UNIFAC (NIST-Modified) Activity Coefficient Model: DETAILS

The model is similar to the NIST-KT-UNIFAC model. Details of the parameterization for NIST-KT-UNIFAC are given here: NIST-KT UNIFAC Parameter Details

Distinctions from the NIST-KT UNIFAC model follow.

Parameters A, B, and C for each pair of main groups for the particular binary mixture are available as part of the TDE output.

Temperature dependence of interaction parameters is given by:

Ψ_nm = exp((A_nm + B_nm × T + C_nm × T^2) / T)

The combinatorial contribution is represented by:

γ_i^c = 1 - Φ'_i + ln(Φ'_i) - 5 × q_i × (1 - (Φ_i / θ_i) + ln(Φ_i / θ_i))

where

Φ_i = r_i / summation over j of [x_j × r_j]; θ_i = q_i / summation over j of [x_j × q_j]; Φ'_i = (r_i)^3/4 / summation over j of [x_j × (r_j)^3/4]

Indexes i and j (above) belong to the components of the mixture. The molecular volume and surface area fi and qi are calculated from subgroup contributions (specific for NIST-modified UNIFAC) Rk and Qk, where nik is the number of occurrences of the k-th subgroup in molecule i.

r_i = summation over k of [n_ik × R_k]; q_i = summation over k of [n_ik × Q_k]