Predicted induction periods

The Landolt reaction (iodate + reductant) is prototypical of an autocatalytic clock reaction. During the induction period, the absence of the feedback species (Irere iodide ion, assumed to have virtually zero initial concentration and fomred from the reactant iodate only via very slow initiation steps) causes the reaction mixture to become kinetically frozen . There is reaction, but the intemiediate species evolve on concentration scales many orders of magnitude less than those of the reactant. The induction period depends on the initial concentrations of the major reactants in a maimer predicted by integrating the overall rate cubic autocatalytic rate law, given in section A3.14.1.1. 

Hansen L D and Hart R M 1978 Shelf-life prediction from Induction period J. Electrochem. Soc. 125 842... 

Owing to the widespread use of ultrasonic cleaning baths, it is not surprising that many early sonochemical experiments were directed at reactions where dirty metal surfaces were thought to be the cause of inefficiencies. Reactions typified by Grignard and Simmons Smith reactions (Scheme 7.11) are often not predictable, sometimes having long induction periods followed by violent exotherms. Frequently, small... 

Previous reports on FMSZ catalysts have indicated that, in the absence of added H2, the isomerization activity exhibited a typical pattern when measured as a function of time on stream [8, 9], In all cases, the initial activity was very low, but as the reaction proceeded, the conversion slowly increased, reached a maximum, and then started to decrease. In a recent paper [7], we described the time evolution in terms of a simple mathematical model that includes induction and deactivation periods This model predicts the existence of two types of sites with different reactivity and stability. One type of site was responsible for most of the activity observed during the first few minutes on stream, but it rapidly deactivated. For the second type of site, both, the induction and deactivation processes, were significantly slower We proposed that the observed induction periods were due to the formation and accumulation of reaction intermediates that participate in the inter-molecular step described above. Here, we present new evidence to support this hypothesis for the particular case of Ni-promoted catalysts. 

The Responses for 9, 9jjo> and are shQvm in Fig 12. The value of 9fl, which is initially 0.925, decreased during the period of NO exposure at the same time that the value of 0jjq increases rapidly from zero. It should be noted that the predicted rate of accumulation of adsorbed NO is qualitatively consistent with the dynamics of the band appearing at 1680 cm-, associated with No shown in Fig. 4. It is seen, though, that while the experimental results exhibit a short induction period followed by a rapid rise in the absorbance of the 1680 cm-- - band to a saturation level, the predicted curve shows a smooth monotonic increase in 0jjo ... 

The self-heating and ignition of baled or loose wool in bulk storage is discussed and analysed, and steady state thermal explosion theory is applied to the prediction of critical masses and induction periods for storage and transportation situations in relation to ambient temperature. Results obtained were consistent with current safety practices. 

Figure 2.32. Predicted dependences of a dimensionless induction period in a non-isothermal process on dimensionless shear rate at different values of the parameter i 0 (curve 1) 0.6 (curve 2) 1.2 (curve 3) 1.8 (curve 4).

The correspondence between the calculated results based on the model of heat dissipation due to viscous flow and the experimental data in the decrease of the induction period at high shear rates proves that the observed effect is adequately explained by this mechanism. The effects of shearing itself on the kinetics of curing are either absent or of secondary importance. If the experimentally observed decrease in the induction period is more pronounced than predicted by the dissipative model, then it is reasonable to consider additional heat sources, for example, the exothermal effects of a reaction. Heat flux from the surroundings can also influence the kinetics of... 

Figure 2.34. Comparison of theoretical predictions (curve, calculated from Eq. (2.8S) according to the dissipative model of non-isothermal curing) with experimental data on the decrease of the induction period at high shear rates for phenolic-based compounds (vertical bars) and silicon-based composites at different initial temperatures 150°C (1) 170°C (2) and 190°C (3).

This simple kinetic model predicts the presence of an induction period in isothermal runs, where (3 gets close to (3eq, followed by a first-order kinetics, as experimentally observed. 

Hansen LD, Eatough DJ, Lewis EA, Bergstrom RG, Degraft-Johnson D, Cassidy-Thompson K. Shelf-life-prediction from induction period calorimetric measurements on materials undergoing autocatalytic decomposition. Can J Chem 1990 68 2111-2114. 

Thus it appears that the simple models for the ideal adsorbed layer can be used to predict the critical effects that have not previously been found experimentally. In particular, the results [146-148] permit us to interpret (and predict) the induction period for the reaction with long-term relaxation near critical transition (points of bifurcation) observed in the experimental studies of reactions characterized by critical effects [164, 169],... 

A simple model of the chemical processes governing the rate of heat release during methane oxidation will be presented below. There are simple models for the induction period of methane oxidation (1,2.>.3) and the partial equilibrium hypothesis (4) is applicable as the reaction approaches thermodynamic equilibrium. However, there are apparently no previous successful models for the portion of the reaction where fuel is consumed rapidly and heat is released. There are empirical rate constants which, due to experimental limitations, are generally determined in a range of pressures or concentrations which are far removed from those of practical combustion devices. To calculate a practical device these must be recalibrated to experiments at the appropriate conditions, so they have little predictive value and give little insight into the controlling physical and chemical processes. 

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