The evolution, chemistry and applications of chelated iron hydrogen sulfide removal and oxidation processes

Abstract

The evolution of the use of Fe(III) as a regenerable oxidant for the conversion of H 2S to S is traced from inception as solid phase oxidation processes employing Fe 2O 3, through aqueous, alkaline Fe(OH) 3 suspensions and [Fe(CN) 6] based processes to modern, aqueous, homogeneous redox catalysts using amino polycarboxylate chelated iron. In the current process, H 2S bearing gas streams are contacted with a dilute ([Fe] = 5 mM to 0.5 M), aqueous, mildly alkaline (pH 7 to 9) amino or polyamino polycarboxylato iron (III) solution using a variety of application dependent gas-liquid contactors. S is precipitated and continuously removed. The Fe(III)L is concomitantly reduced to Fe(II)L and is regenerated by aeration in the same or a separate vessel, depending on the need to prevent admixture of reoxidation air with the H 2S free process gas stream. Basic process chemical reactions are reviewed and both beneficial and troublesome side reactions, including ligand degradation by dioxygen reduction products and control thereof by in-situ generated S 2O 2− 3, are discussed together with associated mechanistic studies. Selection criteria for ligands suitable for use in this process are described and new chelating agents with significantly increased in-process stability are disclosed. The current global scope of applications is presented, advantages and limitations of the process relative to alternate technologies are discussed and the present status and future expectations are reviewed.