Modeling Adsorption Data of Hydrocarbons on 13X Zeolite using the Gaussian Adsorption Isotherm Model

Abstract

Subcritical and supercritical adsorption data available in the literature for alkanes, alkenes and aromatics on 13X zeolite are correlated using the Gaussian adsorption model. This model is specified by three parameters: qmax, the saturation loading; P50, adsorption pressure at 50% of the maximum loading; and σ, the standard deviation. A semi log plot of pressure vs. the inverse of the Gaussian cumulative distribution function produces a linear relation with a slope equal to the standard deviation of the isotherm and an intercept equal to the logarithm of the pressure at 50% of the maximum loading. The model is applied to 138 alkane isotherms, 17alkene isotherms and 44 aromatic isotherms from the literature on 13X zeolite. The isotherms are collected from 17 different studies and fitted to the model using a pre-specified maximum saturation loading value for the supercritical isotherms and a calculated value using crystallographic zeolite data and the Rackett or modified Rackett equation for the subcritical isotherms. The mean standard deviation is found to be equal to 1 for reduced temperatures greater than 0.65 and greater than three for reduced temperatures less than 0.65. Plots of the pressure at 50% of the maximum loading vs. the inverse of the temperature are constructed. The slopes of the generated lines are used to calculate the values of the heat of adsorption at 50% loading for each species. The values are either equal or less than the values published in literature at 0% loadings for isotherms with a sigma value of (0.5 to 1) and are remarkably higher for isotherms with a sigma value above 3. The Gaussian model is found to be a better fit for supercritical data than for subcritical data especially for reduced temperatures below 0.75 where some inconsistencies are encountered. Keywords: Isotherm, Gaussian model, alkanes, alkenes, aromatics, 13X zeolite.