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Eingeladener Vortrag
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Explosion protection of oxidation reactions in micro reactors was investigated. Lange et al. (2014) reported on the possibilities of operating oxidation reactions in catalyst coated micro reactors within the explosion regime, but also warned about hotspot induced thermal runaway and detonation ignition at certain conditions. Methane and ethene, representing the explosion groups IIA1 and IIB (DIN EN ISO 16852), were used in stoichiometric oxygen mixtures with respect to total oxidation, which represents the worst case scenario in terms of safety assessment. Using laser radiation on a ceramic target inside of the micro channel, an artificial, controllable hotspot was generated. The ignition temperatures of fuel gas/oxygen mixtures inside a micro reactor were measured and their dependencies on initial pressure, initial temperature, volumetric flow rate, and micro channel height were examined. Deflagration reactions prior to the detonation were observed for the first time inside a micro reactor.
Investigations on primary, secondary and tertiary explosion protection have been performed for micro reactors with slit-like channels which are coated with catalyst. Ethene/oxygen mixtures have been used as model gas mixtures representing explosion group IIB (DIN EN ISO 16852). It could be shown that the explosion regime can be significantly reduced when using micro reactors (primary explosion protection). Furthermore, safe Operation inside the explosion regime is possible with micro reactors (secondary explosion protection), but, at certain conditions thermal runaway and ignition of detonations are possible. An unexpected behaviour has been found, because thermal runaway occurred when Volumetrie flow rate was increased. This behaviour is completely opposite to that of multi tube fixed bed reactors which show an increase of safety when increasing volumetric flow rate. Micro reactors can be constructed pressure-resistant even when detonations occur inside (tertiary explosion protection). For the example of o-xylene oxidation it could be shown that space time yield can be increased significantly when entering explosion regime.
The selective gas phase oxidation of o-xylene was investigated inside of the explosion regime using a microstructured reactor. The V2O5/TiO2 catalyst was used as microfixed bed and as catalyst coating. There were no significant losses of the selectivity to phthalic anhydride with feed compositions up to 7 vol % o-xylene observable. Above 7 vol % the selectivity decreased due to total oxidation, especially for the microfixed bed, which was probably caused by a hotspot.
Der vorliegende Beitrag behandelt spezifische Aspekte der Sicherheitstechnik bei heterogen katalysierten Oxidationsreaktionen. Ergebnisse von Explosionsuntersuchungen an Ethen-Sauerstoffgemischen in einem kontinuierlich betriebenen Mikroreaktor, die beispielsweise für den Ethylenoxidprozess relevant sind, werden vorgestellt. Der Anfangsdruck der untersuchten Eduktgemische lag zwischen 1000 hPa und 10000 hPa bei Anfangstemperaturen zwischen Umgebungstemperatur und einer Temperatur bis zu 673 K. Mikrostrukturierte Reaktoren bieten einen erweiterten Bereich von Betriebsbedingungen. Die Untersuchungen zielen auf den sicheren Betrieb eines Mikroreaktors bei Bedingungen, die bei konventionellen Reaktoren als innerhalb des Explosionsbereichs eingestuft werden, ab. In bestimmten Grenzen kann eine Unterdrückung von Explosionen in einem Mikroreaktor erreicht werden. Es ist jedoch nicht möglich, einen Mikroreaktor unter allen Bedingungen sicher zu betreiben. Aus diesen Gründen wurde die Explosionsausbreitung durch einen mikrostrukturierten Reaktor hindurch und die Zündung einer Gasphasenexplosion durch Hot-Spots in dem Reaktor untersucht. Die angewandten Untersuchungsmethoden sind Gegenstand aktueller Normungsaktivitäten.
The present contribution reports on specific aspects of safety engineering in heterogeneously catalysed oxidation reactions. Results for ethene-oxygen-mixtures in a continuous-flow micro reactor are reported related to the safety issues of the ethylene oxide process. Initial pressure is ranging from below 1 bar up to 10 bar at initial temperatures ranging from room temperature up to 673 K. Micro structured reactors offer an extended range of operating conditions. The key issue to be discussed in the present contribution is how to safely operate a micro reactor at conditions in conventional devices to be characterised as inside the explosion region. Within certain limits suppression of explosion inside a micro reactor can be achieved.
This holds true for chain reactions as well as runaway reactions. Nevertheless it is not possible to safely operate micro structured reactors at any condition. Therefore, explosion propagation through a Micro structured reactor and initiation of gas phase explosions by hot spots inside the reactor were investigated.
The investigation methods applied are subject to actual standardization.
Although catalytic reductions, cross-couplings, metathesis, and oxidation of C=C double bonds are well established, the corresponding catalytic hydroxylations of C–H bonds in alkanes, arenes, or benzylic (allylic) positions, particularly with O2, the cheapest, 'greenest', and most abundant oxidant, are severely lacking. Certainly, some promising examples in homogenous and heterogenous catalysis exist, as well as enzymes that can perform catalytic aerobic oxidations on various substrates, but these have never achieved an industrial-scale, owing to a low space-time-yield and poor stability. This review illustrates recent advances in aerobic oxidation catalysis by discussing selected examples, and aims to stimulate further exciting work in this area. Theoretical work on catalyst precursors, resting states, and elementary steps, as well as model reactions complemented by spectroscopic studies provide detailed insight into the molecular mechanisms of oxidation catalyses and pave the way for preparative applications. However, O2 also poses a safety hazard, especially when used for large scale reactions, therefore sophisticated methodologies have been developed to minimize these risks and to allow convenient transfer onto industrial scale.
The present issue reports on a specific aspect of safety engineering of the heterogeneously catalysed oxidation of ethene in a continuous-flow microreactor. Conversion of ethene and the corresponding surface temperature of the catalyst were monitored for different reactor temperatures and total flow rates with a stoichiometric ethene/oxygen mixture for total oxidation. Safe operation of the highly exothermic oxidation was possible, but not in the whole parameter range. At high reactor temperatures and flow rates, ignition of an explosion inside the microreactor caused by a hot spot on the surface of the catalyst was observed for the first time.
A method for the determination of safety properties for micro reactors and micro structured components is presented.
Micro structured reactors are not inherently safe but the range of safe operating conditions of micro reactors are
extended since the explosion region is reduced. The λ/3 rule was demonstrated to be applicable to micro scale
tubes for stoichiometric mixtures of ethane–oxygen and ethane–nitrous oxide. Furthermore first results from an investigation concerning detonation propagation through a micro reactor of non-ideal geometry are shown. Initial
pressure investigated is ranging from low pressure up to 100 kPa.