Emergency relief vent sizing for runaway chemical reactions using different methods

The sizing of emergency relief vents to mitigate runaway chemical reactions in batch or semi-batch reactors is important for the protection of the workers and the process unit. This has been a preoccupation in industry for many years and different design methods have been proposed depending on the type of system (gassy, vapor, or hybrid) and the flow characteristics of the reacting fluid. Industry needs simple, reliable and accurate sizing methods so as not to undersize nor oversize the vent area. In the present work, runaway reactions in two chemical systems were studied experimentally with various apparatuses in volumes from 30 µl up to 10 liters (DSC, PHI-TEC I, ARSST, VSP2, Vent Sizing Simulator, 800 ml reactor, and the standard 10 liter vessel used for the United Nations test for sizing rupture disks for vessels containing self-reactive materials). The two systems were the decomposition of a solution of DiButylPeroxide in butyrate and the esterification of acetic anhydride by methanol. The first system is virtually purely gassy, meaning that the overpressure in the vessel during runaway is produced by incondensable gas while the second is purely vapor, meaning that the overpressure is due to the vapor pressure of the reaction mass as the temperature rises. The data recorded were used to determine the A/V ratio for the emergency relief vent or rupture disk for a 1m3 vessel. Two categories of sizing methods were used. The similarity method was employed, whereby the A/V ratio is directly determined experimentally for a given set of conditions, with three reactors : 0.11, 0,8 and 10 liters. The calculation method was based on the DIERS approach and was employed using the data from bench-scale pseudo-adiabatic calorimeters. The results show that the similarity method predicts vent sizes which are much smaller than those predicted by the DIERS approach