Reaction gassy

Reactive System Screening Tool (RSST) The RSST is a calorimeter that quickly and safely determines reactive chemical hazards. It approaches the ease of use of the DSC with the accuracy of the VSP. The apparatus measures sample temperature and pressure within a sample containment vessel. Tne RSST determines the potential for runaway reactions and measures the rate of temperature and pressure rise (for gassy reactions) to allow determinations of the energy and gas release rates. This information can be combined with simplified methods to assess reac tor safety system relief vent reqiiire-ments. It is especially useful when there is a need to screen a large number of different chemicals and processes. 

Using Equation 12-30 for a gassy reaction, the charge amount is ... 

As propagation rates of gassy reactions are pressure dependent, so are gasless reactions temp dependent. This temp dependence has been... 

Gassy or nontempered systems—the total pressure is due to the fact that the reaction produces permanent gases, and... 

A simplified VSP procedure which enables a quick estimate of the required vent diameter is discussed in [191]. The venting characteristics in connection with runaway chemical reactions can be related to vapor and gassy (including hybrid) systems. 

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... 

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. 

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... 

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. 

SIZING METHODS FOR BOTTOM RELIEF (DUMPING) OF GASSY REACTIONS... 

WORKED EXAMPLE OF RELIEF SYSTEM SIZING FOR A GASSY RUNAWAY REACTION... 

A reactor of volume 3.5 m3 has a design pressure of 14 barg. A worst case relief scenario has been identified in which a gassy decomposition reaction occurs. The mass of reactants in the reactor would be 2500 kg. An open cel test has been performed in a DIERS bench-scale apparatus, in which the volume of the gas space in the apparatus was 3,800 ml, and the mass of the sample was 44.8 g. The peak rate of pressure rise was 2,263 N/m2s at. a temperature of 246°C, and the corresponding rate of temperature rise was 144°C/minute. (These values include corrections for thermal inertia.) The pressure in the containment vessel corresponding to the peak rate was 20.2 bara. 

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 ... 

In an open test, the pressure measured will be,the pressure in the containment vessel. If the test is done with the containment vessel closed and the nitrogen pressure control system off, then any permanent gas produced by the reaction will cause the containment vessel pressure to rise. Thus a constant containment vessel pressure indicates a vapour pressure system and a rising containment vessel pressure indicates a gassy or hybrid system. 

As with vapour pressure and gassy systems, for continuous or semi-batch reactions it may be possible to reduce the relief system size by taking into account the reduced accumulation of reacting mass. See A2.4.1 and reference 10. 

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. 

First, different typologies of nonideal batch reactors are considered. In particular gas-liquid reactors are discussed, which may be used for different industrial applications (e.g., reactions of oxidation) and are often encountered in the case of gassy reactions (i.e., liquid-phase reactions which do not produce significant thermal effects but in which the production of gaseous products may lead to explosions). 

Pressure is more directly connected to the concept of explosion nevertheless, it is less directly connected to the reactor status, since, for liquid-phase reactors, pressure nonlinearly depends on temperature (trough the vapor pressure relationship) and concentration (through the activity coefficients in liquid phase). Moreover, since pressure measurements are usually less accurate than temperature measurements, they are to be considered in particular for gassy reactions, i.e., when the runaway produces small temperature effects but large amounts of incondensable products in gaseous phase. 

A survey within the Fine Chemical Manufacturing Organization of ICI has shown that gassy reaction systems predominate due to established processes such as nitrations, diazotizations, sulphonations, and many other types of reactions [22], Very few vapor pressure systems have been identified that also generate permanent gas (i.e., hybrid type). 

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

Huff, J. E., Emergency Venting Requirements for Gassy Reactions from Closed System Tests, Plant/Operations Progress, 3(1), 50-59 (Jan. 1984). 

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