Mathematical Modeling and Genetic Algorithm Optimization of Reactive Absorption of H 2 S A rate-based mathematical model was developed for the reactive absorption of H 2 S in NaOH, with NaOCl or H 2 O 2 as the chemical oxidant solutions in a packed column. A modified mass transfer coefficient in the gas phase was obtained by genetic algorithm and implemented in the model to correct the assumption of instantaneous reactions. The effects of different operating variables including the inlet H 2 S concentration, inlet gas mass flux, initial NaOH, concentrations of the chemical oxidants in the scrubbing solutions, and liquid-to-gas ratio on the H 2 S removal efficiency were studied. A genetic algorithm was employed to optimize the operating variables in order to obtain maximum removal efficiency of H 2 S. The model results were in good agreement with the experimental data. Keywords: Genetic algorithm, H 2 S removal, Mathematical modeling, Packed-bed column, Reactive absorption Received: May 08, 2014; revised: July 08, 2014; accepted: August 11, 2014 DOI: 10.1002/ceat.201400288 1 Introduction Nowadays, the removal of gaseous pollutants from the flue gas streams of chemical industries has received considerable atten- tion because of their adverse health and environmental effects. One of the most dangerous gaseous pollutants is hydrogen sul- fide (H 2 S). H 2 S is a poisonous, flammable, and explosive gas, which creates environmental, health, and corrosion hazards. It is a colorless gas with the characteristic foul odor of rotten eggs which one can easily detect at a concentration of 0.4 ppb. In- haling H 2 S causes headaches, fatigue, irritability, insomnia, and gastrointestinal disturbances in low concentration. Exposure to H 2 S concentrations greater than 300 ppm for 30 min is fatal for humans due to the detrimental effect on the central nervous system [1, 2]. Several technologies have been developed for the removal and conversion of H 2 S to a more beneficial and safer species. H 2 S contamination may be treated via several physicochemical processes, such as scrubbing, adsorption, biological treatment, condensation, and oxidation [3]. Among these, the scrubbing process of H 2 S using sodium hydroxide (NaOH) solution is a well-established procedure [4]. NaOH is a very active and effi- cient absorbent; however, it is not easily regenerable. This defi- ciency limits the application of the NaOH scrubbing process to a small concentration of H 2 S in acid gases [5]. The first investigation on the H 2 S absorption process was done by Astarita and Gioia [6]. They studied the chemistry of absorption of H 2 S in aqueous hydroxide solutions and reported that during the absorption of H 2 S, two equilibrium reactions of sodium bisulfide (NaHS) and sodium sulfide (Na 2 S) formation took place [4, 6]: H 2 S (aq) + NaOH (aq) > NaHS (aq) +H 2 O (l) (1) NaHS (aq) + NaOH (aq) > Na 2 S (aq) +H 2 O (l) (2) Experimental observations and results indicate that in order to increase the absorption of H 2 S in NaOH solution, the pH of the solution must be maintained approximately higher than 13 [5]. If, in any way, the pH of scrubbing solution decreased, reaction (1) is driven back and H 2 S is released to the gas phase. Therefore, various chemical oxidant compounds such as sodium hypochlorite (NaOCl), hydrogen peroxide (H 2 O 2 ), chlorine (Cl 2 ), and ozone (O 3 ) have been applied to prevent odor emission of H 2 S. Among these, NaOCl and H 2 O 2 are commonly used with NaOH scrubbing solution because of their low cost, availability, and high efficiency. Furthermore, their application avoids formation of harmful oxidation by- products [2, 7]. NaOCl and H 2 O 2 react with Na 2 S according to reactions (3) and (4), respectively: Na 2 S (aq) + 4 NaOCl (aq) > Na 2 SO 4(aq) + 4 NaCl (aq) (3) Na 2 S (aq) +4H 2 O 2(aq) > Na 2 SO 4(aq) +4H 2 O (l) (4) Chem. Eng. Technol. 2014, 37, No. 12, 2175–2184 ª 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cet-journal.com Fatemeh Bashipour 1 Saied Nouri Khorasani 1 Amir Rahimi 2 1 Department of Chemical Engineering, Isfahan University of Technology, Isfahan, Iran. 2 Department of Chemical Engineering, College of Engineering, University of Isfahan, Isfahan, Iran. – Correspondence: Dr. Saied Nouri Khorasani (saied@cc.iut.ac.ir), Department of Chemical Engineering, Isfahan University of Technol- ogy, Isfahan 84156-83111, Iran. Research Article 2175