Transient-response method

A good review of the transient response method in heterogeneous catalysis was published by Kobayashi and Kobayashi (1974). These authors credit Bermett (1967) for applying this previously microcatalytic research technique to recycle reactors and thereby, in view of this author, to engineering problems. 

Gal-Or and Hoelscher (G5) have recently developed a fast and simple transient-response method for the measurement of concentration and volumetric mass-transfer coefficients in gas-liquid dispersions. The method involves the use of a transient response to a step change in the composition of the feed gas. The resulting change in the composition of the liquid phase of the dispersion is measured by means of a Clark electrode, which permits the rapid and accurate analysis of oxygen or carbon dioxide concentrations in a gas, in blood, or in any liquid mixture. 

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. 

Three flow systems with different gas composition were prepared so that the transient response experiments could be completed for three different gas mixtures within a few minutes. A more detailed description of transient response method used in this study can be found elsewhere (.6, 7, 8). ... 

Lietti and co-workers studied the kinetics of ammonia adsorption-desorption over V-Ti-O and V-W-Ti-O model catalysts in powder form by transient response methods [37, 52, 53[. Perturbations both in the ammonia concentration at constant temperature in the range 220-400 °C and in the catalyst temperature were imposed. A typical result obtained at 280 °C with a rectangular step feed of ammonia in flowing He over a V2O5-WO3/TiO2 model catalyst followed by its shut off is presented in Figure 13.5. Eventually the catalyst temperature was increased according to a linear schedule in order to complete the desorption of ammonia. 

The adsorption of NO, under lean conditions was studied by imposing a step change of NO and NO2 feed concentrations in the presence and absence of excess oxygen over the reference catalysts in a fixed-bed flow microreactor operated at 350 ° C and analyzing the transient response in the outlet concentrations of reactants and products [transient response method (TRM)[. The adsorption/desorption sequence was repeated several times in order to condition the catalytic systems fully due to the regeneration procedure adopted (either reduction with 2000 ppm H2 + He or TPD in flowing He), BaO was the most Ba-abundant species present on the catalyst surface. FT-IR spectroscopy was used as a complementary technique to investigate the nature of the stored NO species. 

There have been several attempts to use the transition response method for the evaluation of the mixing performance of operations/equipment. In the transition response method for the flow system, a tracer is injected into the inlet and the change in its concentration at the outlet with time is measured. On the other hand, in the case of the batch system, the tracer is injected into some specific position, and the change in the spatial distribution of the concentration in the equipment with time is measured. In the following discussion, a method based on information entropy to evaluate the mixing operations/equipment on the basis of the transient response method is discussed. 

Let us deal with the transient response method. The transient response method is a method of measuring the characteristics of a system in particular, this method is effective when the dynamic characteristics of the system are investigated. In order to clarify the characteristics of the system, a comparison between the input and the output wave forms is useful. In general, the following three input wave forms have been widely used (Figure 2.1) ... 

The mixing capacity for the case where only the inflow from a specific o-region to each region occurs is a modified index that is obtained by setting 1 = 0 and then multiplied by VT/V0 in Eq. (2.30). The expression of the modified index becomes identical to the definition of the mixedness in the case of the transient response method in the previous section. This is only the contact point with the mixedness based on the transient response method. The CPD can be calculated if the transient probability of the inner substance is known. 

This fact implies that the mixedness by using the transient response method is a special case of the multi-component mixedness12. 

In order to evaluate the mixing capacity of batch mixing operation/equipment absolutely, the transient probability of an inner substance from one region to another region should be considered because the result of the transient response method depends on the region where the tracer is injected. 

In an attempt to contribute to the discussion we have applied the transient response method (5) to iron catalysts in order to get first hand information about the rate of some reaction steps in the synthesis. 

Rival Kinetic Models in the Oxidation of Carbon Monoxide over a Silver Catalyst by the Transient Response Method... 

The oxidation of carbon monoxide by nitrous oxide and oxygen over a silver catalyst at 20°C was analysed by both the Hougen -Watson procedure and the transient response method. The rival models derived from both procedures were clearly distinguished by the mode of the transient response curves of C02 or N caused by the concentration jump of CO, 02 or N20. 

It is very convenient and helpful in the selection of a sound kinetic model to have a simple experimental technique which can seriously distinguish between the rival kinetic models. To meet this necessity, our recent works have proposed the transient reponse method [7 ]. In the present study, the transient response method is typically applied to distinguish between the rival kinetic models in CO oxidation over a silver catalyst derived from the Hougen-Watson procedure. It is also shown how the best kinetic parameter-set can be determined among the rival parameter-sets by using the transient response method. 

Figure 2. Test for the discrimination of rival kinetic models by the transient response method. Key --------, Model 1 in Table 1 — Model 2 in Table l and...

Figure 60 indicates that the overall mass-transfer coefficient kob b is obtained from the use of transient-response methods, and this approach has been utilized extensively. However, the approach seems to be more appropriate to small-diameter beds. In fact, for large-diameter beds is mostly governed by the term megEjs/f c. but only little influenced by... 

Nuclear spins with different electronic environments may be brought into resonance by either one of two techniques. In the frequency-sweep method, the spectrum is recorded by sweeping the applied radiation frequency. In the transient-response method, the transient signal for its component frequencies is sorted/transformed after an induction (by pulse(s) of an applied field in terms of angles, e.g. 90° or 180°) of transient response in the system. The transient signal is changed into a normal spectrum by Fourier transformation in the transient-response method, which is used by most modem NMR spectrometers. There are four parameters that define the NMR spectrum ... 

The reality of such processes has been confirmed experimentally by at least two other independent methods. Temperature Programmed Reaction — TPRI I and Transient Response Method. ... 

The mechanism, however, may vary from one type of catalyst to another, or it may depend on the kind of hydrocarbons used as reductant as well as the reaction conditions. In this study, we studied the reaction mechanism on Fe-silicate catalyst using a technique of transient response method. 

Mechanism of NO reduction by Propene We studied the reaction mechanism of NO reduction by C3H6 on Fe-silicate catalyst by means of transient response method in the following manner. First, the reaction was carried out using a mixture of NO, C3H6, and O2 at 573 K (Reaction A). After the N2 formation rate reached the steady-state, the reactant mixture was switched for various components. 

Kobayashi, M., Kobayashi, H., 1972a. Application of transient response method to the study of heterogeneous catalysis I. Nature of catalyticaUy active oxygen on manganese dioxide for the oxidation of carbon monoxide at low temperatures. J. Catal. 27, 100-107. 

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