(205d) Assessment of the O2/N2 Separation Ability of Carbon Molecular Sieves By Spontaneous Liquid-Gas Imbibition

Objective:

The
kinetics of spontaneous water-oxygen or nitrogen imbibition at 303.2K on carbon
molecular sieves (CMSs) were carried out to establish
an assessment method for the O₂/N₂ separation performance
of the CMSs in pressure swing adsorption (PSA).

Method:

The
spontaneous water-gas imbibition was conducted this way: in the sample holder
with a constant volume, CMS particles saturated with gas were immersed into
water at a given temperature, and the pressure of the gas with time on
stream, which is a measure of the volume of the gas recovered by water, is recorded.
The results of the spontaneous water-oxygen imbibition on eight CMSs are shown
in Fig.1.

Fig. 1. Kinetics of the spontaneous
water-oxygen imbibition at 303.2K on CMS-1°«CMS-8.

Results and discussion:

The
spontaneous water-gas imbibition on CMSs involves the surface adsorption of
water, the diffusion of the gas along the micropores,
and the diffusion of the gas through the barrier at the micropore
entrance of the CMS. To elucidate the rate-controlling step of the process, the
pseudo-second-order (PSO) kinetic model and the linear driving force (LDF)
model has been used to describe the spontaneous water-gas imbibition.

For
CMS-1°«CMS-6,
the kinetics of the water-oxygen imbibition obeys the PSO model, indicating the
surface adsorption is the rate-controlling step. On the contrary, for CMS-8 the
diffusion of oxygen through the barrier at the micropore
mouth is the rate-controlling step with the kinetics following the LDF model. For
CMS-7, up to 200s, the dynamics obeys the LDF model, and over 200s it follows
the PSO model. Hence, the size distribution of the micropore
mouths for CMS-7 is far less uniform than that of other CMSs. The Kinetic
parameters of the two models demonstrate that the
average sizes of the micropore mouths are in the
order of CMS-1>CMS-2>CMS-3>CMS-7>CMS-4>CMS-5> CMS-6>CMS-8.

The
kinetics of the water-nitrogen imbibition on the CMSs indicate
that CMS-1 and CMS-2 fit with the PSD model and the physical adsorption process
is the rate-controlling step. For CMS-3°«CMS-5,
the rate-controlling step is mainly the diffusion of nitrogen through the micropore mouth. For CMS-6 and CMS-8, their micropore sizes are small enough to make the kinetics of
the process controlled by not only the barrier resistance at the micropore mouth but also the diffusion in the micropore interior. And for CMS-7, the kinetics is
controlled by all three steps mentioned above. In addition, the same order of
the average sizes of the micropore mouths of the CMSs
are  obtained
from the kinetics of the water-nitrogen imbibition as that of the water-oxygen
imbibition.

An
assessment method for the O₂/N₂ separation ability of
CMSs has been established, which has been verified by the PSA of air:

The
micropore mouths of CMSs such as CMS-1 are too large
to separate O₂/N₂ sufficiently
on which the rate-controlling steps of the water-O₂ or N₂
imbibition are both the surface adsorption.

The
micropore mouths of CMSs are too small to adsorb O₂
within a short time in SPA cycles leading to lower selectivity of O₂/N₂.
Since the diffusion in the micropore interior is one
of the rate-controlling steps in the water-nitrogen imbibition on them.

CMSs
whose micropore mouths dimensions are not uniform as
CMS-7 are unsuitable for the kinetic separation of O₂/N₂.

CMSs
owning
proper and uniform micropore mouths
dimensions such as CMS-3°«CMS-5
are favorite adsorbents for the O₂/N₂ separation by PSA.