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For the industrial production of chemicals a safe process design is required to avoid harm to people and
environment. It becomes tremendous important if one or more of the following points are characteristic for
the synthesis: high heat release, explosive atmosphere, presence of toxic and/or of thermal unstable substances.
One substance group, known for being unstable, is the group of organic peroxides. They are potential
high energetic substances. Many syntheses of organic peroxides are carried out in semi-batch
mode to control the heat release in a good manner. With the aim to increase process control, the advantages
of continuous reaction mode, combined with micro reaction technology, are used for the synthesis
of one specific organic peroxide in this work.
This approach is not only characterised by shorter residence time, good heat transfer, but also by smaller
quantities of managed chemicals, and, therefore, by a lower hazard potential. Often yield and selectivity
can be improved additionally.
For the study a peroxyester, namely tert-Butyl peroxy-2-ethylhexanoate (TBPEH), with known thermal
hazard potential was chosen. The two phase liquid/liquid reaction was carried out in three types of reactors.
A small scale tubular reactor, stressed by ultrasound, and two different micro structured reactors were
used. One of them had a meandering and the other a split-and-recombine channel structure. Synthesis
temperature was also varied.
From preliminary studies it was known that the side reaction of the involved carboxylic acid chloride to the
corresponding acid can become more important in continuous mode compared to semi-batch mode. The
effect of reactor type and temperature on selectivity and therefore yield of TBPEH was analysed and compared.
In result the reaction could be carried out in a safe manner. The high heat-exchanging efficiency of the
used reactors and the short residence time allowed synthesis temperature near the onset temperature of
decomposition of TBPEH. In relation to the results of the capillary tube and of the reactor with meandering
channel structure (MR), the results for the split-and-recombine reactor (SAR) showed the best trend line.
Further studies have to underline the result. Compared to the traditional reaction path (the semi-batch
mode), a higher space time yield could be achieved. This promising information is accompanied by the fact
of low reaction volumes. In consequence the productivity is low without a numbering up of reaction channels
or a scaling up of it. The nowadays existing philosophy of producing a specific quantity of product by a
company, followed by the transport to the costumer may change. Just organic peroxides, used in small
quantities, e.g. as initiators for polymerisation processes or as resin hardeners, provide the opportunity for
an on-demand-production.
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.
The synthesis of peroxycarboxylic esters, as one subgroup of organic peroxides, is characterized by a high thermal hazard potential regarding process safety. In case of failure in the production process, e.g., if the heat of reaction cannot be removed sufficiently fast, decomposition reactions can be triggered, and as a result, remarkable amounts of heat and gas can be released and can cause a high extent of damage. Multifarious technical and organizational measures are necessary to ensure the safe industrial production of peroxides. With the introduction of microreaction technology plenty of possibilities have been opened to carry out highly exothermic reactions in smaller volumes and with more efficient heat removal. In this paper we report the application of three different microstructured reactors, representing different mixing strategies, to synthesize two peroxymonocarboxylic esters, namely tert-butyl peroxypivalate and tert-butyl peroxy-2- thylhexanoate. The following reactor types were considered: an orifice microreactor, a split and recombine microreactor and a capillary tube reactor in combination with ultrasonication. The efficiency of the two Phase liquid/liquid reaction is expressed in comparison of conversion and selectivity. With microreaction technology a remarkable increase in space-time-yield, ranging from 12,500 kg·m−3·h−1 to 414,000 kg·m−3·h−1, is achieved.
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.
Exothermic reactions involving organic peroxides carry a high potential hazard and must be considered with care. A safe handling requires, among others, the assessment of thermal process safety, for which safety characteristics like overall heat production and the resulting adiabatic temperature rise are essential. The article presents the results of the calorimetric investigation of the synthesis of four peroxycarboxylic esters, three tert-Butyl and one tert-Amyl peroxycarboxylic ester. In the two-step synthesis the second one clearly shows the higher exothermic potential. The overall heat production lies in the range of 126135 kJ/mol and is nearly independent of the carboxylic acid residual in the tert-Butyl peroxycarboxylic ester. The calculated adiabatic temperature rise is 7080 K. Influence of temperature and feed rate on the heat generation is discussed for one species. A grading of the synthesis with respect to temperature levels according to the criticality classes by Stoessel leads to the most critical for an exothermic reaction.