Transient experiments, analysis

The severity of the transient conditions can be established on the basis of past experience or data collected from similar installations. However, for large and more critical installations, such as a generating station or a large switchyard, it is advisable to carry out transient network analysis (TNA) or electromagnetic transient programme analysis (EMTP) with the aid of computers. For more details refer Gibbs et al. (1989) in Chapter 17. Where this is not necessary, the system may be analysed... 

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]. 

Table 4 summarizes the rate constants kj - Aj exp(-Ej / RT) for the forward and the reverse reaction derived from our microkinetic analysis of the steady-state and transient experiments with the three catalysts, i.e. Cs-Ru/MgO, Ru/MgO, and Ru/AlaOs catalyst [24]. 

Analysis of the dynamics of SCR catalysts is also very important. It has been shown that surface heterogeneity must be considered to describe transient kinetics of NH3 adsorption-desorption and that the rate of NO conversion does not depend on the ammonia surface coverage above a critical value [79], There is probably a reservoir of adsorbed species which may migrate during the catalytic reaction to the active vanadium sites. It was also noted in these studies that ammonia desorption is a much slower process than ammonia adsorption, the rate of the latter being comparable to that of the surface reaction. In the S02 oxidation on the same catalysts, it was also noted in transient experiments [80] that the build up/depletion of sulphates at the catalyst surface is rate controlling in S02 oxidation. 

This chapter reports the results from transient experiments (mainly, TPD or TPSR) coupled with on-line analysis of reaction mixture at the outlet of a well-stirred reactor. It means that the gas composition detected at the outlet of the reactor is in contact with the catalyst inside the reactor. Catalytic runs in isothermal conditions were also proceeded in order to avoid strong adsorptions of reactants or intermediates. 

Temporal analysis of products (TAP) reactor systems enable fast transient experiments in the millisecond time regime and include mass spectrometer sampling ability. In a typical TAP experiment, sharp pulses shorter than 2 milliseconds, e.g. a Dirac Pulse, are used to study reactions of a catalyst in its working state and elucidate information on surface reactions. The TAP set-up uses quadrupole mass spectrometers without a separation capillary to provide fast quantitative analysis of the effluent. TAP experiments are considered the link between high vacuum molecular beam investigations and atmospheric pressure packed bed kinetic studies. The TAP reactor was developed by John T. Gleaves and co-workers at Monsanto in the mid 1980 s. The first version had the entire system under vacuum conditions and a schematic is shown in Fig. 3. The first review of TAP reactors systems was published in 1988. 

Transient experiments also require that the analysis of the outlet gas mixture must be continuous this determines the choice of suitable gas analyzers with high time resolution, which should allow to monitor the temporal evolution of the largest possible number of species involved in the considered reactions. Measured composition dynamics typically need to be corrected for the transfer functions of the test rig and of the analyzers, as done, e.g. by Oh and Cavendish (1985), Siemund et al. (1996) and Nova et al. (2006a) on the basis of blank composition step change experiments. The important role of suitable gas analyzers in understanding the dynamic behavior of SCR systems is specifically discussed in (Ciardelli et al., 2007b). 

In 2001, Mirodatos et al. [89] stressed the importance of transient studies as an alternative to steady continuous reactor operations. A combination of microkinetic analysis together with transient experiments should allow the determination of the global catalytic conversion from elementary reaction steps. Prerequisite for such analysis is the correlation of experimental data with the data of a model. Compliance between the data helps to derive the reaction mechanism. 

XPS (x-ray photoelectron spectroscopy) utilizes photoionization and energy-disperse analysis of the emitted photoelectrons to study the composition and electronic state of a region of the surface of a zeolite. However, aU these techniques are destructive ones, and for that reason other methods such as isotopic-transient experiments or reflectance [16] and fluorescence [17] imaging can be used to estimate the effective membrane thickness. 

In order to use the transient experiments to calculate the surface coverage of coke, we first introduce the continuous deactivation function. We define the deactivation function as the activity related to the first analysis after 5 minutes and compensated for the changes in reaction conditions. The compensation is carried out by assuming that the degree of deactivation immediately before and after a change in reaction conditions is the same. To be able to do so, a few assumptions have to be made (see Discussion) ... 

The model for a—j3 intersubunit communication indicates that it is the formation of the aminoacrylate species that leads to activation of the a reaction. When both serine and IGP are added simultaneously to the enzyme in a single enzyme turnover experiment, there is a lag in the cleavage of IGP that is a function of the reaction of serine to form the aminoacrylate species. Accordingly, amino acids other than serine that can undergo dehydration to form the aminoacrylate such as cysteine should serve as alternate substrates but should lead to a longer lag for the a subunit activation as determined by transient kinetic analysis. Cysteine does... 

In the following section, we will discuss the kinetic implications of the dynamical changes in catalyst morphology during methanol synthesis. First, we will present an analysis of steady state kinetic experiments using a static, microkinetic model where it is assumed that the number of sites are constant. Then, we will introduce the dynamic aspect into the microkinetic modeling and also discuss some recent transient experiments. 

In the present communication we report on the influence of water on the FT synthesis studied by SSITKA and conventional kinetic experiments. Steady-state isotopic transient kinetic analysis (SSITKA) has proved to be a powerful technique for this work. The technique involves switching between CO and " CO in the feed gas and analyzing the transients with respect to the formation of products containing C and C. This technique allows the determination of the true turnover frequency of the active site, decoupled from site coverage. Applied to the FTS over metal promoted cobalt catalysts SSITKA has shown that the true turnover frequency of cobalt always remains the same, regardless of the second metal [6-8]. 

The transient kinetic analysis of the isotopic exchange H/D is a direct and useful method to investigate the transformation of the catalytic surfaces under different reaction conditions. These experiments prove that there is a strong variation among the OH population over the oxides in the hydrogen stream zis a function of the temperature and of the nature of the oxidic support of Ni catalysts. 

Since the isotopic transient technique involves the number and type of intermediates on the catalyst surface, independent transient experiments (with or without the use of isotopes) have also been used to determine these parameters. The simplest reaction for analysis by the isotopic transient kinetic technique for the conversion of syngas is the production of methane. Studies of methanation provide a background to the isotopic transient kinetic studies and independent justification for the number and type of adsorbed species involved in FTS. Furthermore, the production of methane is undesirable for FTS and an understanding of the mode of its production will aid in FTS catalyst and process design. 

The analysis of DMA data is illustrated in Fig. 4.160 with the example of shear deformation. The periodicity of the experiment is expressed in frequency, either in hertz, v, (dimension s ), or in w (dimension radians per second). Periodic experiments at frequency (o in DMA, as also in TMDSC, are comparable to a nonperiodic, transient experiment with a time scale of t 1/w = 0.159 s. 

The single-pellet diffusion reactor can be employed for transient experiments. Cannestra et al. [65] give an example. Gas composition was measured at the center of one-dimensional pellets. The standard single-pellet diffusion reactor was modified to allow continuous gas analysis and miniaturized in order to reduce the time constants of gas flow mixing. A rather simple model for data evaluation used by the authors was not able to predict major features of the response measurments at the pellet center but gave qualitatively correct results of the external concentration responses. This demonstrates the necessity of an elaborate modeling of instationary multicomponent diffusion and porous structure for this type of reactor. 

Matsumoto et al [11] used a transient analysis to study the methantion reaction on a fused magnetite catalyst (96,5% Fe304 2,5% AI2O3 and 0,6% K2O as promoters 0,4% Si02 as support). During transient experiments the only hydrocarbon observed was methane. After cleaning the surface with He, and subsequent dosix with H2, methane was the main product observed. However, ethane and propane were also observed in smaller amounts. 

Pulse intensities in vacuum experiments range from 10 to 10 molecules per pulse with a pulse width of 250 ps and a pulse frequency of between 0.1 and 50 pulses per second. Such a spectrum of time resolution is unique among kinetic methods. Possible experiments include high-speed pulsing, both single-pulse and multipulse response, steady-state isotopic transient kinetic analysis (SSITKA), temperature-programmed desorption (TPD), and temperature-programmed reaction (TPR). 

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