Enhancing safety in the storage of hazardous molecules: The case of hydroxylamine

Handling large quantities of thermally unstable compounds in storage vessels can result in severe accidents due to runaway reactions. Therefore, developing inherently safe design strategies for storage equipment is crucial for enhancing chemical plant reliability and preventing hazardous scenarios. The Frank-Kamenetskii theory (FKT) of self-heating offers practical tools for designing procedures to address phenomena that could lead to uncontrolled chemical reactions. This study introduces a design procedure based on the FKT for creating intrinsically safe storage vessels. An expanded FKT version incorporating parametric sensitivity analysis has been developed to improve the method's reliability. To understand self-heating phenomena, stability and performance diagrams were created, relating critical design parameters (e.g., the critical value of the Frank-Kamenetskii number) and verification parameters (e.g., maximum reached dimensionless temperature) to the dimensionless activation energy ($\gamma$). Additionally, the proposed design strategy includes a procedure for designing relief systems to mitigate the risk of equipment explosions from runaway reactions. The applicability of this procedure was tested using two cases: (I) an aqueous solution containing 50% w/w hydroxylamine (HA) and (II) a 50% w/w HA aqueous solution with 1% w/w hydroxylamine hydrochloride (HA-derived salt). Results indicate that for large $\gamma$ values, the traditional FKT formulation and the expanded theory yield similar vessel designs. However, for finite $\gamma$ values ($\gamma \le 100$), the refined FKT version allows for less conservative storage equipment design. Combining DIERS guidelines with standard procedures for relief systems results in a more versatile and consistent protocol. However, incorporating relief systems is often impractical for large storage vessels due to the excessively large venting areas required for runaway reactions. In such cases, intrinsically safe vessel designs become the only feasible solution to prevent catastrophic incidents.