Assessment of a redox alkaline/iron-chelate absorption process for the removal of dilute hydrogen sulfide in air emissions

The oxidative absorption of hydrogen sulfide (H2SH2S) into a solution of ferric chelate of trans-1,2- diaminocyclohexanetetraacetate (CDTA) was studied in a counter-current laboratory column randomly packed with 15 mm plastic Ralu rings. The present investigation takes concern about the Kraft pulping situation where dilute H2SH2S concentrations are omnipresent in large-volume gas effluents. A fractional two-level factorial approach was instigated to determine the significance of six operating variables, namely the solution's alkalinity (pH; 8.5–10.5), the liquid mass flow rate (L;1.73L;1.73–5.19kgm-2s-1), the solution's ionic strength (IC;0.01IC;0.01–0.1moldm-3), the gas mass flow rate (G;0.19G;0.19–0.57kgm-2s-1), the inlet H2SH2S concentration (CH2S,0;70CH2S,0;70–430 ppm) and the initial ferric CDTA concentration (CFe,0;100CFe,0;100 –400μmoldm-3). Initially, a Plackett–Burman design matrix of seven duplicated experiments revealed that pH is the leading factor controlling the H2SH2S conversion rate while the ionic strength and ferric CDTA concentration effects remained negligible within the factorial domain. Surface response analysis based on 11 duplicated factorial experiments plus 10 central composite trials revealed that the H2SH2S conversion significantly increases with liquid flow rate but decreases with growing H2SH2S load up. Further examination about the influence of ferric CDTA on H2SH2S absorption rate was set up over a broader concentration range (CFe,0;0(CFe,0;0–2000μmoldm-3) at pH of 9.5 and 10.5. It showed good potential at 2000μmoldm-3 as H2SH2S conversion increased by a significant 25% for both pH values in comparison to pure alkaline solutions containing no ferric CDTA.