Enthalpy vaporisation

AHv the vaporisation enthalpy of the liquid at 298° K, T the absolute temperature and R the gas universal constant [Hildebrand and Scott, 1950]. For apolar or moderately polar compounds, the vaporization enthalpy can be estimated by their boiling points (bp, ° K) as... 

Note that the fluid used in a vapour compression cycle should have large vaporisation enthalpy, moderate vapour pressure, as well as acceptable usage properties, as non-toxic, non-inflammable and non-corrosive. 

In the above relations and Cp y are the molar heat capacity of liquid and vapour, AHy is the molar vaporisation enthalpy, while Tp, T, are the actual feed temperature, feed bubble point and feed dew point. 

There are many compounds in existence which have a considerable positive enthalpy of formation. They are not made by direct union of the constituent elements in their standard states, but by some process in which the necessary energy is provided indirectly. Many known covalent hydrides (Chapter 5) are made by indirect methods (for example from other hydrides) or by supplying energy (in the form of heat or an electric discharge) to the direct reaction to dissociate the hydrogen molecules and also possibly vaporise the other element. Other known endothermic compounds include nitrogen oxide and ethyne (acetylene) all these compounds have considerable kinetic stability. 

The fact that water is a liquid at room temperature with high enthalpies of fusion and vaporisation can be attributed to hydrogen bond formation. The water molecule is shown in Figure 10.3. 

If H is the enthalpy of the humid gas per unit mass of dry gas, Ha the enthalpy of the dry gas per unit mass, Hw the enthalpy of the vapour per unit mass, Ca the specific heat of the gas at constant pressure, Cw the specific heat of the vapour at constant pressure. 0 the temperature of the humid gas, 0a the reference temperature, X the latent heat of vaporisation of ihe liquid at 6 0 and the humidity of the gas. 

It can be seen from Figure 13.5 that for the air-water system a straight line, of slope equal to the enthalpy of dry saturated steam (2675 kJ/kg), is almost parallel to the isothermals. so that the addition of live steam has only a small effect on the temperature of the gas. The addition of water spray, even if the water is considerably above the temperature of the gas, results in a lowering of the temperature after the water has evaporated. This arises because the. latent heat of vaporisation of the liquid constitutes the major part of the enthalpy of the vapour. Thus, when steam is added, it gives up a small amount of sensible heat to the gas, whereas when hot liquid is added a small amount of sensible heat is given up and a very much larger amount of latent heat is absorbed from the gas. 

The vapour enthalpies are calculated from the molar heat capacity functions for the vapour components and the latent heats of vaporisation at standard temperature. The vapour overflow, V, is then obtained from the energy balance as... 

In many flash processes the feed stream is at a higher pressure than the flash pressure and the heat for vaporisation is provided by the enthalpy of the feed. In this situation the flash temperature will not be known and must be found by trial and error. A temperature must be found at which both the material and energy balances are satisfied. 

In the case of solids, the enthalpy of atomisation includes the enthalpies of fusion and vaporisation in the case of liquids, it includes the enthalpy of vaporisation. 

R. Sabbah, I. Antipine, M. Coten, L. Davy. Quelques Reflexions de la Mesure Calorimetrique de l Enthalpie de Sublimation ou Vaporisation. Thermochim. Acta 1987, 775, 153-165. 

In practice, the quantity vaporised is not fixed directly but it depends upon the enthalpy of the hot incoming feed and the enthalpies of the vapour and liquid leaving the separator. For a given feed condition, the fraction vaporised may be increased by lowering the pressure in the separator. 

Substance Melting point (K) Enthalpy of crystallisation (kl/kg) Boiling point (K) Enthalpy of vaporisation (kJ/kg)... 

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