Effectiveness of Hydrogen Peroxide in H2S Removal by a Packed High SpecificSurface Area Bed Scrubber

G. Moussavi,a,* K. Naddafi,b A. Mesdaghinia,b and M. Mohsenica Department of Environmental Health, School of Medical Sciences,Tarbiat Modares University, Tehran, IranbDepartment of Environmental Health, School of Public Health,Tehran University of Medical Sciences, Tehran, IrancDepartment of Chemical and Biological Engineering,University of British Columbia, 2360 East Mall,Vancouver, BC V6T 1Z3, Canada

Removal of H2S from waste air streams was investigated in a chemical scrubberpacked with new low-cost and high specific surface area media and in the presence ofH2O2 in scrubbing liquid. The experimental conditions included superficial gas velocitiesof = 500 and = 840 m h1, inlet H2S volume fraction in the range of = 50 to250 106, scrubbing liquid flow rate ranging from QL = 1 to 10 L min

1 and liquid phasepH of 7 to 12. The results showed H2S removal efficiencies of = 97.5 % to more than = 99 % at liquid flow rate of QL = 5 L min

1 and pH of 10 to 11. Overall, it was deter-mined that the media tested and H2O2 can be used in scrubbers for efficient H2S removalwithout the possibility of forming any toxic byproducts.

Key words:Hydrogen sulfide, treatment methods, chemical scrubber, hydrogen peroxide

Introduction

Hydrogen sulfide (H2S) is a toxic, odorous, andcorrosive colorless gas that is released to the atmo-sphere from different industrial processes includingpetroleum refining, sour gas sweetening, pulp andpaper manufacturing, tanning and waste manage-ment facilities such as wastewater treatment andcomposting plants.1 Chemical scrubbing, physicaladsorption, electrochemical treatment, andbiofiltration techniques have been used to purifyH2S from waste gas streams.2,3 Chemical scrubbingis among the most popular and successful of thesetechnologies because it is very effective at remov-ing H2S and is flexible with respect to the use ofmany different scrubbing solutions.4 The mostwidely applied oxidant in the scrubbing solutionsfor the control of odorous compounds has been thevarious forms of chlorine.4 However, the use ofchlorine has a major drawback of producing chlori-nated byproducts such as halomethanes, which areknown toxics.5 To avoid forming these toxic by-products, research has been focused on finding anefficient surrogate oxidant. Hydrogen peroxide(H2O2), which is the strongest oxidant after O3, hasbeen considered a suitable replacement for chlorinecompounds. H2O2 is a very selective and clean oxi-dant that does not produce toxic and corrosive by-

products.6 In addition, it is relatively safe to handleand use.

According to eqs. (1) and (2), and dependingon the pH of the liquid phase, H2O2 oxidizes H2S tomolecular sulfur or sulfate.2

H2O2 + H2S S + 2 H2O pH < 8.5 (1)

4 H2O2 + H2S H2SO4 + 4 H2O pH > 8.5 (2)

The aim of this research was to investigate theeffectiveness of H2O2 as a surrogate oxidant to chlo-rine in a packed-bed chemical scrubber reactorpacked with low-cost and high specific surface areamedia for H2S removal.

Materials and methods

Experimental setup and operation procedure

Fig. 1 shows a schematic diagram of the over-all experimental setup. A bench scale packed-bedscrubber was constructed from Plexiglas and had adiameter of D = 13.5 cm and a total height of H =120 cm. The packing materials were low-cost highspecific surface area corrugated plastic tube parts.The total bed height and volume of scrubber wereD = 70 cm and V = 10 L, respectively. The porosityof the bed was 92 %. The scrubber bed medium hada specific surface area of S = 625 m2m3 which was

G. MOUSSAVI et al., Effectiveness of Hydrogen Peroxide in H2S Removal by a Packed , Chem. Biochem. Eng. Q. 22 (1) 914 (2008) 9

* Corresponding author: G. Moussavi, e-mail: moussavi@modares.ac.ir

Original scientific paperReceived: November 21, 2006Accepted: December 10, 2007

much more than that of the media commonly usedin scrubbers. Special characteristics of the appliedmedia were low density, low cost, high specific sur-face area, and corrosion resistance.

The scrubber operated in a counter- currentpattern with the polluted air and scrubbing liquidflowing up and downward, respectively. Air wassupplied by an air compressor, which was firstpassed through a GAC column to eliminate the oiland other impurities. The inlet air flow rate to thescrubber was controlled and monitored by a flowmeter and the provided valves. Pressure drop acrossthe scrubber bed was determined by a water ma-nometer. A perforated plate, placed in the middle ofthe bed, helped reduce the channeling of the fluidsthrough the bed.

Contaminated air stream with the desired frac-tion of H2S was created by blending the compressedclean air with a concentrated H2S stream from acustom-made H2S generator. The generation of H2Shas been described elsewhere.7 The H2S-laden gasstream was then passed through the scrubber whereH2S was absorbed in the scrubbing liquid flowingdownward and finally oxidized by H2O2. The re-moval of H2S in the scrubber was monitored bymeasuring the fractions of H2S in the inlet and out-let streams.

The section under the bed served as a liquidsump to hold the scrubbing liquid for recirculation.This liquid was recirculated from the sump by acentrifuge pump and sprayed on the top of the bedthrough a nozzle. The scrubbing liquid for each test

consisted of V = 5 L fresh tap water, H2O2, andNaOH. A dosing pump (Prominert B0415, GFG)was used to add the oxidant (H2O2) and pH buffer(NaOH), at a controlled flow rate. H2O2 mass con-centration in the spent scrubbing liquid was con-trolled around = 50 mg L1 to make sure thatenough oxidant was available in the liquid phasefor reaction with the absorbed H2S in the bed. Sev-eral pH ranging between 7 and 12 were tested dur-ing the investigation to find the optimum pH of thescrubbing liquid for the highest H2S removal.

Gas contact time in the bed, inlet H2S fraction,and scrubbing liquid flow rate and pH are generallyconsidered the most important operational factorsaffecting H2S removal efficiency. Therefore, the ef-fect of these quantities on the removal of H2S in thepresence of H2O2 was investigated. The rangestested for the selected quantities are, superficial gasvelocity between = 500 and 840 m h1, H2S frac-tion from = 50 to 250 106, scrubbing liquid ve-locity from = 4 to 40 m h1 and scrubbing liquidpH from 7 to 12. Under these conditions, gas con-tact times in the bed were between t = 5 and 3 s andspent liquid retention time in the sump was t = 5 to0.5 min. For each experimental condition, triplicateruns were conducted and the performance of thescrubber at removing H2S was calculated accordingto the average value of the experimental data.

Analytical methods

In each experiment, H2S concentration wasmeasured in samples simultaneously taken from theinlet and outlet sampling ports of the scrubber. H2Swas sampled by aspirating a measured volume ofthe air through an alkaline suspension of cadmiumhydroxide.8 Sampling set up consisted of twoimpingers, an air flow meter, an air vacuum pump,tubing, and valves. V = 50 mL of alkali absorptionsolution was pipetted into each impinger. To protectthe solution from light effect, the impingers werewrapped up entirely in aluminum foil. Sampling airflow rate was QL = 1 L min1 and sampling timewas in the range of t = 5 to 30 min depending onthe H2S concentration. After sampling, the impingersolutions were analyzed using spectrophotometer(Lambda 25, Perkin Elmer) at the wavelength of = 670 nm.

Samples of the liquid phase were taken fromthe scrubbing liquid recirculation line to measureH2O2, total sulfide, and pH. Liquid pH was deter-mined using a pH meter (E 520, Metrohm Herisau)and sulfide concentration in the spent scrubbingsolution was measured by the methylene bluemethod.9 H2O2 concentration was performed basedon the colorimetric titration using potassium per-manganate.10 The amount of H2O2 demand per unit

10 G. MOUSSAVI et al., Effectiveness of Hydrogen Peroxide in H2S Removal by a Packed , Chem. Biochem. Eng. Q. 22 (1) 914 (2008)

1 compressor, 2 GAC column, 3 H2S cylinder, 4 mixer, 5 recator sump, 6 scrubber bed, 7 perforated plate, 8 recycle liquidpump, 9 recycle liquid line, 10 nozzle, 11 oxidant solution con-tainer, 12 recycle liquid pump, 13 outlet air line, 14 monometer,15 sampling port, 16 waste liquid line, 17 waste liquid container

F i g . 1 Schematic drawing of the investigated packed bedscrubber

H2S oxidized was calculated from the H2S loading,the percent removal of H2S in the scrubber, and theexperiment time.

Results and discussion

The effects of superficial gas velocity, H2S in-let fraction, scrubbing liquid flow rate, and pH onH2S removal were investigated in different runs in-volving H2O2 as oxidant in the packed-bed scrub-ber. As a result, a range of operational conditionswas established in which the highest H2S removalefficiency in the presence of H2O2 in the scrubbingliquid was achieved.

Effect of scrubbing liquid pH on H2S removal

The effect of pH on the H2S removal efficiencyof the scrubber was studied over a range of 7 to12 and at superficial gas velocities of = 500 and840 m h1 and H2S inlet fractions of = 50 and250 106 each. According to eq. (2), the oxidationof H2S by H2O2 in alkaline pH produces sulfuricacid. So, to regulate and control the pH of the liquidphase at a specific value, NaOH solution was in-jected into the liquid recirculation line. Fig. 2 pres-ents the results of H2S removal efficiencies vs. pHat the inlet H2S fraction of = 50 106. With thepH of the scrubbing solution increasing from 7 to10, the removal of H2S increased from about 20 %and 30 % to more than 99 % at the air velocities of = 500 m h1 and 840 m h1, respectively. With fur-ther increase of the pH, the H2S removal efficiencywas unchanged.

Similar experiments were also carried out athigh H2S inlet fraction. Fig. 3 shows the effectof scrubbing liquid pH on H2S removal efficiencyat the inlet fraction of = 250 106 for the

above-mentioned gas velocities. It can be seen thatthe increase of pH from 7 to 10 resulted in increas-ing the removal efficiency from = 38 % to morethan = 99 %, and from = 27 % to more than =99 % at the superficial gas velocities of = 500 and840 m h1, respectively. As with the inlet fraction of = 50 106 (Fig. 2), when the inlet fraction was = 250 106, the scrubber performance at both gasvelocities was constant at pH greater than 10.Therefore, it is obvious that there is a minimumscrubbing liquid pH below which the H2S removalefficiency is highly pH-dependent. This behaviorcan be related to the changes occurring in the per-centage of H2S ionic form in a liquid as function ofits pH. The percentage of HS increases fromaround 50 at pH 7 to more than 99.9 in pH 10which results in the enhancement of the H2S solu-bility and so mass transfer between the gas and theliquid phases.

Fig. 3 also indicates that the removal efficiencyof H2S at pH 7 at superficial gas velocity of = 840m h1 is 11 % lower than that at = 500 m h1.Comparing Fig. 2 and 3 reveals that inlet H2S frac-tion does not affect the removal efficiency at super-ficial gas velocity of = 500 m h1, which is corre-sponding to t = 5 s contact time in the scrubber bed.However, the removal efficiency dropped from =40 % to 28 % at the superficial gas velocity of =840 m h1 when concentration was increased to =250 106. This can be due to the decreasing contacttime in the bed.

From the results of this part of the study it canbe inferred that the pH of the scrubbing liquid playsa vital role in H2S removal by scrubbers and thatthe removal efficiency of more than 99 % can bereached at high pH of at least 10. Charron et al.11

have reported H2S removal efficiency of around =97 % at pH of more than 9 for the scrubbing solu-tion in a chemical scrubber in which H2O2 was usedas oxidant.11 Given the results obtained in this re-search and those from the literature, it can be con-

G. MOUSSAVI et al., Effectiveness of Hydrogen Peroxide in H2S Removal by a Packed , Chem. Biochem. Eng. Q. 22 (1) 914 (2008) 11

F i g . 2 H2S removal efficiency vs. pH at inlet fraction of = 50 106

F i g . 3 H2S removal efficiency vs. liquid phase pH at inletfraction of = 250 106

cluded that the optimum liquid pH in the scrubbertreating H2S in the presence of H2O2 as oxidant is inrange of 10 to 11. Therefore, the subsequent experi-mental runs were carried out with adjusting the pHof scrubbing solution in this optimum range.

Effect of scrubbing liquid flow rateon H2S removal

The selection of the scrubbing liquid condi-tions is critical in designing an efficient scrubber. Inaddition to pH, the flow rate is another key parame-ter for scrubbing liquid, because absorption is di-rectly proportional to the amount of liquid exposedto the gas stream. In this regard, the effect of flowrate of the scrubbing liquid containing H2O2, inrange of QL = 1 to 10 L min1 at the optimum pHwas studied. These flow rates correspond to liquidvelocities of = 4 to 20 m h1 in the scrubber. Thisstep of the study was carried out at two different su-perficial gas velocities of = 500 and 840 m h1

and the inlet H2S fractions of = 50 and 250 106

for each velocity. Fig. 4 shows the removal effi-ciency of H2S at two gas velocities of = 500 and840 m h1 with = 50 106 H2S in the inlet air.The H2S percent removal increased proportionallywith the increase of the scrubbing liquid velocityfrom = 4 to 20 m h1 in both gas velocities tested.H2S removal efficiency increased from = 72 % tomore than 99 % and from 63 % to more than 99 %at gas velocities of = 500 and 840 m h1, respec-tively. Liquid velocities of greater than = 20 m h1

did not affect the performance of the scrubber forboth superficial gas velocities. These experimentswere repeated for the inlet H2S fraction of =250 106 to determine the combined effects of liq-uid flow rate and fraction on the H2S removal effi-ciency of the scrubber.

The results shown in Fig. 5 reveal that the H2Sremoval efficiency had the same trend vs. liquid ve-

locity. That is, the percent removal increased from = 69 % to around = 99 % at gas velocity of =500 m h1 and from = 59 % to about = 99 % atgas velocity of = 840 m h1, while scrubbing liq-uid velocity was increased from = 4 to 20 m h1.Therefore, the results indicate that the change in theliquid flow rate had direct effect on the scrubberperformance; suggesting that mass transfer resis-tance in the liquid phase is considerable under theseconditions. Wetted surface and thickness of the liq-uid film on the scrubber packing can be adjusted bymanipulating the liquid flow rate to achieve an opti-mum condition in a scrubber. Nonetheless, H2S re-moval efficiency for liquid velocities lower than =20 m h1 was around = 10 % lower at gas velocityof = 840 m h1 than that at = 500 m h1. Above = 20 m h1 liquid linear velocity did not affect theH2S removal efficiency in the scrubber. It is possi-ble that at liquid velocities above = 20 m h1,which is the minimum value to achieve high andconsistent removal efficiency, the gas to liquid masstransfer limitation due to wetting media ratio hasbeen overcome. According to these findings, effi-cient scrubbing of H2S, with H2O2 as oxidant, wasachieved with minimum scrubbing liquid velocityof = 20 m h1.

Some other researchers have also investigatedthe effect of liquid flow rate on scrubber perfor-mance for H2S removal. Charron et al.11 found noreal trend between the liquid to gas ratio in therange of 2.3 to 6.2, with respect to the removal effi-ciency of H2S in a chemical scrubber using H2O2 asthe oxidant.11 Chen et al.12 studied the effect of liq-uid to gas ratio in the range of 1.25 to 3.75 in ascrubber with NaOCl in liquid phase as oxidant.They concluded that the efficiency of H2S removalwas almost constant for the liquid to gas ratios stud-ied.12

12 G. MOUSSAVI et al., Effectiveness of Hydrogen Peroxide in H2S Removal by a Packed , Chem. Biochem. Eng. Q. 22 (1) 914 (2008)

F i g . 4 H2S removal efficiency vs. liquid flow rate at inletfraction of = 50 106

F i g . 5 Removal efficiency vs. liquid flow rate at H2S inletfraction of = 250 106

Effect of air flow rate and inlet fractionon H2S removal

From the engineering viewpoint, the air flowrate and pollutant concentration, as two main char-acteristics of waste air streams that are directlylinked to pollutant contact time and loading, playcritical roles on the reactor performance and thus,reactor sizing and design. On the other hand, H2Sremoval in an oxidative scrubber is a two-step pro-cess where H2S is first absorbed and then oxidizedin the liquid phase containing H2O2. So, the air ve-locity through the bed and the inlet concentrationare expected to affect the H2S mass transfer fromthe gas to the liquid phase, thereby influencing thescrubber performance. In this regard, the effect ofsuperficial gas velocity (air flow rate) and inlet con-centration on scrubber performance were investi-gated by applying two velocities of = 500 and 840m h1 under optimum conditions of scrubbing liquidand variable inlet H2S fractions ranging from =50 106 to = 250 106.

As shown in Fig. 6, H2S removal efficiencywas above = 99 % for all the inlet fractions at su-perficial gas velocity of = 500 m h1, correspond-ing to t = 5 s contact time in the scrubbing bed. Theresults indicate that the H2S removal efficiency inthe scrubber operating at t = 5 s contact time wasnot affected by the inlet fraction in the range of =50 106 to = 250 106. This high performancewas likely due to the use of media with high spe-cific surface area, optimized scrubbing liquid con-dition, and relatively long retention time of gas inthe scrubber bed.

Statistical analysis of the results presented inFig. 6 reveals that with a confidence limit of 95 %,superficial gas velocity (contact time) and inletfraction had no significant effect on performance ofthe scrubber under the operational conditions inves-tigated. Hence, it can be concluded that the scrub-

ber with the tested media can be used for efficienttreatment of gas streams containing H2S, achievinga high performance. In addition, increases in H2Sremoval efficiency with increasing the superficialair velocity and inlet fraction can be interpreted insuch a way that mass transfer from gas to the liquidphase is likely an important rate limiting step in theprocess. Chen et al.12 investigated the effect of gasmass flow rate on H2S removal efficiency at inletfraction around = 200 106 in a scrubber usingNaOCl as the oxidant. That study showed that theremoval efficiency dropped from = 99.2 % to95.1 % when gas mass flux increased from 3.859 103 to 1.157 104 kg m2 h1. Charron et al.11 alsoreported the removal efficiencies in range of =89 % to = 99 % in a scrubber using H2O2 at airflow rate of Qa = 300 m3 h1 and other given condi-tions.11 Compared to these reports, the developedscrubber in this study performed better and showedgreater efficiency.

The results of sulfide measurement in the spentscrubbing liquid samples taken from recirculationline indicated that sulfide concentration was verylow (i.e., undetectable) over the range of experi-mental runs at the optimum conditions. This obser-vation indicates that all the absorbed H2S was con-verted to the end product, which is sulfate. There-fore, it can be inferred that liquid retention time of1 min in the scrubber sump is high enough to oxi-dize the H2S absorbed. Complete oxidation of H2Sin the spent scrubbing liquid before recirculationwill overcome the problem of being stripped H2Sabsorbed in the recycle liquid to the outlet,otherwise removal efficiency will decrease.

Contribution of dissolved oxygen in oxidationand consumption of H2O2

Oxygen is an oxidant with the redox potentialof +0.28.13 As the air passes through the column,oxygen is dissolved into the scrubbing liquid.Hence, the dissolved oxygen (DO) may oxidize thesulfide in the liquid phase. To determine the contri-bution of DO in sulfide oxidation, a set of batch ex-periments was performed without H2O2 in the liquidphase that was saturated with oxygen. The result in-dicated that DO could only remove less than 5 % ofthe sulfide after t = 60 min reaction time in alkalinepH. Since, the contact time of the contaminated airin the scrubbers is much lower than this time; onecan conclude that the DO has no significant contri-bution in sulfide oxidation in this experimentalcondition.

The consumption of H2O2 during the H2S elim-ination was measured for the optimum operationalconditions, i.e., air flow rate of Qa = 200 L min1,liquid flow rate of QL = 5 L min1; pH 10 for the

G. MOUSSAVI et al., Effectiveness of Hydrogen Peroxide in H2S Removal by a Packed , Chem. Biochem. Eng. Q. 22 (1) 914 (2008) 13

F i g . 6 Removal efficiency vs. H2S inlet fraction at twodifferent superficial air velocities

scrubbing liquid, and inlet H2S fraction of =100 106. The measurements data showed that 4.9mole H2O2 was consumed per mole of oxidizedH2S. Despite being greater than the stoichiometricvalue (eq. (2)), this is close to what is reported inthe literature by Couvert et al.14

Conclusion

An experimental study was conducted on theeffectiveness of H2O2 as an oxidant for the removalof H2S in a chemical scrubber with a new packingmedium. The results have demonstrated that theH2O2 in the packed bed scrubber successfullytreated the air containing H2S in range of = 50 to250 106 at bed contact time of t = 3 s, with re-moval efficiencies between = 97.5 % and morethan = 99 %. To achieve this high performance inthe scrubber, minimum liquid flow rate and pHwere QL = 5 L min1 and 10, respectively. The mostimportant advantages of this chemical scrubber arehigh removal efficiency, no production of toxic by-products, and efficient low cost media. H2O2 is anoxidant as effective as NaOCl, with the advantageof being much safer and more environmentallyfriendly for application in scrubbers for H2S re-moval. In addition, corrugated tube parts are veryefficient media to increase contact surface of thegas and liquid phases where packed into scrubbers.

L i s t o f s y m b o l s

D diameter, cmH height, cmQa volume air flow rate, L min

1

QL volume liquid flow rate, L min1

S specific surface area, m2 m3

t time, s superficial gas velocity, m h1

V volume, L efficiency, % wavelength, nm volume fraction, 106

R e f e r e n c e s

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5. Gabriel, D., Deshusses, M. A., PANS 100 (2003) 6308.6. http://www.H2O2.com/intro/overvies/html7. Moussavi, G., Naddafi, K., Mesdaghinia, A., Deshusses,

M. A., Environ. Technol. 28 (2007) 987.8. Lodge, J. P., Methods of air sampling and analysis, 3rd edi-

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14. Couvert, A., Charron, I., Laplanche, A., Renner, C., Pat-ria, L., Requieme, B., Chem. Eng. Sci. 22 (2006) 7240.

14 G. MOUSSAVI et al., Effectiveness of Hydrogen Peroxide in H2S Removal by a Packed , Chem. Biochem. Eng. Q. 22 (1) 914 (2008)