In this work, direct air capture (DAC) via adsorption is studied through the design and analysis of two temperature–vacuum swing adsorption (TVSA) cycles. In the first part, a novel way of describing the adsorption of $${\hbox {CO}}_{2}$$ CO 2 in presence of water vapor is proposed for co-adsorption kinetic and thermodynamic data gathered from the literature. Secondly, two TVSA cycle designs are proposed: one with a desorption step via external heating, and one with a steam purge. A schematic method for the determination of the cycle step times is proposed and a parametric study on the operating conditions is performed via cycle simulations using a detailed, first principles model. Finally, the two cycles are compared in terms of $${\hbox {CO}}_{2}$$ CO 2 production and energy consumption. The parametric study on the desorption time shows that there is a desorption time yielding the highest $${\hbox {CO}}_{2}$$ CO 2 production at low energy consumptions. Low evacuation pressures are necessary to reach high $${\hbox {CO}}_{2}$$ CO 2 production, but higher evacuation pressures show to be always favorable in terms of specific electrical energy requirements. A steam purge requires an additional thermal energy cost, but it not only allows decreasing the specific electrical energy consumptions, it also enhances $${\hbox {CO}}_{2}$$ CO 2 desorption kinetics and allows reaching higher $${\hbox {CO}}_{2}$$ CO 2 productions at milder evacuation pressures. The results of this work present the possibility to directly relate the availability of power and heat to the design of the cycle.
The optimization of the air−solid contactor is critical to improve the efficiency of the direct air capture (DAC) process. To enable comparison of contactors and therefore a step toward optimization, two contactors are prepared in the form of pellets and wash-coated honeycomb monoliths. The desired amine functionalities are successfully incorporated onto these industrially relevant pellets by means of a procedure developed for powders, providing materials with a CO 2 uptake not influenced by the morphology and the structure of the materials according to the sorption measurements. Furthermore, the amine functionalities are incorporated onto alumina wash-coated monoliths that provide a similar CO 2 uptake compared to the pellets. Using breakthrough measurements, dry CO 2 uptakes of 0.44 and 0.4 mmol g sorbent −1 are measured for pellets and for a monolith, respectively. NMR and IR studies of CO 2 uptake show that the CO 2 adsorbs mainly in the form of ammonium carbamate. Both contactors are characterized by estimated Toth isotherm parameters and linear driving force (LDF) coefficients to enable an initial comparison and provide information for further studies of the two contactors. LDF coefficients of 1.5 × 10 −4 and of 1.2 × 10 −3 s −1 are estimated for the pellets and for a monolith, respectively. In comparison to the pellets, the monolith therefore exhibits particularly promising results in terms of adsorption kinetics due to its hierarchical pore structure. This is reflected in the productivity of the adsorption step of 6.48 mol m −3 h −1 for the pellets compared to 7.56 mol m −3 h −1 for the monolith at a pressure drop approximately 1 order of magnitude lower, making the monoliths prime candidates to enhance the efficiency of DAC processes.
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