Solid Vapor Equilibrium SVE

At temperatnres below its triple point, a pure solid can vaporize. Solid/vapor equilibrium for a pure species is represented on a PT diagram by the subhmation curve (see Fig. 3.1) here, as for VLE, the eqnilibrinm pressure for a particnlar temperature is called the (solid/vapor) saturation pressure P 

We consider in this section the eqnilibrinm of a pnre solid (species 1) with a binary vapor mixhire containing species 1 and a second species (species 2), assnmed insolnble in the solid phase. Since it is nsnally the major constitnent of the vapor phase, species 2 is conventionally called the solvent species. Hence species 1 is the solnte species, and its mole fraction yi in the vapor phase is its solubility in the solvent. The goal is to develop a procedure for computing yi as a function of T and P for vapor solvents. 

Only one phase-eqnilibrinmeqnation can be written for this system, because species 2, by assumption, does not distribute between the two phases. The solid is pure species 1. Thus, 

Equation (11.41) for a pnre liquid is, with minor change of notation, appropriate here  

Combining the tliree preceding equations and solving for yi gives  

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Solubilitiesattemperaturesand pressures above the critical values of the solvent liave important applications for supercritical separation processes. Examples are extraction of caffeine from coffee beans and separation of asplraltenes from heavy petroleum fractions. For a typical solid/vapor equilibrium (SVE) problem, tire solid/vapor saturation pressure P is very small, and the saturated vapor is for practical purposes an ideal gas. Hence 0 for pure solute vapor at this pressure is close to unity. Moreover, exceptfor very low values of the system pressure P, the solid solubility yj is small, and can be approximated by j, the vapor-phase fugacity coefficient of the solute at infinite dilution. Finally, since is very small, the pressure difference P — in the Poyntingfactor is nearly equal to P at any pressure where tins factor... 

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