System gassy

The major method of vent sizing for gassy system is two-phase venting to keep the pressure eonstant. This method was employed before DIERS with an appropriate safety faetor [34], The vent area is expressed by ... 

Gassy system In gassy systems, the pressure is due to a permanent gas that is generated by the reaetion. 

Data may be simple physical properties or sophisticated calorimeter data to characterize two phase flow (including gassy systems and/or high viscosity laminar flow systems). 

For some gassy systems (Ap/At driven), viscosity considerations become important which make testing difficult if not prohibitive. 

Gassy systems are "untempered". Removal of gas from the relief system will not stop the temperature from rising and the volumetric rate of gas generation will continue to increase. The- relief system should be designed to cope with the maximum rate of gas generation that can occur before the vessel empties. For untempered systems, it is important to check (by testing) whether or not, as the temperature rises, secondary reactions or decompositions occur (see also Figure... 

For gassy systems, this will depend on the rate of pressure rise in a closed system (rate of gas generation). Since gassy systems are untempered, the rate of interest is the maximum rate. This is likely to be at close to the maximum temperature reached by the runaway. 

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. 

Fauske s method for vapour pressure systems (see A5 3) and-the sizing method for gassy systems (see Chapter 7) have been, used to do a very.approximate relief sizing. Alternatively for screening purposes, nomographs153 could be used. "... 

For case (iii) the gassy system method from Chapter 7 has been used . ... 

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). 

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). 

Gassy systems are untempered. This means that the operation of the relief system cannot control the rate of the runaway reaction, but simply acts to remove material from the reactor. For untempered systems, homogeneous flow in the reactor (see... 

Figure 7.1 POSSIBLE BEHAVIOUR OF A GASSY SYSTEM DURING RELIEF...

Figure 7.2 STRATEGY FOR RELIEF SYSTEM SIZING FOR GASSY SYSTEMS...

In spite of the above qualitative reasons for preferring a low relief temperature, the simple sizing equation (see 7.3, below) for gassy systems does not enable any benefit (in terms of a reduced Relief size) to be obtained from either a low relief temperature or a low relief pressure. Alternative relief sizing methods (see 7.4) do allow such factors to be taken 1 into account and for a smaller relief size to be obtained. 

ALTERNATIVE RELIEF SYSTEM SIZING METHODS FOR TOP VENTING OF GASSY SYSTEMS... 

It is important that the computer code chosen is suitable for carrying out physical property calculations for pure gassy systems. Most simulation codes require the reaction mechanism to be sufficiently well understood that data including stoichiometric coefficients for the reaction and the molecular weight of the evolved gas(es) can be supplied. It is recommended that these data be derived from suitable adiabatic experiments (see Annex 2). A few codes make direct use of adiabatic experimental data, so that a full understanding of the reaction is not required. Most codes assume that the evolved gas can be treated as ideal, and, if this is not the case, an appropriate code must be found. 

For gassy systems, the Omega parameter is given by (see Annex 8) ... 

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 ... 

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 ... 

As for gassy systems, it may be beneficial to provide bottom relief rather than relief from the top of the reactor, and use of a bursting disc may be preferable to a safety... 

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)... 

For gassy systems, G should be calculated assuming non-flashing two-phase flow, sometimes called "frozen flow V Possible methods for the calculation of G for gassy systems (using the.homogeneous frozen flow model (HFM) which is a version of the HEM) are ii ... 

The capacity of the relief system can be obtained from a two-phase flow calculation for nozzle flow. If the flow is not choked, then the Omega method (see Annex 8) or suitable computer code must be used to calculate flow capacity. For choked flow a larger range of methods may be applicable, e.g. ERM for vapour pressure systems (see 9.4.2) or Tangren et al. s method for gassy systems (see 9.4.3), together with the application of a discharge coefficient. The capacity can then be obtained from ... 

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