4.4 CO 2 Separations
In the previous section we mentioned that zeolite adsorbents were not used for
adsorption of CO 2 in the context of H 2 purification because the CO 2 selectivity was
not critical for the purity of H 2 . However, the context changes when we want to
combine H 2 production with CO 2 capture from the synthesis gas. For the production
of two pure products from the synthesis gas mixture, a two-stage PSA process is
needed. Such two-stage PSA processes, which could desorb pure CO 2 for utilization
or storage, as well as produce pure H 2 , were already proposed in the 1980s [18–
20]. The first stage of such a PSA process must selectively separate CO 2 from the
other components of the synthesis gas mixture. For this purpose, zeolite adsorbents
are more appropriate than the fairly unselective activated carbon materials [21].
The problem of selective CO 2 adsorption at medium pressure also arises in the
purification of natural gas or biogas. The objective in this application is to produce
pure CH 4 , with CO 2 contents below the legislative norms. Producing high purity
CH 4 is not difficult as such, but the challenge is to minimize the loss of CH 4 in the
waste CO 2 stream, which is caused by co-adsorption of CH 4 on the adsorbent
material. As mentioned in the introduction, the descriptor that best describes the
minimization of CH 4 losses is the separation factor, i.e., the ratio of the working
capacities of CO 2 and CH 4 . On the other hand, the working capacity of CO 2 alone
will be strongly related to the bed size factor.
We have demonstrated how the working capacity and the separation factor in
CO 2 /CH 4 separations can be nicely tuned via the content of extraframework cations
in FAU or LTA zeolites (which in turn is related to the framework Si/Al ratio)
[22]. CO 2 being a quadrupolar molecule, which strongly interacts with the
extraframework cations, its adsorption constant is exponentially related to the Na
+
content of the zeolite framework, while that of CH 4 is quite insensitive to
extraframework cation content (Fig. 6).
Theory allows us to calculate the value of the CO 2 adsorption constant that will
optimize working capacity and separation factor
1 (Fig. 7), and we can, thus, deduce
the optimal extraframework cation content. The predictions from theory were fully
validated by experimental measurements of working capacity and selectivity. A
method for experimentally screening working capacity and selectivity (the separation factor is the product of both) is described in [23].
We note, however, that PSA processes for selective CO 2 separations from natural
gas, biogas, or synthesis gas mixtures are in competition with other separation
processes, i.e., CO 2 absorption by solvents or membrane separations. Whether
PSA or solvent absorption processes are preferred depends on scale and on the
availability of thermal energy. Since most solvent processes rely on high temperature
steam for regeneration, the availability of such a heat source on site may give
absorption processes a cutting edge.
1 We will treat the reasons for the different optima of working capacity and separation factor in more
detail in a separate section of this chapter.
Industrial Zeolite Applications for Gas Adsorption and Separation Processes
207
In the previous section we mentioned that zeolite adsorbents were not used for
adsorption of CO 2 in the context of H 2 purification because the CO 2 selectivity was
not critical for the purity of H 2 . However, the context changes when we want to
combine H 2 production with CO 2 capture from the synthesis gas. For the production
of two pure products from the synthesis gas mixture, a two-stage PSA process is
needed. Such two-stage PSA processes, which could desorb pure CO 2 for utilization
or storage, as well as produce pure H 2 , were already proposed in the 1980s [18–
20]. The first stage of such a PSA process must selectively separate CO 2 from the
other components of the synthesis gas mixture. For this purpose, zeolite adsorbents
are more appropriate than the fairly unselective activated carbon materials [21].
The problem of selective CO 2 adsorption at medium pressure also arises in the
purification of natural gas or biogas. The objective in this application is to produce
pure CH 4 , with CO 2 contents below the legislative norms. Producing high purity
CH 4 is not difficult as such, but the challenge is to minimize the loss of CH 4 in the
waste CO 2 stream, which is caused by co-adsorption of CH 4 on the adsorbent
material. As mentioned in the introduction, the descriptor that best describes the
minimization of CH 4 losses is the separation factor, i.e., the ratio of the working
capacities of CO 2 and CH 4 . On the other hand, the working capacity of CO 2 alone
will be strongly related to the bed size factor.
We have demonstrated how the working capacity and the separation factor in
CO 2 /CH 4 separations can be nicely tuned via the content of extraframework cations
in FAU or LTA zeolites (which in turn is related to the framework Si/Al ratio)
[22]. CO 2 being a quadrupolar molecule, which strongly interacts with the
extraframework cations, its adsorption constant is exponentially related to the Na
+
content of the zeolite framework, while that of CH 4 is quite insensitive to
extraframework cation content (Fig. 6).
Theory allows us to calculate the value of the CO 2 adsorption constant that will
optimize working capacity and separation factor
1 (Fig. 7), and we can, thus, deduce
the optimal extraframework cation content. The predictions from theory were fully
validated by experimental measurements of working capacity and selectivity. A
method for experimentally screening working capacity and selectivity (the separation factor is the product of both) is described in [23].
We note, however, that PSA processes for selective CO 2 separations from natural
gas, biogas, or synthesis gas mixtures are in competition with other separation
processes, i.e., CO 2 absorption by solvents or membrane separations. Whether
PSA or solvent absorption processes are preferred depends on scale and on the
availability of thermal energy. Since most solvent processes rely on high temperature
steam for regeneration, the availability of such a heat source on site may give
absorption processes a cutting edge.
1 We will treat the reasons for the different optima of working capacity and separation factor in more
detail in a separate section of this chapter.
Industrial Zeolite Applications for Gas Adsorption and Separation Processes
207