Is your ARC data misleading? Heat-transfer limitations and reaction rate underestimation in Accelerating Rate Calorimetry

Abstract

Accelerating Rate Calorimetry (ARC) is a widely used adiabatic calorimetry technique to evaluate thermo-kinetics of hazardous chemical reactions. Most ARCs rely on measurement of temperature at the external wall of the ARC sphere to evaluate the temperature rise rate, and therefore exothermic reaction rate. We report large temperature gradients between the center and wall for a granulated solid sample tested using an un-stirred ARC that leads to varying spatiotemporal temperature profiles. Such gradients were found to be significantly lowered in organic liquid samples, enabling classifying some granulated solid samples as heat transfer-limited systems. This study reveals that such temperature gradients obviate estimation of representative exothermic reaction rate, manifesting in hazard underestimation through erroneous pressure and apparent conversion profiles. Next, the addition of an inert was demonstrated to reduce the magnitude of such gradients, thereby enabling relatively accurate hazard assessment and offering a potential solution. Lastly, several strategies are outlined to allow researchers to identify and mitigate the influence of gradients during hazard assessment. Researchers must be cognizant of the potential for gradients when analyzing and interpreting ARC data for thermo-kinetic modeling and hazard evaluation. During hazard evaluation, researchers should confirm that no significant gradients exist or find ways to de-sensitize their hazard assessment parameters if gradients exist and cannot be minimized.