Superheating nucleation point

Classic nucleation theory must be modified for nucleation near a critical point. Observed supercooling and superheating far exceeds that predicted by conventional theory and McGraw and Reiss [36] pointed out that if a usually neglected excluded volume term is retained the free energy of the critical nucleus increases considerably. As noted by Derjaguin [37], a similar problem occurs in the theory of cavitation. In binary systems the composition of the nuclei will differ from that of the bulk... 

Boiling in the bulk of the fluid generally takes place at submicron nucleation sites as impurities, crystals, or ions. When there is a shortage of nucleation sites in the bulk of the liquid, its boiling point can be exceeded without boiling then the liquid is superheated. There is, however, a limit at a given pressure above which a liquid cannot be superheated, and when this limit is reached, microscopic vapor bubbles develop spontaneously in the pure liquid (without nucleation sites). 

For liquid products (solvents), only polar molecules selectively absorb microwaves, because nonpolar molecules are inert to microwave dielectric loss. In this context of efficient microwave absorption it has also been shown that boiling points can be higher when solvents are subjected to microwave irradiation rather than conventional heating. This effect, called the superheating effect [13, 14] has been attributed to retardation of nucleation during microwave heating (Tab. 3.1). 

In this article, we suggest that a modified superheated-liquid model could explain many facts, but the basic premise of the model has never been established in clearly delineated experiments. The simple superheated-liquid model, developed for LNG and water explosions (see Section III), assumes the cold liquid is prevented from boiling on the hot liquid surface and may heat to its limit-of-superheat temperature. At this temperature, homogeneous nucleation results with significant local vaporization in a few microseconds. Such a mechanism has been rejected for molten metal-water interactions since the temperatures of most molten metals studied are above the critical point of water. In such cases, it would be expected that a steam film would encapsulate the water to... 

The nucleation characteristics of samples were examined by the relation between superheating temperature JTu ( = Tu —Tm) and supercooling temperature JTsc ( = Tm—Tsc). Where, Tm is the melting point of CaCl2 6H20 sample, that is, 302K. 

Similarly, when rhombic red a-sulfur is heated above 100°C, it usually fails to exhibit the expected thermodynamic conversion to yellow /3-sulfur at 96°C. Instead, it persists as a superheated metastable phase up to 114°C (dashed line), where it exhibits an apparently normal melting point to the liquid form (unless extreme patience or a nucleating seed crystal of /3-sulfur is employed). The dashed lines in Fig. 7.5 therefore mark out metastable phase transition boundaries between forms of sulfur that are not true Gibbs free energy minima at the cited temperature and pressure (e.g., superheated a-sulfur and supercooled liquid sulfur at 114°C, 1 atm). The metastable phase domains can overlap stable phase domains in a quite complex and confusing manner. A kinetically facile metastable phase boundary will often appear more real and relevant to actual chemical phenomena than will the idealized boundary between (kinetically inaccessible) phases of lowest Gibbs free energy. 

As long as the wall temperature stays below that required for the formation of vapour bubbles, heat will be transferred by single-phase, forced flow. If the wall is adequately superheated, vapour bubbles can form even though the core liquid is still subcooled. This is a region of subcooled boiling. In this area, the wall temperature is virtually constant and lies a few Kelvin above the saturation temperature. The transition to nucleate boiling, is, by definition, at the point where the liquid reaches the saturation temperature at its centre, and with that the thermodynamic quality is r h = 0. In reality, as Fig. 4.53 indicates, the liquid at the core is still subcooled due to the radial temperature profile, whilst at the same time vapour bubbles form at the wall, so that the mean enthalpy is the same as that of the saturated liquid. As explained in the previous section, the... 

The embryos that trigger vapor formation in a superheated liquid are microscopic bubbles small regions where the density is smaller than in the bulk. To calculate the rate of homogeneous nucleation in a superheated liquid according to the classical theory, one must therefore consider the energetics of bubble formation. The contents of vapor embryos can be treated as an ideal gas except near the critical point. Let P be the pressure inside the critical nucleus. Then, P being the bulk pressure in the superheated... 

The theoretical superheating limiting conditions at which spontaneous homogeneous nucleation will exist in all the liquid mass can be established from the tangent line to the vapor pressure-temperature curve at the critical point. This represents the limit to which the liquid may be heated before spontaneous nucleation occurs with a vapor explosion (Reid, 1979). This is shown in Eig. 22.2. 

Instead, if during the heating process the liquid temperature reaches, for example, 89°C (point R in Fig. 22.2), during the depressurization the tangent line will be reached (point S in Fig. 22.2). In this case the conditions required (superheating) by the aforementioned spontaneous homogeneous nucleation would exist and a BLEVE explosion would occur. 

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