Regeneration Characteristics and Kinetics of Modified Semi-coke

Oct 23, 2012 - According to the model, the apparent activation energy and the corresponding frequency factor ... Materials Science Forum 2017 896, 155...
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Regeneration Characteristics and Kinetics of Modified Semi-coke Supported (Fe, Zn, Ce) Desulfurization Sorbents Jie Mi, Meng Yu, Fenyun Yuan, and Jiancheng Wang* Key Laboratory of Coal Science and Technology of Shanxi Province and Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, Shanxi, People's Republic of China ABSTRACT: The effects of the regeneration temperature and sulfur dioxide concentration on the regeneration behavior of modified semi-coke supported (Fe, Zn, Ce) sorbent were investigated using a fixed bed reactor and a thermogravimetric analyzer in the present work. Regeneration kinetic studies were also performed at regeneration temperatures of 650−750 °C. The results show that the optimum regeneration temperature and SO2 concentration are 700 °C and 12 vol %, respectively. The reaction order of regeneration with respect to SO2 is first-order. The equivalent grain model can be effectively used to correlate with the experimental data. In the early stage of reaction (x < 35%), the regeneration is controlled by the chemical reaction, while it is controlled by the diffusion through the product layer in the latter stage (x > 85%). According to the model, the apparent activation energy and the corresponding frequency factor of the chemical reaction are 12.45 kJ/mol and 0.255 m/s, vs 27.08 kJ/mol and 1.05 × 10−4 m2/s for the diffusion reaction. The chemical reaction, ZnS + FeS + CeS2 + 6SO2 → ZnFe2O4 + CEO2 + Sx, for regeneration over our sorbent are obtained according to the XPS and XRD results.

1. INTRODUCTION Coal gasification is one of cleaning coal technologies, while the hydrogen sulfide is generated during the coal gasification.1 To avoid the corrosion of the equipment, hydrogen sulfide must be reduced to below several ppmv. Therefore, the sorbents with high sulfur capacity, good mechanical strength, and good performance of desulfurization and regeneration are being developed in the purification.2 Composite metal oxides, such as zinc ferrite, combine different properties of different metal oxides to promote the regeneration properties and desulfurization precision.3,4 Focht pointed out that the zinc sulfate can subsequently be removed by thermal decomposition in an inert atmosphere or by direct exposure to a reducing atmosphere at atmospheric pressure over a temperature range of 550−850 °C, and the structural property would be changed because of serious sintering.5 Galvin also showed that the supported mixed-metal oxide sorbents for hot-gas desulfurization are capable of withstanding multiple sulfidation/regeneration cycles.6 The combination of mixed-metal oxides displayed synergism in enhancing efficiency for H2S removal and improved the crush strength on the pellets. Jothimurugesan pointed out that modified zinc titanate sorbents prepared by coprecipitation with Cu, Ni, Co, Fe, and Mo at levels up to 10 wt % could reduce the regeneration temperature.7 The maximum reduction in regeneration temperature was achieved for a zinc titanate sorbents with 5 wt % Co and 5 wt % Ni, which could be regenerated at temperatures as low as 475 °C. In parallel with the development of single element sorbents, other sorbents studies have conducted with various mixed metal oxides, such as Cu−Mn, Cu−Fe, Cu−Mo, Zn−V, Zn−Ti, Zn−Fe−Ti, and Zn−Fe−V.8−14 The results showed that the reactivity and regenerability of the sorbents were improved when the mixed metal oxides were loaded on suitable support.15,16 For example, alumina supports show good performance of capture H2S and regeneration.17,18 It implies that the supported sorbents could be very promising candidates for high temperature desulfurization. © 2012 American Chemical Society

To satisfy the industrialized demand, hot gas sorbent should have favorable regeneration performance and good stability in cycle use, the sulfidation/regeneration is influenced by the characters of the sorbent the mode of the subsequent recovery of resources of sulfur is dependent on operation conditions and regeneration gas atmosphere during regenerative process. So, it is of practical importance to know the regeneration behavior and kinetics of the spent sorbent. Many studies on the sorbents of different metals-based have shown that the kinetics of O2 regeneration are faster than regeneration using H2O or the mixtures of O2 and H2O, but SO2 and sulfate are usually produced in the O2 regeneration, which is generally accepted as detrimental to the long-term sulfur removal capacity of the sorbent.5 Consequently, SO2 can react with H2S or metal sulfide to form elemental S over proper metal catalytic action known as the Claus reaction. It is essential to investigate the regenerability of the sorbent using SO2 as a feeding gas. In our previous work, the performance of modified semi-coke supported (Fe, Zn, Ce) sorbents were investigated, and it was found that the modified semicoke supports and ultrasonic irradiation increase the surface area of sorbents, this combined with the addition of CeO2 improves the performance of sorbents.19 In the present work, the modified semi-coke supported (Fe, Zn, Ce) sorbents was chosen as desulfurizer. The regeneration performance of sulfided sorbent was investigated in the fix bed reactor and the thermogravimetric analyzer in order to study the regeneration behavior and kinetics in SO2 atmosphere, respectively.

2. MATERIALS AND EXPERIMENTAL TECHNIQUES The active components supported by modified semi-coke were prepared by a coprecipitation method with assistance of ultrasonic irradiation. First, the raw semi-coke (40−80 mesh) from Shanxi Yuling, China, was Received: July 21, 2012 Revised: October 22, 2012 Published: October 23, 2012 6551

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eliminate the resistance of gas film diffusion, the flow rate of the reactant gas was determined at 150 mL/min.21 The regeneration conversion rate of the sorbent can be calculated according to TGA curves. The regeneration conversion rate of x is w −w x= 0 × 100 w0 − we (1)

treated with nitric acid. The Zn(NO3)2, Fe(NO3)3, and Ce(NO3)2 were used as precursors of metal oxides and ammonia (17 vol %) was used as precipitant. The coprecipitation was carried out under the ultrasonic wave. Then, the cylindrical extrudates with about 3 mm in diameter and 3 mm long were extruded by using the metal oxides/MSC power and binder. The properties of the sorbents are listed in Table 1. The details of the preparation methods and the characteristics of the raw materials and the sorbents were given in our previous work.19

Surface areas of sorbents were measured by using a Micromeritics TriStar-3000, applying adsorption isotherms of nitrogen at −196 °C. The morphology of the sorbents was recorded by scanning electronic microscopy (SEM, LEO-438VP). The XPS spectra of the sorbents were obtained using a PHI5000C spectrometer equipped with an Al Kα source operating at 250 W and 93.9 eV passed energy. Energy calibration was performed by using the C 1s peak at 284.6 eV.

Table 1. Properties of the Fresh Sorbent Zn (wt %) Fe (wt %) Ce (wt %) specific surface BET (m2·g−1) pore volume (cm3·g−1) bulk density (g/m3) pore size (m)

6.69 5.85 3.76 75.07 0.16 1.45 × 105 1.5 × 10−3

3. RESULTS AND DISCUSSION In the regeneration, the metal sulfide reacted with sulfur dioxide to produce metal oxide, resulting in a decrease in the weight of the sorbent. While the side reaction is that the metal sulfide reacted with sulfur dioxide to produce the sulfate with an increase in the weight of sorbent. As the temperature rises, the sulfate decomposes to metal oxide and the weight of sorbent decreases. 3.1. Effect of Temperature and SO2. The regeneration was investigated using 12% SO2 (vol) with the programmed temperature (heating rate at 10 °C/min). The TGA curve of the regeneration for the sorbent with programmed temperature is shown in Figure 2. It shows that the weight of sorbent changed

Prior to the regeneration test, the sulfidation of the sorbents was conducted in a vertical quartz fixed bed reactor (19 mm in diameter, about 650 mm in length). The temperature of sorbents was measured by a C-type thermocouple fixed in the center of the sorbents bed. The desulfurization was performed at 500 °C using the simulated Texaco gas,20 containing (vol) 0.3% H2S, 39% H2, 27% CO, 12% CO2, 10% H2O, and balance N2, at SV = 2000 h−1. A gas chromatograph with a flame photometric detector (FPD) was used to analyze the outlet and inlet gases from the reactor. When the desulfurization efficiency decreased to 80%, the reaction was stopped. Two regeneration methods were investigated. One is carried out at the same conditions as those of the sulfurization reactors in order to obtain the regeneration ability of the present samples; the other was carried out in a thermogravimetric analysis (TGA) instrument (NETZSCH-STA409C Germany) because its isothermal conditions make it possible to study the regeneration kinetics of the sorbents. The device for the TGA experimental apparatus was shown in Figure 1.

Figure 2. TGA curve of the sorbent with temperature programmed regeneration.

slightly below 250 °C, indicating that the regeneration reaction cannot be performed at low temperature. The sample weight then increases rapidly as the temperature rises to 450 °C. However, it then changes slightly until 550 °C. Finally, it decreases consistently above 550 °C, and after 600 °C, the weight decreases rapidly. This is because the sulfate decomposition rate is higher than the sulfate formation rate at higher temperatures. To study the sorbent regeneration at different temperatures, the TGA tests were carried out at 650, 675, 700, and 750 °C, respectively. As shown in Figure 3, the weight change has the similar trend at different regeneration temperatures when the SO2 concentration is 12 vol %. There is a continuous weight loss over the entire temperature range. At the initial stage, the weight of the sorbent decreases quickly. However, along with the

Figure 1. Apparatus of thermogravimetric analyzer. The regeneration reaction was carried out in the temperature range of 650−750 °C. The reactant gas was composed of 8−16 vol % SO2 in N2 as the balance and the sample was 20 mg. The regeneration reaction of metal oxide sorbent is a typical noncatalytic gas−solid reaction. The reaction processes include outward diffusion, inward diffusion, the adsorption on the solid surface, and the regeneration reaction. According to the preliminary experiments to 6552

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Figure 3. Effect of different temperatures and SO2 content on the regeneration performance.

reaction, the weight decreases slowly. From the Figure 3a, it can be found that the optimum temperature is 700 °C. At the initial stage, the chemical reaction is the control step, the reaction rate increases as the reaction temperature increases. As the reaction continues, the diffusion process becomes the limiting step, the sintering of the sorbent of the regeneration at higher temperatures (750 °C) becomes serious, and the pore structure of the sorbent will be broken. The result is that the regeneration rate is worse than that at 700 °C. Figure 3b shows the effect of the SO2 concentration on the regeneration performance at 700 °C. It can be seen that the best SO2 concentration is 12 vol %. The reason for the nonlinear changes between the reactant concentration and the reaction rate may be attributed to the different O2-containing functional group on the surface of the sorbent after the addition of the modified semi-coke; the O2-containing functional groups have different reactivity with SO2. 3.2. Regeneration Kinetics of the Sorbents. 3.2.1. Reaction Order of SO2. During the initial regeneration stage, the gaseous reactant is considered to be uniformly dispersed throughout the granular solid. Konttinen has reported that the regeneration rate of the sulfided sorbent follows the uniform conversion model,22 which was chosen to analyze the reaction order in this work. The reaction order could be confirmed by examining the average rate of regeneration reaction at different SO2 concentrations. The uniform conversion model equation is expressed as

dx n = ks(1 − x)CSO 2 dt

Figure 4. Regeneration rate vs SO2 content.

regeneration rate of the sulfided sorbent can be effectively described with the uniform conversion model at the initial regeneration; yet, as the regeneration reaction is being processed, the regeneration changed into one being controlled by the inward diffusion, which could be described with the equivalent grain model.23 For cylindrical particles, the equivalent grain model can be expressed as follows: The chemical reaction control,

(2)

in which n could be determined by the regression of the initial conversion rate with different sulfur dioxide concentrations at 700 °C. As shown in Figure 4, a straight line and n of 1.03 were obtained. Therefore, the order of the regeneration reaction with respect to sulfur dioxide is nearly 1. 3.2.2. Apparent Kinetic Parameters. According to Siriwardance and Woodruff, the impact of sulfate was ignored in the process of analyzing the kinetic data, because the sorbents regeneration rate is obviously higher than the sulfate generation rate.10 Moreover, the high regeneration temperature can inhibit the sulfate formation. In this study, the regeneration temperature was investigated between 650 and 750 °C to determine the rate control step and kinetic parameters. On the basis of physical property analysis, the

t = AG(x)

(3)

G(x) = 1 − (1 − x)1/2

(4)

A=

ρ0 R 0 ksC0

(5)

The particle inner diffusion control, t = B1 + B2 F(x)

(6)

F(x) = x + (1 − x) ln(1 − x)

(7)

B1 = 6553

ρ0 D (1 − ε)C0

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Figure 5. G(x) and F(x) vs t for sorbent regeneration.

B2 =

ρ0 R 02(1 − ε) 6DeC0

(9)

The density of metal sulfide particles ρ0 is 1.45 × 10 g/m3; the radius of metal sulfide particles R0 is 1.5 × 10−4 m; the regeneration gas concentration C0 is 9 g/m3; the porosity of metal sulfide particles ε is 0.66 according the mercury porosimetry. The relationships between G(x) and F(x) and t were shown in Figure 5, respectively. The experimental results show that when the regeneration conversion rate of sorbent is below 35%, the G(x) (Figure 5a) is a good linear to t and the regeneration is controlled by the chemical reaction. When the conversion rate is above 85%, the F(x) (Figure 5b) is linear to t and the reaction is controlled by the particle inner diffusion. From the slopes of the fitted lines, as shown in 5, the chemical reaction rate constant ks and the effective diffusion coefficient De can be calculated via eqs 5 and 9, respectively. According to the Arrhenius equation, 5

ks = ks0 exp−Ea / RT

(10)

De = De0 exp−Ep / RT

(11)

Figure 6. ks and De vs 1/T for sorbent regeneration.

groups on the modified semicoke surface of the sorbent have different active reaction with SO2.25 3.3. Fixed Bed Regeneration of the Sorbent. Three desulfurization/regeneration cycle experiments over the sorbent are carried out after sulfidation at the fixed bed regeneration; the results of the breakthrough curves and sulfur capacity are shown in Figure 7. The fresh sorbent has a breakthrough time of 35 h (Figure 7a) and a sulfur capacity of 25.3 g S/100 g sorbent (Figure 7b). After the third regeneration, the performance remains to be 32.5 h and 25.0 g S/100 g sorbent, respectively; almost the same as the fresh sorbent. It shows that this sorbent is regenerable for the removal of H2S from hot coal gas. In the future, more tests of the desulfurization/regeneration cycle will be performed in order to fulfill the industrial requests. 3.4. Change of the Sorbents Properties. It is well-known that the textural structures of the sorbents, such as pore structure and specific surface area, will undergo great changes due to both sulfidation and regeneration subjected to the thermal treatments. It is important to investigate these changes of chemical and physical properties by using SEM (scanning electron microscopy), N2 adsorption, XPS (X-ray photoelectron spectroscopy), and XRD (X-ray diffraction). First, the fresh, the sulfided and the first regenerated sorbents were identified by SEM, and the results are shown in Figure 8. The SEM images of the fresh (Figure 8a) and the first regenerated

A linear equation of ln ks = ln ks0 −

Ea RT

and ln De = ln De0 −

Ep RT

can be obtained from the logarithm forms of eqs 10 and 11. Their Arrhenius plots are shown in Figure 6. By calculating the slopes and intercepts of the two lines shown in Figure 6, the chemical reaction activation energy and corresponding frequency factor are 12.45 kJ/mol and 0.255 m/s, respectively, while the diffusion activation energy and corresponding frequency factor are 27.08 kJ/mol and 1.05 × 10−4 m2/s, respectively. The calculated activation energy is obviously lower than that of some references;22 it may be attributed to the addition of the Ce into the sorbent and modified semi-coke, as Ce has been proven to react with SO2 and CeSx to produce elemental sulfur.24 Additionally, the O2-containing functional 6554

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Figure 7. Breakthrough curves and sulfur capacity absorbents for fresh and for three sulfide cycles.

Figure 8. SEM photograph of the fresh, the sulfided, and the regenerated sorbents.

The broad Fe2p3/2 peak in the sulfided sorbent (Figure 9a) can be resolved into two shoulders at 710.8 eV and 713.3 eV, which can be assigned to FeS and a more complicated FeSx, respectively.26 The broad Fe2p3/2 peak of the regenerated sorbent (Figure 9b) can be resolved into two at 711.3 eV and 713.7 eV, which can be assigned to Fe2O3 and Fe2(SO4)3, respectively.26,27 In Figure 9c and d, the Zn2p3/2 peaks of the sulfided and regenerated sorbents can be observed at 1022.5 and 1022.0 eV, which can be assigned to ZnS and ZnO, respectively.28 From Figure 9e and f, the main binding energies of Ce3d5/2 in the sulfided and regenerated sorbents are 882.7 and 886.1 eV, which can be assigned to Ce2O3 and CeO2, respectively.24 Only a weak peak around 881.4 eV for the Ce3d5/2 in the sulfided sorbent is assigned to the Ce species in CeS2.24 Figure 10 shows the XRD patterns of the sulfided and the thirdtime-regenerated sorbents. Only diffractions peaks ascribed to FeS, ZnS, and CeS2 are observed for the sulfided sorbent. The diffraction peaks assigned to crystalline ZnFe2O4 and CeO2 are observed for the regenerated sorbents. Combined the XPS and XRD results, it can be found that regeneration makes FeS, ZnS and CeS2 transform into the active components (ZnFe2O4 and CeO2) of the fresh sorbents.29,30 Also, yellow elemental sulfur is found to form in exit of the quartz reactor. It suggests that the metal sulfide reacts with SO2 mainly to generate the elemental sulfur, and the major chemical reaction of regeneration is

sorbents (Figure 8a) are similar, with the regenerated sorbent having larger agglomerated particles. It indicates that the regeneration method under SO2 atmosphere can restore the physical structure of the sorbent, while the particle sizes of the sulfided sorbent became larger and there were more particle agglomerate in the sulfided sorbent (Figure 8b). Table 2 shows the BET (Brunauer−Emmett−Teller) surface areas and pore volumes of the fresh, sulfided, and regenerated Table 2. Physical Parameters of Samples samples

BET surface area (m2/g)

pore specific volume (cm3/g)

fresh sorbent sulfided at 500 °C regenerated at 700 °C

75.07 17.75 78.99

0.16 0.07 0.18

sorbents. It can be seen that the sulfided sorbent has less pore volume and surface area. However, the pore volume and surface area can be restored after regeneration; it is important to refresh the abilities for adsorption H2S. The chemical states of the surface Fe, Zn, and Ce species for the sorbents of the sulfidation and the first regeneration process were investigated using XPS, and the results are presented in Figure 9. It is found that the sulfided and the regeneration sorbents exhibit different Fe2p3/2, Zn2p3/2, and Ce3d5/3 features.

ZnS + FeS + CeS2 + 6SO2 → ZnFe2O4 + CeO2 + Sx 6555

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Figure 9. XPS spectra of the sulfide and the regenerated sorbents.

is important for the smooth operation of the whole process. Currently, we have used a cold trap near the exit to remove the resulting sulfur during the regeneration using SO2 as a feeding gas.

The beneficial effect of the direct production of elemental sulfur during the above regeneration is that a separate sulfur postprocessing step can be eliminated. It should be noted that the sulfur separation 6556

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Figure 10. XRD patterns of the sulfided and the regenerated sorbents.

4. CONCLUSIONS The effects of the regeneration temperature and the SO2 concentration on the regeneration performance and kinetic of modified semi-coke supported (Fe, Zn, Ce) sorbent were investigated in the present work. By correlating experimental data with the kinetic model, we can conclude the following: (1) The optimum regeneration temperature and SO2 concentration are 700 °C and 12 vol %, respectively. (2) The regeneration reaction with respect to SO2 can reasonably be assumed as firstorder reaction. (3) The apparent activation energy and the corresponding frequency factor of the chemical reaction are 12.45 kJ/mol and 0.255 m/s, respectively. For the diffusion activation process, 27.08 kJ/mol and 1.05 × 10−4 m2/s are obtained, respectively. Moreover, the physical structure and chemical components of sorbents can be effectively restored after SO2 regeneration. The regeneration reaction could be expressed as ZnS + FeS + CeS2 + 6SO2 → ZnFe2O4 + CEO2 + Sx; the solid sulfur will simplify the sulfur postprocessing.

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n = the order with respect to sulfur dioxide R0 = sorbent grain radius, m W = sorbent weight during the regeneration, g W0 = initial sorbent weight, g We = final sorbent weight after complete regeneration, g

AUTHOR INFORMATION

Corresponding Author

*Telephone: 86-351-6018482. E-Mail: [email protected] (J.C.W.), [email protected] (M.Y.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (No. 2012CB723105), the National Natural Science Foundation of China (51272170), and the Key Programs for Science and Technology Development of Shanxi Province under Contract No. 20080322035.

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NOMENCLATURE SV = space velocity, h−1 CSO2 = the concentration of SO2, g/m3 De = effective diffusion coefficient, m2/s De0 = frequency factor of diffusion coefficient, m2/s Ea = chemical reaction activation energy, kJ/mol Ep = diffusion activation energy, kJ/mol ks = apparent chemical reaction rate constant, m/s ks0 = frequency factor of chemical reaction rate constant, m/s 6557

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