Temperate systems

An equation representing the relief behavior for a length L/D < 400 as in the tempered system is given by [33]... 

Vent sizing for two-phase (runaway reactions) flow vent sizing for a tempered system... 

For a pure vapor system (AT/At driven), where the runaway reaction can be kept under control by the latent enthalpy of evaporation (tempered system), a relatively simple expression can be used for the estimation of the necessary vent diameter. 

For gassy and for tempered systems, the flow rate can also be measured in a simulated vent line (same 1/d ratio) of diameter do. Additional calculation formulas are given in [191]. 

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. 

For tempered systems, the pressure relief system will almost always need to be bigger if two-phase flow occurs, and DIERS[1] recommended that two-phase relief should normally be assumed for vent sizing purposes using the type of hand calculation methods given in this Workbook. This is explained in 4.3.2, Subsection... 

For a vapour pressure system (and any other tempered system) which is to relieve a two-phase mixture, there are two reasons why a low relief pressure is beneficial ... 

HFM is applied to a system in which the liquid is volatile, then it assumes a high degree of non-equilibrium since the model does not account for any flashing and the flow could be greatly overestimated. The model is therefore not recommended for relief sizing for tempered systems. ... 

As described in Annex 3, the amount of level swell increases with gas or vapour superficial velocity. At bench-scale, the superficial velocity due to runaway will be very low compared with that at plant-scale, so that very little level swell occurs in bench scale tests, even for Inherently foamy fluids. However, some level swell at bench-scale is required to determine whether the reacting system is inherently foamy or not. For tempered systems, this can be achieved by rapid depressurisation of the test cell to give flashing and a consequent high superficial velocity, and this is one of the ways in which the DIERS bench-scale apparatus can be used. (Note that... 

Where it is uncertain whether thle system is inherently foamy, it is recommended that the worst case assumption is used (see 4.3.2(1)). For tempered systems, the worst case will be inherent foaminessl Where tempered systems are not inherently foamy, the level swell calculations described in this Annex may lead to a reduction in calculated relief system size. For untempered systems, the worst case is vapour/ liquid disengagement causing reduced mass loss from the reactor during relief. In this case, dynamic simulation (see A3.4) may be needed to take account of level swell in relief sizing. 

For tempered systems, the following assumptions are conservative r a) Two-phase rather than vapour relief. 

Before beginning the series of runs to determine the relief size, the physical property and kinetic data need to be correlated in the form required, by the code. In some cases, the code may already have the components required on a database. In all other cases, physical property data must be found, estimated or measured and correlated in the appropriate form. Some codes have a front-end program for curve fitting of data. For tempered systems, the vapour/ liquid equilibrium models are of critical importance since errors will cause the code to open the relief system at the wrong temperature and reaction rate. It is therefore worthwhile to spend time to ensure reasonable behaviour of the vapour pressure predictions. Check that all correlations behave sensibly over the entire temperature range of relevance for relief sizing. A good test for the physical property and kinetic data supplied to the code is to first model the (unrelieved) adiabatic calorimetric test which was used to obtain the kinetic data.. . ... 

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. 

In some cases, direct scale-up may be impracticable, for example because of blockage of the small-scale relief line. The requirement for complete emptying of the small-scale reactor by two-phase relief may also not be met in practice. If this occurs for a tempered system, the problem could be overcome by using a small-scale relief system from the bottom of the test reactor to simulate one at the top of the large-scale reactor. This procedure would not be safe for untempered reactions. 

This method1161 is the recommended D1ERS method for untempered systems, and is given in detail for that case in 7.3. The method can also be safely used for tempered systems but tends to greatly oversize unless the available overpressure during relief is very small. At zero overpressure, Leung s method for vapour pressure systems (see equation (6.5)) is identical with this method. 

For a tempered system, this rate can be evaluated at the relief pressure because the relief system will then be designed to limit the pressure (and temperature and reaction rate) to this value. For untempered systems, the peak rate attained during the course of the runaway must be used. 

Fauske[20,21] has produced nomographs (graphs) for the purpose of relief sizing. That for tempered systems is based on Fauske s sizing method for vapour pressure... 

However, for tempered systems, relief via a bursting disc may give rise to two-phase relief due to flashing as the reactor depressurises. Although this does not affect the sizing of the relief system, it does increase the mass loss from the reactor and has implications for the disposal system design. Use of a safety valve, rather than a bursting disc, can prevent this. 

The required relief rate for tempered systems is given by ... 

Figure 10.5 Typical temperature and pressure course during pressure relief. Left tempered system, Right gassy system.

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. 

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