8
0.6
0.2 0.3 0.4 0.5 0.6 0.7
CO2/CH4
(c)
0.2 0.3 0.4 0.5 0.6 0.7
CO2/CH4
(d)
14.50
13.15
11.80
10.45
9.10
7.75
6.40
5.05
3.70
8.34
7.40
6.46
5.53
4.59
3.65
2.71
1.78
0.84
1.4
1.2
1.0
0.8
0.6
1.4
1.2
1.0
0.8
0.6
0.2 0.3 0.4 0.5 0.6 0.7
CO2/CH4
0.2 0.3 0.4 0.5 0.6 0.7
CO2/CH4
Fig. 5. Content of CH in the synthesis gas (%) as a function of CO /CH and H O/CH molar ratios under a pressure of 20 atm
4
2
4
2
4
and temperatures of (a) 700, (b) 800, (c) 900, and (d) 1000°C
.
The optimized ranges of CH4/CO2/H2O molar sponding parameters of the synthesis gas are listed in
ratios for the steam–carbon dioxide conversion proꢀ Table 2. As can be seen in Table 2, the optimum
cess at different temperatures and pressures are listed CO2/CH4 and H2O/CH4 molar ratios fall as the temꢀ
in Table 1. In each region, the CH4/CO2/H2O molar perature increases, while they rise with an increase in
ratios are found for which the content of impurities in pressure. Under a pressure of 5 atm over the range of
the synthesis gas reaches their lowest values at the temperatures, the content of methane and carbon
specified temperature and pressure. The resulting dioxide in the obtained synthesis gas is less than 7%.
optimized molar ratios for the process of steam–carꢀ Raising the temperature in gradients of 100°С leads to
bon dioxide conversion of methane and the correꢀ reduction in 2.5–3 times of the CH4 and CO2 ratios in
Table 2. Optimized CH4/CO2/H2O molar ratios for the steam–carbon dioxide conversion of methane and corresponding
parameters of the synthesis gas
T, °C
p
, atm
CH4/CO2/H2O
CO2,%
CH4,%
H2/CO
Δ
H, kcal/mol
800
5
20
5
1/0.45/1.11
1/0.59/1.32
1/0.32/0.91
1/0.42/1.22
1/0.31/0.81
1/0.34/1.05
6.72
14.15
2.16
6.43
0.86
2.86
5.16
13.78
2.17
6.43
0.82
2.85
2.20
2.19
2.18
2.20
2.11
2.18
44.7
44.6
44.6
44.8
43.8
44.5
900
20
5
1000
20
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
Vol. 85
No. 2
2011

OPTIMIZING THE PARAMETERS OF THE STEAM–CARBON DIOXIDE CONVERSION
209
(a)
(b)
26.1
23.7
21.3
18.9
16.5
14.2
11.8
9.4
16.65
15.13
13.60
12.08
10.55
9.03
7.50
5.98
4.45
1.4
1.2
1.0
0.8
0.6
1.4
1.2
1.0
0.8
0.6
7.0
0.2 0.3 0.4 0.5 0.6 0.7
CO2/CH4
(c)
0.2 0.3 0.4 0.5 0.6 0.7
CO2/CH4
(d)
10.44
9.39
8.34
7.28
6.23
5.18
4.13
3.07
2.02
7.74
6.86
5.98
5.11
4.23
3.35
2.47
1.60
0.72
1.4
1.2
1.0
0.8
0.6
1.4
1.2
1.0
0.8
0.6
0.2 0.3 0.4 0.5 0.6 0.7
CO2/CH4
0.2 0.3 0.4 0.5 0.6 0.7
CO2/CH4
Fig. 6. Content of CO in the synthesis gas (%) as a function of CO /CH and H O/CH molar ratios under a pressure of 20 atm
2
2
4
2
4
and temperatures of (a) 700, (b) 800, (c) 900, and (d) 1000°C
.
the synthesis gas, and at 1000°С, the content of impuꢀ ture was shown to be accompanied by considerable
rities is less than 1%. Under a pressure of 20 atm and coke formation. The degree of coke formation, the
at 800°С, the total volume fraction of methane and
carbon dioxide in the synthesis gas reaches 28%, and
we cannot recommend the process of steam–carbon
dioxide be performed under such conditions. Raising
the temperature to 900°С allows us to obtain synthesis
gas with an impurity content of less than 7%, while at
1000°С, the volume fractions of CH4 and CO2 in the
synthesis gas turn out to be less than 3%. The data in
Table 2 indicate that in producing synthesis gas with a
module of 2.1–2.2 by steam–carbon dioxide converꢀ
sion with the optimized CH4/CO2/H2O molar fracꢀ
tions, the thermal effect of the reaction remains virtuꢀ
ally constant as temperature and pressure change and
totals ~44 kcal/mol of CO.
content of methane and carbon dioxide in the syntheꢀ
sis gas, and the synthesis gas H2/CO ratio were plotted
as a function of the CO2/CH4 and H2O/CH4 molar
ratios in the initial mixture at different temperatures
and pressures. The regions of the optimum
CH4/CO2/H2O molar ratios for steam–carbon dioxꢀ
ide conversion were determined. There was no coke
formation in these regions, the synthesis gas H2/CO
ratio lay within a range of 2.1–2.2, and the content of
impurities was reduced. The optimized molar
CH4/CO2/H2O fractions characterized by the miniꢀ
mum content of methane and carbon dioxide in the
synthesis gas were found for each region. In compariꢀ
son to the stoichiometric composition of the initial
mixture, the use of initial mixtures with the optimized
CH4/CO2/H2O molar ratio allowed us to reduce the
temperature of coke formation by more than 150°С
and to lower the methane content in synthesis gas by
CONCLUSIONS
The thermodynamic equilibrium for the steam–
carbon dioxide conversion of methane was studied by more than half. The calculation data show that under
Gibbs energy minimization. The use of the process a pressure of 5 atm and using the optimized
with a stoichiometric composition of the initial mixꢀ CH4/CO2/H2O molar ratios, the process of steam–
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
Vol. 85
No. 2
2011

210
MIKHAILOV et al.
8. T. A. Semelsherger and R. L. Borup, J. Power Sources
155, 340 (2006).
9. N. A. S. Amin and T. C. Yaw, Int. J. Hydrogen Energy
32, 1789 (2007).
10. Y. Li, Y. Wang, X. Zhang, and Z. Mi, Int. J. Hydrogen
carbon dioxide conversion can be performed at a temꢀ
perature as low as 800°С; under a pressure of 20 atm,
however, the temperature must be raised to 900°С in
order to bring the total fraction of impurities in the
synthesis gas to levels of below 13%.
Energy 33, 2507 (2008).
11. C. Huang and A. TꢀRaissi, J. Power Sources 163, 637
REFERENCES
(2007).
1. H. HolmꢀLarsen, Stud. Surf. Sci. Catal. 136, 441
12. S. Gordon and B. J. McBride, Computer Program for the
Calculation of Complex Chemical Equilibirum Composiꢀ
tions with Applications; 1, Analysis (NASA Ref. Publ.,
1994), p. 1311.
(2001).
2. M. Nurunnabi, Y. Mukainakano, S. Kado, et al., Appl.
Catal. A 308, 1 (2006).
3. A. Shamsi and C. D. Johnson, Catal. Today 84, 17
13. D. Li, T. Shishido, Y. Oumi, et al., Appl. Catal. A 332
98 (2007).
,
(2003).
4. K. Tomishige, Catal. Today 89, 405 (2004).
5. J. Zhu, D. Zhang, and K. D. King, Fuel 80, 899 (2001).
14. D. Li, I. Atake, T. Shishido, et al., J. Catal. 250, 299
(2007).
15. J. H. Jeong, J. W. Lee, D. J. Seo, et al., Appl. Catal. A
6. K. Faungnawakij, R. Kikuchi, and K. Eguchi, J. Power
302, 151 (2006).
Sources 164, 73 (2007).
7. K. Faungnawakij, R. Kikuchi, and K. Eguchi, J. Power 16. V. R. Choudhary and K. C. Mondal, Appl. Energy 83
,
Sources 161, 87 (2006).
1024 (2006).
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
Vol. 85
No. 2
2011