Dissolution kinetics of magnesium hydroxide for CO2 separation from coal-fired power plants

Abstract

The dissolution of magnesium hydroxide in water for the release of magnesium and hydroxyl ions into the solution to maintain suitable alkalinity is a crucial step in the Mg(OH)2-based CO2 absorption process. In this study, the rate of dissolution of Mg(OH)2 was investigated under different operating conditions using a pH stat apparatus. The dissolution process was modeled using a shrinking core model and the overall Mg(OH)2 dissolution process was found to be controlled by the surface chemical reaction of Mg(OH)2 with H+ ions. Under the chemical reaction control regime, the dissolution of Mg(OH)2 in alkaline conditions was found not to follow a first-order reaction, and the fractional order of reaction was estimated to lie between 0.20 and 0.31. This suggests that the dissolution reaction is a non-elementary reaction, consisting of a sequence of elementary reactions, via most likely forming a surface magnesium complex. The true activation energy value of 76±11kJ/gmol was found to be almost twice as much as the observed activation energy value of 42±6kJ/gmol determined at pH 8.6, and was comparable with the previously reported values. The particle sizes predicted from the intrinsic kinetics determined from the model were in good agreement with the experimentally measured particle sizes during the dissolution process.