Protection from runaway reactions

Containment systems are only rarely designed with sufficient pressure and temperature rating to fully contain a runaway reaction. For this reason, overpressure protection is of obvious critical importance as a last line of defense against loss events that can result from runaway reactions. The latter sections in this chapter on Pressure Relief Systems and on Emergency Relief Device Effluent Collection and Handling address design basis selection, relief calculations, and effluent treatment system configurations for reactive system overpressure protection. 

The protection principle of pressure relief is based on limiting the pressure to which equipment might be exposed by the removal of gaseous or multi-phase material flows from, in the case of an explosion or a runaway reaction by allowing certain predetermined openings to be opened in such a way that the pressure in the vessel does not exceed a predetermined permitted value. 

Where the hazard is an exothermic runaway reaction, there are number, of alternative measures that may be used either to prevent or control runaway. Informatipn on the various options is in Annex 1. In the UK, one of the most commonly selected measures used to protect reactors from exothermic runaway is an emergency relief system. These have a number of advantages ... 

It is clearly not safe to test unknown reactions or compounds in a full-size reactor, as a vigorous exotherm may overcome the protection systems provided. Various theoretical techniques and small-scale tests have therefore been devised to provide data on the likelihood and severity of a runaway reaction. They vary from simple calculations and basic heating tests to sophisticated simulations of full-size plant. This chapter describes the main theoretical techniques and experimental tests available for identifying chemical reaction hazards, and suggests how to select a suitable test regime. 

Few chemical reactors are operated without any pressure relief system, but it is not uncommon for the relief system to be sized only to deal with overpressures from service fluids or fire engulfment. In such situations, where the relief system does not protect against the consequences of a runaway reaction, the safety of the reactor system must be assessed thoroughly to demonstrate compliance with legal requirements and recommended standards. The company operating the process should be able to justify its decision not to provide adequate overpressure relief. ... 

Thermal stability testing showed that the reaction mixture could decompose exothermically with self heating occurring on the plant scale from 145°C (the boiling point of the mixture is about 160°C). Decomposition of the reactant mass would lead to a runaway reaction with the generation of a toxic and irritant gas. This would be vented safely, as far as protecting the reactor from overpressure is concerned, by the emergency relief vent, but would cause a serious toxic and corrosive aerosol emission. 

A measure is preventive if it prevents the occurrence of a runaway, a decomposition, or a hazardous secondary reaction. The system conditions remain close to operating conditions. Excessive increases in temperatures and/or pressures are avoided. Preventive measures include feed rate control systems, interlocks to prevent the reaction from starting unless sufficient diluent is present or the cooling system is working, and tests for the presence of catalysts or unwanted impurities. Preventive measures are always to be preferred over protective or mitigating (defensive) measures. 

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