(223g) Evaluation of Structured Adsorbents for Carbon Capture Applications

Carbon capture and storage has been advocated as a near term technology for capturing CO₂ from large stationary sources like power plants¹. Different process like adsorption, absorption and membrane separation are being evaluated as potential candidates for capturing and concentration CO₂ from these sources. Regarding adsorption, the research focus can be classified into two broad categories: 1: Synthesizing novel materials such as metal organic frameworks (MOFs), amine supported adsorbents, zeolite imidazolate frameworks (ZIFs) with high CO₂ capacity and selectivity, and 2: Cycle design and process optimization of adsorption processes. A traditional adsorption process employs packed columns containing adsorbent pellets or extrudates which are few millimeters in size. The main challenges associated with packed beds are large pressure drops and mass and heat transfer limitations at high flowrates. This would result in significant increase in energy consumption and a decrease in productivity of the capture process. In recent years, structured adsorbents have gained significant interest owing to the potential advantages of lower pressure drop and faster mass transfer over conventionally shaped adsorbents, allowing significant improvement in energy consumption and productivity.

Most of the studies on these structured adsorbents are confined to adsorption equilibrium, kinetic studies and pressure drop tests and to the best of our knowledge, we have not seen in the open literature, how much of an improvement in productivity and energy consumption can be achieved over conventional pelletized sorbents. This information can be obtained from rigorous process optimization studies. The goal of the present work is therefore to evaluate the performance of structured sorbents against conventionally shaped pellets. The structured adsorbents used in this study were obtained by 3D printing. The advantage of this method is that, it is possible to obtain different shapes of adsorbents, with controlled channel sizes and geometries. We have considered two cases with different CO₂ concentrations 1. Post-combustion CO₂ capture from a coal fired power plant containing 15% CO₂, 5% H₂O and 80% N₂ and 2. The PSA tail gas of the hydrogen purification unit which contains 51% CO₂, 10-15% CO,10-15% CH₄ and 20-25% H₂ and a small amount of H₂O.

In this study, we have used a six-step vacuum swing adsorption (VSA) cycle which has been studied in literature², to evaluate the performance of the aforementioned two types of adsorbents. We have chosen supported amine sorbents in this study, owing to their tolerance to moisture in comparison with zeolites and MOFs. The amines were first grafted on to the silica beads and 3D-printed silica structure. In the next step, single component isotherms of the different gases were obtained using a commercial volumetric apparatus and dynamic column breakthrough experiments were carried out to measure the kinetic constants. Detailed optimization of the VSA cycle was then carried out to arrive at operating conditions satisfying 95% CO₂ purity and 90% recovery constraints with maximum productivity and minimum energy consumption.

References:

1. IPCC, 2014: Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change; IPCC, Geneva, Switzerland.
2. Khurana, M.; Farooq, S., Simulation and optimization of a 6-step dual-reflux VSA cycle for post-combustion CO2 capture. Chemical Engineering Science 2016, 152, 507-515.