Membrane Gas Separation

5.10
Conclusion 
Tailoring the pore size distribution within materials has been suggested as a means of 
fi ne tuning the transport properties in membranes [61,62] . To facilitate the development 
CO
2
Permeability (arbitrary units)
S
electiv
ity
CO
2
/C
H
4
10
1
10
2
10
3
10
4
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Surface diffusion
Knudsen diffusion
Parallel transport
Resistance in Series transport
Figure 5.14 Model predictions of CO 
2
 /CH 
4
selectivity versus CO 
2
permeability for 
varying temperature T , within a pore of size d = 10 Å for all mechanisms expect for the 
resistance in series transport for which a pore size of d = 6.5 Å was chosen. Arrows 
indicate the direction of increasing temperature, from 70 to 500 K

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106
Membrane Gas Separation
of gas separation membranes, the modelling of the transport of individual gas molecules 
through pores has been undertaken by various groups. The interactions between the gas 
molecules and the pore wall at the pore opening are considered to be of great importance 
and from which certain information about the gas kinetics can be obtained. A combination 
of theories provides the theoretical determination of the size of pores in which different 
modes of diffusion may dominate for each gas. Critical pore sizes d
min
and d
K
indicate 
the division of the three diffusion regimes, namely, activation diffusion, surface diffusion 
and Knudsen diffusion. By using this model one can predict the separation outcome for 
a variety of membranes in which the pore shape, size and composition are known, and 
conversely one can predict pore characteristics with known permeation rates. Further, one 
can also have a desired separation in mind and use this model to guide the design of the 
micro - structure of the membrane material.