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080103__CHE_101_Full_Class_Notes_2 - 2.2.4 Multi-Component...

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2.2.4 Multi-Component Phase Equilibrium Applictions and processes: > Distillation >Any other separation technique >Absorbers/strippers Previously we have focused on system with only one pure component or only one condensable component. Now we will discuss gas-liquid systems where multiple components exist in the gas and the liquid phase. Much of the utility of these systems is for separations. We will be dealing with two types of systems. The first are liquid-liquid systems where the species involved are similar (water and ethanol, benzene and toluene). With these systems Raoult’s law applies (Ideal case). The second type of system is where species are not similar (gas dissolved in a liquid, oil and water). With these systems we will need to use Henry’s law. This is essentially a special case of Raoult’s law (non-ideal case). We will first study the ideal case where components are similar before moving onto the special cases. IDEAL LIQUID – LIQUID MIXTURES: Raoult’s law for ideal liquid mixtures: ) ( * T p x P y P A A T A A = = These systems differ significantly from systems of one condensable components. As these mixtures boil the composition of the liquid changes. As the composition changes, the boiling point changes. Figure 7: Multi-component mixtures 1
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Bubble point and dew point calculations We will begin using Raoult’s law to determine the bubble point and dew point of a mixture of liquids. Bubble point temperature : Recall single phase system: the bubble point was where the vapour pressure of the component was equal to the total pressure above it. However now we have multiple components in the liquid phase, each of which are contributing to the total vapour pressure to varying degrees. Assuming an ideal solution, Raoult’s law says that the total pressure above a multicomponent liquid is equal to the mole fraction of the components times their vapour pressures at the temperature in question.
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