A TVSA Adsorbent Screening Tool for Post-Combustion Carbon Capture
Department of Chemical Engineering, Imperial College London, UK
Metal-organic frameworks (MOFs) have received significant attention by the research community for CO2 capture. However, there has been little focus on the industrial application of these materials in cyclic processes. This is unsurprising given that MOFs generally have low synthesis yields and comparatively higher costs, which can hamper technology development cycles leading to delayed commercialisation.
In this work, we aim to identify the MOF properties and process conditions that yield better performance for post-combustion carbon capture, from both power and industrial emissions. The identification of these could better direct research efforts, leading to more efficient development process. To achieve this, we first developed a simplified pressure-vacuum swing adsorption (PVSA) model with integrated process economics . The PVSA model is based on a one bed, three-step equilibrium cycle which reflects the non-isothermal nature of adsorption while still allowing rapid solutions . Temperature swing adsorption (TSA) would traditionally have been disregarded for this application due to prohibitively long cycle times, rapid TSA technologies are gaining popularity and they may enable the deployment of TSA to these very large-scale applications.
For the PVSA case, we found that improving selectivity by reducing N2 adsorption, and improving CO2 working capacity by having moderate enthalpies of adsorption yield the greatest process improvements both technically and economically. For TSA, there are a much wider range of isotherms which yield good performance. This is due to the fact that adsorbents which perform well for PVSA are those that do not show significant thermal effects, whereas for TSA, thermal effects are desired to take advantage of the temperature driving force.
MOFs that displayed poor performance for PVSA such as Ni- and Mg-MOF-74, and HKUST-1, showed significantly improved separation performance in TSA due to their higher enthalpies of adsorption. When TVSA is employed for a flue gas containing ≈4 %mol CO2, the DOE targets of 95 %mol purity and 90 % recovery were able to be achieved by some adsorbents (Mg-MOF-74, Ni-MOF-74, UTSA-16, zeolite 5A, zeolite 13X). With one (UTSA-16) being able to match the energy consumption of conventional MEA-based amine absorption processes. This is facilitated by TVSA allowing lower vacuum levels to be used during regeneration, reducing electrical energy consumption.
The process economics for the TSA/TVSA case are currently being finalised, however, initial results show that higher working capacities can be obtained which leads to lower adsorbent inventories. Reduced vacuum requirements are also advantageous. However, whether these can offset the longer cycle times due to bed cooling is to be determined.