Rapid Thermal Swing Adsorption Gas Separation for Carbon Capture

Abstract

The large column inventory enforced by the long regeneration times prevented the use of thermal swing adsorption for gas separation. But with shorter regeneration times the technology would have a good potential due to the low operation cost enabled by the use of low grade waste heat. The key to shorter regeneration times is a rapid thermal swing adsorption (RTSA) process. This process has been developed in adsorption heat pumps that offer a clean technology for cooling or heating utilizing waste or renewable heat as driving energy. RTSA constitutes a similar step improvement as the rapid pressure swing process that reduced the investment cost for many gas purification and separation processes including oxygen separators used in treating COVID patients. A crucial component to reduce cycling times is the use of hierarchically structured sorbents. As part of the development of better adsorption heat pumps we have devised an inexpensive and scalable magnetic alignment route for the fabrication of adsorptive coatings with vertically open channels and thermal bridges improves mass transport more than threefold [1]. This combined with methods to improve thermal transport accelerate the thermal swing process strongly reducing column inventory and cost. A novel controlled fast temperature jump setup (TJS) accurately measures temperatures and sorption processes based on chamber pressure as function of time following temperature steps. The sorption material is mounted as beads or layers onto aluminum carries that are temperature controlled with high performance microchannel heat exchangers. The TJS allows fast acquisition of cycled mass, thermal resistance and mass transport resistance to screen and optimize material and process libraries. Measurements were carried out as function of wide choice of sorbent materials, gas type (CO2, N2, H2O) including mixtures, gas pressure, cycling time, temperature gradient, layer thickness, and layer structuring. A selection parameter was defined that contains exchanged mass per cycle, adsorption and desorption speed as well as layer thickness: Parameters that maximize the performance of an RTSA gas separation process. The maximum achievable specific exchanged mass per cycle could be determined by gravimetric method through Dynamic Vapor Sorption experiments. All adsorbents were tested in both mono and multicomponent gases showing that maximum CO2 adsorption capacity alone does not sufficiently represent the potential of a material, as residuals of adsorbed water in the pore can significantly decrease the cycling capacity. The same methods, as well as full adsorption isotherms, were applied to the adsorber coating materials (including binders, surfactants and thermally conductive materials) to provide a benchmark for the evaluation of the enhanced coatings tested with different cycle times. According to the results obtained with the TJS experiments, the initial coating formulation and process was improved aiming to increase the active mass use and the CO2 cycled per unit area, and to decrease the adsorption and regeneration times. As a result, a protocol ensuring good adsorbent adhesion, good thermal contact and fast kinetics was obtained. It was demonstrated that a proper integration strategy of pore formation by magnetically aligned emulsions is as significant as the choice of the adsorbent material itself, leading improvement factors between 2 and 10. The material screening and integration tailoring methods proposed proved to quickly highlight the most promising direction on which to build on scale-up and further research activities. We thus can quickly select best materials and structuring processes to establish RTSA as a viable alternative to other carbon capture processes. [1] J. Ammann, P. Ruch, B. Michel, and A. R. Studart, High-Power Adsorption Heat Pumps Using Magnetically Aligned Zeolite Structures, ACS Appl. Mater. Interfaces 2019, 11, 27, 24037–24046.