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

Abstract

Supercritical adsorption data available in the literature for C 1 to nC 12 n 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: q max , the saturation loading; the 50% adsorbate loading pressure P 50 ; 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 log P 50 . The temperature dependency of P 50 is deduced from a plot of log P 50 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 q max 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/ P 50) 1/ σ . The isosteres of P 50 indicate heats of adsorption (−Δ H 50) that are equal to or greater than literature values at zero loading (−Δ H 0); the difference between the two is negligible for low carbon numbers rising to over 10 kcal/mol at higher carbon numbers ( nC 10). This is expected as the heat of adsorption is calculated at 50% loading. The values of P 50 are predicted by correlating the slope and intercept of the isosteres with carbon number; the slope derives from the heat of adsorption (−Δ H 50). Using q max = 9 g/100 g Z, σ = 1, and predicted P 50 values, any supercritical isotherm may be predicted. The Henry constant K H may also be predicted using P 50, σ and θ values <0.05. The isotherms of nC 5 are predicted and plotted with the experimental data to indicate the accuracy of the fit. Also predicted are isotherms for nC 9 for which no experimental data exists.