Are you underestimating thermal hazards?  Heat transfer-limited systems in Adiabatic Calorimetry

Accelerating rate calorimetry (ARC) is a widely used adiabatic calorimetry technique employed in academic and industrial laboratories to evaluate the thermokinetics of hazardous chemical reactions. ARC records the rate of temperature rise of a sample as it is heated and then uses this temperature rise to calculate the self-heat rate (SHR) as a function of temperature to evaluate reaction kinetics. This produces reasonably accurate results for low viscosity liquids. However, ARC has the potential to underestimate SHR in the case of high-viscosity liquid or solid samples due to lack of or minimal mixing. Inadequate mixing is exacerbated by the fact that temperature in many ARC instruments is measured externally at the wall of the ARC and in some cases, may lead to temperature gradients within the ARC sphere. Such gradients lead to the measured SHR being lower than the actual SHR and consequently, lead to an underestimation of the reaction hazard.

In this work, we have measured the extent of SHR underestimation for both liquid and solid samples. ARC experiments were designed to compare the measured SHR with actual SHR for situations expected to have zero, moderate and high SHR values. Further, simulations of conjugate heat transfer coupled with chemical reaction have been used to model the spatio-temporal temperature profile inside an ARC sphere under a variety of operating modes. Learnings from such simulations were used to identify and propose strategies to minimize underestimation of reaction hazards. Lastly, attempts have been made to validate such strategies through calorimetric testing.

The importance of precisely measuring thermal hazards of chemical reactions cannot be overstated. Additionally, many violent chemical reactions tend to occur in solid-phase (e.g., powdery azo- and nitro-containing compounds), the latter possessing the tendency to build up significant temperature gradients during testing in an un-stirred ARC given the low bulk thermal conductivity of powdery samples. Incidents in the recent past, including the tragic 2020 explosion in Beirut point to the necessity of accurately estimating exothermic SHR occurring in solid and high-viscosity liquid phases. We hope that the findings presented here-in serve to catalyze further research into this area and ensure accuracy of thermokinetics data to prevent incidents that may arise from underestimation of hazards.