ZHANG Jie

Experimental results and a kinetic modeling were previously reported for the gas phase oxidation of methane in the presence of NO2 at short residence time (20-80 ms) in a microreactor. A detailed kinetic analysis is carried out in the present study in means of flow rate and sensitivity analysis, in order to investigate the role of NO2. A comparison is made of the reaction systems with and without NO2. The concentration profiles of several key species are presented and analyzed in the present study. The CH3O center dot radicals are present in far larger quantities in the CH4/O-2/NO2 system than in the CH4/O-2 system. With NO2, the dominant route to HCHO is through CH3O center dot, primarily involving the reaction: CH3 center dot + NO2 double left right arrow CH3O center dot + NO, followed by: CH3O center dot (+M) double left right arrow CH2O + H center dot (+M) and CH3O center dot + O-2 double left right arrow HO2 center dot + CH2O. The promoting effects of NO2 can be explained by the interconversion NO2/NO, which is mainly related to: HO2 center dot + NO double left right arrow NO2 + OH center dot, NO2 + H center dot double left right arrow NO + OH center dot, NO + CH3O2 center dot double left right arrow CH3O center dot + NO2 and NO2 + CH3 center dot double left right arrow CH3O center dot + NO.

This study deals with the modelling of a tubular flow microreactor and an annular flow microreactor that are used for kinetic studies of thermal reactions (873-1273K) at very short residence times (10-100 ms). The construction of a kinetic model on the basis of experimental reaction data requires the precise characterization of the thermal behaviour and the hydrodynamics of the reactor. In kinetic studies, the reactor is usually considered as an isothermal Plug Flow Reactor. In this paper, we demonstrate that an annular flow microreactor used at very short residence time not only leads to a temperature profile which is better controlled than in a tubular flow microreactor, but also to less axial dispersion for laminar flow. The comparison between both reactors is performed on the basis of temperature measurements, RTD measurements at room temperature, dimensionless numbers calculations and Computational Fluid Dynamics in typical reaction conditions.

Formaldehyde, one of the basic products of chemistry, is industrially synthesized by a multi-step process, in which the energy efficiency is limited. Thus, a synthesis by direct oxidation of methane in gas phase, which could be more advantageous, has been studied experimentally and by kinetic modeling, in the framework of this work. To favor the production of formaldehyde, an intermediate product of methane oxidation, low flow times (< 100 ms) were considered. An annular microreactor (0.5 mm annular space) made of quartz was used, in which the reaction was studied by varying the following operating parameters: temperature (600-1000°C), passage time (20-80 ms), XO2/XCH4 ratio (0.5-15) and added NO2 content (0-0.6%). Without NO2, HCHO selectivities decrease with conversion and the maximum yield without recycling is 2.4% (950°C, 60 ms and XO2/XCH4 = 8). The addition of NO2 decreases the required temperature by 300°C, and increases the best HCHO yield to 9% (700°C, 30 ms and XO2/XCH4 = 7 and 0.5% NO2). At low advancement, the reaction without NO2 can be modeled with the Gri-Mech 3.0 mechanism without any adjustments. For the reaction with NO2, after some corrections and modifications based on a literature review, the mechanism of Zalc et al. (2006) can correctly account for the experimental results. The flow analysis showed that the inter-conversion between NO2 and NO plays an important role in the reaction medium. It continuously forms the reactive OH- radicals, and converts CH3- and CH3O2- radicals to CH3O- radicals.