(260b) The Rigid Adsorbent Lattice Fluid Model: A New Thermodynamic Framework for Multicomponent Adsorption

In this contribution a new Rigid Adsorbent Lattice Fluid (RALF) model is presented and simple expressions for adsorption of mixtures are derived from a corrected version of the Lattice Fluid (LF) model of Sanchez and Lacombe [1-3]. The LF model, which formulates the properties of the mixture in its close-packed state, is specialised to the case in which the total volume of the lattice corresponds to the volume occupied by a microporous adsorbent, ie. the solid volume including the micropores [4]. The RALF model bears some similarities to the Non Equilibrium Lattice Fluid (NELF) of Doghieri and Sarti [5], which has proven to be a very successful thermodynamic framework to model pure and mixed sorption of light gases and solvents in glassy polymers, but the modifications to the LF model lead to different expressions for the fugacity coefficient of the adsorbed phase.

In presenting the RALF framework, the special case of the frozen solid is discussed in detail and shown to correspond to a type I isotherm. Explicit expressions for the Henry law constant, the zero loading adsorption energy and the saturation capacity are derived. These can be used to determine from adsorption data the 3 model parameters for an adsorbent. As a case study adsorption on silicalite is considered, given the availability of data in the literature. The parameters for silicalite are obtained from Henry law constants, zero loading adsorption energies and saturation capacities for 6 molecules [6] and data on n-paraffins are then predicted from the remaining two binary interaction parameters present in the RALF model. This should be compared to a standard Langmuir model that requires 3 parameters (Henry law constant, saturation capacity and the adsorption energy).

One of the advantages of the RALF framework is that it is derived as a multicomponent model, therefore prediction of adsorbed mixtures can be carried out either estimating the interaction parameter from fluid phase data or assuming that the interaction parameter between adsorbed molecules is zero. Literature data for CH₄-C₂H₆ on silicalite [7] will be used to demonstrate the application of the RALF framework to binary system.

The talk will conclude with a discussion on how to extend the RALF framework to heterogeneous adsorbents.

References

[1] Sanchez, IC and Lacombe, RH Elementary Molecular Theory of Classical Fluids - Pure Fluids. J. Phys. Chem. 80, 2352-2362 (1976).

[2] Lacombe, RH and Sanchez, IC Statistical Thermodynamics of Fluid Mixtures. J. Phys. Chem. 80, 2568-2580 (1976).

[3] Sanchez, IC and Lacombe, RH Statistical Thermodynamics of Polymer Solutions. Macromolecules 11, 1145-1156 (1978).

[4] Brandani, S, Mangano, E, and Sarkisov, L Net, Excess and Absolute Adsorption of Helium. Adsorption 22,261-276 (2016).

[5] Doghieri, F and Sarti, GC Non-Equilibrium Lattice Fluids: A Predictive Model for the Solubility in Glassy Polymers. Macromolecules 29, 7885-7896 (1996).

[6] Golden, TC and Sircar, S Gas Adsorption on Silicalite. J. Colloid Interface Sci. 162, 182-188 (1994).