(698d) Prediction of Surface Tensions and Adsorption Isotherms of Alkane Isomers

One of the challenging tasks in process engineering is the separation of substances having similar thermodynamic and chemical properties. Realization of suitable processes is expensive in cost and time. Therefore, process development and design relies more and more on support by computer simulations that require accurate physical models. However, parameterizing these models requires experimental data. To reduce the experimental effort, it is desirable to have models that are able to conduct predictions of thermodynamic data based on as less as possible data points.

For achieving high purities, adsorption may be the method of choice and a model to calculate adsorption isotherms utilizing only a few data points is given the classical Density Functional Theory (DFT) [1,2,3,4]. In combination with an external potential and a suitable equation of state, within the DFT framework density profiles are calculated by minimizing the grand potential function. To increase the predictive capability of the theory, the equation of state Lattice Cluster Theory (LCT) [5,6,7,8,9] can be used, since it is able to predict isomer properties just based on adjustments to the linear molecules. Previous work [8] showed the principle of this formalism and this work aims to verify the calculated adsorption isotherms by comparing them to experimental and molecular simulation data. Since the theoretical framework has been described using a local density approach, the theory could not be utilized to additional calculation of surface tensions. Therefore, this works also extends the previous approach by implementing a weighted density approach meaning an evaluation of the grand potential function using information about the local and nearby values of the densities with a suitable weighing function. Consequential, the theory provides a description of interfaces between solids and liquids as well as within liquids i.e. in liquid-vapor or liquid-liquid decomposed systems.

In this new framework, the three parameters of the LCT are adjusted to phase equilibrium data, the parameter of the weighing function requires one experimental surface tension of the pure substance and for the calculation of the adsorption isotherms the parameters of the external are adjusted using a few points of one isotherm. This work will show that, once parameterized for the linear molecule i.e. n-pentane, the surface tensions of neo- and iso-pentane as well as the adsorption of them on the similar adsorbent can be predicted. Moreover, the model is able to predict the physical behavior of mixtures of isomers.

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