(395o) New Insights On Sulfidation and Oxidizing Regeneration Reactions of Metal Oxides Used As Sulfur Sorbents

Abstract

Fischer-Tropsch based XTL processes and IGCC technologies are attractive alternatives for future energy production. In order to prevent unit corrosion or FT catalysts poisoning, deep desulfurization of syngas is achieved with metal oxides able to react with H₂S to form sequestrated sulfide.

The present work focused on the solid-state transformations involved in metal oxides sulfidation and oxidizing regeneration reactions. Reactions kinetics were studied through thermogravimetric experiments. Regarding ZnO sulfidation, in opposition to common reaction modelings described in the literature, which assume an inward development of ZnS (shrinking core model), we propose a mechanism with ZnS outward growth, based on TEM and SEM observations. In agreement with the experimental data, an expression of the reaction rate has been determined.

The regeneration properties of various metal oxides were also studied. We will show that mixed metal oxides are for this purpose more adapted, and that lower regeneration temperature can be reached.

Introduction

Fischer-Tropsch (FT) based XTL processes and IGCC technologies are attractive alternatives for future energy production [1,2]. In order to prevent unit corrosion or FT catalysts poisoning, deep desulfurization of syngas is achieved with metal oxides, like zinc oxide, which reacts according to the reaction:

ZnO(s) + H₂S(g) → ZnS(s) + H₂O(g)

The use of metal oxides as sulfur sorbents allows to reach very low H₂S levels in the treated gases, due to the highly favored sulfidation reactions. For IGCC applications, regenerable sulfur sorbents attract a great deal of interest, regarding the amount of H₂S to be removed from the gases and the process operating conditions. However the way back to the oxide is not favored at low temperature, as the formation of sulfate phases is usually observed during oxidative regeneration processes [3-5].

Although the ZnO sulfidation reaction has also been widely studied [6], little is known about the transformation into ZnS at the crystal scale, its detailed mechanism and kinetics. In this communication, we will present last advances in this field resulting from work performed at IFP Energies Nouvelles in collaboration with the Centre SPIN and IRCELyon laboratories. Especially, we will present the main results obtained in a study focused on the sulfidation of ZnO as model metal oxide. We will show evidences for an outward growth process of the resulting ZnS phase, which are in contradiction with the previously admitted assumption of an inward growth on ZnS with diffusion of sulfur species.

The regeneration properties of various metal oxides have also been studied, which will be presented. We will show that mixed metal oxides are for this purpose more adapted, and that lower regeneration temperature can be reached.

Experimental

The sulfidation reaction has been studied through thermogravimetric experiments (TGA) performed under variable H₂S partial pressures (0.001 to 1 bar) in helium, in a temperature range from 200°C to 400°C. TGA has also been used to study regeneration properties, under O₂ containing atmosphere (5-15% in N₂), between 100 and 700°C.

Materials characterizations have been performed by scanning and transmission electron microscopies (SEM, TEM), electron tomography (3D-TEM), X-Ray Diffraction (XRD), N₂ and Kr adsorption isotherm measurements, and real-time in situcharacterizations, such as XRD, Extended X-Ray Absorption Fine Structure (EXAFS) and Fourier Transform Infrared spectroscopy (FTIR).

Results and Discussion

ZnO sulfidation study.Mono-crystalline ZnO materials have been synthesized and the sulfidation reaction has been studied through thermogravimetric experiments in order to establish a kinetic rate model. Kinetic tests based on thermogravimetric measurements have been performed and show that the kinetic rate can be expressed according to the following expression (rate-limiting step assumption) [7]:

dα/dt = Φ(T,P_i).S_m(α)

where Φ (mol.m^-2.s^-1) depends on the nature of the rate-limiting step and is a function of temperature T and partial pressures P_i of the reacting gases, and S_m(α) (m².mol^-1) is a function of the fractional conversion related to the size and geometry of the reaction zone where the rate limiting step is spatially located.

SEM and TEM characterizations of sulfided ZnO particles reveal voids formation that evidence an external growth based mechanism for sulfidation reaction. Therefore the diffusion step is more likely to involve a diffusion of zinc and oxygen atoms from the ZnO/ZnS internal interface to the outer surface of the ZnS particle [8]. Related characterizations will be presented.

A reaction mechanism will be proposed that accounts for these results, and related possible rate laws determined upon approximation of the rate-determining step [9]. Thermogravimetry experiments performed in a wide range of H₂S and H₂O partial pressures have shown that ZnO sulfidation reaction rate exhibits a non linear variation with H₂S partial pressure, and that there is no significant influence of water vapor on reaction kinetics.

The formation of voids at the ZnO/ZnS internal interface, characterized by TEM and electron tomography, has been shown to influence the sulfidation reaction rate. The impact of the decreasing ZnO/ZnS internal interface on reaction kinetics, as well as the outward growth development of the resulting ZnS phase have been taken into account in a modeling, which will be presented in the communication.

Study of regeneration properties of mixed metal oxides.The sulfidation and regeneration properties of various single oxides have been studied. Based on the results obtained on single oxides, various zinc based mixed oxides were selected and synthesized, and their reactivity toward both sulfidation and oxidative regeneration was evaluated.

In particular after sulfidation step, Zn-Mo mixed oxides were shown to require lower regeneration temperature under O₂containing atmospheres than zinc oxide (from 150 to 200°C lower). Regeneration of sulfided mixed oxides was thus shown to exhibit synergetic effects resulting from exothermic oxidative reactions of molybdenum phases [10].

Moreover, textural modifications also occur during the sulfidation and regeneration processes. More characterizations on the modifications of the structure and texture of the mixed-oxide sorbents will be presented during the communication.

Conclusions

The researches performed at IFP Energies Nouvelles have allowed to bring new insights on the solid-state transformations involved during metal oxides sulfidation and regeneration processes. These results question generally admitted assumptions used for previous sulfidation reaction kinetics modeling. Kinetic model based on an outward growth development will be presented.

The sulfidation and regeneration properties of various single and mixed oxides have also been investigated. A decrease of 200°C of the regeneration temperature has been observed for some mixed metal oxides, and mechanisms allowing such temperature decrease will be presented.

In fine, this better understanding of the sorbents sulfidation and regeneration properties should allow to improve the efficiency of metal oxides sorbents-based desulfurization processes.

References

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(10) V. Girard, D. Chiche, A. Baudot, D. Bazer-Bachi, C. Geantet, To be published