Improvement of the UNIQUAC Combinatorial-Entropy Term by Adjusting the Standard Segment

A standard segment is used to determine the structural parameters r and q of the UNIQUAC combinatorial-entropy term. Generally, it is assumed that the choice of the standard segment and the influence of the absolute value of q are negligible. The standard segment area, however, does not cancel out in the model equation and, therefore, is a model parameter. In this work this parameter is determined by fitting the model to experimental data of binary n-alkane mixtures. To account for residual effects of the n-alkane mixtures, a suitable correlation was developed. From the fit a new standard segment resulted, which is about a factor of 3 smaller than the original UNIQUAC standard segment. The new standard segment significantly improves the performance of the UNIQUAC combinatorial-entropy term.