Enthalpy of Vaporization Latent Heat

The latent heats of vaporization of the more commonly used materials can be found in the handbooks and in Appendix C. 

Avery rough estimate can be obtained from Trouton s rule (Trouton, 1884), one of the oldest prediction methods  

More accurate estimates, suitable for most engineering purposes, can be made from knowledge of the vapor pressure-temperature relationship for the substance. Several correlations have been proposed see Reid et al. (1987). 

The equation presented here, due to Haggenmacher (1946), is derived from the Antoine vapor pressure equation (see Section 8.11)  

If an experimental value of the latent heat at the boiling point is known, the Watson equation (Watson, 1943) can be used to estimate the latent heat at other temperatures  

---

The two components have equal and constant enthalpies of vaporization (latent heats). 

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. 

The total furnace heat absorption may be estimated by using the calculated furnace exit gas temperature and analysis to determine the enthalpy (excluding the latent heat of water vapor) and thus deducting the heat rejection rate from the net heat input rate. 

Achener, P. Y., 1964, The Determination of the Latent Heat of Vaporization, Vapor Pressure, Enthalpy, and Density of Liquid Rubidium and Cesium up to 1,800°F, Proc. 1963 High Temperature Liquid Metal Heat Transfer Technology Meeting, Vol. 1, pp. 3-25 USAEC Rep. ORNL-3605. (2) Achener, P. Y, 1965, The Determination of the Latent Heat of Vaporization, Vapor Pressure of Potassium from 1,000-1,900°F, Aerojet-General Nucleonics Rep. AGN-8141. (2)... 

Because the latent heat A//vap(r, P) is nearly constant over the usual effective range of T and P variations, we assume that it can be replaced by thz fixed molar enthalpy of vaporization AH°wap for standard state conditions (e.g., 25°C, 1 atm) ... 

Enthalpy—Total heat content the sum of the sensible heat of the air and water vapor and of the latent heat of vaporization. 

There is another derivation of Clapeyron s equation which is very instructive. This is based on the use of the Gibbs free energy G. In the last section we have seen that this quantity must be equal for two phases in equilibrium at the same pressure and temperature, and that if one phase has a lower value of G than another at given pressure and temperature, it is the stable phase and the other one is unstable. Wo can verify these results in an elementary way. We know that in going from liquid to vapor, the latent heat L is the difference in enthalpy between gas and liquid, orL = // — Hi. Bui if the change is carried out in equilibrium, the heat absorbed will also equal T (IS, so that the latent heat will be T(SU — Si). Equating these values of the latent heat, we have ... 

The enthalpy of vaporization, AH, is also termed the latent heat of vaporization. AH is the difference between the enthalpy of the saturated vapor and that of the saturated liquid at the same temperature. The enthalpy of vaporization data is used in process calculations such as the design of relief systems involving volatile compounds. In distillation, heat of vaporization values are needed to find the heat loads for the reboiler and condenser. Watson [ 12] has expressed a widely used correlation in calculating AH,. 

The liquid enthalpies were obtained by use of the vapor enthalpies and the latent heats of vaporization as follows... 

Gibbs free energy of formation Latent heat of fusion Enthalpy change at the triple point Heat of vaporization Enthalpy of vaporization Volume change for liquid-solid transformation... 

It is worth remembering that if the dryer is working efficiently, the outlet gas should have a temperature close to the wet bulb temperature and also be at high humidity. Therefore, the majority of the gas enthalpy is the latent heat in moisture vapor, and heat recovery should, when possible, include moisture condeusation from the drying gas. This method is applied in heat pump dehumidifiers. 

This amount of energy is also called latent heat of vaporization, to indicate that it is not accompanied by any temperature change. Writing H=U+ PV for the enthalpy, the enthalpy of vaporization can also be expressed as... 

An alternative graphical method for handling binary mixtures is that of Ponchon and Savarit, and while more cumbersome to use than McCabe-Thiele, it dlows for variations in the molar latent heat of vaporization and thus removes the principal assumption of the McCabe-Thiele method. As basic information it requires not only a y -x equilibrium relationship but also data on enthalpy of vaporization as a function of composition, and, except for a few mixtures, such data are not readily available. 

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

It is appropriate to conclude this section with a few remarks on the magnitude of the latent heats. For most liquids the enthalpy of vaporization amoimts to 10-100 kJ mol its value at the boiling-point is related to the temperature of boiling on the absolute scale by the following approximate relation due to Trouton ... 

This relation is the origin of the obsolescent terms latent heat of vaporization and fusion for what are now termed the enthalpy of vaporization and fusion. 

In addition to the recovery of the latent heat of vapor streams, in many cases it is practical to recover part of the sensible heat in the column bottom product and steam condensate by exchange with column feed. Such schemes have been used in the chemical and petroleum industries for years. Since feed flow is typically set by level controllers or flow-ratio controllers, its flow rate will not be constant. The feed enthalpy or temperature, therefore, is apt to be variable. This may make column-composition control difficult unless one employs either feedforward compensation or a trim heater with control for constant temperature or enthalpy. (See Chapters 5 and 11.)... 

---