The main problem of the exponential models is its incorrect behavior in the low-pressure zone where all the exponential isotherms fail to fit the Henry constant [22]. To overcome this difficulty for the breakthrough modeling, a four-parameter equation, the Toth model was used for data interpretation [16]. Here, for high-pressure data, the pressure has to be replaced by the fugacity. This model has been extensively used for gas adsorption on microporous adsorbents [23] and specifically for propane–propylene adsorption on zeolites [24]. It is Langmuir reducible when the heterogeneity parameter, n, is unity (homogeneous adsorbent). The other three parameters have the same physical meaning as in the Langmuir isotherm: heat of adsorption (DH), maximum adsorbed phase concentration (qm) and infinite adsorption constant (K ). The Toth model is expressed by