Kinetic Study on the Sulfidation and Regeneration of Manganese

Article pubs.acs.org/IECR

Kinetic Study on the Sulfidation and Regeneration of ManganeseBased Regenerable Sorbent for High Temperature H2S Removal Bing Zeng, He Li, Tao Huang, Changjun Liu, Hairong Yue, and Bin Liang* Multi-phases Mass Transfer and Reaction Engineering Laboratory, College of Chemical Engineering, Sichuan University, Chengdu 610065, China ABSTRACT: This paper describes a kinetic study on the sulfidation and regeneration of Mn/Al2O3 sorbent for high temperature H2S removal. The influence of the reactant gas compositions and temperatures on the sulfidation and regeneration behavior was systematically investigated in a thermogravimetric apparatus. The results show that the shrinking core model can be used to correlate with the experimental data. The sulfidation is mainly dominated by diffusion. Through the regressions of the initial sulfidation reaction data, the chemical reaction order, activation energy, and pre-exponential factor are 1, 37.42 kJ/mol, and 4.16 × 10−2 m/s, respectively, and the diffusion activation energy and pre-exponential factor are 57.23 kJ/mol and 6.41 × 10−5 m2/s, respectively. O2 regeneration has the same situation as sulfidation; the chemical reaction order, activation energy, and preexponential factor are 1, 24.62 kJ/mol, and 0.17 m/s, respectively, and the diffusion activation energy and pre-exponential factor are 122.44 kJ/mol and 8.51 × 10−4 m2/s, respectively. For SO2 regeneration, the regeneration rate is controlled by the chemical reaction in the early stage, and then it is controlled by the diffusion in the latter stage; the chemical reaction order, activation energy, and pre-exponential factor are 1, 124.82 kJ/mol, and l.62 × 102 m/s, respectively, and the diffusion activation energy and pre-exponential factor are 224.83 kJ/mol and 2.33 × 10−6 m−2/s, respectively.

1. INTRODUCTION The integrated gasification combined cycle (IGCC) is one of the most promising processes for advanced electric power generation,1 which possesses several advantages, such as environmental friendliness, good economy, and high efficiency.2−4 In IGCC, coal is gasified and the hot coal gas is purified to serve as the fuel of gas turbine to generate electricity. H2S in the hot coal gas needs to be removed in purification in order to protect the turbine from corrosion and guarantee the low sulfur emission of flue gas. A coal gasifier typically is operated under a temperature higher than 1273 K and the hot coal gas is 1073−1173 K.5 It requires a desulfurization temperature around 1123 K to achieve high efficiency. High temperature H2S removal of hot coal gas is also vital in sponge iron production. The shaft furnace for sponge iron production operates at 1123 K, in which the ferrous ore is directly reduced with hot coal gas. However, the hot coal gas must be desulfurized below 15 ppm of H2S to avoid the formation of sulfides.6 A great number of compound sorbents such as Fe containing compounds,7,8 zinc ferrite,9−15 zinc titanate,16−19 and copper based20−22 sorbent were developed for the high temperature desulfurization. However, these metal oxides are likely to be reduced to their metallic states or corresponding metal carbides in reducing atmosphere. It results in the decrease of the mechanical strength and sulfur capacity of the sorbent. Manganese-based sorbent is a promising acceptor of H2S for hot coal gas. Pure manganese oxide can react with H2S and remove sulfur from the coal gas. However, the manganese sulfide cannot be simply regenerated at the same temperature with SO2 or steam. The addition of alumina can improve the regeneration of the sorbent with SO2 or steam. In fact, alumina is the catalyst of the Claus reaction. The alumina supported manganese oxide sorbent for high temperature H2S removal © 2015 American Chemical Society

was extensively investigated at Delft University of Technology. Wakker investigated the impregnated sorbent at 873 K.23,24 It was suggested that 8% manganese content would give optimum performance. The sulfided sorbent was proved to be easily regenerable with steam at 873 K. Liang investigated the influence of manganese content and showed that the sulfur capacity of the sorbent linearly increased with the increase of the manganese content.25,26 The manganese sorbent prepared by repeated impregnation exhibited excellent performance in 11 successive sulfidation-regeneration cycles with appropriate S/ Mn molar ratios. Bakker prepared a Mn/Al2O3 sorbent with a 32−40 wt % Mn loading by wet impregnation.27Then they regenerated the sulfided sorbent with 50 mol % SO2 at 1123 K and directly produced elemental sulfur during regeneration. In our previous work,28 Mn−Al based sorbent with different manganese contents was prepared via coprecipitation method and used for high temperature H2S removal. The results showed that the sulfur capacity of the sorbent linearly increased with the manganese content. During five successive sulfidation−regeneration cycles, the sorbent showed stable capacities. Coprecipitation method is an easier preparation method compared with the repeated impregnation method, in which the manganese content can be precisely controlled. However, in order to obtain spherical sorbent with high mechanical strength, we used the shaped alumina pellets as the carrier and prepared the sorbent by impregnation. In our previous work, we also investigated the impregnated Mn/Al2O3 sorbent using the shaped alumina pellet as the carrier.29 Received: Revised: Accepted: Published: 1179

August 16, 2014 December 25, 2014 January 6, 2015 January 6, 2015 DOI: 10.1021/ie503233a Ind. Eng. Chem. Res. 2015, 54, 1179−1188

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Industrial & Engineering Chemistry Research

sorbent in reducing atmosphere, we can calculate the manganese content of the sorbent, and the calculated results show that the calculated values are the same as the measured values. Before sulfidation, the sorbent was reduced in H2; the reduced sorbent was reacted with the H2S contained gas. According to our previous experimental results, CO2 and H2O have little effect on the sorbent conversion, so the sulfidation kinetic was measured with the H2S/H2/N2 gas mixtures at 1023−1173 K. The H2S content changed as 0.5, 1, 2, and 3 vol % by using N2 as balance gas. During the regeneration with oxygen, regeneration conditions must be carefully controlled. Lower regeneration temperature may lead to the formation of sulfate, sulfate decomposes slowly, and the regeneration efficiency may be decreased. On the other hand, oxygen regeneration is a strong exothermic reaction; higher oxygen concentration may result in the sintering of the sorbent. In our kinetic measurement, it was found that the desulfurization activity of the sorbent regenerated with >10 vol % O2 decreased compared with the fresh sorbent. Furthermore, when the regeneration temperature is higher than 923 K, no sulfate is observed in regenerated sorbent. In order to avoid the formation of sulfate and the sintering of the sorbent, the regeneration kinetic was measured with the O2/N2 gas mixtures at 1098−1173 K, and the O2 content changed as 1, 2, 3, and 4 vol % by using N2 as balance gas. During regeneration with sulfur dioxide, the sulfided sorbent reacted with the SO2 contained gas. The regeneration kinetic was measured with the SO2/N2 gas mixtures at 1073−1193 K. The SO2 content changed as 18.6, 27.1, 40.1, and 58.4 vol % by using N2 as balance gas. Conversion Calculations of the Sulfidation and Regeneration. During the sulfidation and regeneration cycles, the chemical reactions can be expressed as follows: Sulfidation:

Through 6 sulfidation-regeneration cycle tests, the sorbent maintained good mechanical stability and sulfidation-regeneration activity. On the whole, the alumina supported MnO has excellent sulfidation-regeneration performance, high sulfur capacity, high desulfurization efficiency, and high structure stability at up to 1123 K. In this work, we focused on the kinetic behaviors of the Mn/ Al2O3 sorbent prepared by wet impregnation. The data were fitted with different kinetic models. According to the crystal phase changes and characterization, the rate-controlling steps were analyzed. The sulfidation and regeneration kinetics are most useful to the scale up and commercialization of this process.

2. EXPERIMENTAL SECTION Analysis and Characterization of the Sorbent. The manganese content of the sorbent was analyzed by ammonium iron||sulfate titrimetric method (Chinese analysis standard method, GB1506-2002-T). The pore volume and the specific surface area of the sorbent were analyzed with N2 adsorption on an ASAP 2020 BET system. Particle morphology was recorded by scanning electronic microscopy (SEM, JSM-7500E, Japan). The distribution of sulfur element content associated with the sulfided and regenerated sorbent was detected by energy dispersive spectrometer (EDS, X-Max50). Preparation of Mn/Al2O3 Sorbent. The porous Al2O3 pellets with a 1.7 mm diameter were used as the carrier of the sorbent. Manganese(II) acetate tetrahydrate (Mn(Ac)2·4H2O, AR) was used as the impregnation salt. Sorbent was prepared by repeated wet impregnation, drying, and calcination. The carrier was impregnated by a 2 mol/L Mn(Ac)2 for 18 h, then was dried in a microwave oven at a power of 280 W for 30 min (compared with the conventional drying, the microwave drying has the characteristics of high efficiency and energy saving26), and then was calcined at 1123 K for 6 h in a high temperature oven. The above process was repeated for 12 times to obtain the sorbent with a manganese content of 37.71%. The composition and properties of the manganese oxide sorbent are listed in Table 1.

MnO + H 2S → MnS + H 2O

O2 regeneration: MnS + 7/4O2 → 1/2Mn2O3 + SO2

Table 1. Chemical and Physical Properties of the Sorbent

(2)

SO2 regeneration:

MnO/Al2O3 sorbent Mn (wt %) specific surface area (m2/g) pore volume (cm3/g) bulk density (g/cm3) particle size (mm)

(1)

2MnS + SO2 → 2MnO + 3/2S2

37.71 2.6 0.16 3.36 1.7

(3)

The sulfidation and regeneration of MnO/Al2O3 sorbent are typical gas−solid reactions. Before sulfidation, the sorbent was reduced with H2 and the Mn2O3 phase was converted to MnO in order to react with H2S. Reaction 1 is the sulfidation, in which MnO reacts with H2S to form MnS. The sorbent becomes heavier during sulfidation due to the molecular weight changes from MnO to MnS. Thus, the conversion of the sorbent can be estimated by a gravimetric measurement

Thermogravimetric Analysis Procedure. The TGA tests were divided into three sections, the first for sulfidation studies, the second for O2 regeneration studies, and the third for SO2 regeneration studies. Before sulfidation, the sorbent stability under reducing conditions has been investigated by passing through the themogravimetric apparatus a simulated reducing gas without H2S, using identical operating conditions as in sulfidation. The results show that all Mn2O3 in sorbent is reduced to MnO in 3 min, it indicates that the complete reduction time is far less than that of sulfidation (the sulfidation time is greater than 120 min in our experiments); MnO is the active component for the removal of H2S. Furthermore, the weight loss ratio of the sorbent is about 5.5%. According to the weight loss ratio of the

⎛ final weight‐initial weight ⎞ %MnO(x) = 100⎜ ⎟ ⎝ 0.108 × initial weight ⎠

(4)

Reaction 2 is the reversal reaction of MnS to Mn2O3, in which MnS reacts with O2 to form Mn2O3. The conversion of MnS can be estimated ⎛ initial weight − final weight ⎞ %MnS(x) = 100⎜ ⎟ ⎝ 0.091 × initial weight ⎠ 1180

(5)

DOI: 10.1021/ie503233a Ind. Eng. Chem. Res. 2015, 54, 1179−1188

Article


when the flow rate is higher enough not to change with increasing flow rate any more. In Figure 1, the sulfidation rate increases with increasing the gas flow rate from 300 to 1000 mL/min. When the gas flow rate is beyond 1000 mL/min, the conversion does not change any more. In kinetic measurements of sulfidation, in order to make sure the elimination of mass transfer limitation, the flow rate was 1700 mL/min. Similarly, we chose the flow rates being 1500 and 1800 mL/min in the kinetic measurements for O2 regeneration and SO2 regeneration, respectively.

When the sulfided sorbent is regenerated with SO2 as reaction 3, the conversion of MnS can be estimated: ⎛ initial weight − final weight ⎞ %MnS(x) = 100⎜ ⎟ ⎝ 0.184 × initial weight ⎠

(6)

Gas Flow Rate. In order to eliminate the influence of mass transfer on the kinetics, the experimental gas flow rate was pretested as shown in Figure 1. By gradually increasing the gas flow rate, the reaction rate increases due to the mass transfer enhancement. The flow rate of the kinetics measurements would be chosen in the range

3. RESULTS AND DISCUSSION Sorbent Reactivity via Thermogravimetric Analysis. The TGA tests were conducted for three stages: sulfidation, oxygen regeneration and sulfur dioxide regeneration. The results are shown in Figures 2−4. Figure 2 shows the results of sulfidation, in which the left demonstrates the influence of H2S content and the right demonstrates the influence of temperature. The sulfidation rate increases with increasing H2S content in the range 0.5−3 vol % at 1123 K. At a low H2S concentration of 0.5% at 1123 K, the conversion of the sorbent is very small and a long reaction time is necessary for completed sulfidation of the sorbent. The right figure shows the influence of temperatures at the range of 1023−1173 K under a constant concentration of 2% H2S. We can find the significant influence of the temperature in the experimental range on the sulfidation, the sulfidation conversion increases because of the increase of reaction rate constant with increasing the sulfidation temperature. The influence of gas concentration and temperature on the O2 regeneration is shown in Figure 3. The O2 content greatly affects the regeneration rate, and the results show that the regeneration rate with 1% O2 at 1123 K is relatively slow. However, when the O2 concentration is in the range of 2−4%, the regeneration rate is much quicker than sulfidation. Oxygen regeneration can be easily completely converted. Figure 3 also shows the influence of temperature at 1098−1173 K under 2% O2. In such a temperature range, the temperature does not show strong influence on the regeneration because the oxygen regeneration is a strong exothermic reaction. Figure 4 illustrates the SO2 regeneration of sulfided sorbent. The influence of SO2 concentration on the regeneration conversion is shown on the left and the influence of temperature is shown on the right. The regeneration with SO2 is much slower than the regeneration with oxygen. The influence of temperature in the range of 1073−1193 K is also shown in Figure 4. The results show that higher temperature will be helpful for the regeneration and the influence of temperature on the SO2 regeneration is more significant than oxygen regeneration, because the SO2 regeneration is an endothermic reaction. SEM Characterization. Sulfidation and regeneration cycles change the pore structure and specific surface area of the sorbent with the active component converting from MnO to MnS in sulfidation reaction, from MnS to Mn2O3 in oxygen regeneration reaction, and from MnS to MnO in sulfur dioxide regeneration reaction. By cutting the sorbents along the radial direction, the sectional surfaces were observed under SEM. The SEM images of the sectional surfaces of the fresh, sulfided, regenerated sorbent with O2 or SO2 are compared in Figure 5. It can be seen that the fresh and regenerated sorbents are more porous than the sulfided sorbent.

Figure 1. Influence of gas flow rate on the conversion: (a) sulfidation, (b) regeneration with O2, and (c) regeneration with SO2. 1181


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Figure 2. Influence of concentration and temperature on sulfidation: (a) concentration and (b) temperature.

Figure 3. Influence of concentration and temperature on oxygen regeneration: (a) concentration and (b) temperature.

Figure 4. Influence of concentration and temperature on sulfur dioxide regeneration: (a) concentration and (b) temperature.

The disappearance of micro pores means the sulfided sorbent possesses smaller density and expanded crystalline phase. The density of manganese oxide is 5.45 g/mL, while the density of manganese sulfide is 3.99 g/mL. So, when MnO converts to MnS in sulfidation, the micro pores are blocked due to 67% volume expansion. Table 2 shows the BET surface area and pore volume of the fresh, sulfided, and regenerated sorbent. It can be seen that sulfided sorbent has less surface area and pore volume because

of the formation of MnS with larger molecular volume. However, the pore volume and surface area can be restored after regeneration. Compared with the sulfided sorbent, the regenerated sorbent has larger surface area. It means more micropores opened during regeneration. It also means that the whole structure of the sorbent is stable in the sulfidationregeneration cycles. Furthermore, we chose a partially sulfided sorbent and partially regenerated sorbent to observe the changes of the 1182


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Figure 5. SEM images of the sorbents: (a) reduced sorbent, (b) sulfided sorbent, (c) regenerated sorbent with O2, and (d) regenerated sorbent with SO2.

content in the sorbent. By cutting the spherical sorbent along the radial direction, a spherical cross-section was obtained. The content of sulfur was scanned by EDS from the center of the spherical cross-section to the edge along the radial direction, the scanning interval was about 40 μm. Figure 7a is the scanning schematic diagram. The radial distributions of sulfur element of sorbents with 85% sulfidation conversion, O2 regeneration conversion of 85% and SO2 regeneration conversion of 40% are shown in Figure 7. The sorbent can be roughly divided into two regions: reaction region and unreacted region. From Figure 7b, it can be seen that the sulfur content is about zero in the unreacted region, and then the sulfur content increased sharply from about zero to about 20% in reaction region. From Figure 7c,d, it can be seen that the sulfur content is about zero in the reaction region, and then the sulfur content increased sharply from about zero to about 20% in the unreacted region. The existence of reaction region and unreacted region in sorbent is according with shrinking core model. Kinetic Model. The sulfidation and regeneration of Mn/ Al2O3 sorbent are typical gas−solid reactions. During the sulfidation and regeneration, the sorbent size does not change and the reactions proceed gradually from the out to the center of the sorbent. During sulfidation, MnO gradually converted to MnS, and during regeneration MnS gradually converted to Mn2O3 in oxygen and to MnO in sulfur dioxide. Evidences from a wide variety of situations indicate that the shrinking core model can approximately describe the behavior of the reactions. The shrinking core model simplified the situation to a model that the reaction occurs only on the sharp interface between reactant gas and unreacted core. It is suitable for porous-free solid sorbent or fast reaction process. According to the previous

Table 2. BET Surface Area and Volume of Samples sorbent reduced sorbent sulfided sorbent at 1123 K regenerated sorbent with O2 at 1123 K regenerated sorbent with SO2 at 1153 K

BET surface area/ (m2/g)

Pore volume/ (cm3/g)

12.61 1.45 3.13

0.26 0.19 0.24

7.14

0.25

sectional surfaces. Visible reaction zone and unreacted zone are observed inside the sorbents. SEM images of sectional surfaces for the sorbent with 25% sulfidation conversion, 50% O2 regeneration conversion and 50% SO2 regeneration conversion are shown in Figure 6. From the SEM images, both sides cross the boundary layer are according to the sulfided and regenerated sorbent. The results suggest that the both sulfidation and regeneration reactions on the boundary area are much quicker and the reaction process can be described with a shrinking core model. For the sulfided sorbent with 25% conversion, it can be seen that the A region is more porous than the B region, and this indicates that the sulfidation reaction happened in the A region considerably. Conversely, the sulfidation reaction did not happen in the B region or happened inconsiderably. For the regenerated sorbent with 50% conversion with O2 at 1123 K or with SO2 at 1153 K, the D region is more porous than the C region and the F region is more porous than the E region, these indicate that the regeneration reaction happened in the D and F regions considerably. Conversely, the regeneration reaction did not happen in the C and E regions or happened inconsiderably. EDS Characterization. Energy dispersive spectrometer (EDS) was used to detect the distribution of sulfur element 1183


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(1) Solid product formed in the outside layer and the unreacted core is shrinking, (2) Reaction between reactant gas and unreacted core only occurs inside the sharp interface, (3) Sorbent size keeps unchanged during sulfidation and regeneration. The gas−solid reaction is described as (1) Transfer of gaseous reactant through the gas film surrounding the exterior surface of sorbent, (2) Diffusion of gaseous reactant through the newly formed product layer to the surface of unreacted core, (3) Gaseous reactant reacts with solid reactant on the surface of unreacted core and forms gaseous product and solid product. According to the shrinking core model, the t-x relationship can be expressed as follows: For chemical reaction control: t = AG(x) = A[1 − (1 − x)1/3 ]

(7)

bksCA0 ρR 0

(8)

ks = ks0exp( −Ea /RT )

(9)

1/A =

For interior diffusion control: t = BF(x) = B[3 − 2x − 3(1 − x)2/3 ]

1/B =

(10)

6bDeCA0 ρR 0 2

(11)

De = De0exp( −Ep/RT )

(12)

where t, time (s); R0, sorbent radius (m); ks, rate constant of surface reaction (m/s); ks0, pre-exponential factor of ks (m/s); CA0, bulk concentration of gas reactant (mol/Nm3); p, concentration of active component in sorbent (mol/N m3); b, stoichiometric coefficient; De, effective diffusion coefficient (m2/s); De0, pre-exponential factor of diffusion coefficient (m2/ s); n, reaction order; Ea, chemical reaction activation energy (kJ/mol); Ep, diffusion activation energy (kJ/mol); x, conversion. By fitting the time−conversion data with eqs 7 and 10, we can determine the rate-controlling step. Sulfidation Kinetics. The reaction rate is defined as the number of reacted moles of the sorbent on the unit surface area at the unite time, as follows: Figure 6. SEM images of (a) sulfided sorbent with 25% conversion, (b) O2 regenerated sorbent with 50% conversion, and (c) SO2 regenerated sorbent with 50% conversion.

−ρ

drg dt

= ksCAn

(13)

In the beginning stage of the sulfidation or regeneration, the gaseous reactant is considered to be present uniformly throughout the sorbent and little product is formed, so it can be assumed that the internal diffusion is negligible and CA CA0, and the reaction rate of the sulfidation or regeneration is reaction-controlled, the reaction conversion x= (rg03 − rg3)/rg03. Equation 13 can be expressed as follows:

characterizations of SEM and EDS, there is a narrow boundary layer in the sorbent when the sorbent was sulfided or regenerated. In the two sides of boundary layer, one side is the unreacted core and the other is the product layer. Compared with the radius of the sorbent, the thickness of the boundary layer is negligible. Moreover, the sorbent has small pore volume and surface area as shown in Table 2; the shrinking core model is adopted in this work. The schematic diagram of the shrinking core model is shown in Figure 8. Assuming:

⎛ E ⎞ n dx 3 ks0 exp⎜ − a ⎟CA0 = ⎝ RT ⎠ dt ρrg0

(14)

where n is determined by the regression of the initial conversion rate at the different gaseous reactant concentration 1184


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Figure 7. Radial distribution of sulfur in spherical sorbent: (a) the scanning schematic diagram, (b) sulfided sorbent with 85% conversion, (c) O2 regenerated sorbent with 85% conversion, and (d) SO2 regenerated sorbent with 40% conversion.

Table 3. Kinetic Parameters of Sulfidation and Regeneration kinetic parameters

sulfidation

O2 regeneration

SO2 regeneration

n Ea, kJ/mol ks0, m/s Ep, kJ/mol De0, m2/s

0.98 37.42 4.16 × 10−2 57.23 6.41 × 10−5

0.94 24.62 0.17 122.44 8.51 × 10−4

1.07 124.82 1.65 × 102 224.83 2.33 × 10−6

According to the results of EDS and SEM, the sulfidation reaction process is in agreement with the shrinking core model. For the reaction between H2S and MnO, the molecular volume of the sulfidation product is enlarged because of the formation of MnS. The porosity of the sorbent becomes very small and the product layer is compact, causing the change of the ratecontrolling step of the sulfidation from the chemical reaction control in the beginning stage to the diffusion control. Moreover, the rate-controlling step can be also identified according to the relationship between G(x) or F(x) and time. Figure 9 shows that F(x)−t curve follows a good linear relationship in the entire sulfidation process. According to the slopes of F(x)−t, De can be calculated. According to eq12, the diffusion activation energy and pre-exponential factor can be obtained by plotting ln De to 1/T, and the regression results are showed in Table 3 Oxygen Regeneration Kinetics. The chemical reaction order n of O2 regeneration can be obtained by plotting ln(dx/ dt) to ln CA0, ks0, and Ea can be obtained by plotting ln(dx/dt) to 1/T, according to eq 14. The linear regression results are shown in Table 3. From Table 3 we can see the reaction order

Figure 8. Schematic diagram of the shrinking core model.

at the constant temperature, ks0 and Ea are determined by the regression of the initial conversion rate at different temperature at the constant gaseous reactant concentration. The chemical reaction order n can be obtained by plotting ln(dx/dt) to ln CA0; ks0 and Ea can be obtained by plotting ln(dx/dt) to 1/T. The linear regression results are shown in Table 3. From Table 3 we can see the reaction order of the sulfidation is 0.98. Therefore, the reaction order of the sulfidation is nearly 1. 1185


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good linear relationship. According to the slopes of the F(x)−t curve, De can be calculated. According to eq 12, the diffusion activation energy and pre-exponential factor can be obtained by plotting ln De to 1/T, the regression results are shown in Table 3. SO2 Regeneration Kinetic. According to eq 14, the chemical reaction order n of SO2 regeneration can be obtained by plotting ln(dx/dt) to ln CA0. The linear regression result is shown in Table 3. From Table 3 we can see the reaction order of regeneration with SO2 is 1.07. Therefore, the reaction order of the SO2 regeneration is nearly 1. Plotting G(x) to t and F(x) to t, Figure 11a shows G(x) and t has a good linear relationship in the early stage of reaction, while Figure 11b shows F(x) and t has a good linear relationship in the latter stage. These indicate that the regeneration rate of SO2 regeneration is controlled by the chemical reaction in the early stage, while it is controlled by the diffusion through the product layer in the latter stage. According to the slopes of G(x)−t, ks can be calculated. According to eq 9, the chemical reaction activation energy and pre-exponential factor can be obtained by plotting ln ks to 1/T, and the chemical reaction activation eq 14 can be used to calculate the chemical reaction activation energy and the preexponential factor of SO2 regeneration; the calculated results are 127.91 kJ/mol and l.56 × 102 m/s m/s, respectively. The calculated results of eqs 9 and 14 are similar; the relative error is less than 8%. We chose the average value of two calculated values as the chemical reaction parameters, and these parameters are listed in Table 3. According to the slopes of F(x)−t, De can be calculated. According to eq 12, the diffusion activation energy and pre-exponential factor can be obtained by plotting ln De to 1/T, and the regression results are shown in Table 3. Comparison of Model Calculation Values and Experimental Values. In order to verify the accuracy of the shrinking core mode, model calculation values and experimental values are listed in Table 4. As shown in Table 4, the model calculation values adhere to the experimental values perfectly, the relative errors are less than 10%. Therefore, the shrinking core model can be used to depict the kinetic behavior of sulfidation and regeneration approximately.

Figure 9. Reaction rate data for sulfidation, plotted according to eq 10 for diffusion control.

of O2 regeneration is 0.94. Therefore, the reaction order of O2 regeneration is nearly 1. For the regeneration with O2, the reaction activation energy is low and the reaction rate constant is great, and regeneration rate is very fast. When the sulfided sorbent reacts with O2, product is formed, O2 must diffuse through the product layer to react with MnS, and diffusion is the main rate-controlling step of regeneration reaction with O2. Plotting F(x) to t, Figure 10, shows that F(x)−t curve has a

4. CONCLUSIONS Evidences from a wide variety of situations indicate that the shrinking core model can approximately describe the kinetics

Figure 10. Reaction rate data for O2 regeneration, plotted according to eq 10 for diffusion control.

Figure 11. Reaction rate data for SO2 regeneration: (a) plotted according to eq 7 for chemical reaction control, (b) plotted according to eq 10 for diffusion control. 1186


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Industrial & Engineering Chemistry Research Table 4. Comparison of Calculated Values According to the Shrinking Core Model and Experimental Valuesa sulfidation T = 1148 K

a

O2 regeneration H2S% = 2%

T = 1163 K

SO2 regeneration T = 1173 K

O2% = 2%

SO2% = 58.4%

x

t1

t2

(t1 − t2)/t2

x

t1

t2

(t1 − t2)/t2

x

t1

t2

(t1 − t2)/t2

16.48 20.98 41.23 55.21 66.23 74.58 79.43 85.49 90.20

9.2 13.9 27.9 42.1 56.8 77.4 93.9 117.3 141.2

10 15 30 45 60 75 90 110 130

−8 −7.3 −7 −6.4 −5.3 3.2 4.3 7.3 8.6

17.54 28.28 45.03 58.62 70.46 79.94 87.53 93.69 98.11

4.4 8.9 18.3 28.3 39.2 52.7 64.9 76.7 88.4

5 10 20 30 40 50 60 70 80

−12 −11 −9.5 −5.7 2 5.4 8.2 9.6 10.5

15.23 23.89 37.23 48.65 59.23 66.43 72.51 79.32 83.79

8.2 16.4 34.4 54.8 73.9 87.5 101.1 117.8 134.7

7..5 15 32.5 52.5 72.5 87.5 102.5 122.5 142.5

10.9 9.3 5.8 4.4 2.3 0 −1.4 −3.8 −5.5

Note: x = conversion (%); t1 = caculated time (min); t2 = experimental time (min); (t1 − t2)/t2 = relative error (%). (5) Liang, B.; Korbee, R.; Gerritsen, A. W.; van den Bleek, C. M. Influence of manganese content on the properties of high temperature regenerative H2S sorbent. Fuel 1999, 78, 319. (6) Yang, R. Y. The discussion of sponge iron production with coal gas. Iron. Steel. Technol. 2007, 5, 1. (7) Chang, L. P.; Zhang, Z. Y.; Ren, X. R.; Li, F.; Xie, K. C. Study on the stability of sorbent removing H2S from hot coal gas. Energy Fuels 2009, 23, 762. (8) Raul, E. A.; Donald, W. M. Characterization and long-range reactivity of zinc ferritein high-temperature desulfurization processes. Ind. Eng. Chem. Res. 1991, 30, 55. (9) Xu, H. Y.; Liang, M. S.; Li, C. H. A study of the desulfurization and regeneration behavior of zinc ferrite for hot coal gas clean-up. J. Chin. Soc. Electr. Eng. 2004, 4, 198. (10) Lu, Z. Y.; Li, J. L.; Wang, Z. Development of a hot gas desulfurization agent. Ind. Catal. 2002, 10, 34. (11) Kobayashi, M.; Shirai, H.; Nunokawa, M. Elucidation sulfidation mechanisms of zinc ferrite in a reductive gas environment by in situ Xray diffraction analysis and mǒssbauer spectroscopy. Ind. Eng. Chem. Res. 2000, 39, 1934. (12) Kobayashi, M.; Shirai, H.; Nunokawa, M. High-temperature sulfidation behavior of reduced zinc ferrite in simulated coal gas revealed by in situ X-ray diffraction analysis and mǒ ssbauer spectroscopy. Energy Fuels 2002, 16, 601. (13) Kobayashi, M.; Shirai, H.; Nunokawa, M. Measurements of sulfur capacity proportional to sulfidation on sorbent containing zinc ferrite−silica composite powder in pressurized coal gas. Ind. Eng. Chem. Res. 2002, 41, 2903. (14) Ahmed, M. A.; Alonso, L. Structural changes in zinc ferrites as regenerable sorbent for hot coal gas desulfurization. Solid State Ionics 2000, 138, 51. (15) Ikenaga, N.; Ohgaito, Y.; Matsushima, H.; Suzuki, T. Preparation of zinc ferrite in the presence of carbon material and its application to hot gas cleaning. Fuel 2004, 83, 661. (16) Hatori, M.; Sasaoka, E.; Uddin, M. A. Role of TiO2 on oxidative regeneration of spent high-temperature sulfurization sorbent ZnO− TiO2. Ind. Eng. Chem. Res. 2001, 40, 1884. (17) Jun, H. K.; Lee, T. J.; Ryu, S. O.; Kim, J. C. A Study of Zn−Tibased H2S removal sorbent promoted with cobalt oxides. Ind. Eng. Chem. Res. 2001, 40, 3547. (18) Jun, H. K.; Koo, J. H.; Lee, T. J.; Ryu, S. O.; Yi, C. K.; Ryu, C. K. A study of Zn−Ti-based H2S removal sorbent promoted with cobalt and nickel oxides. Energy Fuels 2004, 18, 41. (19) Ryu, S. O.; Park, N. K.; Chang, C. H.; Kim, J. C.; Lee, T. J. Multi cyclic study on improved Zn/Ti-based desulfurization sorbent in midtemperature conditions. Ind. Eng. Chem. Res. 2004, 43, 1466. (20) Desai, M.; Brown, F.; Chamberland, B.; Jalan, V. Copper based sorbent for hot gas clean up. Am. Chem. Soc, Div Fuel. Chem. 1990, 35, 87. (21) Abbasian, J.; Simane, R. B. A regenerable copper based for H2S removal from coal gases. Ind. Eng. Chem. Res. 1998, 37 (7), 2775.

behavior of the sulfidation and regeneration reactions. Through correlating experimental data with the shrinking core model, the following conclusions can be drawn: The sulfidation is mainly dominated by diffusion. Through the regressions of the initial sulfidation reaction data, the chemical reaction order, activation energy, and pre-exponential factor are 1, 37.42 kJ/ mol, and 4.16 × 10−2 m/s, respectively, and the diffusion activation energy and pre-exponential factor are 57.23 kJ/mol and 6.41 × 10−5 m2/s, respectively. O2 regeneration has the same situation as sulfidation: the chemical reaction order, activation energy, and pre-exponential factor are 1, 24.62 kJ/ mol, and 0.17 m/s, respectively, and the diffusion activation energy and pre-exponential factor are 122.44 kJ/mol and 8.51 × 10−4 m2/s, respectively. For SO2 regeneration, the regeneration rate is controlled by the chemical reaction in the early stage, and then it is controlled by the diffusion through the product layer in the latter stage: the chemical reaction order, activation energy, and pre-exponential factor are 1, 124.82 kJ/mol, and l.62 × 102 m/s, respectively, and the diffusion activation energy and pre-exponential factor are 224.83 kJ/mol and 2.33 × 10−6 m−2/s, respectively.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86 28 85405220. Fax: +86 28 85405220. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was jointly supported by the National Natural Science Foundation of China (No. 50876121, 21336004) and Baosteel Group.

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REFERENCES

(1) Du, X. R. High temperature coal gas desulfurization and regeneration characteristics research. Coal Conversion 2003, 2, 1. (2) Ben-Slimane, R.; Hepworth, M. T. Desulfurization of hot coalderived fuel gases with manganese-based regenerable sorbent. 1. Loading (Sulfidation) tests. Energy Fuels 1994, 8, 1175. (3) Ben-Slimane, R.; Hepworth, M. T. Desulfurization of hot coalderived fuel gases with manganese-based regenerable sorbent. 2. Regeneration and multi cycle tests. Energy Fuels 1994, 8, 1184. (4) Ben-Slimane, R.; Hepworth, M. T. Desulfurization of hot coalderived fuel gases with manganese-based regenerable sorbent. 3. FixedBed Testing. Energy Fuels 1995, 9, 372. 1187


Article

Industrial & Engineering Chemistry Research (22) Slimane, R. B.; Abbasian, J. Copper-based sorbent for coal gas desulfurization at moderate temperatures. Ind. Eng. Chem. Res. 2000, 39, 1338. (23) Hüsnü, A..; Wakker, J. P.; Gerritsen, A. W.; van den Berg, P. J. Removal of H2S from fuel gases at high temperature using MnO/γAl2O3. Fuel 1995, 74, 187. (24) Hüsnü, A..; Wakker, J. P.; Gerritsen, A. W.; van den Berg, P. J. Regeneration of MnO/γ-Al2O3 used for high temperature desulfurization of fuel gases. Fuel 1996, 75, 373. (25) Liang, B.; Korbee, R.; Gerritsen, A. W.; van den Bleek, C. M. Preparation of the Mn/γ-Al2O3sorbent for high temperature regenerative H2S removal. Can. J. Chem. Eng. 1999, 77, 483. (26) Liang, B. Preparation of the Mn/γ-Al2O3 sorbent for high temperature regenerative hydrogen sulfide removal. Research report. Deflt University of Technology: Delft, The Netherlands, 1998. (27) Bakker, J. W.; Kapteijn, F.; Moulijn, J. A. A high capacity manganese-based sorbent for regenerative high temperature desulfurization with direct sulfur production conceptual process application to coal gas cleaning. Chem. Eng. J. 2003, 96, 223. (28) Wang, J.; Liang, B.; Richard, P. Manganese-based regenerable sorbent for high temperature H2S removal. Fuel 2013, 107, 539. (29) Liang, B.; Wang, J.; Zeng, B. Regenerable sorbent and desulfurization process for hot coal gas. J. Sichuan Univ. 2011, 6, 197.

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Kinetic Study on the Sulfidation and Regeneration of Manganese

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