(671b) Trade-Off between CO2/CH4 Membrane Permeability and Selectivity: Mesoscale Insights from Molecular Simulation

Trade-off
between CO₂/CH₄ membrane permeability and selectivity: Mesoscale insights from molecular simulation

Xiaohua Lu

College
of Chemistry and Chemical Engineering, State Key Laboratory of
Materials-Oriented Chemical Engineering, Nanjing Tech University,
Nanjing, 210009, PR China

The
separation of CO₂/CH₄ is important in both biogas
upgrading and natural gas sweetening.
In
the past two decades, gas membrane separation technology has become much more
advanced due to interest in the technology¡äs reliability, low-energy
consumption, and minimal capital investment. However, most membranes used in
industry today are still limited due to an inherent upper bound limitation on
their performance, showing a conventional trade-off behavior between
permeability and selectivity ^[1].

It
is believed that membrane process is a compromise in competition between two
dominant mechanisms: the permeability mechanism and the selectivity mechanism^[2].
In essence, the system behavior is dominated by the complex structure of the
membranes and the complex interaction between the gas and the membranes.
However, no quantitative results of the permeability and selectivity with the
structure structural properties and the interaction were obtained, makes it
different to revealed the inherent mechanism to break the Robeson upper bound.

In
this work, a novel method based on linear nonequilibrium
thermodynamics^[3] and equilibrium molecular simulation was developed
to quantitatively predict selectivity
and permeability for CO₂/CH₄ mixtures based on the
structural properties of materials such as effective contact surface area. By
adjusting the potential parameters (¦Å, ¦Ò) artificially,
different effective contact surface area was obtained and the optimal region to
break the Robeson upper bound was obtained.

A new
channel was formed by the combination of the CO₂ molecules with the
carbon membranes, and the original CH₄ channel was changed. The CH₄
confinement space becomes smaller, which improves the selectivity and breaks
through the Robeson limit. The formation of the new "limited domain"
structure is mainly affected by the solubility of CO₂ and CH₄.

The
research methods developed in this paper can be further extended to other
systems, such as mass transfer in porous materials of N₂, H₂S,
H₂O. It can also be expanded to other systems such as catalytic
reaction system.

Keywords: molecular
simulation, linearized non-equilibrium thermodynamics model, membrane
separation

References:

[1] H. B. Park, J. Kamcev,
L. M. Robeson, M. Elimelech, B. D. Freeman,
¡°Maximizing the right stuff: The trade-off between membrane permeability and
selectivity¡±, Science, 2017,  356, 1137.

[2] J. H. Li, ¡°Exploring the Logic and Landscape of
the Knowledge System: Multilevel Structures, Each Multiscaled with Complexity
at the Mesoscale¡±, Engineering, 2016,
2, 276-285.

[3] W. L. Xie, X. Y. Ji, X. Feng, X. H. Lu,
¡°Mass-Transfer Rate Enhancement for CO₂ Separation by Ionic Liquids:
Theoretical Study on the Mechanism¡±, AIChE Journal, 2015, 61, 4437-4444.