A new simplified pressure/vacuum swing adsorption model for rapid adsorbent screening for CO2 capture applications

Abstract

A large number of promising adsorbent materials for CO2 capture are reported almost daily. Unfortunately, the assessment of an adsorbent in a process is far more challenging. Statements on expected performance are usually confined to visual inspection of isotherms or calculations of pure component selectivities. These are poor indicators of performance in an actual capture process. We present here a new simplified pressure/vacuum swing adsorption model which can be used to quickly screen adsorbents for use in CO2 capture applications. The model strikes a balance between full adsorption simulation (which requires detailed knowledge of PSA operation and is time consuming) and simple visual inspection of isotherms and calculations of selectivities (which is incorrect and misleading in many cases). Our model has been validated against analytical PSA models, full adsorption numerical simulations, and experiments. Using post-combustion VSA as an example, we use the model to compare several types of adsorbents (zeolite 13X, Mg-MOF-74, Activated Carbon, PEI/MCF chemisorbent). Our analysis shows that 13X remains the best adsorbent in VSA applications (for dry flue gas of 12% composition) even though Mg-MOF-74 shows considerably higher CO2 capacity. We have also conducted a sensitivity study to determine which properties are most important to improving performance and we estimate the limits of PSA performance. Adsorbent selectivity and thermal effects have a more significant effect on the specific power consumption than does CO2 adsorption capacity. The optimal heat of adsorption of CO2 for PSA application is between 35 and 45kJ/mol regardless of N2 heat of adsorption. Furthermore, continual increase in surface area is not necessarily beneficial to overall performance, becoming more detrimental as the heat of adsorption of N2 increases. As an estimate of an upper limit of material performance, a hypothetical material with the same surface area as MOF-177, no N2 adsorption, and a CO2 heat of adsorption of 35kJ yields a 68% increase in working capacity and an increase in purity from 78% to 94% when compared to 13X.