Phase change heat involved

Quantitative Aspects of Phase Changes Heat Involved in Phase Changes Equilibrium Nature of Phase Changes Phase Diagrams... 

Values of the correction factor F according to Ref. 4 are plotted in Figs. 10-8 to 10-11 for several different types of heat exchangers. When a phase change is involved, as in condensation or boiling (evaporation), the fluid normally... 

When a sample changes phase, the heat involved is given a name associated with that particular phase change. For example,... 

Many heat-sensitive and oxygen-sensitive solids, such as camphor, are purified by warming under vacuum. The solid vaporizes directly, and the vapor crystallizes on a cool surface. What phase changes are involved in this method ... 

J. D. Cox and G. Pilcher, Thermochemistry of Organic and Organometallic Compounds, Academic Press, London New York, 1970. The major feature is a tabulation of experimental measurements of A H°, heats of reaction, for organic and organometallic compounds. Values of AfH° are derived for each experimental result where phase changes are involved measurements of AH (heat of vaporization) are tabulated. Experimental uncertainties are assessed. Introductory chapters discuss the basics of thermochemistry, the types of experimental measurements involved, their accuracies and limitations. Additional chapters examine theoretical aspects of thermochemistry and various schemes for relating thermochemical properties to structural parameters. 

A heating curve can also be used to explain why hikers stranded by blizzards are warned not to consume snow in an effort to stay hydrated. When you drink cold water, your body expends energy to warm the water you consume to body temperature. If you consume snow, your body must first expend the energy necessary to melt the snow, and then to warm it. Because a phase change is involved, the amount of energy required to assimilate snow is much greater than the amount necessary to assimilate an equal mass of water—even if the water is ice-cold. This can contribute to hypothermia, a potentially dangerous drop in body temperature. 

Heat transfer and mass transfer occur simultaneously whenever a transfer operation involves a change in phase or a chemical reaction. Of these two situations, only the first is considered herein because in reacting systems the complications of chemical reaction mechanisms and pathways are usually primary (see HeaT-EXCHANGETECHNOLOGy). Even in processes involving phase changes, design is frequendy based on the heat-transfer process alone mass transfer is presumed to add no compHcations. But in fact mass transfer effects do influence and can even limit the process rate. 

The integral terms representing AH and AH can be computed if molal heat capacity data Cp(T) are available for each of the reactants (i) and products (j). When phase transitions occur between T and Tj for any of the species, proper accounting must be made by including the appropriate latent heats of phase transformations for those species in the evaluation of AHj, and AH terms. In the absence of phase changes, let Cp(T) = a + bT + cT describe the variation of (cal/g-mole °K) with absolute temperature T (°K). Assuming that constants a, b, and c are known for each species involved in the reaction, we can write... 

There are two heat factors involved in cUiy phase change. They are "heat capacity", i.e.- Cp or Cy, and "heat of transformation", usually denoted as H. 

Which of these phase changes does NOT involve the absorption of heat energy ... 

If any component undergoes a phase change in the unit, the heat required is computed from the latent heat (at 25°C) and the quantity involved. 

FOR EACH REACTION OR PHASE CHANGE INPUT THE HEAT OF REACTION OR THE LATENT HEAT, kJ/kmol AND QUANTITY INVOLVED kmol/h REMEMBER HEAT ENVOLVED POSITIVE HEAT ABSORBED NEGATIVE... 

A reduced velocity (Boure, 1966 Ishii and Zuber, 1970) involving the flow rate-versus-heating power ratio or its reciprocal [phase change number in Ishii and Zuber (1970)]. This group includes the specific volume ratio in Boure (1966) and in Ishii and Zuber (1970). 

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. 

A case of a thermally induced phase change involving ring inversion was recently described by Kaftory (31). He found that a crystal of the exo isomer of the adduct, 7a, of ll-cyano-l,6-methano[10]annulene with 4-methyl-1,2,4-triazoline-3,5-dione is transformed to the endo isomer 7b on heating to 175°C. The process involves nucleation and growth of the product phase, but maintains... 

You just learned about the enthalpy changes that are associated with phase changes. Another type of physical change that involves a heat transfer is dissolution. When a solute dissolves in a solvent, the enthalpy change that occurs is called the enthalpy of solution, Affsoin- Dissolution can be either endothermic or exothermic. 

The product is enriched in one or more constituents of the mixture, leaving a solution of higher or lower concentration on the high pressure side of the membrane. No heating of the membrane is necessary, and no phase change in product recovery is involved in this separation process. 

---