(489f) Sorbate Densities On 13X Zeolite

The extensive equilibrium adsorption data of sorbates on 13X zeolite is examined in order to deduce an equation for prediction of the saturation loadings using as input the crystallographic parameters of the zeolite and the thermodynamic parameters of the sorbates. Deducing the saturation loading qmax for 13X is more qualitative than a similar procedure for 5A zeolite. In the case of 5A, the isotherms plateaued as Psat is approached and the corresponding qmax for 5A can be easily established. For the n alkane sorbates in 5A, the saturation loadings were satisfactorily calculated using the Rackett equation up to a critical adsorbate reduced temperature Tcar different from Tr = 1 for VLE data together with the 5A crystallographic data. Above Tcar for 5A, a constant value may be employed for the saturation loading.

For 13X, the extensive data does not appear to plateau as Psat is approached. The extensive data includes n-alkanes, iso alkanes, alkenes, aromatics, halogen compounds, and inorganics. However, there is a lot less n alkane data published on 13X than is available for the 5A zeolite. For some unknown reason, the saturation loading in 13X appears to keep on rising. It may even appear to come to a sudden stop as is observed with ammonia on 13X after 5 orders of magnitude in pressure. Although most of the data is measured in pellets of 13X as there is very limited crystal data, capillary condensation does not appear to be the cause of the sustained rise as the change from concavity to convexity is not sharp even at high mPa. This makes establishment of a suitable saturation loading model very difficult to establish definitively.

Calculations so far indicate the Rackett thermodynamic model may be satisfactory when combined with the crystallographic 13X data, but that the larger 13X cavity may behave similarly to VLE liquids up to the critical point unlike the smaller 5A cavity. An accessibility factor may have to be used with some sorbates such as iso alkanes. At present, we have not established the appropriate thermodynamic equation to use above Tr = 1 for 13X data but expect to have done so by the conference date.