A new procedure for the prediction of the isothermal behaviour of the solid-gas system from non-isothermal data is suggested.
It bypasses the use of various approximations of the temperature integral that ground the integral methods of prediction.
The procedure was checked for: (1) simulated data corresponding to a first order reaction; (2) experimental data obtained
in the isothermal and non-isothermal decompositions of ammonium perchlorate. For the simulated data, a very good agreement
between calculated isotherms and those evaluated by means of the suggested procedure was obtained. A satisfactory agreement
(errors in time evaluation corresponding to a given degradation lower than 18%, for 0.10a0.37 and lower than 10% for 0.37a0.70)
was obtained for the experimental data corresponding to the decomposition of ammonium perchlorate. In this last case, the
mentioned differences between experimental and calculated data can be due both to the inherent errors in the evaluation of
the decomposition isotherms and to the dependence of the activation energy on the conversion degree.
Authors:Zhou Bao-xue, Zhong Wei-ke, Zou Li-zhuang, and Wang Xiao-ling
Barium(II) tetraphenylborate, Ba(Bph4))2·4H2O was prepared, and its decomposition mechanism was studied by means of TG and DTA. The products of thermal decomposition were examined by means of gas chromatography and chemical methods. A kinetic analysis of the first stage of thermal decomposition was made on the basis of TG and DTG curves and kinetic parameters were obtained from an analysis of the TG and DTG curves using integral and differential methods. The most probable kinetic function was suggested by comparison of kinetic parameters. A mathematical expression was derived for the kinetic compensation effect.
A differential method is proposed which uses local heating rates to evaluate non-isothermal kinetic parameters. The method
allows to study the influence of the deviation of the true heating rate with respect to the programmed one on the values of
the kinetic parameters. For application, the kinetic parameters of the following solid-gas decomposition reaction were evaluated:
[Ni(NH3)6]Br2(s)→[Ni(NH3)2]Br2(s)+4NH3(g). The results obtained revealed significant differences between the values of the non-isothermal kinetic parameters obtained
by using local heating rates and those obtained by using the programmed heating rate. It was also demonstrated that the kinetic
equation which makes use of the local heating rates permits a better description of the experimental (α, t) data than the
kinetic equation which uses the programmed constant heating rate.
Authors:M. Badea, R. Olar, E. Cristurean, D. Marinescu, M. Brezeanu, C. Calina-Soradi, and E. Segal
Data concerning the thermal behaviour of four heteropolynuclear compounds with the general formula [CuML(CH3COO)3] whereM=Ni(II), Zn(II), Mn(II) and Co(II); LH=2-amino-5-mercapto-1,2,3-thiadiazole were obtained. For the kinetically workable decomposition steps the values of the kinetic
parameters were estimated.
The adsorption of n -butane on extruded cylindrical activated carbon grains is studied providing two kinds of information: the influence of the
temperature and the hydrocarbon partial pressure on the adsorption dynamics (kinetic study) and on the adsorption capacities
(thermodynamic study). The thermodynamic aspect could be interpreted by a Langmuir model. From a kinetic point of view, we
have experimentally proved that strong temperature variations occur inside the particles during the adsorption. In this paper,
a kinetic model including both mass and heat transfer phenomena is proposed. Good agreement is found between the kinetic model
predictions and the experimental mass and temperature variations inside the grain during the hydrocarbon adsorption.
Summary The thermal behavior of KH2PO4, NaH2PO4 and Na2HPO4 under non-isothermal conditions using TG method with different heating rates was studied. The values of the reaction rate were processed by means of Friedman’s differential-isoconversional method. A dependence of the activation energy vs. conversion was observed. Therefore a procedure based on the compensation effect (suggested by Budrugeac and Segal) was applied. A less speculative data processing protocol was offered by the non-parametric kinetics method suggested by Serra, Nomen and Sempere. Three steps were observed by non-isothermal heating: a dehydration, a dimerization and a polycondensation. The differences in the intimate reaction mechanism are determined by the initial number of water molecules.
Authors:P. Budrugeac, D. Homentcovschi, and E. Segal
The differential and integral isoconversional methods for evaluation the activation energy, described in the first note of
this series, were applied for:
a) simulated data for two successive reactions;
b) dehydration of calcium oxalate monohydrate.
It was shown that for these systems the activation energy depends on the conversion degree as well as on the method of evaluation.
The condensation approximation (CA) and numerical regularization procedure (RP) methods used to solve a Fredholm integral
equation of the first kind describing the adsorption equilibria on a heterogeneous solid surface under isothermal conditions
have been adopted in the present study to evaluate desorption energy distributions from temperature-programmed desorption
(TPD) spectra. From comparisons of the computational results obtained by means of these methods on the basis of simulated
TPD spectra, it follows that the CA gives stable solutions for wide desorption energy distributions and it can be used successfully
for calculations from wide and clear resolved peaks in the TPD spectra. The use of the RP is more advantageous for acquisition
of the distributions from closely related narrow peaks in the TPD spectra.
Summary Many years ago, thermal analysis earned its place as a current instrumentation technique in assisting/solving the analytical problems of pharmaceuticals. A relative new trend is the study of the thermal stability of food additives in connection with the molecular structure. The studied compounds were: natrium and potassium glutamate, respectively natrium, potassium and calcium benzoate. The thermogravimetric data (TG) were obtained in dynamic nitrogen atmosphere, with open Pt crucible and heating rates of 5, 7, 10 and 12 K min-1, using a Perkin-Elmer TGA7 equipment. In order to estimate the non-isothermal kinetic parameters, the Friedman's differential-isoconversional method and the method suggested by Budrugeac and Segal (based on the compensation effect) were used. A variation of the activation energy vs. conversion was observed by using Friedman's method. The discrimination between the different reaction steps was performed by the non-parametric kinetic method, suggested by Sempere, Nomen and Serra. This is due to a complex process. The thermal stability data are very important for avoiding a possible misuse by processing of the studied food additives.
An improved version of the Coats-Redfern method of evaluating non-isothermal kinetic parameters is presented. The Coats-Redfern approximation of the temperature integral is replaced by a third-degree rational approximation, which is much more accurate. The kinetic parameters are evaluated iteratively by linear regression and, besides the correlation coefficient, the F test is suggested as a supplementary statistical criterion for selecting the most probable mechanism function. For applications, both non-isothermal data obtained by theoretical simulation and experimental data taken from the literature for the non-isothermal dehydration of Mg(OH)2 have been processed.