Theoretical consideration has been made of the non-isothermal kinetics of consecutive reactions based on the superposition
principle. In the model the first reaction product reacts to form the final product and the two reactions proceed independently.
The amount of the first reaction product and the production rate of the final product have been obtained as a function of
time for isothermal cases and as a function of the reduced times for non-isothermal cases.
The authors continue their considerations concerning the validity of the steady-state approximation in non-isothermal kinetics.
A sequence of two first-order consecutive reactions with an active intermediate was subjected to kinetic analysis by numerical
solution of the corresponding differential kinetic equations for a number of particular cases. The results demonstrated that
the rate of change of concentration of the active intermediate is negligibly small if the assumption made in the isothermal
case is also accepted for the non-isothermal case, i.e. k2(T(t))>>
The integral methods, which are obtained from the various approximations for the temperature integral, have been extensively
used in the non-isothermal kinetic analysis. In order to obtain the precision of the integral methods for the determination
of the activation energy, several authors have calculated the relative errors of the activation energy obtained from the integral
methods. However, in their calculations, the temperature integral at the starting temperature was neglected. In this work,
we have performed a systematic analysis of the precision of the activation energy calculated by the integral methods without
doing any simplifications.
The results have shown that the relative error involved in the activation energy determined from the integral methods depends
on two dimensionless quantities: the normalized temperature θ=T/T0, and the dimensionless activation energy x0=E/RT0 (where E is the activation energy, T is the temperature, T0 is the starting temperature, R is the gas constant).
The most debatable and discrepant viewpoints of non-isothermal kinetics are discussed in the form of twelve questions and answers. The reputation of non-isothermal kinetics when carried out by thermoanalysts; the consequences of simplified concepts transferred from the kinetics of homogeneous reactions; the physical meaning of basic kinetic parameters in solid-state processes; the kinetic compensative effect and interdependence of kinetic parameters using the Arrhenius rate constant; the mutual usefulness of differential and integral methods of kinetic data evaluation; their accuracy and correctness; the reliability of DTA measurements; non-isothermal versus isothermal investigations; equilibrium and kinetic data and their mutual effect; the extended discussion initiated by MacCallum and Tanner; non-isothermal data publication policy; and finally the use of computers.
Authors:Eda Elmas, Kenan Yildiz, Nil Toplan, and H. Özkan Toplan
The effects of mechanical activation on the thermal behavior of kaolinite – alumina ceramic system and the non-isothermalkinetics of mullite formation were investigated and the following results were obtained.
In the kaolinite
A number of 1145 sets of kinetic parameters derived in our earlier papers from TG curves have been worked up. The apparent activation energy and pre-exponential factor values have been found to obey a linear compensation law (isokinetic relation) if the thermal decomposition begins in the same temperature interval, irrespective of the nature of the chemical reaction. The isokinetic temperatureTi has been found to be very close to the mean value of the temperaturesT0.1 at which the conversion becomes equal to 0.1 and atTi the rate constant has been found to be approximately equal to 10−3s−1 in allT0.1 intervals investigated. It is concluded that the kinetic compensation effect observed in heterogeneous non isothermal TG kinetics is not a true one.
The object of this work is the quantitative explanation of linear correlation between activation energy (E), initial decomposition temperature (Ti) and ionic potential (Vi), observed for thermal degradation of some complexes of transitional metals.
The proposed model allowed the evaluation of characteristic parameter proportional to the activation free enthalpy and also
the variation of effective electrical charge (ΔQ*) of ligand, in the formation process of the activated complex.
These results are satisfactory, taking into account that we utilized many simple hypotheses for deduction of Arrhenius equation.