Tempered hybrid systems

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  

Qv and Qg for use in equation (A2.6) must be calculated at the same temperature, usually the tempering temperature corresponding to the relief pressure. 

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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). 

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. ... 

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 ... 

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. 

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

In some hybrid systems, the generated vapor in a vented reaetion is high enough to remove suffieient latent heat to moderate or temper the runaway (i.e., to maintain eonstant temperature). This subsequently gives a smaller vent size. 

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 ... 

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

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

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. 

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. 

A few codes are available which evaluate the simple relief sizing calculations, as an alternative to evaluation using a pocket calculator. An example is VSSP t10] from Fauske and Associates Inc. The VSSP code gives an option to calculate the relief size for tempered systems with churn-turbulent or bubbly flow. Another code, VSSPH [11], calculates a relief size for hybrid systems. 

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 key factor of success in the design of emergency pressure relief systems lies in a good understanding of the behavior of the reaction under relief conditions. The first point in this context is the cause of pressure increase. This may be the vapor pressure of the reaction mass, the so-called tempered system. Pressure increase may also be due to gas release by a reaction, the so-called gassy system. There are also cases where the pressure stems from both vapor pressure and gas release, the so-called hybrid system, which may or may not be tempered. 

Hybrid systems, where gaseous decomposition occurs before reaching boiling, but the rate of reaction is tempered by vapor stripping. The pressure developed in the system is due to the vapor pressure of the volatile components and to the partial pressure of noncondensible gases. 

During the relief of some hybrid systems and all vapour pressure systems the reaction tempers — that is. vaporization removes sufficient heat to hold the temperature constant, as shown in Figure 6.3. The rate at which material needs to be vented is therefore governed by the rate of reaction after the relief device has opened. 

In hybrid systems that do not temper and gassy systems, the temperature continues to rise and the rate of reaction continues to increase even after the... 

Advanced power generation cycles that combine high-temper-ature fuel cells and gas turbines, reciprocating engines, or another fuel cell are the hybrid power plants of the future. As noted, these conceptual systems have the potential to achieve efficiencies greater than 70% and projected to be commercially ready by the year 2010 or sooner. The hybrid fuel cell/turbine (FC/T) power plant will combine a high-temperature, conventional molten carbonate fuel cell or a solid oxide... 

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. ... 

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). 

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