Temperature Dependence of the Nonequilibrium Kinetic Model That Describes the Adsorption and Desorption Behavior of CO2 in K-Promoted HTlc

A nonequilibrium kinetic model developed previously by the authors to describe the reversible adsorption and desorption behavior of CO2 in a K-promoted hydrotalcite-like compound (HTlc) was extended to account for temperature effects. This model involves three steps and four phases that reversibly undergo adsorption or reaction with CO2 in the structure. The model parameters were obtained by fitting it to experimental adsorption and desorption cycling data carried out with in-house-synthesized K-promoted HTlc at 11 temperatures ranging from 300 to 500 C. A single adsorption (in CO2 at 1 atm) and desorption (in He at I atm) cycle having a 700 min half-cycle time was obtained at each temperature and fitted successfully to the model. Then, using the same set of parameters, the model was used to successfully predict similar shorter cycle time experiments carried out with a 60 min half-cycle time for eight cycles at each temperature. For both the long and short cycle time data, the model captured all three kinetic regimes, the absolute CO2 capacity, the CO, working capacity, the periodic behavior, and their temperature dependence. The deviations that did occur between the model and experiments were generally due to irreversible losses observed with the short cycle time runs at 480 and 500 C that could not be predicted by the model; they were thus considered to be inconsequential. Both the model and experiments showed that temperature played an important role, with optimum temperatures being in the 380-420 degrees C range. Overall, this new model further validated that CO2 uptake and release in K-promoted HTlc is associated with a combination of completely reversible adsorption, diffusion, and reaction phenomena, through a three-step, four-phase process that has a strong temperature dependence.