Chemisorption working capacity and kinetics of CO2 and H2O of hydrotalcite-based adsorbents for sorption-enhanced water-gas-shift applications

Abstract

The adsorption behavior of carbon dioxide and water on a K-promoted hydrotalcite based adsorbent has been studied by thermogravimetric analysis with the aim to better understand the kinetic behavior and mechanism of such material in sorption enhanced water-gas shift reactions.The cyclic adsorption capacity was measured as a function of temperature (300–500°C), pressure (0–8bar) and the cycle time. Both species interact at elevated temperatures with the adsorbent. The history of the adsorbent (pretreatment/desorption conditions) has a profound influence on its sorption capacity. Slow desorption kinetics determine the sorption capacity during cyclic operation, where a high temperature during the desorption and long half-cycle times can increase the cyclic working capacity for both CO2 and H2O significantly. Accounting for the sorbent history and the definition of adsorption capacity are very important features when comparing sorption capacities to values reported in literature. The adsorbent shows very high capacities for H2O compared to CO2 which has not been reported in the literature up to now. The mechanism for H2O and CO2 adsorption seems to be a different one. Whereas H2O adsorption seems to follow the principles of a simple physisorption mechanism, CO2 adsorption can only be explained by a chemical reaction with the adsorbent. Working isotherms (cyclic working capacity at isothermal conditions at different pressures) of both CO2 and H2O were measured up to 8bar total pressure. Higher partial pressures increase the cyclic working capacity of the adsorbent up to 0.47mmol/g for CO2(PCO2=8bar) and 1.06mmol/g for H2O (PH2O=4.2bar) at 400°C after 30min of adsorption followed by 30min of dry regeneration with N2.