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

3.A.K. RAJAGOPALAN; A.M. AVILA; A. RAJENDRAN

San Francisco

Congreso; 2016 AIChE Annual Meeting; 2016

AIChE

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 focusing 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.