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A new model has been developed for predicting mixed-gas adsorption equilibria from multicomponent gas mixtures based on the dual-process Langmuir (DPL) formulation. It predicts ideal, nonideal, and azeotropic adsorbed solution behavior from a knowledge of only single-component adsorption isotherms and the assertion that each binary pair in the gas mixture correlates in either a perfect positive (PP) or perfect negative (PN) fashion on each of the two Langmuir sites. The strictly PP and strictly PN formulations thus provide a simple means for determining distinct and absolute bounds of the behavior of each binary pair, and the PP or PN behavior can be confirmed by comparing predictions to binary experimental adsorption equilibria or from intuitive knowledge of binary pairwise adsorbate-adsorbent interactions. The extension to ternary and higher-order systems is straightforward on the basis of the pairwise additivity of the binary adsorbent-adsorbate interactions and two rules that logically restrict the combinations of PP and PN behaviors between binary pairs in a multicomponent system. Many ideal and nonideal binary systems and two ternary systems were tested against the DPL model. Each binary adsorbate-adsorbent pair exhibited either PP or PN behavior but nothing in between. This binary information was used successfully to predict ternary adsorption equilibria based on binary pairwise additivity. Overall, predictions from the DPL model were comparable to or significantly better than those from other models in the literature, revealing that its correlative and predictive powers are universally applicable. Because it is loading-explicit, simple to use, and also accurate, the DPL model may be one of the best equilibrium models to use in gas-phase adsorption process simulation.

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In situ FTIR spectroscopy was used to study carbonate transformations during adsorption and desorption of CO 2 in K-promoted HTlc at 450 °C just after activation. It revealed one irreversible process associated with the slow formation of polydentate carbonate during both adsorption and desorption, which explains CO 2 capacity fade with cycling. It also revealed several reversible processes associated with the disappearance of free carbonate ion and the formation of unidentate, bidentate, and bridged (surface) carbonates during the adsorption of gaseous CO 2 on active sites (highly basic, metal-bound unsaturated oxygen atoms) and vice versa during desorption. As the active sites became depleted during adsorption, free carbonate also transformed into bidentate, bidentate formation continued throughout adsorption, and unidentate and bridged carbonates began to disappear, possibly transforming irreversibly into polydentate. Once enough active sites became available during desorption, bidentate also began to transform back into free carbonate, unidentate and bridged carbonates continued to disappear throughout desorption, and bidentate disappearance and free carbonate formation both ceased. The slow formation of bidentate during adsorption and the slow disappearance of unidentate and bridged carbonates during desorption explains the never ending CO 2 uptake and release. Changes in active site and carbonate basicity were the driving force behind K-promoted HTlc being a reversible adsorbent for CO 2 at around 450 °C.

A nonequilibrium kinetic model was developed to describe the reversible adsorption and desorption behavior of CO 2 in a K-promoted hydrotalcite-like compound (HTlc). The model consisted of three reversible reactions. Two of the reactions were of the Langmuir-Hinshelwood type with slow and intermediate kinetics, and one was a mass-transfer-limited chemisorption process with very fast kinetics. To calibrate and test this model, a K-promoted HTlc was synthesized and studied to determine its dynamic behavior during CO 2 adsorption and desorption cycles carried out at 400 °C. A long cycle time adsorption (700 min) and desorption (700 min) experiment was carried out with a sample activated at 400 °C for 12 h in helium. With this experiment approaching equilibrium at the end of each step, it proved that the adsorption and desorption behavior of CO 2 in K-promoted HTlc was completely reversible. Then, the effect of the half-cycle time (15, 30, 45, 60, and 75 min) was studied with samples activated for 12 h in helium at 400 °C and cycled four times each, and the effect of the activation time (8, 12, 16, and 20 h) was studied with samples cycled twice with a 45-min half-cycle time. The former set of experiments proved that periodic behavior was achieved very quickly with cycling even when far removed from an equilibrium state; the latter set proved that the CO 2 working capacity was independent of the activation time. The model was fitted successfully to the long cycle time experiment. It then predicted successfully the dynamic and cyclic behavior of both the much shorter cycle time and different activation time experiments. This kinetic model accurately simulated the reversible adsorption and desorption behavior of the very fast, intermediate, and slow kinetic processes; the approach to periodic behavior during cycling; and the independence between the CO 2 working capacity and activation time. It also proved that the adsorption and desorption behavior was due to a combination of completely reversible adsorption, diffusion, and reaction phenomena.