Adsorption Equilibrium in Multicomponent Systems

In Chapter 8 the equilibrium factor for a single adsorbed species was defined by analogy with the relative volatility. This definition is easily extended to a binary or multicomponent system. For competitive sorption the binary separation factor is defined by 

Thus ipi = A , = mole fraction in adsorbed phase and if no inerts arc present = T = mole fraction in fluid phase. It is apparent that 

The equilibrium factor measures the affinity of the adsorbent for a particular component relative to the same component in the fluid phase whereas the binary separation factor measures the relative preference of the adsorbent for two different competing adsorbates. If the equilibrium obeys the multicomponent Langmuir model  

With the definition given above, the smaller the value of the more strongly held is component / relative to component j. Following Helfferich and Vermeulen it has become conventional to define the separation factor for a multicomponent system (a, ) as the reciprocal of Py. 

FIGURE 9.1. Orthogonal concentration coordinates for a three-component system and representation as a triangular composition diagram. 

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