Keywords Carbon capture · Diffusion in zeolites · Theory and experiment ·
Zero length column technique
1 Introduction
Separation processes based on adsorption are typically based on equilibrium selectivity. As an example consider carbon capture applications where the main components are carbon dioxide, CO 2 and nitrogen, N 2 [1]. The strong quadrupole moment
of CO 2 can be targeted, for example, using accessible cations in zeolites,
i.e. adsorbing CO 2 and obtaining almost pure N 2 . The challenge in this case is the
fact that CO 2 is the heavy component, and to be able to recover it at high purity, one
requires a high selectivity or even better that N 2 is completely excluded [2]. A
possible approach in this direction is to tailor small pore zeolites to achieve molecular sieving, i.e. combine the equilibrium selectivity; CO 2 is already more strongly
adsorbed than N 2 , with a kinetic selectivity.
Measuring gas transport kinetics is very important in adsorption separations that
are kinetically controlled, because the design of these separation processes relies
heavily on the reliability of the diffusion coefficients, which have to be determined
experimentally. Over the years, several techniques have been developed for this
purpose, and existing methodologies have been adapted to meet the requirements of
more advanced material and new separation challenges [3]. All the techniques can be
grouped into two main categories: microscopic measurements are techniques that
look at diffusion at the molecular level by following the movement of several
molecules in order to obtain the root mean squared displacement; macroscopic
measurements are techniques that change the external concentration at the surface
of the material and measure the molar fluxes at the outer boundary of the solid.
Belonging to the first class are, for example, pulsed field gradient (PFG) NMR and
neutron scattering. Among the macroscopic methods, the most commonly used are
gravimetric, volumetric/piezometric and chromatographic methods and the
corresponding frequency response techniques. Only for very few systems, it is
possible to measure diffusion coefficients using both methods. This is due to the
fact that for microscopic techniques, the measured signal increases with mobility;
therefore the interpretation of the results is more straightforward and direct with fast
moving molecules. The opposite is true for macroscopic techniques, where the
limitations come from the rate at which the external concentration can be varied.
Therefore in this case, it is better to slow down the system to have an accurate
measurement. In practice the use of large crystals is the key requirement to have
systems where both approaches can be used, but when the pore size becomes very
close to the limit of size exclusion, macroscopic techniques have a clear advantage.
The main challenge when assessing the transport properties of nanoporous
adsorbents using macroscopic techniques is to decouple the secondary effects that
122
E. Mangano and S. Brandani
Zero length column technique
1 Introduction
Separation processes based on adsorption are typically based on equilibrium selectivity. As an example consider carbon capture applications where the main components are carbon dioxide, CO 2 and nitrogen, N 2 [1]. The strong quadrupole moment
of CO 2 can be targeted, for example, using accessible cations in zeolites,
i.e. adsorbing CO 2 and obtaining almost pure N 2 . The challenge in this case is the
fact that CO 2 is the heavy component, and to be able to recover it at high purity, one
requires a high selectivity or even better that N 2 is completely excluded [2]. A
possible approach in this direction is to tailor small pore zeolites to achieve molecular sieving, i.e. combine the equilibrium selectivity; CO 2 is already more strongly
adsorbed than N 2 , with a kinetic selectivity.
Measuring gas transport kinetics is very important in adsorption separations that
are kinetically controlled, because the design of these separation processes relies
heavily on the reliability of the diffusion coefficients, which have to be determined
experimentally. Over the years, several techniques have been developed for this
purpose, and existing methodologies have been adapted to meet the requirements of
more advanced material and new separation challenges [3]. All the techniques can be
grouped into two main categories: microscopic measurements are techniques that
look at diffusion at the molecular level by following the movement of several
molecules in order to obtain the root mean squared displacement; macroscopic
measurements are techniques that change the external concentration at the surface
of the material and measure the molar fluxes at the outer boundary of the solid.
Belonging to the first class are, for example, pulsed field gradient (PFG) NMR and
neutron scattering. Among the macroscopic methods, the most commonly used are
gravimetric, volumetric/piezometric and chromatographic methods and the
corresponding frequency response techniques. Only for very few systems, it is
possible to measure diffusion coefficients using both methods. This is due to the
fact that for microscopic techniques, the measured signal increases with mobility;
therefore the interpretation of the results is more straightforward and direct with fast
moving molecules. The opposite is true for macroscopic techniques, where the
limitations come from the rate at which the external concentration can be varied.
Therefore in this case, it is better to slow down the system to have an accurate
measurement. In practice the use of large crystals is the key requirement to have
systems where both approaches can be used, but when the pore size becomes very
close to the limit of size exclusion, macroscopic techniques have a clear advantage.
The main challenge when assessing the transport properties of nanoporous
adsorbents using macroscopic techniques is to decouple the secondary effects that
122
E. Mangano and S. Brandani