Liquids phase changes

Solid-solid phase changes. Solid-solid phase changes have the same characteristics as solid-liquid phase changes, but usually do not posses a large phase change enthalpy. However, there are exceptions. 

Figure 101. Heat storage as latent heat for the case of melting (solid-liquid phase change)...

Chickos, J.S., Acree, W.E., Jr., Liebman, J.F. (1999) Estimating solid-liquid phase change enthalpies and entropies. J. Phys. Chem. Ref. Data 28, 1535-1673. 

As air diffuses through the samples during the controlled stripping in the test, the concentration of the more volatile components of the liquid phase changes with time as the stripping continues. The presence of two volumes of liquid in series is sufficient for most components to prevent alteration of the vapour/liquid equilibrium, but for some very volatile substances, the concentration in the exit air will inevitably change with time. 

Note that the mass flux of component i in the liquid phase changes due to chemical conversion, whereas this flux remains constant in the vapour/gas phase since it has been assumed that no reaction occurs in the vapour/gas phase. For both phases the conservation for thermal energy equation is given by... 

Middlebrook, A. M., L. T. Iraci, L. S. McNeill, B. G. Koehler, M. A. Wilson, O. W. Saastad, and M. A. Tolbert, Fourier Transform-Infrared Studies of Thin H2S04/H20 Films Formation, Water Uptake, and Solid-Liquid Phase Changes, J. Geophys. Res., 98, 20473-20481 (1993). 

This difference in bond strength is also responsible for many other differences in the physical properties of these two substances. For instance, whereas the melting point of NaCl is 801°C, that of KCI is only 770°C. The 31°C difference is easy to explain in terms of what happens when a solid-to-liquid phase change occurs the particles of the solid have to be pried apart from one another. The weaker ionic bonds in KCI mean the ions separate more easily, and the macroscopic evidence of this is the lower melting point of KCI. 

Knowing the values of AH and AS for a phase transition makes it possible to calculate the temperature at which the change occurs. Recall from Section 8.14 that AG is negative for a spontaneous process, positive for a nonspontaneous process, and zero for a process at equilibrium. Thus, by setting AG = 0 and solving for T in the free-energy equation, we can calculate the temperature at which two phases are in equilibrium. For the solid — liquid phase change in water, for instance, we have... 

The dilation (or dilatation) of a partly solidified fat sample at a given temperature is the increase in specific volume (volume per unit mass) that occurs during isothermal complete melting at that temperature. The melting dilation of the fat at the same temperature is the increase in specific volume that would occur on complete melting if the sample was initially 100% solid. Dilation is the result of the expansion that occurs as a result of the solid to liquid phase change the specific volume of molten fat is about 10% greater than that of solid fat. 

The method is usually adopted when the transferring component is 02, since it is possible to monitor the dissolved oxygen concentration with a polarographic electrode(Clark cell). The method requires some precautions. The probe has an intrinsic time constant tp (the time needed to attain 63% of the reading after a stepwise concentration change) so that its output is not directly related to the actual concentration, especially when the concentration in the liquid phase changes rapidly. Indeed the measurement may be... 

Flow Models In a real column the composition of the vapor and liquid phases changes due to flow across a tray or over and arouncf packing. Thus, the bulk phase mole fractions that appear in the rate equations (13-69) and (13-70) vary with position and should not automatically be assumed to be equal to the average exit mole... 

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