Development of Kinetic Models Part I

Safe storage, handling, and transportation of reactive chemicals is challenging. Characterization of both desired and undesired chemistries requires a variety of methods including theoretical and computational screening, testing, and detailed modeling. A multitude of process and environmental conditions can influence reaction rates such as contamination, reactant accumulation, loss of agitation, loss of cooling, etc. Identification and characterization of undesired chemistries are often missed at the development stage and/or not communicated properly to toll manufacturers and production facilities during scale-up. Safety data sheets may not be adequate and cannot be solely relied upon for safe storage, handling, or transportation of reactive chemicals. Exothermic runaway reactions can cause loss of containment, significant loss of property and life, and environmental impact.

Adiabatic and isothermal calorimetry are often used to characterize chemical reactions. Calorimetry data can then be reduced and used for direct scale-up or to develop simple and detailed chemical reaction kinetic models. Kinetic models are coupled with fluid dynamics for the assessment of thermal stability, process optimization, and pressure relief systems and vent containment design.

Two types of kinetic models can be developed, (a) simple or isoconversion models, and (b) detailed models. Isoconversion models are easy to develop, do not require information about stoichiometry, phase change, or vapor/liquid equilibrium, but cannot be used for pressure relief design. They are mostly used for thermal stability assessments where phase change can be neglected and where there is no mass exchange with the system boundaries.

Detailed models require the development of reaction stoichiometry, and detailed thermophysical and transport properties. They are mostly used for modeling the dynamics of pressure relief systems and vent containment design, process dynamics, as well as thermal stability assessments. Although they are more complex to develop than isoconversion models, they can be used to extend limited test data to wider ranges of composition, temperature, and pressure. The use of detailed kinetic models is preferred over the use of direct scale-up models because they often result in practical designs and better risk reduction. Once a detailed kinetic model is developed, it can be used over and over again in many process design and modeling applications.

In this paper we provide an overview of how simple kinetic models are developed from calorimetry data and how they are used for the modeling of thermal stability and thermal explosion theory.

Our Process Safety Office^® SuperChems™ software can be used to evaluate thermal stability for storage and transportation of chemicals and chemical mixtures for simple and complex packaging geometries, under a variety of ambient and heating/cooling conditions.