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Catalytic oxidation of carbon monoxide in a fixed bed reactor
Separation and Purification Technology 34 (2004) 105–108
Catalytic oxidation of carbon monoxide in a fixed bed reactor Huiping Zhang a , Xijun Hu b,∗ a
Department of Chemical Engineering, P.O. Box 1925, Xiamen University, Xiamen 361005, China b Department of Chemical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR, China
Abstract A fixed bed reactor is used to study the catalytic oxidation of carbon monoxide (CO) using activated carbon impregnated with poly-metals (copper, chromium and silver) as the catalyst. The conversion factor of carbon monoxide was measured under different operation conditions such as reactor bed height, catalyst particle diameter, and temperature. The conditions for which the mass transfer resistance could be eliminated are studied. With such conditions satisfied the surface reaction on the catalyst will be the rate controlling step, and the oxidation of CO was found to follow a first-order catalytic reaction. The reaction activation energy of CO in the copper impregnated carbon bed is found to be 107.2 kJ/ml. © 2003 Elsevier B.V. All rights reserved. Keywords: Reaction kinetics; Activated carbon; fixed bed reactor; Carbon monoxide
1. Introduction Activated carbon is widely used in air purification, water and wastewater treatment, chemical separation processes, owing to its abundant micropore and mesopore volumes as well as high specific surface area. As a protective material it takes a very important role in the chemical warfare protection such as gas masks. However, it does not remove the small molecular size gases such as cyanogen chloride (CNCl) and hydrogen cyanide (HCN) in a satisfactory manner. By impregnating only one metal of either copper, chromium or silver onto activated carbon makes little difference, but when all three metals are impregnated the carbon’s performance to remove CNCl or HCN is signif∗ Corresponding author. Tel.: +85-2-2358-7134; fax: +85-2-2358-0054. E-mail address: [email protected] (X. Hu).
icantly enhanced. The activated carbon impregnated with copper, chromium and silver is also a good catalyst for carbon monoxide (CO) oxidation at reasonable low temperature (<70 ◦ C). Right now the Cu/Cr/Ag carbon catalyst has been widely used in gas mask for industrial and military applications for over 50 years. The removal performance of activated carbon simultaneously impregnated with copper, chromium and silver for cyanogen chloride and hydrogen cyanide or other smaller molecular size gases will decrease with the usage time. It is necessary to determine nondestructively the residual life of carbon filters such as gas masks for economic as well as safety purposes by tracing methods. Although there have been many test methods available [1–6], the nondestructive detection of chemical reactivity of activated carbon impregnated with copper, chromium and silver is still a difficult problem. Zhang [5] proposed a method by using the catalytic reaction kinetics data of carbon monoxide
1383-5866/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S1383-5866(03)00182-5
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oxidation on the impregnated activated carbon. This method proposed a mathematical model of removing CNCl and compared the model solution with the experimental reaction kinetics data of CO oxidation to set up a useful relationship between CO conversion and the residual life (time) for CNCl in the impregnated carbon bed. The predicting relationship can be expressed as t0.5 =a(1 − XCO ) + b, where a and b are constants, while the residual life for CNCl and CO conversion in the carbon bed are denoted by t and XCO , respectively. Therefore, knowing the catalytic oxidation kinetics one can predict the residual life of Cu/Cr/Ag/carbon for CNCl. However, there is little information available in the literature regarding the catalytic reaction kinetics of CO oxidation on the commercial Cu/Cr/Ag carbon catalyst bed. Hence, it is the purpose of this study to investigate the intrinsic reaction kinetics of CO oxidation on the impregnated carbon.
2. Reaction mechanism The oxidation of carbon monoxide (CO) on the catalyst involves a few steps such as film mass transfer, intraparticle diffusion, adsorption of reactants onto the catalyst surface, surface reaction, and the desorption of products. Usually the adsorption and desoprtion processes are fast enough to assume equilibrium. In order to understand the intrinsic reaction kinetics, the internal and external diffusion effects must be eliminated. By ignoring the mass transfer resistance, the mechanism of CO oxidation on activated carbon impregnated with copper, chromium and silver can be considered to proceed as below: O2 + 2S ↔ 2OS
(1)
CO + OS ↔ CO2 S
(2)
CO2 S ↔ CO2 + S
(3)
where S is the active site. The adsorption of oxygen (Eq. (1)) and the desorption of CO (Eq. (2)) are assumed fast so that instant equilibrium can be established. Therefore, the surface CO oxidation reaction is considered as the rate determining step, which is a pseudo-first order reaction. This will be verified by the kinetic experimental results presented later. The rate
equation according to the Langmuir model and mass action law can be obtained as below [7]: rA =
dXco = k(1 − XCO ) dt
(4)
where XCO , t and k are the conversion factor of CO, reaction time and reaction rate constant, respectively.
3. Experimental The commercial Type 111 Cu/Cr/Ag impregnated (coal-based) carbon manufactured by Xinhua chemical plant (China) was used in our experiments. This carbon has an average particle diameter of 0.9 mm, a specific surface area of 1080 m2 /g, a bulk density of 0.7 g/cm3 . The hardness of the carbon is over 75%. The schematic diagram of the fixed bed reactor for studying the catalytic reaction kinetics of CO oxidation by using the activated carbon impregnated with copper, chromium, silver as catalysts is shown in Fig. 1. The reactor bed diameter is 2 cm. The CO inlet concentration was kept at 180 ppm. All experiments were done under dry conditions for both gases and carbon catalysts. The analysis of CO was done with a QGS-08 Infrared Gas Analyzer made by Beijing Analysis Instrument Plant.
4. Results and discussion 4.1. Confirmation of the elimination of mass transfer resistance In order to confirm that the external mass resistance is negligible, the conversion factors of CO at 323 K versus residence time were measured for different reactor bed heights in the range of 1.5–3.0 cm. In this study, the commercial activated carbon catalyst with its original particle size (0.9 mm diameter) was used. The experimental results are shown in Fig. 2. It is seen that the conversion kinetics is nearly independent of the reactor bed height, hence, confirming that the external mass transfer resistance is eliminated under these conditions. To determine the critical particle size for which the intraparticle diffusion could be eliminated, the reaction kinetics was measured for different catalyst particle
H. Zhang, X. Hu / Separation and Purification Technology 34 (2004) 105–108
107
Fig. 1. Schematic diagram of the fixed bed reactor for catalytic oxidation of CO.
sizes. The reactor height is 2.0 cm which falls in the region that the external mass transfer can be ignored. The catalyst particle diameter (dp ) examined ranges from 0.2 to 0.75 mm. The other reaction conditions are: 323 K, inlet concentration of 180 ppm CO and residence time (W/F) of 5 min. The experimental re-
sults are shown in Fig. 3. It is observed that the CO conversion factor is almost constant if the catalyst particle diameter is smaller than 0.6 mm. This confirms that the internal diffusion is negligible when the catalyst particle diameter is smaller than 0.6 mm.
Fig. 2. Effect of reactor bed height on the conversion factor of CO.
Fig. 3. Effect of catalyst particle size on the conversion of CO.
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temperatures and the results are shown in Fig. 4. As expected, the conversion kinetics becomes faster at a higher temperature. The reaction rate is calculated from the tangent line of the reaction kinetics curves (Fig. 4) and the results are shown in Fig. 5. The relationship between the reaction rate and the conversion factor is described by Eq. (4). By fitting Eq. (4) to the data of Fig. 5, the reaction rate constant, k, can be obtained for the four temperatures. Assuming the reaction constant follows the Arrhenius relationship, the surface reaction activation energy and the pre-exponential factor were found to be 107.23 kJ/mol and 8.5076 × 1016 min−1 , respectively.
5. Conclusions Fig. 4. Effect of temperature on the conversion kinetics of CO.
4.2. The intrinsic reaction kinetics After investigating the effect of mass transfer on the reaction kinetics, the following conditions were used to ensure that the mass transfer resistance can be eliminated so that the reaction kinetics is under surface reaction control. The bed diameter, height and catalyst particle diameter used are 2.0 cm, 2.0 cm, and 0.3 mm, respectively. The conversion factors of CO versus reaction time were measured at four different
When carbon impregnated with copper, chromium and silver is used as the catalyst in the catalytic oxidation process of carbon monoxide in a fixed bed reactor of 2 cm high and 2 cm diameter, the intraparticle diffusion is negligible if the catalyst particle diameter is smaller than 0.6 mm. The external mass transfer can also be ignored for this system so that the catalytic oxidation of CO is controlled by the surface reaction, which follows a first-order catalytic reaction. The activation energy for the surface reaction of CO in the copper impregnated carbon bed is found to be 107.2 kJ/ml. The kinetics information of catalytic oxidation of CO on the impregnated Cu/Cr/Ag carbon is useful for the determination of residual life of this carbon for removing CNCl.
References
Fig. 5. The dependence of reaction rate on the CO conversion at different temperatures.
[1] B. Xie, A study of gas protection carbon catalysts, The Third National Activated Carbon Symposium of China, Beijing, China, 1987. [2] L.A. Graceffo, Carbon 27 (1989) 441. [3] C.T. Chiou, P.J. Reucroft, Carbon 15 (1977) 49. [4] P.J. Reucroft, C.T. Chiou, Carbon 15 (1977) 285. [5] H. Zhang, D. Cheng, F. Gao, Carbon 28 (1990) 657. [6] H. Zhang, D. Cheng, Carbon 38 (2000) 877. [7] H.S. Fogler, Elements of Chemical Reaction Engineering, Prentice-Hall, New Jersey, 1992.
Catalytic oxidation of carbon monoxide in a fixed bed reactor Huiping Zhang a , Xijun Hu b,∗ a
Department of Chemical Engineering, P.O. Box 1925, Xiamen University, Xiamen 361005, China b Department of Chemical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR, China
Abstract A fixed bed reactor is used to study the catalytic oxidation of carbon monoxide (CO) using activated carbon impregnated with poly-metals (copper, chromium and silver) as the catalyst. The conversion factor of carbon monoxide was measured under different operation conditions such as reactor bed height, catalyst particle diameter, and temperature. The conditions for which the mass transfer resistance could be eliminated are studied. With such conditions satisfied the surface reaction on the catalyst will be the rate controlling step, and the oxidation of CO was found to follow a first-order catalytic reaction. The reaction activation energy of CO in the copper impregnated carbon bed is found to be 107.2 kJ/ml. © 2003 Elsevier B.V. All rights reserved. Keywords: Reaction kinetics; Activated carbon; fixed bed reactor; Carbon monoxide
1. Introduction Activated carbon is widely used in air purification, water and wastewater treatment, chemical separation processes, owing to its abundant micropore and mesopore volumes as well as high specific surface area. As a protective material it takes a very important role in the chemical warfare protection such as gas masks. However, it does not remove the small molecular size gases such as cyanogen chloride (CNCl) and hydrogen cyanide (HCN) in a satisfactory manner. By impregnating only one metal of either copper, chromium or silver onto activated carbon makes little difference, but when all three metals are impregnated the carbon’s performance to remove CNCl or HCN is signif∗ Corresponding author. Tel.: +85-2-2358-7134; fax: +85-2-2358-0054. E-mail address: [email protected] (X. Hu).
icantly enhanced. The activated carbon impregnated with copper, chromium and silver is also a good catalyst for carbon monoxide (CO) oxidation at reasonable low temperature (<70 ◦ C). Right now the Cu/Cr/Ag carbon catalyst has been widely used in gas mask for industrial and military applications for over 50 years. The removal performance of activated carbon simultaneously impregnated with copper, chromium and silver for cyanogen chloride and hydrogen cyanide or other smaller molecular size gases will decrease with the usage time. It is necessary to determine nondestructively the residual life of carbon filters such as gas masks for economic as well as safety purposes by tracing methods. Although there have been many test methods available [1–6], the nondestructive detection of chemical reactivity of activated carbon impregnated with copper, chromium and silver is still a difficult problem. Zhang [5] proposed a method by using the catalytic reaction kinetics data of carbon monoxide
1383-5866/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S1383-5866(03)00182-5
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H. Zhang, X. Hu / Separation and Purification Technology 34 (2004) 105–108
oxidation on the impregnated activated carbon. This method proposed a mathematical model of removing CNCl and compared the model solution with the experimental reaction kinetics data of CO oxidation to set up a useful relationship between CO conversion and the residual life (time) for CNCl in the impregnated carbon bed. The predicting relationship can be expressed as t0.5 =a(1 − XCO ) + b, where a and b are constants, while the residual life for CNCl and CO conversion in the carbon bed are denoted by t and XCO , respectively. Therefore, knowing the catalytic oxidation kinetics one can predict the residual life of Cu/Cr/Ag/carbon for CNCl. However, there is little information available in the literature regarding the catalytic reaction kinetics of CO oxidation on the commercial Cu/Cr/Ag carbon catalyst bed. Hence, it is the purpose of this study to investigate the intrinsic reaction kinetics of CO oxidation on the impregnated carbon.
2. Reaction mechanism The oxidation of carbon monoxide (CO) on the catalyst involves a few steps such as film mass transfer, intraparticle diffusion, adsorption of reactants onto the catalyst surface, surface reaction, and the desorption of products. Usually the adsorption and desoprtion processes are fast enough to assume equilibrium. In order to understand the intrinsic reaction kinetics, the internal and external diffusion effects must be eliminated. By ignoring the mass transfer resistance, the mechanism of CO oxidation on activated carbon impregnated with copper, chromium and silver can be considered to proceed as below: O2 + 2S ↔ 2OS
(1)
CO + OS ↔ CO2 S
(2)
CO2 S ↔ CO2 + S
(3)
where S is the active site. The adsorption of oxygen (Eq. (1)) and the desorption of CO (Eq. (2)) are assumed fast so that instant equilibrium can be established. Therefore, the surface CO oxidation reaction is considered as the rate determining step, which is a pseudo-first order reaction. This will be verified by the kinetic experimental results presented later. The rate
equation according to the Langmuir model and mass action law can be obtained as below [7]: rA =
dXco = k(1 − XCO ) dt
(4)
where XCO , t and k are the conversion factor of CO, reaction time and reaction rate constant, respectively.
3. Experimental The commercial Type 111 Cu/Cr/Ag impregnated (coal-based) carbon manufactured by Xinhua chemical plant (China) was used in our experiments. This carbon has an average particle diameter of 0.9 mm, a specific surface area of 1080 m2 /g, a bulk density of 0.7 g/cm3 . The hardness of the carbon is over 75%. The schematic diagram of the fixed bed reactor for studying the catalytic reaction kinetics of CO oxidation by using the activated carbon impregnated with copper, chromium, silver as catalysts is shown in Fig. 1. The reactor bed diameter is 2 cm. The CO inlet concentration was kept at 180 ppm. All experiments were done under dry conditions for both gases and carbon catalysts. The analysis of CO was done with a QGS-08 Infrared Gas Analyzer made by Beijing Analysis Instrument Plant.
4. Results and discussion 4.1. Confirmation of the elimination of mass transfer resistance In order to confirm that the external mass resistance is negligible, the conversion factors of CO at 323 K versus residence time were measured for different reactor bed heights in the range of 1.5–3.0 cm. In this study, the commercial activated carbon catalyst with its original particle size (0.9 mm diameter) was used. The experimental results are shown in Fig. 2. It is seen that the conversion kinetics is nearly independent of the reactor bed height, hence, confirming that the external mass transfer resistance is eliminated under these conditions. To determine the critical particle size for which the intraparticle diffusion could be eliminated, the reaction kinetics was measured for different catalyst particle
H. Zhang, X. Hu / Separation and Purification Technology 34 (2004) 105–108
107
Fig. 1. Schematic diagram of the fixed bed reactor for catalytic oxidation of CO.
sizes. The reactor height is 2.0 cm which falls in the region that the external mass transfer can be ignored. The catalyst particle diameter (dp ) examined ranges from 0.2 to 0.75 mm. The other reaction conditions are: 323 K, inlet concentration of 180 ppm CO and residence time (W/F) of 5 min. The experimental re-
sults are shown in Fig. 3. It is observed that the CO conversion factor is almost constant if the catalyst particle diameter is smaller than 0.6 mm. This confirms that the internal diffusion is negligible when the catalyst particle diameter is smaller than 0.6 mm.
Fig. 2. Effect of reactor bed height on the conversion factor of CO.
Fig. 3. Effect of catalyst particle size on the conversion of CO.
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temperatures and the results are shown in Fig. 4. As expected, the conversion kinetics becomes faster at a higher temperature. The reaction rate is calculated from the tangent line of the reaction kinetics curves (Fig. 4) and the results are shown in Fig. 5. The relationship between the reaction rate and the conversion factor is described by Eq. (4). By fitting Eq. (4) to the data of Fig. 5, the reaction rate constant, k, can be obtained for the four temperatures. Assuming the reaction constant follows the Arrhenius relationship, the surface reaction activation energy and the pre-exponential factor were found to be 107.23 kJ/mol and 8.5076 × 1016 min−1 , respectively.
5. Conclusions Fig. 4. Effect of temperature on the conversion kinetics of CO.
4.2. The intrinsic reaction kinetics After investigating the effect of mass transfer on the reaction kinetics, the following conditions were used to ensure that the mass transfer resistance can be eliminated so that the reaction kinetics is under surface reaction control. The bed diameter, height and catalyst particle diameter used are 2.0 cm, 2.0 cm, and 0.3 mm, respectively. The conversion factors of CO versus reaction time were measured at four different
When carbon impregnated with copper, chromium and silver is used as the catalyst in the catalytic oxidation process of carbon monoxide in a fixed bed reactor of 2 cm high and 2 cm diameter, the intraparticle diffusion is negligible if the catalyst particle diameter is smaller than 0.6 mm. The external mass transfer can also be ignored for this system so that the catalytic oxidation of CO is controlled by the surface reaction, which follows a first-order catalytic reaction. The activation energy for the surface reaction of CO in the copper impregnated carbon bed is found to be 107.2 kJ/ml. The kinetics information of catalytic oxidation of CO on the impregnated Cu/Cr/Ag carbon is useful for the determination of residual life of this carbon for removing CNCl.
References
Fig. 5. The dependence of reaction rate on the CO conversion at different temperatures.
[1] B. Xie, A study of gas protection carbon catalysts, The Third National Activated Carbon Symposium of China, Beijing, China, 1987. [2] L.A. Graceffo, Carbon 27 (1989) 441. [3] C.T. Chiou, P.J. Reucroft, Carbon 15 (1977) 49. [4] P.J. Reucroft, C.T. Chiou, Carbon 15 (1977) 285. [5] H. Zhang, D. Cheng, F. Gao, Carbon 28 (1990) 657. [6] H. Zhang, D. Cheng, Carbon 38 (2000) 877. [7] H.S. Fogler, Elements of Chemical Reaction Engineering, Prentice-Hall, New Jersey, 1992.