Transient microreactor experiments

The first stage of the investigation includes steady and transient microreactor experiments for the determination of the intrinsic kinetics of the various NH3 NO t reactions. According to this approach, a part of the initial extruded or washcoated catalyst is crushed into a fine powder form and placed inside a microreactor. 

This study presents kinetic data obtained with a microreactor set-up both at atmospheric pressure and at high pressures up to 50 bar as a function of temperature and of the partial pressures from which power-law expressions and apparent activation energies are derived. An additional microreactor set-up equipped with a calibrated mass spectrometer was used for the isotopic exchange reaction (DER) N2 + N2 = 2 N2 and the transient kinetic experiments. The transient experiments comprised the temperature-programmed desorption (TPD) of N2 and H2. Furthermore, the interaction of N2 with Ru surfaces was monitored by means of temperature-programmed adsorption (TPA) using a dilute mixture of N2 in He. The kinetic data set is intended to serve as basis for a detailed microkinetic analysis of NH3 synthesis kinetics [10] following the concepts by Dumesic et al. [11]. 

Fig. 37. Transient SCR microreactor experiments with step feed of NH3 (0-1,000ppm) in NO (1,000ppm)/He + H20 (1% v/v) + He + 02 (2% v/v) (A) and 02 (6% v/v (B) and at different temperatures. Symbols measured concentrations of NH3, NO, N2 at reactor outlet. Lines kinetic fitting using the modified redox rate law, Eq. (57). Reprinted with permission from Nova et al. (2006a).

Fig. 38. Transient SCR microreactor experiments with high frequency NH3 feed pulses (l,000ppm) in flowing NO (l,000ppm) + 02 (2% v/v) and H20 (1% v/v) + He at 180°C, with pulse frequency 5min on/5min off. Symbols outlet concentration of ammonia (circles), NO (squares) and N2 (triangles)—Dotted lines feed ammonia concentration. Solid lines (A) simulation using the MR rate law, (B) simulation using the ER rate law.

The TAP-2 reactor system [4] was used to perform transient response experiments under vacuum and at temperatures ranging from 300 to 400°C. A carbon loading of 100 mg was placed between two layers of quartz particles (0.2-0.3 mm particle size). Neon was used as an internal standard for calibration and as a reference for diffusion. Nitric oxide and neon were introduced by pulses in the microreactor (25.4 mm in length and 4 mm in diameter) in a volume ratio of 1 1. The reactor was continuously evacuated and the response of the pulses as a function of time was analysed on -line by a quadrupole mass spectrometer. 

Transient response experiments were performed immediately after oxidation treatments or steady-state reaction studies without exposing the catalyst to ambient conditions. All pulse response experiments using n-butane were performed with a gas blend containing a mixture of 78% n-butane and 12% argon. Typical pulse intensities were in the range of 10 molecules per pulse. The argon pulse response was determined to be independent of the pulse intensity. Under these conditions gas transport through the microreactor can be described by Knudsen diffusion. 

The key feature which distinguishes it from other pulse experiments is that no carrier gas is used and gas transport is the result of a pressure gradient. A TAP pulse response experiment is a special type of transient response experiment that involves injecting a small gas pulse of veiy short duration into an evacuated microreactor containing a packed bed of particles. When the number of molecules in the pulse is sufficiently small ( 10 10 mol) convective flow disappears, gas transport occurs by Knudsen diffusion, and the molecules move through the... 

The development of global reaction kinetics is a stage wise scale-up approach. The various stages of this multiscale approach are summarized in Fig. 13.6 and include (a) microreactor experiments over powdered catalyst for the determination of the intrinsic reaction kinetics, (b) synthetic gas bench experiments over small monolith samples to account for intraporous diffusion of species, and (c) validation on steady-state and transient engine tests. 

Apparatus and Procedure. The kinetic studies of the catalysts were carried out by means of the transient response method (7) and the apparatus and the procedure were the same as had been used previously (8). A flow system was employed in all the experiments and the total flow rate of the gas stream was always kept constant at 160 ml STP/min. In applying the transient response method, the concentration of a component in the inlet gas stream was changed stepwise by using helium as a balancing gas. A Pyrex glass tube microreactor having 5 mm i.d. was used in a differential mode, i.e. in no case the conversion of N2O exceeded 7 X. The reactor was immersed in a fluidized bed of sand and the reaction temperature was controlled within + 1°C. 

In the first stage of the investigation the catalyst can be considered in the form of powder in order to derive intrinsic transient kinetics of all the relevant reactive processes. To this purpose, dynamic reactive experiments can be performed in a simple tubular fixed-bed microreactor over small quantities (50-200 mg) of finely powdered catalyst in principle, this guarantees negligible transport limitations and more controlled conditions (e.g. isothermal catalyst bed), hence enabling a direct estimation of intrinsic rate parameters by kinetic fit. Internal diffusion limitations are particularly relevant to the case of bulk (extruded) monolith catalysts, such as vanadium-based systems for NH3/urea SCR however, they... 

The transient experiments herein described were carried out over powdered catalyst in a microreactor a portion consisting of several grams from the original extruded monolith was crushed and sieved to a powder (140-200 mesh). One hundred and sixty milligrams of this powder, diluted with 80 mg of quartz were eventually loaded in the microreactor. Intraparticle gradients and gas-solid mass transfer limitations were ruled out by theoretical criteria (Mears, 1971). 

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