PhaseChangesPartitioning - CEE 320 Environmental...

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CEE 320 Phase changes and partitioning Environmental Engineering 1 Phase changes and partitioning – a study guide to supplement your textbook – (see also Learning Objectives present in lecture) (Text and examples adapted from Nazaroff and Alvarez-Cohen 2001) Introduction Here we will consider transformation processes that involve a change of phase among solid, liquid, and gaseous states. In environmental engineering processes and applications, we must consider many phase-change processes. These include the dissolution of species (whether gas, liquid, or solid) into water, the adsorption of gaseous and liquid species onto solid surfaces, the evaporation of species from liquid to gas, and the volatilization of gaseous species from the aqueous phase. Vapor pressure This concept is not covered well in your textbook. Definition: The vapor pressure of a pure liquid species is the equilibrium partial pressure of the gas molecules of that species above the surface of the pure liquid. Vapor pressure increases with increasing temperature for all liquids. When the partial pressure of the species is less than the vapor pressure, the liquid will tend to evaporate; this condition is called subsaturation . Conversely, under supersaturation conditions the partial pressure exceeds the vapor pressure, and there will tend to be net condensation of the gas. When equilibrium is satisfied, we say that the gas phase is saturated with the species. The equilibrium condition between a species in the vapor phase and a flat surface of the pure liquid of that same species can be expressed by: P o i (T) = constant(T) Where P i o represents the equilibrium partial pressure of species i and the “constant” is an equilibrium constant known as the vapor pressure , which is a function of temperature. The most important substances that volatilize and condense in environmental engineering applications are water and volatile organic compounds. The amount of water vapor in air is commonly expressed as the relative humidity, RH. This parameter is defined by the actual partial pressure of water vapor divided by the saturation vapor pressure. RH = 100 % × P H 2 O P o H 2 O ( T ) When the vapor pressure exceeds the total air pressure applied to the liquid surface, the liquid will boil. The boiling point of water at sea level, where the atmospheric pressure is 1 atm, is 100 o C. At higher elevations, the air pressure is lower and the boiling point is correspondingly reduced.
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CEE 320 Phase changes and partitioning Environmental Engineering Dissolution of species in water Whenever liquid water comes into contact with other materials, there is a tendency for some molecules of these other substances to dissolve into the water. In this section, we consider each of
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