Reaction studies in a micro reactor exemplified by the oxidation of isobutane to TBHP
The partial oxidation of hydrocarbons is currently the usual method for the industrial production of a variety of chemical products. Such processes are frequently characterized by low conversions and yields, which are mostly related to mass and heat transfer problems. Due to their hazard potentials, such processes are still not sufficiently investigated. Micro structured reactors, e.g. micro capillary reactors, have high potentials for process intensification due to enhanced mass and heat transfer as well as for optimized and inherently safe process control.
An example for a typical non-catalyzed alkane oxidation process is the production of tert.-butyl hydroperoxide (TBHP) by the partial oxidation of isobutane. TBHP is an important initiator for industrial processes, like the oxidation of propylene to propylene oxide, known as the oxirane process, which belongs to the 18 most important chemical production processes worldwide. Propylene oxide is an important product for the manufacturing of polymers, anti-freezing products etc. The hydro¬peroxide is also used as a fine chemical for many purposes in organic synthesis. However, the partial oxidation of isobutane to TBHP has still a big potential of improvement.
The overall reaction is quite exothermic with a reaction en¬thalpy of -100 to -250 kJ/mol. The reaction pathways of the isobutane oxidation to TBHP show a high degree of complexity consisting of many reaction steps and including an autocatalyzed radical mechanism.
The low selectivity is due to the decomposition of TBHP to further products like e.g. isobutene, tert.- butanol, acetone and methanol. The overall reaction can be described by the TBHP formation and formation of by-products.
In the frame of the Helmholtz Energy Alliance “Energy Efficient Multiphase Processes”, a modular micro capillary reactor facility has been deve¬loped and constructed  in order to apply it to partial oxidation processes of hydrocarbons to enhance¬ their yield and product selectivities by exploring new process windows using a Taylor flow as well as supercritical conditions. The aims are to analyse the reaction course at varying flow and mass transfer conditions as well as thermodynamic process windows and to develop new concepts for process control.
The production of TBHP by partial oxidation of liquid isobutane is investigated as an example for the non-catalyzed oxidation of hydrocarbons with oxygen with the goal to reduce the percentage of by-products, being the consequence of thermal decomposition. In particular, the application of new process windows is in the focus of the studies. To study the influence of the residence time, oxygen concentration, initiator, additives, temperature and pressure on the conversion and product selectivity of the partial isobutane oxidation, the reaction is performed for the first time as a two phase process in a capillary reactor in an especially wide range of temperatures (110 to 145 °C), pressures (35 to 80 bar) and residence times (2 to 10 h).
Since many initiators and the peroxidic reaction products are sensitive to metals, the micro reactor and further parts of the lab facility needed to be coated. Therefore, a variety of materials has been investigated by a micro-calorimetric method (TAM). Among the investigated ma¬terials especially silicon has been determined as the most suitable material . Therefore, steel has been coated by amorphous silicon. To analyse the reaction mixture, a GC/MS method has been developed . A typical example for the gas chromatogram of the resulting product mixtures is given below.
The reaction has been investigated under two phase conditions and supercritical conditions using DTBP as initiator. Current works concern the use of other initiators, e.g. TBHP in water and in decane, and other coatings of the reactor wall.
-  T. Willms, H. Kryk, M. Wiezorek, U. Hampel
Development of a modular microreactor for the partial hydrocarbon oxidation.
Chem. Eng. Com. (submitted 28.2.2017)
-  T. Willms, H. Kryk, J. Oertel, X. Lu, U. Hampel
Reactivity of t-butyl hydroperoxide and t-butyl peroxide toward reactor materials measured by a micro calorimetric method.
J. Thermal Analysis and Calorimetry 128(1), 319-333.
-  T. Willms, H. Kryk, U. Hampel
The gas chromatographic analysis of the reaction products of the partial isobutane oxidation as a two phase process.
J. Chromatography A 1458 (2016), 126–135.