Sorbate Density in 13x Zeolite

Abstract

The objective of this study on sorbate densities on 13X or NaX zeolite is to deduce a model equation to predict the intrinsic isotherm saturation loadings. The input information for the model equation is the crystallographic zeolite information which is abstracted from the literature and the thermodynamic sorbate density. The modified Rackett equation is used to theoretically model the sorbate density below the critical point; above the critical point an empirical relation is derived. Data for the adsorption of various hydrocarbons and polar and non-polar gases on 13X or NaX zeolite over a wide temperature and pressure range is collected from the literature. The hydrocarbon gases include alkanes, alkenes, cyclic alkanes, aromatics and acetates from 22 different studies. Polar and non polar gases include water, carbon dioxide, carbon monoxide, oxygen, nitrogen, argon, sulfur hexafluoride and iodine from 33 different studies. The total isotherms collected are approximately 600. Untabulated adsorption data on NaX or 13X is extracted by digitizing the appropriate figures. The data was screened for consistency. Isotherms with unknown % binder and crossing or misplaced isotherms are deleted from further consideration. This reduces the isotherms to 350. Emphasis is placed on the saturation loadings. This is deduced from the horizontal plateauing of the isotherms as Psat is approached, or from the highest data points on the isotherms providing dlog(q)/dlog(p) is approaching zero. The author and his two advisers systematically examined all data assigning a full black circle to saturated isotherms which appeared totally plateaued, a full white circle to doubtful isotherms and removed all isotherms which are clearly not saturated. The results indicate that the model saturation loading equation for qmax approaches within 5-10 % of the experimental data for the subcritical isotherms. A tuning parameter is derived based on steric considerations to account for the lower observed experimental data. For the supercritical data, the experimental results appeared to be modeled best by saturation densities calculated equivalent to the subcritical value of 0.98Tr.