Hazard assessment strategies for reduction reactions.
Reduction reactions involving heterogeneous catalytic hydrogenations,
complex metal hydrides, and to lesser degree hydrogen-transfer reactions, are
regularly scaled-up in pilot plants. Unfortunately, thermal runaway incidents
involving reduction reactions do occur, despite best efforts to prevent them
through the application of a chemical reaction hazard assessment strategy.
A review of the literature, plant incidents, thermochemical and calorimetric
techniques, identified the requirements for a unique assessment strategy for
reduction reactions. The preference was to safeguard the plant using
preventive measures first which were supported by adequate protective
measures. The basis of safety was defined by, the boiling point of the reaction
mass, the process temperature and the adiabatic temperature rise for the
desired and/or adverse reactions including other kinetic data, e. g., "time to
A number of instrumental and thermochemical procedures were adopted for
the hazard identification portion of the strategy. The DSC capillary and
ampoule techniques were used for substrate thermal decomposition and air
oxidation determinations including reaction solution thermal stability
studies. An estimation technique (Yoshida) used DSC exothermic data to
predict a substrate's susceptibility of being shock sensitive and/or explosion
propagating. An evolved gas mass flow detector was coupled to a reaction
calorimeter to determine the maximum off-gas rate.
A modified stirred ARC for hydrogenations and a stirred-micro-calorimeter
for the quantification of the adverse reaction were developed. Adiabatic
determinations for quantification of the adverse reaction were variable. The
heat losses were unacceptable for a controlled hydrogenation in a modified
stirred ARC. Results for the stirred-micro-calorimeter were satisfactory.
However, adverse reactions for hydride decompositions and "shot additions"
yielded adequate calorimetric results.
A series of controlled experiments by reaction calorimeter coupled with an insitu
FTIR, characterised the thermochemistry, reaction kinetics, mass transfer
coefficient and reaction mechanism for the desired and inhibited
hydrogenations. A customised What-If? /Checklist process hazard analysis
technique was developed for reduction reactions and two worked examples
A hazard assessment strategy with appropriate hazard identification
procedures was developed. Eight case studies (three hydrogenations, three
hydride reductions and two hydrogen-transfer reactions) were used as
examples to validate the reduction assessment strategy and hazard