Equilibrium adsorption of an oxygen-nitrogen mixture on zeolite NaX

Abstract

Dependences of the Helmholtz energy, entropy, and internal energy in single micropores on the total number of guest molecules and the oxygen to nitrogen ratio were found by analysis of experimental data on adsorption of the binary oxygen--nitrogen mixture on zeolite NaX using the statistical model. A substantially non-ideal character of the adsorption phase, which exhibits negative deviations from Raoult's taw, can result from heterogeneity of the adsorption field inside single cavities.Key words: zeolite, equilibrium adsorption, binary mixtures, thermodynamic functions, molecular statistics.Multicomponent mixtures are widely used for calculations of industrial adsorption processes. Accuracy of such calculations depends on the degree to which the equilibrium adsorption of gaseous or vapor mixtures can be adequately described. Several models of equilibrium multicomponent adsorption are introduced, such as the model of the ideal adsorption solution (lAST) developed by Myers and Prausnitz, 1 two variants of the model of the vacancy solution, 2-5 and the model of the adsorption solution in which an adsorbent serves as one of the components (stoichiometric adsorption theory). 6,7 These models ignore the mechanism of adsorption in single micropores and assume that parameters such as the composition and pressure are the same for any equilibrium state through the whole volume of the adsorption phase, i.e., the adsorption space is considered as energetically uniform. The same limitation was used 8,9 in the statistical model where the array of zeolite cavities is treated as a large canonical ensemble of quasirindependent open systems. The difference in the composition in different points of the volume of a given adsorption phase was taken into account in the Arnold theory t~ and the theory of heterogeneous adsorption space ll-13 that uses the Polanyi model for analysis of multicomponent systems. However, in these theories, the microscopic size of pores is ignored as well.A common opinion about an adequate definition of the ideal phase is still lacking. For example, the vacancy models and the stoichiometric theory consider as ideal the adsorption solution, with behavior obeying the Langmuir or, in the case of adsorption of a mixture, the Markham--Benton equation. This implies that almost all adsorption systems are nonideal. By contrast, in the IAST model, the assumption that the adsorption phase is ideal is frequently used. Comparing the results obtained by processing the same data ill the framework of different models shows that a particular system can be ideal for one theory and nonideal for another. Heterogeneity of the adsorption field inside a micropore can cause the system to deviate from ideal behavior. In addition, the character of the interaction of molecules of different components (e.g., nonpolar molecules and molecules with a quadruple moment) with the adsorption field can be different. As a result, molecules in the volume of a micropore can be rearranged to change their total internal energy and entropy. Theref...

Nonideality of binary mixtures in the adsorbent micropores

“…Some ordering arises in the mutual arrangement of the component molecules, which results in a decrease in the entropy and internal energy as compared to the case of random distribution. Taking into account expression (9). the excess internal energy can be determined as follows:…”