adsorbed upon “breathing” transition, it gets displaced by CO 2 with the introduction
of further CO 2 . According to the same group, a mixed gas stream of 40% CH 4 /10%
CO 2 /50% He can be purified using various cation forms of merlionite, with
K-merlionite exhibiting the most rapid and efficient response to the introduction of
CO 2 . While such a “breathing” mechanism is considered present in the RHO family
of embedded isoreticular zeolites as well [61], the complicated structures of the
higher RHO family members, i.e., ZSM-25 and PST-20, make this impossible to
discern.
4 Small-Pore Zeolites for Small Hydrocarbon Separation
Olefin/paraffin separation is one of the most important, yet most energy consuming,
processes in the petrochemical industry [62], because olefin/paraffin mixtures with
same carbon number, especially ethylene/ethane and propylene/propane, are quite
similar in many of their physicochemical properties, e.g., boiling point, molecular
weight, polarizability, kinetic diameter, etc. [63, 64]. Among the olefins, ethylene
and propylene play a significant role because they are not only used for manufacturing of polymers, i.e., polyethylene and polypropylene, but are also major feedstocks
for the chemical industry. Olefins are produced by several processes such as steam
cracking, catalytic cracking, and catalytic dehydrogenation of paraffins. Of these, the
most widely utilized process for the production of olefins is the cracking of
C 4 -hydrocarbons fractions followed by dehydrogenation, where the corresponding
paraffins are also produced [65]. Thus, to fully utilize these light hydrocarbons, it is
essential to separate olefins (ethylene and propylene) from the corresponding paraffins (ethane and propane).
The energy-intensive cryogenic distillation or condensation processes are currently being used for olefin/paraffin separation for the production of polymer-grade
ethylene and propylene [66]. To afford some perspective of this energy consumption, therefore, olefin/paraffin separation processes usually need two huge splitter
columns to separate the raw vapor mixtures. For example, C 2 hydrocarbon separation is carried out on 160 tray columns at 243 K and 22 bar, whereas C 3 hydrocarbon
separation requires 220 tray columns at 243 K and 2 bar [67]. Considering the high
capital and operation cost associated with such refrigerated distillations, however,
there is enough motivation for researchers to explore cost- and energy-effective
alternative technologies. Among the energy-efficient technologies proposed for
olefin/paraffin separation so far, on the other hand, adsorptive separation using
porous materials has been considered the most promising alternative. The two
major categories of this separation technology are the kinetic and π-complexationdriven separations. In the former case, the difference in the diffusion rates between
olefin and paraffin with the same number of carbon atoms into zeolite pores separates
the mixture: the olefin normally has a faster diffusion rate because of its slightly
smaller kinetic diameter. In the latter case, the olefin is selectively adsorbed from the
Small Gas Adsorption and Separation in Small-Pore Zeolites
15
of further CO 2 . According to the same group, a mixed gas stream of 40% CH 4 /10%
CO 2 /50% He can be purified using various cation forms of merlionite, with
K-merlionite exhibiting the most rapid and efficient response to the introduction of
CO 2 . While such a “breathing” mechanism is considered present in the RHO family
of embedded isoreticular zeolites as well [61], the complicated structures of the
higher RHO family members, i.e., ZSM-25 and PST-20, make this impossible to
discern.
4 Small-Pore Zeolites for Small Hydrocarbon Separation
Olefin/paraffin separation is one of the most important, yet most energy consuming,
processes in the petrochemical industry [62], because olefin/paraffin mixtures with
same carbon number, especially ethylene/ethane and propylene/propane, are quite
similar in many of their physicochemical properties, e.g., boiling point, molecular
weight, polarizability, kinetic diameter, etc. [63, 64]. Among the olefins, ethylene
and propylene play a significant role because they are not only used for manufacturing of polymers, i.e., polyethylene and polypropylene, but are also major feedstocks
for the chemical industry. Olefins are produced by several processes such as steam
cracking, catalytic cracking, and catalytic dehydrogenation of paraffins. Of these, the
most widely utilized process for the production of olefins is the cracking of
C 4 -hydrocarbons fractions followed by dehydrogenation, where the corresponding
paraffins are also produced [65]. Thus, to fully utilize these light hydrocarbons, it is
essential to separate olefins (ethylene and propylene) from the corresponding paraffins (ethane and propane).
The energy-intensive cryogenic distillation or condensation processes are currently being used for olefin/paraffin separation for the production of polymer-grade
ethylene and propylene [66]. To afford some perspective of this energy consumption, therefore, olefin/paraffin separation processes usually need two huge splitter
columns to separate the raw vapor mixtures. For example, C 2 hydrocarbon separation is carried out on 160 tray columns at 243 K and 22 bar, whereas C 3 hydrocarbon
separation requires 220 tray columns at 243 K and 2 bar [67]. Considering the high
capital and operation cost associated with such refrigerated distillations, however,
there is enough motivation for researchers to explore cost- and energy-effective
alternative technologies. Among the energy-efficient technologies proposed for
olefin/paraffin separation so far, on the other hand, adsorptive separation using
porous materials has been considered the most promising alternative. The two
major categories of this separation technology are the kinetic and π-complexationdriven separations. In the former case, the difference in the diffusion rates between
olefin and paraffin with the same number of carbon atoms into zeolite pores separates
the mixture: the olefin normally has a faster diffusion rate because of its slightly
smaller kinetic diameter. In the latter case, the olefin is selectively adsorbed from the
Small Gas Adsorption and Separation in Small-Pore Zeolites
15