Untempered hybrid systems

A similar test to that for gassy systems (see A2.5 above) should be used to determine the permanent gas evolution rate. The rate of temperature rise should also be measured so that the rate of vaporisation can be calculated using equation (A2.7) above. 

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As for gassy systems (see 7.2), the operation of the relief system cannot control the temperature or the reaction rate of untempered hybrid systems. Consequently, these will continue to rise to their peak values. However, a low relief pressure can still be beneficial because ... 

It is important that these sizing methods are only used if the hybrid is tempered and remains tempered until the reaction is complete in an open test (see 4.2 and Annex 2). If the methods in this section are used for untempered hybrid systems, the calculated relief size is likely to be inadequate. 

Table A2.1 summarises the information which can be obtained from the experiments described above, and the conclusions which can be made about characterising the system. It can be seen that it is not possible to distinguish experimentally between a gassy system and an untempered hybrid system. Some indication can be obtained from the vapour pressure of the reactants and products. If the mixture might be expected to boil at the temperatures experienced during runaway, then the system may be a hybrid.

Hybrid systems may be either tempered or untempered. Generally, untempered systems require much larger relief systems than tempered systems. It is often important that advantage is taken of this in the design of relief systems for tempered hybrids. 

Gassy systems are untempered in that pressure relief will not control the temperature or the reaction rate. Hybrid systems can be either tempered or untempered depending on the relative rates of vapour and gas production at the chosen pressure. Lowering the pressure during relief normally increases the likelihood of tempering because the vapour pressure becomes a higher proportion of the total pressure. However, in some cases, this can also increase the likelihood that all of a solvent would be vaporised, either by the reaction itself or by external fire, before the reaction reaches completion. Hybrid systems can be treated as gassy systems if the vapour pressure is low (less than about 10% of the total pressure). 

The sizing method to be used for a hybrid system depends on whether that system is tempered or untempered under the relief conditions of interest. See 4.2 and Annex 2 for discussion of how to determine whether or not a system is tempered. 

In general terms, tempered hybrids behave in a similar way to vapour pressure systems (see Chapter 6) and untempered hybrids behave in a similar way to gassy systems (see Chapter 7). However, many of the sizing methods developed for vapour pressure and gassy systems are inapplicable for hybrid systems because ... 

It may also be possible to use computer simulation in cases where the reaction is initially tempered but stops tempering later in the runaway when a volatile solvent has boiled off. However, a good understanding of the reacting system would be required in order to have confidence in the results of such a simulation. Alternatively, the reaction could be treated as an untempered hybrid (see 8.4). 

RELIEF SYSTEM SIZING FOR UNTEMPERED HYBRIDS WITH TWO-PHASE RELIEF. ... 

As for gassy systems, relief from the bottom of the reactor (dumping) may be a better option than relief from the top, for untempered hybrids. This is discussed in... 

If top relief is to be used, DIERS proposed the following simple sizing method. This method has the same assumptions and conditions of applicability as the equation proposed for gassy systems (see section 7.3). The version for untempered hybrids... 

As for gassy systems, detailed computer simulation or direct scale-up (if applicable) can be used as alternative relief sizing methods for untempered hybrids. These methods are further discussed in section 7.4. 

Relief sizing for untempered hybrids is similar to that for gassy systems, for which a worked example is given in section 7.6)... 

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