Vent sizing

The evaluation of thermal stabihty requires the determination of the temperature at which an exothermic reaction occurs, the rate of such a reaction as a function of temperature, and the heat generated per unit mass of material by the reaction. In many cases, data on the increase of pressure during a reaction are also required, especially for vent sizing. The term onset temperature Toa%et is used in two contexts ... 

Vent Sizing Package (VSP) The VSP is an extension of ARC technology. The VSP is a bench-scale apparatus for characterizing runaway chemical reactions. It makes possible the sizing of pressure relief systems with less engineering expertise than is required with the ARC or other methods. 

Pressure build-up Provide adequate venting and dust filtration on downstream of receiving vessel vent mill (risk of com-, Where liquefied gas (nitrogen or CO9 ) is used ponent failure, ensure adequate vent sizing and limit particularly in gas liquefied gas feed-rate to mill conveying systems). Internal pressure may also force product out of the mill. CCPS G-11 CCPS G-22 CCPS G-23 CCPS G-29... 

Anonymous, Vent sizing Software, Eauske and Assoc., Burr Ridge, IL. 

As the seale of operation inereases, the effeet of the heat eonsump-tion by the plant typieally deelines. Therefore, the extent to whieh the kineties of the runaway reaetion is influeneed by the plant is redueed. For plant seale vessels, the ([t-faetor is usually low (i.e., 1.0-1.2) depending on the heat eapaeity of the sample and the vessel fill ratio. Laboratory testing for vent sizing must simulate these low ([t-faetors. If the laboratory ([t-faetor is high, several anomalies will oeeur ... 

The rate of reaetion will be redueed (i.e., giving ineoiTeet reaetion rate data for vent sizing). 

The vent sizing package (VSP) was developed by Fauskes Associates, Inc. The VSP and its latest version VSP2 employ the low thermal mass test cell stainless steel 304 and Hastelloy test cell with a volume of 120 ml contained in a 4-1, high-pressure vessel as shown in Figure 12-13. The typical ([t-factor is 1.05-1.08 for a test cell wall thickness of 0.127-0.178 mm. Measurements consist of sample temperature Tj and pressure Pj, and external guard temperature Tj and... 

Figure 12-13. Vent sizing package (VSP) apparatus. (Source Fauskes Associates Inc.)...

The RSST can rapidly and safely determine the potential for runaway reactions. It also measures the temperature rates and, in the case of gassy reactions, pressure increases to allow reliable determinations of the energy and gas release rates. This information is combined with analytical tools to evaluate reactor vent size requirements. This is extremely useful when screening a large number of different chemicals and processes. 

Figure 12-16 gives a recommended test sequence for identification of reactive system type and vent sizing. 

The PHI-TEC II adiabatic calorimeter as shown in Figure 12-17 was developed by Hazard Evaluation Laboratory Ltd. (UK). The PHI-TEC can be used both as a high sensitivity adiabatic calorimeter and as multi-purpose vent sizing device [17,18]. The PHI-TEC employs the principles established by DIERS and includes advanced features compared to the VSP. It also provides important information for storage and handling and provides useful insight into the options suitable for downstream disposal of vented material. 

It is the rate of temperature inerease (i.e., power output) between the set pressure and the maximum allowable pressure, whieh determines the vent size and not the peak rate. Boiling is attained before potential gaseous deeomposition (i.e., the heat of reaetion is removed by the latent heat of vaporization). The reaetion is tempered, and the total pressure in the reaetor is equal to the vapor pressure. The prineipal parameter determining the vent size is the rate of the temperature rise at the relief set pressure. 

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. 

Equation 12-18 gives a eonservative estimate of the vent area, and the simple design method represents overpressure (AP) between 10%-30%. Eor a 20% absolute overpressure, a liquid heat eapaeity of 2,510 J/kg K for most organies, and eonsidering that a saturated water relationship exists, the vent size area per 1,000 kg of reaetants is ... 

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. 

Figure 12-36. Vent sizing model for high vapor pressure systems due to nonequilibrium effects turnaround in temperature is assumed to coincide with the onset of complete vapor disengagement.

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 ... 

Equation 12-31 is valid if disengagement oeeurs before the pressure would have turned over during homogeneous venting, otherwise it gives an unsafe (too small) vent size. Therefore, it is neeessary to verify that... 

If Fauske s method yields a signifieantly different vent size, then the ealeulation should be reviewed. 

Choose the smaller of the vent size from the two methods. 

Determine the vent size for a vapor pressure system using the following data and physical properties. 

Using Eauske s nomograph from Eigure 12-35 at a self-heat rate of 6.3 K/min and a set pressure of 217.5 psia, the corresponding vent size area per 1,000 kg of reactants = 0.0008 m. ... 

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

Gustin, J. L., Choiee of Runaway Reaetion Seenarios for Vent Sizing Based on Pseudo-adiahatie Calorimetrie Teehniques, Int. Symp. on Runaway Reaetion, Pressure Relief Design, and Effluent Handling, AIChE, pp. 11-13, 1998. 

Singh, J., PHI-TEC Enhaneed Vent Sizing Calorimeter Appliea-tion and Comparison with Existing Deviees, Proeeeding of the Int. Symp. on Runaway Reaetions, AIChE/CCPS, pp. 313-330, Boston, MA, Mareh 7-9, 1989. 

---