SAFE RADIUS OF HEAT GENERATING SUBSTANCES hemical and physical transformations which arise during handling and storage of unstable substances may be accompanied by heat generation to such an extent that no stationary temperature distribution can exist. Due to the temperature rise, the rate of decomposition increases, consequently followed by an increase of the heat generation. Such a self-heating process results in an accelerated decomposition process, which may manifest itself as an explosion. This hazard is of particular importance in relation to the necessary precautions for production, handling, and storage of materials with explosive properties such as smokeless powder (5),rocket propellants (7), high explosives ( 8 ) , organic peroxides (5), and distinct types of fertilizers (4). For safe operational use of these substances, it is also important to know to the extent to which material can withstand high temperatures. T o date, it is not fully known which precautions are necessary for storage and handling of unstable substances to sufficiently limit the temperature rise of the heat generating substances. No stationary temperature distribution is possible when the heat production remains higher than the heat loss to the surroundings. This situation will occur if the dimension of the stored material exceeds a distinct critical value. For a heat generating substance, contained in a distinct geometrical shape (sphere, cylinder, or slab), this critical value is called the critical radius r,. If the storage radius is smaller than or equal to r,, a stationary temperature distribution is possible. However, when the radius exceeds r,, no stationary temperature distribution is possible, and the temperature will continue to rise. I n 1939 Frank-Kamenetzky ( 3 ) calculated the value of rc using some simplifications concerning the physical and chemical parameters which rule the thermal selfheating process. He assumed that the heat production depends only on the temperature and can be described by the Arrhenius law:
C
W
=
C--E/RT
(11
If the heat transport in the material takes place by con24
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
duction only and the material is homogeneous and it is assumed T - T,
