Energy & Fuels 2007, 21, 1863-1871
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Kinetics of Zinc Oxide Sulfidation for Packed-Bed Desulfurizer Modeling Jian Sun,*,† Shruti Modi,‡,§ Ke Liu,‡,| Roger Lesieur,‡,§ and John Buglass‡,⊥ School of Chemical and EnVironmental Engineering, Shanghai UniVersity, 149 Yanchang Road, Shanghai 200072, China, and HydrogenSource LLC (A Joint DeVelopment of UTC Fuel Cells and Shell Hydrogen), 60 Bidwell Road, South Windsor, Connecticut 06074 ReceiVed October 18, 2006. ReVised Manuscript ReceiVed February 14, 2007
The sulfidation process of porous zinc oxide sorbent with hydrogen sulfide can be described in five steps after external mass transfer and pore diffusion. They are surface adsorption of hydrogen sulfide gas on zinc oxide sorbent and dissociation of the gas molecule on the sorbent surface, followed by reversible surface reactions, sulfide ion migration under the surface, and sulfidation penetration into the solid crystallite. On the basis of the understanding of this chemistry, an empirical rate law for the intrinsic kinetics of the sulfidation process was derived in this study. Kinetics modeling results using this reversible, adsorption, and ion-migration (RAIM) model were found, consistent with selected experiments of single-particle sulfidation. Modeling results were also comparable with several well-defined sulfidation models in the literature. The intrinsic kinetics of a porous ZnO sorbent G-72E were measured in a microflow packed column and calculated using the RAIM model, using a finite difference approach. The effective pore diffusivity of gaseous hydrogen sulfide in the porous zinc oxide pellet was calculated using the general process modeling system (gPROMS). Finally, design calculation for a full-scale packed desulfurizer was performed using the gPROMS distributed reactor model. Case studies were presented for hydrogen sulfide removal from natural gas in a simulated fuel-processing train for syngas production.
Introduction Porous ZnO is a commodity sorbent material widely used in refinery for the removal of gaseous H2S because it has well-predictable reaction kinetics and adsorption (or absorption) capacity. The production cost of ZnO is low compared with other sorbent materials, such as molecular sieves or zinc-titanium oxide. Sorbent regeneration is usually not economical for ZnO at present in the chemical industry. In addition, spent ZnO sorbent is considered nonhazardous for solid waste disposal. ZnO has another important feature that cannot be ignored. Unlike other metal oxide sorbents from copper or iron, ZnO is relatively more stable and not readily reduced in the presence of H2. Therefore, it is widely used for desulfurization in fuel processing for the fuel cell industry. The objective of this study is to develop a ZnO sulfidation kinetics equation, integrate it into a distributed reactor model, and design a desulfurizer for the process of natural gas fuel processing. There are basically four well-defined kinetic models in the literature to describe the sulfidation of ZnO or ZnO-type sorbents with H2S. They are the generalized * To whom correspondence should be addressed. E-mail: sunjkl@ shu.edu.cn. † Shanghai University. ‡ HydrogenSource LLC. § Present address: UTC Power, 195 Governor’s Highway, South Windsor CT 06074. | Present address: GE Global Research Center, FCL-EPT, 18A Mason, Irvine CA 92618. ⊥ Present address: Shell Global Solutions International BV, Carel van Bylandtlaan 23, 2596 HP Den Haag, The Netherlands. (1) Yu, H. C.; Sotirchos, S. V. A generalized pore model for gas-solid reactions exhibiting pore closure behavior. AIChE J. 1987, 33, 382.
random pore (RPM) model,1 unreacted shrinking-core (USC) model,2 grain model,3 and overlapping grain (OGM) model.4 The OGM model has been frequently studied in recent publications.5-8 In this study, let us look into the sulfidation chemistry from another perspective. An empirical rate law for the intrinsic kinetics was developed. Modeling results were compared with those obtained from the sulfidation models in the literature. Using these comparison results as a reference or initial values for a parameter estimate, the intrinsic sulfidation kinetics and effective diffusivity for a porous ZnO sorbent G-72E were measured in a microflow packed column and calculated with the reversible, adsorption, and ion-migration (RAIM) rate law. Modeling results were also applied to the design calculation of a full-scale desulfurizer. (2) Zevenhoven, R.; Yrjas, P.; Hupa, M. Hydrogen sulphide capture by limestone and dolomite at elevated pressure. II. Particle conversion modelling. Ind. Eng. Chem. Res. 1996, 35, 943. (3) Gibson, J. B.; Harrison, D. P. Ind. Eng. Chem. Process Des. DeV. 1980, 19, 231. (4) Sotirchos, S. V.; Yu, H. C. Overlapping grain models for gas-solid reactions with solid product. Ind. Eng. Chem. Res. 1988, 27, 836. (5) Lew, S.; Sarofim, A. F.; Flytzani-Stephanopoulos, M. Modeling of the sulfidation of zinc-titanium oxide sorbents with hydrogen sulfide. AIChE J. 1992, 38, 1161. (6) Konttinen, J.; Zevenhoven, R.; Hupa, M. Hot gas desulfurisation with zinc titanate sorbents in a fluidised bed 1. Determination of sorbent particle conversion rate model parameters. Ind. Eng. Chem. Res. 1997, 36, 23322339. (7) Konttinen, J.; Zevenhoven, R.; Hupa, M. Hot gas desulfurisation with zinc titanate sorbents in a fluidised bed 2. Reactor model. Ind. Eng. Chem. Res. 1997, 36, 2340-2345. (8) Konttinen, J.; Zevenhoven, R.; Yrjas, P.; Hupa, M. Modelling of sulfided zinc titanate regeneration in a fluidised bed reactor 1. Determination of the solid conversion rate model parameters. Ind. Eng. Chem. Res. 1997, 36, 5432-5438.
10.1021/ef060521t CCC: $37.00 © 2007 American Chemical Society Published on Web 06/19/2007
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Figure 1. Sulfidation chemistry: surface reaction.
Figure 2. Sulfidation chemistry: ion migration and sulfidation penetration.
Sulfidation Chemistry According to the specifications from ZnO manufacturers (Sud-Chemie or SCI), sulfidation of porous ZnO usually applies at temperatures ranging from 150 to 450 °C. With an increasing temperature, sulfidation kinetics generally increase and the capacity increases to a maximal value followed by a reduction as the temperature goes above 450 °C as a result of the sulfidation equilibrium. In a process free of external mass transfer and pore diffusion, sulfidation of solid ZnO by gaseous H2S on the sorbent surface can be described by the following steps as shown in Figure 1: In step 1, a H2S molecule adsorbs and dissociates at two active sites on the solid surface. The two sites can be ZnO or ZnS depending upon the surface structure and degree of sulfidation. This reaction can be reversible especially when ZnS is the dominant species on the solid surface, i.e., when sulfidation reaches a high conversion stage.
H2S + 2• S H+• + HS-• In step 2, adsorbed HS- reacts with ZnO, which is on the surrounding site if the adsorption site is ZnS or can be the adsorption site ZnO itself.
What happens if all ZnO on the grain surface is converted to ZnS? Literature suggests that gaseous H2S diffuses through the solid ZnS layer followed by the reaction with ZnO underneath.4-8 The diffusion of gas in solid is modeled in the literature as a very slow process compared with the initial surface reaction. This assumption can be deduced from the commonly used thermogravimetric analysis (TGA) graphs (sorbent weight gain or ZnO conversion over time) for ZnO sulfidation, which can be elaborated with two slopes of kinetic behavior: the first being the fast surface reaction and the second being gas diffusion in the solid. Let us not scrutinize the validity of this assumption that gaseous molecules, such as H2S, can diffuse through a dense crystal structure, such as ZnS. On the other hand, the ionmigration or ion-exchange theory has been recognized in other fields, such as corrosion engineering for decades.9 Ion migration appears to be a valid alternative approach to interpret the sulfidation mechanism, because it can be illustrated by the following steps, i.e., steps of sulfidation of ZnO under ZnS skin layer as shown in Figure 2: In step 1, H2S adsorbs on two active sites of ZnS on a completed sulfided grain surface.
H2S + 2• S H+• + HS-•
HS-• + ZnO ) ZnS + OH-•
In step 2a, H+ dissociates the adsorption site that is ZnS.
HS-• (ZnO) ) OH-• (ZnS)
H+• (ZnS) ) Zn2+• + HSOH-
In step 3, water is formed from yielded and the neighboring adsorbed H+ and active sites are reclaimed for the next cycle through water desorption.
OH-• + H+• ) H2O + 2•
Note that this HS- is now in (or under) skin and ready to react with ZnO underneath
HS- + ZnO ) ZnS + OHfollowed by outward ion migration of OH- and combining with the adsorbed proton formed in step 2b.
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In step 2b, adsorbed HS- on the surface is readily available to combine with Zn2+ dissociated by the proton.
HS-• + Zn2+• ) ZnS + H+• + • This initiates the sulfidation of the ZnO second layer. In step 3, water is formed and active sites are liberated through water desorption. This can be achieved after either OHdiffusing out, followed by water formation
OH- + • ) OH-• OH-• + H+• ) H2O + 2• or the proton diffusing in, forming water in the solid, followed by water diffusing out thereafter.
H+• ) • + H+; OH- + H+ ) H2O Parts a and b of step 2 presumably take place simultaneously. Ion migration to the third layer and below can be interpreted on the basis of a similar mechanism, starting with the inward diffuse of the proton to the second sulfided layer. The Rate Law Let us assume that adsorption and dissociation of H2S on active sites is the rate-limiting step. This is reasonable because all other steps described above are basically near-irreversible acid-base reactions. It is more so when the ZnO grain reaches a high conversion. In other words, H2S adsorption on the solid surface becomes less favorable because of the increasing disappearance of ZnO. Hence, the sulfidation rate law can be simply represented as
r ) k1CN2
(1)
where r is the rate, k1 is the intrinsic rate constant, C is the concentration of gaseous H2S, and N is the number of active sites on the surface. N can be a function of the amount of ZnO in the grain because H2S has less affinity to ZnS. It also depends upon the temperature because a higher temperature increases the ion migration (exchange) activity, so that the sulfidation of ZnO under skin is accelerated. Ion migration can very well be the other rate-limiting step after the majority of surface ZnO is consumed. Because it has a direct impact on N, the effect of ion migration can be lumped into N. Hence, N is a function of the amount of surface ZnO accessible to H2S, which depends upon both the total ZnO amount and rate of ion migration. The rate law can be rewritten as
r ) k1C(k2k3F)2
(2)
where F is the bulk (or particle) density, representing the changing amount of ZnO in solid grain. k2 is a constant to linearly correlate N with F, and k3 is a constant to correlate N with the ion-migration rate. The temperature dependence in k3 can be presented generally as an exponential term, as in the Arrhenius law. Combining k values and using moles per unit volume in the bulk density
r ) kCF2
(3)
where k is the apparent rate constant (equivalent to a thirdorder reaction) that includes the kinetic factors discussed above. (9) Fontana, M. G. Corrosion Engineering, 3rd ed.; McGraw-Hill: New York, Nov 1985.
Figure 3. TGA experiment of ZnO conversion (X) over time and a model comparison of RPM and RAIM. Experimental data and RPM modeling results were reproduced from Yu and Sotirchos.1 Sulfidation conditions: 0.5% H2S in N2 on powdered G-72D (50 mesh or 300 µm particle size) at 400 °C. The intrinsic rate of G-72D is calculated as 1 L2 mol-2 s-1 for the RAIM model. Note that mass-transfer resistance is minimal or film diffusion would result in a linear conversion over time.
For the sake of convenience, this rate law is named the RAIM rate law for ZnO sulfidation. Sulfidation equilibrium needs to be included when performing the reactor design calculation. On the basis of the reverse analogue, the reverse reaction of sulfidation (i.e., steam oxidation of ZnS; in fact, it is used to regenerate the spent zinc titanate sorbent with the addition of O2) can be written as
r′ ) k′CF2 ) kCF2/Keq
(4)
where Keq is the sulfidation equilibrium constant. To model ZnO conversion X in TGA experiments for single-particle sulfidation, eq 3 is integrated to
X)
kF0Ct 1 + kF0Ct
(5)
where F0 is the initial ZnO bulk density and t is the sulfidation time. The reverse rate in eq 4 is not considered during TGA graph modeling because its run time is limited. Comparison with the Literature Intrinsic Kinetics. To evaluate intrinsic kinetics for model comparison, select sulfidation experiments of ZnO or the ZnOtype sorbent and their modeling data were reproduced from Yu and Sotirchos.1 These experiments were usually run on TGA at a controlled temperature and fixed H2S partial pressure. External mass-transfer (film diffusion) resistance is minimal for the sulfidation of ZnO powder as concluded from the literature. Figure 3 plots the experimental data and model calculations for the sulfidation of G-72D (a predecessor of G-72E), a ZnO sorbent made by United Catalyst (now SCI). The RPM calculation is included in Figure 3 for comparison, and it appears to overpredict the kinetics after ∼60% conversion. This can be attributed to the RPM, presumably not considering the layer diffusion of H2S (kinetics slower than the surface reaction as suggested in the literature) through initially formed ZnS skin on the sorbent surface. USC is a conventional gas-solid reaction model and welldocumented in the literature.10 Many modified USC models for sulfidation have been developed.2,11 Nevertheless, the USC model does not take the internal surface [as indicated from the Brunauer-
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Figure 4. Sulfidation experiment and model comparison of OGM and RAIM. Experimental and OGM data were reproduced from Lew et al.5 The symbols 0 and O stand for the experimental data points at 400 and 500 °C, respectively. Reaction conditions: a powdered zinctitanium oxide sorbent at 400 and 500 °C in 2% H2S and 1% H2 in N2.
Emmett-Teller (BET) surface area measured for porous materials] into account, presumably resulting in an underprediction at the beginning and overprediction toward the end of sulfidation, a result similar to RPM. In comparison with USC, OGM realistically describes the surface reaction simultaneously over the external solid and internal pore surfaces. It also includes the reduction of the surface area because of the molar volume increase of the solid (from ZnO to ZnS) during sulfidation. Its sulfidation mechanism, however, assumes that the reaction surface boundary retreats from the skin surface at the same rate for all grain sizes. This assumption can result in a rate contradiction at the intersections of grains of different sizes. Using a sophisticated numerical modeling process, the OGM model is the latest sulfidation model and frequently studied in the literature. In another case, Figure 4 plots the comparison of OGM and RAIM on the TGA graph for sulfidation with a zinc-titanium oxide sorbent at 400 and 500 °C. The RAIM model gives a good prediction for sulfidation at 400 °C, while its prediction at 500 °C is not as satisfactory. This can be related to two factors. The existence of titanium in the sorbent can change the ZnO sulfidation mechanism, and the applicable temperature range for ZnO sulfidation is below 450 °C as recommended by the manufacturers. In sulfidation of the zinc-titanium oxide experiment at 500 °C, the reaction order for ZnO density in eq 3 may change with a decreasing surface adsorption (with an increasing temperature). It seems that the RAIM rate law applies best for ZnO sulfidation at temperatures below 400 °C, which is near the simulation temperature for the sulfidation reactor design in our case. Note that the rate constant is the only changing variable in eq 5 for curve fitting in Figures 3 and 4, an advantage for the sulfidation intrinsic kinetics study. Pore Diffusion. Pore diffusion is the intermediate step for a modeling continuation from the kinetics rate law to the packed bed simulation. Sulfidation modeling of the porous ZnO pellet using the RAIM rate law was carried out with pore diffusion on the general process modeling system (gPROMS) platform (pseenterprise.com). Modeling formulations for pore diffusion are presented in the Supporting Information. Using the apparent sulfidation rate constant obtained from the TGA experiments, effective pore diffusivity of H2S is a modeling parameter in pellet sulfidation. (10) Levenspiel, O. Chemical Reaction Engineering, 2nd ed.; John Wiley and Sons, Inc.: New York, 1972. (11) Fan, H.; Li, C.; Guo, H.; Xie, K. Microkinetics of H2S removal by zinc oxide in the presence of moist gas atmosphere. J. Nat. Gas Chem. 2003, 12, 43-48.
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Figure 5. TGA results for G-72C ZnO (1.25/16” pellets) conversion with 1% H2S in N2 at 375 °C fitted by the gPROMS model. Experimental data were from Gibson and Harrison.3
Figure 6. Simulated radial ZnS density (mol/L) profile in a spent G-72C pellet after 15 min of sulfidation. With an estimated fixed saturated capacity, the effective diffusivity changes at 1 × 10-5 (b), 1 × 10-6 (9), and 1 × 10-7 (2) m2/s.
TGA results were also available in the literature for ZnO G-72C (another predecessor of G-72E) pellets at 375 °C.3 Figure 5 reproduces the TGA results of G-72C pellet conversion in 1% H2S in N2 at 375 °C. It also plots the corresponding gPROMS modeling results. The gPROMS model combines the RAIM rate law for intrinsic kinetics and pellet diffusion with two fitting parameters, effective diffusivity of gaseous H2S in pores and saturated sulfur adsorption capacity of the G-72C pellet. In theory, the saturated capacity of the ZnO grain of 100% purity is the stoichiometric capacity for ZnO at 100% conversion or 39.5% by weight. For pellets, its saturated sulfidation capacity or maximal conversion is always lower than the stoichiometric value because its sulfidation service time is limited. As a modeling parameter, the saturated capacity for the G-72C pellet at the specified condition is calculated at about 24% (or 60% in ZnO conversion). The diffusivity was around 1.67 × 10-6 m2/s. The simulated radial ZnS concentration profile for the sulfided pellet obtained in this study with a changing effective diffusivity of 1 × 10-5, 1 × 10-6, and 1 × 10-7 m2/s is illustrated in Figure 6. The calculated diffusivity of 1.67 × 10-6 m2/s was found consistent with the observed sulfur profile on the micrograph.3 Experimental Apparatus. A 3/8” microflow packed reactor column (Figure 7) was constructed for sulfidation experiments with porous G-72E ZnO sorbent from SCI. Two cylindrical pellets of
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Figure 7. Schematic of the bench-scale sulfidation reactor for powdered and pellet G-72E. “P” stands for the pressure gauge; “V” stands for valves; “FM” stands for flowmeters; and “MFC” stands for mass-flow controller. Water saturator controls the humidity of the gas stream to simulate the field operating conditions.
Figure 8. (a, top) Breakthrough curve of powdered G-72E with no steam addition at 350 °C. Reaction conditions: 0.165 g (0.11 cm3) of powder (-200 mesh or
