(13f) Ignition Behavior of Methane-Oxygen Mixtures at Elevated Conditions

New processes
for hydrocarbon oxidation aim at increasingly severe operating conditions. Examples
are ethylene production by oxidative coupling of methane (OCM) and the
production of synthesis gas by methane partial oxidation (POM), with process
temperatures ranging from 700°C-1000°C [1,2]. Air or pure oxygen are generally
favored as oxidizing agents for these heterogeneously catalyzed gas phase
reactions. Mixing of methane and oxygen is needed at some point in the process
which comprises a potential explosion hazard. The knowledge on explosion
properties at these elevated conditions is sparse. This is mainly attributed to
experimental difficulties at high temperatures and pressures [3]. Most studies
treat conditions below 600°C, only air as oxidizing agent or lean gas mixtures
[4-6].

One of the
determining process variables is the molar ratio of methane to oxygen, R:

Operation at
high R values with rich gas mixtures is aimed to suppress ignition at these
conditions. This increases process safety but limits the possible per-pass
conversion in a reactor. A detailed understanding is therefore needed to
determine safe and at the same time economically viable process conditions.

In a first
approach, a high-temperature autoclave was set up for investigations of the explosion
properties. Measurements of the auto-ignition behavior of methane-oxygen
mixtures were conducted in a temperature range from 600 to 850 °C. The
influence of variations in feed rate, gas mixture composition R and inert gas
addition is investigated. The tests show that considerably high R-values are needed
to suppress an ignition or to limit its outcome. Heating-up tests show that
ignition is possible at temperatures that are significantly lower than the
values published in literature. This can be attributed to pre-ignition
reactions that produce more instable intermediates.

This work is part of the Cluster of
Excellence ?Unifying Concepts in Catalysis? coordinated by the Technische
Universität Berlin. Financial support by the Deutsche Forschungsgemeinschaft
(DFG) within the framework of the German Initiative for Excellence is
gratefully acknowledged (EXC 314).

[1] Stansch, Z; Mleczko, L; Baerns, M, Ind. Eng. Chem. Res. 36, 1997, 2568-2579.

[2] Zaman, J. Fuel Process. Technol.,
{1999}, {58}, {61-81}.

[3] Pasman, H.; Pekalski, A.; Braithwaite,
M.; Griffiths, J.; Schroeder, V. & Battin-Leclerc, F. Process Saf. Environ.
Prot., {2005}, {83}, {317-323}.

[4] Norman, F. Dissertation Leuven (2008) ISBN:
978-90-5682-960-5.

[5] Caron, M. et al.  J.
Hazard. Mater., Vol. 65, No. 3, pp. 233-244 Elsevier Verlag (1999).

[6] SAFEKINEX Deliverable No. 5, Contract  No.
EVG1-CT-2002-00072 Berlin (2005).