Predicting Adsorption Isotherms of Systems Containing Isomers

The separation of mixtures containing different isomers is a challenging task in chemical and process engineering. If components have similar vapor pressures and melting points as well as solubilities in a given solvent, adsorption can be the method of choice to separate the mixture. Then, adsorption isotherms are needed for the process development and intensification. In principle, it is possible to measure them but it evokes a high technical and time effort. Therefore, a rigorous thermodynamic model for the prediction of adsorption isotherms is desirable. One method to calculate adsorption isotherms is given by the Density Functional Theory, developed by Ebner, including an external potential as proposed by Tarazona and Evans. Within this theory, an equation of state is needed to describe the bulk fluid phase. For the description of isomers, the Lattice Cluster Theory, introduced by Freed and co-workers, can be applied. Due to the theory being developed for polymer systems, it is modified in this work to describe small molecules and it is parameterized for various alkanes and finally inserted into the Density Functional Theory. The modification of the Lattice Cluster Theory leads to an enhanced predictive capability concerning vapor pressures and densities of isomers. Furthermore, within the Density Functional Theory framework in combination with the Lattice Cluster Theory the parameters of the external potential have to be adjusted to a single given adsorption isotherm. Consequently, adsorption isotherms of isomers and under different conditions can be predicted. In this work, three different parameterizations of the external potential are investigated and the predictive capability is tested by comparing calculated isotherms with experimental data from literature. The qualitative prediction succeeds from throughout. For high values of pressure, predictions and experiments are in good agreement. Furthermore, the possibility to predict adsorption isotherms of mixtures is presented.