Subcooled vapors

RELATIONSHIP BETWEEN ABSORPTION AND SUBCOOLED VAPOR PRESSURE OF SOC... 

Actually, the PV behavior predicted in this region by proper cubic equations of state is not wholly fictitious. When the pressure is decreased on saturated liquid devoid of vapor-nucleation sites in a carefully controlled experiment, vaporization does not occur, and the liquid phase persists alone to pressures well below its vapor pressure. Similarly, raising the pressure on a saturated vapor in a suitable experiment does not cause condensation, and the vapor persists alone to pressures well above the vapor pressure. These nonequilibrium or metastable states of superheated liquid and subcooled vapor are approximated by those portions of the PV isotherm which lie in the two-phase region adjacent to the saturated-liquid and saturated-vapor states. 

A feed composition in the metastable set is stable to infinitesimal composition disturbances but is unstable to finite ones hence, for such a composition phase splitting can only occur by nucleation, and not simply by Brownian motion (which, at most, supposedly results in infinitesimal composition disturbances). Hence, a metastable composition may be observed as a one-phase system in the laboratory Superheated liquids and subcooled vapors are elementary one-component examples. In contrast with this, one-phase spinodal compositions, by virtue of being unstable to infinitesimal perturbations, will never be observed in the laboratory. 

For the more complicated shell and tube exchangers (condensers with superheat and/or subcooling, vaporizers with superheating) the usual methods of calculating At or MTD do not give correct answers. 

If reflux cannot be cut back (e.g., in an unrefluxed stripper, in azeotropic distillation, or when the packed section above the feed is close to its minimum wetting limit), boilup will need to be raised to compensate for the excess subcooling. Vapor and liquid traffic below the feed and reboiler duty will rise and effectively lower the column feed capacity. Premature flooding may result. If the lower capacity or higher reboiler duty cannot be tolerated, feed preheating (Fig. 12.5a)... 

If no liquid level is maintained in the condenser, and the liquid is not substantially subcooled, vapor bubbles are likely to be entrained in the liquid condensate. The outlet line should therefore be sized for self-venting flow (Fig. 4.5). Moore (290) has suggested a slightly more conservative rule of thumb for gravity drainage of condensers (assuming no liquid accumulation)... 

Isotherms on an A-V diagram are useful to illustrate the origin of p-V diagrams, superheated liquids and subcooled vapors in metastable states, unstable states between the spinodal points, critical points, and the requirement of mechanical stability. It is reasonable to assume that the slope of A vs. V at constant temperamre and composition is negative, because... 

Metastable states equilibrium states within the binodal region where (dp/dV)T,aa Ni < 0. Superheated liquids and subcooled vapors represent examples of metastable states. They can be isolated and studied because small fluctuations in system properties that normally trigger phase separation are quenched. 

Compound Approximate number of components Water solubility (gm ) Henry .s Law Constant (Pa mol ) Log Kcrw Subcooled vapor pressure (Pa)... 

Still other examples of observable metastabilities include subcooled phases, such as subcooled vapors to make liquids, subcooled liquids to make solids, and subcooled solids to make other solids. The lifetimes of such metastable phases can be substantial, because the nucleation of new phases may require particular kinds of fluctuations that occur only rarely. Lastly we mention the huge number of observable conformational metastabilities that can be exhibited by large molecules such as proteins. 

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