Evaluation of the cycling performance of a sorbent for H2S removal and simulation of desulfurization-regeneration processes

Chemical Engineering Journal 326 (2017) 1255–1265

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Evaluation of the cycling performance of a sorbent for H2S removal and simulation of desulfurization-regeneration processes Yu Feng a, John Wen a,b, Yongfeng Hu a,c, Bin Wu a, Mengmeng Wu a, Jie Mi a,⇑ a

Key Laboratory of Coal Science and Technology of Shanxi Province and Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, Shanxi, PR China Department of Mechanical and Mechatronics Engineering, University of Waterloo, 200 University Avenue West, Waterloo, ON N2L 3G1, Canada c Canadian Light Source, 44 Innovation Boulevard, Saskatoon, SK S7N 2V3, Canada b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


12 cyclic tests were performed to

study the reusability of desulfurization sorbent.
Mass transfer and breakthrough behavior of H2S removal were simulated and predicted.
Simulated data proves the reliability of COMSOL in desulfurization and regeneration.

a r t i c l e

i n f o

Article history: Received 1 March 2017 Received in revised form 14 May 2017 Accepted 15 May 2017 Available online 15 June 2017 Keywords: H2S removal Sorbent Cycling performance Simulation Modeling

⇑ Corresponding author. E-mail address: [email protected] (J. Mi). http://dx.doi.org/10.1016/j.cej.2017.05.098 1385-8947/Ó 2017 Elsevier B.V. All rights reserved.

a b s t r a c t High temperature gas desulfurization is an efficient and environmentally-friendly process for syngas purification. This paper investigated the cycling behaviors of iron oxide/red clay desulfurization sorbents within 12 desulfurization-regeneration cycles in a fixed-bed quartz reactor. The results showed that the sulfur capacities of the regenerated sorbents reduced to less than 50% with all the regeneration rates of the sorbents exceeded 70.15%. The surface and structural properties of the sorbents in the desulfurization-regeneration cycles were characterized by XRD, XPS, SEM, BET and elemental mapping analyses. According to XRD analyses, the particle size of the sorbents gradually increased with the number of desulfurization-regeneration cycles. These cycles reduced the surface area and blocked the pore structure, which have a contribution on the degenerated desulfurization performances of regenerated sorbents. The XPS and EDS spectra indicated the existence of the sulfur-containing compounds in the cycled sorbents, contributing to the lowered regeneration rate of the sorbents. The concentrations of iron and oxygen on the surface decreased over the repeated cycles, which are adverse to the adsorption of acidic H2S. A simulation model was created using COMSOL Multiphysics software to predict the performance and characteristics of the desulfurization-regeneration processes and elucidate its specific mechanism in the desulfurization and regeneration reactions. The comparison between predictions using COMSOL and experimental results were also conducted. Although there are deviations observed in the comparison, the simulated results match the experimental data to a large degree and show the feasibility of the COMSOL Multiphysics in the study of desulfurization and regeneration. Ó 2017 Elsevier B.V. All rights reserved.

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1. Introduction Hydrogen sulfide is a well-known catalyst toxicant in many industrial processes such as natural gas reforming, coal gasification, coking and flue gas treatment [1,2]. Hydrogen sulfide in the products of coal gasification not only results in the corrosion of equipment and piping but also causes serious environmental pollution when discharged into the atmosphere. Two major desulfurization technologies, namely, wet cleaning and dry desulfurization, have been developed to address these issues [3,4]. Dry desulfurization is an attractive solution that simplifies the syngas treatment. It is more efficient and potentially reduces the cost of producer gas when compared with wet cleaning. It allows for both mid- and high-temperature desulfurization processes and can reduce the H2S concentration to a low level [5]. During these desulfurization processes, a sorbent, usually a metal oxide or a carbonate, is reacted with H2S to remove it. Westemorland et al. conducted a pioneering thermodynamics study of various oxides and carbonates and the affinity of these sorbent for H2S adsorption [6]. Among them, iron oxide has been investigated to develop a durable desulfurization sorbent for H2S removal [7,8]. It shows a high desulfurization capacity within a broad temperature range and can be conveniently regenerated through oxidization in air or N2-diluted air at considerably lower temperatures, which improves the quality of the products and reduces the operation cost. Since iron oxide is naturally abundant, iron oxide-based sorbents are potentially promising candidates for H2S desulfurization with better economic and environmental benefits. In addition to desulfurization capacities of sorbents, to satisfy the industrial demand, hot coal gas desulfurization sorbents must exhibit desirable regeneration performance and a good reusability [8]. During regeneration, the metal sulfide converted back into metal oxide. Since the regeneration process is usually timeconsuming, the overall efficiency of the desulfurizationregeneration procedure can be significantly improved if the reaction rate of regeneration could be increased. The regeneration of high temperature desulfurization sorbents using O2 requires a shorter time (i.e., a faster conversion rate) than using steam, SO2 or the mixtures of steam and O2 [8]. Moreover, oxygen vacancies on the metal oxide surface can be repaired to enhance the desulfurization performance. Thus, it has great benefits using O2 as feeding gas in regeneration. Fan et al. prepared iron oxide sorbents from red mud using mixed clay as the binder to increase the resistance to powdering and durability of sorbent over multiple desulfurization and regeneration cycles [9]. There was almost no decline in sulfur capture capacity for the first 1–4 cycles, however, a relatively low sulfur capture capacity for the fifth cycle was observed which was probably associated with pore volume changes and pore distribution changes due to accumulated thermal sintering in successive cycles. Gupta et al. carried out 100 desulfurization-regeneration cycles with zinc ferrite sorbent and reported that the loss in capacity of sorbent was likely due to a combination of the following two factors: (1) loss in reactivity caused by changes in the chemical and physical characteristics of the sorbent, and (2) loss of sorbent in the reactor caused by elutriation and loss of mechanical strength [1]. In addition, in our previous paper, iron oxide/arenaceous clay sorbent has been intensively investigated both in its optimization of preparation conditions and cycling performance in multiple desulfurization-regeneration cycles [10]. The sorbent performed well in its initial desulfurization process and the desulfurization capacity deteriorated obviously after 3 rounds desulfurizationregeneration cycle due to the decreased mechanical strength. Thus, red clay, a kind of clay with high mechanical strength was introduced as the carrier in the sorbent to improve the stability and

reusability of sorbent in the successive desulfurizationregeneration cycles. Multiscale modeling approaches have been utilized to determine the specific design and control procedure of operating parameters and predict the experimental data [11–16]. COMSOL multiphysics software was used to compute the different fixedbed adsorption models [17,18] focusing on the adsorption of CO2 and ethanol. In addition, Ortiz et al. firstly modeled and simulated the breakthrough curves for hydrogen sulfide adsorption from biogas using COMSOL Multiphysics software. The predicted breakthrough curves matched the experimental data and were also compared with those predictions obtained in their previous work by Aspen Adsorption assuming ideal plug flow [12,19]. The key feature of multiphysics simulations is its ability to handle different kinds of coupled non-linear partial differential equations (PDE) and differential algebraic equations (DAE) within a single platform. It is expected that the research, development and production costs of sorbent regeneration will decrease, and its energy efficiency will increase accompanied with an improved understanding of complex physical and chemical phenomena associated with the process of interest. In the literature, 3D COMSOL Multiphysics model has been used to achieve this goal [11–13]. This work evaluates the cyclic performance of a high temperature desulfurization sorbent (Fe2O3) for H2S removal through analyses of chemical and structural properties of the reactants and products from both desulfurization and regeneration. A series of cyclic desulfurization-regeneration experiments were performed in a quartz-fixed-bed reactor with H2S removal efficiency was characterized. The surface and structural characteristics of the sorbents existing in the successive desulfurization and regeneration cycles were characterized by XRD (X-ray diffraction), XPS (X-ray photoelectron spectroscopy), N2 adsorption-desorption, SEM (scanning electron microscopy) and elemental-mapping to create a comprehensive picture of the phase and properties changes. Simulation results obtained from COMSOL Multiphysics software were validated by contrasting them to experimental results, and these were used to elucidate the reaction behaviors and transport processes during desulfurization and regeneration processes on a single pellet sorbent. 2. Experimental 2.1. Preparation of iron oxide/red clay sorbent FeC2O42H2O, the precursor of Fe2O3, was prepared by the solidstate method [9] using FeCl24H2O and H2C2O42H2O. The raw materials were mixed with a molar ratio of 1:1 and homogenized in an agate mortar before being dried at 80 °C for 1 h. Next, the mixture was dried at 100 °C for 2 h for complete removal of water. Iron oxide sorbent was then prepared from the mixture of FeC2O42H2O and the red clay binder (Fe2O3:binder = 3:7) which was homogenized in a ball mill for 2 h. In order to achieve a thick paste, water was added to the mixture powder with the appropriate consistent dropping speed which was controlled by a syringe. Then, the sorbent was extruded in a cylindrical extruder (3 mm  3 mm for multi-cycle experiments and 25 mm  10 mm for single pellet test and simulation) and then dried at 90 °C for 3 h. Finally, the sorbent was calcinated in flowing air (300 ml min1), heated up from ambient temperature to 500 °C with a heating rate of 5 °C min1, and maintained for 30 min at 500 °C by microwave heating (The as-prepared sorbent from this process is denoted as CF, which implies the sorbent with a certain composition and preparation conditions). The detailed parameters (chemical composition and preparation conditions) are the same between the particle sorbent used in desulfurization-regeneration cycles and the single pellet

Y. Feng et al. / Chemical Engineering Journal 326 (2017) 1255–1265

sorbent used in modeling and simulation. The only difference between the two kinds of sorbents is the dimension, 3  3 mm for the particle sorbent and height = 25 mm, diameter = 10 mm for the single pellet sorbent. In the in-house microwave heating system, a quartz tube was inserted in the microwave tube furnace (Qingdao MKW Microwave Applied Technology Co.). The temperature of the sorbent bed was measured using a thermocouple (type K) placed inside of the quartz tube and near its middle position. 2.2. Desulfurization experiment Prior to the regeneration test, the desulfurization experiments of the sorbent were conducted in a fixed-bed quartz reactor (20 mm in diameter, about 650 mm in length), as shown in Fig. 1. There are two kinds of desulfurization tests. One was for the evaluation of cycling performance of sorbent (3  3 mm for sorbent and 20 mm for the outside diameter of reactor). Totally, 10 g of sorbent (about 50–55 mm in height in the quartz tube) was used in each desulfurization experiment. The other one desulfurization experiment was conducted with the single pellet sorbent (height = 25 mm, diameter = 10 mm for sorbent and inner diameter (i.d.) = 13 mm, length = 450 mm for the reactor). In the reactor, the desulfurization sorbent was suspended in the reaction tube in order to ensure that the various surfaces of the desulfurizer are exposed to the feeding gas. The temperature of sorbent was measured by a thermocouple (type K) fixed at the center of the sorbent bed. The desulfurization experiments were performed at 500 °C using the mixed gas, containing 2500 ppm H2S and balance N2, at the volume space velocity = 2000 m3 (h m3)1. Besides, the effect of other gases exist in the syngas was not investigated in the present paper, the chemical and physical properties of sorbents were only tested and discussed under the mixed atmosphere of H2S and N2. H2S concentrations at the inlet and outlet tail gases were measured by a GC equipped with a thermal conductivity detector (TCD) and a flame photometric detector (FPD). When the concentration of H2S in the outlet gas reached 500 ppm (20% of the inlet concentration of H2S), the reaction stopped. The sulfur capacity (SC) of sorbent was calculated by the following equation:

SC ¼ WHSV 

MS  VM

Z

t

ðC in  C out Þdt  104

H 2S

1

5

4

5

GC

7

8 10

9

11 12

14

13

12

TC 15

14

GC O2

2.3. Regeneration experiment In order to obtain the regeneration ability of the present samples, the regeneration reaction was carried out with the same conditions, 650 °C, 4 vol.% O2 (N2 as the balance gas) and the volume space velocity of 3000 m3 (h m3)1. The iodometry was used to periodically detect the concentration of SO2 in the outlet gas. The regeneration was considered to be complete when SO2 was not traced in the outlet gas from the reactor. The regeneration rate (Xr) was defined as:

Xr ¼

C rb  C ra  100% C rb

ð2Þ

where, Crb and Cra are sulfur contents (g) of the sorbents before and after regeneration, respectively, both of which were determined through a. 2.4. Characteristic of sorbent The crystal structures of the sorbents were examined by an Xray diffraction device (JSM-6360 LV) using Cu Ka radiation. Scanning electron microscopy (SEM, JSM-26360LV) was used to characterize the morphology of samples. XPS data of samples were collected on a PHI5000C spectrometer using an Al Ka source operating at 250 W and 93.9 eV passed energy. The analysis of pore structure was conducted on a Porosity Analyzer (Micromeritics TriStar-3000). The surface area was calculated based on the Brunauer-Emmett-Teller (BET) theory. The content of sulfur in the sorbent was measured by KZDL-8F-type fast-smart sulfur instrument made by Hebi Xianke Coal Instrument Co., Ltd. 3. COMSOL Multiphysics modeling

6

N2

where WHSV is the hourly mass space velocity (g-H2S g1sorbent h1); MS stands for molar weight of H2S (34 g mol1); VM is molar volume of H2S at 1 atm and 25 °C (24.5 L mol1), Cin and Cout represent the concentration of H2S in the inlet and outlet gas, respectively, and t is the breakthrough time of desulfurization process.

ð1Þ

0

2 3

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16

Fig. 1. Schematic figure of the desulfurization and regeneration processes of sorbents (1-cylinder; 2-valve; 3-flow meter; 4-water bath; 5-furnace; 6-thermocouple; 7-temperature controller; 8-inlet gas concentration detection; 9-absorption solution; 10-gas concentration detection; 11- sorbent; 12-quartz tube; 13-tail gas).

COMSOL Multiphysics is based on the finite element method (FEM) and analyzes the coupling of physical models at different scales. A 3D simulation model for the desulfurization and regeneration processes using a fixed-bed reactor is shown in Fig. 2. The reaction system was simulated at atmospheric pressure. A quartz tube (inner diameter (i.d.) = 13 mm, length = 450 mm) was inserted in the middle of the furnace on the fixed-bed reactor, and the CF sorbent (height = 25 mm, diameter = 10 mm) was placed in the tube. The main reactions occurred in desulfurization and regeneration that were considered are Fe2O3 + H2S(g) ? FeS + H2O and FeS + O2(g) ? Fe2O3 + SO2(g), respectively. The operating conditions used in the modeling and simulation within the desulfurization and regeneration reactions were designed to be the same as those of the experimental process. The reaction temperatures of desulfurization and regeneration were fixed at 500 and 650 °C, while the inlet concentrations of H2S and O2 were set to 2500 ppm and 4%, respectively. Based on this, a geometric model was established, and then the finite element mesh was generated. The complete mesh was composed of 118914 domain units, 15036 boundary units and 552 edge units. The relative tolerance and absolute tolerance of the used model are 1  103 and 1  104, respectively. The nomenclature of variables and parameters used in COMSOL are summarized in Tables 1 and 2. The initial and boundary conditions are the following:

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Fig. 2. The modeling and meshing of the reaction in the porous media model using the finite element method.

Table 1 The nomenclature of variables and parameters used in COMSOL.

Table 2 Parameters used in COMSOL implementation.

Nomenclature NA CH2S CN2 C0,H2S C0,N2 Ci CFe2O3 Cp Cp D De Ea F H I kL kP kfj krj M n Ni p p0 qL qP Qbr QL QP Qsv r Rg Ri Sr S TL TP T0 u

vij

frequency factor (s1) concentration of H2S in gas phase concentration of N2 in gas phase inlet concentration (mol/m3) inlet concentration (mol/m3) concentration of species i (mol/m3) concentration of Fe2O3 in solid phase specific heat capacity of fluid (J/(kg K)) specific heat capacity of solid (J/(kg K)) diffusion coefficient (m2/s) effective diffusion in porous media (m2/s) activation energy (J/mol) volume force vector (N) heat capacity (J) identity matrix () thermal conductivity of fluid (W/(m K)) thermal conductivity of solid (W/(m K)) forward rate constants ((mol/m3)1r s1)) reverse rate constants ((mol/m3)1r s1)) molecular mass (g/mol) temperature exponent () flux vector (mol/(m2 s)) pressure (Pa) outlet pressure (Pa) heat flux in fluid (w/m2) heat flux in solid (w/m2) mass source (kg/(m3 s)) heat source in gas phase (W/m3) heat source in solid phase (W/m3) flow rate (m3/s) reaction order () gas constant (8.314 J/(mol K)) reaction rate of i (mol/(m3 s)) entropy change (J mol1 K1) strain-rate tensor () temperature of fluid (°C) temperature of solid (°C) temperature of furnace (°C) velocity vector (m/s) stoichiometric coefficients ()

Greek

ep q qp s sL l j hp

porosity () density of fluid (g/cm3) density of solid (g/cm3) viscous stress tensor (Pa) tortuosity factor () viscosity of the fluid (Pa s) permeability of the porous medium (m2) solid volume fraction ()

Parameters

Value

A Cp Cp,p

7.36E2, 4.43E2 Calculated by chemical module Calculated by chemical module

D

4.79E4, 1.03E04

Ea n

5690, 16080 0

kL kP Qsv

Calculated by chemical module Calculated by chemical module 5.55E6, 5.55E6,

q qp ep j l

Calculated by chemical module Calculated by chemical module 0.48, 0.18 1.00E12, 1.00E13 Calculation by chemical module 0.52, 0.82

hp

For desulfurization: t = 0: u = 0 m/s, p = 1 atm, Tp = TL = 773.15 K, CFe2O3 = 2570 mol/m3, CH2S = 0, CN2, balance gas; C0,H2S = 2500 ppm, C0,N2, balance gas, T0 = 773.15 K, p0 = 1 atm. For regeneration: t = 0: u = 0 m/s, p = 1 atm, Tp = TL = 923.15 K, CFeS = 5140 mol/m3, CO2 = 0, CN2, balance gas; C0,O2 = 4%, C0,N2, balance gas, T0 = 923.15 K, p0 = 1 atm. 3.1. Model equations 3.1.1. Chemical reaction equations The reaction models in COMSOL are based on the mass action law. The reaction rate r i can be described by the mass action law [20]: f

r j ¼ kj

Y

v ij

ci

ð3Þ

i2react

Here, kfj denotes the rate constant. The concentrations of species i are denoted as ci. The stoichiometric coefficient is denoted vij and defined as negative for reactants. In addition to the concentration dependence, the temperature dependence of the reaction rates can be included by using the pre-defined Arrhenius expression for the rate constants:

k ¼ AT n expðEa =Rg TÞ

ð4Þ

Y. Feng et al. / Chemical Engineering Journal 326 (2017) 1255–1265

Here, A denotes the frequency factor (s1), n is the temperature exponent, Ea is the activation energy (J/mol), and Rg is the gas constant, 8.314 J/(mol K) [21]. 3.1.2. Momentum equations The free and porous media flow interface uses the NavierStokes equations to describe the flow in open regions and the Brinkman equations to describe the flow in porous regions [22]. On one hand, the single-phase fluid flow interfaces are based on the Navier-Stokes equations, which in their most general form appear below:

q@u=@t þ qðu  rÞu ¼ r  ½pI þ s þ F

ð5Þ

Eq. (5) is a vector equation that represents the conservation of momentum [23]. For a Newtonian fluid, which has a linear relationship between the stress and strain, Stokes deduced the following expression [24]:

s ¼ 2ls  2=3lðr  uÞI

ð6Þ

In these equations, S is the strain-rate tensor [27]:

S ¼ 1=2ðru þ ðruÞt Þ

ð7Þ

On the other hand, the flow in porous media is governed by a combination of the continuity equation and momentum equation, which together form the Brinkman equations results in [26,27]:

q=ep ð@u=@t þ ðu  rÞu=ep Þ ¼ rp þ r  ½1=ep flðru þ ðruÞT Þ  2=3lðr  uÞIg  ðj1 l þ Q br =e2p Þu þ F ð8Þ The dependent variables in the Brinkman equations are the Darcy velocity and the pressure. In these equations, q is the density, u is the velocity vector, p is pressure, s is the viscous stress tensor, F is the volume force vector, l is the dynamic viscosity of the fluid, ep is the porosity, I is the hydraulic gradient, and j is the permeability of the porous medium. 3.1.3. Mass balance equation

qðr  uÞ ¼ Q br

ð9Þ

Eq. (9) is the continuity equation and represents the conservation of mass of incompressible flow. Here, Qbr is a mass source.

ep @ci =@t þ r  ðDrci Þ þ u  ci ¼ Ri

ð10Þ

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3.1.4. Heat transfer equation The heat transfer equation for porous media is derived from the mixture rule on energies appearing in the solid and fluid heat transfer equations. For non-deformable immobile solids, the related Eq. (14) is shown below [32]:

qp C p;p @T P =@t þ r  qp ¼ Q p

ð14Þ

For a fluid domain where the pressure work and viscous dissipation are neglected, the equation becomes [33]:

qC p @T f =@t þ qC p uL  rT L þ r  qL ¼ Q L

ð15Þ

The mixture rule applies by multiplying the Eq. (15) by the solid volume fraction, hp, multiplying the Eq. (16) by the porosity, ep , and summing up the resulting equations. The solid volume fraction and porosity have the following relationship [34]:

ep þ hp ¼ 1

ð16Þ

The local thermal equilibrium hypothesis assumes the equality of the temperature in the fluid and solid phases [35–37]:

TL ¼ Tp

ð17Þ

In these equations, q is the density, Cp is the specific heat capacity at a constant stress, q is the heat flux by conduction, Q contains additional heat sources, TL is the absolute temperature in the fluid phases, TP is the absolute temperature in the solid phases, and u is the velocity vector. 4. Results and discussion 4.1. Cycling performance of iron oxide/red clay sorbents Thermogravimetric analysis was used to investigate the characteristics of the iron oxide/red clay sorbent during the regeneration process. The TG/DSC curves of the oxidized regeneration process at a heating rate of 10 °C min1 with 4 vol.% O2 are illustrated in Fig. 3. Two weight loss stages were clearly observed: a weight loss in the range of room temperature to 240 °C (which most likely results from the dehydration of the adsorbed water and crystal water in the iron sulfide) and an even faster process starting near 625 °C (corresponding to the mass change of the reaction between iron sulfide and oxygen) [9]. The weight loss at the second stage was 2.71%, which is close to 2.73%—the theoretical value corresponding to the mass change of converting iron sulfide into iron oxide.

Eq. (10) in its form above includes the transport mechanism’s diffusion and convection. Here, ci is the concentration of the species, D denotes the diffusion coefficient in gas phase, Ri is a reaction rate expression for the species, and u is the velocity vector [28]. The flux vector Ni is associated with the mass balance equation above and used in boundary conditions and flux computations. When the diffusion and convection are the only transport mechanisms, the flux vector is defined as

Ni ¼ Drc þ uc

ð11Þ

The effective diffusion in porous media, De, depends on the structure of the porous material and phases involved. Depending on the transport of the species occurring in the saturated porous media, the effective diffusivity is defined as [29,30]:

De ¼ ep =sL D

ð12Þ

Here, D is the single-phase diffusion coefficient in pure liquid, and

sL is the corresponding tortuosity factor, which in the Millington and Quirk model and is defined as [31]:

sL ¼ ep1=3

ð13Þ

Fig. 3. Thermogravimetric analysis of the regeneration process of iron oxide/red clay sorbent.

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Twelve successive desulfurization-regeneration cycles using the CF sorbent (3  3 mm) as the original sample were carried out in a gas containing hydrogen sulfide at 500 °C to investigate the desulfurization ability and cycling performance of the sorbent. The desulfurization breakthrough curves and regeneration rates for these 12 successive cycles are demonstrated in Fig. 4. We found that the desulfurization precision of the sorbent matched the increase of the desulfurization-regeneration cycle times, however, the breakthrough time was shortened nearly 50% after 12 cycles. The breakthrough sulfur capacity corresponding to the H2S concentrations below 500 ppm in the fresh CF sorbent decreased from a maximum value of 11.36 (g S/100 g sorbent) slowly to 11.06 (g S/100 g sorbent) in cycle 2, 10.29 (g S/100 g sorbent) in cycle 3 and 9.98 (g S/100 g sorbent) in cycle 4. This value dropped to 9.43, 9.05, 8.61, 7.73, 7.44, 6.92, 6.65 and 5.62 (g S/100 g sorbent) in cycles 5–12, respectively. The decrease of the breakthrough sulfur capacity in cycles 1–12 could be attributed to the unwanted changes in the surface and structural properties of the sorbent. All regeneration rates were above 70.15% (see inset in Fig. 4). However, pulverization and some cracks in the sorbent CF sorbent were found after the seventh regeneration, and more sorbents lost their strength and transformed into powder during the eleventh regeneration. This may be caused by the physical pulverization of the sorbent due to replacement of the oxygen ions by the sulfur ions (oxides M sulfides) and subsequent molecular volume expansion of iron sulfide during the desulfurization reaction. Repeated contractionexpansion of the microstructure during the successive desulfurization-regeneration cycles has a great adverse effect on the mass transfer and diffusion efficiency. In addition, the sintering that resulted from the repeated calcination at high temperatures and the sulfates accumulated in the desulfurization-regeneration cycles might also be responsible for the declined desulfurization performances and regeneration rates. 4.2. Changes in the sorbent structure Fig. 5 illustrates the N2 adsorption-desorption isotherms and BJH desorption pore size distribution plots of the sorbents in the desulfurization-regeneration multi-cycle tests. Fig. 5 shows that according to IUPAC, all the sorbents exhibited typical I adsorption isotherm for mesoporous materials [38]. Fig. 5 clearly shows that although the differential pore volume distribution patterns were similar to these samples before and

Fig. 5. N2 adsorption-desorption isotherms plots and BJH pore distribution plots of the sorbents in desulfurization-regeneration cycles.

after the multi-cycle tests, the peak differential pore volume went down gradually over the repeated cycles. Fig. 5 and Table 3 indicate that the pore volume and surface area were mostly restored after regeneration. However, the adsorption capacity of the desulfurizer decreased over repeated cycles, and the surface areas reduced slowly from 27.605 m2/g for the fresh sorbent to 16.563 m2/g for the sorbent after 12 cycles. The sulfide sorbent had the lowest pore volume and surface area. The radius of sulfur is bigger than that of oxygen, and thus sulfur displaces oxygen in the desulfurization process and contributes to smaller pores indicating that the surface area and pore volume are diminished. Table 3 shows that the pore volume was the highest for the fresh sorbent at about 0.0736 cm3/g and then dropped to 0.0373 cm3/g after 12 cycles. The progressive reduction of the pore volume during the cycles suggests that the sorbent particles aggregated and the sintering of internal structure. This suggests the change in the pore structure is detrimental to desulfurization. The side product, Fe2(SO4)3, was easily formed during the cycles, and the formation of iron sulfate could plug the pores hindering the mass transfer of H2S into the core of the sorbent particles and thus reducing the H2S removal efficiency. Additionally, the production of Fe2(SO4)3 could also clog the pores because the molar volume of the Fe2(SO4)3 species (39.6 cm3 mol1) was much higher than that of the Fe2O3 species (28.9 cm3 mol1). Fig. 5 implies that although the differential pore size distribution pattern was similar to the sorbent pattern before and after the multi-cycle tests, there was a continual decrease of the percentage of the pores in the size range between 2 nm and 10 nm when the desulfurizer was reused 12 times. Progressive reductions in the surface area, pore volume and pore size in the regenerated sorbents were observed.

Table 3 Surface area pore volume and average pore size of sorbents in desulfurizationregeneration cycles.

Fig. 4. Breakthrough curves and regeneration rates of sorbents in twelve desulfurization-regeneration cycles.

Samples

S (m2 g1)

V (cm3 g1)

Average pore size (nm)

Fresh Sulfide 1st Reg 3rd Reg 5th Reg 7th Reg 9th Reg 11th Reg

27.605 12.488 24.121 23.737 21.601 20.085 19.088 16.563

0.074 0.022 0.066 0.064 0.059 0.047 0.043 0.037

13.487 (±1.021) 6.320 (±0.587) 15.116 (±1.116) 15.246 (±1.393) 14.154 (±1.182) 11.756 (±0.961) 12.487 (±1.025) 11.223 (±0.865)

(±2.313) (±0.985) (±2.158) (±1.926) (±1.657) (±1.686) (±1.427) (±1.067)

(±0.009) (±0.002) (±0.006) (±0.004) (±0.006) (±0.003) (±0.003) (±0.005)

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4.3. XRD patterns of the sorbents formed in the desulfurizationregeneration cycles Fig. 6 depicts the X-ray diffraction peaks of the fresh, sulfide and some regenerated sorbents. The peaks for the fresh sorbent are attributed to Fe2O3 [PDF# 33-0664, #47-1409] (all the PDF refers to the PDF-2 2004, JCPDS) and SiO2 [PDF# 31-1233, #42-1404, #31-1234] [8,9]. After desulfurization, the peaks for sulfide switch to FeS [PDF# 23-1121, #49-1632, #23-1120] and Fe1xS [Fe0.88S: PDF# 24-0220, Fe0.9S: PDF# 29-0726] [9]. Trace amounts Fe2(SO4)3 were observed in the sulfide sorbent, which might be due to the oxidation of SO2 absorbed on the sorbent [39,40]. After these regeneration processes, the XRD patterns of the samples were similar to that of the fresh sorbent except for the appearance of Fe2(SO4)3. In addition, compared to the XRD pattern of the fresh sorbent, the peaks of the regenerated sorbents became much sharper implying increased particle size of the active components in the regenerated sorbents, which possibly results from the relatively high regeneration temperature [9]. The increased particle size may reduce the surface area and block the pore structure, which hinders mass transfer and alters the reaction rates of the desulfurization-regeneration cycles. Like those observed in the sulfide sorbent, the peaks of the sulfur-containing compounds, mainly as Fe2(SO4)3 [PDF# 42-0229], were also observed in the regenerated sorbents, and this may also account for its decreased regeneration rate [39].

4.4. XPS analyses of the sorbents formed in the desulfurizationregeneration cycles The chemical states of the surface Fe species for the fresh sorbent and various reaction stages in the desulfurizationregeneration process were investigated using XPS. Fig. 7 and Table S1 show the binding energy (BE) and elemental quantification. The peaks near 710.5 eV (Feb) most likely belong to Fe3+ [8]. The peak at 707.1 eV (Fea) corresponding to Fe2+ is also confirmed [40]. The peak (at 711.0 eV, assigned to the Fe 2p3/2 orbital of the surface Fe atom) of the fresh sorbent shifted toward a higher BE (711.1 or 711.2 eV) after desulfurization indicating stronger conversion of Fe2O3 to metal sulfide. The broad Fe2p3/2 peak of the sorbent after desulfurization was divided into 2 peaks at about 710.6 (Fec) and 712.6 eV (Fed), which were assigned to iron sulfide and iron sulfate, respectively [41,42].

Fig. 7. Fe2p XPS spectra of sorbents formed in desulfurization-regeneration cycles.

Fig. 7 shows that the XPS spectra after the regeneration processes are similar, but exhibit different shapes and widths compared to fresh sorbent. The broad Fe2p peaks of the sorbents after individual regeneration process were resolved into 3 peaks at 710.9, 711.8 and 713.6 eV, which can be assigned to Fe2O3 and Fe2(SO4)3, respectively. For the regenerated sorbents, their broad Fe 2p3/2 peaks shift slightly toward the high BE region compared to that of the fresh sorbents, which leads to the increase in the outer layer electron density of the Fe atoms. However, the lower binding energy of metal atoms raises the acidity of the Fe ion and facilitates the adsorption of acid H2S on the surface of the desulfurizer (according to the Lewis acid-base electron theory) [8]. Table S1 indicates that the atomic percentage of Fe3+ related to Fe2O3 dropped gradually from the fresh sorbent to the regenerated sorbents, however, a continual increment in the percentage of SO2 4 was also observed. The increased binding energy, formation of sulfates and decreased content of Fe3+ should be responsible for the deteriorating regeneration rate and desulfurization performance of the regenerated sorbents (Fig. 5). The XPS analyses of S2p and O1s of the iron oxide sorbents were also conducted. The XPS spectra and count analyses are shown in Figs. S1–S3 and Table S1. 4.5. FT-IR analyses of the sorbents formed in the desulfurizationregeneration cycles The FT-IR spectra of the eight representative sorbents are compared in Fig. 8. The functional groups of these sorbents were found in the far-infrared region. The characteristic bands of O–H in the sorbents were observed at around 3355 cm1 due to the adsorption of water molecules. In the FT-IR spectra of the fresh sorbent, a strong band appeared at 1043 cm1 along with the closely spaced bands at 468 cm1 belonging to the characteristic stretching vibration of Fe–O [43]. These bands disappeared after desulfurization, while the Fe–S vibration bands at around 482 cm1 were observed [44]. However, a trace amount of ferric sulfate was detected in the regenerated sorbents as supported by the small band located at around 1161 cm1 [45]. The results agree well with the XRD results and XPS analyses. 4.6. SEM-EDS-elemental mapping analyses of the sorbents formed in the desulfurization-regeneration cycles

Fig. 6. XRD patterns of the fresh, sulfide and regenerated sorbents formed in representative desulfurization-regeneration cycles.

Fig. 9 shows the SEM images for the fresh sorbent and sulfide sorbent after the first, third, fifth, seventh, ninth, and eleventh cycles, respectively. Fig. 10 shows that the fresh desulfurizer con-

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Fig. 8. FT-IR spectra of fresh, sulfide and regenerated sorbents formed in desulfurization-regeneration cycles.

Fig. 9. SEM images and elemental-mapping graphs of sorbents in desulfurization-regeneration cycles.

Fig. 10. The elemental-mapping graphs of the concentration distributions of element oxygen in single pellet sorbent in desulfurization (A) and regeneration (B) for different time.

tained some clusters with a good macroporosity and were composed of numerous non-spherical particles (see Table 3 for the

details), while the morphologies of the fresh sorbent and sulfide sorbent differed significantly. The sizes of the particles in the sulfide sorbent were larger, and the structure of the surface was much compact. This is mainly due to the agglomeration and blockage of pores caused by the formation of metal sulfides during desulfurization. After the first cycle, a slight growth was observed in the average size of the clusters. Then, after the third, fifth, and seventh cycles, the structure of the desulfurizers remained, and the surface of the particles was loosely covered with some fine fragments. The more uniform compact surface of the reacted particles shown in Fig. 9 was formed as a consequence of the long-term alternating desulfurization and regeneration as well as the thermal stress in the ninth and 11th cycle regenerated sorbents. All SEM images suggest that the porosity of the desulfurizer particles decreased to some extent, and the distinct tendency of a mild agglomeration also appeared during the long-term tests.

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Fig. 9 also provides elemental mapping data for the fresh sorbent and regenerated sorbents after the first, third, fifth, seventh, ninth, and 11th cycles. Fig. 9 shows that O atoms were spread well in all the sorbents, while the elemental mapping spectrum of the sorbent after desulfurization possesses a sulfide phase, and the intensity of the oxygen decreases to some extent, which could be attributed to the replacement by sulfur. The elemental mapping graphs of the regenerated sorbents show that the intensity of the oxygen phase obviously increased compared to that of the sulfide sorbent, but the intensities of the sulfur-containing species in all the regenerated sorbents were detected more strongly than with fresh sorbent. Additionally, the composition of the various elements was similar for the fresh samples and the samples after the regeneration period indicating that the sorbent was regenerated with a greater efficiency over the multi-cycle tests. Table S2 reports EDS elemental analysis to support the mapping data. The sulfur content increased from fresh sorbent to the sulfide one and decreased in the sorbent of the first regenerated sorbent, however, the sulfur content in the regenerated sorbents continually climbed from the first to the eleventh regenerated sorbent. 4.7. Modeling and simulation of desulfurization-regeneration processes The reactions between Fe2O3 and H2S, FeS and O2 are typical non-catalytic gas-solid reactions. The dynamic models describe these processes including the grain model, equivalent grain model, and shrinking core model [47,48]. The shrinking core model assumes that the reactions occur at a sharp interface, which divides the reacted outer shell and the unreacted core of the solid making it suitable for the pore-free solid sorbent or fast reaction process. Researchers have shown that the shrinking core model can successfully predict experimental data for high temperature desulfurization and regeneration [47,48]. A desulfurization experiment was conducted to investigate the pathway of the reaction using a single pellet sorbent. The single pellet sorbent reacted for different periods after the desulfurization reaction commenced and proceeded along the radial and axial directions. Fig. 10 shows that the cross section can be divided into 2 separate areas: the reacted area and inner unreacted area. The reacted area is full of blank indicating the desulfurization product FeS was formed, while the unreacted area (in red) shows that Fe2O3 was not transformed into iron sulfide. Fig. 10 also shows the elemental mapping data of O during the process of desulfurization. In the mapping graphs, the zone with

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sparse green dots represents the reacted region (i.e., the oxygen atoms were replaced by sulfur atoms). The inner area with high densities of green dots is the unreacted core region. This observation indicates that the reaction proceeded in a step-by-step manner from the surface to the center core of the sorbent during the desulfurization. This process gradually converted the iron oxides into iron sulfides. A similar phenomenon was observed in the process of regeneration using the single pellet sorbent, which exhibited a contrast process compared to desulfurization reaction. Thus, the results suggest that the shrinking core model could fit the desulfurization and regeneration reactions. A sketch map of the shrink core model in the process of desulfurization upon single pellet sorbent is shown in Fig. 11A and B, and the results of the transient simulations using COMSOL Multiphysics are shown in Fig. 12A and B. The desulfurization and regeneration reactions are demonstrated by a replacement between iron oxide and iron sulfide. A good compatibility between the experimental characterizations of the desulfurization and regeneration reactions and simulation results obtained from 3D modeling was observed. The results indicate that the COMSOL Multiphysics software and the current simulation model can be successfully used to simulate the desulfurization and regeneration processes. The experimental and predicted breakthrough curves of outlet concentrations of H2S and SO2 are shown in Fig. 13A and B. The predicted and experimental values coincide and match each other in general, heat transfer in porous medium and chemistry, although there are still some degree of deviations with the predictions in both the desulfurization and regeneration processes especially in the initial section of the curves that show a larger increased trend and a larger reaction rate compared to that of practical experiments. There observations are because a lower driving force but also a higher resistance to diffusion through the pores. When the pores become saturated in the later period of reaction, the overall mass transfer coefficient decreases [19]. On the other hand, the fluid flow and chemical reactions are much more complicated in the experiments. The predicted reactions proceed in a more idealization way than the actual situation, means that the chemical reaction degree of the prediction is greater in the same time period. However, the experimental one needs to undergo a complex and a variable flow and reaction process (side reactions) before the gas flows out of the reactor both in the free and porous medium domains. These differences have a contribution to the deviation between experimental and predicted results. Further work will be done to relieve and eliminate the influence of these factors to get a more accurate prediction of the process.

Fig. 11. Sketch map of the shrink core model in the processes of desulfurization (A) and regeneration (B) upon single pellet sorbent.

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Fig. 12. Demonstrations of shrinking core model using COMSOL Multiphysics in desulfurization (A) and regeneration (B) processes.

Fig. 13. Experimental and simulated breakthrough curves of desulfurization (A) and regeneration (B) processes.

5. Conclusion We investigated the cycling behaviors of iron oxide/red clay desulfurization sorbents in a fixed-bed quartz reactor. XRD, XPS, SEM-elemental mapping, EDS and BET were used to analyze the surface and inner structural properties of the sorbents before and after regeneration. The characterization data suggests that the desulfurizer particles showed both a good multicycle stability and recyclability. However, the degraded desulfurization performances of the regenerated sorbents were observed, which can be attributed to the unfavorable changes occurring on the surface and inside the inner structure. XPS analysis implies the sulfurcontaining compounds were generated during the regeneration, contributing to the lowered desulfurization activity and regeneration rate of the sorbent. The small amount of surface lattice oxygen participating in the desulfurization reaction also contributed to this effect, which was detrimental to the adsorption of acidic hydrogen sulfide and the desulfurization reaction on the surface of sorbent. In addition, the N2 adsorption and SEM data reveal that

a high surface area and pore volume of the sorbent played important roles in the diffusion and adsorption of H2S molecules via the interaction of highly dispersed active sites with H2S. A 3D mathematical model was created using the COMSOL Multiphysics for the desulfurization and regeneration processes. Modeling of the fixedbed desulfurization-regeneration processes upon single pellet sorbent was simulated from both the view of shrink core model and the prediction of breakthrough curves by COMSOL. The simulated processes agree with the real situation in the term of shrink core model in both desulfurization and regeneration reactions. This indicate the validation in the qualitative analysis of COMSOL. The results of simulation imply that the desulfurization and regeneration of sorbents follow the shrink core model. On the other hand, the predicted breakthrough curves also matched the experimental values both in the process of desulfurization and regeneration. This simulator is a powerful computational tool that can predict the performance and characteristics of desulfurization and regeneration processes. The most important finding in this work was the possibility of accurately modeling the breakthrough behavior of the fixed-bed desulfurization sorbent using the kinetic and thermodynamic parameters obtained from the batch experiments. More detailed and specific work is needed to increase the match degree. Acknowledgment This work was supported by the National Natural Science Foundation of China (51272170/21276172). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2017.05.098. References [1] X. Meng, W.D. Jong, R. Pal, In bed and downstream hot gas desulphurization during solid fuel gasification, A review, Fuel Process. Technol. 91 (2010) 964– 981.

Y. Feng et al. / Chemical Engineering Journal 326 (2017) 1255–1265 [2] F.J.G. Ortiz, P.G. Aguilera, P. Ollero, Biogas desulfurization by adsorption on thermally treated sewage-sludge, Sep. Purif. Technol. 123 (2014) 200–213. [3] F.J.G. Ortiz, P.G. Aguilera, High performance regenerative adsorption of hydrogen sulfide from biogas on thermally-treated sewage-sludge, Fuel Process. Technol. 145 (2016) 148–156. [4] J.G. Speight, The Chemistry and Technology of Coal, third ed., CRC Press, New York, 2012. [5] D. Liu, W. Zhou, J. Wu, CeO2–MnOx/ZSM-5 sorbents for H2S removal at high temperature, Chem. Eng. J. 284 (2016) 862–871. [6] P.R. Westmoreland, D.P. Harrison, Evaluation of candidate solids for hightemperature desulfurization of low-Btu gases, Environ. Sci. Technol. 10 (1976) 659–661. [7] F. Yin, J. Yu, S. Gupta, S. Wang, D. Wang, J. Dou, Comparison of desulfurization characteristics of lignite char-supported Fe and Fe–Mo sorbents for hot gas cleaning, Fuel Process. Technol. 117 (2014) 17–22. [8] Y. Feng, J. Mi, B.W. Chang, Regeneration performance and characteristic of iron oxide/arenaceous sorbents in the atmosphere of O2/N2, Fuel 186 (2016) 838– 845. [9] H.L. Fan, C.H. Li, Testing of iron oxide sorbent for high-temperature coal gas desulfurization, Energy Source Part A 27 (2005) 245–250. [10] Y. Feng, T. Hu, M. Wu, J. Shangguan, H. Fan, J. Mi, Effect of microwave irradiation on the preparation of iron oxide/arenaceous clay sorbent for hot coal gas desulfurization, Fuel Process. Technol. 148 (2016) 35–42. [11] G. Chabot, R. Guilet, P. Cognet, A mathematical modeling of catalytic millifixed bed reactor for Fischer-Tropsch synthesis: influence of tube diameter on Fischer Tropsch selectivity and thermal behavior, Chem. Eng. Sci. 127 (2015) 72–83. [12] F.J.G. Ortiz, P.G. Aguilera, P. Ollero, Modeling and simulation of the adsorption of biogas hydrogen sulfide on treated sewage-sludge, Chem. Eng. J. 253 (2014) 305–315. [13] J. Chang, H. Tian, J. Jiang, Simulation and experimental study on the desulfurization for smelter off-gas using a recycling Ca-based desulfurizer, Chem. Eng. J. 291 (2016) 225–237. [14] F.J.G. Ortiz, A simple realistic modeling of full-scale wet limestone FGD units, Chem. Eng. J. 165 (2010) 426–439. [15] F.J.G. Ortiz, A. Serrera, S. Galera, Experimental study of the supercritical water reforming of glycerol without the addition of a catalyst, Energy 56 (2013) 193– 206. [16] F.J. García-Mateos, R. Ruiz-Rosas, M.D. Marqués, Removal of paracetamol on biomass-derived activated carbon: modeling the fixed bed breakthrough curves using batch adsorption experiments, Chem. Eng. J. 279 (2015) 18–30. [17] A. Elsayed, S. Mahmoud, R. Al-Dadah, Experimental and numerical investigation of the effect of pellet size on the adsorption characteristics of activated carbon/ethanol, Energy Procedia 61 (2015) 2327–2330. [18] M.S. Shafeeyan, M.A.W.D. Wan, A. Shamiri, Modeling of carbon dioxide adsorption onto ammonia-modified activated carbon: kinetic analysis and breakthrough behavior, Energy Fuels 29 (2015) 6565–6577. [19] P.G. Aguilera, F.J.G. Ortiz, Prediction of fixed-bed breakthrough curves for H2S adsorption from biogas: importance of axial dispersion for design, Chem. Eng. J. 289 (2016) 93–98. [20] A.J. Lotka, Undamped oscillations derived from the law of mass action, J. Am. Chem. Soc. 8 (2002) 1595–1599. [21] J.A. Tamada, A.S. Kertes, C.J. King, Extraction of carboxylic acids with amine extractants. 1. Equilibria and law of mass action modeling, Ind. Eng. Chem. Res. 29 (1990) 1319–1326. [22] P.M. Gresho, R.L. Sani, Incompressible flow and the finite element method, Isothermal Laminar Flow, vol. 2, John Wiley & Sons, (2000). [23] F. Augier, F. Idoux, J.Y. Delenne, Numerical simulations of transfer and transport properties inside packed beds of spherical particles, Chem. Eng. Sci. 65 (2010) 1055–1064.

1265

[24] A.B. Metzner, R.E. Otto, Agitation of non-Newtonian fluids, AIChE J. 3 (1957) 3– 10. [26] D. Nield, A. Bejan, Convection in Porous Media, third ed., Springer, 2006. [27] M. Le Bars, M.G. Worster, Interfacial conditions between a pure fluid and a porous medium: implications for binary alloy solidification, J. Fluid. Mech. 550 (2006) 149–173. [28] J.H. Cushman, Fractional advection-dispersion equation: a classical mass balance with convolution-Fickian flux, Water Resour. Res. 36 (2000) 3763– 3766. [29] R.D. Burnett, E.O. Frind, An alternating direction galerkin technique for simulation of groundwater contaminant transport in three dimensions: 2 dimensionality effects, Water Resour. Res. 23 (1987) 695–705. [30] B. Noetinger, D. Roubinet, A. Russian, et al., Random walk methods for modeling hydrodynamic transport in porous and fractured media from pore to reservoir scale, Transp. Porous Media 115 (2016) 345–385. [31] R.J. Millington, J.M. Quirk, Permeability of porous solids, Trans. Faraday Soc. 57 (1961) 1200–1207. [32] S. Sachdev, S. Pareek, B. Mahadevan, A. Deshpande, Modeling and Simulation of Single Phase Flow and Heat Transfer in Packed Beds, Proceedings of the COMSOL Conference, Bangalore, (2012). [33] R.B. Bird, W.E. Stewart, E.N. Lightfoot, Transport Phenomena, second ed., John Wiley & Sons, 2007. [34] D.A. Nield, A. Bejan, Convection in porous media, Convection Heat Transfer, fourth ed., John Wiley & Sons Inc, Hoboken, NJ, USA, 2013. [35] D.A. Nield, Effects of local thermal non-equilibrium in steady convective processes in a saturated porous medium: forced convection in a channel, J. Porous Media 1 (1998) 181–186. [36] W.J. Minkowycz, On departure from local thermal equilibrium in porous media due to a rapidly changing heat source: the sparrow number, Int. J. Heat Mass Transfer 42 (1999) 3373–3385. [37] A. Amiri, K. Vafai, Transient analysis of incompressible flow through a packed bed, Int. J. Heat Mass Transfer 41 (1998) 4259–4279. [38] K. Cassiers, T. Linssen, M. Mathieu, M. Benjelloun, K. Schrijnemakers, P. Van Der Voort, P. Cool, E.F. Vansant, A detailed study of thermal, hydrothermal, and mechanical stabilities of a wide range of surfactant assembled mesoporous silicas, Chem. Mater. 14 (2002) 2317. [39] G. Bo, L.P. Chang, K.C. Xie, Desulfurization behavior of cerium–iron mixed metal oxide sorbent in hot coal gas, Ind. Eng. Chem. Res. 53 (2014) 8874–8880. [40] K. Liu, Q. Chen, H. Hu, et al., Pressure acid leaching of a Chinese laterite ore containing mainly maghemite and magnetite, Hydrometallurgy 104 (2010) 32–38. [41] R. Al-Gaashani, S. Radiman, N. Tabet, A.R. Daud, Rapid synthesis and optical properties of hematite (a-Fe2O3) nanostructures using a simple thermal decomposition method, J. Alloys Compd. 550 (2013) 395–401. [42] Y. Feng, J. Dou, A. Tahmasebi, et al., Regeneration of Fe–Zn–Cu sorbents supported on activated lignite char for the desulfurization of coke oven gas, Energy Fuels 29 (2015) 7124–7134. [43] X. Ren, L. Chang, F. Li, K. Xie, Study of intrinsic sulfidation behavior of Fe2O3 for high temperature H2S removal, Fuel 89 (2010) 883–887. [44] L.O.B. Benetoli, C.M.D.D. Souza, K.L.D. Silva, et al., Amino acid interaction with and adsorption on clays: FT-IR and mössbauer spectroscopy and X-ray diffractometry investigations, Origins Life Evol. Biosphere 72 (2015) 239–240. [45] Y. Liu, J. Terry, S.S. Jurisson, Pertechnetate immobilization with amorphous iron sulfide, Radiochim. Acta 96 (2008) 823–833. [47] B. Zeng, H. Li, T. Huang, C. Liu, H.R.J. Yue, B. Liang, Kinetic study on the desulfurization and regeneration of manganese-based regenerable sorbent for high temperature H2S removal, Chem. Res. 54 (2015) 1179–1188. [48] J.J. Huang, J.T. Zhao, X.F. Wei, Y. Wang, X.P. Bu, Kinetic studies on the desulfurization and regeneration of zinc titanate desulfurization sorbent, Powder Technol. 180 (2008) 196–202.