Thermodynamic Fundamentals of Mixtures

3 Thermodynamic Fundamentals of Mixtures 2.1.3.3.1 Equilibria, ideal - nonideal 

In order to describe the principles of distillation, the following thermodynamic concepts should be taken into consideration [15-17]. The liquid phase is in equilibrium with the vapour phase when their chemical potentials are equal. Since the chemical potential is dependent on temperature and pressure, one parameter is always kept constant for the thermodynamic description of distillation. 

Mixtures of liquids exhibit ideal behaviour when their intermolecular forces are equal among and between themselves, their partial enthalpies are independent of concentration and equal to the molar enthalpies of the pure components. In this case they obey Raoult s law, which states that the partial vapour pressure of a component is proportional to its mole fraction in the liquid mixture  

As the partial vapour pressures of the individual components add up to the total pressure, the more volatile component accumulates in the vapour phase. This fact constitutes the basis of distillation. The ideal relative volatility factor then results from the vapour pressures of the pure components  

Graphical depiction of an ideal binary mixture results in the equilibrium phase diagram shown in Fig. 2.32. 

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