Reanalysis of Supercritical and Subcritical Ethane Adsorption Data On 5A Zeolite

Data was collected from the literature for ethane adsorption on 5A zeolite, subjected to a critical screening, and subsequently reviewed and analyzed.  Seven studies provided 21 isotherms, which were screened to determine the consistency between the different groups.  Isotherms from four studies were found to be consistent whereas three were observed to be inconsistent with other studies.  The consistent four studies provided a total of 15 isotherms, 5 at supercritical temperatures and 10 at subcritical temperatures.  The isotherms were linearized using the Gaussian adsorption model.  Values for the maximum loading are calculated using crystallographic zeolitic information and Rackett’s equation for the adsorbate density below a reduced temperature of 0.96; above a reduced temperature of 0.96, a constant value of 9 g/100 g is used.  The model fitted the data well, with regression coefficients ranging from 0.974 to over 0.999 for the linearized Gaussian model.  The linearized isotherms display a point of intersection for the subcritical isotherms.  All isotherms are readily plotted on a single normalized characteristic curve using the fitted model.  Parameters for the Gaussian model are σ, the standard deviation; P_r,50, the midpoint reduced pressure corresponding to a loading of 50% of the maximum; and q_max, the saturation loading.  A plot of sigma vs reduced temperature indicates decreasing values of sigma until it attains a constant value of approximately 0.75 at reduced temperatures of 0.96 and above.  To consider the pressure parameter, it is first necessary to compare pressures from each fitted model at a constant loading, taken as 50% of the maximum loading for supercritical adsorption, 4.5 g/100 g.  Since q_max differs for the subcritical isotherms, each fitted model is used to predict the reduced pressure for a loading of 4.5 g/100 g.  The log of these values were plotted vs 1/T to provide an isostere, which was linear (R=0.98).  The heat of adsorption at this loading was found from the slope of this line to be 7.0 kcal/mole, which compares favorably with the literature values.