B. Beck et al. / Catalysis Today 228 (2014) 212–218
217
with an equilibrium constant of 1.01E + 06. Therefore, OCM is
highly exothermic and not thermodynamically limited in practice.
The heat capacity of the gas mixture was calculated with Eq. (1).
Cp = x0,CH4 Cp,CH + x0,O Cp,O
4
2
2
For a mixture of 5% oxygen and 95% methane the heat capac-
ity is 71.81 J/(mol K). The adiabatic temperature increase is 507 K
under the assumption of a selectivity of 70% ethene and 30% to
−ꢀHRcA,0
ꢀTad
=
(2)
vAꢁc¯p
To estimate the critical tube diameter of the fixed bed reactor
Eqs. (3) and (4) from [41,42] are used.
8 Ge L R Tw2
Da EaꢀTAd
dcrit
=
(3)
(4)
Fig. 9. Simulated yields of C2 products as a function of methane conversion for
Mn/Na2WO4/SiO2 calculated using the kinetic parameters obtained from the OCM
reported C2 yields from literature [30–39] (cross symbol).
0.125dp/d
ꢀ
Ge =
ꢁ
2
2 − 1 − 2dp/d
batch reactor model as predicted by the model of Sun and Marin,
which is the only available microkinetic model for magnesia based
catalysts [14]. The maximum C2 yield is predicted to be around 42%
tal data of other research groups with this model leads to a good
prediction of the methane conversion, but a large overestimation
of the C2 selectivity [25].
In Eq. (3) the Damköhler number (Da) was added under the
assumption of a first order reaction, caused by a quasi stationary
methane concentration. The geometry factor (Ge) describes the vol-
ume properties of particles in a tube. Typical dimensions of tubular
reactors are given by the length/diameter ratio of 100 and a particle
size ratio to tube diameter of 0.1. With a wall temperature of 700 ◦C
the critical tube diameter is 8 mm.
Takanabe et al. [11] simulated the C2 yield dependent of the
methane conversion (Fig. 9) for Mn/Na2WO4/SiO2 in a recirculating
batch reactor at atmospheric pressure including direct oxidation
pathways form methane and ethane to carbon oxides. The maxi-
mum C2 yield is located around 24% at a methane conversion of 55%.
Comparison of the simulated C2 yield with those available in litera-
ture using various reactor setups and reaction conditions exhibits a
conspicuous consistency. It is reasonable that suppression of uns-
elective gas phase activation of methane, ethane and ethene will
lead to notable higher C2 yields as already shown for magnesia.
Independent of the type of reactors, different kinetic models
used (TAP or normal pressurized reactor), and the type of cata-
lysts, a methane conversion between 55% and 75% seems to be
optimal to maximize the C2 yield. Due to the explosion limit of
methane/oxygen mixtures and the suppression of undesired gas
phase reactions, the oxygen partial pressure must be kept at a low
level. Therefore, a distributed oxygen feed along the reactor would
be a good alternative which could be implemented by means of a
membrane or stage-type reactor. Additionally, the formation of hot
spots by the strongly exothermic total oxidation reaction has to be
prevented.
An increase of the oxygen mole fraction to 0.15 leads to a reduc-
tion of the critical tube diameter to 2 mm. Taking into account
that upscaling leads to similar problems, the fixed bed reactor is
unsuitable for OCM reactions at industrial conditions. The reason
is the fast heat production and the low thermal conductivity of the
catalyst bed. The absolute value of the critical tube diameter is dom-
inated by the product distribution. Side reactions resulting in the
formation of carbon monoxide and hydrogen will shift it to slightly
higher values. Consecutive reactions resulting in the formation of
carbon dioxide and water will shift it to lower values.
4. Conclusion
The experiments at elevated pressures point out that the yield
of C2+ components increases with increasing pressure at the same
residence time. Without a catalyst the yield is passing through a
maximum, which is caused by the consecutive oxidation of C2+
.
This occurs in homogenous gas phase reaction as well as heteroge-
neously catalyzed surface reaction at the catalyst itself, the reactor
wall, and quartz packing. It is evident that the unselective gas phase
reactions are more accelerated by higher pressures than surface
catalyzed reactions. Thus it is reasonable that each specific reactor
setup will pass through a maximum yield of C2+ components with
increasing pressure. The methane conversions to maximize the C2
yield are mostly in the 60–75% range. Sufficient oxygen has to be
provided at a constant, but relatively low oxygen partial pressure.
Kinetic models for OCM are developed for plug flow reactors with a
relatively high oxygen feed fraction. Therefore for reactor simula-
tions it is necessary to extrapolate these models to low fractions of
oxygen leading to inaccuracies in the prediction of conversion and
selectivity. Nevertheless, it is not possible to prevent the formation
of hotspots inside the catalyst bed, due to the strongly exothermic
total combustion. Jasˇo et al. [30] showed that the use of a fluidized
bed reactor can overcome this flaw by providing isothermal condi-
tions even at high oxygen to methane ratios. On the other hand, the
broad residence time distribution of the gas will favor consecutive
reactions, which are lowering the C2 yield. Additionally, the phys-
ical strain could destroy the catalyst material during fluidization,
but this was not observed in the case of Mn/Na2WO4/SiO2. Another
From this qualitative insight into the reaction mechanism and
the thermodynamical data some indication for the reaction engi-
neering and reactor design can be drawn. The complexity of the
reaction mechanism prevents accurate simulations of the derived
reactor concepts. Therefore these have to be tested and investigated
in detail before further upscaling.
3.4. Thermodynamics and reaction engineering aspects
Industrial application of heterogeneous catalyzed gas phase
reactions are often realized in tubular reactors. In the case of
exothermic reactions the heat production can lead to hot spot
formations and in the worst case to a thermal runaway of the
reactor. The critical tube diameter is a safety-related parameter
giving the upper limit to assure the necessary heat transfer. The
thermodynamic data from the NIST database [40] are used to
calculate the reaction parameters. The enthalpy of reaction for the
OCM in the temperature range of 500 to 1000 ◦C is −176 kJ/mol