(67b) Modeling Monomer Runaway Scenarios for Hazard Assessment

Hazard assessment for runaway scenarios of monomers pose a unique challenge. This is due to the variety of chemical pathways that exothermic polymerization of monomers may proceed by as well as the complex interplay of polymer physics and transport phenomena that has to be un-entangled in order to assess peak exothermic self-heat and pressure rise rates. This complexity hinders scale-up of adiabatic calorimetry data of monomers derived from laboratory experiments as is commonly performed for many other non-monomer chemical systems. As a result, hazard assessment performed on monomer runaway scenarios leans upon lumped n^th order kinetic models that typically over-estimate self-heat rates, manifesting in higher installed capital due to requirement of enhanced safety instrumentation and larger relief devices.

One specific source of technical complexity relates to auto-acceleration of certain monomers (known as Norrish-Trommsdorff or gel effect[1]) which refers to the large (several orders of magnitude) drop in the polymerization termination constant above the ~ 30-40% monomer conversion regime that leads to a sudden rise in conversion rate and molecular weight build-up. This results in a higher exothermic self-heat rate and viscosity build-up respectively, which further breeds heat/mass transport limitations. Evaluation of this non-linear rise in molecular weight and conversion by coupling temperature rise with chain propagation/termination necessitates incorporation of polymerization kinetic models into conventional thermo-kinetic evaluation. While several efforts have focused on modeling monomer auto-acceleration under well-controlled isothermal conditions,[2] major gaps exists in the area of applying such models for thermo-kinetic hazard evaluation to evaluate monomer runaway scenarios.

In this work, we have demonstrated a path forward to bridge this gap using the case of free-radical initiated exothermic polymerization of methyl methacrylate (MMA). A first-principles model describing monomer auto-acceleration under isothermal conditions was adopted from existing literature[3] and its predictions of reaction rate and gel index compared to experimental isothermal calorimetry data of MMA polymerization at various temperatures and inhibitor concentrations. Then, this model was modified to predict the reaction rate and gel index trajectory under adiabatic conditions and these predictions were compared to adiabatic calorimetry data. The comparison serves as a case study to illustrate how coupling first-principles polymerization kinetic models with adiabatic calorimetry data can help predict self-heat rates without the need for significant over-estimation during auto-acceleration of monomers under runaway conditions.

Monomers remain much less well-understood in their exothermic behavior under adiabatic conditions as compared to other hazardous chemical classes due to coaction of increasing reaction rates accompanied by building molecular weight. Through this study, the authors hope to demonstrate that integration of polymerization kinetics with thermo-kinetic modeling can help estimate reasonable self-heat rate values while obviating the need for lumped kinetic models. Such an approach would significantly contribute to rational safety instrumentation and relief system design while assessing monomer runaway scenarios.

[1] Journal of applied polymer science 30, no. 10 (1985): 3985-4012.

[2] Polymer Engineering & Science 35, no. 16 (1995): 1290-1299.; Macromolecular theory and simulations 16, no. 4 (2007): 319-347; Journal of Polymer Science: Polymer Chemistry Edition 14, no. 4 (1976): 883-897.

[3] Macromolecules 25, no. 14 (1992): 3739-3750