The Mathematical Treatment of Low-Pressure VLE Data

The tables and plots shown in the previous section are clear and have some inmitive content. But in the age of computers we wish to reduce all such information to equations, which can then be used by computers to interpolate and extrapolate the data and to apply it to practical problems. Furthermore, these equations will allow us to estimate VLE for systems for which we have little or no data, often with considerable success. 

The normal way of representing and correlating such data is via equations for the fugacity coefficient in the gas and the activity coefficient in the liquid. We know from chapter 7 that for each species in a VLE,/) yjiiquid pg ies i. 

Substituting the definitions of the fugacity coefficient, in the vapor and the activity coefficient, in the liquid we find 

The right hand side of Eq. 8.3 is called the phi-gamma representation of VEE, and is used almost universally in chemical engineering (and in the rest of this book). For most of low-pressure VLE the vapor phase is close enough to being an ideal gas that we set (j)i= 1.00, which makes our working equation 

Example 8.2 Estimate the liquid-phase activity coefficients for acetone and water from the data in Table 8.1. 

---