Fractional Distillation Using Enthalpy-Concentration Method

6 FRACTIONAL DISTILLATION USING ENTHALPY-CONCENTRATION METHOD 

6 Fractional Distillation Using Enthalpy-Concentration Method 

The diagram at a given constant pressure is based on arbitrary reference states of liquid and temperature, such a 273 K (32°F). The saturated liquid line in enthalpy h kJ/kg (btu/lbn,) or kJ/kg mol is calculated by 

6-3) the pure liquid is heated from Tq to Ti,a, vaporized at T a, and then cooled as a vapor to Tq. Similarly, Eq. (11.6-4) also holds for A. For convenience, the reference temperature Tq is often taken as equal to the boiling point of the lower-boiling component A. This means A,4 = A, ,. Hence, only X t, must be corrected to Xg. 

EXAMPLE 11.6-1. Enthalpy-Concentration Plot for Benzene-Toluene Prepare an enthalpy-concentration plot for benzene-toluene at 1 atm pressure. Equilibrium data are given in Table 11.1-1 and Figs. 11.1-1 and 11.1-2. Physical property data are given in Table 11.6-1. 

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Sec. 11.6 Fractional Distillation Using Enthalpy-Concentration Method... 

Introduction. In Section 11.4B the McCabe-Thiele method was used to calculate the number of theoretical steps or trays needed for a given separation of a binary mixture of A and B by rectification or fractional distillation. The main assumptions in the method are that the latent heats are equal, sensible heat differences are negligible, and constant molal overflow occurs in each section of the distillation tower. In this section we shall consider fractional distillation using enthalpy-concentration data where the molal overflow rates are not necessarily constant. The analysis will be made using enthalpy as well as material balances. 

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