Chemical absorption and degradation kinetics of iron (III) chelate in H2S removel process

Hydrogen sulfide (H2S) removal using chemical absorption by iron chelate solution catalyzed by Fe(III)EDTA is an economically promising technique. Fe(III)EDTA can be easily regenerated by bubbling air into the absorbing liquid. In this study a chemical oxidation using Fe(III)EDTA was selected for the treatment of high H2S concentration in biogas which produced from wastewater of concentrated latex rubber industry (CLRI). Experiments were performed using a pilot packed column with diameter and packed height of 0.5 and 0.8 m, respectively. The biogas flow rate and H2S concentration were in the range of 5.16 x10-5.61x10 m/s and 0.35-0.77 mol/m, respectively. Experimental results indicated that Fe(III)EDTA solution was effective at removing H2S from biogas with a maximum removal efficiency of about 97%. Suitable operating conditions, including Fe(III)EDTA concentration, flow of Fe(III)EDTA and air flow rate were determined. In addition, no side-reaction of Fe(III)EDTA with valuable methane was found. A mathematical model of the absorption and the reaction between H2S and Fe(III)EDTA in a packed column was proposed and verified against the experimental data. The results confirmed the potential use of the model to design packed column for H2S removal from biogas using absorption coupled with oxidation by Fe(III)EDTA. A counter-current laboratory packed column was used to study the process variables which were known to influence the absorption and reaction rate such as scrubbing liquid flow rate (0.167-0.833 mL/s), initial Fe(III)EDTA concentration (10-30 mol/m), gas flow rate (0.033-0.167 L/s), inlet H2S concentration (0.025-0.167 mol/m), and height of packed bed (0.15-0.45 m). The effects of these variables on absorption and reaction performance were analyzed via evaluating the absolute removal efficiency. A central composite design was used in vi vii the design of experiments. The H2S removal efficiency was modeled statistically and optimized using linear Regression method. A quadratic model was suggested and validated experimentally with the coefficient of determination equal to 0.872. All significant variables were presented in the model and the interaction effects between variables were found. Results showed that the developed regression model provides a better understanding of the interactions involved in the studied H2S removal process. Moreover, the effect of H2S molar flow rate, the initial concentration of Fe(III)EDTA and the presence of sodium citrate in Fe(III)EDTA solution on the degradation of Fe-EDTA were investigated. The semibatch reactor with continuous flow of H2S containing biogas was used under a wide range of experimental conditions; initial pH = 7.0, H2S molar flow rate (1.08x10-3.40 x10 mol/h), the initial concentration of Fe(III)EDTA (2.17-8.16 mol/m) and the concentration of sodium citrate (0-310 mol/m). The result showed that sodium citrate acted as stabilizer with a good ability to reduce the degradation rate. The degradation rate of Fe-EDTA was found to follow pseudo first order kinetics. The correlation model between degradation rate constant and H2S molar flow rate, the initial concentration of Fe(III)EDTA and the concentration of sodium citrate was developed and can be used to predict the degradation rate of Fe-EDTA for H2S removal from biogas. The precipitated solid, called sulfur cake precipitated during the reaction was also recovered and analyzed for its compositions. The result revealed that the sulfur cake contained more than 98% sulfur element almost balances with iron and no significant EDTA was degraded into the solid form. Finally, the cost of H2S removal from biogas using the packed column were analyzed. The investment cost for removing 1,300 mg/m of H2S from biogas at 2 L/min was about 10,000-15,000 Baht while the operating cost was 0.81 Baht/m biogas. The cost comparison of using Fe(III)EDTA with other oxidant, KMnO4 and NaOCl was investigated and it revealed that the operating cost for Fe(III)EDTA system was slightly lower than KMnO4 system but about two times lower than that of NaOCl.