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Cite This: Ind. Eng. Chem. Res. 2019, 58, 9246−9256
Reaction Kinetics of HCl Catalytic Oxidation over a Supported Cu-Based Composite Industrial Catalyst Jigang Zhao,*,†,§ Daiqi Fu,† Nan Song,‡ Xiangqian Yuan,‡ and Xiaotao Bi§ †
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International Joint Research Center of Green Energy Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China ‡ State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China § Chemical and Biological Engineering Department, University of British Columbia, Vancouver, British Columbia V6T1Z3, Canada ABSTRACT: The chlorine from HCl can be recycled by catalytic oxidation during the industrial process, but excessive heat will be produced during the HCl catalytic oxidation, which leads to catalyst deactivation. Thus, a study of the reaction kinetics of HCl catalytic oxidation over a Cu-based composite catalyst is necessary. The influence of T values of 360−400 °C, nHCl/nO2 values of 1−4, and FHCl0/W values of 0.01−60 h−1 on HCl conversion and reaction rate was studied, and a kinetic model was established by a MATLAB parameter fit and a statistical test. Results show that, before chemical equilibrium, HCl conversion and reaction rate increase with an increase in T or a decrease in nHCl/nO2. With a decrease in FHCl0/W, HCl conversion increases while the HCl reaction rate decreases. After chemical equilibrium, HCl conversion decreases with an increase in T and nHCl/nO2 or a decrease in FHCl0/W. The kinetic model that assumes O2 adsorption is the rate-controlling step can suitably describe the reaction behavior of HCl catalytic oxidation over a Cu-based composite catalyst.
1. INTRODUCTION Cl2 is the primary raw material for the production of various chlorine-containing products and is widely used in the chloralkali industry. Most chlorine-containing products are produced by a substitution or addition reaction in an industrial synthesis process. However, the maximum level of atomic utilization of Cl2 is 50% in a substitution reaction. A large amount of chlorine is converted to HCl, instead of being converted to a chlorine-containing product, which causes a large amount of the Cl2 resource to be wasted.1−4 Therefore, to efficiently utilize HCl and alleviate the shortage of Cl2, developing a chemical process to convert HCl to Cl2 is urgently required. Thus far, two kinds of chemical processes have been applied in the chlor-alkali industry: (1) electrolysis (ODC process) and (2) catalytic oxidation (Deacon process).5−7 Compared with electrolysis, the advantages of catalytic oxidation are a higher conversion (≥90%), being less expensive, and consuming less energy.8 It is a process in which HCl is oxidized by O2 over a catalyst to generate Cl2 and H2O. The reaction is illustrated as eq 1. HCl(g) +
deactivation. Researchers generally studied this problem from two perspectives: (1) improvement of catalyst stability and activity and (2) optimization of reactor and reaction conditions. For the first perspective, catalysts generally used in HCl catalytic oxidation, including Cu-, Cr-, Ru-, and Ce-based composite catalysts, were studied to increase the stability and decrease the activity temperature.9−15 Sun et al.16 studied the CuKSmx compound catalyst, and their results showed that addition of SmCl3 promoted dispersion of copper species and provided more active sites of copper species, which increased the catalytic activity of the CuKSmx catalyst. Feng et al.17 studied Cu−K−La/γ-Al2O3 catalysts, and their results showed that Cu, K, and La species highly dispersed on the surface of γ-Al2O3 and addition of KCl provided more active sites of Cu2+ species and improved the catalytic activity and stability. For the second perspective, researchers have studied reaction kinetic models of HCl catalytic oxidation over various catalysts to describe the reaction behavior accurately.18,19 Two kind of reactors have usually been used in reaction kinetic studies: (1) fixed-bed reactors and (2) nongradient reactors. Dai at al.20 studied the macrokinetics of HCl oxidation over a supported CuO−CeO2 composite oxide catalyst under lean oxygen conditions in a fixed-bed reactor, and the model of
catalyst, Δ 1 1 1 O2 (g) ←⎯⎯⎯⎯⎯⎯→ H 2O(g) + Cl 2(g) 4 2 2
ΔHr,298 K = −28.56 kJ/mol
Received: Revised: Accepted: Published:
(1)
However, the high reaction temperature and the large reaction heat of HCl catalytic oxidation may cause catalyst © 2019 American Chemical Society
9246
January 14, 2019 May 7, 2019 May 9, 2019 May 9, 2019 DOI: 10.1021/acs.iecr.9b00239 Ind. Eng. Chem. Res. 2019, 58, 9246−9256
Article
Industrial & Engineering Chemistry Research
Figure 1. Process flow diagram of the HCl catalytic oxidation experiment: (1−3) N2, HCl, and O2 cylinders, respectively, (4) fixed-bed reactor, (5) heating belt, (6) nongradient reactor, (7) heating bath circulator, (8) PTFE valve, (9) KI solution, and (10) NaOH solution.
macrokinetics assuming O2 adsorption as the rate-controlling step is shown as eq 2. Tang et al.21 studied the intrinsic kinetics of HCl oxidation over a Cu/Ce composite oxide catalyst in a nongradient reactor, and the model of kinetics assuming surface reaction as the rate-controlling step is shown as eq 3. rO2 =
2.862 × 104e−74260/ RT (PO2 − PCl2 2PH2O2) 1 + 7.88e12210/ RT PHCl + 36.38e1271/ RT PCl2
mol g −1 min−1 (2)
4
rHCl =
5.118 × 106e−8.21 × 10
/ RT
(PHCl 2PO2 0.5 − PCl2PH2O/ K /P0 )
(1 + 3.656e966/ RT PHCl + 1.854e238/ RT PO2 0.5 + 4.79e2277/ RT PCl2)3 mol g −1 min−1
(3) Figure 2. Influence of external diffusion under different angular velocities of a nongradient reactor (T = 400 °C; W = 4 g; nHCl/nO2 = 2; angular velocity = 2000, 2400, or 2800 rpm).
In this paper, a supported Cu-based composite industrial catalyst is used to study the reaction kinetics of HCl catalytic oxidation. Generally, the external diffusion effect has been eliminated under the industrial production conditions to ensure production efficiency. To simulate industrial reaction conditions, external diffusion is eliminated. Meanwhile, because the catalyst used in the experiments is provided by Shanghai Chlor-Alkali Chemical Co., Ltd., and the catalyst diameter has been fixed at 2 mm for the industrial production requirement, internal diffusion is not considered. The results of a preliminary experiment showed that external diffusion can be eliminated but HCl conversion can reach only 50% in a nongradient reactor while HCl conversion can reach 90% but external diffusion cannot be eliminated in a fixed-bed reactor. Thus, the reaction behavior of HCl catalytic oxidation over this supported Cu-based composite industrial catalyst is further studied in the device combined with a fixed-bed reactor and a nongradient reactor. The model of reaction kinetics is established to accurately describe the reaction behavior of HCl catalytic oxidation, which will play an important role in the optimization of the industrial reactor and reaction conditions.
active components and promoters are supported on the γ-Al2O3 carrier22 with a diameter of 2 mm. In the prior test performed by Chlor-Alkali Chemical Co., Ltd., via simulation of the industrial reaction conditions,23 the long period performance of the catalyst activity has been evaluated on a single-tube reactor.24 The results show that the average HCl conversion is 75% and there is no obvious change in the long duration, which indicates the catalyst activity is stable. 2.2. Reaction Apparatus. The process flow diagram of the HCl catalytic oxidation experiment is shown in Figure 1. Experimental data were measured under the following conditions: FHCl0/W = 0.01−60 h−1, T = 360−400 °C, nHCl/nO2 = 1−4, and atmospheric pressure. The catalyst was loaded in a fixed-bed reactor (10 mm in diameter, 0.6 m in length) and a nongradient reactor (15 mm in diameter, 45 mm in length). HCl (>99.999% pure) and O2 (>99.9% pure) were mixed and first added to the fixed-bed reactor through which the gas reactants were partially converted to Cl2. Then, gas reactants were added to a nongradient reactor and continued to be converted to Cl2. The off-gas of reactants was absorbed by a NaOH solution (5 mol/L), while the sample of gas reactants was absorbed by a KI solution (0.2 mol/L).
2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. A supported Cu-based composite industrial catalyst is used in the experiment. The catalyst is provided by Shanghai Chlor-Alkali Chemical Co., Ltd. The 9247
DOI: 10.1021/acs.iecr.9b00239 Ind. Eng. Chem. Res. 2019, 58, 9246−9256
Article
Industrial & Engineering Chemistry Research
Figure 3. Influence of reaction temperature and HCl space velocity on HCl conversion.
PHCl =
P0yA0 (1 − xA ) 1 + δAyA0 xA
PCl 2 = PH2O =
, PO2 =
P0yB0 − 0.25P0yA0 xA 1 + δAyA0 xA
,
0.5P0yA0 xA 1 + δAyA0 xA
(6)
where δA = (1 + 1 − 2 − 0.5)/2 = −0.25 is the variation of the total molecular number in eq 1.
3. RESULTS AND DISCUSSION 3.1. Elimination of External Diffusion. The influence of external diffusion is studied in a nongradient reactor by using different angular velocities. The result is shown in Figure 2. It shows that different xHCl ∼ W/FA0 curves coincide exactly when the angular velocity is >2400 rpm, while different xHCl ∼ W/FA0 curves appear different when the angular velocity is 2400 rpm. 3.2. Reaction Behavior. The influence of the reaction temperature (T), gas reactant molar ratio (nHCl/nO2), and HCl space velocity (FHCl0/W) on HCl conversion (xHCl) and HCl reaction rate (rHCl) is studied. 3.2.1. Influence of Reaction Temperature. The influence of reaction temperature on HCl conversion and reaction rate is studied at T values of 360−400 °C, nHCl/nO2 values of 4/2, and FHCl0/W values of 0.01−60 h−1, and the results are shown in Figures 3 and 4. As one can see in Figure 3, HCl conversion increases with an increase in the reaction temperature before reaching chemical equilibrium. The HCl conversion at 400 °C is 1.5 times the HCl conversion at 380 °C. However, HCl conversion decreases with the increase in reaction temperature
Figure 4. Influence of reaction temperature and HCl space velocity on HCl reaction rate.
2.3. Performance Evaluation. To measure the molar quantities of generated Cl2 (nCl2 and n′Cl2) and unreacted HCl (nHCl and n′HCl), the sample of gas reactants was titrated with a Na2S2O3 standard solution (0.1032 mol/L) and a NaOH standard solution (0.1024 mol/L), respectively. Then, the HCl conversion (xHCl and x′HCl), HCl reaction rate (rHCl), and partial pressure of each component (PHCl, PO2, PCl2, and OH2O) can be evaluated with eqs 4−6. x HCl = rHCl =
2nCl 2 2nCl 2 + nHCl
, x′HCl =
FHCl0(x HCl − x′HCl ) W
2n′Cl 2 2n′Cl 2 + n′HCl
(4)
(5)
Figure 5. Influence of gas reactant molar ratio and HCl space velocity on HCl conversion. 9248
DOI: 10.1021/acs.iecr.9b00239 Ind. Eng. Chem. Res. 2019, 58, 9246−9256
Article
Industrial & Engineering Chemistry Research
conversion increases with the decrease in the gas reactant molar ratio. The HCl conversion at an nHCl/nO2 of 4/4 is almost twice the HCl conversion at an nHCl/nO2 of 4/2. This condition indicates that the increase in the O2 partial pressure is beneficial for the positive reaction. As one can see in Figure 6, the HCl reaction rate slightly increases with the decrease in the gas reactant molar ratio but not significantly. The HCl reaction rate at an nHCl/nO2 of 4/4 increases by only 20% compared with the HCl reaction rate at an nHCl/nO2 of 4/2. 3.2.3. Influence of HCl Space Velocity. The influence of HCl space velocity on HCl conversion and reaction rate is studied under at T values of 360−400 °C, nHCl/nO2 values of 1−4, and FHCl0/W values of 0.01−60 h−1, and the results are shown in Figures 5−8. As one can see in Figures 5 and 7, HCl conversion increases with the decrease in HCl space velocity before reaching chemical equilibrium. However, HCl conversion decreases with the decrease in HCl space velocity after reaching chemical equilibrium. This condition indicates that the lower concentration of gas reactants caused by the lower HCl space velocity is not conducive to the positive reaction. At an nHCl/nO2 of 4/2 and an FHCl0/W of ∼0.22 h−1, maximum conversions of HCl of 87%, 85%, and 83% can be reached at 360, 380, and 400 °C, respectively. As one can see in Figures 6 and 8, the HCl reaction rate increases with the increase in HCl space velocity and is strongly influenced by HCl space velocity. Because the effects of HCl space velocity on HCl conversion and HCl reaction rate are completely opposite before reaching
Figure 6. Influence of gas reactant molar ratio and HCl space velocity on HCl reaction rate.
after reaching chemical equilibrium. HCl catalytic oxidation is a strong exothermic reaction; thus, a higher reaction temperature leads to lower equilibrium conversion. Therefore, HCl conversion can be improved in an appropriate reaction temperature range. As one can see in Figure 4, the HCl reaction rate is strongly influenced by reaction temperature, which increases with the increase in reaction temperature. The HCl reaction rate at 400 °C is almost twice the HCl reaction rate at 380 °C. 3.2.2. Influence of the Gas Reactant Molar Ratio. The influence of the gas reactant molar ratio on HCl conversion and reaction rate is studied at a T of 380 °C, nHCl/nO2 values of 1−4, and FHCl0/W values of 0.01−60 h−1, and the results are shown in Figures 5 and 6. As one can see in Figure 5, HCl
Figure 7. Comparison of the experimental HCl reaction rate vs the predicted HCl reaction rate. 9249
DOI: 10.1021/acs.iecr.9b00239 Ind. Eng. Chem. Res. 2019, 58, 9246−9256
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Industrial & Engineering Chemistry Research
Figure 8. Distribution of the residuals.
θHCl + θO + θCl 2 + θ V = 1
chemical equilibrium, the optimum HCl space velocity is chosen for industrial production to ensure both HCl conversion and reaction rate remain at high levels to improve production efficiency. At a T of 380 °C and an nHCl/nO2 of 4/2, when FHCl0/W is ∼2 h−1, HCl conversion can reach 60% while the HCl reaction rate is ∼0.2 × 10−3 mol g−1 min−1. In summary, for the device combined with a fixed-bed reactor and a nongradient reactor, external diffusion can be eliminated and HCl conversion can reach 1−90%, the reaction can reach stable for short periods of time, and experimental data are quite repeatable. Thus, this combined device is suitable for the study of reaction kinetics. 3.3. Model of Reaction Kinetics. HCl catalytic oxidation is a gas−solid two-phase catalytic reaction; both HCl and O2 adjacent reactant molecules are adsorbed on the catalyst surface.30−33 Thus, the model of kinetics is established according to the Langmuir−Hinshelwood mechanism (eq 7):
2HCl + 2* ←⎯⎯⎯⎯⎯⎯⎯→ 2HCl*
′ θHCl 2 r1 = kHClPHCl 2θV 2 − kHCl
′ k O2 , k O 1 2 O2 + * ←⎯⎯⎯⎯⎯→ O* 2
r2 = k O2PO2 0.5θV − k O′ 2θO
2HCl* + O* k r , k r′ ←→ ⎯ Cl 2 * + H 2O + 2* ′ , k Cl k Cl 2 2
Cl 2* ←⎯⎯⎯⎯⎯⎯→ Cl 2 + *
kp =
k O2 k O′ 2
, and K Cl 2 =
k Cl2 ′2 k Cl
. Thus, the stand-
′2 kO k Cl k rKHCl 2K O2 kHCl k × 2 × r × = ′ kHCl k O′ 2 k r′ k Cl 2 k r′K Cl 2
(9)
As shown in Table 1, four different models of kinetics (eqs 10−13) are established by assuming different element reactions are the rate-controlling step.34 Arnold and Kobe35 reported the relationship between the HCl oxidation equilibrium constant and temperature as shown in eq 14. 5881.7 − 0.9303 log T + 1.37014 × 10−4T T
log K p =
− 1.7584 × 10−8T 2 − 4.1744
(14)
Equilibrium constant Kp is based on the reaction 4HCl + O2 ↔ 2Cl2 + 2H2O, while kp is based on the reaction 2HCl + 1 /2O2 ↔ Cl2 + H2O. Therefore, the relationship between kp and K is shown in eq 15:
r3 = k rθHCl 2θO − k r′θCl2PH2OθV 2 ′ 2θCl2 − k Cl2PCl2θV r4 = k Cl
, K O2 =
ard equilibrium constant can be expressed as eq 9.
1 2HCl + O2 ↔ Cl 2 + H 2O (− rA) = kPHCl 2PO2 0.5 − k′PCl2PH2O 2 ′ kHCl , kHCl
kHCl ′ kHCl
KHCl =
(8)
kp =
Kp P0
(15)
3.4. Estimation of the Kinetic Parameters. As shown in eq 16, reaction rate constant k and adsorption equilibrium constants KHCl, KO2, and KCl2 in kinetic models can be expressed by the Arrhenius equation.
(7)
According to the normalization method, the relationship of θHCl, θO, θCl2, and θV can be expressed as eq 8. 9250
DOI: 10.1021/acs.iecr.9b00239 Ind. Eng. Chem. Res. 2019, 58, 9246−9256
Article
Industrial & Engineering Chemistry Research Table 1. Models of Kinetics of HCl Catalytic Oxidation over a Cu-Based Composite Catalyst rate-controlling step
model I
HCl molecular adsorption
model II
O2 molecular adsorption
model III
surface reaction
model IV
rHCl =
rHCl =
rHCl =
Cl2 molecular desorption
expression of k
HCl consumption rate for Deacon reaction
rHCl =
jij jj j k
PCl2PH2O y z
i k jjjjPHCl 2 − k
KHCl 2PCl2PH2O k pPO2 0.5
zy + K O2PO2 0.5 + K Cl2PCl2 + 1zzzz { k pPHCl 2
KO2PCl2PH2O k pPHCl
k = kHCl
2
(10)
PCl2PH2O y z
i k jjjjPO2 0.5 − k
KHClPHCl +
zz {
k pPO2 0.5 z
2
zz z {
k = kO2
+ K Cl2PCl2 + 1
(11)
PCl2PH2O y
i k jjjPHCl 2PO2 0.5 − k zzz p k { (KHClPHCl + K O2PO2 0.5 + K Cl2PCl2 + 1)3
k = krKHCl2KO2
(12)
y i k pPHCl 2PO 0.5 k jjjj P 2 − PCl2zzzz H2O { k
KHClPHCl + K O2PO2 0.5 +
k pKCl2PHCl 2PO2 0.5 PH2O
k = kCl2
+1
(13)
Table 2. Estimated Kinetic Parameters of Models of HCl Catalytic Oxidation Kinetics kinetic model model model model model
k0 (mol g−1 min−1 kPa−n)
Ea (kJ/mol)
KHCl,0 (kPa−1)
QHCl (kJ/mol)
KO2,0 (kPa−0.5)
QO2 (kJ/mol)
× × × ×
63.5 97.4 14.2 8.92
