Phase changes water vaporization

The slope of the line allows for the determination of the enthalpy of vaporization of water, A//Vap, and the y intercept yields the entropy of vaporization, A. S vap As both the enthalpy and the entropy of water increase as the phase change liquid — vapor occurs, the slope and y intercept of the Clausius-Clapeyron equation are negative and positive, respectively. At 373 K these thermodynamic quantities have values of AHvap = 40.657 kJ mol-1 and ASvap = 109.0 J K-1 mol-1. The leavening action due to water vapor or steam arises from the increased amount of water vapor that forms as pastry temperatures initially rise in the oven and then from the increased volume of the water vapor as temperatures continue... 

The pressure-temperature phase diagrams also serve to highlight the fact that the polymorphic transition temperature varies with pressure, which is an important consideration in the supercritical fluid processing of materials in which crystallization occurs invariably at elevated pressures. Qualitative prediction of various phase changes (liquid/vapor, solid/vapor, solid/liquid, solid/liquid/vapor) at equilibrium under supercritical fluid conditions can be made by reference to the well-known Le Chatelier s principle. Accordingly, an increase in pressure will result in a decrease in the volume of the system. For most materials (with water being the most notable exception), the specific volume of the liquid and gas phase is less than that of the solid phase, so that... 

During the dehydration of a salt hydrate both the structure and the composition of the initial crystal change to produce the new crystalline phase and water vapor. The processes are therefore more complex than the transformation at constant composition of crystals such as tin ( ), sulfur ( ), and silver nitrate ( ). 

Molecular Nature of Steam. The molecular stmcture of steam is not as weU known as that of ice or water. During the water—steam phase change, rotation of molecules and vibration of atoms within the water molecules do not change considerably, but translation movement increases, accounting for the volume increase when water is evaporated at subcritical pressures. There are indications that even in the steam phase some H2O molecules are associated in small clusters of two or more molecules (4). Values for the dimerization enthalpy and entropy of water have been deterrnined from measurements of the pressure dependence of the thermal conductivity of water vapor at 358—386 K (85—112°C) and 13.3—133.3 kPa (100—1000 torr). These measurements yield the estimated upper limits of equiUbrium constants, for cluster formation in steam, where n is the number of molecules in a cluster. 

The heat accompanying the phase change (2) is 1.44 kcal/mole. This is much less than the molar heat of vaporization of water, 10 kcal/mole. Table 5-II contrasts the melting points and the heats of melting per mole (the molar heat of melting, or the molar heat of fusion) of the same pure substances listed in Table 5-1. 

Let us compare the behavior of these two systems during a phase change. Consider, first, how water acts when it is frozen or vaporized. Pure water freezes at a fixed temperature, 0°C. If we freeze half of a water sample to ice, remove the ice, melt it in another container, and compare the separate samples, we find that the two fractions of the original sample are indistinguishable. 

Carey van P (1992) Liquid-vapor phase-change phenomena. An introduction to the thermophysics of vaporization and condensation processes in heat transfer equipment. Hemisphere, New York Celata GP, Cumo M, Mariani A (1997) Experimental evaluation of the onset of subcooled flow boiling at high liquid velocity and subcoohng. Int J Heat Mass Transfer 40 2979-2885 Celata GP, Cumo M, Mariani A (1993) Burnout in highly subcooled water flow boiling in small diameter tubes. Int J Heat Mass Transfer 36 1269-1285 Chen JC (1966) Correlation for boiling heat transfer to saturated fluids in convective flow. Ind Eng Chem Process Des Develop 5 322-329... 

Latent heat is the energy associated with phase changes. Evaporation of water requires an energy input of 2.5 x 10 J per kilogram of water at 0°C, almost 600 times the specific heat. When water vapor is transported via atmospheric circulation and recondensed, latent heat energy is released at the new location. Atmospheric transport of water vapor thus transfers both latent and sensible heat from low to high latitudes. 

A substance vaporizes from a condensed phase (liquid or solid) into the gas phase, so A ra (gas) = 1 mol. The change in volume for this type of transformation is almost equal to the volume of the resulting gas. For example, according to the ideal gas equation, one mole of water vapor at 373 K has a volume of 30.6 L ... 

The dashed lines on Figure 11-39 show two paths that involve phase changes for water. The horizontal dashed line shows what happens as the temperature increases at a constant pressure of 1 atm. As ice warms from a low temperature, it remains in the solid phase until the temperature reaches 273.15 K. At that temperature, solid ice melts to liquid water, and water remains liquid as the temperature increases until the temperature reaches 373.15 K. At 373.15 K, liquid water changes to water vapor. When the pressure is 1 atm, water is most stable in the gas phase at all higher temperatures. The vertical dashed line shows what happens as the pressure on water is reduced at a constant temperature of 298 K (approximately room temperature). Water remains in the liquid phase until the... 

Pervaporation. Pervaporation differs from the other membrane processes described so far in that the phase-state on one side of the membrane is different from that on the other side. The term pervaporation is a combination of the words permselective and evaporation. The feed to the membrane module is a mixture (e.g. ethanol-water mixture) at a pressure high enough to maintain it in the liquid phase. The liquid mixture is contacted with a dense membrane. The other side of the membrane is maintained at a pressure at or below the dew point of the permeate, thus maintaining it in the vapor phase. The permeate side is often held under vacuum conditions. Pervaporation is potentially useful when separating mixtures that form azeotropes (e.g. ethanol-water mixture). One of the ways to change the vapor-liquid equilibrium to overcome azeotropic behavior is to place a membrane between the vapor and liquid phases. Temperatures are restricted to below 100°C, and as with other liquid membrane processes, feed pretreatment and membrane cleaning are necessary. 

In pharmaceutical systems, both heat and mass transfer are involved whenever a phase change occurs. Lyophilization (freeze-drying) depends on the solid-vapor phase transition of water induced by the addition of thermal energy to a frozen sample in a controlled manner. Lyophilization is described in detail in Chapter 16. Similarly, the adsorption of water vapor by pharmaceutical solids liberates the heat of condensation, as discussed in Chapter 17. 

What are the mechanisms and the rates of phase changes between liquid water, water vapor, surface water in porous substrates, and strongly bound interfacial water in PEMs ... 

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