zinc oxide sorbents for the removal of hydrogen sulfide from syngas

Zinc Oxide Sorbents for the Removal of Hydrogen Sulfide fromSyngas

Ilaria Rosso,* Camilla Galletti, Massimo Bizzi, Guido Saracco, and Vito Specchia

Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, Corso Duca degli Abruzzi,24-10129 Turin, Italy

Several pure zinc oxide materials, prepared with different methods (combustion synthesis anda modified version of the citrates method) and calcined at different temperatures, wereinvestigated as low-temperature desulfurizing sorbents from gaseous streams (syngas, inparticular). Comparative tests with a commercial sorbent were also carried out. The sulfidationperformance was investigated in a fixed-bed reactor in terms of breakthrough curves at 250 °C.Fresh and sulfided samples were characterized by X-ray diffraction, scanning electronmicroscopy-energy dispersion spectroscopy, BET, pore volume, pore size, and pore sizedistribution analyses. The ZnO sorbent prepared by the citrated method calcined at 400 °Cshowed the most durable effectiveness in reducing sulfur from 100 to less than 1 ppm: itsbreakthrough time is about 9 h measured at a space velocity of about 105 h-1. Its regenerabilitywas evaluated by subsequent sulfidation-thermal regeneration cycles. A numerical model wasalso developed and validated on the experimental data of the ZnO sorbent calcined at 400 °C;a good agreement was obtained. The internal mass-transfer resistance resulted in the rate-limiting step of the process. Sulfur sorption was found to be confined to the external layers ofthe pellets (because it was difficult for H2S to reach the adsorbent core), and a maximization ofinternal open porosity to improve the system performance was pointed out as the main routefor further developments and improvements.

Introduction

Fuel cells are receiving attention from the thermalefficiency1 and/or environmental points of view. Theirhigh efficiency reduces CO2 emission (greenhouse gas)per unit of electric power without the emission of NOxand SOx. Numerous types of fuel cells have beendeveloped: the differently used materials and operationtemperatures make them suitable for several uses.2Polymeric-electrolyte-membrane fuel cells (PEMFCs)work at low temperature (80-100 °C) and are promisingfor vehicle applications. The use of fuel cells on cars hasbeen subjected to intense development efforts in recentyears because of the significant advantages of the long-term possibility to reach zero emissions of pollutants,particularly important in large metropolitan areas.Gasoline and other hydrocarbon fuels do not haveadequate electrochemical reactivity to be used directlyin PEMFCs, so a catalytic fuel processor is required toconvert these fuels to hydrogen-rich, sulfur-free fuelgases.3 Because the hydrogen demand of the fuel cellstack varies with the electric load required, fuel proces-sors for automotive fuel cell engines must be able tostart up quickly, follow the power demand rapidly, andoperate efficiently over a wide range of conversion rates.Fuel conversion needs to be essentially complete overthe entire load range, and, no less important, automo-tive fuel processors must be very compact and low incost. The development of a complete on-board fuelprocessor for the production of clean hydrogen fromcommercial gasoline is the specific objective of anindustrial-type EU project, PROFUEL, which involves,beyond our group, several partners from the automotive

and catalyst manufacturing industries. The fuel proces-sor is schematically described in Figure 1. It consists ofthe following units: a dosing system, an autothermalreformer, a high-temperature water-gas-shift (HTWGS)reactor, a desulfurizer, a low-temperature water-gas-shift (LTWGS) reactor, and a selective CO-oxidation(PROX) unit.

Although the oil companies are expected to follow thelead entered by BP-Amoco and progressively reduce thesulfur content in gasoline (in Europe, 50 ppm areplanned for 2005 against the actual 150 ppm), thecomplete removal of sulfur is unlikely to happen in theshort term. It has been observed that more than 1 ppmof sulfur could dramatically shorten the lifetime ofLTWGS catalysts as well as that of the noble metals(Pt and Pt-Ru) used for the selective CO oxidation andfor the fuel cell electrodes. In this scenario, the develop-ment of long-life, low-cost adsorbers effective in reducingH2S in the reformed syngas to less than 1 ppm isessential. In this context, the trapping efficiency andcapacity should be maximized to reduce as much aspossible the size and weight of the on-board desulfurizer.

The literature reports several studies on metal oxidesas candidate desulfurization sorbents: pure oxides (e.g.,

* Corresponding author. Tel: +39-011-5644710. Fax: +39-011-5644666. E-mail: [email protected].

Figure 1. Schematic description of a gasoline for hydrogenproduction. The average operating temperatures of each processstage are indicated.

1688 Ind. Eng. Chem. Res. 2003, 42, 1688-1697

10.1021/ie0208467 CCC: $25.00 © 2003 American Chemical SocietyPublished on Web 03/13/2003

ZnO, CuO, CaO, Fe2O34-6), solid mixtures of metal

oxides that react with H2S (e.g., ZnO-Fe2O3, ZnO-MnO, CuO-Fe2O3

6-9), or mixtures of an inert oxide witha solid reactant (ZnO-TiO2, MnO-Al2O3, Fe2O3-Al2O3,CaO-MgO6,10,11) have been investigated as sorbents forthe removal of H2S, especially at high temperatures. Theaddition of an inert solid in the ZnO sorbent is, in fact,expected to stabilize the metal oxide against its reduc-tion to the metal form and/or volatilization.9

Among the different tested oxides, zinc oxide has thehighest equilibrium constant for sulfidation, yieldingH2S removal down to even fractions of 1 ppm. Itsprincipal limitation is that in the highly reducingatmosphere of synthesis gas it is partially reduced toelemental zinc, volatile above 600 °C, with consequentsorbent loss; for this reason the addition of TiO2, whichhas a stabilizing effect, is recommended for high-temperature applications. In contrast, if the operatingtemperature is low, pure zinc oxide has been singledout as the sorbent of choice for desulfurization of coalgas.

Considering the fuel processor described in Figure 1,the desulfurizer could be located either before or afterthe HTWGS unit. In the first case, a more complexsorbent, stable at high temperature, i.e., ZnO-TiO2,would be required; protection of HTWGS catalystsagainst sulfur poisoning would also be guaranteed.However, in the second case, as represented in Figure1, a simpler and lower in cost sorbent, i.e., pure ZnO,can be used and, anyway, protection against sulfurpoisoning of the most sulfur-sensitive catalysts of thefuel processor (LTWGS and CO-PROX catalysts) issecured. Furthermore, the low-temperature operationbrings about another important advantage for vehicleapplications: the consequent low gas viscosity implieslow pressure drops. Taking into account the quite highsulfur tolerance of the HTWGS catalysts, the locationof the desulfurizer after the HTWGS unit was chosenby the PROFUEL partnership, and an adsorption mate-rial able to reduce the sulfur content down to 1 ppm atabout 250°C with the highest possible capacity was fixedas the main research target.

In this paper the development of a long-life zinc oxidesorbent is described. Specifically, the effect of theprocedure followed to prepare the sorbents on their H2Sremoval ability is evaluated, in comparison with acommercial zinc oxide sorbent, by means of sulfidationexperiments performed on a fixed bed of sorbent par-ticles. A model of the H2S adsorber based on a ZnOpellets bed was developed as well, with the mainpurpose of enlightening the mechanisms governing thesystem performance and outlining pathways for furtherimprovements.

Experimental Section

Sorbent Preparation. A commercial sample of ZnOwas obtained from Aldrich. Conversely, a series of ZnOsorbents were prepared following two different experi-mental procedures:

(i) The glycerine method, a modified version of theso-called “citrates method”:12 an amount of Zn(NO3)2‚6H2O (from Aldrich) was mixed with a 40 wt % amountof glycerin (from Fluka) and a 40 wt % amount of water.The mixture was slowly heated to 120 °C until a slightNOx emission started, then rapidly poured into a stain-less steel vessel, and kept in an oven at 180 °C for 30min. Under such conditions, NOx, CO2, and water vapor

form in huge amounts, thus causing the formation of asolid scum, quite friable and porous. Each sorbent wasthen finely ground in an agate mortar and calcined inan electric oven in calm air for 2 h at 300 °C (ZnO300),at 400 °C (ZnO400), at 500 °C (ZnO500), and at 600 °C(ZnO600).

(ii) The combustion synthesis method13 (the ureamethod): appropriate amounts of Zn(NO3)2‚6H2O andCO(NH2)2 (both from Aldrich) were dissolved in theminimum amount of water possible (about 5 mL for 1 gof final ZnO). After a few minutes of stirring on aheating plate, to ensure proper homogeneity, the so-prepared solution was transferred in a capsule, whichwas placed into an oven kept at the constant tempera-ture of 500 °C (ZnOu500) or of 700 °C (ZnOu700). First,the aqueous solution underwent dehydration, and thenthe mixture frothed and swelled, until a fast andexplosive reaction took off:

and large amounts of gases evolved. The whole processwas over after 5-6 min, but the time occurring betweenthe actual ignition and the end of the reaction was lessthan 10 s. A foamy and easily crumbled material wasobtained to give a fine and volatile powder. Each sorbentwas then finely ground in an agate mortar and stabi-lized by calcination in an electric oven for 1 h at 600 °Cin calm air.

Sorbent Characterization. X-ray diffraction (XRD)analyses (Philips PW1710 apparatus equipped with amonochromator for the Cu KR radiation) were per-formed on all fresh sorbents to check the crystallizationof zinc oxide, on the sulfided ones (see the next section)to check the formation of zinc sulfide, and on theregenerated ones (see the following section) to check thedisappearance of zinc sulfide.

The specific surface area, determined by the Brunau-er-Emmett-Teller (BET) method using N2 (Micromer-itics ASAP 2010M apparatus), together with the totalpore volume, the pore size, and the pore size distribu-tion, was measured on all of the sorbents in pelletizedform, as prepared, after sulfidation, and after regenera-tion.

Fresh, sulfided, and regenerated sorbents were alsoexamined by scanning electron microscopy (SEM) andenergy dispersion spectroscopy (EDS) (Philips 515 SEMequipped with an EDAX 9900 EDS) to investigate thepossible morphology variation before and after sulfida-tion as well as the elemental distribution over thepellets. Because the morphological analysis (SEM) needsgold metallization of the sample and the X-ray emission(EDS) of gold overlaps those of sulfur, simultaneousEDS and SEM measurements on the same sample werenot possible.

Sulfidation Apparatus and Procedure. The de-sulfurization performance of all of the sorbents wasinvestigated in a bench-scale reactor. A fixed bed ofabout 2 cm in length containing 0.25 g of sorbentparticles (obtained by pressing at 125 MPa the sorbentpowders into tablets, by crushing the tablets, and bysieving the produced particles to separate 0.25-0.425mm granules) was enclosed in a glass tube (i.d. of 4 mm)and sandwiched between two glass-wool layers. Thereactor was placed in a PID (PID)-regulated oven andoperated at 1 bar of outlet pressure. A thermocouple wasinserted in the packed bed for oven regulation purposes.

3[Zn(NO3)2 + 6H2O] + 5CO(NH2)2 f

3ZnO + 8N2 + 5CO2 + 28H2O (1)

Ind. Eng. Chem. Res., Vol. 42, No. 8, 2003 1689

After 30 min at 250 °C in nitrogen flow, as a commonpretreatment, a gas flow rate of 400 Ncm3 min-1

(composition: H2S, 100 ppm; He, balance) was fed tothe reactor. The composition and the flow rate of thefeed gas were controlled by mass flow controllers, anda pressure transducer checked the pressure of the inletand outlet gases in order to overlook any possibleundesired packing of the sorbent fixed bed. A constanttemperature of 250 °C was maintained during the test.The outlet gas stream was sampled every 15 min andanalyzed through a gas chromatograph-mass spectrom-eter (GC-MS; Varian 3400 Saturn 4D). Three aliquotsof 1000 µL of the same sampled volume were injectedinto a fused silica capillary column (Poraplot Q; 25 mlength × 0.32 mm i.d.) kept at 40 °C and eluted with ahelium flow of about 2 mL/min. The appearance of apeak relative to the mass/charge ratio equal to 34 and36, due to the molecular ions H2S+ and H2

34S+, respec-tively (retention time of 4 min), monitored by the massspectrometer, permitted a quantitative evaluation ofH2S. Therefore, H2S concentration plots versus timewere obtained. The time of the abrupt change of the H2Sconcentration (from 0 to 2 ppm, the minimum detectorsensitivity) in the product gas was called “breakthroughtime”, whereas the so-obtained time-dependent curveswere called “breakthrough curves”.

Regeneration Procedure. Sulfided sorbents wereregenerated by flowing air at 625 °C for 15 min in thesame apparatus as that used for sulfidation experi-ments. Six successive sulfidation-regeneration cycleswere performed on the same sample of the ZnO400sorbent.

Model Equations

In the system under investigation, a reaction takesplace between the gas-phase reactant and the solid-phase surface:

As the reaction proceeds, the solid particle storagecapacity is progressively consumed and the unreactedcore of the particle shrinks, thus increasing the difficultyof the H2S molecules to reach the fresh sorbent material.

The model developed in the present study consistedof the mass conservation equations for the gas and solidphases. Because the system was operated isothermallyand the H2S concentration was very low, the energybalance equation was neglected. The momentum equa-tion was also neglected because of the considerably lowpressure drop that occurred in the experimental setupunder investigation (a few tens of pascals). The spatialvelocity was about 105 h-1. Under these conditions, thelongitudinal Peclet number is as high as 300, and aplug-flow assumption was considered to be acceptablein the model formulation.14 Therefore, the followingpartial differential equation represented the H2S bal-ance in the gas phase:

The RH2S that appears in this equation is the specificadsorption rate of H2S that can be expressed as

The oxygen mass balance on the solid phase wasemployed to determine the amount of fresh sorbent ona single catalyst pellet. By stating that the variation ofthe oxygen content of the sorbent particle equals thenet oxygen consumption due to the heterogeneousreaction, one obtains the equation describing the varia-tion of the radius of the unreacted particle core:

In eq 5, the density value should be that of theunreacted core of the sorbent particle, whereas the Wparameter represents the oxygen content of the unre-acted core, expressed in moles of oxygen per unit massof fresh solid.

The overall kinetic constant of eqs 4 and 5 wasobtained by considering that the oxygen consumptionrate should be determined by accounting for the con-tribution of several transport resistances in series. Toreach the fresh sorbent core, the H2S molecule must betransferred from the bulk of the gas phase to the particleexternal surface (external mass transport), then mustdiffuse across the internal pores of the particle (internalmass transfer), and finally undergo a chemical reactionwith the unreacted core. By considering these processesto be first order versus the H2S concentration and byassuming the Fick’s law with an appropriate effectivediffusivity for the internal diffusion, one obtains thefollowing expression for the overall coefficient:

An exhaustive analysis of the reaction kinetics of H2Sremoval by oxide sorbents was thoroughly performedin the work of Garcia et al.15 A first-order kineticexpression with the following Arrhenius-type kineticconstant was suggested:

The external mass transport coefficient Kext of H2Sin the fixed-bed system was evaluated by means of thefollowing equations reported in the work of Yoshida etal.:16

where17

As far as the internal mass transport is concerned,the pore dimension of the sorbent particles could be such

H2S + ZnO f H2O + ZnS (2)

FS

∂CH2S

∂z+ εfb

∂CH2S

∂t)

RH2SAi

Vp(1 - εfb) (3)

AiRH2S ) -(Ka)ovcH2S (4)

∂ri

∂t) -

(Ka)ovcH2S

4πWFsri2

(5)

1(Ka)ov

) 14πri

2Kr

+ 14πre

2Kext

+re - ri

Deff4πrire(6)

Kr ) 1107 exp(-3257.17/T) (7)

{Jd ) 0.91Re-0.51ψ 0.01 < Re < 50Jd ) 0.61Re-0.41ψ 50 < Re < 1000

(8)

Re )Gdp

6µ(1 - εfb)(9)

Jd ) ShRe × Sc1/3

(10)

Sh ) Kextdp/D (11)

Sc ) µ/FD (12)

1690 Ind. Eng. Chem. Res., Vol. 42, No. 8, 2003

that both molecular and Knudsen mechanisms couldplay an important role in the diffusion process. There-fore, the following equation to calculate the diffusioncoefficient18 was employed:

The molecular diffusivity Dm value was calculated bythe Chapman-Enskog equation.17 The Knudsen diffu-sivity was calculated with the following expression, usedalso by other authors in the present research field:18

The porosity used in this expression should be theintraparticle value, namely, the internal open porosityof the sorbent pellet. It must be noticed, moreover, thatthe density value to be employed in this expressionshould be that of the reacted external layers, which haveto be crossed by the H2S molecules in order to reachthe unreacted core.

Once the D value was determined, the effectivediffusivity in a sorbent particle could be calculateddepending on the diffusion coefficient and on the sorbenttortuosity:

Because no means to evaluate τ were available, therandom pore model was then employed19 as a firstapproximation. This model relates tortuosity and poros-ity and states that τ is the inverse of ε. The effectivediffusivity expression therefore becomes

Results and Discussion

Experimental Procedures. The diffraction patternsof sorbents prepared with the glycerin method (ZnO300,ZnO400, ZnO500, and ZnO600) are shown, as anexample, in Figure 2. The diffraction peaks of ZnO

(JCPDS card 80-0075) are clearly seen in each pattern,but they become higher and sharper from ZnO300 toZnO600 (from curve a to curve d, respectively), becausethe increasing calcination temperature brings aboutlarger crystals. No other phases were observed. Thecalcination temperatures were chosen considering thedecomposition temperature of zinc carbonate (300 °C20),which forms in large quantity during the reaction of theglycerin method. The diffraction patterns of commercialZnO and of sorbents prepared via combustion synthesis,the urea method (ZnOu500 and ZnOu700), not shown,are similar to the ZnO600 diffraction pattern presentingwell-defined ZnO diffraction lines.

The BET data of all fresh sorbents are listed in Table1. ZnOu500 and ZnOu700 sorbents, prepared with theurea method, have a very low specific surface area(about 2.5 m2/g), whereas the sorbents prepared withthe glycerin method have a higher specific surface area,which increases by decreasing the calcination temper-ature, in good agreement with their diffraction patterns.The urea method, even if very quick, does not permitone to obtain immediately the ZnO phase at both 500and 700 °C reaction temperatures, so that 1 h ofcalcination at 600 °C is required and low specific surfacearea materials are consequently obtained. On thecontrary, the glycerin method is more time-consumingbut permits one to obtain sorbents with higher specificsurface area even when calcination is performed at 600°C (about 10 m2/g of ZnO600 against the 2-2.5 m2/g ofZnOu500 and ZnOu700 obtained with the urea method).

Figure 2. XRD patterns of fresh ZnO sorbents prepared with the glycerin method: (a) ZnO300; (b) ZnO400; (c) ZnO500; (d) ZnO600.

1D

) 1Dm

+ 1Dk

(13)

Dk )19400εi

AsFs xTM

(14)

Deff ) Dεi/τ (15)

Deff ) Dεi2 (16)

Table 1. Breakthrough Time, Specific Surface Area(BET), and Pore Volume of Fresh and Sulfided Pellets ofZnO Sorbents

fresh sulfided

sorbentBET

(m2/g)

porevolume(cm3/g)

breakthroughtime (min)

BET(m2/g)

porevolume(cm3/g)

ZnO300 50.3 0.20 550 12.4 0.07ZnO400 43.3 0.30 430 12.2 0.10ZnO500 13.1 0.13 20 11.3 0.08ZnO600 10.1 0.13 10 11.3 0.04ZnOu500 2.5 0.06 5 2.5 0.06ZnOu700 2.2 0.04 5 2.2 0.04ZnO (Aldrich) 7.8 0.07 20 6.5 0.06


Moreover, the BET area of all of the sorbents preparedwith the glycerin method is higher than that of thecommercial ZnO. Pore-volume data of pellets of freshsorbents, also listed in Table 1, are directly related toBET data: sorbents with the highest specific surfacearea have generally also the highest pore volume.

The breakthrough times obtained in sulfidation ex-periments on each sorbent are listed in Table 1, as well.ZnOu500 and ZnOu700 sorbents show the shortestbreakthrough times (5 min), whereas ZnO600, ZnO500,ZnO400, and ZnO300 sorbents show progressively longertimes (from 10 to 550 min, respectively). The commercialZnO has a breakthrough time of only 20 min. Bycomparison of these results with the BET data and thepore volume of fresh sorbents, the important role ofpellet structural properties, i.e., pore volumes andspecific surface areas, in sulfur trapping capacity canbe easily deduced: as expected, the higher are thespecific surface area and the pore volume, the higher isthe storage capacity.

Table 1 also lists the BET data and the pore volumesof sulfided sorbents. The sorbents with low BET area,low pore volume in their fresh state, and consequentlylow breakthrough time show a negligible variation inthe BET area and pore volume after the sulfidationexperiments. On the contrary, the sorbents with highBET area, pore volume, and breakthrough time in theirfresh state (specifically ZnO300 and ZnO400) show astrong reduction in both the specific surface area andpore volume after sulfidation. A pure thermal effect at250 °C in the absence of H2S was evaluated on thespecific surface area and pore volume of ZnO300 (thesorbent calcined at the lowest temperature, 300 °C, closeto the temperature of sulfidation experiments, 250 °C):the BET area diminished from 50.3 to 37.0 m2/g over aperiod of 1450 min, whereas the pore volume shows anegligible variation. Hence, sulfidation seems to be themain cause of BET and pore-volume reduction. It is wellassessed that the chemical reaction involved in sulfi-dation is only the one reported in eq 2. Because themolar volume of the sulfide product is greater than thatof the oxide reactant (the ratio between the molarvolume of ZnS and ZnO is 1.67), morphological andstructural property changes do accompany the sulfida-tion process.

The relationship between the characteristics of poreand H2S sorptivity has been investigated by calculationof the pore size distribution by the BJH method fromthe adsorption isotherm. As an example, Figure 3reports the ratio of the pore volume per unit mass/porediameter, which can be considered to be proportionalto the specific surface area, versus the pore diameter ofthe ZnO300 sorbent both in the fresh state (solid line)and after sulfidation (dashed line). The average pore

Figure 3. Pore size distribution of the ZnO300 sorbent in the fresh (solid line) and sulfided (dashed line) states calculated by the BJHadsorption method.

Figure 4. SEM micrographs of pellets of the ZnO400 sorbent:(A) fresh; (B) sulfided.


diameter, calculated by the BJH adsorption method,increases from 17.2 nm in the fresh state to about 26nm after sulfidation, confirming that the average porediameter is essentially inversely proportional to thesurface area. In particular, Figure 3 shows that in thefresh state (solid line) the pore area is provided by alarge amount of small pores of about 1.8 nm and bylarger pores of about 20 nm, whereas in the sulfidedstate (dashed line) the pore area is mainly given by fewlarge pores of about 25 nm. This means that small poresare completely blocked by H2S and only a few largepores are kept after sulfidation.

Figure 4 shows SEM micrographs of the ZnO400sorbent in its fresh state (Figure 4A) and after sulfida-tion (Figure 4B): the small, quite uniform grains of ZnOin its fresh state become larger and less uniform aftersulfidation, whereas EDS analyses confirmed the pres-ence of a large amount of sulfur in the sulfided samples(about 20 wt %).

XRD analyses, performed on all of the sulfided sor-bents, show the appearance of peaks of the ZnS phase(JCPDS card 72-0162). Figure 5 shows, as an example,the diffraction patterns of sulfided ZnO300-ZnO600sorbents (curves a-d, respectively). ZnO and ZnSphases are present in all curves, but the intensity ofthe ZnS peaks is very low for the ZnO600 sorbent (curved) and higher for the other sorbents. At the same time,if the ZnO peaks of fresh and sulfided sorbents arecompared (curves a-d of Figure 2 with curves a-d ofFigure 5), it can be noticed that their intensity is lowerin the sulfided sorbents; the formation of the ZnS phaseinvolves a progressive disappearance of the ZnO peaks,according to reaction (2). This occurrence is morepronounced for the ZnO300 and ZnO400 sorbents(curves a and b of Figure 5) thanks to their higher sulfurtrapping capacity (Table 1).

The breakthrough curves of ZnO300-ZnO600 sor-bents are shown in detail in Figure 6. The outlet H2Sconcentration rises from 0 ppm and reaches 100 ppmrapidly after only 269 min for ZnO600 and after 388min for ZnO500. The sorbents ZnO400 and ZnO300,instead, show a much higher sulfur capture capabilityas the outlet H2S concentration reaches about 100 ppmat about 1500 min for ZnO400 and well above 1600 minfor ZnO300.

The total and breakthrough time amounts of sulfuradsorbed by each sorbent can be calculated consideringthe total amount of H2S fed to the sorbents during thetime of sulfidation experiments (the time required bythe outlet H2S concentration to pass from 0 to 100 ppm),the one at breakthrough conditions, and the trend ofthe breakthrough curves. The results, listed in Table2, confirm that ZnO300 and ZnO400 sorbents have thehighest capacity of sulfur trapping and are in very goodagreement with the data obtained by EDS analyses onthe sulfided sorbents. The ZnO300 sorbent has the

Figure 5. XRD patterns of sulfided ZnO sorbents prepared with the glycerin method: (a) ZnO300; (b) ZnO400; (c) ZnO500; (d) ZnO600.Legend: (2) ZnO; (b) ZnS; ([) sample support (Al).

Figure 6. Breakthrough curves of ZnO sorbents prepared withthe glycerin method.

Table 2. Total (Outlet S Concentration Equal to the InletOne) and Breakthrough Time Amount of SulfurAdsorbed by ZnO Sorbents during SulfidationExperiments

sorbenttotal S adsorbed

(mg of S/g of sorbent)

S adsorbed atbreakthrough time

(mg of S/g of sorbent)

ZnO300 48 31.4ZnO400 48 24.5ZnO500 12 1.14ZnO600 7.0 0.57ZnOu500 1.7 0.28ZnOu700 2.5 0.28ZnO (Aldrich) 12.5 5.0


highest breakthrough time (550 min), according to itshighest specific surface area; the ZnO400 sorbent,however, is singled out as the best and more reliablefor application in syngas purification because it has ahigher structural stability than ZnO300, deriving fromthe higher calcination temperature (400 °C), and itssulfur trapping capacity (breakthrough time of 430 min)is very high as well.

On the selected sorbent, ZnO400, the regenerationperformance was investigated. It is well-known21 that

an oxygen atmosphere gives the following regenerationreaction:

Preliminary experiments were carried out in order toinvestigate the regeneration performance as a functionof temperature, time, and O2 concentration. The bestregeneration of the ZnO400 sorbent was achieved in 15min at 625 °C in 20% by volume of O2. Multicycle testsconsisting of seven sulfidations with six interveningregenerations were carried out on the same aliquot of anew preparation of the ZnO400 sorbent (breakthroughtime in the fresh state of 390 min). The breakthroughtimes after each sulfidation (186, 92, 73, 60, 57, and 57min, respectively) show that the ZnO400 sorbent recov-ers about 50% of its starting sulfur trapping capacityafter the first regeneration and about 25% after thesecond regeneration, and it stabilizes on a value of about15% from the third to the sixth regeneration. BET datademonstrate a progressive diminution of the specificsurface area after each cycle. Starting from a value of29.2 m2 g-1 in its fresh state, it becomes 21.3 m2 g-1

after the first sulfidation and 20.7 m2 g-1 after the firstregeneration; it maintains 20.7 m2 g-1 after the secondsulfidation, diminishes to 19.4 m2 g-1 after the secondregeneration, and at the end of the seventh sulfidationis equal to 19.2 m2 g-1.

If sulfidation brings about inevitably morphologicaland structural property changes, such changes should

Figure 7. Breakthrough curve. Comparison between the experi-mental ZnO400 curve (b) and model calculations (s). Fittingparameter Deff ) 5 × 10-8 m2/s.

Figure 8. Results of model calculations: concentration profiles (a) and breakthrough curve (b) without internal mass transfer (Deff equalto 5 × 10-5 m2/s); concentration profiles (c) and breakthrough curve (d) in the presence of internal mass transfer (Deff equal to 5 × 10-8

m2/s). Concentration profiles along the sorbent bed are calculated every 2.5 h.

ZnS + 3/2O2 f ZnO + SO2 (17)


be at least partially reversed during the regenerationstep. XRD analyses on regenerated sorbents show thetotal disappearance of the zinc sulfide phase, accordingto reaction (17); however, small traces of sulfur aredetected by EDS analysis on the same materials. Moreimportantly, the unsuccessful recovery of the specificsurface area after the regeneration step shows thatirreversible structural changes occur. The high temper-ature requested by the regeneration step (625 °C), muchhigher than the calcination temperature of the sorbent(400 °C), is likely the main reason for the unavoidableprogressive sintering of the sorbent, which, however,stabilizes after the second regeneration step with con-sequent progressive asymptotic behavior of the break-through times of successive sulfidation steps.

Modeling and Related Issues. The first step in thenumerical analysis was the model validation, carriedout on a data set relative to the most promising ZnOsorbent: ZnO400. The comparison between the modelcalculations and the experimental breakthrough curveis presented in Figure 7. The model was solved consid-ering the effective diffusivity as a fitting parameter thatwas tuned to achieve the best agreement betweenexperiments and calculations. The optimal Deff valuewas 5 × 10-8 m2/s. Considering that the sample had asurface area of 43.3 m2/g, this diffusivity value entails,according to eq 16, an internal open porosity of 25%,which appears to be quite a reasonable value if com-pared with the original 45% porosity of the sorbentpellets measured by helium picnometry. This is a clearsign that sulfidation affects the internal mass-transferproperties of the sorbent despite the fact that sulfurappears to be preferentially captured in the nanosizedpores instead of the larger ones, which should likely hostmost of the diffusive flow rate toward the core of thepellets.

The rather flat shape of the breakthrough curvesuggests that the internal diffusion process should limitthe system performance. Figure 8 shows the calculated

concentration profiles and the corresponding break-through curves obtained by computer simulation withDeff values of 5 × 10-5 m2/s (Figure 8a,b) and 5 × 10-8

m2/s (Figure 8c,d) closely related to the presentedexperimental data (Figure 7). The former Deff is equalto the molecular diffusivity of oxygen in air at 523 K,and the simulation with this value represents the caseof diffusion within the sorbent pores without the ratelimitation due to the Knudsen regime and to internaltortuosity. In this case, the gas stream would easilyaccess the sorbent material at the particle core thatmust be completely consumed before the H2S concentra-tion profile can move downstream. As a result, plug-shaped profiles can be observed, and the breakthroughcurve reflects this behavior, taking the same qualitativeshape. On the other hand, in the case of considerabletransport resistance due to internal mass transfer, thesorbent material is mainly employed at its externallayers, and its core can hardly be reached. As a result,the concentration profiles turn into a different shape,which reveals how the H2S molecules need a freshexternal surface to be readily adsorbed and that the coreof the particle needs longer residence times to bereached. To further enlighten this point, the three termsof eq 6 were separately calculated and represented inan Arrhenius plot in Figure 9, at different values of theri parameter. An overall transport coefficient, deducedfrom (Ka)ov according to eq 18, was represented on thesame diagram:

Moreover, for a direct comparison with the othertransport coefficients, the internal mass transport coef-ficient was defined as

Figure 9. Comparison between the different terms of the overall transport process: (a) ri/re ) 99.95%; (b) ri/re ) 99.5%; (c) ri/re ) 99%;(d) ri/re ) 97%.

Kov ) (Ka)ov/4πre2 (18)

Ki )Deff

re - ri(19)


It can be noticed that with the fresh sorbent particles(Figure 9a) the transport resistance due to internaldiffusion is relatively negligible. Under these conditions,the system is limited by reaction kinetics at low tem-perature and by external mass transport at highertemperature. In particular, Figure 9a shows that, at theoperating temperature of 523 K (1/T ) 0.0019), the freshsorbent particles operate under an external transportcontrolled regime. Parts b-d of Figure 9, obtained byprogressively decreasing the ri value, show that therelative importance of the internal mass transportincreases as the sorbent particles are consumed. Inparticular, it can be observed that already when the rireaches the 99-97% of the initial particles radius(Figure 9d), the system operates in an internal masstransport controlled regime. This remark provides afurther enlightenment on the shape of the breakthroughcurve because it reveals that even a minor particleconsumption already leads the system under the controlof internal mass transport. Therefore, the importanceof the internal morphology of the sorbent particles onthe system performance has to be underlined.

Obviously, the most remarkable improvement on thesystem performance could be achieved by an appropriatepreparation method of the sorbent particles, intendedto maximize their internal open porosity and to mini-mize their tortuosity, in line with the observationsderived by the investigation on the pore size distribu-tion. More in detail, a multimodal pore structure, i.e.,a sort of “clusterized” pore distribution with macroporesentering the pellets from its surface with side mi-cropores exploiting the high specific surface area, shouldimprove the effectiveness factor of the pellets andconsequently the system performance. The calculationsshown in Figure 8 indicate that the breakthrough timecould even be doubled in theory. Finally, this optimizedpore structure could determine the possibility of afurther increase in the performance of the system if itis operated at higher space velocities (see Figure 10,where the experimental conditions of our texts are alsodrawn). Experimental studies are in progress on thisperspective.

Conclusions

Several pure zinc oxide materials were preparedfollowing two different routes, characterized and tested

as low-temperature desulfurizing sorbents; break-through curves in a fixed-bed reactor equipped withsorbent pellets of about 0.34 mm in diameter wereobtained. ZnO300 and ZnO400 sorbents, calcined at thelowest temperatures and with the consequent highestspecific surface area and pore volume, showed thehighest sulfur trapping capacity, with breakthroughtimes varying from 430 to 550 min measured at a spacevelocity of 105 h-1; these are very good results, especiallyif compared to the breakthrough time of the commercialsorbent: 20 min at the same space velocity. The ZnO400sorbent is singled out as the best and more reliable forapplication in syngas purification because of its higherstructural stability than ZnO300. On the ZnO400 sor-bent, muticycle tests of sulfidation and regenerationwere carried out to provide information concerning itsdurability. Regeneration performed for 15 min at 625°C in air brings about only a partial recovery of thestarting sulfur trapping capacity, because of an un-avoidable progressive sintering of the sorbent.

A numerical model of the system was developed andvalidated, with a good agreement, by fitting the experi-mental data set of the ZnO400 sorbent. Under theconsidered operating conditions, the rate of ZnO pelletsulfidation was limited by the internal mass-transferresistance, because of the difficulty of H2S moleculesreaching the adsorbent core, while the influence of theexternal mass transfer became rapidly irrelevant. Thismeans that only a portion of the sorbent pellets, theexternal layers, is consumed and that a maximizationof their internal open porosity could improve the sorbentperformance. The increase of the internal porosity ofsorbent pellets, by exploiting a multimodal pore distri-bution with macro- and micropores, represents themajor pathway for our future research efforts.

Nomenclature

A ) area [m2]cH2S ) molar concentration of H2S in the gas phase [kmol/

m3]d ) diameter [m]D ) diffusivity [m2/s]F ) gas flow rate [m3/s]G ) specific mass flow rate [kg/s‚m2]Jd ) mass-transfer factor

Figure 10. Effect of open porosity (εi) and space velocity (GHSV) on the sorbent performance: (b) experimental conditions of our tests.


K ) kinetic constant (gas-phase transport or surfacereaction) [m/s]

(Ka)ov ) overall kinetic constant multiplied by a meanexchange area [m3/s]

M ) molecular weight of gas-phase species [kg/mol]ri ) radius of the internal unreacted core of a sorbent

particle [m]re ) initial radius of the sorbent particle [m]RH2S ) specific adsorption rate of H2S [kmol/m2‚s]Re ) Reynolds numberS ) reactor cross section [m2]Sh ) Sherwood numberSc ) Schmidt numberT ) temperature [K]t ) time [s]V ) volume [m3]W ) oxygen content of the unreacted core [kmol/kg]z ) reactor axial coordinate [m]

Greek Letters

ε ) porosityµ ) gas viscosity [kg/m‚s]F ) density [kg/m3]τ ) tortuosityψ ) shape factor in the mass-transfer coefficient equation

Subscripts

eff ) effectiveext ) external mass transferfb ) fixed bedi ) intraparticle value (limit of the internal unreacted core)m ) moleculark ) Knudsenov ) overallp ) particler ) chemical reactions ) solid phase

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Received for review October 25, 2002Revised manuscript received February 10, 2003

Accepted February 13, 2003

IE0208467


zinc oxide sorbents for the removal of hydrogen sulfide from syngas

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