Optimization of H2S absorption using impeller-based scrubber with electrochemical regeneration of FeEDTA

Abstract

This study proposes an innovative Fe(Ⅲ)EDTA-based H2S removal technology using an impeller-based scrubber that maximizes the reaction between the gas and the liquid. H2S can be captured in an alkaline envi-ronment, and then oxidized into element sulfurs by Fe(Ⅲ)EDTA. However, Fe of Fe(Ⅲ)EDTA is precipitated under high pH, which causes loss of absorbent and clogging problem of the impeller mesh. For this reason, iron precipitation should be considered. The optimal conditions for H2S removal in which precipitation of Fe is not an issue were derived to be pH 9 and 25 mM Fe(Ⅲ)EDTA with EDTA/Fe = 1.1, 1 of L/G ratio, and 2000 rpm of impeller rotation rate. Under the optimal conditions, 20 ppmv H2S could be removed to 100%. However, 100 ppmv H2S could be removed up to 98%, and the removal efficiency decreased to 80% after 3 hours since a lot of sulfur precipitation not only occurred, but also Fe3+ was not enough to keep high removal efficiency. Thus, the sulfur treatment process and the regeneration of Fe2+ are required for the sustainable removal of H2S. Sulfur precipitates are well separated through common solid-liquid separation methods such as filtration and sedi-mentation. The Fe(Ⅱ)EDTA regeneration usually is proceeded via contact oxygen. However, when Fe(Ⅱ)EDTA reacts with oxygen, hydroxyl radical can be produced as a byproduct through the Fenton reaction, and it leads to EDTA degradation. For a more stable regeneration, an electrochemical operation was introduced. In an elec-trolytic cell, inactive Fe(Ⅱ)EDTA is forced to become active Fe(Ⅲ)EDTA at the anode, and water is reduced to hydrogen at the cathode. The optimal voltage for Fe(Ⅱ)EDTA oxidation was determined to be 0.6 V, and it was better to use the catalyst only for the cathode considering the high cost of the precious Pt catalyst. In addition, the electrolysis cell was applied to the absorption system at the point when the removal efficiency starts to drop to prevent the decrease in pH. Thus, this study provides a demonstration of many desirable aspects for field implementation, such as effective removal of H2S and the introduction of Fe regeneration in the electrolysis cell.