Enthalpy of evaporation

It carries very large amounts of heat as enthalpy of evaporation or latent heat in relatively small weights of steam ... 

Then if hfg = enthalpy of evaporation of the steam at the pressure involved. 

High-pressure condensate forms at the same temperature as the high-pressure steam from which it condenses, as the enthalpy of evaporation (latent heat) is transferred from it. When this condensate is discharged through a steam trap to a lower pressure the energy it contains is greater than it can hold while remaining as liquid water. The excess... 

The boiling point of water is 100 °C (212 °F) at atmospheric pressure, and at this same point the enthalpy of evaporation (latent heat of vaporization) is 970 Btu/lb. 

Feh r and Hitzemann [30] determined the enthalpies of evaporation AyfP293 of the sulfanes H2S with n=2-5 calorimetrically. Interestingly, there exists a linear relationship between H°293 (in kj mor ) and the number of sulfur atoms ... 

At a given ambient water vapor pressure (usually the level found in the open atmosphere), the temperature of the material is raised so that the equilibrium water vapor pressure over the hydrated material is higher than the ambient water vapour pressure. Generally, heating up to 400 °C is sufficient to remove all the water of crystallization from materials. This removal of water yields a material which may contain some more strongly bound water. To remove this water, the material requires to be heated to a higher temperature (400-600 °C) so that the equilibrium water vapour pressure exceeds the ambient water vapour pressure. For near-complete removal of the last traces of water, temperatures as high as 1000 °C may be required. In addition to the heat required to raise the temperature of the material, heat is also required for the evaporation of water, which is an endothermic process. The enthalpy of evaporation increases as the water content, and hence the equilibrium water vapor pressure, decreases. 

By using open or vented cells, data are obtained to determine if the full-scale reactor vent flow will be single or two-phase, and if a tempered or nontempered reaction occurs [192]. A tempered reaction is one that can be controlled through the latent enthalpy of evaporation. 

For a pure vapor system (AT/At driven), where the runaway reaction can be kept under control by the latent enthalpy of evaporation (tempered system), a relatively simple expression can be used for the estimation of the necessary vent diameter. 

Large enthalpy of evaporation— to minimize the mass flow rate for given power output. 

The subsequent additions of water molecules are associated with significantly smaller releases of enthalpy and are consistent with the formation of hydrogen bonds. The total enthalpy change for the production of the ion H+(H20)6(g) is —1116 kJ mol-1, a value not significantly different from that adopted for the enthalpy of hydration of the proton. Further additions of water molecules to the HT(H20)(,(g) ion are associated with enthalpy changes that arc not significantly different from the enthalpy of evaporation of water (44 kJ mol - ) and do not appear to add to the stability of the hydrated proton. 

In comparison to the high enthalpy of evaporation of water (57.36 kj moh1), the respective values are much lower for organic solvents, which are thus much easier to separate than water. Table 12.1 lists some boiling points and enthalpies of evaporation for common organic solvents. 

Table 12.1 Boiling points and enthalpies of evaporation for common organic solvents.

By increasing the temperature, the vapor pressure of the reaction mass may increase. The resulting pressure can be estimated by the Clausius-Clapeyron law, which links the pressure to the temperature and the latent enthalpy of evaporation A Hv ... 

In Equations 2.9 and 2.10, the enthalpy of evaporation used is the specific enthalpy of evaporation, expressed in kjkg 1. These expressions only give the amount of solvent evaporated, which is a static parameter. They give no information about the vapor flow rate, which is related to the dynamics of the process, that is, the reaction rate (see Section 9.4). This aspect is discussed in the chapter on technical aspects of reactor safety. 

The boiling temperature of water is 373.15 K at normal atmospheric pressure. If the enthalpy of evaporation at this temperature is 40.656 kJ mol-1, calculate the pressure at which water boils at 90 °C. 

Note The tabulated value for ps of water at 90 °C is 701.1 mbar (ref. (e), p. 150). The difference is probably because the temperature-dependence of the enthalpy of evaporation has not been taken into account in this calculation. 

The analysis performed in [3] has revealed that fullerenes dissolve best in the solvents for which specific enthalpy of evaporation referring to the specific volume of a solvent molecule is close to that for fiillerene. 

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