Example 8.11 SLE of the System CCl₄ - p-Xylene With a 1:1 Adduct

problem:

Calculate the SLE behavior of the binary system CCl₄ (A) – p-xylene (B) which forms an 1:1 adduct AB. Perform the calculation assuming ideal behavior using the following experimental melting temperatures and enthalpies of fusion:

T_m,A = 250.77 K Dh_m,A = 2528.4 J/mol
T_m,B = 286.45 K Dh_m,B = 16790 J/mol
T_m,AB = 269.15 K Dh_m,AB = 27698 J/mol

general

constants

and

definitions:

input data

Solution

The equilibrium calculation for this case is very similar to the general case of a eutectic mixture except that a further component (the adduct) has to be taken into account.

As the adduct consists of one molecule A and one molecule B, the respective activity a_AB in the liquid phase is 4*a_A*a_B and thus equal to one at x₁=0.5, where the liquid can be considered pure adduct..

The activity of the pure solid phases as function of temperature can again be calculated using the function:

The function for the SLE equilibrium temperature has been modified assuming that the last component is a 1:1 adduct. The activity coefficient function was kept in place to allow more simple later extension.

The starting value for the adduct crystallization temperature has been set to 4*x₁*x₂. At the optimal adduct composition of x₁=0.5, the starting value is thus the melting temperature of the adduct. This factor is also required for the liquid activity calculation as discussed above.

Using this function for the SLE equilibrium temperature, the results can now be shown in a plot. As the components were specified in opposite order to diagram in the textbook, the calculated behavior is a mirror of the representation in Fig. 8.11 a).