Fuel Processing Technology 91 (2010) 777–782

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Fuel Processing Technology

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Removal of carbonyl sulfide at low temperature: Experiment and modeling

Wang Li a,⁎, Wang Shudong b, Yuan Quan b

a Research Institute of Industrial Catalysis, East China University of Science & Technology, Shanghai, 200237, Chinab Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China

⁎ Corresponding author. Tel./fax: +86 21 64253824.E-mail address: wangli@ecust.edu.cn (W. Li).

0378-3820/$ – see front matter. Crown Copyright © 20doi:10.1016/j.fuproc.2010.02.013

a b s t r a c t

a r t i c l e i n f o

Article history:Received 31 July 2009Received in revised form 10 February 2010Accepted 20 February 2010

Keywords:COSsimultaneous removalkinetic modelingdeactivation

A mathematic model of carbonyl sulfide (COS) removal at low temperature with fouling of catalyst has beendeveloped based on experimental results. Kinetic studies were conducted in a fixed bed reactor underatmospheric pressure and at low temperature (40-70 °C). Experimental results of breakthrough curves wereused to obtain kinetic parameters accounting for axial dispersion, external and internal mass-transferresistances as well as the sulphur deposition on inner-face of catalyst. Initial bulk porosity of particle (ɛP0),deactivation coefficient (α), sulfide deposition coefficient (β) were used to quantify the behavior of COSremoval at different operating conditions. Adsorption heat of H2O and activation energy of COS removal was21.5 and 62.4 kJ/mol respectively. The effects of flow rate, COS inlet concentration, temperature and relativehumidity(RH) were analyzed, and it was found that relative humidity carried a heavier weight thantemperature on εP0, α, β within our experimental conditions. The model agreed well with the experimentalbreakthrough curves and satisfactorily predicted the fixed-bed reactor performance, and this model can beused as a reliable tool for process design and scaling-up of similar system.

Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.

1. Introduction

Carbonyl sulfide (COS) is unique among the sulfur-containingimpurities in hydrocarbon feedstocks due to its odourless, tastelessand colourless characters. The presence of COS in industrial productstreams has always been an important consideration to chemicalengineers because the existence of COS will cause compliance-testingfailures, especially in liquefied petroleum gas (LPG) industry. Carbonylsulfide will hydrolyze to hydrogen sulfide (H2S), which will causecatalyst deactivation. It has been reported that only 4 mg sulfur pergram on the Fe-Cu-K catalyst would result in a loss of 50% catalystactivity [1]. Apart from affecting catalyst, H2S also lead to reactorcorrosion increased.

The conventional way to remove COS is hydrogenation andhydrolysis[2], COS can be reduced to H2S in the presence of hydrogenor water at 200-400 °C and under 30-50 bar. Subsequent processesincluding scrubbing or adsorption using activated carbon or zinc oxide(ZnO) is necessary to remove H2S generated from COS reduction/hydrolysis. Most of studies are all concentrated on COS removal atoperation temperature in excess of 100 °C, and operational cost andenergy consumption will be high due to high operation temperationfor COS removal and the extra H2S removal process.

Recently, there are extensive studies using mild reaction conditionsthat are more economical preferable in environmental protection point

10 Published by Elsevier B.V. All rig

of view. The studies can be divided into two part, one part is focusing onCOS hydrolysis at low temperature; and the other part is onsimultaneously removing both COS and H2S. West[3] found that theaddition ofNi and Zn can efficently promote COS hydrolysis onγ−Al2O3

at 30 °C, and compesation effect was found when Guo[4] studied COShydrolysis at low temperature ( 45 - 100 °C) in alkali metal oxides andalkali earthmetal oxides. Catalyst deactivation model of COS hydrolysisin presence of oxygen at ambient temperature was also studied byWang[5] After COS hydrolysis to H2S, H2S must be removed throughanother treatment; namely, two processes are required to remove COS.Anewmethod to removeCOSbya single processutilizingoneadsorbentor one catalyst at mild conditions was suggested. Miura[6] presented asingle COS removal process using an iron oxide catalyst at around 50 °C.During this process Fe2O3 was the catalyst for COS hydrolysis and theadsorbent for H2S removal. In another study, ZnO was used tosimultaneously remove both COS and H2S by Research TriangleInstitute[7]. Zinc oxide was the adsorbent for COS and H2S removal.Above way for simultaneously removing COS and H2S was throughcatalytic hydrolysis– adsoption or adsorption-asdorptionprocess,whilethe coupling catalytic hydrolysis – oxidation process would be moreefficently in simultaneously removing COS and H2S.

In our lab, simultaneous removal of COS and H2S was accom-plished by a bi-functional catalyst (Al-Na-Activated Carbon), bywhichCOS can be finally converted to sulphur (S) and deposited on support[8]. There are two different active sites on the support, hydrolysissites and oxidation sites; COS is hydrolyzed to H2S on the hydrolysissites, while H2S is oxidized to elemental S or sulfuric acid on theoxidation sites and then deposit on the porous support. With our

hts reserved.

778 W. Li et al. / Fuel Processing Technology 91 (2010) 777–782

bi-functional catalyst, H2S did not appear at all in the outlet gasand COS was the only detected compound when the catslyst wasbreakthrough. The whole process could be described as follow:COS→H2O H2S→O2 S = SO2−

4 ↓. Highly efficient COS removal (COS emis-sion on ppb lever) and high sulfur capacity up to 10%g/g cat. can beobtained with this bi-functional catalyst.

The deposition of elemental sulfur on catalyst continuouslydamages the catalyst effectiveness due to the catalyst texturechanging and the active site losing[9]. The S deposition may putthe whole reaction system into unsteady condition. In a study con-ducted by Kaliva the amount of sulfur formed (Ws) was calculated bygraphical integration of the breakthrough curves, and the reciprocalWs is plotted against the reciprocal time to obtain the initial reactionrate[10]. Although this study can acquire the initial reaction rate withempirical method, the behavior of catalyst fouling was not clearlydescribed in modeling point of view.

The deactivation model of H2S oxidation [11] and coupling carbondisulfide (CS2) removal [12,13] were also developed. The catalystdeactivating model used breakthrough curves to obtain kinetic pa-rameters accounting for axial dispersion, external and internal mass-transfer resistances as well as element S deposition on inner-face of thecatalyst. The deactivation function φ(ms)and deactivation coefficient αwere proposed and used to denote the function of S depositon (ms) onthe support, which means φ(ms) could present the decrease rate of thenumber of active sites. According to H2S oxidation and coupling CS2removal, it could be understood that the two different reaction rateexpression would cause different deactivation function form.

The study of two different catalyst system for coupling CS2 removalat two different temperature range, 85-125 °C and 30-70 °C respec-tively, was shown that the two deactivation function for CS2 removalwere in the same form, namelyφ(ms)=(1−αms)2. Although the designfor coupling removal of organic sulfide (CS2 and COS) was all throughhydrolysis-oxidation process and the intrinsic rate functions of hydro-lysis of CS2 were found to be best fit as the proposed rate functions ofCOS hydrolysis[14], further work were still needed to confirm whetherthe deactivation function expression was in the same form for couplingCS2 and COS removal. The reason why catalysts had different sulfurcapacity in S-compounds removal also need to be explained. In thispaper, the deactivation function expression for coupling COS removalwas studied and the influences of operating variables on the perfor-mance of coupling COS removal were also investigated.

2. Experiments

The catalyst (Al-Na-Activated Carbon) was prepared using aconventional incipient impregnation method. First, the commercialactivated carbon (AC) was dipped in a solution of 10(wt)% aluminumnitrate (Al(NO3)3), dried (100 °C, 2 h) and calcined in nitrogen (N2)atmosphere (450 °C, 2 h), followed by doping sodium carbonate(Na2CO3) and dried at ambient temperature.

The catalysts were tested in a fixed-bed reactor with a nitrogen gasspace velocity (SV) of 3450 h-1. Thewater concentration in the feedwasregulated by controlling the temperature of carrier gas (N2), in whichsaturated water vapor was provided all together. COS with concentra-tion from200 to1000 ppmwasdilutedwithN2. 3.7 gAl-Na-AC catalystswith particle diameter 1.5 mm were used in each experiment. Thereactor outlet gas was analyzed with a sulfur Micro-coulomb analyzer(WC-2, China) and a gas chromatograph (Varian 3800, USA) with apulsed flame photometric detector.

3. Mathematical model

In the modeling, the following assumptions were made:(a) the process is operated under isothermal conditions and the

packed porosity is constant along the longitudinal axis of the reactorbed; (b) An axial dispersion plug-flowmodel is adopted to account for

non-ideal flow in reactor bed; (c) Mass transfer between the bulkphase and the solid particle is described by the external-film masstransfer coefficient kf; (d) Intraparticle mass transfer is characterizedby the pore diffusion coefficient Dp.

3.1. Reactor model

The overall mass balance around the fixed bed can be expressed as

∂C∂t + u

∂C∂z = Dz

∂2C∂z2

+3kfRP

1−εbεb

Cp j r=RP−C

� �ð1Þ

The initial and Danckwerts boundary conditions at the both endsof the bed are given by the following equations

at: t = 0 C = 0 ð2Þ

at: t N 0 z = 0; Dz∂C∂z = uðC−CinÞ ð3Þ

z = L;∂C∂z = 0 ð4Þ

The Wakao-Funazkri correlation[15] was adopted to estimate thefilm mass transfer coefficient Dm and axial dispersion coefficient Dz:

Shp = 2 + 1:1Sc1 = 3Re0:6pεbDz

Dm= 20 + 0:5ScRep ð5Þ

where Shp and Rep are the Sherwood and Reynolds numbers relativeto particle and can be calculated as follow:

Shp = kf dp =Dm Rep = dpUρf = μf ð6Þ

Schmidt number (Sc) can be calculated according to:

Sc = μf = ρf Dm ð7Þ

The molecular diffusivity in multicomponent gas, Dm, wascalculated by the Fuller-Schettler-Gridding method:[16]

DAB =0:00143T1:75

PM1 = 2AB ∑VAð Þ1=3 + ∑VBð Þ1=3

h i2 ð8Þ

3.2. Intraparticle mass transfer

The mass transfer within a catalyst microsphere can be expressedwith the following reaction-diffusion equation:

∂ εpCp

� �∂t =

1r2

∂∂r r2Dp r; tð Þ ∂Cp

∂r

!−RCOS ð9Þ

The sulfide deposition in the particle was governed by

∂ms

∂t = RCOS ð10Þ

The random pore model[17] was used to estimate the porediffusion coefficient Dp, which is calculated by the followingequation:

Dp r; tð Þ = Dmε2p ð11Þ

where εp is given by

εpðr; tÞ = εp0−βmsðr; tÞ ð12Þ

Fig. 1. Experimental (dots) and modeled (line) outlet concentration of COS for differentinlet concentrations at T=40 °C and RH=12%.

779W. Li et al. / Fuel Processing Technology 91 (2010) 777–782

together with the initial conditions

at t = 0 ; Cp = 0; ms = 0 ð13Þ

The mass transfer boundary conditions on the surface of thecatalyst are expressed as

at t N 0 ; r = Rp; Dp∂Cp

∂r = kf ðC−Cp j r=RPÞ ð14Þ

The symmetry conditions at the center of theparticles are as follows:

r = 0;∂Cp

∂r = 0 ð14Þ

3.3. Kinetic model

Compared to COS hydrolysis, H2S oxidation is more easily occurreddue to its active character at ambient temperature. So it could besurmised that the rate of COS hydrolysis is the rate-determining stepduring this coupling process. In several experiments, differentconcentrations of COS and H2S were supplied simultaneously to thecatalyst bed. Although the H2S/COS ratio in the feedstockwas adjustedfrom 1 to 5, it was found that the removal efficiency of COS did notchange at all. Therefore, it can be concluded that there are differentactive sites on the catalyst. Since the sulfur capacity of H2S oxidationon the catalyst was 5 times than that of COS removal, it can be knownthat H2S oxidation rate was faster than that of COS removal. Thatexperiment results also confirmed that COS hydrolysis is the rate-determining step during COS coupling removal. Since sulfur capacityof COS removal with the catalyst is 10 times than that of COSadsorption capacity, hydrolysis-oxidation reaction is the key point forCOS removal and then the COS adsorption process can be excluded.

Since the rate of H2S oxidation is faster than that of COS hydrolysis,COS hydrolysis is hypothesized to be the rate-determining step in thecoupling removing COS, and so COS coupling removal rate would betaken as the rate of COS hydrolysis. West[18] studied COS hydrolysisat different temperature, and found that the reaction mechanismat low temperature ( 30–60 °C ) is contrasted with the previousstudies at higher temperatures. COS hydrolysis at ambient tem-perature proceeds by the reaction of adsorbed COS with a surfacehydroxyl species on γ -Al2O3. The surface hydroxyls are replenishedby adsorption of water on the surface; however, water adsorptionleads to significant inhibition of the reaction rate for the low tem-perature reaction. COS hydrolysis is observed to follow Langmuir-Hinshelwood kinetics[19]:

RCOS =kCOSCCOS

1 + KH2OCH2O

ϕ msð Þ ð15Þ

where φ(ms) is the deactivation function, expressed as follows:

φðmsÞ = 1= ð1 + αmsÞ2 ð16Þ

The method of finite volume was applied to discretize the spacevariables z and r of the partial differential Eqs. (1) and (8) , and theresulting ordinary differential equations were numerically integratedby Gear's method.

The values of εp0,α,β, kCOS andKH2Owere determined byminimizing

the difference between calculated and experimental outlet concentra-tions of COS, using the objective function of

ϕ = ∑Ne

i=1∑Ns

j=1ððCexp

out;j;i−Ccalout;j;iÞ=Cexp

in;i Þ2 ð17Þ

where Ne is the number of experimental runs, and Ns is the numberof samples per run. The BFGS algorithm was adopted to search forthe optimum fitting value of the parameters.

4. Results and discussion

4.1. Effects of temperature

The outlet COS over the time as a function of temperatures at 40,50, 60 and 70 °C was plotted in Figs. 1–4. When the temperaturesincreased from 40 °C to 70 °C, the breakthrough time for COS atalmost same concentration level (2468 ppm) increased from 0.2 h to4 h. During the coupling process for COS removal, the followingtwo reactions occur almost at the some time: COS+H2O→H2S, H2S+O2→S. COS hydrolysis is endothermic reaction while H2S oxidation isan exothermic reaction, but two reactions are highly thermodynam-ically favorable to product side. COS hydrolysis is more sensitive to thetemperature[20] than H2S oxidation is. The rate of COS hydrolysis isthe rate-determining step during this coupling process, so with theincreasing temperature COS hydrolysis rate increased and the COSremoval efficient did too.

When the inlet COS concentration increased from 2468 ppm to4562 ppm at 70 °C shown in Fig. 4 , the breakthrough curves becamesteeper which means more COS detected in the effluent stream. Witha lower inlet COS concentration, the amount of S deposited on thecatalyst was lesser which resulted in lower deactivation rate[21].

At high temperature and with lower inlet COS, the amounts ofsulfur and/or sulfate deposited on the catalyst was lesser whichresulted in a lower deactivation rate. For the faster initial reaction rate,the more sulfur and/or sulfate is deposited on the support, whichcauses a lower mass-transfer flux from the bulk to the particle, andfaster decrease of active sites.

4.2. Effects of relative humidity

Relative humidity (RH) on COS removal was conducted at 60 °Cplotted in Fig. 5. RH plays different functions on COS hydrolysis andH2S oxidation. COS hydrolysis proceeds by the reaction of adsorbedCOS with a surface hydroxyl species on γ -Al2O3. The surfacehydroxyls are replenished by adsorption of water on the surface;however, water adsorption leads to significant inhibition of the re-action rate for the low temperature reaction. However, more waterwould inhibit COS hydrolysis for the competitive adsorption betweenCOS and water on active sites, and adsorbed water can act as aninhibitor for the COS hydrolysis reaction simply by blocking access tothe active sites.

With the increase of RH in the feed the creation of water films onactivated carbon could be significantly enhanced and then some

Fig. 2. Experimental (dots) and modeled (line) outlet concentration of COS for differentinlet concentrations at T=50 °C and RH=12%. Fig. 4. Experimental (dots) and modeled (line) outlet concentration of COS for different

inlet concentrations at T=70 °C and RH=12%.

780 W. Li et al. / Fuel Processing Technology 91 (2010) 777–782

micropores were inaccessible which caused the change of bulk ofinitial bulk porosity of particle (εP0 ). The presence of a water filmenables the dissociation of H2S molecules to hydrogen sulfide ions HS-,which can then be oxidized to S[22]. The creation ofwater films also canprovide more space for the oxidation product S.

Although RH has an effect on both COS hydrolysis and H2Soxidation, COS hydrolysis is the rate-determining step during thiscoupling process. According to Fig. 5, when RH increases, the COSremoval rate decreases. Thus, the negative impact of water compet-itive adsorption with COS on the catalyst surface predominates overthe positive impact of water supplying more hydroxyl species.

4.3. Effects of space velocity

Space velocity on COS removal was investigated and shown inFig. 6. COS conversion increased with the decreasing of space velocitywithin the experimental range. At lower space velocity, the residencetimes of COS and H2Owere longer and there was a great probability ofcollisions on the surface, resulting in an increase in COS conversion.

The flow rate and, thus, the linear velocity of the fluid phase affectsthe thickness of the boundary layer surrounding the particles and theexternal mass-transfer coefficient that is a function of the latter. Theeffect of the flow rate on this coupling process is characterized by theexternal-film mass-transfer coefficient. Within the experimental

Fig. 3. Experimental (dots) and modeled (line) outlet concentration of COS for differentinlet concentrations at T=60 °C and RH=12%.

range, the external diffusion was also a mass-transfer control mech-anism. The value of kf is suitable at low Reynolds numbers.

4.4. Model parameters analysis

The mathematical model describing the kinetics of the oxidationprocess was used to fit the experimental breakthrough curves in orderto determine the parameters by minimization of the objectivefunction, as previously described. The best fit values of the parametersfor different operating temperature and relative humiditywere shownin Tables 1 and 2.

It is shown in Table 1 that increasing temperature increases thereaction rate constant, although the adsorption rate constant de-creases, as would be expected. However, temperature has little effecton α and β. The Arrhenius equation was used to obtain the param-eters of H2O adsorption and COS coupling removal, and the kinetic

parameters are: KH2O = 2:4 × 10−4e21497RT , kCOS = 6:3 × 109e

−62384RT re-

spectively. H2O adsorption heat on the bi-functional catalyst was21.5 KJ/mol, and it was found to be positive as it is an exothermicprocess; therefore, KH2O decreased from 29.13 to 9.78 with theincrease of temperature from 40 to 70 °C. COS removal activationenergy was 62.3 KJ/mol, and with an increase in the operatingtemperature yielded higher values of reaction rate constant.

Fig. 5. Experimental (dots) and modeled (line) outlet concentration of COS for differentrelative humidity at T=60 °C and COS=2500 ppm.

Fig. 6. Experimental(dots) and modeled(line) outlet concentration of COS for differentspace velocity at T=60 °C and RH=12%.

Table 2Optimum model parameters at different relative humidity values T=60 °C COS=2500 ppm.

RH εp0 α×103 β×105 kCOS KH2O

12% 0.40 7.17 1.29 22.15 17.6621% 0.37 6.83 1.04 22.01 17.0337% 0.36 6.13 0.75 21.98 17.5146% 0.33 5.04 0.49 22.22 16.91

781W. Li et al. / Fuel Processing Technology 91 (2010) 777–782

The effect of relative humidity on kinetic parameters was shown inTable 2.With the increase of relative humidity in the feed, the creationof water films can be significantly enhanced which cause the decreaseof ɛp0. COS hydrolysis proceeds by the reaction of adsorbed COS witha surface hydroxyl species. Although the surface hydroxyls are re-plenished by adsorption of water on the surface, water adsorptionleads to significant inhibition of the reaction rate for the low tem-perature reaction. Since COS hydrolysis is the rate-determining stepsduring coupling COS removal process, much more water leads to thedecreasing of COS removal.

Thepresence of awaterfilmenables thedissociationofH2Smoleculesto hydrogen sulfide ions HS-, which are then oxidized to sulfur polymershaving chain or ring-like shapes soluble in water [23]. The water playsan important role in COS hydrolysis and H2S oxidation; however, theadsorption equilibrium constant KH2O is not influenced by the relativehumidity. The presence ofwater slows down the deactivation process bypromoting deposition and adsorption of sulfur on different carbon sites,and/or bymechanically removing sulfur from active siteswhich all resultin the decrease of α, β. With the increasing of relative humidity, it wouldinhibit catalysts deactivation, while it has slight effect on kCOS and KH2O.The presence of water dissolved sulfur species also was observed in theregeneration of exhausted carbon feasible[24].

5. Conclusions

Amathematicmodel was developed for COS removal using couplingreactions on a bi-functional catalyst, accounting for external-film andpore-diffusion mass transfer mechanism and axial dispersion. Themodel was used to fit the COS removal experimental breakthroughcurves, and best fitting parameters were attained. Although the designfor coupling removing organic sulfide (CS2 and COS) were all throughhydrolysis-oxidation process and the intrinsic rate functions ofhydrolysis of CS2 were found to be best fit as the proposed ratefunctions of COShydrolysis, the deactivation function expessionwasnotin the same form for CS2 and COS removal. The best fit deactivation

Table 1Optimum model parameters at different operating temperatures RH=12% COS=2500 ppm.

T εp0 α×103 β×105 kCOS KH2O

40 °C 0.41 7.17 1.29 6.02 29.1350 °C 0.39 7.07 1.29 15.11 22.860 °C 0.38 7.16 1.29 22.15 17.6670 °C 0.40 7.11 1.29 39.18 9.78

funcnction for COS removal is φðmsÞ = 1= 1+αmsð Þ2; while CS2 isφ(ms)=(1−αms)2. It is shown from the expression of φ(ms) for COSand CS2 removal that the deactivation rate for COS removal is muchslower than that for CS2. The different expression of deactivationfunction also suggestes that theworking sulfur capacity for COS removalis much higher than that for CS2.

Water adsorption heat and COS removal activation energy were21.5 and 62.3 kJ/mol respectively obtained by the Arrhenius equation.The parameters εP0, α, β with respect to the catalyst was used toquantify the performance of the process at different operatingconditions. The temperature favors the effectiveness of the reactionrate constant, H2O adsorption equilibrium constant decrease in theseconditions as expected. The efficiency increases with relative humidity,as the parameters εP0, α, β decrease. It was also observed that the effectof relative humidity on εP0, α, β is significant than that of temperature inthe study range.

The model described satisfactorily the experimental breakthroughcurves and explained the performance of coupling COS removal. Theapplication of the model to COS removal in a fixed bed can provide areasonable basis for analysis and scale-up of similar industrial gaspurification systems.

Notation

C concentration in the bulk phase, g m-3

Cm concentration at catalyst external surface, g m-3

Coutcal calculated outlet concentration, g m-3

Coutexp experimental outlet concentration, g m-3

Cp internal concentration in particle, g m-3

dp particle diameter, mDm film mass transfer coefficient, m2 s-1

Dp pore diffusion coefficient relative to particle, m2 s-1

Dz axial diffusion coefficient, m2 s-1

kf external mass transfer coefficient, m s-1

KH2O adsorption constant of H2O, Pa-1

ms function of sulfide deposition, mol g-1

M molecular weight, g mol-1

Ne number of experiment runsNs number of samplesP pressure, Par radius relative to particle, mRep Reynolds number of particleRCOS rate of reaction, mol h-1 g-1

Sc Schmidt numberShp Sherwood number of particlet time, hT temperature, Ku velocity, m s-1

U flow rate∑V special diffusion parametersz bed length, m

Greek lettersα deactivation coefficientβ sulfide deposition coefficientεb bulk porosity

782 W. Li et al. / Fuel Processing Technology 91 (2010) 777–782

εp0 initial bulk porosity of particleεp bulk porosity of particleϕ(ms) deactivation functionφ objective function for optimization Eq. (17)ρf particle density, Kg m-3

μf dynamic viscosity, Pa s

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