Binary Systems

11.5.1. The heat balance

In previous sections, the case of constant molar latent heat has been considered with no heat of mixing, and hence a constant molar rate of reflux in the column. These simplifying assumptions are extremely useful in that they enable a simple geometrical method to be used for finding the change in concentration on the plates and, whilst they are rarely entirely true in industrial conditions, they often provide a convenient start for design purposes. For a non-ideal system, where the molar latent heat is no longer constant and where there is a substantial heat of mixing, the calculations become much more tedious. For binary mixtures of this kind a graphical model has been developed by Ruhemann(33), Ponchon(34), and Savarit(35) , based on the use of an enthalpy-composition chart. A typical enthalpy-composition or H — x chart is shown in Figure 11.24, where the upper curve V is the dew-point curve, and the lower curve L the boiling-point curve. The use of this diagram is based on the geometrical properties, as illustrated in Figure 11.25. A quantity of mixture in any physical state is known as a "phase" and is denoted by mass, composition and enthalpy. The phase is shown upon the diagram by a point which shows enthalpy and composition, though it does not show the mass. If m is the mass, x the composition and H the enthalpy per unit mass, then the addition of two phases A and B to give phase C is governed by:

Similarly, if an amount Q of heat is added to a mass mA of a phase, the increase in enthalpy from to HC will be given by:

and:

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