Article pubs.acs.org/IECR
HCl Oxidation To Recycle Cl2 over a Cu/Ce Composite Oxide Catalyst. Part 1. Intrinsic Kinetic Study Jihai Tang,† Xian Chen,† Zhaoyang Fei,†,‡ Jianhui Zhao,† Mifen Cui,† and Xu Qiao*,†,‡ †
Jiangsu Key Laboratory of Industrial Water-Conservation & Emission Reduction, College of Chemistry and Chemical Engineering, and ‡State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, Nanjing 210009, People’s Republic of China ABSTRACT: Intrinsic kinetics of HCl oxidation over a Cu/Ce composite oxide catalyst have been studied using an integral tubular reactor. The kinetic data were measured after elimination of the influence of external and internal diffusion at 390−420 °C, with nHCl/nO2 ranging from 1 to 4 and W/FA0 ranging from 50 to 350 g·min/mol under an atmospheric pressure. The empirical intrinsic kinetic model assuming the surface reaction as the rate-controlling step has been established. The kinetic parameters were estimated by the Levenberg−Marquardt method. The predicted results agree well with the experimental data. The activation energy over a Cu/Ce composite oxide catalyst is 82.10 kJ/mol. This kinetic model has laid an important foundation for reactor designation and optimization. studied,29,30 which can maintain a relatively high and stable conversion rate for about 1200 h in a laboratory-stabile experiment. In this paper, the reaction kinetic behavior of HCl oxidation over a Cu/Ce composite oxide catalyst was further studied. The intrinsic kinetic data were collected in an integral fixed-bed reactor for kinetic parameter regression. The results will lay an essential engineering foundation for reactor simulation, scale-up, and optimization of an industrial catalytic process.
1. INTRODUCTION Chlorine (Cl2) is a key basic chemical raw material to create important products of the chemical industry such as pharmaceutical, pesticide, poly(vinyl chloride), polyurethane, etc. However, Cl2 is reduced to hydrogen chloride (HCl) or chloride salts in most of those processes. The prominent example is the synthesis of isocyanate production involving phosgene in which Cl2 is used as an oxidizing agent. All of Cl2 is converted into the byproduct HCl, which causes an overcapacity of hydrochloric acid, resulting in a serious toxic-waste disposal problem. In this case, seeking efficient routes for recycling Cl2 from HCl to design closed process recycles would be an important technique in realizing energy savings and emission reduction in industrial chemistry involving Cl2. Electrolysis1−7 and catalytic oxidation8−13 are the major ways to perform the recycling process of HCl to Cl2. Because of its simple operation and low energy consumption, the environmentally friendly catalytic oxidation route has attracted more and more attention.14−24 In the past few years, noble metal elements such as gold14 and ruthenium15−18 have been used as catalysts for HCl oxidation. The noble metal catalysts can show super catalytic activity even at temperatures as low as 600 K.19 However, the high and dramatically fluctuating market price limits the large-scale application of those catalysts in industry. Copper compounds and ceria are relatively cheap catalysts used in HCl oxidation.20−23 The adsorption mechanism of HCl and water over CuO was studied.24−27 The reaction mechanism study over CeO2 suggests that dissociative adsorption of HCl occurs on the surface oxygen vacancies, activation of the Cl atom from lattice vacancies to surface positions is the most energy-demanding step, and Cl2−oxygen competition for the available active sites may render reoxidation as the ratedetermining step.28 Furusaki9 studied the reaction rate of HCl oxidation over CuCl2/SiO2 according to the L−H mechanism in an integral and differential tubular reactor. In our prior work, a possessed industrial application ability of the Cu/Ce composite oxide catalyst has been prepared and © 2013 American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Apparatus. A HCl oxidation reaction device schematic diagram is shown in Figure 1. The flow rates of gas reactants HCl and O2 were controlled by an anticorrosive mass flowmeter. Via a mixer, the reactive gases were interred into
Figure 1. Schematic diagram for an intrinsic kinetic experiment of HCl oxidation. Received: Revised: Accepted: Published: 11897
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an isothermal integral fixed-bed reactor. The equivalent diameter and length of the quartz tube reactor are 10 mm and 0.5 m, respectively. The catalytic performance of a Cu/Ce composite oxide catalyst was tested at a constant temperature within ±1.5 °C. The off-gas from the reactor was absorbed by an alkali solution. A three-way valve was installed before alkali absorption for gas sampling. The reacted product gas was absorbed by an excessive potassium iodide solution and tested by iodometry and acid−base titration to detect the generated Cl2 and unreacted HCl. The conversion rate was calculated according to the detected results. 2.2. Catalysts. 2.2.1. Catalyst Preparation. The catalyst was prepared by incipient wetness coimpregnation with an aqueous solution of the corresponding metal salts. The details are as follows: 13.5 g of Cu(NO3)2·3H2O (Sinopharm Chemical Reagent Co., Ltd.) and 17.5 g of Ce(NO3)3·6H2O (Sinopharm Chemical Reagent Co., Ltd.) were absolutely dissolved in 70 mL of deionized water to form an aqueous solution. Then, 50 g of commercial Y-zeolite (Wenzhou Huahua Co. Ltd.) was impregnated with it for 24 h and then extracted as 2 mm of diameter. The sample was dried at 100 °C in an oven for 24 h and calcined at 550 °C for 3 h. Finally, the catalysts were grated to the required diameter distribution according to the kinetic test. 2.2.2. Catalyst Stability. The 1200 h stability test was carried out to ensure that the intrinsic kinetic data were measured within the catalytic stable period. The experimental results are shown in Figure 2. The average conversion of HCl is 85.2% and
Figure 3. Effect of the external diffusion on the HCl catalytic oxidation: reaction temperature = 420 °C; nHCl/nO2 = 1.
indicates that the linear velocity effect on the reaction rate can be ignored and the influence of external diffusion is eliminated in this condition. Meanwhile, there are hardly differences when W/FA0 is higher than 275 g·min/mol (linear velocity under 62.2 m/h), indicating that the reaction rate is obviously affected by the external diffusion when W/FA0 is higher than this value. In view of the above, the subsequent intrinsic kinetic study was carried out with a linear velocity above 62.2 m/h. 2.3.2. Elimination of Internal Diffusion. Another premise for the intrinsic kinetic study is the elimination of internal diffusion. The effect of the particle size of the catalyst on the conversion of HCl was tested under the given conditions (Table 1). As can be seen from Table 1, the relative variation is Table 1. Effect of the Internal Diffusion on the HCl Catalytic Oxidation mesh size
7−10
10−14
14−20
20−32
32−42
42−60
xAa/%
37.3
38.5
40.6
43.0
42.8
43.2
Reaction temperature: 420 °C. Catalyst quantity: 1 g. nHCl/nO2 = 1/1. W/FA0 = 160 g·min/mol. a
less than 0.5% with a decrease of the particle size when the particle size is above 20−32 mesh, implying that the internal diffusion is eliminated. In this case, the subsequent intrinsic kinetic study was carried out with particle sizes of 42−60 mesh. Figure 2. Stability test over the Cu/Ce composite oxide catalyst: reaction temperature = 430 °C; catalyst quantity = 5 g; nHCl/nO2 = 1; HCl flow = 80 mL/min.
3. RESULTS AND DISCUSSION 3.1. Intrinsic Kinetic Data. The intrinsic kinetic data were measured at a linear velocity above 62.2 m/h and the particle size of 42−60 mesh to limit of external and internal diffusion influences. A total of 112 sets of intrinsic kinetic data were determined (Figure 4). As can be seen from Figure 4, the temperature can remarkably influence the conversion of HCl. The reaction rate at higher temperature will be faster, resulting in a greater conversion rate in kinetics. On the other hand, the high temperature is undesirable according to the thermodynamics; HCl oxidation is a reversible exothermic reaction. From Figure 4, a rising trend of the conversion rate of HCl was observed with an increase of W/FA0 and the residence time, indicating that the reaction is far from thermodynamic equilibrium, and the reaction is controlled by the kinetics in our test condition.
there was no obvious change in the long duration, suggesting that the Cu/Ce composite oxide catalyst possessed preliminary industrial application ability and the intrinsic kinetic data available. 2.3. Kinetic Tests. 2.3.1. Elimination of External Diffusion. In order to eliminate the external diffusion of catalysts, the influence of external diffusion was investigated under different catalyst quantities of 0.6 and 1 g (average particle size of 42−60 mesh and diluted 30 times with quartz of the same diameter), and the results are shown in Figure 3. Two xA−W/FA0 curves exactly coincide under different catalyst amounts when W/FA0 is lower than 275 g·min/mol (linear velocity above 62.2 m/h), which 11898
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θHClσ, θOσ, and θCl2σ represent the coverage ratios of HCl, O2, and Cl2 on the active sites, which can be calculated according to the adsorption and desorption rates, respectively. θHClσ = KHClPHClθ V
(7)
θOσ = K O2PO2 0.5θ V
(8)
θCl 2σ = K Cl 2PCl 2θ V
(9)
where KHCl, KO2, and KCl2 represent the adsorption equilibrium constants of HCl, O2, and Cl2, respectively. θV = 1/(1 + KHClPHCl + KO2PO20.5 + KCl2PCl2) represents the uncoverage ratio of the active sites of the catalyst. PHCl, PO2, PCl2, and PH2O are the partial pressures of HCl, O2, Cl2, and H2O, respectively. When the adsorption and desorption rate equations (7)−(9) are substituted into eq 6, the surface reaction rate can be described as eq 10. rA =
kR KHCl 2K O2PHCl 2PO2 0.5 − kR ′K Cl 2PCl 2PH2O (1 + KHClPHCl + K O2PO2 0.5 + K Cl 2PCl 2)3
(10)
2
The variable k defined as k = kRKHCl KO2 (mol/g·min·kPa2.5) and another variable kp defined as kp = kRKHCl2KO2/kR′KCl2 (kPa−0.5) are the positive reaction rate constant and reaction equilibrium constant, respectively. Equation 10 can be simplified into
Figure 4. Intrinsic kinetic data on the catalyst particle size 42−60 mesh, a linear velocity above 62.2 m/h, temperatures ranging from 390 to 420 °C, and nHCl/nO2 = 1−4.
nHCl/nO2 also significantly influences the conversion of HCl. Reducing nHCl/nO2 can increase the oxygen concentration adsorbed on the surface of the catalyst and thereby increase the conversion rate of HCl. 3.2. Intrinsic Kinetic Model. In this work, the empirical intrinsic kinetic model was established. The chemical equation of the catalytic oxidation of HCl is as follows: 2HCl + 0.5O2 ⇌ Cl 2 + H 2O
rA =
(1)
k O2 ′
kR ′
k Cl2
kp =
(2)
5881.7 − 0.9303 log T + 1.37014 × 10−4T T (12)
K P0
(13)
P0 is the atmospheric pressure, 100 kPa. 3.3. Estimation of the Kinetic Parameters. The intrinsic kinetic data were measured at atmospheric pressure, assuming that react-gases are the ideal gases and the fluid is plug flow. When eq 13 and Arrhenius equations (14)−(17) are substituted into eq 11, the ordinary differential equation (18) of the reaction rate can be obtained.
(3)
(4)
(5)
where σ is the active site of the catalyst and HClσ, Oσ, and Cl2σ are the adsorptive HCl, O2, and Cl2, respectively. Assuming that the surface reaction (eq 4) is the ratecontrolling step, the reaction rate of catalytic oxidation of HCl in eq 1 can be described as follows: rA = kR θHClσ 2θOσ 0.5 − kR ′θCl 2σPH2O
(11)
The equilibrium constant K is based on the reaction equation 4HCl + O2 ⇌ 2Cl2 + 2H2O in which the stoichiometric coefficient of HCl is 4, 2 times the value of eq 1. Obviously, the relationship between kp and K is as follows:
k Cl2 ′
Cl 2σ HoooI Cl 2 + σ
⎠
− 1.7584 × 10−8T 2 − 4.1744
kR
2HClσ + Oσ HooI Cl 2σ + H 2O + 2σ
⎟
(1 + KHClPHCl + K O2PO2 0.5 + K Cl 2PCl 2)3
log K =
k O2
0.5O2 + σ HoooI Oσ
kp
Arnold and Kobe reported the equilibrium constant of HCl oxidation, and the function of the reaction equilibrium constant to temperature is as follows:
kHCl
kHCl ′
PCl2PH2O ⎞
31
The following assumptions are made: (1) The energies of all active sites are equivalent. (2) Adsorption of the reactants is monolayer ideal, and interaction of the adsorbed reactants is negligible. (3) There is molecular adsorption of HCl and Cl2, dissociative adsorption of O2, and no adsorption of H2O. The following element reaction steps can be established: HCl + σ HooooI HClσ
⎛ k ⎜PHCl 2PO2 0.5 − ⎝
(6) 11899
k = k 0e−E / RT
(14)
KHCl = KHCl,0eQ HCl / RT
(15)
K O2 = K O2,0eQ O2 / RT
(16)
K Cl 2 = K Cl 2,0eQ Cl2 / RT
(17)
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Table 2. Estimated Kinetic Parameters according to Equation 18 k0 (mol/g·min·kPa2.5)
E (kJ/mol)
KHCl,0 (kPa−1)
QHCl (kJ/mol)
KO2,0 (kPa−0.5)
QO2 (kJ/mol)
KCl2,0 (kPa−1)
QCl2 (kJ/mol)
5.118 × 10 2.373 × 105
82.10 2.62
3.656 0.567
0.966 0.088
1.854 0.299
0.238 0.091
4.790 0.742
2.277 0.876
6
value standard error
Figure 5. Predicted conversion under different experimental conditions.
dxA
rA = d
=
In order to obtain the kinetic parameters, a nonlinear leastsquares method is used, and the objective function is defined as follows:
( ) W FA0
⎛ k 0e−E / RT ⎜PHCl 2PO2 0.5 − ⎝
PCl2PH2O ⎞ K / P0
n
⎟
⎠
SSR =
Equation 22 means minimization of the sum of squared residuals (SSR) between the calculated values (xi,c) and experimental data (xi,e) of HCl conversion. The ordinary differential equation (18) is solved by the Runge−Kutta method to calculate HCl conversion versus W/ FA0. Then, the kinetic parameters are fitted by the Levenberg− Marquardt method using the 112 sets of data obtained in section 3.1. All of the numeric routes are developed in the Matlab 2010a program. The estimated intrinsic kinetic parameters are listed in Table 2. Then, the intrinsic kinetic model is as follows:
3
(18)
E, QHCl, QO2, and QCl2 in eq 18 are the reaction activation energy of HCl oxidation and the adsorption activation energies of HCl, O2, and Cl2 (kJ/mol), respectively. Because of the decrease of the molecular number in oxidation of HCl to Cl2, the partial pressures of HCl, O2, Cl2, and H2O are changed in the wake of conversion of HCl, defined as xA; thus PHCl =
PO2 =
P0yA0 (1 − xA ) 1 + δAyA0 xA
(19)
PCl 2 = PH2O =
d (20)
=
0.5P0yA0 xA 1 + δAyA0 xA
dxA
rA =
P0yB0 − 0.25P0yA0 xA 1 + δAyA0 xA
(22)
i=1
(1 + KHCl,0eQ HCl / RT PHCl + K O2,0eQ O2 / RT PO2 0.5 + K Cl 2,0eQ Cl2 / RT PCl 2)
∑ (xi ,c − xi ,e)2
( ) W FA0
4 ⎛ 5.118 × 106e−8.21 × 10 / RT ⎜PHCl 2PO2 0.5 − ⎝
966/ RT
(1 + 3.656e (21)
PHCl + 1.854e
PCl2PH2O ⎞
238/ RT
K / P0
PO2
0.5
⎟
⎠
mol
3
+ 4.79e 2277/ RT PCl 2)
where δA = (1 + 1 − 2 − 0.5)/2 = −0.25 is the variation of the total molecular number in reaction (1).
/g·min 11900
(23)
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The predicted results under different experimental conditions are shown in Figure 5; the outlet conversion of HCl was calculated by the intrinsic kinetic model equation (23) under atmospheric pressure with the temperature ranging from 390 to 420 °C and W/FA0 from 50 to 350 g·min/mol. As can be seen in Figure 5, the predicted values according to eq 23 are in good agreement with the experimental HCl conversion and SSR is 3.62 × 10−2. This is also confirmed by the parity plot, as shown in Figure 6. All of the predicted HCl oxidation rates are closely
Figure 7. Distribution of the residual plot.
±5% and obey the random normal distribution, which indicates that the kinetic model is reliable. The reaction activation energy of a Cu/Ce composite oxide catalyst is 82.10 kJ/mol, much lower than the values of the metal oxide catalysts listed in Table 4. It is close to the Table 4. Comparison of the Activation Energy of HCl Oxidation over the Catalysts Reported in the Literature
Figure 6. Comparison of experimental versus predicted rates.
distributed around the diagonal line, which suggests that the difference between the predicted values and experimental data is very little and the intrinsic kinetic model obtained according to the above-mentioned deduction can exactly describe the intrinsic kinetic behavior of HCl catalytic oxidation over a Cu/ Ce composite oxide catalyst. ρ2 is the relativity coefficient, calculated by eq 24.
catalyst
E (kJ/mol)
ref
Cu/Ce composite oxide CuAlO2 CeO2 Cr2O3 RuO2/SnO2
82 100 90 97 69
this work 27 28 32 33
N
ρ2 = 1 −
∑i = 1 (xi ,e − xi ,c)2 N ∑i = 1 xi ,e 2
activation energy of the RuO2-based catalyst (69 kJ/mol).33 Here, the synergetic effect between CuO and CeO2 is considered to take charge of the high catalytic activity, which will be carried out in the upcoming studies. The results indicate that a Cu/Ce composite oxide catalyst appears to be the most possible cost-effective alternative to the RuO2-based catalysts for large-scale Cl2 recycling.
(24)
F is the proportion of the regression sum of mean square to the residual error sum of mean square, calculated by eq 25. N
F=
N
[∑i = 1 xi ,e 2 − ∑i = 1 (xi ,e − xi ,c)2 ]/Np N
∑i = 1 (xi ,e − xi ,c)2 /(N − Np)
(25)
4. CONCLUSIONS The intrinsic kinetics of HCl oxidation over a Cu/Ce composite oxide catalyst were studied at the temperature ranging from 390 to 420 °C, nHCl/nO2 ranging from 1 to 4, and W/FA0 ranging from 50 to 350 g·min/mol under atmospheric pressure in an integral tubular reactor. The intrinsic kinetic data were tested within a catalytic stable period of the catalyst and on the premise of the elimination of external and internal diffusion influences. The reaction rate equation was established on the assumption that the surface reaction is the reactioncontrolling step. The kinetic parameters are estimated, and the reaction activation energy of a Cu/Ce composite oxide catalyst is 82.10 kJ/mol. The predicted conversion of HCl according to the intrinsic kinetic model is in good agreement with the experimental data. The model can exactly describe the behavior of the reaction intrinsic kinetics of HCl oxidation, which will play an important role in the industrial reactor design and scaleup.
FT is the value of the F table corresponding to a 95% confidence level. The results of the statistical test of intrinsic kinetic model equation (23) are shown in Table 3. Normally, the model is Table 3. Statistical Test of the Intrinsic Kinetic Model eq 23
Np
N − Np
ρ2
F
FT(Np, N−Np)
8
104
0.9966
3801.09
2.04
suitable when ρ2 > 0.9 and F > 10FT. As shown in Table 3, the values calculated by eq 23 are in good agreement with the experimental results because both prerequisites (ρ2 > 0.9 and F > 10FT) are satisfied. The relative errors of 112 sets of data between calculated values and experimental data of HCl conversion are shown in Figure 7. As can be seen, the residuals of the predicted values and experimental data are kept within 11901
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +86 25 83172298. Fax: +86 25 83587168. E-mail: qct@ njut.edu.cn. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by National Key Technology Research and Development Program of the Ministry of Science and Technology of China (Grant 2011BAE18B01), Natural Science Foundation of Jiangsu Province (Grant BK2010083), and Technology Research and Development Program of Jiangsu Province (Grant BE2011830).
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NOMENCLATURE W= weight of the catalyst, g FA0= molar flow rate of HCl, mol/min rA= reaction rate of HCl, mol/g·min xA= conversion of HCl Σ= active sites of the catalyst PHCl, PO2, PCl2, PCl2= pressure of HCl, O2, Cl2, and H2O, respectively, kPa kHCl= adsorption rate constant of HCl, kPa−1 kHCl′= desorption rate constant of HCl kO2= adsorption rate constant of O2, kPa−0.5 kO2′= desorption rate constant of O2 kR= positive rate constant of the surface reaction, mol/ g·min kR′= reverse rate constant of the surface reaction, mol/ g· min·kPa kCl2= adsorption rate constant of Cl2, kPa−1 kCl2′= desorption rate constant of Cl2 KHCl= adsorption equilibrium constant of HCl, kPa−1 KO2= adsorption equilibrium constant of O2, kPa−0.5 KCl2= adsorption equilibrium constant of Cl2, kPa−1 θHCl= coverage ratio of the active sites of HCl θO= coverage ratio of the active sites of O2 θCl2= coverage ratio of the active sites of Cl2 k= positive reaction rate constant, mol/g·min·kPa2.5 E= positive reaction activation energy, kJ/mol R= gas constant, 8.314 J/mol·K T= reaction temperature, K QHCl= adsorption activation energy of HCl, kJ/mol QO2= adsorption activation energy of O2, kJ/mol QCl2= adsorption activation energy of Cl2, kJ/mol δA= variation of the total molecular number in the system when one molecular HCl is reacted P0= atmospheric pressure, 100 kPa xc= calculated value of HCl conversion xe= experimental data of HCl conversion N= number of experiments Np= number of parameters in the equation of model
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