Membrane Gas Separation

14.3
Membranes Applications in GS
The key factors to qualify the performance of a specifi c membrane material for GS appli-
cations are the permeability and the selectivity. It is generally recognized that there is a 
trade - off limitation for polymeric materials between these two parameters: as selectivity 
increases, permeability decreases and vice versa. Robeson showed such empirical upper -
bound relationships for different gas separations, in 1991 [8] . In 2008, Robeson [30]
published an upload of these trade - off diagrams. This diagram virtually summarized all 
the existing membrane materials, giving an indication of the best performance achievable 
(at present) by a polymeric membrane material for a specifi c gaseous mixture. 
The upper bound correlation follows the relationship that is valid in double logarithmic 
scale:
Permeability
k Selectivity
i j
n
=
(
)
,
where k is referred to as the ‘ front factor ’ and n is the slope of the log – log plot of the 
noted relationship [30] . Figure 14.3 shows a plot of selectivity versus permeability, the 
upper bound trade - off of the most important type of separations. With respect to the fi rst 
Robeson ’ s plots [8] , a signifi cant shift of the upper bound was obtained for several sepa-
rations, due to the creation of novel high performance membrane materials; in many cases 
such materials are perfl uorinated polymers. Furthermore, some new trade - off limits, like 
those relative to the CO 
2
/N 
2
and N 
2
/CH 
4
separations, were added. A more detailed analysis 

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Membrane Engineering: Progress and Potentialities in Gas Separations
287
on the state of the art of the membrane GS fi eld in which many experimental results rela-
tive to different membrane materials are reported can be found in [10,31] .