Presenting Experimental VLE Data

A separate value of A,- exists for each species in a mixmre being distilled, and its value changes with changes in tem-peramre, pressure, and composition. One also frequently sees 

Example 8.1 Compute the two A factors and the relative volatility for a liquid with 0.05 mol fraction (5 mol%) acetone, balance water. 

We see that the individual As can take on values greater or less than one, but never negative values. As x, 1.0, Kt — 1.0 for each species. The relative volatility is always positive and greater than 1 if there is no azeotrope present (Section 8.4.4). For quick estimates of the difficulty of a separation by distillation, the relative volatility a is the chemical engineer s favorite. If a is greater than 1.5 to 2 over the whole range of composition values, then distillation will almost always be the cheapest separation method. If a is less than 1.1, then we seriously consider other separation 

FIGURE 8.5 Awater. Aacetune, and the relative volatility a plotted as a function of acetone mol fraction in the hquid at 1 atm pressure, using data from Table 8.1. The curves are simple interpolations. 

Most distillation and equilibrium parts of process-design computer programs (e.g.. Aspen, ChemCad, ProMax) report A, values for each species in the VLE data for mixtures, and some distillation programs report a. It must be clear from their definitions that they are made up from values of the y, and Xi. The computer programs mostly first compute the y and Xi, by the methods shown in the rest of this chapter and the next, and then compute and report the A, and a values for the convenience of users who prefer that formulation the, x, and y are the basic values. The common hand-calculation methods of estimating VLE are presented in terms of the A values in Section 8.9. 

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