Equilibria and kinetics of CO2 adsorption on hydrotalcite adsorbent

The equilibria and kinetics of high temperature CO2 adsorption on hydrotalcite adsorbent have been studied using semi-technical and bench-scale elution apparatus. The former unit enabled the simulation of the adsorption, depressurisation and purge steps of a pressure swing adsorption-based process. Conditions of measurements were chosen to depict those of the separation enhanced steam methane reforming process, i.e. temperatures up to 753 K and in the presence of water vapour. These conditions are also appropriate to some flue gas CO2 recovery processes. At 753 K and in the presence of water vapour, adsorption saturation capacities of similar to 0.58 mol/kg were measured, and found to be insensitive to the actual concentration of feed water. Under dry feed conditions, a small reduction in the capacity of the fresh adsorbent (similar to 10%) was observed, however, both dry and wet feed cases could be adequately described by Langmuir models. Measurements also suggest rapid and irreversible chemisorption on freshly packed adsorbent, followed by reversible and relatively weak adsorption on the material thereafter; adsorption appears to be promoted in the presence of water. Nevertheless, a temporal decline in the reversible adsorption capacity was also observed, which was particularly severe for dry feed operation. For example, at 673 K, a steady-state capacity 30% to 40% less than that of the fresh adsorbent was measured. A steam purge was found to partially reactivate the adsorbent, but some irreversible loss in capacity was indicated for very long times-on-stream (e.g. > 90 d at 673 K). A mathematical model based on a linear driving force description of mass transfer, but in which the non-linearity of the isotherm is accounted for, was found to give a good description of the adsorption, depressurisation and purge steps of operation. The model also accounts for non-isobaric and non-isothermal adsorption/desorption, and is thus suitable for the purposes of large-scale design and process analysis. (C) 2000 Elsevier Science Ltd. All rights reserved.