Liquid, superheated

So far the emphasis has been on substituting hazardous materials or using less, i.e., intensification. Let us now consider use of hazardous materials under less hazardous conditions, i.e. at less extreme temperatures or pressures or as a vapor rather than superheated liquid or diluted, in other words, attenuation. ... 

Tsup = temperature of the superheated liquid BT = normal boiling point If the mass of liquid vaporized is rriy, then... 

In fact, the true fire load will be greater than the energy release calculated in Example 9.1. In practice, such a release of superheated liquid generates large amounts of fine spray in addition to the vapor. This can double the energy release based purely on vaporization. 

The action of this and other anti-bumping devices e.g., minute carborundum chips) is dependent upon the fact that the transformation of a superheated liquid into the vapour will take place immediately if a vapour phase e.g., any inert gas) is introduced. The effect may be compared with that produced by the introduction of a small quantity of a solid phaM into a supercooled liquid, e.g., of ice into supercooled water. 

For example, vaporization may occur as a result of heat absorbed, by radiation and convection, at the surface of a pool of hquid or as a result of heat absorbed by natural convect ion from a hot wall beneath the disengaging surface, in which case the vaporization takes place when the superheated liquid reaches the pool surface. Vaporization also occurs from falling films (the reverse or condensation) or from the flashing of hquids superheated by forced convec tion under pressure. 

Pressure/temperature Operate at moderate temperature and pressure where possible. Avoid superheated liquids, which will flash-off, if practicable Allow for effects of over-/under-temperature, over-/under-pressure. Following assessment (e.g. by HAZOP)... 

In this section, the phenomenon of BLEVE is discussed according to theories proposed by Reid (1976), Board (1975), and Venart (1990). Reid (1979, 1980) based a theory about the BLEVE mechanism on the phenomenon of superheated liquids. When heat is transferred to a liquid, the temperature of the liquid rises. When the boiling point is reached, the liquid starts to form vapor bubbles at active sites. These active sites occur at interfaces with solids, including vessel walls. 

After vessel rupture, the superheated liquid vaporized in a white cloud consisting of vapor and fine droplets. After ignition, the flame propagated through the cloud, forming a fireball. Fireball size increased as combustion proceeded, and the fireball was lifted by gravitational buoyancy forces. 

Experiments by Schmidli et al. (1990) were focused on the distribution of mass on rupture of a vessel containing a superheated liquid below its superheat-temperature limit. Flasks (50-ml and 100-mI capacity) were partially filled with butane or propane. Typically, when predetermined conditions were reached, the flask was broken with a hammer. Expansion of the unignited cloud was measured by introduction of a smoke curtain and use of a high speed video camera. Large droplets were visible, but a portion of the fuel formed a liquid pool beneath the flask. Figure 6.5 shows that, as superheat was increased, the portion of fuel that... 

A vessel filled with a pressurized, superheated liquid can produce blasts upon bursting in three ways. First, the vapor that is usually present above the liquid can generate a blast, as from a gas-filled vessel. Second, the liquid will boil upon depressurization, and, if rapid boiling occurs, a blast wiU result. Third, if the fluid is combustible and the BLEVE is not fire induced, a vapor cloud explosion may occur (see Section 4.3.3.). In this subsection, only the first and second types of blast wiU be investigated. 

Theoretical Work. Theoretical work on the blast from superheated liquid addresses two questions ... 

Thus, the BLEVE theory predicts that, when the temperature of a superheated liquid is below T, liquid flashing cannot give rise to a blast wave. This theory is based on the solid foundations of kinetic gas theory and experimental observations of homogeneous nucleation boiling. It is also supported by the experiments of BASF and British Gas. However, because no systematic study has been conducted, there is no proof that the process described actually governs the type of flashing that causes strong blast waves. Furthermore, rapid vaporization of a superheated liquid below its superheat limit temperature can also produce a blast wave, albeit a weak... 

In the preceding subsections, bursting vessels were assumed to be filled with ideal gases. In fact, most pressure vessels are filled with fluids whose behavior cannot be described, or even approximated, by the ideal-gas law. Furthermore, many vessels are filled with superheated liquids which may vaporize rapidly, or even explosively, when depressurized. 

Equation (6.3.15) is not accurate for the calculation of explosion energy of vessels filled with real gases or superheated liquids. A better measure in these cases is the work that can be performed on surrounding air by the expanding fluid, as calculated from thermodynamic data for the fluid. In this section, a method will be described for calculating this energy, which can then be applied to the basic method in order to determine the blast parameters. 

The above methods assume that all superheated liquids can flash explosively, yet this may perhaps be the case only for liquids above their superheat-limit temperatures or for pre-nucleated fluids. Furthermore, the energies of evaporating liquid and expanding vapor ate taken together, while in practice, they may produce separate blasts. Finally, in practice, there are usually structures in the vicinity of an explosion which will reflect blast or provide wind shelter, thereby influencing the blast parameters. 

Reid s theory that a superheated liquid which flashes below its homogeneous nucleation temperature T i will not give rise to strong blast generation has not been verified. 

Schmidli, J., S. Baneijee, and G. Yadigaroglu. 1990. Effects of vapor/aerosol and pool formation on rupture of vessel containing superheated liquid. J. Loss Prev. Proc. Ind. 3(1) 104-111. 

Kirkw ood [30] describes Bleves referenced to flammable liquids as occurring when a confined liquid is heated above its atmospheric boiling point by an external source of heat or fire and is suddenly released by the rupture of the container due to overpressurization by the expanding liquid. A portion of the superheated liquid immediately... 

Physical examples of the three types are afforded by a gas contained in a cylinder under an external pressure equal to the gas pressure, by a superheated liquid, and by a mixture of water and saturated steam, under the same conditions respectively. 

States such as superheated liquid and supercooled vapour are known as metastable, they are not of themselves unstable, but become so on introduction of a small amount of the stable phase. 

A centrifugal pump is to be used to circulate liquid (density 800 kg/m3 and viscosity 0.5 mN s/nr) from the reboiler of a distillation column through a vaporiser at the rate of 400 em /s, and (o introduce the superheated liquid above the vapour space in the reboiler which contains liquid to a depth of 0.7 in. Suggest a suitable layout if a smooth bore 2.5 mm pipe is to be used. The pressure of the vapour in the reboiler is i tcN/nr and the NPSH required by the pump is 2 in of liquid. 

Thermodynamic and mechanical equilibrium on a curved vapor-liquid interface requires a certain degree of superheat in order to maintain a given curvature. Characteristics of homogeneous and heterogeneous nucleation can be estimated in the frame of classical theory of kinetics of nucleation (Volmer and Weber 1926 Earkas 1927 Becker and Doring 1935 Zel dovich 1943). The vapor temperature in the bubble Ts.b can be computed from equations (Bankoff and Flaute 1957 Cole 1974 Blander and Katz 1975 Li and Cheng 2004) for homogeneous nucleation in superheated liquids... 

These studies consider the dynamics of a single bubble that grows in infinity space, which is filled by superheated liquid. Under these conditions the bubble expansion depends on inertia forces or on intensity of heat transfer. In the case when inertia forces are dominant the bubble radius grows linearly in time (Carey 1992) ... 

Catalytic hydrogen supply from a decalin-based chemical hydride under superheated liquid-film conditions... 

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