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Microreactor Studies for Efficient Organic Oxidation Processes

Willms, T.; Kryk, H.; Hampel, U.


The industrially important partial oxidation of hydrocarbons is frequently characterized by low conversions and yields, which are mostly related to mass and heat transfer problems. Due to the reaction conditions, such processes include also important safety risks and are still not sufficiently investigated. To study the influence of the process parameters on the product selectivity, a lab facility including a silicon-coated micro reactor of stainless steel has been developed and constructed. Due to its modular construction (e.g. replaceable capillary), it permits for the first time to perform catalyzed and non-catalyzed oxidations of hydrocarbons with oxygen as a two-phase process in a capillary reactor in a wide range of residence times (some mins to 24 h), temperatures and pressures.
As an example the oxidation of isobutane to tertiary butyl hydroperoxide (TBHP) is studied. TBHP, as an intermediate for the production of propylene oxide according to the Oxirane process, is currently produced at industrial scale by the partial oxidation of liquid isobutane at temperatures of 120 to 140 °C and pressures of 25 to 37 bars at high residence times of up to 12 hours using bubble columns or bubble tray reactors. The conversion is limited to 35 to 50 % in order to obtain a TBHP selectivity of 50 to 60 % minimizing the formation of by-products, which are caused by the decomposition of the TBHP due to the complex reaction mechanism. Besides safety aspects, the high reaction enthalpy of the oxidation as well as heat and mass transport problems are further issues of this process. In the frame of the Helmholtz-Energy-Alliance project “Energy efficient chemical multiphase processes“, this reaction has been investigated for the first time as a Taylor-Flow process in a micro reactor with the aim to enhance the space-time yield of the process. The advantages of micro reactors are the high surface – volume ratio for an efficient heat transfer and the improved, nearly inherent, safety. This permits to investigate yet unexplored process windows, for instance within the explosive region of a reaction mixture.
The reaction has been studied varying flow rates, temperature, pressure, the molar ratio of the starting products using two different initiators, namely TBHP and di-t-butyl peroxide (DTBP). For all experiments the selectivity of the reaction products and the conversion of the reaction have been studied by sampling and analyzing the reaction by GC/MS. The use of TBHP as initiator increased the selectivity of the reaction for the target product TBHP. TBHP as initiator seems to give a better selectivity since at high temperatures as they are necessary for DTBP, the formation of propanone already becomes important which favours the decomposition of TBHP. The replacement of the initiator diluent water by decane resulted in a very fast reaction and the highest non-catalyzed isobutane conversion (~18 %) obtained ever in a microreactor but the selectivity of TBHP dropped to zero. In this experiment the coating of the capillary was damaged, so a more stable material will be necessary.



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