(72d) Explosion Hazards In Membrane Reactors

Catalytic
oxidation processes have attracted significant attention of researchers in
academia and industry [1]. One trend for these reactions aims towards increasingly
severe operating conditions which can cause major safety problems. The
oxidative coupling of methane (OCM) to form ethylene is bound to be operated at
process conditions up to ten bar and temperatures from 700-950°C [2]. Concepts
to overcome the selectivity and safety problems of this reaction are reactors
consisting of two concentric tubes separated by a membrane [3].

A packed-bed of
catalyst in the inner tube is surrounded by an empty annulus between membrane
and outer wall (figure 1). This configuration allows separate feeds of the
reactants and therefore higher oxygen concentrations than tolerable in conventional
packed-bed reactors. The explosion hazard is therefore drastically reduced in
normal operation.

Fig.1: Membrane reactor

Little attention
has been given to the fact that unreacted hydrocarbons can permeate to the
shell side of the reactor when non-selective porous membranes are used. A
potentially explosive gas mixture can form in the hot oxygen-rich atmosphere
when consumption by gas-phase reactions and dilution by convective flow are
insufficient to prevent substantial accumulation. An ignition on the shell side
of the reactor is particularly critical, as no solid catalyst is present to
extinguish flames or to hinder propagation in the reactor and into down
streaming units [4,5].

A detailed
simulation study is presented to investigate the influence of key reactor
design parameters on the shell concentration profiles. The interaction of flow,
mass transport and chemical reactions in the catalytic bed and in the shell gas
phase is described. The influence of reactor geometry, membrane permeability,
reactor inlet and reaction conditions is illustrated. A threshold value for
maximal hydrocarbon concentration is defined and operating limits are derived. The
membrane permeability and the respective flow rates in tube and shell side are
identified as main design parameters. The results can be used in further design
considerations for this promising reaction device.

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

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

[3] J. Coronas, J. Santamaria, M. Menendez,
Chem. Eng. Sci. 49 (1994) 2015.

[4] Khakpour T; Holst N; Holtappels K;
Steinbach J: 2011 AIChE Spring Meeting & 7th Global Congress on Process
Safety Chicago, IL March 13-16, 2011 (submitted).

[5] Stuenkel S; Holst N; Repke J; Steinbach
J; Schomäcker R; Wozny G. PSE'09 August 16-20, 2009 Salvador-Bahia.