 Example 8.05 SLE of the Eutectic System CCl4 - n-Octane With Solid Phase Transition

problem:

Calculate the SLE diagram of the eutectic system CCl4 (1) – n-octane (2) with the help of the modified UNIFAC model and the Wilson model. Please take into account that CCl4 shows a phase transition at 225.35 K. general

constants

and

definitions:    input data

Number of components and range variables:    Pure component melting temperatures, heats of fusion and solid transition temperature and enthalpy:    Wilson parameters:  Solution

To calculate the equilibrium between the liquid and solid phase, expressions for the fugacity or activity of the components in both phases are required. Due to the existance of a solid phase transition point in case of CCl4, the function for the solid activity has to be extended:  The behavior of the pure solid activities as function of temperature is shown in the following plot. Both values are zero at the respective melting temperatures and the change in slope in case of the first components at its transition point is clearly visible.  To calculate the activity of the components in the liquid, the activity coefficient is required. In this example it is first calculated via the Wilson model:

Calculation of the interaction parameters DL as function of temperature: Calculation of the activity coefficient as function of temperature and composition can be performed using the expression given in Table 5.6: The solid liquid equilibrium for a given mole fraction can now be calculated using the following function:   In order to calculate the equilibrium using mod. UNIFAC, some information about the components as well as the required model parameters have to be defined first:              The function to calculate the activity coefficients using modified UNIFAC is in the area below.

The function for the SLE equilibrium temperature defined above can also be used for modified UNIFAC. Note that the lower bound for the root function was changed. With quadratic temperature dependence of the interaction parameters, it is possible that the activity coefficients decrease drastically at very low temperatures outside the range of applicability of the modified UNIFAC model. This can lead to a second solution of the solid-liquid equilibrium at very low temperatures. Using the two functions for the SLE equilibrium temperature defined above, the results of both models can now be shown in one plot.  This result compares well with Fig. 8.7 in the textbook: 