HOW Well DO YOU MEASURE? WHEN Disaster CAN Hide Behind REACTIONS In SERIES with AN Exothermic STAGE FOLLOWING AN Endothermic ONE

V. H. Carreto-Vazquez¹, T. Adamopoulou², E. Kounalakis², M. Papadaki², and M.S. Mannan¹

¹Mary Kay O’Connor Process Safety Center, Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843-3122, USA

²Department of Environmental and Natural Resources Management, University of Ioannina, Seferi 2, Agrinio, GR 30100, Greece

    ABSTRACT

Calorimetry is an excellent experimental tool for the measurement of the effective enthalpy of reactions and the empirical assessment of the consequences of a potential reaction runaway. However, calorimetric measurements are relative and the information they provide, unless used in combination with other techniques and computational methods can be misleading.

In semi-batch processes, where gradual addition of one reactant is used as a means of controlling reactant accumulation, accidents have occurred because of an unnoticed drop in reactor temperature resulting in increased reactant accumulation [1]. A similar effect could be result if the process involves unidentified multiple reactions between which endothermic stages do exist. An endothermic stage preceding an exothermic one, can often pass unnoticed as frequently reaction testing focuses on the detection of exotherms and the measurement of the heat released by a reaction. However, an unknown endothermic stage can cause a drop in the process temperature, which, can either pass unnoticed resulting in reactant accumulation or can be interpreted differently; it can be considered as a cooling malfunction for instance. An attempt to increase the temperature by increasing the heat supplied to the reactor –or by reducing the heat removed from it- can result in disaster, as such processes are often followed by vigorous exothermic stages. The powerful heaters employed in adiabatic calorimetry aiming at the compensation of the heat losses and the small samples employed in adiabatic calorimetry tests can conceal such a stage, thus seriously compromising the reaction safety.

In this work, aqueous solutions of hydroxylamine, a highly energetic compound extensively used in the chemical and nuclear industry, have been left to decompose under the same conditions, using different experimental set-ups. It is shown how the measurement of the energy content of the reaction depends on the measuring device or the experimental set-up. Different runaway scenarios are considered and presented, comparing safety implications if the endothermic stage is unnoticed as opposed to being known.

    REFERENCES

1.         Partington, S. and Waldram, S. Runaway reaction during production of an azo-dye. in Hazards IVI, Symposium series No 148, 2001.

  ACKNOWLEDGEMENTS

The financial support of the Mary Kay O'Connor Process Safety Center is greatly acknowledged.