(395r) Adsorption of Alkenes On 13X Zeolite: Heats of Adsorption and Henry Constants From the Gaussian Isotherm Model

Adsorption of Alkenes on 13X Zeolite:

Heats of Adsorption and Henry Constants from the Gaussian Isotherm Model

Adsorption data available in the literature for three alkenes (ethylene, propylene and 1-butene) on 13X zeolite is modeled using the Gaussian Adsorption isotherm. This isotherm model has three parameters: qmax, the saturation loading; the 50% adsorbate loading pressure ; and the standard deviation σ.

Methods: Values for qmax are calculated as specified in Al-Mousa [1]. For subcritical sorbates the sorbate density was calculated using Rackett’s equation in combination with crystallographic data; for supercritical sorbates, a constant value was used. The remaining two parameters are deduced from a log plot of the isotherm pressure P versus the inverse Gaussian cumulative frequency distribution of ϴ. The slope of this plot is σ and the y-intercept is log P₅₀. The heat of adsorption at 50 % loading is deduced from the slope of  log P₅₀versus 1/T.  Henry constants for supercritical isotherms are calculated using the Gaussian parameters in a method described in a forthcoming publication. 

Z_H is a point within the range where the Gaussian model is consistent with Henrys Law. In this manner, the Henry constant can be deduced from isotherm data regardless of whether the experimental data extends into the Henrys Law region.

Results:  The results were remarkably similar for all three alkenes.  The standard deviation σ is observed to be strongly dependent upon temperature for subcritical adsorption, and only weakly dependent upon temperature for supercritical adsorption. The relationship between σ and reduced temperature is similar between the three alkenes. 

The heats of adsorption (-ΔH₅₀) are 9.02, 10.87, and 9.4 kcal/mole for ethylene, propylene, and 1-butene, respectively.  These values compare well to literature values at zero loading (-ΔH₀.).

The Henry constants for the supercritical isotherms range from 0.24 to 2.9 g/100 g-kPa for ethylene and from 0.3 to 3.6 g/100 g-kPa for propylene.  There were no supercritical isotherms for 1-butene.  The calculated Henry constants will be compared to values deduced from a virial plot of each isotherm.

[1] Alaa Al Mousa, K.F. Loughlin and D. Abouelnasr. Review and Analysis of Saturation Loadings for Subcritical Alkane Adsorption On 13X Zeolite, 2011 Annual Meeting of the AIChE, Minneapolis, 2011.