(144h) Application of ZIF-8 and Zeolite Y in a Pressure-Temperature Swing Adsorption Process for Simultaneous H2 Purification and CO2 Capture

By enabling carbon-neutral hydrogen (H₂), the coupling of fossil fuel-based H₂ production with carbon capture and storage (CCS) is one of the greatest opportunities to fast-track the decarbonisation of Europe’s energy system¹. At the industrial scale, H₂ recovery and purification is carried out using a Pressure Swing Adsorption (PSA) process, whereby additional parallel adsorbers can be added to capture carbon dioxide (CO₂) from the tail gas². Recent studies show that co-purification of CO₂ and H₂ at high purities and recoveries can be achieved in a single process unit deploying a single vacuum PSA (VPSA) cycle^3,4. However, these novel schemes use only one adsorbent and are limited to cycles driven solely by pressure and vacuum. We contend that opportunities for process improvement exist, when one considers: (i) pressure-temperature swing adsorption cycles as well as (ii) novel adsorbents or adsorbent combinations that can handle the impurities present in the feed gas. In this endeavor, a major limitation is the scarcity of adsorption equilibrium data for CO₂, H₂ and the relevant impurities (e.g., CH₄, N₂) over a sufficiently wide range of temperature (25-120° C) and pressure (< 30 bar), and for a range of adsorbent materials. These data are not only essential input parameters for process simulators, but can also be used for rapid material screening purposes within equilibrium-based process modelling and optimization workflows⁵.

In this study, we systematically compare the ability of adsorbents to co-purify of H₂ and CO₂ within a single PTSA process. To this end, we have measured equilibrium adsorption isotherms of CO₂, H₂, CH₄ and N₂ on a commercial zeolitic imidazolate framework, ZIF-8, and Y-type zeolites (both H⁺ and cation exchanged forms) in a Rubotherm magnetic suspension balance in the pressure range 0 – 30 bar and at temperatures between 25 – 120° C. We show that these materials present favorable characteristics for a PTSA process, because they feature a ‘gentle’ uptake at low pressure, while providing sufficient working capacity at higher pressures.

We have also assessed the potential of these adsorbents, and of those for which literature data are available, for H₂ purification with CO₂ capture using an adsorption-equilibrium model combined with multi-objective optimization. To this end, we have considered a range of feed gas conditions corresponding to different fossil fuel based H₂ production processes. A simple process configuration is chosen that includes any combination of the following steps: adsorption, blowdown, heating, evacuation and pressurization. As such, process optimization has been carried out by considering both VPSA and VPTSA cycle schemes by using the evacuation and blowdown pressure as well as the desorption temperature as adjustable variables. We have evaluated material performance in terms of purity and recovery of CO₂ and H₂, working capacity and energy consumption. We show that for adsorbents, such as zeolite 13X and activated carbon, the addition of a temperature swing step can reduce the energy consumption up to 30%, while maintaining the required working capacity. The improvement is more evident for ZIF-8 because it possesses a higher adsorption capacity at the feed pressure, while featuring a moderately steep isotherm at low pressure, thereby reducing the vacuum work. Dynamic adsorption studies will be conducted on these promising adsorbents to provide experimental demonstration of their performance as well as the parameterization needed for detailed process modelling and scale up.

References

1. IEA. The Future of Hydrogen. Futur. Hydrog. (2019) doi:10.1787/1e0514c4-en.
2. Sircar, S. & Golden, T. C. Purification of hydrogen by pressure swing adsorption. Sep. Sci. Technol. 35, 667–687 (2000).
3. Streb, A., Hefti, M., Gazzani, M. & Mazzotti, M. Novel Adsorption Process for Co-Production of Hydrogen and CO2 from a Multicomponent Stream. Ind. Eng. Chem. Res. 58, 17489–17506 (2019).
4. Streb, A. & Mazzotti, M. Novel Adsorption Process for Co-Production of Hydrogen and CO₂ from a Multicomponent Stream-Part 2: Application to Steam Methane Reforming and Autothermal Reforming Gases. Ind. Eng. Chem. Res. 59, 10093–10109 (2020).
5. Danaci, D., Bui, M., Mac Dowell, N. & Petit, C. Exploring the limits of adsorption-based CO2 capture using MOFs with PVSA-from molecular design to process economics. Mol. Syst. Des. Eng. 5, 212–231 (2020).