Homogeneous two-phase venting

ICI [34] developed a method for sizing a relief system that aeeounts for vapor/liquid disengagement. They proposed that homogeneous two-phase venting oeeurs and inereases to the point of disengagement. Their derivations were based on the following assumptions ... 

For a tempered36 system, assuming homogeneous, two phase venting, vent area based on the DIF.RS method as developed by Ixrung37 can be expressed as ... 

Equation (7.1) takes no account,of any mass loss through the relief system before the peak reaction rate is reached. This is approximately true in the case where gas-only venting occurs right until the peak rate. It could be expected to be very conservative in cases where homogeneous two-phase relief occurred, for example due to inherent foaminess of the reacting mixture (see Annex 3). 

A description of level swell is gjjven in 4.3.1. Level swell calculations do not apply to inherently foamy systems as these always vent a homogeneous two-phase mixture. 

The Design Institute for Emergency Relief Systems (DIERS) has developed methods to predict the extent of level swell -. but these are valid only for systems which are not natural foamers. Naturally-foaming systems fill the reactor with a homogeneous two-phase mixture during relief and will always vent as a two-phase mixture. Since only trace quantities of certain substances... 

The first two eases represent the smallest and largest vent sizes required for a given rate at inereased pressure. Between these eases, there is a two-phase mixture of vapor and liquid. It is assumed that the mixture is homogeneous, that is, that no slip oeeurs between the vapor and liquid. Furthermore, the ratio of vapor to liquid determines whether the venting is eloser to the all vapor or all liquid ease. As most relief situations involve a liquid fraetion of over 80%, the idea of homogeneous venting is eloser to all liquid than all vapor. Table 12-3 shows the vent area for different flow regimes. 

The data required for the emergency vent design includes [191] (1) the thermokinetic and pressure history monitored under near adiabatic conditions, (2) the character of the type of vented system (vapor, gassy, or hybrid), (3) the phase of the vented material (vapor, liquid, or two-phase), and (4) the degree of two-phase disengagement (turbulent, bubbly, or homogeneous). To determine these characteristics, the VSP defines the system as viscous (100 cp) or nonviscous, and also whether or not it has a foaming tendency. 

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