Example P05.04 VLE Calculation For the System Ethanol - Benzene Using

the Wilson Model

problem:

Calculate the Pxy-diagram at 70 °C for the system ethanol (1) – benzene (2) assuming ideal vapor phase behavior using the Wilson equation. The binary Wilson parameters L_{12} and L_{21} should be derived from the activity coefficients at infinite dilution (see Table 5.6). Experimentally the following activity coefficients at infinite dilution were determined at this temperature:

general

constants

and

definitions:

input data

Solution

In the first step the Wilson binary interaction parameters have to be calculated from the activity

coefficients at infinite dilution. As there is no analytical solution to the system of two equations with two unknowns, a Given-Find Block is used:

starting values:

The activity coefficients can now be calculated using the following function:

In addition the pure component vapor pressures are required for the VLE calculation:

Vapor pressure constants for the Wagner equation (Appendix A):

Verification:

The following functions then yield the partial pressures, the total pressure and the vapor phase composition:

The functions are tested here on a single data point:

Finally the functions can be used to calculate the requested Pxy-diagram: