A Refined Divergence-based Runaway Detection Criterion for Tubular Reactors

The thermal management of highly exothermic reactions in tubular reactors is a critical issue in chemical process design and operation. Classical intrinsic stability criteria, such as the Strozzi-Zaldivar criterion, have been extensively applied to batch systems to detect the onset of runaway conditions, whereas their application to plug flow reactors (PFRs) remains limited, despite the well-known structural analogy between batch time evolution and steady axial behaviour in PFRs. This work aims to: (i) extend the Strozzi-Zaldivar criterion to plug flow reactors; (ii) formulate a divergence-based indicator based on the local Jacobian trace to detect runaway-prone regions along the reactor length; and (iii) evaluate its suitability as a foundation for future real-time model-based control strategies for runaway prevention. The methodology is applied to a fixed-bed CO2 methanation reactor, a strongly exothermic system of interest for synthetic natural gas production. Results show that the batch-PFR analogy enables a pointwise assessment of thermal stability and that the divergence-based indicator provides a computationally efficient and predictive tool for early runaway detection in spatially distributed reactors.