(535d) A Model to Describe Mixtures of Cross-Associating Structural Isomers

One of the most common problems in process engineering is the separation of substances from a mixture. The shift towards the use of more renewable resources in the chemical industry results in an increasing number of different components in product mixtures. A large number of these are structural isomers with functional groups. A sufficiently detailed experimental investigation of mixtures of such isomers is not possible. They are not commercially available in sufficient purity. The ability to predict the phase behavior of these mixtures is therefore an interesting prospect. Most conventional thermodynamic models cannot be applied to such systems. They do not distinguish between structural isomers and require a separate set of parameters for each isomer. The lattice cluster theory (LCT) of Freed et al [1] has been shown to be able to describe the phase behavior of mixtures of branched molecules based on their linear analogues. LCT is only able to do this if the functional groups occupy the same position on the structure.

A novel model based on the LCT is developed, which is able to describe structural isomers with functional groups. An approach similar to the Chemical Association Lattice Model (CALM) of Browarzik [2] is chosen to model the association as a chemical association. To cope with the large number of association clusters required, we apply the continuous thermodynamics of Rätsch and Kehlen [3].

The new model is able to take into account the structure of the isomers as well as the structure of the association clusters. It requires only one set of parameters depending on the size and functional group of the molecule. The model is used to correctly predict the liquid-liquid equilibrium phase behavior of mixtures of different alkane isomers and alkanols. It can also predict the liquid-liquid equilibrium phase behavior of cross-associating mixtures of several alkanols. In addition, we give an outlook on the possible application to vapor-liquid equilibria of associating structural isomers.

[1] J. Dudowicz und K. F. Freed, „Effect of Monomer Structure and Compressibility on the Properties of Multicomponent Polymer Blends and Solutions: 1. Lattice Cluster Theory of Compressible Systems,“ Macromolecules, 24, pp. 5076-5095, 1991.

[2] D. Browarzik, „Calculation of excess functions and phase equilibria in binary and ternary mixtures with one associating component,“ Journal of Molecular Liquids, 146, p. 95–104, 2009.

[3] M. Rätsch und H. Kehlen, „Continuous Thermodynamics of Polymer Systems,“ Progress in Polymer Science, 14, pp. 1-46, 1989.