Nonequilibrium kinetic model that describes the reversible adsorption and desorption behavior of CO2 in a K-promoted hydrotalcite-like compound

A nonequilibrium kinetic model was developed to describe the reversible adsorption and desorption behavior of CO2 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 CO2 adsorption and desorption cycles carried out at 400 degrees C. A long cycle time adsorption (700 min) and desorption (700 min) experiment was carried out with a sample activated at 400 degrees 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 CO2 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 degrees 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 CO2 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 CO2 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.