(60a) Evaluating Kinetic Parameters for Solid Substances Exhibiting Complex Self-Heating Behavior

Authors: 
Ogle, R., Exponent, Inc.
R. Morrison, D., Exponent, Inc.


Determining safe storage conditions for a self-heating material requires knowledge of the reaction kinetics governing its decomposition under potential storage conditions. Several thermal hazards analysis techniques are available, but most require kinetics analysis at a high temperature and micro-scale quantities. In contrast, isothermal calorimetry (i.e., isothermal oven tests) has proven to be a convenient method for conducting experiments with material quantities in the 0.1 to 1000 kilogram range at conditions more closely resembling the potential storage environment. The isothermal oven test protocol determines the critical temperature for several different sample sizes. The resulting data are then analyzed using the Frank-Kamenetskii variables to yield estimates for the activation energy and the pre-exponential factor. The classical Frank-Kamenetskii solution of the self-heating problem assumes that heat generation, described by zero order reaction kinetics, is balanced by heat loss from the self-heating body via conduction at steady state. However, potential influences of other rate processes are neglected.

In this paper we describe the decomposition behavior of two solid oxidizers, calcium hypochlorite and sodium percarbonate, that do not conform to the classical Frank-Kamenetskii model. The two oxidizers exhibit complex self-heating behavior including temperature oscillations, particle agglomeration, gas generation, melting, and cavity growth within the self-heating body.

Other investigators have extended the classical Frank-Kamenetskii model to include factors such as multiple chemical reactions, solid melting and gas evolution. The objective of this research task is to determine if the Frank-Kamenetskii model, suitably modified to account for these complicating factors, can be applied to thermal ignition data for these solid oxidizers. The applicability of the modified Frank-Kamenetskii model to these materials is discussed and compared with previously reported studies published in the literature.

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