(617e) Pvsa and Tvsa Adsorbent Screening Tools for Post-Combustion and Industrial Carbon Capture

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 and industrial carbon capture. 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 [1]. 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 [2]. While temperature swing adsorption (TSA) would traditionally have been disregarded for this style of 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. Therefore, an analogous study was carried out for TSA for which a simplified equilibrium temperature-vacuum swing adsorption (TVSA) model was developed. The separation performance output from these are then used to size and cost the process equipment required to process flue gas of a given flow rate. In addition to the traditional metrics of purity, recovery, working capacity, and specific energy, the cost of capture is a significant metric for technology selection and deployment and allows the identification of trade-offs not otherwise seen.

We surveyed the literature for MOFs with measured isotherms of CO₂ and N₂ at, at least, three temperatures. We estimated other required inputs such as density, porosity, and heat capacity by their respective methods. While the adsorption cycle used may not reflect the greatest attainable performance for each MOF, it does allow for rapid evaluation of the adsorbent-process ensemble.

We exploited this feature to determine the ideal MOF isotherms for a set of post-combustion and industrial flue gases. 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.

The capture costs are dominated by process capital costs which are primarily comprised of vacuum requirements and adsorbent inventory due to low working capacities.

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. Furthermore, by applying modest vacuum (30 – 50 kPa_a) the total energy consumed by the TVSA process was reduced compared to TSA while still maintaining product purity. The process economics for the TSA/TVSA case are currently in progress, however, initial results show that higher working capacities can be obtained which would lead to lower adsorbent inventories. Whether this increased working capacity can offset longer cycle times is to be determined.

The performance of published MOFs were evaluated for post-combustion and industrial carbon capture applications in PVSA, TSA, and TVSA processes. Ideal MOF isotherms were proposed for a selection of carbon capture scenarios, and areas to focus adsorbent development on were proposed considering both separation performance and process cost.

[1] – D. Danaci et al., Mol. Syst. Des. Eng., 2020, Advance Article

[2] – B.J. Maring, P.A. Webley, Int. J. Greenh. Gas. Con., 15, 16-31, 2013