Adsorption behavior and kinetics of H2S on a potassium-promoted hydrotalcite

Abstract

Adsorption of H2S and the influence of steam on its adsorption capacity and kinetics were studied on a commercial potassium-promoted hydrotalcite. The sorbent shows a very high cyclic working capacity for H2S compared to CO2 and H2O, even at lower partial pressures and at different operating temperatures ranging between 300 and 500 °C. The operating temperature does not significantly influence the cyclic working capacity for half-cycle times of 30 min. The adsorption mechanism, however, changes at higher temperatures. At lower temperatures (300 °C) a fast adsorption with a fast approach to steady state was observed. At higher operating temperatures, H2S reacts with the hydrotalcite structure, forming strongly bonded sulfuric species on the sorbent. When using dry regeneration conditions, the first cycles in cyclic operation at higher temperatures show a significantly higher adsorption of H2S (especially the first cycle), which cannot be desorbed during regeneration with N2. After the first fast initial adsorption rate a continuous slow adsorption of H2S occurs, probably caused by a surface reaction between H2S and the hydrotalcite structure. This reaction is, however, reversible if steam is used.The adsorption mechanism for H2S and H2O was determined using multiple cyclic experiments comparable to previous studies performed for CO2 and H2O adsorption. It is evident that the adsorption mechanism developed for CO2 on the same sorbents is also valid for H2S, indicating that the developed mechanism is consistent for sour gas adsorption on this type of sorbents. The cyclic working capacity can be significantly increased if steam is used during the regeneration step of the sorbent. The mechanistic model developed for the adsorption of CO2 and H2O was successfully validated with more than 160 different TGA experiments. An operating temperature of 400 °C seems to be optimal to achieve a high cyclic working capacity for H2S, because at higher temperatures the regeneration of the formed sulfuric species seems to be hindered resulting in a significant decrease in the cyclic working capacity.