(680g) The Importance of Nitrogen Co-Adsorption on Effectiveness of Post-Combustion CO2 Capture Materials: A Process Optimization Study

Adsorption-based carbon capture is considered to be a potential alternative to current technologies such as absorption. Research in this area has concentrated on the development of new materials and novel processes¹. On the materials front, many forms of metal-organic frameworks, zeolites and activated carbons have been synthesized. On the process front, many novel processes have been developed and pilot scale demonstrations have proven the capability of adsorption to achieve targets set by US Department of Energy².

Most adsorbents prescribed for CO₂ capture show significantly higher affinity for CO₂ compared to N₂ (the major component in post-combustion flue gas). Hence, during adsorbent development it has been customary to ignore the affinity of N₂ while focussing on enhancing CO₂ capacity. However, from the process perspective, the high concentration of N₂ in the flue gas makes it challenging to design processes to achieve high purity CO₂, without sacrificing CO₂ recovery. Hence, understanding the impact of CO2 and N2 affinities on any sorbent is hence key to developing materials that can perform well at a process level.

In this paper we study the trade-off between improving CO₂ adsorption and reducing N₂ adsorption by considering several hypothetical materials with varying CO₂ and N₂ affinity and understanding how the performance of a vacuum swing adsorption process is impacted by these parameters. These hypothetical materials are subject to detailed process modeling and multi-objective optimization which take into account process configuration, constraints, temperature and mass transfer effects. The multi-objective optimization framework³, which has been validated using our pilot-plant results², provides Pareto plots of key performance indicators such as CO₂ purity vs. recovery, and energy vs. productivity. These Pareto plots provide an objective comparison of the performance of various adsorbents.

The study provides many interesting results. Firstly, it appears that minimum selectivity required for achieving CO₂ purity and recovery in excess of 95% is much lower than what is exhibited by conventional materials such as Zeolite 13X. Secondly, increasing the CO₂ affinity of an adsorbent shows minimal impact on the process performance. However, reducing the N₂ adsorption has a significantly higher impact on process performance. In summary, it appears that the efficacy of an adsorbent is limited by its ability to release N₂ rather than by its ability to capture CO₂. These results are illustrated using a graphical approach that provides contours of CO₂ purity, recovery and specific energy consumption as a function of CO₂ and N₂ affinity. The graphical approach provides a direction for the design of new adsorbents that can satisfy process requirements and result in lower parasitic energy.

REFERENCES:

1. M.E. Boot-Handford, et al, Energy & Env. Sci. 2014, 7, 130-189.
2. S. Krishnamurthy et al., AIChE J. 2014, 60, 1830-1842.
3. R. Haghpanah et al., AIChE J.  2013, 59, 4735-4748.