Isotherms for Single and Mixed Gases

The Langmuir-type isotherms remain to be the most widely used for practical applications. The Langmuir isotherm for pure component adsorption is 

The parameter a is the sticking probability or accommodation coefficient for adsorption (upon a collision on the surface), and P is the rate constant for desorption. The constant B is called Langmuir constant. At low pressures, the Langmuir isotherm reduces to a linear form, or Henry s law form  

All isotherms should reduce to the Henry s law form at extreme dilution. The Henry s constant is the most important factor for purification. From Eq. 3.1a, B is exponentially proportional to Q, the heat of adsorption. Here Q(Q = -AH) is positive by definition, because AH, the enthalpy change for physical adsorption, must be negative (Yang, 1987). For physical adsorption, Q is equal to the bond energy between the adsorbate molecule and the sorbent. Therefore, the bond energy is critical for purification. Strong bonds are needed for ultrapurification. 

Zeldowitsch (1934), assuming an exponentially decaying function of site density with respect to Q, obtained the classical empirical isotherm  

The Langmuir isotherm for pure-component adsorption can readily be extended to an n-component mixture, known as the extended Langmuir isotherm (Yang, 1987)  

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Potential Theory Isotherms for Single and Mixed Gases... 

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