Novel design of a microstructured reactor allowing fast temperature oscillations

Two types of stainless steel microstructured reactors for catalytic gas-phase reactions have been developed and characterized with respect to their thermal behaviour under non-stationary temperature conditions. One of the reactors used (FTC-I) allowed periodic temperature changes up to 100K with a frequency of 0.05 Hz. However, a broad emperature gradient of 80K developed inside the reactor. A second reactor (FTC-II) enabled periodic temperature variations of maximum 60K with a frequency of 0.06 Hz while avoiding temperature non-homogeneity. The CO oxidation taken as a test reaction was carried out over a Pt/Al2O3 catalyst in the FTC-II reactor. In this way it was possible to study the effect of non-stationary temperature conditions on the reactor performance. A significant increase in CO conversion was observed with periodic temperature cycling as compared to values obtained under steady-state conditions.

Fast temperature cycling of the catalytic CO oxidation using microstructure reactors

Martin Luther

The possibility to increase the performance (productivity or selectivity) of a chemical reactor by using periodic variations of reaction parameters (e.g. reactant concentration or temperature) has been theoretically envisaged since the beginning of the 70th. The experimental validation of the predicted positive effects was successful in the case of concentration variation but failed for temperature variation. This was mainly due to the high thermal inertia of the conventional chemical reactors used for the measurements which prevented to create variations having a sufficiently high frequency. Microstructure reactors own, at the contrary to conventional reactors, a very low thermal inertia and allow to generate temperature oscillations with an amplitude of about ten to hundred Kelvin at a frequency in the order of magnitude of 10-1 to 4 Hz. These properties, coupled with the possibility to introduce a catalytic active material within the devices, seem to make them well suited for the study of the effects of fast periodic temperature variations of a catalytic reaction. The objective of this work was to demonstrate that non-stationary temperature conditions may increase the reaction rate of a heterogeneously catalyzed reaction up to values not predicted by the classical Arrhenius dependency towards the temperature. Two different types of microstructure reactors have been used. The reaction was taking place in the first one (FTC-type 2) in microstructured channels on whose walls a catalytic layer was deposed. In the second one (FTC-type 3), the reaction was taking place on a piece of sintered metal fibres (SMF) plate placed in a reaction chamber. The catalytic active material was deposed on the SMF plate filaments. Both devices where permanently heated by electrical resistors and periodically cooled down with water flowing through cooling channels incorporated in the devices. The test reaction chosen for the experimental measurements was the CO oxidation reaction, heterogeneously catalyzed by platinum supported on alumina (Pt/Al2O3). The reaction behaviour under stationary thermal conditions was consistent with the one predicted by the Arrhenius law. The dependency of the reaction rate with the temperature was exponential with an apparent activation energy of 104 kJ·mol-1, a negative partial reaction order for CO and a positive one for O2. The measurements effectuated under quasi-stationary thermal conditions (slow temperature ramps) have shown that a temperature change rate between 7 and 14 K·min-1 was not sufficient to observe any non-trivial effect of the temperature. The reaction behaviour was always predicted by the Arrhenius law. The surface coverage of the reactive species is, in this case, always able to follow the slow temperature changes and the reaction behaves as being always under steady-state conditions. The experiments realized under non-stationary thermal conditions using the FTC-type 2 reactor have also failed to demonstrate any non-trivial effects of the temperature oscillations. This device allows indeed to generate temperature oscillations with an amplitude of up to 120 K with a frequency of 0.1 Hz but unfortunately correlated with a very high thermal inhomogeneity. A temperature difference of up to 80 K was measured between a cold spot and a hot spot inside the device. Due to their very low local reaction rate the colder areas of the reactor have attenuated the product concentration (CO2) oscillations which should have been created by the temperature variations. This attenuation prevented the temperature oscillations to have any positive effect compared to the stationary thermal conditions. The FTC-type 3 reactor allowed temperature changes of lower amplitude but the thermal homogeneity was much better. The maximal temperature difference measured between two points within the reactor was only 15 K. Under temperature oscillation conditions, the measured instantaneous CO2 concentration was higher for any temperature within the oscillation range compared to the one recorded under stationary thermal conditions. The increase obtained in the mean CO2 concentration was ranging from 34% for a frequency of the oscillations of 0.035 Hz to 85% for a frequency of 0.052 Hz. The amplitude of the oscillations was kept constant at approximately 40 K and the mean temperature value at 437 K. The simulations effectuated using a theoretical model for the catalytic CO oxidation including a feed-back step in the form of the oxidation-reduction of the catalyst have shown that the experimentally observed increase may be qualitatively explained. Above a certain frequency, the temperature oscillations are fast enough compared to the characteristic time of the feedback step and are able to perturb the stationary established reactive species surface coverage. The formation of a transient surface coverage more favourable for the reaction than the surface coverage at high temperature allows during the transient period to reach an instantaneous surface reaction rate values higher than the one at high temperature. This reaction rate peak is then responsible for the increase of the mean reaction rate under non-stationary thermal conditions and, thus, for an increased yield.

EPFL2008

N2O Decomposition over Fe-ZSM-5 Studied by Transient Techniques

Bryan Bromley, Lioubov Kiwi, Petra Prechtl, Albert Renken

N2O decomposition to gaseous N-2 and O-2 catalyzed by a commercial Fe-ZSM-5 has been studied by different transient techniques: (i) via the transient response methods at ambient pressure, (ii) via the temporal analysis of products (TAP) reactor under vacuum, and (iii) by temperature-programmed desorption (TPD) under vacuum. The catalyst was activated in He at 1323 K. Two main steps can be distinguished within the transient period of N2O decomposition under constant N2O feed at 603 K: the first step consists of molecular N-2 formation and surface atomic oxygen (O)(Fe). It follows a period of stoichiometric N2O decomposition to gaseous N-2 and O-2 with increasing conversion until steady state is reached. The observed rate increase is assigned to a slow accumulation on the surface of NOx,ads species formed from N2O and participating as co-catalyst in the N2O decomposition. The NOx,ads species accelerates the atomic oxygen recombination/desorption, which is the rate-determining step of N2O decomposition. The formation and accumulation of NOx,ads species during N2O interaction with the catalyst was confirmed by TAP studies. The amount of NOx,ads was found to depend on the number of N2O pulses injected into the TAP reactor. In the presence of adsorbed NOx on the catalyst surface (NOx,ads ) the desorption of dioxygen is facilitated. This results in a shift of the oxygen desorption temperature from 744 K to considerably lower temperatures of 580 K in TPD experiments. Pulses of gaseous NO had a similar effect leading to the formation NOx,ads thus facilitating the oxygen recombination/desorption.

2009

Performance enhancement by unsteady-state reactor operation: Theoretical analysis for two-sites kinetic model

Lioubov Kiwi, Albert Renken

Theor. anal. of the reactor performance under unsteady-state conditions was carried out. The reactions are described by two kinetic models, which involve the participation in catalytic reaction of two types of active sites. The kinetic model I assumes the blocking of one of the active sites by a reactant, and the kinetic model II suggests a transformation of active sites of one type into another under the effect of the reaction temp. The unsteady-state conditions on the catalyst surface are supposed to be created (i) by forced oscillations of temp. and concn. in the reactor inlet (periodic operation of reactor) and (ii) by catalyst circulation between two reactors in a dual-reactor system (spatial regulation). The effect of various parameters like concn. of reactant, cycle split, length of period of forced oscillations, temps. and the ratio of catalyst vols. in the dual-reactor was investigated with respect to the yield of the desired product. It is shown that for both cases of unsteady-state conditions (periodic reactor operation as well as in a dual-reactor system), a mean reaction rate predicted by the kinetic model I was up to two times higher than the steady-state value. The kinetic model II shows a 20% increase of the selectivity towards the desired product. [on SciFinder (R)]

2003

Fast temperature cycling of the catalytic CO oxidation using microstructure reactors

Martin Luther

EPFL2008