Tempered hybrids

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  

For both tempered and uritempered hybrids, a low. temperature and correspondingly low reaction rate at the relief pressure is desirable in order to reduce the relief system size.  

The discussion for vapour pressure systems in 6.2 also applies to tempered hybrids. A low relief pressure is beneficial because  

STRATEGY FOR RELIEF SYSTEM SIZING FOR HYBRID SYSTEMS 

Will theftybrid system remain tempered until the, vented reaction reaches completion 7 (see 4.2 and Annex 2) 

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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. 

Most screening tests are likely to lump all reactions that generate gas together. Tempered hybrid systems will not be distinguished but these will require a smaller relief area than a gassy system with the same gas generation rate. If the worst case is subsequently-found to be a tempered hybrid reaction, rather than a gassy system, then some reiteration to check that it is still the worst case may be required. 

The use of the sizing method above for gassy systems assumes that case (iii) is not a tempered hybrid. If, during detailed relief sizing, case (iii) does turn out to be a tempered hybrid system, and the vent size is significantly smaller, then the worst case would need to be reassessed, by carrying out detailed relief sizing for both cases (ii) and (iii). 

Size using Leung s method for tempered hybrids (see 8.3.1), if applicable... 

RELIEF SYSTEM SIZING FOR TEMPERED HYBRIDS WITH TWO-PHASE FLOW... 

The relief, size should also be calculated using Leung s method for vapour pressure systems (see 6.3) and the larger of the vapour pressure system and tempered hybrid system relief sizes should be used121. 

The evaluation of the two-phase mass relief capacity per unit area, G, is discussed in Chapter 9. The additional parameters which are required for tempered hybrid systems are PJP, the ratio of the vapour pressure to the absolute pressure, and , the closed vessel temperature rise as the pressure rises from the relief pressure to the maximum pressure. 

Alternative relief system sizing methods for tempered hybrid systems... 

Leung[341 has proposed an alternative sizing method for tempered hybrid systems (see A5.11). This method makes the same assumptions as that above, except that the conservative assumption that the allowable temperature rise is the same as that in a closed vessel does not need to be made. The method is therefore likely to yield smaller relief sizes than the method above. However, the method is more time-consuming to evaluate as it requires a trial and error procedure. ... 

WORKED EXAMPLE OF RELIEF SYSTEM SIZING FOR A TEMPERED HYBRID RUNAWAY REACTION... 

It is required to size a bursting disc system with a maximum specified bursting pressure of 2.2 barg (3.2 bara) for a reactor of volume 1.5 m3 and design pressure 3 barg (maximum accumulated pressure = 4.3 bara). The frictional resistance of the bursting disc system in this case is equivalent to 4fL/D = 5. The worst case reaction has been identified as a tempered hybrid, and an open system calorimetric test has demonstrated that it will continue to temper until the reaction is complete. For the worst case reaction, the mass in the reactor would be 860 kg. 

The relief system must also be sized assuming the tempered hybrid is a vapour pressure system, and the larger relief diameter taken. 

The larger relief size is that obtained using the method for tempered hybrids, of 0.00812 m2 and this should be used for design. 

The data required for a tempered hybrid system is similar to that for vapour pressure systems. However, because permanent gas is being generated by the reaction, an open test should be used. This is because ... 

The open test method for tempered hybrid systems is the same as that given for vapour pressure systems in A2.4.3 above. However, in addition to measuring the test cell temperature, the rate of pressure rise in the closed containment vessel during tempering should also be measured. The rate of heat release per unit mass, q, can be obtained from measured dT/dt data, suitably corrected for thermal inertia (e.g. by using equation (A2.12)). Equation (A2.4) can be used to determine the rate of permanent gas evolution, QG. As the containment vessel provides a large heat sink, vapour is likely to condense, so that the rate of pressure rise is due only to the non-condensible gas., ... 

Leung s method for relief system sizing for tempered hybrid systems (see 8.3.1) requires the ratio of the vapour pressure to the total pressure, P P. This is approximately given by ... 

LeungI15] gives an alternative method for tempered hybrid systems (see 4.2) which is slightly more rigorous than that given in 8.3.1 but takes longer to evaluate. The method assumes ... 

The first term is the vaporisation rate and the second term is the rate of production of permanent gas by the reaction The second term is only required for tempered hybrids... 

A reaction has been characterised as a tempered hybrid system and it has been determined that the system will relieve single-phase gas/, vapour. Relief sizes for both a safety valve and a bursting disc system are required. The reactor contains a charge of 3000 kg. Data for relief sizing have been compiled in Table A6.2. -The type of safety valve selected has a de-rated discharge coefficient under BS 6759131 of 0.87. 

This worked example calculates G for the tempered hybrid relief sizing worked example given in Chapter 8. It uses the mixing rule for hybrids and therefore shows example Omega calculations for vapour pressure, gassy and hybrid systems. 

The relief sizing equation for tempered hybrids, in which the calculated value of G is to be used (see Chapter 8) assumes that the reactor fills with a homogeneous two-phase mixture during relief. Thus ... 

For vapour pressure and tempered hybrid systems, venting the reactor at as low... 

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