Reaction Kinetics of Ethylene Combustion in a Carbon Dioxide Stream

Dec 26, 2012 - ABSTRACT: The intrinsic kinetics of the catalytic combustion of a trace ... mechanism to explain the ethylene combustion reaction on th...
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Reaction Kinetics of Ethylene Combustion in a Carbon Dioxide Stream over a Cu−Mn−O Hopcalite Catalyst in Low Temperature Range He Li,† Hong Chen,†,‡ Mingfa Yao,‡ and Yongdan Li*,†,‡ †

Tianjin Key Laboratory of Applied Catalysis Science and Technology and State Key Laboratory of Chemical Engineering (Tianjin University), School of Chemical Engineering, Tianjin University, Tianjin 300072, China ‡ State Key Laboratory of Engines, Tianjin University, Tianjin 300072, China S Supporting Information *

ABSTRACT: The intrinsic kinetics of the catalytic combustion of a trace amount of ethylene in a CO2 stream over a Cu−Mn− O catalyst prepared with a coprecipitation method is investigated. The experiments are carried out in a fixed-bed reactor with 0.3 g of catalyst in a low temperature range (470 to 620 K) and varying the concentration of C2H4 and O2 in the feed stream. The power rate law, Langmuir−Hinshelwood (LH), Eley−Rideal (ER), and Mars−van Krevelen (MVK) models are compared. The residual error distribution of the ethylene conversion is employed to optimize the model equations. The extended MVK model containing desorption terms of the combustion products fit the data well. The pilot test with a fixed-bed reactor and a commercial feed stream is carried out, and the macro kinetic equations are obtained. Combined with the extended MVK model equations of the intrinsic kinetics, the effectiveness factor is calculated, which gives further prediction of the performance of the extruded catalyst under commercial conditions.

1. INTRODUCTION Carbon dioxide with a trace amount of ethylene is the major exhaust of the ethylene/glycol oxidation (EO/EG) process. By removing the ethylene, food grade carbon dioxide can be obtained which is widely used in industry as a food additive and protecting gas.1 Meanwhile, carbon dioxide recovery technologies are the basis for the reduction of greenhouse gas emissions. A number of technologies including absorption, adsorption, and catalytic combustion have been reported for the removal of ethylene.2,3 The absorption technology is suitable for high hydrocarbon concentration and low temperature conditions, while the adsorption is not a continuous process and depends on the selectivity of the adsorbent to adsorb the volatile organic compounds (VOCs).4−6 Comparatively, catalytic combustion in a low temperature range is a more attractive and efficient approach because of the continuous process operation, elimination of all kinds of organic compounds, and the high selectivity for carbon dioxide and water. Catalytic combustion of VOCs (e.g., propane, toluene, benzene) has been applied extensively.7−9 Noble metal catalysts such as platinum and palladium have been proved as the active catalysts.10,11 However, they are capital intensive for most of the applications. Transition metal oxide catalysts, therefore, have a great commercial potential, because they are inexpensive and have possibly higher thermal stability and better resistance to poisons.12 Chen et al.13 reported that a Cu−Mn−O Hopcalite catalyst doped with Mg and Al shows a promising catalytic performance with a low activation temperature (570 K) and a high stability. They proposed a possible reaction mechanism to explain the ethylene combustion reaction on the Cu−Mn−O catalyst and the deactivation of the catalyst. © 2012 American Chemical Society

In our previous work, we prepared a Cu−Mn−O catalyst under commercial conditions with a coprecipitation method, and the catalyst showed good activity and stability in the laboratory test. However, to apply the catalyst in industry, its performance under commercial conditions should be predictable. The intuitive method for the catalyst test is to investigate its performance under actual reaction conditions. However, that is too expensive. The reaction kinetic model predicting the influences of the temperature, pressure, and space time on the reaction rate helps to solve the problem. Recent work on kinetic studies of VOC combustion mainly focuses on propane, methane, and benzene etc., and work on the kinetics of ethylene catalytic combustion is still a rarity. Meanwhile, reliable experimental data and a kinetic model are demanded for industrial reactor design. In this work, the intrinsic kinetics of the ethylene combustion reaction with a trace concentration in a carbon dioxide stream on a modified Hopcalite Cu−Mn−O catalyst is investigated. A number of model equations deduced from different mechanisms are compared. The best fit kinetic equations are selected to combine with the macro kinetic model equation, and the effectiveness factor of the catalyst bed in a side-steam test is obtained.

2. EXPERIMENTAL AND THEORETICAL BASIS 2.1. Catalyst Preparation. The Cu−Mn−O catalyst was prepared with a coprecipitation method. A 0.5 M mixed nitrate Received: Revised: Accepted: Published: 686

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to a fixed-bed reactor with a diameter of 7.6 cm. The feed stream came from the exhaust gas of the EO/EG process with 50 ppmv to 300 ppmv ethylene. Oxygen was added, with the control of a mass flow meter, in the form of air. The temperature of the catalyst bed was monitored with a thermocouple in an insertion well in an axial direction and another one in a radial insertion well in the catalyst bed. To analyze the experimental data with a simple reactor model, the ratio of the catalyst bed height (L) and the diameter of the catalyst (dp) should be far more than 50, and the ratio of the diameter of the reactor and the diameter of the catalyst must be more than 8−10, which helps to reduce the reactor wall effect. For the experiments both in laboratory and in plant, the two conditions were satisfied. 2.5. Kinetic Models. The power rate law, Langmuir− Hinshelwood (LH), Eley−Rideal (ER), and Mars−van Krevelen (MVK) models were examined. The power rate law model is composed of a rate constant, and the reactant partial pressures are raised to a certain power as shown in eq 1

aqueous solution of copper and manganese was dropped into 1 M Na2CO3 solution under vigorous stirring until the pH reached 9. Then a potassium permanganate solution was dropped into the slurry. The mole ratio of potassium permanganate and manganese nitrate was 1:4. The final ratio of manganese and copper in the precipitate was 1:2. After being aged for 6 h at 353 K, the precipitate was washed with hot water for several times. The material was then extruded into a cylindrical extrudate with a diameter of 3.8 mm and dried overnight and calcined at 673 K for 5 h. 2.2. Catalytic Activity Test. 2.2.1. Experimental Conditions. The ethylene combustion experiments were carried out in a fixed-bed reactor with an internal diameter of 8.6 mm under atmospheric pressure. A blank experiment was carried out first to make sure the experimental conditions were suitable. A carbon dioxide stream with 0.18 vol% ethylene and 0.54 vol% oxygen flowed through the reactor filled with quartz sand particles. The outlet gas was analyzed online with a gas chromatograph equipped with a FID detector and a 3 m packed GDX-102 column. The length of the uniform-temperature zone was about 45 mm along the reactor, which meets the needs of the experiments. Meanwhile, the reactor wall and quartz particles had no catalytic effect on the ethylene combustion. For the catalyst activity test, the extruded catalyst was crushed into different meshes and 300 mg of catalyst diluted with quartz particles was loaded in the fixed-bed reactor. The repeatability of the experimental data was verified in several experiments with the same ethylene conversion under the same experimental conditions. 2.2.2. Elimination of the External and Internal Diffusion Effects. The conversion of ethylene as a function of the gas flow rate at a fixed Q0/W at different temperatures is plotted in Figure S1, Supporting Information. The catalyst extrudates were crushed into 60−80 mesh, and 150 mg of catalyst was tested at different temperatures. Consecutive tests were carried out with the increase of both the reactant flow rate and the catalyst volume while Q0/W was the same.14 At 553 K, the flow rate was 350 mL/min, when the conversion was nearly the same, implying that there was little diffusion effect. In this work, a gas flow of 400 mL/min, with linear rate equal to 0.12 m/s, was chosen in the experiment. Figure S2, Supporting Information, presents the influence of the particle size on the conversion of ethylene under a gas flow rate of 400 mL/min in a fixed-bed-reactor with 0.3 g of catalyst at 553 K. The conversion is constant despite the changing particle size. This indicates that the effect of the internal diffusion is eliminated. Particles with diameters of 0.18−0.25 mm were selected in the following experiments. 2.3. Intrinsic Kinetics Experiment. A wide range of experimental conditions were employed to obtain effective and sufficient experimental data in the intrinsic kinetics measurements. The feed stream was carbon dioxide with different concentrations of ethylene, from 0.18 vol% to 1.8 vol%, and oxygen, from 0.54 vol% to 5.4 vol%, with nitrogen in balance. A 0.3 g amount of catalyst with diameters of 0.18−0.25 mm was used for each test. The reaction temperature was higher than 493 K. The experiments were carried out at an ethylene to oxygen mole ratio of 1:3. The effect of oxygen on the catalyst activity was also tested by varying the content of oxygen in the inlet gas. The ratio of oxygen to ethylene was from 1.5:1 to 15:1. 2.4. Side-Stream Tests. For the side-stream tests, the catalyst was extruded with a diameter of 3.8 mm and was added

r = ksPenPom

(1)

where r is the reaction rate, ks the reaction rate constant, Pe and Po the partial pressures (atm) of ethylene and oxygen, and n and m the reaction orders for ethylene and oxygen, respectively. The LH model considers that ethylene and oxygen are adsorbed first on the active sites and then the reactions occur between the two kinds of adsorbed species. For surfacereaction-controlling cases, ethylene and oxygen may adsorb on two different kinds of active sites (eq 2) or on the same kind of active sites, causing a competitive adsorption (eq 3). r=

r=

ksKoKePoPe (1 + KoPo)(1 + KePe)

(2)

ksKoKePoPe (1 + KoPo + KePe)2

(3)

in which ks is the surface reaction rate constant. The ER model considers that the controlling step is the reaction between the adsorbed oxygen and the ethylene molecule in the gas phase (eq 4) or between the adsorbed ethylene and the oxygen molecule in the gas phase (eq 5). r=

ksKoPePo 1 + KoPo

(4)

r=

ksKePePo 1 + KePo

(5)

In these equations, Ko and Ke are equilibrium adsorption constants for oxygen and ethylene molecules. The Mars−van Krevelen (MVK) model (eq 6) is a two-step redox reaction model and has been used to explain the rates of oxidation reactions for heterogeneous oxide catalysts for over 50 years.15 The model conveys that the catalyst surface is in a redox cycle, reduced here by the ethylene and oxidized by oxygen. The oxidation rate is equal to the reduction rate in a stationary state, and ν is the stoichiometry factor. The oxygen in the catalyst could be either chemisorbed or lattice oxygen.16 r=

kokePoPe koPo + νkePe

(6)

Catalyst reduction: 687

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Cat−O + E → Cat + CO2

Article

metal catalyst and in a high temperature range.19 Here the parameters of the LH model equations, rows 2 and 3 in Table S1, with one and two active sites meet the criteria mentioned above, but the degree of fitting indicated with R2 is not as good as that for the power rate law model for both model equations. Of all the kinetic model equations examined, the ER model equations have the worst fit to the experiment data, and they are no longer considered in further work. The initial Mars−van Krevelen model equation, model 6 in Table S1, fits the experimental better than the LH and power rate law model equations with a smaller residual sum of squares. When the O atom is considered in the MVK model, the equation, model 7 in Table S1, provides a worse fit than the initial model equation. The extended MVK model which considers the desorption of the reaction product, model 8 in Table S1, provides the best result among all kinetic models, with the smallest RSS of 2.01 × 10−2 and the largest R2 of 9.91 × 10−1. The best equation is:

(7)

Catalyst oxidation: ko

2Cat + vO2 → 2Cat−O

(8)

17

Vannice insisted that the initial MVK model has no physical relevance and can only be viewed as a mathematical fitting function if the oxygen molecule is not adsorbed on a single site. While oxygen atoms are proposed as the reactive oxygen species rather than oxygen molecules, the form of the equation is expressed as eq 9. r=

kokePePo0.5 koPo0.5 + νkePe

(9) 18

Heynderickx et al. proposed an extended MVK model equation for propane oxidation, which considers desorption of combustion products. For the ethylene combustion, the model equation could be derived as eq 10. In parameter regression, [2/kw + 2/kc] can be taken as a single parameter as 2/kp, where kw and kc are desorption rate constants for H2O and CO2. r=

kokePoPe koPo + νkePe + kokePoPe[2/k w + 2/kc]

5

r = (1.54 × 106e−1.24 × 10 /RT PoPe) 4

4

/(3.66 × 10e−4.54 × 10 /RT Po + 1.25 × 105e−7.86 × 10 /RT Pe 4

+ 6.17 × 105e−5.85 × 10 /RT PoPe)

(10)

(12)

The residual error distributions of the ethylene conversion calculated with the power rate law, the initial MVK model equation, and the extended MVK model equation are shown in Figure 1. The residual errors appear as a double curve

2.6. Parameter Regression Analysis. In kinetic parameter analysis, the rate constants, activation energies, and adsorption coefficients must be positive and the heat of adsorption must be negative.19 The reaction rate differential balance equation and one of the above equations are simultaneous equations and are solved with the Runge−Kutta method. The integration is along the reactor bed. The integral initial conditions are W = 0, x = 0. The Nelder−Mead simplex method20 is applied for the parameter optimization, which satisfies the least-squares criterion through minimizing the objective function for the residual sum of squares (RSS). In the equation, xe,j is the experimental conversion of ethylene, and xc,j is the calculated result. n

Fobj =

∑ (xe,j − xc,j)2 i=1

(11)

Determining coefficient R2 was selected to evaluate the regression results, which indicates the quality of matching of the model equations with the experimental data. The model equation was further assessed by analysis of variance (ANOVA), which shows the degree of model equations fitting to the experimental data and determines whether the model equation is significant and the calculated conversion is valid.21

Figure 1. Residual error distribution of model equations: (1) the power rate law model; (2) the initial Mars−van Krevelen model; (3) the extended Mars−van Krevelen model. Cc is the calculated conversion, and Ce is the experimental conversion.

3. RESULTS AND DISCUSSION 3.1. Kinetic Model Parameters. The regression results with the kinetic model equations are given in Table S1, Supporting Information. The power rate law model equation (model 1 in Table S1) shows a good fit to the experimental data. The apparent activation energy is about 5.84 × 104 J/mol, with the order for ethylene as 0.22 and the order for oxygen as 0.14. However, it is reported that under different concentrations of reactant or reaction temperatures, the reaction orders may be different.22 Therefore, the power rate law model equation is regarded as a mathematical equation without mechanism information for the reaction, though it gives a good fit. Nibbelke et al. reported that an LH model is suitable for ethylene oxidation, but the work was performed on a noble

relationship for the power rate law model, indicating that this model equation is defective. The residual errors of the initial Mars−van Krevelen model with independent variables are distributed in a divergent way. However, for the extended MVK model equation, the residual errors with independent variables are in random distribution and the points are symmetrically distributed on both sides of the zero coordinates. The extended MVK model with a reaction product desorption item in the equation was the most suitable model for the trace amount of ethylene combustion on Hopcalite catalyst in a carbon dioxide stream. Figure 2 plots the goodness of fit of the calculated ethylene conversion using the extended MVK model equation to the 688

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at the same time on different sites and exhibit a balanced reaction rate, i.e., the ethylene combustion rate (re). ke

C2H4 + O−S → CO2 −S + H 2O−S

(13)

ko

O2 + S → O−S

(14)

kc

CO2 −S → CO2 + S

(15)

kw

H 2O−S → H 2O + S

(16)

H2O and CO2 have been found to significantly inhibit the catalytic combustion of VOCs and induce a competitive adsorption of reactants.23−26 For ethylene combustion in CO2, the two groups occupy parts of the active site of the catalyst. The initial MVK model does not consider the influence of the product desorption, while the extended MVK model takes into account the desorption of the products which inhibits the reaction. 3.3. Calculations for the Side-Stream Test with Catalyst in a Commercial Size. The power rate law model equation is used in the macro kinetic parameter regression for the activity data of side-stream tests. The regression results are given in Table 2. The macro reaction rate is predicted with the

Figure 2. Calculated conversion versus experimental conversion with the extended MVK model (model 8 in Table S1). For example, 1:3 400 mL/min means that the ratio of ethylene and oxygen is 1:3 and the gas flow rate is 400 mL/min. The content of ethylene in all four experiments is 0.18%.

experimental ethylene conversion. The ratio of ethylene and oxygen in the feed is varied from 1:3 to 1:10.5. The experimental values are accurately predicted by the model calculation, regardless of each test set value or of all the experimental data. The F-test was performed with the fitness, and the results (Table 1) show that the F-test statistic value (1.01 × 103) with regard to the extended MVK model equation (model 8 in Table S1) is several hundred times greater than the critical statistical value (3.2) at the confidence level of 99.9%. However, with a residual standard deviation of 0.02, which is smaller than that of any other model equations, model 8 gives the most accurate prediction of the ethylene conversion value with a deviation of ±3.90 × 10−2 at a 95.4% possibility. The regressed model equation is highly significant, and the predictive ethylene conversion is valid. 3.2. Reaction Mechanism. Among all the model equations tested, the extended MVK model shows the best fit to the intrinsic kinetic experimental data, which means that the intrinsic activity can be predicted accurately by this model. In our previous work, the in situ DRIFTS spectra of ethylene combustion over a Hopcalite catalyst showed that the OH groups and CO2 species appear on the catalyst surface in the reaction.13 The amount of OH groups decreases, and no CO2 species exist with the combustion reaction below 473 K. As the temperature increases, the reaction rate increases, the amount of OH groups increases, and the CO2 species appear. These two groups on the catalyst surface are balanced by H2O and CO2 in the reaction products. For ethylene combustion over Hopcalite catalyst, the surface-active sites are in a redox cycle. The sites are reduced by ethylene and oxidized by oxygen as shown in the reactions of eqs 13 and 14. The desorption of H2O and CO2 finally occurs to generate the active sites as shown in the reactions of eqs 15 and 16. The reactions happen

Table 2. Results of Macro Kinetic Model Equation Regression equation expression ksPenPom

kinetic parameters ks = 1.93 × 10exp (−3.79 × 104/(RT)) n = 0.96 m = 0.12

R2

RSS 1.80 × 10

−2

9.89 × 10−1

macro kinetic equation under different reactant concentrations and reaction temperatures. Model 8 in Table S1 is used as the intrinsic kinetic equation. The effectiveness factor is obtained as the ratio between the two reaction rates. The curve in Figure 3 shows that the effectiveness factor decreases rapidly as the temperature increases for 0.015% ethylene in carbon dioxide. The values of the effectiveness factors are in the range of 1.5− 2.5%, a typical range for commercial sized solid catalysts. The second test on the extruded catalyst was carried out with different ethylene concentrations at different temperatures. The measured values of the effectiveness factor decrease along the effectiveness factor curve. The effectiveness factor can be predicted under different industrial conditions, which plays a significant role in reactor design.

4. CONCLUSION The Cu−Mn−O catalyst prepared with a coprecipitation technique used in this work is a stable and suitable catalyst for the trace amount of ethylene combustion in a CO2 stream at low temperature. The intrinsic kinetics is measured to understand the reaction and to meet the demand of scale up of

Table 1. Results of Variance Analysis

regression residual total

sum of squares

freedom

variance

F

F0.01(6, 53)

2.31 2.02 × 10−2 2.27

6 53 59

3.85 × 10−1 3.81 × 10−4

1.01 × 103

3.2

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86-22-27405613. Fax: +86-22-27405243. E-mail: ydli@ tju.edu.cn. Notes

The authors declare no competing financial interest.

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Figure 3. Influence of temperature on the effectiveness factor. (○) Effectiveness factors at different temperatures in the second test.

the catalyst in commercial applications. With the linear velocity of the inlet gas no less than 0.12 m/s and the particle size (diameter) of catalyst no larger than 0.2 mm, the effects of the external and internal diffusions are negligible and the intrinsic kinetics experiments are carried out. The analysis of the intrinsic kinetics experimental data showed that the regression results of LH models and ER models are not much more suitable than that of the power rate law. The MVK model then is applied to fit the data. The extended MVK model equation which contains the effect of the desorption of the reaction products turns out to fit the best to the experimental data as shown in eq 12. Further analysis of the extended MVK model and the in situ IR results of the ethylene combustion reaction on the Cu−Mn−O catalyst show that the model is useful to explain the reaction mechanism. Three redox reactions were balanced on the catalyst surface, viz., the reduction of the catalyst by ethylene, the oxidation of the catalyst by oxygen, and the desorption of CO2 and H2O as products. The combustion reaction products have a significant inhibition effect on the ethylene combustion reaction. The effectiveness factor is calculated with the macro kinetic model equation measured with a side-stream testing reactor with commercial feed gas from the ethylene oxidation process. The effectiveness factor is reasonable and can be used to predict the behavior in the commercial reactor. However, the effectiveness factors are all smaller than 0.03 for the reaction conditions tested, indicating that the apparent activity of the extruded catalyst is significantly controlled by the internal diffusion and a monolithic catalyst may be useful for the process.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Figure S1 shows the results of the external diffusion-excluding experiments. Figure S2 illustrates the results of the internal diffusion-excluding experiments. Table S1 shows the regression results with the kinetic model equations. This material is available free of charge via the Internet at http://pubs.acs.org. 690

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