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Self ignition: Experiments on explosion safety - determination procedures, phenomena and results -
(2006)
Enhancement of process safety and efficiency are the drivers in EU project SAFEKINEX investigating the mechanisms of hydrocarbon oxidation and explosion safety indices of the associated mixtures. The project produces a wealth of data on explosive behavior at elevated pressures and temperatures. This paper describes some of the self-ignition aspects.
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.
Short diffusion paths and high specific interfacial areas in microstructured devices can increase mass transfer rates and thus accelerate multiphase reactions. This effect can be intensified by the application of ultrasound. Herein, we report on the design and testing of a novel versatile setup for a continuous ultrasound-supported multiphase process in microstructured devices on a preparative scale. The ultrasonic energy is introduced indirectly into the microstructured device through pressurized water as transfer medium. First, we monitored the influence of ultrasound on the slug flow of a liquid/liquid two-phase system in a channel with a high-speed camera. To quantify the influence of ultrasound, the hydrolysis of p-nitrophenyl acetate was utilized as a model reaction. Microstructured devices with varying channel diameter, shape, and material were applied with and without ultrasonication at flow rates in the mL min-1 range. The continuous procedures were then compared and evaluated by performing a simplified life cycle assessment.
The heterogeneously catalyzed selective gas phase oxidation of o-xylene was investigated, using a
microstructured fixed bed reactor, inside of the explosion regime. The reaction was carried out with high
amounts of o-xylene in air and stoichiometric with oxygen using a V2O5/TiO2-catalyst prepared through
grafting. There were no significant losses in the selectivity to phthalic anhydride observable, during the
measurements in the explosion regime, with feed compositions of up to 7 vol% o-xylene. Also the
space-time yield was up to 2.3 times higher in comparison to conventional reaction conditions. An
increase of the selectivity to total oxidation products was observed at higher o-xylene concentrations
between 10 and 25 vol% o-xylene, which possibly was caused by the formation of a hotspot. The
investigation of the used catalyst revealed a thermally induced deactivation due to phase transition of
anatase into rutil and formation of crystalline vanadium oxide. Despite that expected hotspot at high oxylene
concentrations, there was no ignition or explosion of the gas mixture observable.
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.