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Eingeladener Vortrag
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Für die Bestimmung der SADT existieren verschiedene Prüfverfahren, die einen Zeitaufwand von mindestens einem Tag bis zu mehreren Wochen erfordern. Es ist deshalb wünschenswert, bereits in einem frühen Stadium von sicherheitstechnischen Untersuchungen die SADT ohne großen zeitlichen Aufwand zu bestimmen bzw. abzuschätzen. Ein klassisches Screening-Verfahren, um Stoffe mit geringem Material- und Zeitaufwand zu charakterisieren ist die Diffenrtial Scanning Calorimetry (DSC). Wie aus der Literatur bekannt, kann bei Anwendung eines definierten Kriteriums für die Bestimmung der Onset-Temperatur in der DSC eine SADT berechnet werden, die im Vergleich zum experimentell bestimmten Wert geringe Abweichungen besitzt. Diese Methodik wird für weitere flüssige und neu für feste organische Peroxide angewendet. Im Ergebnis zeigt sich unter Berücksichtigung eines zusätzlichen Sicherheitsabstandes von 5 K bis auf eine Ausnahme ein gutes Abbild der gemessenen SADT.
Organic peroxides are known for their decomposition behaviour and for considerable heat release connected with pressure build up. Efforts are required to run the industrial synthesis in semi-batch mode on safe condition. With the aim to use advantages as good heat transfer and small hold up, the synthesis of a peroxycarboxylic ester, namely tert-Butyl peroxy-2-ethylhexanoate (TBPEH), was transformed into continuous mode using a tube reactor with inner dimension of 1 mm. An appraisal of the critical diameter of the tube was done in advance. The use of an ultrasonic device allowed for specific reaction parameters a nearly complete conversion of the educts.
Tests according to the UN Recommendations on the Transport of Dangerous Goods for the determination of explosive properties of organic peroxides have been compared with screening criteria for explosivity based on measurements in a closed mini-autoclave (MCPVT). It will be shown that an additional screening test may be helpful but the information obtained from the UN tests are more important to characterise the specific properties of a substance under different conditions.
Autoclave methods are commonly used when pressure progress under thermal stress is of interest. One of them is the mini-autoclave method by Kühner AG. To test the robustness of the equipment under powerful exposure, two known high energetic materials, DNT and TNT, were investigated. The sample mass was increased stepwise in the range from 0.25 g to 1.0 g. For one test (1 g TNT), gas burners instead of the usually employed 2-zone heating block, were used to intensify the test conditions. Results showed the expected slower pressure generation of DNT in comparison to TNT. The mechanical robustness of the apparatus could be approved in all runs. Pressure data were used to calculate the energy release of decomposition. The values were in good agreement with the heat of decomposition, estimated by DSC.
A major item of thermal process safety, concerning the industrial realisation of exothermic reactions, is the determination of a temperature, where a reaction can be carried out without risk for humans, environment, and of economic loss. The so-called limit temperature Texo is a safety parameter for the reaction process and should not be mixed up with the Self Accelerating Decomposition Temperature (SADT). The last one refers to a temperature value for safe transportation including specific conditions like transport time and type of containment.
In Germany a technical guide (Identification and Control of Exothermic Chemical Reactions, TRAS 410, 2012) specifies three approaches for determining Texo. In this work, all these approaches were applied to specify Texo for the synthesis of three tert-Butyl peroxycarboxylic esters. Peroxycarboxylic esters are a subdivision of organic peroxides, known for their high hazardous potential.
Results differed partly significant. Anyhow, all of the used techniques and evaluation methods led to Texo values, which were on the safe side – taken the Texo value estimated by the heat release criterion as reference. This method is considered by the authors as a very accurate one.