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).

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[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.