BIFURCATION ANALYSIS OF METHANE
OXIDATIVE COUPLING WITHOUT CATALYST
Vemuri Balakotaiah1*, Arun Kota
1, Sagar Sarsani
2 and David H. West
2
1University of Houston, Houston, TX 77204, USA
2SABIC Corporate Research & Development, Sugarland, TX-77478, USA
Abstract
We present a detailed bifurcation analysis of methane oxidative coupling in the gas phase (without
catalyst) using a global kinetic model for the various oxidation, reforming and dehydrogenation
reactions. The model predictions are compared with literature and new experimental data to validate the
approach. The model is used to determine the methane conversion (X) and C2+ products selectivity (Y)
under various feed and operating conditions. It is found that (X+Y) can vary in the range 0 to 200% with
the C2 product being mostly C2H2 at highest values of X+Y. The ratio C2H4/C2H2 decreases with higher
X and lower values of CH4/O2 ratio. The best C2+ selectivity of about 80% is obtained at methane
conversions of around 30% for adiabatic operation in a temperature window of 1200-1300K and on
ignited branches close to the extinction point.
Keywords
Oxidative coupling, ethylene, ethane, methane, reforming, dehydrogenation, adiabatic operation
Introduction
* To whom all correspondence should be addressed
Since the pioneering work of Keller and Bhasin [1],
oxidative coupling of methane (OCM) has been
investigated for more than thirty years both experimentally
and theoretically through kinetic modeling and reactor
simulations. However, most of these studies dealt with
OCM in the presence of a catalyst and only a few
considered the same in the absence of catalyst. Most prior
studies dealing with gas phase chemistry were also
restricted to reactor simulations for isothermal operation.
To our knowledge, there are no prior studies that presented
a complete bifurcation analysis (ignition, extinction and
autothermal behavior) of the gas phase OCM process. This
is the main goal of the present work.
It is often reported that the first step in OCM chemistry is
generation of methyl radicals on catalyst surface, while
subsequent reactions happen in gas phase [2]. While
debate continues on the extent of catalytic versus
homogeneous contribution in the subsequent reactions, it
is generally accepted that gas phase reactions may become
significant in OCM when the temperature exceeds about
873K but certainly important (and may be even dominant)
for temperatures above 1173K. Since the oxidation
reactions occurring in OCM are highly exothermic leading
to adiabatic temperature rise of 300 to 1200K (depending
on the methane to oxygen ratio in the feed), a detailed
understanding of the catalytic OCM also requires an
understanding of the contribution of the gas phase
chemistry to the overall process. This is the main
motivation for this study.
We consider various limiting homogeneous reactor
models (adiabatic CSTR, isothermal and adiabatic PFR
models) with global kinetics for various oxidation,
dehydrogenation and reforming reactions and present a
detailed bifurcation analysis. The focus here is on the
determination of the methane conversion and selectivities
of various C2 products (ethane, ethylene and acetylene)
and how these vary with the feed composition, operating
conditions (inlet temperature, space time) and the reactor
type.
Global Kinetic Model
While detailed models consisting of many species and
several hundred reaction steps have been proposed for gas
phase OCM, such models are not suitable for bifurcation
analysis. Here, we consider a simplified model consisting
of nine gas phase species (CH4, O2, CO, CO2, C2H6, C2H4,
C2H2, H2, H2O) and use a global kinetic model consisting
of three groups of reactions as follows:
0
4 2 2 6 2
0
4 2 2 4 2
0
4 2 2
2 2
(i) Parallel oxidation reactions
1(1) 2 42.26 /
2
(2) 2 2 67.38 /
(3) 1.5 2 124.1 /
(4) 0.5
R
R
R
CH O C H H O H kcal mole
CH O C H H O H kcal mole
CH O CO H O H kcal mole
CO O CO
+ → + ∆ = −
+ → + ∆ = −
+ → + ∆ = −
+ →0
0
4 2 2 2
0
2 4 2 2 2
0
2 6 2 4 2
67.64 /
(ii) Dehydrogenation/Pyrolysis reactions
(5) 2 3 89.943 /
(6) 32.772 /
(7) 41.692 /
R
R
R
R
H kcal mole
CH C H H H kcal mole
C H C H H H kcal mole
C H C H H H kcal mol
∆ = −
= + ∆ =
= + ∆ =
= + ∆ =
0
4 2 2
0
2 2 2
(iii) Reforming/Shift reactions
(8) +H O=CO 3 49.269 /
(9) + =CO 9.84 /
R
R
e
CH H H kcal mole
H CO H O H kcal mole
+ ∆ =
+ ∆ =
The global rate equations used for the last five (reversible
reactions) satisfy the thermodynamic constraints. The
kinetic constants for the oxidation reactions were taken
from the combustion literature and minor adjustments were
made based on experimental data on carbon selectivity at
low methane conversions. A similar approach was
followed for dehydrogenation (pyrolysis) and shift
(reforming) reactions.
Results and Discussion
In order to validate our kinetic model as well as
determine the conversion at equilibrium for isothermal as
well as adiabatic operation, the predicted conversion and
selectivity at large space times were compared with the
code of McBride and Gordon [3]. For example, figure 1
shows the equilibrium calculations for an isothermal case
with methane to oxygen feed ratio of 16. Similar
calculations and comparisons were done for adiabatic case
and other feed compositions.
Figure 1. Equilibrium conversion and selectivity of
various products for isothermal operation at 1 bar
(carbon/graphite formation excluded)
The validated model is then used to compare the
selectivity to various species for nearly isothermal reactor experiments. One such comparison is shown
in figure 2 [The experimental data shown in figure 2 is
obtained at SABIC]. Other similar comparisons with
literature data [4] were also made to validate the kinetic
model.
Figure 2. Comparison of model predictions with
new (isothermal) data at various methane to oxygen
ratios.
The validated global kinetic model is used with three
different types of reactor models (mentioned earlier) to
determine bifurcation diagrams of reactor steady-states as a
function of inlet fluid temperature for a fixed residence
time. The bifurcation diagrams were also determined when
residence time was taken as the bifurcation variable (and
inlet temperature is fixed). The main results, which will be
presented in the full manuscript may be summarized as
follows: (i) unlike prior literature claims, it is found that
the sum of methane conversion (X) plus C2+ product
selectivity (Y= sum of ethane, ethylene and acetylene
selectivity), X+Y can vary in the range 0 to 200% for
isothermal operation (which is difficult to achieve in
practice), (ii) at high values of X+Y, the main C2 product
is C2H2 , (iii) the ratio C2H4/C2H2 decreases with higher X
and lower values of CH4/O2 ratio in the feed, (iv) for the
adiabatic case, the best C2+ selectivity of about 80% is
obtained at methane conversions of around 30% for inlet
temperature window of 1200-1300K and on ignited
branches close to the extinction point, and (v) the
exothermic chemistry dominates at short residence times
and the endothermic at longer times.
Acknowledgements:
The work was at University of Houston was supported
by grant from SABIC Global Technologies.
References
[1] G. E. Keller & M. Bhasin, J. Catalysis, 73, 9 [1982].
[2] Tomoyasu I., Ji-Xiang Wang, Chiu-Hsun Lin & J. H.
Lunsford, J. Am. Chem. Soc., 107, 5062 [1985]
[3] B. J. McBride & S. Gordon, NASA-Lewis Publication 1311
[1996].
[4]. Chen, Q., Hoebink, H.B.J., & G. B. Marin, Ind. Eng. Chem.
Res. 30, 2088 [1991].