Development of predictive criteria for supercritical n alkane adsorption data on 5A zeolite using the Gaussian adsorption isotherm model

Supercritical adsorption data available in the literature for C1 to nC12n alkanes on 5A zeolite are correlated using the Gaussian adsorption model and subsequently used to predict any supercritical isotherm for these systems. This isotherm model has three parameters: qmax  , the saturation loading; the 50% adsorbate loading pressure P50P50; and the standard deviation σ. The latter two parameters are deduced from a log plot of the isotherm pressure P vs. the inverse Gaussian cumulative frequency distribution plot Φ−1(θ). The slope of this plot is σ and the y  -intercept is logP50. The temperature dependency of P50P50 is deduced from a plot of logP50 vs. 1/T: this is actually the isosteric heat of adsorption plot at 50% loading with slope proportional to the negative heat of adsorption at 50% loading (4.5 g/100 g Z).The model is tested using 63 supercritical n alkane isotherms from the literature on 5A zeolite. The 63 isotherms from 18 different studies are fitted with the model using a qmax of 9 g/100 g Z applicable to n alkane/5A isotherms in the supercritical temperature range. The mean standard deviation σ is observed to be 0.960 with a standard deviation of 0.127. All 63 isotherms reduce to one characteristic curve when θ is plotted against (P/P50)1/σ.The isosteres of P50 indicate heats of adsorption (−ΔH50) that are equal to or greater than literature values at zero loading (−ΔH0); the difference between the two is negligible for low carbon numbers rising to over 10 kcal/mol at higher carbon numbers (nC10). This is expected as the heat of adsorption is calculated at 50% loading. The values of P50P50 are predicted by correlating the slope and intercept of the isosteres with carbon number; the slope derives from the heat of adsorption (−ΔH50). Using qmax = 9 g/100 g Z, σ = 1, and predicted P50P50 values, any supercritical isotherm may be predicted. The Henry constant KH may also be predicted using P50, σ and θ values <0.05. The isotherms of nC5 are predicted and plotted with the experimental data to indicate the accuracy of the fit. Also predicted are isotherms for nC9 for which no experimental data exists.

Data for 63 isotherms, for C1 through nC10 overlaid on a single normalized isotherm graph.Figure optionsDownload as PowerPoint slideHighlights
► Supercritical data of n alkanes on 5A zeolite are modeled by the Gaussian isotherm.
► Sixty three isotherms for 10 n alkanes are overlaid onto one characteristic curve.
► The Gaussian model predicts isotherms and isobars of n alkane on 5A zeolite.