7/30/2019 JCGQPOSTER1

1/1

ABOUT THE SINGLE HEATING RATE/MULTIPLE HEATING

RATES TGA KINETIC ANALYSISA.Marcilla*, J.C. Garca-Quesada and R. Ruiz.

Chemical Engineering DepartmentUniversity of Alicante. Apdo. 99, E-03080 Alicante, Spain.

Tlf.: +34 965 90 34 00 - Ext. 3789, Fax: +34 965 90 38 26, *E-mail: antonio.marcilla@ua.es

REFERENCES

1] M.E. Brown, M. Maciejewski, S. Vyazovkin, R. Nomen, J. Sempere, A. Burnham, J. Opfermann, R.

Strey, H.L. Anderson, A. Kemmler, R. Keuleers, J. Janssens, H.O. Desseyn, Chao-Rui Li, Tong B. Tang, B.

Roduit, J. Malek, T. Mitsuhashi, Thermochim. Acta 355 (2000) 125-143.

2] M. Maciejewski, Thermochim. Acta 355 (2000) 145-154.

3] S. Vyazovkin, Thermochim. Acta 355 (2000) 155-163.

4] A. K. Burnham, Thermochim. Acta 355 (2000) 165-170.

5] B. Roduit, Thermochim. Acta 355 (2000) 171-180.

6] P. Budrugeac, E. Segal, E. Int. J. Chem. Kinet. 33(10), (2001) 564-573.

ACKNOWLEDGEMENTThe authors of the work wish to thank financial support provided by the Spanish Comisin de Investigacin Cientfica y Tecnolgica de la Secretara

de Estado de Educacin, Universidades, Investigacin y Desarrollo and the European Community (FEDER refunds) (CICYT CTQ2004-02187) and

y the Generalitat Valenciana (project GRUPOS03/159).

fke

dt

dRT

Ea

Figure 1.

0.0

0.2

0.4

0.6

0.8

1.0

500 600 700 800 900

Temperature (C)

Reaction order model

Jander model

2.5C/min

5C/min

10C/min

ABSTRACT

n the latter years, kinetic analysis is a tool which utilization has progressively gained importance in the

hermal degradation field and specially in that of pyrolysis.

During decades authors have been using different procedures in order to elucidate the likely processes

nvolved during materials degradation and in order to calculate the kinetic parameters associated to the

eaction pattern (the constant rate, the activation energy and the conversion degree function):

The methods found in bibliography lie on two categories: single heating rate and multi heating rates

procedures. The latter are preferred and recommended by different authors; for example it is worth

mentioning the second part of a series of papers dealing with the results of the ICTAC (International

Confederation for Thermal Analysis and Calorimetry) Kinetic Analysis Project [1-5].

In this work M. Maciejewski [2] presents an

example where he emphasized the very limited

applicability of the kinetic methods that use

single-heating rate data and stated that to obtain

reliable kinetic descriptions, computational

methods that employ multi-heating rate data

should be used for treating multi-step

processes. The example is based on the fact that

with the appropriate set of kinetic parameters

two different models (reaction order and Jander

models) can represent very similar-T curvesat one heating rate (5C/min), but at other two

different heating rates (2.5 and 10C/min) the

same kinetic parameters used at 5C/min

illustrate very different curves.

This kind of reasoning is also followed by different authors to discard single-heating rate methods and to

tate that multi heating rate method can be used to discern between likely kinetic models [6].

The question that arises is: Does multi-heating rates data analysis ensure that the kinetic model used in

he right one?:

A DEGRADATION PROCESS

FOLLOWS A REACTION

PATTERN THAT CAN BE

REPRESENTED BY A KINETIC

MODEL

DATA CAN BE PROPERLY

CORRELATED AT DIFFERENT

HEATING RATES BY A

KINETIC MODEL

n our opinion the two ways route can not be followed. Data correlated at different heating rates does not

ensure that the kinetic model considered is the one corresponding to the reaction studied. In this work we

present different examples where we illustrate how different models can represent very similar

conversion degree curves.

KINETIC MODELS EMPLOYED

Model Symbol)(f

Reaction order model Fn n)1

SestakeBerggren (empirical kinetic

model)

SBnm mn )1(

Random nucleation and growth of

nuclei (AvramiErofeev equation)

An nn11

)1ln()1(

CONCLUSIONS

Results obtained have revealed that different models are capable to correlate a set of dy

data obtained at different heating rates. Consequently care should be taken when carryin

analysis. Nevertheless, the consideration of data at different heating rates is preferable t

a single heating rate. However, in any case a good correlation of data could be used for

from a mechanistic point of view, if considering that the ability of a model to represen

necessary, but not sufficient condition to validate a kinetic model. To elucidate mec

during degradation processes this kind of analysis should be supported by other techniq

EXAMPLES OF DIFFERENT MODELS THAT CORRE

THE SAME SET OF DATA AT DIFFERENT HEATING

Conversion curves have been obtained from different models and different sets of kineti

integration by the 4th order Runge-Kutta procedure the equation:dT

fe RT

E )(A

T

0

OVERLAPPED PROCESSES

Lets us consider another type of hypothetical situation where a product A can dec

different reactions, as may occur during polymers degradation, and 50% of A follow

while 50 % the second route:

CA

BA

A mere analysis of the overall conversion degree curve or alternatively its derivative c

only one process is taking place (see Figure 5), i.e.:

CBA

The kinetic analysis of these curves can

considering one single step and the reaction

example, there exits a set of kinetic paramet

Ea(kJ/mol)=92.5, n=1.41) that allows a ve

conversion degree curves, as is possible to ob

Figure 4.

Reaction 1 and reaction 2 are described by

the reaction order model. Although their

corresponding kinetic parameters are

different, both processes are not easily

separated by changing the heating rate, aspossible to observe in Figure 4.

Are in consequence the mechanism and the model suggested valid (i.e. one single step

order model)?.

In our opinion kinetic analysis can not be used solely to validate models or reaction m

fit of experimental results is only one condition, but it should not be the unique one. Ofor example infrared or mass spectroscopy can be useful in order to obtain more

processes involved. In this example, the monitoring of likely signals attributed to B

revealed that they are not released concurrently and the suggested single step mechan

wrong .

0.0E+00

5.0E-05

1.0E-04

1.5E-04

2.0E-04

2.5E-04

3.0E-04

100 200 300 400 500 600 700

Temperature(C)

d

/dt

Apparent

Reaction1

Reaction2

0.0E+00

2.0E-04

4.0E-04

6.0E-04

8.0E-04

1.0E-03

1.2E-03

100 200

d

/dt

Rea c t. 1 Rea c t. 2

lnA (s-1) 5.0 17.5

Ea (kJ/mol) 70.7 137.2

n 1 2

Kinetic parameters used

Figure 5.

0.0

0.2

0.4

0.6

0.8

1.0

100 200 300 400 500 600 700

Temperature (C)

Generated considering

two steps

Generated considering

one step

Figure 3.

0.0

0.2

0.4

0.6

0.8

1.0

150 200 250 300 350 400 450

Temperature (C)

2 K /m in 5 K /m in 1 0 K /m in

In this example, it is also pos

how two different models rep

conversion curves at different he

THE NUCLEATION AND THE REACTION ORDER MOD

Fn An

Ln A (s-1

) 16.70 15.50

Ea (kJ/mol) 125.0 105.2

n 1.00 1.09

Figure 2.

0.0

0.2

0.4

0.6

0.8

1.0

200 220 240 260 280

Temperature (C

2 K/ mi n 5 K/ mi n

In this case both models show very similar

conversion degree curves at different heating

rates.

THE NUCLEATION AND THE SESTAKE-BERGGREN MO

An S

Ln A (s-1

) 25.0 24

Ea (kJ/mol) 140.0 13

n 3 0.

m --------- 0.