Heat, latent of vaporization

The latent heat of vaporization of water corresponds to the heat required per unit mass of liquid to vaporize it. It is usually denoted L, and it is expressed in J.kg . The latent heat of vaporization is a function of temperature as shown in Table 19.12. At room temperature, a common value used for designing systems is 2.256 MJ.kg (970 Btu/lb) of water. 

TaMe 19.12. Latent heat of vaporization of water vs. T Temperature Latent enthalpy of vaporization 

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Latent heat of vaporization decreases i.e., reboiler and condenser duties become lower. 

Example 9.1 A process involves the use of benzene as a liquid under pressure. The temperature can be varied over a range. Compare the fire and explosion hazards of operating with a liquid process inventory of 1000 kmol at 100 and 150°C based on the theoretical combustion energy resulting from catastrophic failure of the equipment. The normal boiling point of benzene is 80°C, the latent heat of vaporization is 31,000 kJ kmol the specific heat capacity is 150 kJkmoh °C , and the heat of combustion is 3.2 x 10 kJkmok. ... 

On compression, a gaseous phase may condense to a liquid-expanded, L phase via a first-order transition. This transition is difficult to study experimentally because of the small film pressures involved and the need to avoid any impurities [76,193]. There is ample evidence that the transition is clearly first-order there are discontinuities in v-a plots, a latent heat of vaporization associated with the transition and two coexisting phases can be seen. Also, fluctuations in the surface potential [194] in the two phase region indicate two-phase coexistence. The general situation is reminiscent of three-dimensional vapor-liquid condensation and can be treated by the two-dimensional van der Waals equation (Eq. Ill-104) [195] or statistical mechanical models [191]. 

The most common hydrophobic adsorbents are activated carbon and siUcahte. The latter is of particular interest since the affinity for water is very low indeed the heat of adsorption is even smaller than the latent heat of vaporization (3). It seems clear that the channel stmcture of siUcahte must inhibit the hydrogen bonding between occluded water molecules, thus enhancing the hydrophobic nature of the adsorbent. As a result, siUcahte has some potential as a selective adsorbent for the separation of alcohols and other organics from dilute aqueous solutions (4). 

Physical Properties. Physical properties of anhydrous hydrogen fluoride are summarized in Table 1. Figure 1 shows the vapor pressure and latent heat of vaporization. The specific gravity of the Hquid decreases almost linearly from 1.1 at —40°C to 0.84 at 80°C (4). The specific heat of anhydrous HF is shown in Figure 2 and the heat of solution in Figure 3. 

Fig. 1. (---) Latent heat of vaporization (1,7) and (-) vapor pressure (1,4,7,15) of anhydrous hydrogen fluoride. To convert kPa to psi, multiply by...

This high degree of association results in highly nonideal physical properties. For example, heat effects resulting from vapor association may be significantly larger than the latent heat of vaporization (Fig. 9). Vapor heats of association J for HF to (HF) per mole of (HF) are as follows. To... 

When an atom or molecule receives sufficient thermal energy to escape from a Hquid surface, it carries with it the heat of vaporization at the temperature at which evaporation took place. Condensation (return to the Hquid state accompanied by the release of the latent heat of vaporization) occurs upon contact with any surface that is at a temperature below the evaporation temperature. Condensation occurs preferentially at all poiats that are at temperatures below that of the evaporator, and the temperatures of the condenser areas iacrease until they approach the evaporator temperature. There is a tendency for isothermal operation and a high effective thermal conductance. The steam-heating system for a building is an example of this widely employed process. 

The cross-sectional area of the wick is deterrnined by the required Hquid flow rate and the specific properties of capillary pressure and viscous drag. The mass flow rate is equal to the desired heat-transfer rate divided by the latent heat of vaporization of the fluid. Thus the transfer of 2260 W requires a Hquid (H2O) flow of 1 cm /s at 100°C. Because of porous character, wicks are relatively poor thermal conductors. Radial heat flow through the wick is often the dominant source of temperature loss in a heat pipe therefore, the wick thickness tends to be constrained and rarely exceeds 3 mm. 

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