Wavefront analysis

Last but not least, two epoch-marking technologies have been successfully implemented in 1989 and 1994 active optics, with the 3.5 m ESO New Technology Telescope (NTT), and segmentation, with the 10 m Keck. In the former, real-time adjustments of the primary mirror support forces and of the alignment of the secondary mirror guarantee consistent, optimal performance, and allow relaxation of opto-mechanical fabrication tolerances. These adjustments being derived from wavefront analysis of off-axis stellar sources located outside the scientihc held of view, imply minimal operational overheads at the beneht of reliable, substantial performance improvement... 

Wavefront Analysis—A Special Transient Technique for Kinetic Investigations in Distributed Catalytic Systems... 

Our main motivation to develop the specific transient technique of wavefront analysis, presented in detail in (21, 22, 5), was to make feasible the direct separation and direct measurements of individual relaxation steps. As we will show this objective is feasible, because the elements of this technique correspond to integral (therefore amplified) effects of the initial rate, the initial acceleration and the differential accumulative effect. Unfortunately the implication of the space coordinate makes the general mathematical analysis of the transient responses cumbersome, particularly if one has to take into account the axial dispersion effects. But we will show that the mathematical analysis of the fastest wavefront which only will be considered here, is straight forward, because it is limited to ordinary differential equations dispersion effects are important only for large residence times of wavefronts in the system, i.e. for slow waves. We naturally recognize that this technique requires an additional experimental and theoretical effort, but we believe that it is an effective technique for the study of catalysis under technical operating conditions, where the micro- as well as the macrorelaxations above mentioned are equally important. 

As for any transient technique, it is essential also for wavefront analysis... 

If the step is only equilibrium controlled, a lumped analysis of both fluid and solid phase is possible and the corresponding nonlinear wavefront analysis is straight forward too. Analysis of propagation speed data yield information about the relevant equilibrium sorption mechanism (7). The different form of the transients when a reaction step has been stimulated, is discussed in the Appendix. 

Application of wavefront analysis to watergas shift reaction... 

We report here about the investigation of the low temperature watergas shift reaction on an industrial catalyst (GIRDLER G 66-B and E with copper and zinc oxides as main components) under transient conditions by means of wavefront analysis. After a qualitative analysis to obtain information about the relevant mechanistic scheme the main effort has been concentrated on the dependence of the microkinetics on different oxidation states of the catalyst. The watergas shift reaction in its overall formulation... 

The fitting of space relaxation data using Eq. (1) to this mechanistic scheme (space relaxation data are always isothermal, because transient temperature effects are not relevant for the amplitude change of a concentration disturbance this is just an advantage of wavefront analysis of reaction kinetics), reported in (3, 5, 12), supposing a Langmuir type chemisorption for (CO) and (I O) has confirmed that (see Figure 10, 11) ... 

General Remarks. The wavefront analysis as presented here, is limited only to the chemical relaxations and excludes any dissipation effects. The space relaxation, manifested as the change of the propagating primary wavefront itself, is met with and corresponds to the initial steady state or an equilibrium state of the system. On the other hand, the time relaxation manifests the changes in the system, caused by the primary wavefront. 

Elements of response Wavefront analysis Conventional transient response techniques... 

For the nonideal situation which does not fulfill the above mentioned requirements, the wavefront analysis fails to give a detailed information about the mechanistic scheme. But in any case, the relaxations, measured at the wavefront, correspond at least to the stimulated limiting step. 

Fiolitakis and Hofmann—wavefront analysis supports Eley-Rideal/redox mechanisms. In 1982 and 1983, Fiolitakis and Hofmann231,232 carried out wavefront analysis to analyze the dependence of the microkinetics of the water-gas shift reaction on the oxidation state of CuO/ZnO. They observed three important mechanisms after treatment of the catalyst surface with different H20/H2 ratios. These included two Eley-Rideal mechanisms which converted the reactants via adsorbed intermediates, and a redox mechanism that regulated the oxygen activity, as shown in Scheme 56. The authors indicated that different mechanisms could be dominating at different sections along the length of the fixed bed reactor. 

Fiolitakis, E., Hoffmann, U., and Hoffmann, H. Application of wavefront analysis for kinetic investigation of water-gas shift reaction. Chemical Engineering Science, 1980, 35, 1021. 

Fiolitakis, E. Hoffmann, H. Dependence of the kinetics of the low-temperature water-gas shift reaction on the catalyst oxygen activity as investigated by wavefront analysis. J. Catal. 1983, 80, 328-339. 

Wavefront analysis, a dynamic method for kinetic investiga-tions... 

A similar non-stationary method used for the elucidation of reaction schemes and kinetic analysis of individual reaction steps in complex systems is the so-called wavefront analysis /49/. The experimental set-up consists of an adiabatically operated packed bed reactor with a dosing section able to produce a step disturbance at the entrance and an analysis section, e.g. a mass spectrometer, to follow the front of the transition function at the exit (Fig. 30). 

Fig. 32 Reaction scheme of the watergas shift reaction according to the wavefront analysis /49, 50/.

In fact, wavefront analysis yields much more detailed qualitative and quantitative information than the analysis of stationary operated reactors. Naturally, the technique also has its limits. E.g., if complex reaction systems including molecules of higher molecular weight are involved, the mass-spectrometer data are probably difficult to analyse quantitatively for all reactants and products. 

J.L. Falconer and J.A. Schwarz Temperature programmed desorption and reaction application to supported catalysts Catal.Rev.-Sci.Eng. ( 1983) 141-227 /49/ E. Fiolitakis and H. Hofmann Wavefront analysis, a specific analysis of transient responses for the investigation of continuously operated distributed heterogeneous reaction systems, Catal.Rev.Sci.Eng. 7 (1982) 1 13-157 and ACS Symp.Ser. 7 ... 

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