Oxidation of H2S to elemental sulfur over alumina-based ...scientiairanica.sharif.edu/article_3886_4a517a9b061a54ab52d9c032d73f3e3c.pdfb. Department of Chemical Engineering, Abadan

Scientia Iranica C (2016) 23(3), 1160{1174

Sharif University of TechnologyScientia Iranica

Transactions C: Chemistry and Chemical Engineeringwww.scientiairanica.com

Oxidation of H2S to elemental sulfur overalumina-based nanocatalysts: Synthesis andphysiochemical evaluations

M. Rezaeea, M. Kazemeinia;�, M. Fattahib, A.M. Rashidic and L. Vafajood

a. Department of Chemical and Petroleum Engineering, Sharif University of Technology, Tehran, P.O. Box 11155-9465, Iran.b. Department of Chemical Engineering, Abadan Faculty of Petroleum Engineering, Petroleum University of Technology, Abadan,

Iran.c. Nanotechnology Research Center, Research Institute of Petroleum Industry (RIPI), Tehran, Iran.d. Chemical and Environmental Engineering Group, Islamic Azad University, Tehran South Branch, Tehran, Iran.

Received 3 March 2015; received in revised form 10 August 2015; accepted 16 November 2015

KEYWORDSH2S;Claus process;Al2O3;Spray pyrolysismethod;Wet chemicaltechnique;Nano catalyst.

Abstract. In this paper, oxidation of H2S into elemental sulfur over synthesized alumina-based nanocatalysts was physiochemically investigated and the results were compared witha commercial Claus catalyst. The wet chemical, co-precipitation, and spray pyrolysistechniques were employed to synthesize several alumina nanostructures. Then, the SEM,XRD, and ASAP analysis methods were utilized to characterize in order to choose the bestnanocatalyst. The sulfur and H2S contents were determined through the standard UOPtechniques. Amongst the synthesized materials, Al2O3-supported sodium oxide preparedthrough the wet chemical, and Al2O3 nanocatalysts via spray pyrolysis methods werethe most active catalysts for the purpose at hand. In addition, the TiO2 nanostructureand a hybrid of nano alumina support (made via the wet chemical method), decoratedon the carbon nanotube, were prepared for this goal. Ultimately, the best chemicallycharacterized nanocatalyst was subjected to evaluations in a �xed bed reactor while e�ectsof temperature, metal loading, and GHSV were understudied. It was observed that thealumina nanoparticles prepared through the wet chemical and spray pyrolysis methodsled H2S into elemental sulfur in a reproducible manner with 97 and 98% conversions,respectively. Both of these methods were more desirable than utilizing the commercialcatalysts (i.e. CR-3S and CRS-31) providing nearly 96% conversion.© 2016 Sharif University of Technology. All rights reserved.

1. Introduction

Hydrogen Sul�de (H2S) is a highly poisonous andcorrosive gas causing serious damages to process equip-ments. The so called sour gas containing H2S is usuallyforwarded to a Sulfur Recovery Unit (SRU), throughwhich it is burned into elemental sulfur. There arevarious methods to remove H2S from gas streams,

*. Corresponding author. Tel.: +98 21 66165425E-mail addresses: [email protected] (M. Kazemeini);[email protected], [email protected] (M. Fattahi)

including adsorption, direct conversion, and reversibleabsorption. Moreover, H2S is usually removed by thewell-known Claus process in industry. A Claus SRU isconsidered to be the most common method for conver-sion of H2S into elemental sulfur for streams possessinghigh H2S content. The Claus SRU consists of thermaland catalytic segments [1-4]. This process consists oftwo steps, including thermal oxidation (reaction 1) andcatalytic interaction (reaction 2). At the former step,one-third of H2S is initially burned with air into sulfurdioxide in a waste heat furnace. Then, through thelatter step, SO2 reacts with unconverted H2S to form

M. Rezaee et al./Scientia Iranica, Transactions C: Chemistry and ... 23 (2016) 1160{1174 1161

elemental sulfur over an Al2O3 catalyst. However, dueto the thermodynamic limitations, only 95-97% of H2Scould be converted into sulfur.

2H2S + 3O2 ! 2SO2 + 2H2O; (1)

2H2S + SO2 ! 3nSn + 2H2O: (2)In order to increase fractional conversion of H2S toelemental sulfur in comparison with the Claus process,a single step oxidation being practically irreversiblewith no thermodynamic limitations was developed.Synthesis of active, as well as selective catalysts, wasindeed necessary in order to increase conversion ofH2S to elemental sulfur. Hence, the most importante�ective parameters regarding the selection of propercatalysts for a single-step oxidation of H2S had to beconsidered. Previously, it was demonstrated that thepore size and distribution, particle size and shape, bulkdensity, mechanical strength, and attrition resistanceof the catalyst particles were the main parametersto be considered in such selection [5]. In order tosynthesize the most active catalyst for oxidation ofH2S to elemental sulfur, catalysts containing vanadiumoxide were also utilized both with a stoichiometricand an excess amount of oxygen [6-8]. VBiOx/SiO2was used to improve a long-term deactivation problemnoticed in V/SiO2; however, it needed a dehydrationstep before being sent to a catalytic reactor to increasethe H2S conversion [7]. It was suggested that theformation of less active forms of vanadium, such asvanadyl sulfate, was one cause of such deactivation.Yet, various binary oxides, such as V-Sb, V-Mo, V-Mg,V-Bi, Bi-Mo, and Fe-Sn, were tested in excess oxygenwithout water by some researchers [7,8]. Recently,a TiO2/SiO2 catalyst with stoichiometric amounts ofoxygen demonstrated good activity [3]. The Ce-Vmixed oxide with a V/Ce atomic ratio of one and withthe CeVO4 crystal structure provided very high sulfurselectivity values (i.e. close to one) at almost completeconversion of H2S, for O2/H2S molar ratios close to0.5 [9]. Moreover, Fe supported on silicon carbide(�-SiC) showed to be a highly e�cient catalyst forthe oxidation of H2S into elemental sulfur at reactiontemperatures above the sulfur dew point [10].

On the other hand, a dry catalytic process wasdeveloped for the selective catalytic oxidation of H2Sto elemental sulfur. The commercially developed cat-alysts for this purpose were of the titanium-based ma-terials in the mobil direct oxidation process (MODOP)and of the iron-based ones in the Super Claus pro-cess [11,12]. The vanadium-based mixed oxides [13]and NaX-WO3 mixtures [14] were also utilized as cata-lysts for the oxidation of H2S to elemental sulfur. Thecatalytic performance of vanadia-supported zirconia-pillared clay for the selective catalytic oxidation of

H2S was also reported [15,16]. Besides, the catalyticperformance of Fe-pillared clay and vanadia loadedFe-pillared clay at temperatures ranging between 220-300�C for the selective catalytic oxidation of H2S wasstudied [17]. The aforementioned catalysts exhibitedvery good performance in H2S oxidation without muchSO2 emission. Furthermore, the H2S conversion in-creased with enhanced vanadia content up to 7 wt%.This superior catalytic behavior was related to the uni-form dispersion of vanadia species provided upon thesupport [17]. The e�ect of temperature (in the rangeof 150-250�C), Gas Hourly Space Velocity (GHSV) (of2000-4000 h�1), and sodium: cadmium weight ratio(of 1-5) over synthesized catalysts of sodium silicateand cadmium oxide supported on silicon carbide nano-powders, on the conversion of H2S to elemental sulfur,was studied by the Box-Behnken experimental designmethod. Analysis of the experimental data basedon the above three e�ective parameters showed thattemperature had the most pronounced e�ect as anindependent variable on the H2S conversion [18].

On the other hand, the direct and selective partialH2S oxidation to elemental sulfur at low temperaturemight have been an interesting alternative to thetraditional methods of H2S abatement if an active andselective catalyst was available. The di�erent typeof vanadium-based catalysts supported on the metaloxide (V2O5-TiO2, V2O5-CeO2, V2O5-CuFe2O4) wasperformed in the temperature range of 50-250�C aimingat high H2S conversion and low SO2 selectivity provedthat V2O5/CeO2 was a very solid candidate. Based onthese results, further investigations into V2O5/CeO2catalyst were performed to study the inuence of theinlet concentration of H2S (of 250-1000 ppm), GHSV(of 15000-45000 h�1), and molar feed ratio (of O2/H2S= 0.4-0.5) in order to minimize the selectivity of thesystem towards SO2. The results relevant to the e�ectsof GHSV and the inlet concentrations of H2S showedthat these factors did not signi�cantly reduce theSO2 selectivity. Nonetheless, results in terms of SO2selectivity obtained with a sub-stoichiometric feed ratio(O2/H2S = 0.4) revealed drastically reduced valuesfrom 13 to 4% without any signi�cant lowering of theH2S or O2 conversions [19].

In the Claus process, alumina (Al2O3) and/or ti-tania (TiO2) were employed as the support [3] while Fe,Co, and Ni were usually the catalytic metals depositedon the aforementioned supports [20,21]. Moreover,alumina was the most common support in conversionof H2S to sulfur. Its strength laid in its potentialto disperse active metal catalyst. Indeed, numerousreactions took place between alumina and metal oxidein transition state and some of the produced specieswere too resistant against sul�dation. This was con-sidered to be a drawback since it lowered activityof the catalyst [22]. H2S oxidation, being highly

1162 M. Rezaee et al./Scientia Iranica, Transactions C: Chemistry and ... 23 (2016) 1160{1174

exothermic, caused a major temperature elevation inthe catalytic bed, in turn, leading to troublesomehot spots considering the low thermal conductivity ofalumina [3]. Ultimately, high temperatures favoredthe reverse reaction lowering the conversion of H2Sto elemental sulfur. Tungsten sul�de catalysts weredecorated on single and multi-wall carbon anotubes(SWCNTs & MWCNTs) and activated carbon wassynthesized, characterized, and employed to achievethe conversion of H2S to elemental sulfur. In thisregard, a gas ow, containing 5000 ppm of H2S passedover the catalyst under the GHSV of 5000 h�1, tem-perature of 65�C, steam volume percent of 20, andO2/H2S ratio equal to 2 as the operating conditions,was utilized. The results revealed that the catalystsupported on MWCNTs exhibited higher conversionthan its counterparts. Moreover, the e�ects of GHSV,steam volume percent in the feed, catalyst loading, andtemperature for conversion of H2S to elemental sulfurover tungsten sul�de catalyst decorated on MWCNTswere investigated [23].

In the present study, several nanocatalysts weresynthesized and their activities were determined.Then, respective values were compared with those ofthe commercial Claus catalysts. Nano alumina parti-cles were prepared through three methods, includingwet chemical, co-precipitation, and spray pyrolysis.In the wet chemical technique, the optimum load-ing of sodium oxide was determined based upon theBox-Behnken (i.e. the Response Surface Methodology(RSM)) technique. In the spray pyrolysis method, asetup based on the needs of the aforementioned statis-tical design of experiments was constructed, throughwhich the best alumina-based catalyst with the highestBET surface area for the H2S oxidation was put totest. Beside these, the TiO2 nanoparticles (throughhydrolysis and peptization of titanium isopropoxidemethod [24]) as well as nano hybrid alumina withMWCNT were synthesized and tested in the H2Soxidation setup. Ultimately, it is reiterated that themain goal of the current research was to synthesizenew alumina-based nano materials and determine themost active resulting catalyst(s) for the H2S oxidationto elemental sulfur.

2. Characterization techniques

The prepared catalysts were characterized by the SEM,XRD, and ASAP analyzer techniques. Scanning Elec-tron Microscopy (SEM) images were obtained witha Philips, XL30 device. Gold meshes were utilizedas conductive materials for sample holding. X-RayDi�raction (XRD) measurements were conducted usingstandard powder di�raction procedure carried out witha Philips Di�ractometer (PW-1840, Lump Cu-k�, � =1:54 �A). The pore size and surface area measurements

were performed through a Micromeritics ASAP 2010instrument by adsorption of nitrogen at 77 K. The sul-fur content was determined through a high temperaturemeasurement using IR detector and UOP 864 methodwith a LECO CS600. H2S concentration was analyzedusing a Mettler potentiometer (DL 70 ES) with anaccuracy of �1% of �1 mL equipped with an Ag-Ag2Selectrode (DM 141-SC) and the UOP-163 methods withdetection threshold of 0.5 ppm. Product stream waspassed through a 25 wt.% KOH solution at regular timeintervals of 10 min to measure its H2S content.

3. Catalyst preparation

Several nanocatalysts were synthesized and each onewas tested through the H2S oxidation setup designedand constructed in this research to �nd the most activematerial. The methods of nanocatalyst preparationand the performed characterizations are described be-low.

3.1. Preparation of nano aluminaThe nano alumina was prepared through three meth-ods. First, in the wet chemical method at 55�C, 5.0 gof AlCl3.6H2O (97% supplied by the Aldrich Chemical)was dissolved into 325 mL of distilled water. A mixtureof 5.0 g of ammonia and 2.0 g of sorbitol was added,dropwise, to the �rst solution. The sorbitol acted assurfactant, utilized to accelerate the precipitation. Thedesired pH value of 10-10.5 for the solution was reachedthrough adding NH4OH to the mixture in order toobserve a white precipitate. Then, temperature ofthis solution rose to 70�C under vigorous stirringand insulation with aluminum foil. The resultingprecipitate was left under vigorous agitation for 10 minand then �ltered and washed to get neutralized. Thisprecipitate was washed with a solution of 100 mLethanol and 100 mL distilled water. It was thendried at 60�C for 24 h and calcined at 600�C for4 h to get the -Al2O3. The XRD pattern and SEMimages of materials made through this method werepresented in a previous publication of this researchteam [25]. It was inferred that the prepared particleswere almost spherical, smooth, non-aggregated, andrelatively uniform in size, being of nano-scale (i.e.< 100 nm).

Second, in the co-precipitation method, 5.0 g ofAlCl3.6H2O was dissolved into 325 mL of distilledwater at 55�C. Then, according to stoichiometry ofsodium carbonate with AlCl3.6H2O, based on Eq. (3),3.3 g of sodium carbonate was dissolved into theminimum amount of distilled water at 50�C. Next,sorbitol was added to this solution and the resultingmixture was added, dropwise, to the �rst solution.Afterward, temperature of the �nal solution rose to70�C and it was kept there for 10 min under vigorous


Figure 1. The XRD patterns of the prepared aluminananoparticles through the co-precipitation method.

stirring while being insulated with the aluminum foil.Next, it was �ltered and washed such that its pH wasadjusted to a value of 8-9. Then, the precipitate wasdried at 60�C for 24 h and calcined at 600�C for 4 h toultimately result in the -Al2O3 material.

2AlCl3:6H2O + 3Na2CO3 ! 6NaCl + Al2(CO3)3+ 6H2O: (3)

The powder XRD results obtained for these Al2O3nanoparticles are presented in Figure 1. Two peaksat 2� = 47� and 2� = 67�68� were observed. The �rstpeak was related to the sodium oxide (Na2O). However,since most of the sodium was washed away during thealumina synthesis, its peak was rather weak, while thesecond peak, related to -Al2O3, was more pronounced.

Another technique, through which the aluminananoparticle was synthesized, was the spray pyrolysistechnique. Spray pyrolysis technique was based on theultrasonic generation of micro-sized aerosol dropletsand their decomposition at intermediate temperatures(400-800�C). Due to the fact that evaporation, pre-cipitation, drying, and decomposition occurred in adispersed phase and at a single step, it became pos-sible to control important particle properties such assize, morphology, and chemical composition simply bycontrolling the process parameters. Such parametersincluded the residence time and decomposition tem-perature. Unlike many other �lm deposition tech-niques, spray pyrolysis represented a very simple andrelatively cost-e�ective processing method [26-28]. Inthe present set-up, a pressure nozzle and a staticmixer were utilized for generation of micrometric-sizeaerosol droplets. The spray pyrolysis experimentalset-up is shown in Figure 2. In all experiments,two equimolar solutions of Al(NO3)3.9H2O, (99.997%purity, supplied by the Aldrich Chemicals) and sodiumcarbonate (Na2CO3 99.99%, supplied by the MerckChemicals) were mixed in the static mixer and thenthrough using a pressure nozzle sprayed into the reac-tor. Nitrogen was used as a carrier gas with pressurekept at 2 bar. The particles were water-collected atthe reactor exhaust. The XRD pattern of the as-

Figure 2. Schematic representation of the spray pyrolysistechnique for synthesizing alumina nanoparticles.

prepared catalyst was revealed in a previous workby the authors [25]. The XRD pattern of the bestsample of the Al2O3 nanoparticles, produced throughthis method, possessed the highest BET surface area.This was provided in third row of Table 1. Once again,in previous investigation by this research group intothis material [25], a weak peak at 2� = 47� indicatedthe existence of sodium oxide, while the next peak at2� = 67 � 68� was related to -Al2O3, the materialsought in this work.

3.2. Preparation of titanium dioxideTitanium oxide nanoparticles were prepared by hydrol-ysis and peptization of titanium isopropoxide (TTIP)solution. In this method, 5 mL of TTIP (97% puritysupplied by the Aldrich Chemical) was dissolved in15 mL of isopropanol (99% purity provided by theMerck Chemicals) without heating. Then, the pH of250 mL distilled water was reduced by the concentratednitric acid solution to a value of about 2 and wasadded dropwise to the �rst solution under vigorousagitation in order to produce a gel. In other words,a turbid solution due to hydrolysis of the TTIP wasobtained. This turbid solution was heated to 60-70�C for almost 18-20 h when peptization occurred.After the peptization process, the volume of solutiondecreased to 50 mL and an opaque suspension wasobtained. The obtained precipitation was washed(to prevent agglomeration) with ethanol, dried in avacuum system for several hours at 100�C and calcinedat 200�C for 2 h to get anatase phase of titaniumoxide. It should be noted that the anatase phase wasformed at 200�C while rutile phase formed at 800�Cand the phase transition occurred at 600�C. Furtherdetails of the hydrolysis method for producing TiO2nanoparticles were available in the open literature [24].The powder XRD and SEM of this material arepresented in Figures 3 and 4, respectively. The XRDresults obtained for the TiO2 nanoparticles revealed apeak at 2� = 54 � 55� displaying highly crystallinematerial while emphasizing anatase phase formation.The SEM micrograph recon�rmed the nanostructureof the obtained TiO2.


Table 1. Design matrix and experimental data of the alumina synthesis with spray pyrolysis method used in this study.

Experimentno.

Independent parameters Dependent parameter

A: Temperature(�C)

B: Solutionconcentration

(mol/L)

C: Flowrate(mL/min)

Response:aBET

(m2/g)1 650 0.05 110 3692 500 0.15 110 1883 650 0.1 70 3954 650 0.1 150 2155 575 0.05 150 2466 500 0.05 110 1287 575 0.1 110 1908 500 0.1 150 2109 575 0.15 70 22210 575 0.05 70 23311 500 0.1 70 19912 650 0.15 110 16013 575 0.1 110 19514 575 0.15 150 203

Figure 3. The XRD pattern of the prepared titaniumoxide nanoparticle.

3.3. Preparation of hybrid MWCNT andalumina

The MWCNT prepared over the Co-Mo/MgO catalystswas previously prepared by this research group. Thiswas done through di�erent organic additives using amodi�ed sol-gel method through a Chemical VaporDeposition (CVD) reactor [29]. The same materialswere also utilized in the present research. The carbonnanotubes (CNT) used in this work were meshed in therange of 100 to 120 (i.e. 125 to 149 �m).

For synthesizing Al2O3-CNT with Al2O3 to CNTweight ratio equal to one, the alumina nano-particlecatalysts were prepared by the wet chemical method.Then, 1.1 g of CNT (i.e., based upon calculatingthe mass of alumina in the AlCl3.6H2O material),synthesized at the �nal stage of preparation, was addedto the solution. Next, temperature of solution was

Figure 4. The SEM micrograph of the synthesizedtitanium oxide nanoparticle.

raised to 70�C under vigorous stirring while beinginsulated with aluminum foil. The resulting precipitatewas left under vigorous agitation for 10 min, then�ltered and washed to get neutralized. Then, 100 mLof distilled water was mixed with 100 mL of ethanoland simultaneously poured over the precipitate. Theresulting material was dried at 60�C for 24 h andcalcined at 450�C for 4 h to produce the desiredcatalyst. At temperatures greater than 450�C, theCNT was oxidized; hence, the calcination occurredunder a neutral atmosphere. Figures 5 and 6 demon-strate the powder XRD pattern and SEM image ofAl2O3-MWCNT nanohybrid particles. Several peaks,especially at 2� = 40 � 41�, were observed in thispattern indicating existence of the MWCNT as wellas the amorphous alumina. From the SEM micrographof Figure 6, the mean diameter of these nanoparticles


Figure 5. The powder XRD pattern of the preparedhybrid of Al2O3-MWCNT in this work.

Figure 6. The SEM micrograph of the prepared hybrid ofAl2O3-MWCNT in this research.

was estimated to be lower than 100 nm, which was ingood agreement with the previously determined XRDresults.

4. H2S experimental rig

A schematic ow diagram of the catalytic oxidationprocess is shown in Figure 7. H2S oxidation was car-ried out under isothermal condition and atmosphericpressure. The �xed-bed reactor was constructed by astainless steel-316 tube of 12.5 mm ID and 450 mmlength. The gas mixture was passed downwardsthrough the catalytic bed. The reactor was verticallyplaced in an electrical furnace of which the temperaturewas controlled by a thermal indicator. The ow ratesof the gases (i.e. O2, He, H2S) were monitored andcontrolled with Bronkhorst mass ow meters linked toJUMO (d-TRON 304) electronic control units. Thecatalyst was heated from room temperature to thereaction temperature with slope of 5�C/min in thepresence of helium. At this point, the reactant started

owing into the reactor.

5. Results and discussion

RSM is considered to be a practical and easy way tooptimize a given process based upon the experimentaldata [30,31]. In the current study, Design ExpertsSoftware (Version 8.0.1) was employed for designing ex-periments, mathematical modeling, and optimization.To obtain rational results by the RSM, three stepsought to be considered. The �rst one, in a successfulRSM optimization, is to design the experiments forevaluating the model parameters, e�ciently. Thesecond step is to develop a polynomial model �tted tothe experimental data through regression and then tocheck suitability of the model by applying a statisticaltest (e.g., lack-of-�t, F -test) [31-33]. The third and�nal stage is to determine the value of factors satisfyingthe optimum conditions. Usually, a �rst- or second-order polynomial might be used for the RSM analysis.Furthermore, for responses possessing a curvature, asecond-order polynomial is preferred. The linear formof such polynomial would be as follows:

y = �0 +kXi=1

�ixi +kXi=1

kXj=1i


Figure 7. Schematic diagram of the H2S oxidation rig utilized in the present study.

limited to 14. Hence, 14 tests, according to Table 1,were performed and for each one, the BET surfacearea was measured.

E�ects of the following process variables on thesynthesis of alumina nanoparticles were investigatedthrough the aforementioned design study: (a) tem-perature, (b) concentration of solution, and (c) the

ow rate. The �xed levels of these three variables areprovided in Table 2. The analysis of variance (ANOVA)in the synthesis of alumina nanoparticles is summarizedin Table 3. The F -test statistics were de�ned as F

Table 2. Factors and levels for the Box-Behnken design.

Factor Variable Level

A Temperature (�C) 500 575 650

B Solution concentration(mol/L)

0.05 0.10 0.15

C Flowrate (mL/min) 70 110 150

Table 3. ANOVA for the synthesis of aluminananoparticle by the experimental design.

Factor DF SS MS F P

A 1 21424.5 21424.5 13.80 0.008B 1 5151.1 5151.1 3.31 0.112C 1 3828.1 3828.1 2.46 0.161

AB 1 18090.3 18090.2 11.63 0.011AC 1 9120.3 9120.2 5.86 0.046BC 1 256.0 256.0 0.16 0.697

Error 12.5Total 68762.4

= MSF/MSE, in which the MSF and MSE were themean squares of factors (or interactions) and errors,respectively. If the calculated value of F was greaterthan the value in the F -test at a speci�ed probabilitylevel, a statistically signi�cant factor or interactionwas determined. From a combination of estimatesfor the process variables and the ANOVA results, alinear model with statistical signi�cance might hadbeen generated. The related model in this research,quantitatively elucidating the e�ects of process vari-ables with statistical signi�cance, was presented asfollows:

Surface area =� 2142:53 + 4:23A� 17:93AB� 0:02AC: (5)

The statistical importance of the generated model wasevaluated via the Fisher test (i.e. F -test) and quanti�edby dividing the model mean square by its residualmean square for ANOVA. The results of ANOVA arepresented in Table 4.

Based upon the F value presented in Table 4,being greater than 2.7, and the adequate precisionvalue of 8.5 (being greater than 4), the proposed modelwas determined to be adequately satisfactory over morethan 95% of the signi�cant data compared to thoseavailable in the open literature [36]. As may be seenfrom Table 4 for the response, the p-value related tothe lack of �t test was smaller than 0.05, emphasizingthis to be signi�cant as well.

The 3-D curves related to the impact of theindependent variables of temperature, concentration,and ow rate were demonstrated on the BET surfacearea in Figure 8(a)-(c). As shown in Figure 8(a),


Figure 8. 3-D plot e�ects of temperature, concentration, and ow rate on the speci�c surface area.

Table 4. ANOVA for the linear model proposed in thepresent work.

Parameters of the model Value

Sum of Squares (SS) 57870.3Degree of Freedom (DF) 6.0Mean Square (MS) 9645.0F value 6.2p-value probability > F 0.02Standard deviation 39.5Mean 225.2Coe�cient of Variation (CV) 17.5R2 0.8Adj.-R2 0.7Pre.-R2 0.3Adequate precision 8.5

enhancing the temperature, at lower ow rates, re-sulted in increased speci�c surface area. However,at higher ow rates, lowering of the speci�c surfacearea was the outcome. On the other hand, at a �xedconcentration, with increasing the ow rate, the e�ectof temperature on the speci�c surface area was lesspronounced; whereas at lower ow rates, the e�ect

of temperature on the speci�c area was signi�cant.Figure 8(a) also indicates the e�ects of ow rate onspeci�c surface area as a function of temperature.From this �gure, it is observed that increasing the

ow rate at higher temperatures leads to a decreasedspeci�c surface area since the feed residence time in thereactor is lowered. As such, the reaction is renderedincomplete, resulting in a lowered speci�c surface area.

Figure 8(b) demonstrates that with increased con-centration at higher temperatures, the speci�c surfacearea decreases due to coagulation of the particles, lead-ing to occurrence of the major extent of the reactionin the bulk of the solution. Furthermore, from this�gure, it is seen that the inuence of concentration,as the temperature swings from lower to higher values,becomes considerably less pronounced. Finally, Fig-ure 8(c) reveals that the concentration and ow ratedo not have any sensitive interactions.

From Figure 8(a)-(c), it is observed that athigher temperatures, increasing the concentration val-ues causes the reduction of the surface area and viceversa. As seen from Figure 8(b), at higher tem-peratures, increased concentration lowers the surfacearea of alumina particles while at lower temperatures,increased concentration enhances the BET surface areaof the alumina particles. This emphasizes that there


was an optimum concentration for the purpose athand.

5.2. Screening the parameter a�ectingsynthesized nano-alumina catalysts

Based on previous studies [9,37-39] undertaking theH2S removal, the optimization process variables in thepresent work included: (i) temperature, (ii) metal load-ing, and (iii) GHSV. They were varied in the ranges of200-300�C, 0.2-1.6%, and 5500-7500 h�1, respectively.In all the experiments, concentration of H2S in feedgas was �xed at 6.6%, equivalent to concentration ofH2S at the �rst Claus convertor. Moreover, O2/H2Sratio equaled to the stoichiometric value (O2/H2S =0.5) in all runs. In all experiments, 6 mL of catalystwith mesh of 80-200 �m was used. Steady stateconversion was obtained after 6 h of experiment. Inorder to investigate the interactions, the three-levelBox-Behnken design with the abovementioned threekey variables was developed. Therefore, once again,the total runs were limited to 14. The �xed levels ofthe three operating variables are provided in Table 5.The observation of the fractional conversion of H2S

Table 5. Factors and levels for the 3-level Box-Behnkendesigned in this work.

Factor Variables Level-1 0 1

A Temperature (�C) 200 250 300B Metal loading (%) 0.2 0.9 1.6C GHSV (h�1) 5500 6500 7500

to sulfur through the design matrix is revealed inTable 6.

Data of the fractional conversion of H2S to sulfuris shown in Table 6. It was subjected to regressionanalysis to estimate the e�ects of process variables.The ANOVA results of the fractional conversion of H2Sto sulfur are summarized in Table 7. If the calculatedvalue of F was greater than that obtained in Table 7, ata speci�ed probability level, a statistically signi�cantfactor or interaction was obtained. In the presentcase, A, C, AB, AC, A2 and C2 were determinedas signi�cant model terms. From a combination ofestimates for the process variables and the ANOVAresults, a quadratic polynomial model with statistical

Table 7. ANOVA for the fractional conversion of H2S tosulfur through the experimental design.

Factor DF SS MS F

A 1 248.55 248.55 63.34B 1 1.37 1.37 0.35C 1 87.77 87.77 22.37

AB 1 182.25 182.25 46.44AC 1 225 225 57.33BC 1 23.52 23.52 5.99A2 1 147.42 147.42 37.57B2 1 19.4 19.4 4.94C2 1 33.02 33.02 8.42

Error 2Total 921.68

Table 6. Design matrix for the fractional conversion of H2S to elemental sulfur by the experimental design in this research.

Run no.Independent variables Dependent variable

A: Temperature(�C)

B: Metalloading

(%)

C: GHSV(h�1)

Response:Conversion(mol. %)

1 250 0.2 5500 94.82 250 0.9 6500 973 250 0.9 7500 814 200 0.9 7500 715 200 0.2 5500 966 250 1.6 7500 907 250 0.2 5500 95.58 300 0.9 6500 999 250 0.2 6500 9510 200 0.9 6500 7611 300 0.9 7500 9112 300 0.9 5500 8613 200 1.6 6500 8814 300 1.6 6500 84


Figure 9. Main e�ects of (a) reaction temperature, (b) loading, and (c) GHSV on the conversion of H2S to sulfur.

signi�cance was generated. This model quantitativelyelucidated the e�ects of process variables with statisti-cal signi�cance as follows:

Fractional conversion of H2S = �17:77290+ 0:62857� T + 2:46786� 10�3 �GHSV� 0:19286� T � Loading + 1:5� 10�4 � T�GHSV� 3:46429� 10�3 � Loading�GHSV� 2:71500� 10�3 � T 2 � 5:02551� Loading2

� 3:21250� 10�6 �GHSV2: (6)In Eq. (6), the terms without statistical signi�cancewere deleted from the full e�ect model, based upon theANOVA. These e�ects were considered to be errors inthe experiments and their variances were accordinglypulled into the Sum of Squares of Errors (i.e. SSE).Therefore, the multiple correlation coe�cient squared,R2 = 1-(SSE/SST), equal to 0.983, indicated a verygood �tting to the experimental data of fractionalconversion on factors A, B, and C (Table 8). The main

Table 8. The ANOVA for the proposed model.

Parameters of the model Value

Sum of Squares (SS) 905.99Degree of Freedom (DF) 9Mean Square (MS) 100.67F value 25.65Standard deviation 1.98Coe�cient of Variation (CV) 2.23R2 0.983Adj.-R2 0.945Pre.-R2 0.754Adequate precision 17.067

e�ects (i.e. A, B, and C) and two-factor interactionse�ects (i.e. AB and AC) for fractional conversion ofH2S to sulfur are shown in Figure 9, respectively. Here,the two upper and lower limits of conversion, as afunction of dependent variables, were emphasized whilethe behavioral trends between them were predictedthrough the presented software.

From Figure 9(a), the fractional conversion of H2Sto sulfur is increased by increasing temperature up toabout 265�C; however, it decreases when increasingtemperature from 265 to 300�C. At lower values, theenhancement of temperature leads to increase in therate of reaction; hence, the redox ability of the catalystrises. This causes fractional conversion of H2S toelemental sulfur to increase as well. Of course, someSO2 formation during the reaction predicted throughEqs. (7) and (8) is possible:

1n

Sn + O2 ! SO2; (7)2H2S + 3O2 ! 2SO2 + 2H2O: (8)

Nonetheless, it is obvious that the SO2 formationcaused a decrease in the sulfur selectivity; because ofusing stoichiometric ratio of O2/H2S, there was no SO2formation in the system, even at higher temperatures.This was another reason for decreasing conversion attemperatures higher than 265�C. Decrease in conver-sion of H2S to sulfur might have been partly dueto the changes in surface properties of Na2O/Al2O3nanocatalyst at a higher temperature. At temperaturesover 265�C, a very sharp decrease in H2S conversionwas observed within the �rst few minutes of the reac-tion, and then, the conversion values became stable.This sharp decrease was observed during the �rst fewminutes of the reaction, indicating a change in thesurface properties of the catalyst, including decreasein surface area, pore diameter, and volumes, as well asaggregation of the active sites at temperatures beyond265�C. From Figure 9(b), the fractional conversion of


Table 9. Porosimetry and surface area results of synthesized nanoparticles in this research.

Sample As (BET)(m2/g)

Total porevolume(cm3/g)

Mean porediameter

(nm)

Rp;peak(nm)

Al2O3 (wet chemical) 278.24 0.72 10.31 1.66

TiO2 169.34 0.13 3.11 1.22

Al2O3+CNT 268.00 0.35 4.65 1.66

Al2O3 (spray method) 395.00 0.74 11.52 1.62

H2S to sulfur is increased by increasing loading up toabout 0.9%, and from 0.9 to 1.6%, it is decreased.At lower loadings, the quantity of the active phasewas lower; however, by increasing the metal loading,the ability of the catalyst for oxidation of H2S be-came higher, meaning that the conversion to elementalsulfur was expected to rise. On the other hand,using nanocatalyst instead of conventional ones causedreactant to be trapped and also led to an increasein partial pressure of this species, in turn, enhancingoxidation of H2S to sulfur. Nevertheless, there wasthe possibility of occurrence of catalyst pore blockageat higher loadings; hence, H2S conversion lowered.Figure 9(c) reveals that the fractional conversion ofH2S to sulfur was increased by decreasing the GHSV.By increasing the GHSV, residence time decreasedsuch that enough time for complete reaction was notallowed; thus, the conversion decreased. Although bylowering the GHSV, conversion increased, in an indus-trial Claus process, due to the corrosiveness of H2S,indeed such operating conditions was not desirable.Thus, at a low temperature, high GHSV and low metalloading extent and high and desired conversion mightbe obtained.

It was clear from Table 7 that interactions ABand AC were more signi�cant than others con�rmedthrough the F -test value. It was interesting, however,to note that only the interaction AC was highly signi�-cant according to the F -test value. At constant GHSV,by increasing temperature, conversion enhanced. Fromthe above results and discussion, the key variablesa�ecting the fractional conversion of H2S to sulfurmight have been easily identi�ed by means of thestatistically experimental methodology.

For the present research, the optimum values werefound to be: temperature = 222�C, GHSV = 5591 h�1,and loading = 1.23 wt%. This set gave the highestpredicted H2S conversion of 99.5% for the optimizedresponse. It is revealed that the optimized temperaturevalue leaned toward its lower limit (in comparisonwith Claus conventional reactors) in order to boostconversion of H2S to sulfur. Furthermore, as an addedadvantage, optimizing the GHSV to such a high valuemight lead to alleviation of the corrosive e�ects of H2Son the mechanical instruments.

6. Comparison between the commercial Clauscatalyst and the synthesized nanocatalysts

From the ASAP analyzer outcome (see Table 9), onemight have concluded that the alumina nanoparticles,prepared through the wet chemical and spray pyrolysismethods, resulted in high BET surface areas andtotal pore volume as compared to other synthesizedmaterials in this study, therefore, leading to higherconversions to elemental sulfur. It is observed that adecrease in the total pore volume, synonymous to anincrease in the mass transfer resistance, ended up in aBET surface area reduction, which, in turn, resulted ina lowered reactivity of the nanocatalyst. Moreover, theresults of the ASAP analyzer revealed that the surfacearea of the used catalyst after the reaction at 300�Cwas 230 m2/g, being about 20% less than that of thefresh catalyst (i.e. it was 287.24 m2/g), emphasizingthe provided rationale.

An alumina-based (CR-3S) and a titania-basedClaus catalyst (CRS-31) were tested in H2S oxidationsetup at Claus operating conditions [40]. Speci�cationsof these Claus catalysts are provided in Table 10.Experiments were performed at temperature of 250�C,pressure of 1 atm, 12-14 mesh size of catalyst (1.3 mm),GHSV = 5500 h�1, O2/H2S = 0.5, and 6.6% H2S inthe feed stream. In addition, synthesized nanoparticleswere tested and fractional conversions were calculatedfor each one. Instantaneous fractional conversion ofH2S is de�ned as follows:

Conversion of H2S (%) =[H2S]in�[H2S]out

[H2S]in�100:

(9)

H2S concentration was calculated through utilizinga Mettler potentiometer with detection threshold of0.5 ppm:

ppm H2S =16000�NAg+ � VAg+

M; (10)

where, N+Ag was normality of AgNO3 solution; V+Ag, the

volume consumed; and M , the molecular weight of gassample.

Steady state conversion, demonstrated in Fig-ure 10, was obtained after 6 h for nanoparticles


Table 10. Commercial Claus catalyst speci�cations [40].

Physical properties Unit Typical valueCR-3S CRS-31

Bulk density kg/m3 660 1000Surface area (BET) m2/g 320 130Porosity (macro-porosity) mL/g 0.2 -

Chemical analysisAl2O3> 93:8% TiO2> 85%

Na2O 2000 ppm

Figure 10. The fractional conversion of H2S to elementalsulfur over synthesized nanocatalyst and commercial Clauscatalysts (reaction conditions: T = 250�C, P = 1 atm,12-14 mesh of catalyst (1.30 mm), GHSV = 5500h�1,O2/H2S = 0.5, and 6.6% H2S in the feed stream).

prepared in this research. It is reiterated that eachconversion to elemental sulfur was performed 3 timesand an average value for each point is presented inthis �gure. This emphasizes the reproducibility ofthe data. It was observed that, amongst materialsprepared in this study, alumina nanoparticles preparedthrough the wet chemical and spray pyrolysis methodshad higher activities and resulted in higher conversionsthan that of the commercial Claus catalyst. Becausethe reaction between H2S and O2 was irreversible, nothermodynamic restrictions existed; hence, conversionshigher than 95-97% (usually obtained for the commer-cial catalysts) were achieved.

Furthermore, the total sulfur analysis of theClaus commercial conventional catalyst and the syn-thesized nanocatalyst after reaction at 250�C, GHSV= 5500 h�1, O2/H2S = 0.5, and 6.6% H2S in the feedstream is summarized in Table 11. It is seen thatthe synthesized nanocatalyst in this research had moresulfur adsorbed on its surface. In industry, removal ofH2S and converting it into elemental sulfur are usually

Table 11. Total sulfur analysis of Claus commercialcatalyst and synthesized alumina nanoparticles with wetchemical method in this work.

Samplename

Weightpercent ofsulfur (%)

Commercial catalyst 3.02Alumina nanoparticlessynthesized with wetchemical method in this work

5.94

performed either by the thermal or catalytic techniquespossessing 3-5 steps. However, the system with cata-lysts developed in the current research included onlyone �xed bed reactor. It was shown to be capableof higher interactions of H2S on the surface of thedeveloped catalyst, resulting in higher conversions intoelemental sulfur.

Ultimately, the reactor test results along with theporosimetric properties of materials, understudied inthis work, made it clear that the larger pore volumesand higher surface areas were de�nitely desired forobtaining a better performing catalyst to produceelemental sulfur from H2S.

7. Conclusions

The successful application of the statistical design ap-proach to experiments for the optimization of oxidationof H2S was reported in this research. The wet chemical,co-precipitation, and spray pyrolysis methods wereselected for synthesis of the alumina nanostructuresutilized in this work. The results showed that theAl2O3 prepared with the wet chemical and spraypyrolysis methods provided e�ective catalysts for theoxidation of H2S to elemental sulfur under Clausoperating conditions. In other words, high values(> 98%) of the H2S conversion to elemental sulfur in areproducible manner were obtained for these materials.For the H2S removal, three factors a�ecting catalystperformances were identi�ed and optimized using theBox-Behnken design technique. It was demonstratedthat the factors of temperature and GHSV had more


pronounced e�ects on the conversion of H2S to ele-mental sulfur than others. Furthermore, a sodiumoxide supported alumina nanocatalyst provided totalsulfur conversion (near 100%) in the oxidation of H2Swith a feed stream containing stoichiometric amountsof oxygen and H2S.

Reaction tests and catalyst characterization usingXRD, SEM, and ASAP analyzer techniques showedthat the surface behavior of the nanocatalyst changedduring the reaction such that at higher tempera-tures, catalysts were deactivated perhaps due to fasterpoisoning attributed to the sul�de layer and sulfateformation on the surface of these materials. Moreover,it was shown that temperature was the most importantoperating variable such that at low to intermediatevalues, the highest conversion of H2S to elementalsulfur was reported. Ultimately, it was demonstratedthat utilizing catalysts developed through this study, asingle-stage �xed bed reactor was su�cient to provide ahigh conversion of H2S to elemental sulfur as comparedto the Claus conventional catalyst in a multistageprocess.

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Biographies

Mohammad Rezaee obtained his BS and MS de-grees in Chemical Engineering from the Departmentof Chemical and Petroleum Engineering at Sharif Uni-


versity of Technology, Tehran, Iran. His research in-terests include catalysts preparation, characterizationand evaluation in the �eld of chemical engineering.

Mohammad Kazemeini is professor of Chemical andPetroleum Engineering at Sharif University of Technol-ogy, Iran. His current research interests include nano-catalysts (mostly CNT and CNF based) preparation,characterization and evaluation in the �eld of chemicalengineering and petroleum re�ning.

Moslem Fattahi obtained his BS degree in ChemicalEngineering in 2006 from Razi University, Kerman-shah, Iran, and his MS and PhD degrees in 2008 and2013, respectively, from the Department of Chemicaland Petroleum Engineering at Sharif University ofTechnology, Tehran, Iran. His main research inter-

ests include process modeling, reaction engineering,catalysts preparation, characterization and evaluationin the �eld of chemical engineering and petroleumre�ning.

Ali Morad Rashidi is Associate Professor of Chem-ical Engineering at Research Institute of PetroleumIndustry (RIPI), Tehran, Iran. His main researchinterests include nano-materials (mostly CNT, CNF,and graphene based) preparation, characterization,and evaluation in the �eld of chemical engineering.

Leila Vafajoo is Associate Professor of ChemicalEngineering at Islamic Azad University, Tehran SouthBranch, Tehran, Iran. Her main research interests in-clude process modeling and environmental engineeringin the �eld of chemical engineering.

Documents

Oxidation of H2S to elemental sulfur over alumina-based ...scientiairanica.sharif.edu/article_3886_4a517a9b061a54ab52d9c032d73f3e3c.pdfb. Department of Chemical Engineering, Abadan