Safety Based on Inhibiting Runaway Reaction

Safety based on inhibiting a runaway reaction can take three forms, all of which require a knowledge of the mechanism of the decomposition reaction (see also case histories Al 66-68, page 186). They are  

True inhibition is usually applicable only to intermolecular reactions rather than decompositions. The polymerization of vinyl compounds is an example when a runaway is detected, an inhibitor such as tertiary butyl catechol can be added to stop the reaction by removing the free radicals which propagate it. This method can be applied to other types of reaction, but a laboratory study is always necessary to select the inhibitor and conditions such as inhibitor concentration, mixture temperature and speed of addition. The reliability of the inhibition system should be quantified and demonstrated to be acceptable if inhibition is to be used as the ultimate basis of safety. 

Quenching and dumping are more usual methods of inhibiting a runaway reaction. In both cases a quantity of cold inert diluent is added to the reaction mixture and stops the reaction by cooling it. The ideal substance is water, when applicable, which is cheap, readily available and has a high specific heat. 

In some cases water reacts exothermically with the reaction mixture and an alternative diluent must be used an example is the runaway decomposition of a sulphonation reaction, where cold concentrated sulphuric acid is used as a quenching agent. This avoids an exothermic reaction between water and the hot sulphuric acid in the reaction mass. 

The actual addition of diluent is best carried out by quenching, where the diluent is added rapidly to the reactor from a storage vessel mounted above it. When the runaway is detected in the reactor (for example, by temperature rise) a valve opens automatically and the quench liquid runs rapidly into the reactor under gravity. The quench valve should fail safe (open). It takes a finite time to empty the quench vessel consideration should be given to the course of the runaway reaction during this period. 

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