Research ArticleReceived: 21 December 2011 Accepted: 2 July 2012 Published online in Wiley Online Library:

(wileyonlinelibrary.com) DOI 10.1002/jctb.3903

Oxidative absorption of hydrogen sulfide usingan iron-chelate based process: chelatedegradationGirish M. Deshmukh,∗ Aparna Shete and Deepali M. Pawar

Abstract

BACKGROUND: Oxidative absorption of hydrogen sulfide into a solution of ferric chelates is studied in a stirred cell glass reactor.The experiments were performed to investigate the degradation of chelates sodium salt of nitrilotriacetic acid (NTA) (Merck),ethylenediaminetetraacetic acid diadisodium salt (EDTA) and diethylenetriaminepentaacetic acid (DTPA) at 313 K, pH 6, ironconcentration 10 000 g L−1 and Fe : chelate molar ratio 1 : 2.

RESULTS: Oxidative absorption of hydrogen sulfide into a solution of Fe-NTA was found to be more successful, therefore, furtherexperiments with 10%, 50% and 100% concentrations of hydrogen sulfide were performed. It was shown that this process isapplicable for removal of low and high concentrations of hydrogen sulfide. The effect of antioxidants using sodium thiosulfatewas also studied in order to minimize degradation of NTA. The kinetics were studied and it was observed that the reactionappeared to be first order in ferric chelate with rate constants for 100, 50 and 10% hydrogen sulfide concentration: 0.035, 0.013and 0.019 h−1, respectively.

CONCLUSIONS: Gas sweetening processes have commercial importance in natural gases, refinery of gases and biogas processing.Desulphurization and cleaning (i.e. removal of H2S and CO2) of petroleum gas and biogas is important to make the gas methanerich and to increase the calorific value of fuel. The same techniques of desulphurization and cleaning can be used for treatingnatural gas or petroleum gas. The desulphurization and cleaning processes can minimize the atmospheric emission of gases likeSOx, NOx and CO. As the iron chelate based process is based on the principle of redox reaction of metal chelate with hydrogensulfide, this method is very useful for desulphurization of petroleum gas and biogas. This work studied the effective use ofFe-NTA solution for removal of high to low concentrations of H2S as found in biogas and industrial waste gases.c© 2012 Society of Chemical Industry

Keywords: hydrogen sulfide; absorption; ferric chelate; chelate degradation; nitrilotriacetic acid (NTA)

INTRODUCTIONMany commercial processes are available for the removal ofhydrogen sulfide from gaseous streams. Most of the processesuse gas–liquid contactors in which hydrogen sulfide is contactedwith a reagent to give either another dissolved sulfide containingcomponents (e.g. alkanol-amin or hydroxide based processes) orelemental sulphur as a precipitate. Important representatives ofthe latter type are so called iron chelate based processes. Thedesulphurization technique of bifunctional redox iron chelateprocess can be used for treating natural gas or petroleum gas.The desulphurization and cleaning processes can minimize theatmospheric emission of gases like SOx, NOx and CO. Here theprinciple of redox reaction of metal chelate with hydrogen sulfideis used for desulphurization.

The reaction sequence for oxidative absorption of hydrogensulfide in iron chelate solutions is:

H2S(g) ↔ H2S(aq) (1)

The overall chemistry is simple and consists of two sets ofchemical reactions.

Absorption:

H2S(aq) + 2Fe3+Chelantn− → s ↓ +2H+ + 2Fe2+Chelantn− (2)

Regeneration:

O2(aq) +4Fe2+Chelantn−+2H2O → 4Fe3+Chelantn−+4OH− (3)

The functon of the chelant is to prevent the formation ofinsoluble iron compounds without interfering with the ability ofthe iron to undergo reduction and oxidation.

The number ‘n’ denotes the charge of the chelant anion. In theabsorption step, H2S comes in contact with a liquid containinga soluble ferric iron chelate that is the active ferric chelate(Fe3+chelantn-) and is converted to inactive ferrous chelate

∗ Correspondence to: Girish M. Deshmukh, Department of PetrochemicalEngineering, Dr. Babasaheb Ambedkar Technological University, Lonere(Raigad) – 402103, India. E-mail: gmdeshmukh7@rediffmail.com

Department of Petrochemical Engineering, Dr. Babasaheb AmbedkarTechnological University, Lonere (Raigad) – 402103, India

J Chem Technol Biotechnol (2012) www.soci.org c© 2012 Society of Chemical Industry

www.soci.org GM Deshmukh, A Shete, DM Pawar

(Fe2+chelantn-). The H2S forms sulfide and hydrosulfide ions,which are selectively oxidized to form elemental sulfur and theferric chelate, Fe3+chelant∗ is reduced to the correspondingferrous chelate, Fe2+chelant∗.

In the regeneration step, the ferrous chelate, Fe2+ chelant∗,is reoxidized with air, back to ferric chelate, Fe3+ chelant∗. Theregenerated solution is then recycled to the absorption section.

The set of chemical reactions given below summarizes, thechelant free radical-induced oxidative degradation chemistry andis considerably more complex:

Fe2+chelant∗ + O2 + 2H+ → Fe3+chelant∗ + H2O2 (4)

Fe2+chelant∗ + H2O2 → Fe3+chelant∗ + ∗OH + OH− (5)

Fe2+chelant∗ + ∗OH → Fe2+chelant∗ + OH− (6)

where,

chelant∗ + OH− = Degradation products

The iron chelate can be regarded as a pseudo-catalyst in thereaction of hydrogen sulfide with oxygen. The sulfur that isproduced is easily recoverable from the slurry. Another advantageof iron chelate-based processes is that they essentially operate atambient conditions.1

Most studies have been done on oxidative degradation ofchelate during regeneration of ferrous chelate. Chen et al. useda HPLC techniques to study the degradation of NTA duringregeneration of ferrous chelate.2 It was observed that, chelat-ing agents are less reactive toward the hydroxyl radical asFe3+/Fe2+ redox catalysts for absorption of hydrogen sulfideto sulfur. Some experimental studies reporting on oxidativeabsorption of hydrogen sulfide by ferric chelates are avail-able.

Neumann and Lynn examined the oxidative absorption of hy-drogen sulfide by O2-saturated iron NTA solutions (0.1 moldm−3,pH = 4) in a small co-current wetted wall column.3 The overallsystem was proven effective with an overall efficiency reaching99% for inlet hydrogen sulfide loadings of 40 000–90 000 g L−1.Wubs and Beenackers1 had established the reaction kineticsof hydrogen sulfide with ferric chelates of EDTA and HEDTAin a stirred cell reactor for the temperature range 21–60 ◦Cand pH range 4–10. It was observed that pH plays signifi-cant role in oxidative absorption of hydrogen sulfide in ferricchelates. Piche et al.4 studied the oxidative absorption of hy-drogen sulfide in ferric chelate of diaminocyclohexane tetraacetic acid (CDTA) in a counter current laboratory column ran-domly packed with 15 mm plastic Ralu rings. The maximumobserved hydrogen sulfide conversion in the scrubber ap-proached 91% for 10.5 pH and hydrogen sulfide concentrationof 70 g L−1.

From the literature available it is clear that the chelated ironsolution is modified in such a way to be the best solution fordesulphurization of petroleum gases with minimum drawbacks.The use of liquid based redox process using chelated iron canbe reinvestigated here for desulphurization of gases havinglow to moderate levels of hydrogen sulfide to minimize furtherdegradation or overcome the other drawbacks.

The present experimental work includes study for oxidativeabsorption of hydrogen sulfide using ferric chelates of NTA, EDTAand DTPA, 10%, 50% and 100% concentrations of hydrogen sulfidein Fe-NTA solution and use of sodium thiosulphate in order tominimize degradation of NTA.

EXPERIMENTALMaterialsThe following materials were obtained in the highest purityavailable and were used without purification :

Reagents: Ferric chloride hexahydrate (Merck), sodium salt ofnitrilotriacetic acid (Merck), Ethylene Diamine Tetraacetic aciddisodium salt (EDTA) and diethylene triamine penta acetic acid(DTPA) (Baker Analysed Reagents), sulfuric acid (Merck), HPLCgrade acetonitrile (Merck), HPLC water (Merck), nitric acid (Merck),copper nitrate (Thomas Baker), sodium thiosulphate (Merck), fer-rous sulfide (Merck). Double distilled water was used throughoutthe study for experimentation and HPLC water was used foranalysis.

Gases: Hydrogen sulfide was produced in Kipp’s apparatus. Airwas supplied through an air pump.

Analytical equipmentAnalysis of samples was performed using high performance liquidchromatography (HPLC) (CHEMITO, Mumbai) equipped with aquaternary HP 1050 Pump and HP 1050 series variable wavelengthUV/visible detector. The manual injector is equipped with a sampleloop of 20 µL. Nucleosil 100-5 RP-C18 column, 250 mm in lengthand 4.0 mm in diameter with a particle size of 5 µm.

Chemical preparationThe Fe3+ NTA, Fe3+ EDTA and Fe3+ DTPA, solution were preparedby diluting a predetermined amount of NTA, EDTA and DTPArespectively, in double distilled water (150 mL). The appropriateamount of FeCl3·6H2O (Fe : NTA ratio = 1 : 2 mol/mol, Fe : EDTAratio = 1.1 : 2 mol/mol and Fe : DTPA ratio = 1.1 : 2 mol/mol re-spectively) were added to the NTA, EDTA and DTPA, respectively.The solution was diluted with double distilled water to adjustthe total volume to 1000 mL, giving a clear solution with a pH ofabout 6.

Experimental setupThe kinetic experiments for oxidative absorption of hydrogensulfide into FeNTA aqueous solution were carried out batch wisein a thermostated stirred cell reactor made up of glass havingid 8 cm and volume 300 mL. A four-blade glass turbine impellerof 1.5 cm blade diameter was used to stir the mixture as shownin Fig. 1. Four symmetrically mounted glass baffles increased theeffectiveness of stirring and prevented the formation of a vortex. Aconstant temperature water bath with constant stirring regulatedthe temperature of the liquid mixture during the experiments.The speed of agitation was maintained at 1000 rpm. A calibratedthermometer, tachometer, digital pH meter and rotameter wereused to observe the temperature of solution, speed of stirrer, pHof the sample and flow rate, respectively. A Kipp’s apparatus of 1 Lvolume was used to produce hydrogen sulfide gas.

Experimental procedureThe experimental procedure included the following steps:

1. Charging the Fe3+ NTA solution, of known volume and initialNTA concentration.

2. Liquid heating to a temperature of 313 K.3. Producing hydrogen sulfide gas in Keep’s apparatus by the

reaction of ferrous sulfide sticks and sulfuric acid.

wileyonlinelibrary.com/jctb c© 2012 Society of Chemical Industry J Chem Technol Biotechnol (2012)

Reactors, kinetics and catalysis www.soci.org

Figure 1. Experimental setup for oxidative absorption of hydrogen sulfide in Fe–NTA solution.

4. Initiating the reaction by providing air and H2S. The H2S flowrate was maintained at 0.1 LPM and air flow rate at 1 LPM.Reduction of ferric ions to ferrous is started due to reactionwith hydrogen sulfide, and elemental sulfur is precipitated.Ferrous is converted again to ferric by oxygen present in air.

5. Steady state operation for 10 h to study oxidative absorptionof hydrogen sulfide for various reaction conditions. Thesamples were collected after an interval of 1 h to measurethe concentration of NTA.

RESULT AND DISCUSSIONSeveral iron chelate based processes for the removal of hydrogensulfide from gas streams have been developed over the years andcommercialized. The most important difference between theseprocesses concerned the liquid formulation and the way thedegradation of chelate is suppressed. Obtaining kinetic data foroxidative absorption of hydrogen sulfide in iron chelate complexand the influence of the antioxidants on hydrogen sulfide removal,if applied in this sulfur removal process, will give improved designsfor desulphurization units. This work dealt with these subjects andcontributes to the development of a firm basis for designingdesulphurization unit.

Degradation of various chelates for oxidative absorption ofhydrogen sulfideThe degradation of EDTA, HEDTA, NTA and other aminopolycar-boxylate chelating agents for oxidation of Fe-NTA complex wasreviewed by Chen et al.2 The percentage degradation of HEDTA,EDTA and NTA were 80% within 40 h, 80 h and 100 h, respec-tively. Pekss and Salmas also studied the oxidative degradationof chelates at pH 3–6, and temperature range 30–60 ◦C. Thedegradation of EDTA, DTPA and NTA for oxidative absorption ofhydrogen sufide in iron–chelate complex was studied at 313 K andpH 6 as shown in Fig. 2. The ferric : chelate ratio was taken as 1 : 1and 1 : 2 as per the stability (Stability means Deposition of ferric

Figure 2. Degradation of various chelates for oxidative absorption ofhydrogen sulfide (iron concentration 10 000 ppm, Fe : chelate molar ratio1 : 2 and 1 : 1, temperature 313 K, pH 6, hydrogen sulfide flow rate 0.1 Lmin−1, air flow rate 1 L min−1, speed of agitation 1000 rpm).

chelates) of ferric with the particular chelates, and the initial con-centration of ferric was taken as 10 000 g L−1. The concentrationsof chelates were determined after different time intervals usingHPLC. The percentage degradation of NTA, DTPA and EDTA was30%, 22% and 66%, respectively, after 10 h. The order of degrada-tion was found to be EDTA > NTA > DTPA. It was also observedthat hydrogen sulfide was absorbed only into iron-NTA solution.Ferric chelates of EDTA and DTPA was found unsatisfactory asthere was no separation of elemental sulfur but degradation wasobserved.

It is found that a NTA is far superior ligand for better absorptionof hydrogen sulfide in Fe (III) chelate system.

J Chem Technol Biotechnol (2012) c© 2012 Society of Chemical Industry wileyonlinelibrary.com/jctb

www.soci.org GM Deshmukh, A Shete, DM Pawar

Figure 3. Degradation of NTA for oxidative absorption of various hydrogensulfide concentrations (iron concentration 10 000 ppm, Fe : chelate molarratio 1 : 2, temperature 313 K, pH 6, hydrogen sulfide flow rate 0.1 L min−1,air flow rate 1 L min−1, speed of agitation 1000 rpm).

Degradation of NTA for oxidative absorption of varioushydrogen sulfide concentrationsThe hydrogen sulfide concentration also plays a most importantrole in NTA degradation due to the reduction of ferric into ferrousduring the absorption process. Most investigators have reportedon the successful removal of hydrogen sulfide from biogas usingiron chelate solution. Horikawa et al.5 studied purification of abiogas by chemical absorption of hydrogen sulfide in an iron-chelated solution catalyzed by Fe/EDTA using a continuouscounter-current absorber. It is reported that for a biogas flow-rateof 1000 mL min−1 and 2.2 kgf cm−2 (i.e. 215776 N/m2) pressures,approximately 90% and 70% of the H2S can be removed withthe catalytic solution flowing at 83 and 68 mL min−1, respectively.This means that above a flow-rate of 83 mL min−1 or with anappropriate ratio of gas contacting phases, it is possible to achievetotal removal of H2S.

Biogas contains 2–5% concentration of hydrogen sulfide. How-ever, concentration of hydrogen sulfide may be higher in industrialwaste gases. The effect of hydrogen sulfide concentration forhydrogen sulfide percentages of 10%, 50% and 100% on NTAdegradation was studied at optimized experimental conditions.The various hydrogen sulfide concentrations (10% and 50%) wereprepared by diluting nitrogen gas in pure hydrogen sulfide.

It was observed that degradation of NTA for 10% and 50%hydrogen sulfide concentrations is less than 20% and almost 30%for 100% concentration as shown in Fig. 3. This indicates that theFe–NTA solution with optimized conditions may be suitable forremoval of hydrogen sulfide from gases containing low to highconcentrations.

Effect of antioxidant on degradation of NTA for oxidativeabsorption of hydrogen sulfideFormation of the hydroxyl ion during redox reaction of Fe–NTAmay be responsible for the degradation of NTA. A number ofadditives that function as hydroxyl scavengers have been usedto slow down or prevent radical-induced oxidative degradation.The added material reacts with hydroxyl ions or free radicals,thus preventing them from attacking, hydroxylation, and thusdegrading the organic ligand. There are many such antioxidantsthat may be used, e.g. ethylene glycol, potassium citrate, sodium

Figure 4. Effect of antioxidant on degradation of NTA for oxidativeabsorption of hydrogen sulfide (iron concentration 10 000 ppm, Fe : chelatemolar ratio 1 : 2, temperature 313 K, pH 6, hydrogen sulfide flow rate 0.1 Lmin−1, air flow rate 1 L min−1, speed of agitation 1000 rpm).

thiocynate, 1-butanol and sodium thiosulphate as antioxidants.Chen et al. found 1 mol L−1 NaCl as well as 5% (mol %) of sodiumthiosulphate may greatly reduce the degradation of NTA.2

Hence, sodium thiosulphate was used as antioxidant in orderto study degradation of NTA for oxidative absorption of hydrogensulfide. The effect of sodium thiosulphate concentrations 0%,5% and 10% (mol % based on NTA concentration) was studiedunder optimum conditions. It was observed that the additionof sodium thiosulphate minimizes the degradation of NTA asshown in Fig. 4. It is found that 10% (mol %) concentration ofsodium thiosulphate gives minimum degradation of NTA and isthe optimum concentration.

Kinetics of NTA degradation for oxidative absorption ofhydrogen sulfideWubs and Beenackers had found that reaction of hydrogen sulfideis first order in hydroxyl ferric chelate.1 The kinetic experimentaldata was obtained for degradation of NTA at different hydrogensulfide concentrations ranging from 10% to 100%. The reporteddata was analyzed to identify the order of the reaction. Theregression coefficient values for zero-order, first-order and second-order reaction at different hydrogen sulfide levels were in therange 0.94–0.97, 0.95–0.97 and 0.94–0.96, respectively, as shownin Fig. 5. The reaction order with respect to NTA is found to befirst-order as this gives the best fitting for all three hydrogensulfide concentrations. Rate constant values for 100%, 50% and10% hydrogen sulfide concentration are 0.035 h−1, 0.013 h−1 and0.019 h−1, respectively.

CONCLUSIONThe oxidative absorption of hydrogen sulfide in Fe–chelate wascarried out in a stirred cell glass reactor with four blade turbineimpeller and four leg baffle. NTA was found to be the most suitablechelate compared with EDTA and DTPA for oxidative absorptionof hydrogen sulfide. Also, the degradation of NTA was minimizedusing 10% (mol %) concentration of sodium thiosulphate. Theorder of the reaction obtained from kinetic experimental data isfirst-order in Fe–NTA. This process is applicable for desulfurizationof gases containing low to high concentrations of hydrogen sulfide.

wileyonlinelibrary.com/jctb c© 2012 Society of Chemical Industry J Chem Technol Biotechnol (2012)

Reactors, kinetics and catalysis www.soci.org

Figure 5. First-order kinetics for the degradation of NTA for oxidativeabsorption of various hydrogen sulfide concentrations.

ACKNOWLEDGEMENTWe are grateful to the University Grants Commision, New Delhi,India for providing financial support to this research work.

REFERENCES1 Wubs HJ and Beenackers AACM, Kinetics of H2S absorption into

aqueous ferric solutions of EDTA and HEDTA. AIChE J 40:433–444(1994).

2 Chen D, Martell AE and McManus D, Studies on the mechanismof chelate degradation in iron-based, liquid redox H2S removalprocesses. Can J Chem Eng 73:264–274 (1995).

3 Neumann DW and Lynn S, Oxidative absorption of H2S and O2 by ironchelate solutions. AIChE J 30:62–69 (1984).

4 Piche S, Ribeirs N, Bacaoui A and Larachi F, Assessment of redoxalkaline/iron Chelate absorption process for the removal of H2Sin air emission. Chem Eng Sci 60:6452–6461 (2005).

5 Horikawa MS, Rossi F, Gimenes ML, Costa CMM and Da Silva MGC,Chemical absorption of H2S for biogas purification. Braz J ChemEng 21:415–422 (2004).

J Chem Technol Biotechnol (2012) c© 2012 Society of Chemical Industry wileyonlinelibrary.com/jctb