Kinetics of Thermal Regeneration of SO2-Captured ... - ACS Publications

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Kinetics of Thermal Regeneration of SO2‑Captured V2O5/AC Daojun Zhang, Leiming Ji, Zhenyu Liu, and Qingya Liu* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China S Supporting Information *

ABSTRACT: Carbon-based materials have been used for SO2 removal from flue gases for several decades. In this process, SO2 is captured by storing in the pores of carbons in the form of H2SO4, and regeneration of the SO2-captured materials is necessary to recover SO2 capture ability. V2O5-supported activated coke (V2O5/AC) has been reported to be highly active for SO2 removal, and its regeneration has been investigated from the viewpoint of the reaction mechanism. This work studied the regeneration kinetics with the aid of a thermogravimetric analyzer coupled with a mass spectrometer. The SO2-captured sample was prepared in a fixed-bed reactor with a simulated flue gas containing 1500 ppm of SO2, 5% O2, and 5% H2O. The kinetic equation was obtained by fitting the H2SO4 conversion (α) at different heating rates with nonisothermal kinetic methods called Flynn−Wall− Ozawa and Coats−Redfern. The results indicated that the regeneration kinetic behavior varies with α. At α = 0.1−0.4, regeneration follows first-order reaction model f(α) = 1 − α with an activation energy of about 85.7 kJ/mol. At α = 0.5−0.8, regeneration follows three-dimensional diffusion model f(α) = 1.5(1 − α)2/3[1 − (1 − α)1/3]−1, with the activation energy increasing from 88.9 kJ/mol at α = 0.5 to 112.1 kJ/mol at α = 0.8.

1. INTRODUCTION

C + 2(NH4)2 SO4 = 4NH3 + CO2 + 2SO2 + 2H 2O (2)

Activated coke/carbon (termed AC) has been studied for SO2 and NOx removal from flue gases for several decades,1−3 and the MET−Mitsui−BF technology was put into practice in the 1980s.2 Modification and activation of AC were widely studied in order to further improve the capture capacity and catalytic activity of AC. The vanadium pentoxide supported AC (V2O5/ AC) catalyst−sorbent was formulated and reported to be a promising substitute for AC because of its better performance in both SO2 removal and NO reduction.4 It is generally recognized that SO2 is stored in the micropores of AC or AC-supported catalysts in the form of H2SO4 during the desulfurization process.3−5 The SO2 capture ability of AC will fade when a certain amount of micropores are filled with H2SO4. Regeneration of the catalyst is necessary for recovery of its capture ability when the SO2 concentration in the outlet cannot meet the emission standard. The regeneration process can be divided into water washing and reduction. The former is still in development because of the high consumption of water and the energy-intensive treatment of dilute sulfuric acid. The latter is reduction of H2SO4 stored in the micropores with a reducing agent, such as AC,2 NH3,3,6 and H2,7 to get a highly concentrated SO2, ammonia sulfate, or sulfur. Reduction regeneration using AC as the reductant is usually called thermal regeneration and has been put into practice.2,8 Lizzio and DeBarr9 reported that the main products of thermal regeneration were CO2, SO2, and H2O. Guo et al.10 studied thermal regeneration of SO2-captured V2O5/AC at 653 K and found that the catalyst kept high activities for simultaneous SO2 removal and NO reduction after five reaction−regeneration cycles. The main reactions during the regeneration process can be described as follows: C + 2H 2SO4 = CO2 + 2SO2 + 2H 2O © 2015 American Chemical Society

C + 2NH4HSO4 = 2NH3 + CO2 + 2SO2 + H 2O

(3)

Richter8 investigated the regeneration kinetics of AC loaded with H2SO4 and ammonium sulfate in the differential and backmix reactors. A series of elemental reactions were proposed and assumed to be of first-order. It was found that the activation energies for reactions of H2SO4 ↔ H2O + SO3, SO3 + C → SO2 + C-oxides, and C-oxides → CO2 were 51.2, 46.6, and 35.5 kJ/mol, respectively, regardless of the reactor type. He/she concluded that regeneration in a fluidized bed could also be described by a first-order reaction model. There is little information on the regeneration kinetics of SO2-captured carbon materials except the work of Richter. It is also not sure whether the V2O5 loaded on AC influences the regeneration kinetics. To guide the design and scale-up of the regeneration reactor of a SO2-captured V2O5/AC catalyst, the kinetics of its thermal regeneration is studied with the aid of a thermogravimetric analyzer (TG) coupled with a mass spectrometer (MS).

2. EXPERIMENTAL SECTION 2.1. Preparation of V2O5/AC Catalyst−Sorbent. A coalderived commercial granule AC from Xinhua Chemical Plant (Taiyuan, China) was ground and sieved to 60−80 mesh (0.18−0.25 mm) before being loaded with V2O5. The V2O5/ AC catalyst was prepared by pore-volume impregnation using an aqueous solution containing ammonium metavanadate and oxalic acid. After impregnation, the samples were dried in air at Received: Revised: Accepted: Published:

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June 23, 2015 August 31, 2015 September 14, 2015 September 14, 2015 DOI: 10.1021/acs.iecr.5b02249 Ind. Eng. Chem. Res. 2015, 54, 9289−9295

Article

Industrial & Engineering Chemistry Research

time (min), A is the preexponential factor (min−1), Ea is the apparent activation energy (J/mol), and R is the gas constant (8.314 J/mol·K). Assuming that the initial reaction temperature is T0, the heating rate is β, and the temperature at time t is T, the t is equal to (T − T0)/β. Equation 1 can be transformed to eq 6 by substituting t for (T − T0)/β.

383 K for 5 h, calcined in N2 at 773 K for 5 h, and finally oxidized in air at 523 K for 5 h. The V2O5 loading controlled by the concentration of ammonium metavanadate is 1.0 wt %, which is confirmed by inductively coupled plasma analysis. The SO2-captured V2O5/AC was prepared in a fixed-bed quartz reactor of 55 mm diameter. A total of 20 g of V2O5/AC was put in the middle of the reactor and heated to 423 K in N2. At steady state, a feed stream containing 1500 ppm of SO2, 5% O2, 5% H2O, and balance N2 was switched into the reactor. The total flow rate was 400 mL/min, corresponding to a space velocity of 600 h−1. The concentrations of SO2 and O2 in the inlet and outlet of the reactor were measured online by a flue gas analyzer (Kane KM-9600). SO2 conversion decreased gradually from 100%, and the experiment was terminated at SO2 conversion of 60%. The amount of SO2 captured by 1 g of V2O5/AC (termed sulfur capacity) was calculated to be about 58.0 mg/g according to the desulfurization curve (shown in the Supporting Information, Figure S1). For comparison, SO2captured AC was also prepared at the same conditions, and its sulfur capacity was about 20.5 mg/g according to the desulfurization curve (shown in the Supporting Information, Figure S2). In order to further confirm the sulfur capacity of V2O5/AC, thermal regeneration was carried out in a tube reactor of 20 mm diameter at 673 K in a flow of N2 at 400 mL/min. The amount of SO2 desorbed was determined by passing the exhaust to a 0.05 mol/L iodine solution. The results indicated that the amount of SO2 desorbed averaged 60 mg/g, confirming the reliability of the sulfur capacity. 2.2. TG−MS Experiment. A TG (Setsys Evolution 24, Setaram) coupled with a MS (Oministar QMS 200, Balzers) was used to obtain the data for regeneration kinetic modeling. A preliminary experiment indicated that about 80% of H2SO4 reacted with carbon when the temperature reached 573−673 K at the fastest heating rate of 90 K/min allowable for the TG. Therefore, it is difficult to obtain the data for isothermal kinetic modeling, and the nonisothermal method was selected to study the regeneration kinetics. The SO2-captured V2O5/AC or AC catalyst of about 8 mg (60−80 mesh) was loaded into a quartz crucible of 6 mm diameter to ensure that the catalyst just covered the bottom of the crucible. The crucible was heated to 423 K at a heating rate of 10 K/min in a flow of argon at 150 mL/min, kept for 20 min to purify the catalyst, and then heated to 773 K at a heating rate of 5, 10, 15, or 20 K/min and kept for 5 min. The exhaust of the TG was detected online by the MS during the whole process. The buoyancy effect in TG measurements during the temperature ramping was eliminated using data of the empty crucible under the same conditions. The baseline shift in the MS signals was corrected using the ratio of CO2 (m/e 44), H2O (m/e 18), and SO2 (m/e 64) to the argon (m/e 40) signal obtained at the same time, which was set to zero at time zero.

dα A = f (α) e−Ea / RT β dT

(6)

Transformation of eq 6 and then integration will give eq 7, where g(α) is the integrated form of the reaction rate model, u is Ea/RT, and p(u) is described as eq 8. g (α ) =

∫0

=

A β

α

dα f (α )

∫T

T

0

⎛ −E ⎞ exp⎜ a ⎟ dT ⎝ RT ⎠

⎛ −E ⎞ A T exp⎜ a ⎟ dT ⎝ RT ⎠ β 0 u AEa − e −u = du βR ∞ u 2 AEa = p(u) βR ≈

∫

∫

p(u) =

(7)

⎞ e −u ⎛ 2! 3! 4! ⎜1 − + 3 − 4 + ...⎟ 2 ⎝ 2 ⎠ u u u u

(8)

Ea can be obtained by solving eq 8 with the Flynn−Wall− Ozawa method, and the preexponential factor A can be obtained by the Coats−Redfern method. 3.1. Flynn−Wall−Ozawa Equation. Flynn and Wall12 and Ozawa13 disclosed a method to solve eq 7 by using Doyle’s14 approximation of p(u) when Ea/RT is greater than 20. The final expression is as in eq 9. ⎡ AEa ⎤ E log β = log⎢ ⎥ − 2.315 − 0.4567 a RT ⎣ Rg (α) ⎦

(9)

At the same conversion α, g(α) should be a constant, and then Ea can be obtained from the plot of log β versus 1/T if the plot follows a linear relation. This method has been widely used to calculate Ea in the literature.15−26 3.2. Coats−Redfern Equation. When the value of 1/u (that is, RT/Ea) in eq 8 is far smaller than 1, eq 7 can be approximated to eq 10.27 Substituting u in eq 10 for Ea/RT will give eq 11. g (α ) =

ln

3. NONISOTHERMAL KINETIC ANALYSIS The reaction rate is generally described as eqs 4 and 5.11

g (α ) T

2

AEa e−u βR u 2

= ln

(10)

E AR − a βEa RT

(11) 2

dα = f (α)k(T ) dt

(4)

k(T ) = A e−Ea / RT

(5)

If the expression of g(α) is known and ln[g(α)/T ] versus 1/ T at a given heating rate β follows a linear relation, the preexponential factor A can be obtained from its intercept. Ea can also be obtained from the line’s slope.

4. RESULTS AND DISCUSSION 4.1. TG−MS Results. Taking the result at a heating rate of 20 K/min as an example, the regeneration behavior of SO2-

where α is the conversion of reactant, f(α) is the differential form of the reaction rate model, k(T) is the reaction rate constant (min−1), T is the absolute temperature (K), t is the 9290

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different time or temperature, the H2SO4 conversion (termed α) can be obtained by eq 12.

captured V2O5/AC was analyzed. Figure 1 shows TG and differential thermogravimetry (DTG) curves of the sample

αT =

xT × 196 × 64 × 1000 × 100% 208WSO2 × 98

(12)

where xT (%) is the weight change at temperature T and WSO2 is the sulfur capacity (i.e., the amount of SO2 captured by 1 g of sorbent). WSO2 is about 58.0 mg/g for the V2O5/AC sample and 20.5 mg/g for the AC sample. It should be pointed out that the weight at the end of the pretreatment was set to zero to determine xT. Figure 3 shows the curves of xT versus temperature at the different heating rates. As can be seen, the initial temperature of

Figure 1. TG and DTG curves of the SO2-captured V2O5/AC during the pretreatment at 423 K and the subsequent heating process at a heating rate of 20 K/min.

during the pretreatment at 423 K and subsequent heating from 423 to 773 K. A clear weight loss can be observed from room temperature to 423 K due to H2O release, as evidenced by the MS result shown in Figure 2. The weight remained constant in

Figure 3. Weight changes of the SO2-captured V2O5/AC sample versus temperature at different heating rates.

weight loss increases with increasing heating rate, which is very common and mostly attributed to the delay in heat transfer at a faster heating rate.28,29 On the basis of the data in Figure 3, α was obtained according to eq 12 and is shown in Figure 4, which will be used for the kinetic study. 4.2. Regeneration Kinetic Evaluation. 4.2.1. Calculation of the Activation Energy Ea. The Flynn−Wall−Ozawa method was taken to calculate Ea at a given α, as described in detail in Figure 2. H2O, CO2, and SO2 release during the TG process shown in Figure 1.

the isothermal process at 423 K, indicating that the sample was completely dried. In the subsequent heating process, a weight loss starting at 473 K can be observed, which accompanies the release of H2O, SO2, and CO2, as indicated by the MS result (Figure 2), and this indicates the occurrence of regeneration. No other gases such as SO3 and CO were detected, indicating that the regeneration reaction may be described with the equation 2H2SO4 + C = 2SO2 + 2H2O + CO2, as reported by Guo et al.10 The total weight loss in the regeneration process is about 9.40%, corresponding to SO2 release of 57.8 mg/g according to the regeneration reaction stoichiometry. This value is close to the amount of SO2 captured in the catalyst, 58.0 mg/g (see section 2.1), indicating the complete reduction of H2SO4 and that the regeneration reaction is the only one to cause the weight loss. Therefore, based on the weight loss at

Figure 4. H2SO4 conversion of the SO2-captured V2O5/AC sample versus temperature at different heating rates. 9291

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Industrial & Engineering Chemistry Research section 3.1, where the key is to obtain the plot of log β versus 1/T. On the basis of the data in Figure 4, the plots at different α values were obtained (see Figure S3), and their slopes (that is, Ea) and linear correlation coefficients are shown in Table 1. As

4.2.2. Calculation of the Preexponential Factor. The preexponential factor can be obtained by the Coats−Redfern method if the expression of the integral kinetic mechanism model g(α) is known, which has been described in section 3.2. There are a lot of g(α) expressions in the literature,32,33 which can be classified as chemical reaction control, diffusion control, phase boundary control, and powder functions. In the case of thermal regeneration of the SO2-captured carbon-based materials, it is mainly the reaction between H2SO4 in the liquid phase and carbon in the solid phase to produce gaseous CO2, SO2, and H2O. Because the amount of carbon is wholly far more than that of H2SO4, the regeneration reaction may be first-order with regard to the concentration of H2SO4, as reported by Richter.8 On the other hand, H2SO4 is stored in the micropores, and only the local carbon may be available for the reaction, which may lead to a second-order reaction with regard to H2SO4 and carbon, a bidimensional interface controlled reaction, or a reaction controlled by diffusion of H2SO4. It is also possible that the reaction is controlled by the internal diffusion of gaseous products. Considering the characteristics of the regeneration reaction, the following topochemical reaction kinetic models shown in Table 3 are preliminarily selected. To find the most probable kinetic model, Ea of all of the models were obtained according to eq 11. The model whose Ea is the closest to the values obtained by the Flynn−Wall−Ozawa method (Table 1) will be the most probable one because Ea obtained by the Flynn−Wall−Ozawa method is modelindependent and is considered to be reliable.34,35 As discussed in section 4.2.1, the regeneration reaction may follow the same single kinetic mechanism at α = 0.1−0.4 but other complex ones at α = 0.5−0.8. Therefore, the process was divided into two regions to find the most probable kinetic model and calculate A. 4.2.2.1. Most Probable Kinetic Model and Preexponential Factor at α = 0.1−0.4. Table 4 shows Ea and ln A of all of the models listed in Table 3. It is clear that the absolute values of all of the coefficients except those adopting the second-order reaction model are greater than 0.99, indicating good linear relations of all of the fittings. Compared with the average value of Ea in Table 1, 85.7 kJ/mol, the Ea values calculated according to the first-order reaction model (N1) are the closest, those according to diffusion-controlled models (D1−D5) are much greater, and those according to the second-order reaction model (N2) and interface area control model (R1) are smaller. These indicate that N1 may be the most probable model. To further confirm the most probable reaction model, ln A versus Ea at different β values is plotted according to the kinetic compensation effect equation shown in eq 13, where kiso is the isokinetic rate constant and Tiso is the isokinetic temperature.32−34,36−40 The result is shown in Figure 5, which presents a good linear relationship. According to the slope and intercept of the plot, Tiso and kiso can be obtained, which are 509 K and 0.03558 min−1, respectively. Generally, the kinetic reaction model is reliable if Tiso is between the minimum and maximum temperatures at which α reaches 0.1 and 0.4 in our case. Figure 4 indicates that the minimum temperature is 490 K (α reaches 0.1 at a heating rate of 5 K/min) and the maximum is 575 K (α reaches 0.4 at a heating rate of 20 K/min). Tiso, 509 K, is obviously in the range of 490 and 575 K, confirming that regeneration of SO2-captured V2O5/AC is of first-order with respect to the H2SO4 concentration, which is consistent with the report of Richter.8

Table 1. Regeneration Activation Energies (Ea) of the SO2Captured V2O5/AC Calculated by the Flynn−Wall−Ozawa Equation α

Ea (kJ/mol)

|R|

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

84.3 86.7 85.6 86.3 88.9 95.8 102.9 112.1

0.986 1.000 0.998 0.993 0.991 0.992 0.991 0.988

stated in section 3.1, the Flynn−Wall−Ozawa method is applicable only when Ea/RT is greater than 20.30,31 Ea/RT in our case is between 18.3 and 23.2, indicating applicability of the Flynn−Wall−Ozawa method. The absolute values of linear correlation coefficients at all α values except 0.1 and 0.8 are greater than 0.99, further indicating the reliability of all Ea in Table 1. Ea remains at about 85.7 kJ/mol at α = 0.1−0.4 but increases from 88.9 to 112.1 kJ/mol with increasing α from 0.5 to 0.8. This suggests that regeneration may follow the same kinetic mechanism and can be described by a single model at α = 0.1−0.4 but is kinetically complex and may be a multistep process when α is greater than 0.5. To evaluate the effect of V2O5 on the regeneration process, Ea of the SO2-captured AC was also determined by the Flynn− Wall−Ozawa method, and the results are shown in Table 2. Table 2. Regeneration Activation Energies (Ea) of the SO2Captured AC Calculated by the Flynn−Wall−Ozawa Equation α

Ea (kJ/mol)

|R|

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

65.8 85.1 110.3 108.4 106.4 102.2 115.2 188.7

0.994 0.982 0.998 1.000 1.000 0.999 0.991 0.908

The basic data used for this determination, H2SO4 conversion versus temperature and log β versus 1/T, are shown in Figures S4 and S5 in the Supporting Information. It can be seen from Table 2 that Ea is 65.8 kJ/mol at α = 0.1, 85.1 kJ/mol at α = 0.2, and greater than 100 kJ/mol at α = 0.3−0.7. It is clear that Ea of SO2-captured AC at α = 0.1 is lower than that of SO2-captured V2O5/AC but Ea at α = 0.3−0.7 is higher than that of the SO2captured V2O5/AC. These indicate that V2O5 may promote the reaction of H2SO4 with carbon during the regeneration process. It should be pointed out that all Ea obtained in our case are greater than any of those of the elemental reaction involved in H2SO4 reduction over pure carbon reported by Richter,8 possibly because of the different ACs. 9292

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Industrial & Engineering Chemistry Research Table 3. Possible Topochemical Reaction Kinetic Models during Regeneration of SO2-Captured V2O5/AC reaction mechanism chemical reaction control

diffusion control

interface area control

f(α)

symbol

g(α)

first-order reaction

N1

1−α

−ln(1 − α)

second-order reaction parabolic rule one-dimensional diffusion model Valensi equation cylindrically symmetrical two-dimensional diffusion model (bidimensional particle) Jander equation (2D, n = 2), two-dimensional diffusion model Jander equation (3D, n = 2), three-dimensional diffusion model

N2 D1 D2

(1 − α)2 0.5α−1 [−ln(1 − α)]−1

(1 − α)−1 α2 (1 − α) ln(1 − α) + α

D3 D4

[1 − (1 − α)1/2]2 [1 − (1 − α)1/3]2

anti Jander equation (3D), three-dimensional diffusion model

D5

contraction of the cylinder(n = 2), contracting area, bidimensional shape

R1

(1 − α)1/2[1 − (1 − α)1/2]−1 1.5(1 − α)2/3[1 − (1 − α)1/3]−1 1.5(1 + α)2/3[(1 + α)1/3 − 1]−1 (1 − α)1/2

[(1+ α)1/3 − 1]2 2[1 − (1 − α)1/2]

Table 4. Regeneration Activation Energies of the SO2Captured V2O5/AC Calculated According to the Coats− Redfern Equation at α = 0.1−0.4 model

β (K/min)

Ea (kJ/mol)

ln A

|R|

N1

5 10 15 20 5 10 15 20 5 10 15 20 5 10 15 20 5 10 15 20 5 10 15 20 5 10 15 20 5 10 15 20

80.5 83.9 77.8 82.0 14.2 14.4 12.8 14.1 147.9 154.5 143.8 151.0 154.7 161.6 150.4 158.0 158.4 165.3 153.9 161.8 162.1 169.1 157.4 165.5 138.2 144.6 134.5 141.1 74.9 78.3 72.5 76.3

15.65 16.52 15.06 16.01 0.13 0.72 0.54 1.14 30.30 31.41 28.61 29.98 31.34 32.43 29.51 30.95 31.57 32.66 29.67 31.14 31.67 32.76 29.70 31.21 25.63 26.76 24.14 25.41 14.22 15.10 13.74 14.63

0.999 0.999 1.000 0.997 0.934 0.897 0.904 0.942 0.996 0.999 0.999 0.994 0.997 0.999 0.999 0.995 0.998 0.999 0.999 0.996 0.998 0.999 1.000 0.996 0.995 0.998 0.998 0.992 0.998 0.999 0.999 0.995

N2

D1

D2

D3

D4

D5

R1

ln A = ln k iso +

Ea RTiso

Figure 5. ln A at different heating rate versus the corresponding Ea (ln A and Ea are those of the N1 model, as shown in Table 4).

4.2.2.2. Most Probable Kinetic Mechanism and Preexponential Factor at α = 0.5−0.8. Although the varied Ea at α = 0.5−0.8 suggests a kinetically complex regeneration reaction, as indicated in section 4.2.1, a single-step process is assumed to find the most probable model as adopted by many researchers.15−26 The average Ea value at α = 0.5−0.8 is 99.9 kJ/mol, corresponding to a maximum relative error of 12.2%, which is lower than some literature data.15−20 Because regeneration at α = 0.1−0.4 follows the chemical reaction model of first-order, regeneration at α = 0.5−0.8 may follow models other than that and, therefore, the diffusion and bidimensional interface control models are selected to find the most probable model. Table 5 shows Ea and ln A of these models. As can be seen, the absolute values of almost all of the linear coefficients are greater than 0.99. The Ea values obtained according to D3 (two-dimensional diffusion model) and D4 (three-dimensional diffusion model) seem closer to the corresponding Ea values in Table 1 than those obtained according to other models, suggesting that both D3 and D4 models are possibly the most probable. Similar to the discussion in section 4.2.2.1, the kinetic compensation effect is used to verify the most probable models D3 and D4. Figure 6 shows lnA versus Ea of these two models. The linear correlation coefficient of the D3 model is 0.920 and that of the D4 model is 0.999, suggesting that the D4 model is more reasonable than the D3 model to be the most probable one. According to the fitting equation, kiso and Tiso are calculated as 0.0239 min−1 and 573 K, respectively. Tiso is

(13)

Because Ea obtained by the Flynn−Wall−Ozawa method, 85.70 kJ/mol, is more reliable, substituting it into the fitting equation will give a relatively reliable A, 2.21 × 107 min−1. Therefore, the regeneration kinetics at α = 0.1−0.4 can be described by eq 14. −ln(1 − α) = 2.21 × 107 × e−85700/ RT t

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between the minimum temperature of 525 K (at which α is 0.5 at a heating rate of 5 K/min) and the maximum temperature of 610 K (at which α is 0.8 at a heating rate of 20 K/min), confirming the reliability of the most probable model D4. The D4 model (Jander’s three-dimensional diffusion) has been adopted in many systems, such as thermal decomposition of calix[8]arene,22 decomposition of strontium nitrate,25 reoxidation of Ti−Magnéli phases,41 and oxidation of titanium particles.42 Ea in these cases spans from 68.5 to 343.9 kJ/mol. It is clear that Ea in our case is in the range of literature data. The diffusion involved in the regeneration process includes penetration of H2SO4 to the reactive carbon and diffusion of gaseous products from the pores to the outside. Because the reaction at α = 0.1−0.4 is not controlled by gaseous product diffusion, the subsequent reaction at α = 0.5−0.8 may also not be controlled by it but by diffusion of H2SO4. This result agrees with Richter’s finding that the gas-phase products had no influence on the reaction between H2SO4 and carbon. The increasing Ea at α = 0.5−0.8 may be related to the decreasing amount of H2SO4 as the reaction goes on.

Table 5. Regeneration Activation Energies of the SO2Captured V2O5/AC Calculated According to the Coats− Redfern Equation at α = 0.5−0.8 mechanism

β (K/min)

Ea (kJ/mol)

ln A

|R|

D1

5 10 15 20 5 10 15 20 5 10 15 20 5 10 15 20 5 10 15 20 5 10 15 20

58.2 66.3 68.8 78.9 72.7 82.4 85.7 97.9 82.5 93.3 97.2 110.8 91.9 103.8 108.2 123.2 48.4 55.2 57.4 65.9 36.5 41.8 43.7 50.4

9.37 11.48 12.12 14.29 12.28 14.65 15.38 17.92 13.98 16.52 19.43 20.11 15.45 18.17 19.02 22.06 4.53 6.45 7.05 8.97 4.94 6.61 7.20 8.81

0.988 0.990 0.984 0.991 0.993 0.994 0.990 0.995 0.996 0.997 0.993 0.997 0.997 0.998 0.995 0.998 0.985 0.987 0.981 0.990 0.994 0.996 0.991 0.996

D2

D3

D4

D5

R1

5. CONCLUSION The thermal regeneration kinetics of SO2-captured V2O5/AC under an inert atmosphere was investigated in this work. It was found that the regeneration kinetics may be divided into two stages. At H2SO4 conversion of 0.1−0.4, the reaction was of first-order with respect to the fractional unreacted H2SO4, and its activation energy is 85.7 kJ/mol. At H2SO4 conversion of 0.5−0.8, the activation energy increases from 88.9 to 112.1 kJ/ mol, but Jander’s three-dimensional diffusion model can describe the kinetic behavior very well. The reaction may be controlled by diffusion of the unreacted H2SO4 to the reactive carbon.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b02249. Desulfurization curve of V2O5/AC based on the data obtained during the preparation of a SO2-captured V2O5/ AC sample (Figure S1), desulfurization curve of AC based on the data obtained during the preparation of a SO2-captured AC sample (Figure S2), isoconversional plots of the Flynn−Wall−Ozawa method at different values of conversion of SO2-captured V2O5/AC (Figure S3), H2SO4 conversion of the SO2-captured AC sample versus temperature at different heating rates (Figure S4), and isoconversional plots of the Flynn−Wall−Ozawa method at different values of conversion of SO2-captured AC (Figure S5) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel./Fax: +86-10-64421077. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the Program for New Century Excellent Talents in University (NCET-11-0558) and the National Natural Science Foundation of China (Grant 20976010).

Figure 6. ln A at different heating rates versus the corresponding Ea (ln A and Ea are those of the D3 and D4 models, as shown in Table 5).

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DOI: 10.1021/acs.iecr.5b02249 Ind. Eng. Chem. Res. 2015, 54, 9289−9295

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DOI: 10.1021/acs.iecr.5b02249 Ind. Eng. Chem. Res. 2015, 54, 9289−9295