(621dz) Oxidative Coupling of Methane on Na2WO4-Mn/SiO2: Impact of Reactor Configuration

Oxidative
Coupling of Methane on Na₂WO₄-Mn/SiO₂: Impact
of Reactor Configuration

Aseem, Michael
P. Harold

University of
Houston, Department of Chemical and Biomolecular Engineering, Houston, TX 77204

The
oxidative coupling of methane (OCM) to higher molecular weight hydrocarbons has
great appeal as an alternative route to chemicals and polymers.  The reaction
system is challenging because of large differences in the reactivity of the
reactant methane and desired products (e.g. Ethylene, ethane etc.) at the elevated
temperatures (> 700^oC) needed to activate methane.  Moreover, the
reaction system is highly exothermic and is noted for significant parametric
sensitivity. The goal is to achieve a C₂₊ yield in excess of 30% for
the process to be potentially viable.

In
spite of the large number of catalysts and reactor configurations have been
studied over the past few decades, a catalyst and reactor combination has yet
to be found out that gives sufficiently high C₂₊ yield.  The most
studied catalysts to date for the OCM reaction system have been alkali-promoted
alkaline earth metal oxides, transition metal oxides and rare earth metal
oxides. Na₂WO₄-Mn/SiO₂ catalyst has attracted
attention because of its comparatively favorable catalytic performance [1]. To
date there have been no systematic studies that have compared the performance
of different rector types for the Na₂WO₄-Mn/SiO₂
catalyst.

In
this study a systematic study is underway to evaluate different reactor types
using the Na₂WO₄-Mn/SiO₂ catalyst. We have
evaluated the fixed bed reactor and monolith reactor systems and obtained C₂₊
hydrocarbon yields between ~20% for the former to ~6% for later. The data show
that as the reaction system is oxygen limited beyond a particular temperature,
with the methane conversion limited to ~32%. Thus distributed oxygen feed might
be helpful to enable further reaction with the goal of achieving higher
selectivity for desired products. Previous studies have shown favorable results
through distributed oxygen addition [2].

In
our current work we are using a modified porous asymmetric γ-alumina tubular
membrane to contain the catalyst and provide distributed oxygen to a continuous
stream of methane. The permeability of membrane is modified to achieve an
optimum flux of oxygen through membrane [3]. Membranes are impregnated by
silica sol and characterized by permeability measurements with nitrogen gas.
The fresh membrane has a permeability of 28cm³/cm²minbar.
After impregnation with silica the permeability reduces 4-fold to 6cm³/cm²minbar.
The permeability reduction requires a relatively high imposed transmembrane
pressure gradient which facilitates homogeneous distribution of oxygen along
the reactor.

Figure
1 shows that on increasing the pressure gradient across the membrane nitrogen
flow to the tube side increases which correspondingly decreases the back
diffusion of methane to shell side.  By distributing the feed the aim is to maintain
a low but uniform local concentration of oxygen along the catalyst bed length in
order to avoid complete oxidation of desired products which in turn will
increase the selectivity of C₂₊ hydrocarbons. Experiments are underway
for a range of residence times and temperature. The performance of this fixed bed
membrane reactor will be compared with conventional fixed bed reactor at same
conversion under identical conditions.

References

[1]
Arndt,
S., Otremba, T., Simon, U., Yildiz, M., Schubert,
H., Schomäcker, R, Mn-Na₂WO₄/SiO₂ as catalyst for oxidative coupling
of methane. What is really known?  (2012) Applied
Catalysis A: General, 425-426, pp. 53-61

[2]  Lu Y., Dixon A.G., Moser W.R., Ma
Y.H. ,Oxidative coupling of methane in a modified γ-alumina membrane
reactor (2000) Chem. Eng. Sci., 55 (21), pp. 4901?4912

[3] Papavassiliou, V., Lee, C, Nestlerode, J., Harold, M. P., Pneumatically
Controlled Transport and Reaction in Inorganic Membranes. (1997) Ind. Eng.
Chem. Res., 36, pp. 4954-4954