Intrinsic kinetic phenomena

To summarize, transient kinetic experiments are an established and valuable tool in the investigation of heterogeneously catalysed gas phase reactions. For liquid-phase systems, transient studies are much more rare than for gas-phase systems. It is probably related to slower dynamics and the fact that the intrinsic kinetic phenomena can be obscured by mass transfer effects and catalyst deactivation. As an illustration (Figure 8.11) we will consider three-phase continuous hydrogenation of an organic compound leading to two products over a metal catalyst on a structured support (knitted silica). 

Thermochemical studies of acidity and hydricity are extremely valuable, since they can help determine the energies of potential intermediates in catalytic cycles, and can thus guide the choice of complexes proposed as catalysts. But, since catalysis is a kinetic phenomenon, the kinetics of delivery of a proton or hydride are also important. The kinetics of proton transfer from metal hydrides to amines [21] or metal alkynyl complexes [22], as well as degenerate proton transfers between metal hydrides and metal anions [21] led to the conclusion that proton transfers from metal hydrides have a high intrinsic barrier. 

A significant feature of physical adsorption is that the rate of the phenomenon is generally too high and consequently, the overall rate is controlled by mass (or heat transfer) resistance, rather than by the intrinsic sorption kinetics (Ruthven, 1984). Thus, sorption is viewed and termed in this book as a diffusion-controlled process. The same holds for ion exchange. 

Metal deposition in hydrotreating of heavy oils is one of the most important phenomenon causing catalyst deactivation. Present work focuses on the modeling of hydrodemetallisation catalyst deactivation by model compound vanadyl-tetraphenylporphyrin. Intrinsic reaction kinetics, restrictive diffusion and the changing catalyst porous texture are the relevant phenomena to describe this deactivation process. The changing catalyst porous texture during metal depositon can be described successfully by percolation concepts. Comparison of simulated and experimental metal deposition profiles in catalyst pellets show qualitative agreement. 

The author is well aware of the fact that many aspects which have been treated in the extensive literature on extrinsic crazing have not been considered in this article and that more information is needed for a comprehensive account of the observed craze phenomenon. For instance the recent work on the intrinsic crazing of PC and on related phenomena which has been re wed here has primarily been based on structural considerations. It is believed that future work on the kinetics of craze formation and on the underlying molecular dynamics of the system may contribute considerably to a more detailed account of this phenomenon. Nevertheless, it is hoped that this work has opened up some new paths which may lead to a better understanding of the phenomenon of cavitational plasticity in polymers. 

Steps 3-5 are strictly chemical and consecutive to each other Hougen-Watson-Langmuir-Hinshelwood rate equations describing the rate of the purely chemical phenomenon consisting of steps 3-5 have been derived in Chapter 3 on the kinetics of catalysed reactions. In the transport-limited situation the supply of reactant and/or the removal of reaction product will not be sufficiently fast to keep pace with the potential intrinsic rate, and the concentrations of A and B inside the pores will be different from the corresponding concentrations in the bulk of the fluid phase. 

Multiphase reactions can be significantly affected by how well mixed the system is and how intimately dispersed the phases are. The reason for this is easy to explain, but more difficult to quantify although the course of any reaction is determined exclusively by the local concentrations of the reactants and the intrinsic reaction kinetic rates, in any real reactive system, the local reactant concentrations depend not only on how fast the reactants are depleted by the reaction, but also on how fast they are locally replenished from the bulk of the phases in which they initially reside. The latter phenomenon is directly related to the existence of a mass transfer step (in series with the reaction step), which determines the rate at which the reactants in different phases are brought in contact with each other. In many cases, especially if the rate of reaction is fast with respect to the mass transfer rate, the latter mechanism can become controlling over the former, and the overall reaction process is dominated by mass transfer and, hence, multiphase mixing. 

Diffusional limitations are often analysed through the use of the Weisz modulus, 0, which compares the observed reaction rate to the difiusion rate [18]. When 0 1, the diffusion phenomenon is not significant and the observed reaction rate is equal to the intrinsic reaction rate. When 1, diffusional limitations modify the apparent kinetics, and the observed reaction rate can be very different fi om the intrinsic reaction rate. Since carbon xerogels are composed of two distinct levels, i.e. the pellet level and the microporous nodules level, both with their own pore size and length scale, 0 must be calculated at both levels. This was the object of a complete study [19]. 

The magnitude of EDR can be conveniently expressed by means of the effectiveness factor. The effectiveness factor is a general concept that represents the ratio of rates of a phenomenon under the influence of a factor and freed from that influence. For the present case, it is defined as the ratio of the reaction rate under EDR and that attainable in its absence, this is, the ratio of effective to intrinsic reaction rate. For simple Michaelis-Menten kinetics ... 

Finally, it should be added that the phenomenon of parametric sensitivity, dealt with in this section, is essentially different from the instability encountered in Chapter 10. The tubular reactor with plug flow without recycle as considered here is intrinsically stable in the strict sense (except if kinetic instability would occur). On removal of the perturbation, the reactor will return to its original state. The hysteresis phenomenon encountered in Chapter 10 is not possible in the present case all intermediate steady states are possible. 

The kinetic features of structural relaxation in polymer glasses were first reported by Kovacs in the 1960s [70]. He systematically studied the structural relaxation phenomenon by measuring the volumetric response of polymers subjected to various thermal histories. The findings can be summarized as three ingredients [71] (1) intrinsic isotherm, (2) asymmetry of approaching equilibrium, and (3) memory effect. 

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