Enthalpy-concentration diagram difference

Figure 12.15 Top Isothermal Pxy diagram for a binary mixture of an alkane(l) + an aromatic(2) at 330 K, computed from the Redlich-Kwong equation of state using the phi-phi method. This diagram is the same as in Figure 9.5. Bottom The corresponding isothermal (residual) enthalpy-concentration diagram for the same mixture as at top, also computed from the Redlich-Kwong equation using (12.4.1). Note that differences in liquid and vapor values at Xi = 0 and at Xi = 1 estimate the pure-component latent heats of vaporization.

The Ponchon-Savarit method is summarized in Fig. 5.3-8. The method involves an enthalpy-concentration diagram, and the enthalpies of the saturated liquid and vapor are first plotted on the diagram. Next, the equilibrium tie lines are added, based on phase equilibria. Compositions of feed, distillate, and bottoms are then located on the diagram (in the example shown the feed is mixed vapor-liquid and the distillate and bottoms are saturated liquids). A reflux ratio is chosen, and the enthalpy of the reflux is located as the top difference point, Ag- (The reflux ratio is equal numerically to the vertical distance from the difference poim to the value of y/v> divided by the vertical distance from y to Xg.)... 

The major difference between the McCabe-Thiele method and the Ponchon-Savarit method is that the liquid and vapor flow rates in the latter method are not assumed constant throughout the column. Because of this, the Ponchon-Savarit method is a more general and accurate method. Since the method is based on enthalpy values of fluid mixtures throughout the column, an enthalpy-concentration diagram is used in conjunction with a vapor-liquid equilibrium plot to determine the number of theoretical plates required for a specific separation. Units based on either moles (and mole fractions) or mass (and mass fractions) are acceptable as long as they are consistent. 

Some experimental heat of dilution data for NaCl are shown in Figure 10.10. The negative values show that the enthalpy of a dilute NaCl solution is less than that of a more concentrated one. The data in this diagram are an answer to the question what is the (nonideal) heat of mixing of NaCl and water In this case the NaCl is at two different concentrations - one is 1.0 molal, and the other is as shown on the x-axis. 

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