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Chemical kinetic modeling of ethene oxidation at low and intermediate temperatures
Twenty-Third Symposium (International) on Combustion/The Combustion Institute, 1990/pp. 203-210
CHEMICAL
KINETIC MODELING OF ETHENE OXIDATION AND INTERMEDIATE TEMPERATURES
AT LOW
RICHARD D. WILK
Department of Mechanical Engineering Union College, Schenectady, NY WILLIAM J. PITZ ^ND CHARLES K. WESTBROOK
Lawrence Livermore National Laboratory Livermore, CA AND
NICHOLAS P. CERNANSKY
Department of Mechanical Engineering and Mechanics Drexel University, Philadelphia, 1"A
A detailed chemical kinetic mechanism for ethene oxidation has been dew;loped and used to model low to intermediate temperature oxidation chemistry. The model was used to simulate the reactions in a static reactor at temperatures of 696 K and 718 K, an equivalence ratio of 2.0, and a pressure of 600 torr. The modeling calculations identified some of the key reaction steps at these conditions. The formation of the majority of the reaction intermediates results from two main paths involving the reaction of ethene with HO2 and OH. The addition of HO~ to ethene leads to ethene oxide, which becomes a source of methyl radicals. The methyl radicals lead to the formation of methane and methanol. The addition of OH to ethene leads to formaldehyde, which is subsequently converted to CO and COs. Formaldehyde also results from the O][I abstraction path via the vinyl radical. At temperatures near 700 K, OH abstraction is less important than O1t addition. However, the relative importance of the abstraction path increases with temperature. Modeling results at these conditions were compared with experimental species concentration data. The calculated concentrations of the major products ethene oxide and carbon monoxide are in very good agreement with the experimental data. The calculated concentrations of formaldehyde and methanol are low. The calculated methane concentrations are in good agreement in the early stages of the reaction, but are overpredictcd in the latter stages. The similarities anti differences in agreement between the modeling and experimental results are discussed.
Introduction Efforts are currently underway to develop a detailed chemical kinetic model for the oxidation of higher hydrocarbon fuels which can be applied to address problems in practical combustion systems. A hierarchical approach is used in constructing reaction mechanisms. In one aspect of this approach, a mechanism describing the oxidation of a particular fuel under certain physical conditions (pressure, temperature) is modified and extended for application at different conditions. In another aspect of this approach, mechanisms for smaller fuels, which have been validated experimentally, are used as building blocks (submechanisms) for the formulation of mechanisms for more complex fuels. 203
One of the principal building blocks is the ethene (CzH4) mechanism. Ethene is a major intermediate produced in the oxidation of many hydrocarbon fuels (n-octane, n-pentane, n-butane, propane, propene, and ethane). Its formation is especially important during the oxidation of these fuels in the intermediate and high temperature regimes. At high temperatures (T > ~ 0 K), ethene is formed mainly from the decomposition of higher alkyl radicals. At intermediate temperatures (600 < T < 900 K), ethene forms primarily from the oxidation of ethyl radicals. In either case, ethene is formed at a relatively high rate. Also, because it is a fairly reactive intermediate, it can react readily to form secondary products. Thus, understanding the oxidation mechanism of ethene is an integral step in
204
REACTION KINETICS
the development of chemical kinetic reaction mechanisms for larger hydrocarbons. Detailed mechanisms have been developed for ethene oxidation at high temperatures 1-4 and have been used to model results from high temperature experiments. In the present study, the intermediate temperature reaction regime was modeled and validated with experimental data from a static reactor. Modeling the chemistry in this temperature regime is particularly relevant to the problem of knock in spark ignition engines. The end gas in the cylinder spends appreciable time at intermediate temperatures prior to autoignition and knock. The oxidation chemistry of the fuel at these temperatures can have an influence on the autoignition behavior. Alkenes are unsaturated hydrocarbons characterized by the presence of at least one carbon-carbon double bond in the structure. Because of this, there are certain interesting and unique features of the intermediate temperature oxidation mechanism of alkenes, including ethene. One of these features involves consumption of alkenes by free radical addition. Radicals such as H, O, OH, and HO2 can add at the double bond. These reaction channels lead to products such as aldehydes and oxirans. Incorporation of these additional reaction channels into the mechanism is necessary in order to accurately model the oxidation of these fuels. Previous experimental studies have been conducted to investigate the oxidation process of ethene. Schug et al. 5 used a turbulent flow reactor to examine the high temperature oxidation and pyrolysis under dilute conditions. Thornton and Malte6 used a jet-stirred reactor to study the high temperature oxidation. Wilk et al. 7 used a static reactor to investigate the intermediate temperature oxidation of ethene. Hoffman and Litzinger8 used a high pressure flow reactor to examine pressure effects on the oxidation of ethene at intermediate and high temperatures. A substantial amount of work was done by Baldwin, Walker and coworkers9-12 also using a static reactor to determine the mechanism and certain rate constants in the ethene reaction. Certain elementary reactions important to the ethene mechanism have been studied. The reaction between ethene and OH has been studied by Morris et al.,13 Atkinson and Aschmann, 14 and Bartels et al. 15 Slagle et al. 16A7 studied the reactions radicals and vinyl radicals with O2. The reactions of ethene oxide have been studied by Crocco et al., 18 Benson, 19 Baldwin et al., 2~ and Lorenz and Zellner.21 To date, there have been no modeling studies of the intermediate temperature oxidation of ethene. In this study, ethene oxidation was modeled at temperatures near 700 K. Results were compared with species concentration profiles from a static reactor.
Experiment The experimental data used for comparison with the model was obtained from a static reactor system, details of which have been described previously.22'z3 This system has been used extensively in the past for studying the low and intermediate temperature oxidation of many hydrocarbons. Data for ethene oxidation in the form of pressure, temperature, and species concentration histories were obtained by Wilk et al. 7 For this work, ethene (99.5% pure) and air (99.99% pure) were used as reactants. Experiments were carried out at an initial pressure of 600 torr, with initial temperatures of 696 K and 718 K, and an equivalence ratio of 2.0.
Numerical Model and Reaction Mechanism Ethene oxidation in the static reactor was simulated with the assumptions of constant volume and spatial uniformity over the vessel volume. The HCT programz4 was used to integrate the chemical kinetic rate equations in time. The calculated temperature histories followed the experimentally measured temperatures in the reaction vessel. The detailed chemical kinetic mechanism for ethene was developed based primarily on an earlier high temperature mechanism for ethene 2 and a low temperature mechanism for acetaldehyde.25 Certain portions of an earlier mechanism for propene26 were also incorporated to address the chemical reactions of C3 species. Added to this mechanism were low and intermediate temperature reaction paths involving consumption of ethene, and formation and consumption paths for ethene oxide, a primary intermediate. The combined detailed mechanism for ethene oxidation used in this work is given in Ref. 27. Key Reactions in the Mechanism:
The main initiation reaction for ethene at temperatures near 700 K is: C2H4 + 02--~ C2H3 + HO2
(1)
As soon as HOz radicals are formed, they quickly take over initiation, primarily through the addition reaction: C2H4 + HO2 ~ C2HaOOH
(2)
Baldwin et al. 12 suggest an energy barrier of 17 kcal/ mol for Reaction 2. Slagle et al, 16 suggest a value of 5-7 kcal/mol. We chose an activation energy of 8 kcal/mol for this reaction. We have included three routes for the consumption of the C2H4OOH adduet formed in Reaction
MODELING OF ETHENE OXIDATION 2. The first is dissociation back to reactants (Reaction -2). An activation energy of 20 kcal/mol obtained from Pollard28 was used for this reaction. This value lies between values suggested by Baldwin et al. lz (29 kcal/mol) and Slagle et al. 1~ (17.9 kcal/ mol). The second path for consuming C2H4OOH is by isomerization via internal H-atom transfer, C2H4OOH"-~C2HsO2
(3)
For this reaction, we assigned a value for Ea of 22 kcal/mol, which again is comparable to the values used by Baldwin et al. (28 kcal/mol) and Slagle et al. (17 kcal/mol). The value for A = 2.86 • 1011 s -I was obtained from Pollard. 28 Finally, the primary reaction consuming C2H4OOH is: C2HI4OOH ~ C2H40 + OH
(4)
The reaction sequence (2, 4) is important because it produces ethene oxide, the primary hydrocarbon intermediate observed. Also, it converts HO2 to the more reactive OH radical, and thus accelerates the overall rate of oxidation. At temperatures near 700 K, OH radicals can react with ethene in two ways: abstraction and addition: C2H4 + OH ~ C2Ha + H20
(5)
C2H4 + OH --~ C2H4OH
(6)
For these reactions we used the rate expressions recently obtained by Tully29'3~ (As = 4.8 x 1012 cm3/mol-s; Ea = 5.9 kcal/mol); (A~ = 1.63 • 1012 cmZ/mol-s; Ea = -0.69 kcal/mol). The CzH4OH adduct formed by the addition of OH can dissociate back to reactants or undergo further addition with 02: C2H40H + 02--~ O2C2HI4OH
(7)
The resulting adduct decomposes to formaldehyde and OH: O2C2H4OH--* CH20 + CH20 + OH
(8)
The reaction sequence 6 followed by 7 and 8 is known as the Waddington Mechanism. 31'32 This path is the primary formation route for formaldehyde. It is also a chain carrying path since the OH is regenerated. Other important abstraction reactions involving ethene are: C2H4 + CHa--~ C2Ha + CH4
(9)
C2H4 + CHaO -~ C2Ha + CH3OH
(10)
At intermediate temperatures the vinyl radical reacts with 02 to give formaldehyde and formyl:
C2H3 + O2---~ HCHO + HCO
205 (11)
For this reaction, we use the rate of Slagle et al. 17 (A = 4 • 1012 cma/mol-s; Ea = -0.25 kcal/mol). Since ethene oxide is the major hydrocarbon intermediate in the reaction at these temperatures, it was necessary to incorporate reactions forming and consuming this species. As mentioned earlier, the main reaction path forming ethene oxide results from HO2 addition to ethene. There are also additional steps that form ethene oxide. These are disproportionation reactions which are included in the mechanism: C2H4 + CH302 ~ C2H40 + CH30
(12)
C2H4 + C2H502---> C2H40 + C2H50
(13)
Ethene oxide is consumed by three routes: decomposition, isomerization, and H-atom abstraction. Each of these paths is shown below: C2H40 ~ CH3 + CHO
(14)
C2H40 --~ CH3CHO
(15)
C2H40 + OH ~ C2HaO + H20
(16)
For Reactions 14 and 15, Baldwin et al. 2~ estimated the rate constants at 480~ C. We used these values along with our own estimates for the activation energies to estimate the A factors. The primary radical abstracting H atoms is the OH radical. The abstraction path removes an H atom but keeps the C---O---C structure intact. For Reaction 16, we assigned the same rate parameters as CzH4 + OH. The C2H30 radical formed in Reaction 16 is consumed by two isomerization reactions: C2H30 ~ CH3CO
(17)
C2I-I30 ~ CH2CHO
(18)
We used Ea = 14 kcal/mol for each of these reactions based on the study of Lorenz and Zellner. 21 The ratio of the rate constants for these two reactions was obtained by Baldwin et al. 2~ On the basis of this information we assigned A17 = 8.5 • 1014 cma/mol-s and Als = 1 • 1014 cma/moi-s. One of the primary chain terminating steps in the mechanism is the self reaction to HI02. Results and Discussion Comparisons of the calculated and experimental species concentration profiles are presented in Figs. 1-3. The results for the lower temperature case 696 K are presented in Figs. 1 and 2. The results for the higher temperature case, 718 K, are presented in Fig. 3.
206
REACTION KINETICS 14
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The majority of the fuel consumption at both temperatures occurs by the reaction of ethene with HOz and OH. Reactions are included in the mechanism for both the addition paths involving these radicals and H-atom abstraction by these radicals. At the temperatures investigated in this study, the addition paths dominate the process. At 696 K, OH addition to ethene followed by subsequent reactions accounts for about 55% of the fuel consumed. The OH abstraction path accounts only for about 8%. HO2 addition accounts for approximately 37% of the fuel consumed. At 718 K, the OH abstraction path is more important, accounting for about 14% of the fuel consumption. H atom abstraction by HOe was not found to be important in this temperature range. Likewise, consumption of the fuel by the pyrolysis step, C2H4 + M ~ Cell3 + H + M
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FIc. 2. Concentrations major intermediate species for an initial temperature of 696 K. Curves indicate computed results using numerical model; symbols represent experimental results. is not important at these temperatures due to its very high activation energy (107.5 kcal/mol). For the 696 K case, the calculated fuel consumption becomes faster than that measured in the experiment after about 200 s (Fig. 1), resulting in more fuel being consumed. Since the model predicts a higher degree of fuel consumption, the predicted reaction intermediates and products are affected. In order to isolate the intermediate and product chemistry from the fuel effect, the species concentrations for the 696 K case are plotted against percent fuel consumed (Fig. 2). This approach allows for a comparison of modeling and experimental results on a more common basis from which we can evaluate how well the model predicts quantitatively the underlying intermediate and product chemistry. For the 718 case, the calculated and experimental fuel disappearence profiles are in good agreement (Fig. 3). Thus, for this case, normalization of
207
MODELING OF ETHENE OXIDATION
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the results on the basis of fuel consumed is not really required. The fact that the 718 K case performs better on a time plot than the 696 K case indicates that an adjustment in the activation energies of one or more of the fuel consuming reactions or chain branching reactions may be required. The |ormation of the major intermediates and products in the reaction results primarily from either the HO2 or OH addition paths. The reaction between HO2 and ethene (Reaction 2) determines the amount of ethene oxide produced in the calculation. As seen in Figs. 1-3, for both temperature, cases the ethene oxide concentrations attained during the calculations are in very good agreement with the measured concentration throughout the course of the reaction. Ethene oxide is consumed primarily by the Reactions 14, 15, and 16. Of these three, Reaction 16 is most important followed by the decomposition path (Reaction 14). The isomerization step (Reac-
(20)
Very little ketene was formed in the calculation and ketene was not measured in the experiments. Reaction 17 is the major consumer of CzH30. The acetyl radical CH3CO formed in this reaction is a major source of methyl radicals, CH3CO + M ~ CH3 + CO + M
300
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tion 15) is the least important. Little acetaldehyde was formed in the calculations, and no acetaldehyde was measured in the experiments. The C2H30 radical produced in Reaction 16 undergoes two isomerization reactions. Reaction 18 ultimately leads to ketene by the reaction,
(21)
along with the direct decomposition reaction of C2H40 (Reaction 14). The relatively large amounts of both methane and methanol observed in the calculations and the experiments indicate sul)stantial levels of CH3 in the reaction. There are three principal reactions forming methane at these temperatures. All involve H-atom abstraction by CH3: CH3 + HEO2---->CH4 + t l O z
(22)
CH3 + CzH4 ~ CH4 + C2H3
(23)
CH3 + CHzO ~ CH4 + HCO
(24)
At 696 K, the reaction with hydrogen peroxide (Reaction 22) accounts for about half of the methane formation. Reactions 23 and 24 account for about 25% each. Measurements of the H202 concentration were not made in the experiment. However, the model predicted substantial levels formed in the reaction. The calculated maximum H202 concentration ranged from 0.6% to 1.2%. At 718 K, Reaction 23 is the most important produeer of methane, accounting for about 85% of CH4. At this temperature, Reaction 22 accounts only for about 2% of the CH4. Methanol is formed by H-atom abstraction by methoxy radicals: CH20 + CH30 ~ CH3OH + HCO
(25)
C2H4 + CH30---> CH3OH + C2H3
(26)
Reaction 25 is the most important path at 696 K while reaction with the fuel is most important at 718 K. Some CHaOH is also formed from the reaction of methylperoxy radic',ds: CH3Oz + CH302 CHzO + CHaOH + 02
(27)
REACTION KINETICS
208
A comparison between the calculated and measured concentration profiles of methane and methanol is shown in Figs. 1-3. From Figs. 2 and 3, at both temperatures, the experimental and predicted methane concentrations are in good agreement in the early stages of the reaction (up to approximately 35% fuel consumed). In the latter stages of the reaction, the methane concentration is overpredicted by the model. At 696 K (Fig. 2), the predicted maximum concentration of methane is 2.5 times higher than the measured amount. At 718 K, the predicted maximum is higher by a factor of 1.8. The over prediction of methane in the latter stages of the reaction seems to indicate that methane is not being consumed fast enough. With methanol, the calculated species concentrations are slightly underpredicted by about 20% for both temperature cases. Since a primary step forming methanol is Reaction 25, the underprediction of methanol may be due to only the underprediction of formaldehyde, and not as a result of methanol chemistry. The model correctly predicts the trends in the species profiles for methane and methanol with temperature. Both model and experiment indicate a decrease in the methanol yield and an increase in the methane yield with increasing temperature. The reactions between Oil and ethene (Reactions 5, 6) determine the amount of formaldehyde produced in the calculation. As mentioned earlier, OH addition (Reaction 6) dominates at these temperatures. Following the addition of 0-2 (Reaction 7) the adduct decomposes, yielding two formaldehyde molecules and another OH (Reaction 8). This reaction path accounts for about 90% of the formaldehyde produced. The OH abstraction path also leads to formaldehyde. Hydrogen atom abstraction by OH forms C2H3 (Reaction 5) which reacts mainly with O2 to give CH20 and HCO (Reaction 11). We also consider alternative products from the reaction of 0.2 and C.2H3: C,2tt3 + O2 ~ C2H.2 + HO2
(28)
This minor reaction path is included to account for the small amount of acetylene observed in the 718 K case. Formaldehyde is consumed by reactions with Ott and HO2: CH20 + OH ~ HCO + H20
(29)
CH20 + HO2 ~ HCO + H202
(30)
Comparison of the measured and calculated species concentrations in Figs. 2 and 3, shows that the CH20 concentrations are underpredicted. The calculated maximum CH20 concentration is about 20% low for the 696 K, and about 50% low for the 718
K case. Although, the overall concentrations are lower, the shapes of the calculated profiles follow the experimental profiles very well. The predicted peak formaldehyde occurs earlier in the reaction for the 696 K case. In the 718 case, the predicted and experimental peak CH20 concentrations are coincident with respect to the reaction time. The underprediction of CH20 by the model and the sensitivity of this underprediction to temperature indicates that the rate parameters used for Reactions 6, 7, and 8 may need re-evaluation. Small amounts of propene (0.08%) and propene oxide (0.015%) were observed experimentally in the products. Reactions forming these species were incorporated in the mechanism. Propene is formed from the n-propyl radical which is formed by the addition of CH 3 to ethene: Ctt3 + C2H4 ~ nC3It7
(31)
nC3H7 + 02 ~ C3H6 + HO2
(32)
Propene oxide is formed by the reaction of HO2 with propene: C3H6 + HO2 ~ C3H60 + OH
(33)
Both propene and propene oxide were underpredieted by about a factor of 5, Although these are very minor products, adjustment of the rate parameters for some of the above reactions or adding new reaction paths to form these species may be required to improve the predictions. Finally, the model and experiments show that carbon monoxide is the most abundant product formed from ethene oxidation at these temperatures. CO is formed mainly by the oxidation of the formyl radical: HCO + 02 ~ CO + HO2
(34)
and to a lesser extent by decomposition of the acetyl radical (Reaction 21). One of the characteristics of low and intermediate temperature hydrocarbon oxidation chemistry is that there is not complete conversion of CO to CO2. Consequently, the large heat release that accompanies this conversion, which is observed in high temperature chemistry, is absent at these lower temperatures. At the temperatures of this study, there was partial conversion of the CO to CO2. This occurred by the reaction of CO with OH and HOz: CO + OH ~ CO2 + II
(35)
CO + H O 2 ~ COg + OH
(36)
Reaction 35 is the primary, reaction forming COz at these conditions. Its relative importance increases
MODELING OF E T H E N E OXIDATION with temperature. At 696 K, Reaction 35 accounts for 65% of the CO2, while at 718 K, it is responsible for 73% of the CO2. As seen in Figs. 1-3, both the general shape and the absolute concentrations of the experimental CO data are very well predicted by the model for each temperature case. The calculated results for CO2 indicate a slower initial rate of CO2 formation relative to the data for both temperature cases. One possibility for increasing the calculated CO2 levels in the initial stages of the reaction is to have an early route for CO2 formation, which does not proceed through the CO paths. Such a reaction path has been considered before in low and intermediate temperature modeling studies, ez'z6 This path involves O2 addition to acetyl radicals followed by hydrogenation and decomposition directly to CO2. These reactions are also included in the present mechanism for ethene. It may be necessary to adjust the rates of these reactions to increase the importance of this path and to increase the CO2 production in the early stages of the reaction.
Conclusions
A detailed chemical kinetic mechanism for ethere oxidation has been developed and used to model reactions in a static reactor at temperatures near 700 K. The modeling calculations identified some of the key reaction steps at the present conditions. The formation of the majority of the reaction intermediates results from two main reaction paths. The addition of HOz to ethene leads to ethene oxide, which becomes the source of methyl radicals. The methyl radicals lead to methane and methanol. The addition of OH to ethene leads to formaldehyde, which is subsequently converted to CO and COz. Formaldehyde also results from the OH abstraction path via the vinyl radical. At these temperatures OH abstraction is less important than OH addition. However, the relative importance of the abstraction path increases with temperature.
Acknowledgments The authors would like to thank Dr. Frank Tully of Sandia National Laboratories for the use of his experimentally determined rate expression prior to its publication. The computational modeling portions of this work were supported by the U.S. Department of Energy, Division of Energy Conversion and U t i l i z a t i o n T e c h n o l o g i e s and w e r e performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore National Laboratory Contract No. W-7405-ENG-48 and by Drexel University Contract No. DE-FG0487AL44658. The experimental work was supported
209
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27. WILK, Pt. D., Prrz, W. J., WESTBBOOK, C. K., AND CERNANSKY, N. P.: WSSCI Paper 89-31, Spring Meeting of the Western States Section/ The Combustion Institute, 1989. 28. POLt~BD, R. T.: Comprehensive Chemical Kinetics, (C. H. BAMFOBDAND C. F. H. TIPPER, Eds.), Vol. 17, p. 249, Elsevier, 1977. 29. TULLY, F. P.: Chem. Phys. Lett. 143, 5 (1988). 30. TULLY, F. P.: Private Communication, 1989. 31. RAy, D. J. M., DIAZ, R. R., AND WADDINGTON, D. J.: Fourteenth Symposium (International) on Combustion, p. 259, The Combustion Institute, 1973. 32. BAY, D. J. M. AND WADDINGTON, D. J.: Comb. Flame 21, 327 (1973).
CHEMICAL
KINETIC MODELING OF ETHENE OXIDATION AND INTERMEDIATE TEMPERATURES
AT LOW
RICHARD D. WILK
Department of Mechanical Engineering Union College, Schenectady, NY WILLIAM J. PITZ ^ND CHARLES K. WESTBROOK
Lawrence Livermore National Laboratory Livermore, CA AND
NICHOLAS P. CERNANSKY
Department of Mechanical Engineering and Mechanics Drexel University, Philadelphia, 1"A
A detailed chemical kinetic mechanism for ethene oxidation has been dew;loped and used to model low to intermediate temperature oxidation chemistry. The model was used to simulate the reactions in a static reactor at temperatures of 696 K and 718 K, an equivalence ratio of 2.0, and a pressure of 600 torr. The modeling calculations identified some of the key reaction steps at these conditions. The formation of the majority of the reaction intermediates results from two main paths involving the reaction of ethene with HO2 and OH. The addition of HO~ to ethene leads to ethene oxide, which becomes a source of methyl radicals. The methyl radicals lead to the formation of methane and methanol. The addition of OH to ethene leads to formaldehyde, which is subsequently converted to CO and COs. Formaldehyde also results from the O][I abstraction path via the vinyl radical. At temperatures near 700 K, OH abstraction is less important than O1t addition. However, the relative importance of the abstraction path increases with temperature. Modeling results at these conditions were compared with experimental species concentration data. The calculated concentrations of the major products ethene oxide and carbon monoxide are in very good agreement with the experimental data. The calculated concentrations of formaldehyde and methanol are low. The calculated methane concentrations are in good agreement in the early stages of the reaction, but are overpredictcd in the latter stages. The similarities anti differences in agreement between the modeling and experimental results are discussed.
Introduction Efforts are currently underway to develop a detailed chemical kinetic model for the oxidation of higher hydrocarbon fuels which can be applied to address problems in practical combustion systems. A hierarchical approach is used in constructing reaction mechanisms. In one aspect of this approach, a mechanism describing the oxidation of a particular fuel under certain physical conditions (pressure, temperature) is modified and extended for application at different conditions. In another aspect of this approach, mechanisms for smaller fuels, which have been validated experimentally, are used as building blocks (submechanisms) for the formulation of mechanisms for more complex fuels. 203
One of the principal building blocks is the ethene (CzH4) mechanism. Ethene is a major intermediate produced in the oxidation of many hydrocarbon fuels (n-octane, n-pentane, n-butane, propane, propene, and ethane). Its formation is especially important during the oxidation of these fuels in the intermediate and high temperature regimes. At high temperatures (T > ~ 0 K), ethene is formed mainly from the decomposition of higher alkyl radicals. At intermediate temperatures (600 < T < 900 K), ethene forms primarily from the oxidation of ethyl radicals. In either case, ethene is formed at a relatively high rate. Also, because it is a fairly reactive intermediate, it can react readily to form secondary products. Thus, understanding the oxidation mechanism of ethene is an integral step in
204
REACTION KINETICS
the development of chemical kinetic reaction mechanisms for larger hydrocarbons. Detailed mechanisms have been developed for ethene oxidation at high temperatures 1-4 and have been used to model results from high temperature experiments. In the present study, the intermediate temperature reaction regime was modeled and validated with experimental data from a static reactor. Modeling the chemistry in this temperature regime is particularly relevant to the problem of knock in spark ignition engines. The end gas in the cylinder spends appreciable time at intermediate temperatures prior to autoignition and knock. The oxidation chemistry of the fuel at these temperatures can have an influence on the autoignition behavior. Alkenes are unsaturated hydrocarbons characterized by the presence of at least one carbon-carbon double bond in the structure. Because of this, there are certain interesting and unique features of the intermediate temperature oxidation mechanism of alkenes, including ethene. One of these features involves consumption of alkenes by free radical addition. Radicals such as H, O, OH, and HO2 can add at the double bond. These reaction channels lead to products such as aldehydes and oxirans. Incorporation of these additional reaction channels into the mechanism is necessary in order to accurately model the oxidation of these fuels. Previous experimental studies have been conducted to investigate the oxidation process of ethene. Schug et al. 5 used a turbulent flow reactor to examine the high temperature oxidation and pyrolysis under dilute conditions. Thornton and Malte6 used a jet-stirred reactor to study the high temperature oxidation. Wilk et al. 7 used a static reactor to investigate the intermediate temperature oxidation of ethene. Hoffman and Litzinger8 used a high pressure flow reactor to examine pressure effects on the oxidation of ethene at intermediate and high temperatures. A substantial amount of work was done by Baldwin, Walker and coworkers9-12 also using a static reactor to determine the mechanism and certain rate constants in the ethene reaction. Certain elementary reactions important to the ethene mechanism have been studied. The reaction between ethene and OH has been studied by Morris et al.,13 Atkinson and Aschmann, 14 and Bartels et al. 15 Slagle et al. 16A7 studied the reactions radicals and vinyl radicals with O2. The reactions of ethene oxide have been studied by Crocco et al., 18 Benson, 19 Baldwin et al., 2~ and Lorenz and Zellner.21 To date, there have been no modeling studies of the intermediate temperature oxidation of ethene. In this study, ethene oxidation was modeled at temperatures near 700 K. Results were compared with species concentration profiles from a static reactor.
Experiment The experimental data used for comparison with the model was obtained from a static reactor system, details of which have been described previously.22'z3 This system has been used extensively in the past for studying the low and intermediate temperature oxidation of many hydrocarbons. Data for ethene oxidation in the form of pressure, temperature, and species concentration histories were obtained by Wilk et al. 7 For this work, ethene (99.5% pure) and air (99.99% pure) were used as reactants. Experiments were carried out at an initial pressure of 600 torr, with initial temperatures of 696 K and 718 K, and an equivalence ratio of 2.0.
Numerical Model and Reaction Mechanism Ethene oxidation in the static reactor was simulated with the assumptions of constant volume and spatial uniformity over the vessel volume. The HCT programz4 was used to integrate the chemical kinetic rate equations in time. The calculated temperature histories followed the experimentally measured temperatures in the reaction vessel. The detailed chemical kinetic mechanism for ethene was developed based primarily on an earlier high temperature mechanism for ethene 2 and a low temperature mechanism for acetaldehyde.25 Certain portions of an earlier mechanism for propene26 were also incorporated to address the chemical reactions of C3 species. Added to this mechanism were low and intermediate temperature reaction paths involving consumption of ethene, and formation and consumption paths for ethene oxide, a primary intermediate. The combined detailed mechanism for ethene oxidation used in this work is given in Ref. 27. Key Reactions in the Mechanism:
The main initiation reaction for ethene at temperatures near 700 K is: C2H4 + 02--~ C2H3 + HO2
(1)
As soon as HOz radicals are formed, they quickly take over initiation, primarily through the addition reaction: C2H4 + HO2 ~ C2HaOOH
(2)
Baldwin et al. 12 suggest an energy barrier of 17 kcal/ mol for Reaction 2. Slagle et al, 16 suggest a value of 5-7 kcal/mol. We chose an activation energy of 8 kcal/mol for this reaction. We have included three routes for the consumption of the C2H4OOH adduet formed in Reaction
MODELING OF ETHENE OXIDATION 2. The first is dissociation back to reactants (Reaction -2). An activation energy of 20 kcal/mol obtained from Pollard28 was used for this reaction. This value lies between values suggested by Baldwin et al. lz (29 kcal/mol) and Slagle et al. 1~ (17.9 kcal/ mol). The second path for consuming C2H4OOH is by isomerization via internal H-atom transfer, C2H4OOH"-~C2HsO2
(3)
For this reaction, we assigned a value for Ea of 22 kcal/mol, which again is comparable to the values used by Baldwin et al. (28 kcal/mol) and Slagle et al. (17 kcal/mol). The value for A = 2.86 • 1011 s -I was obtained from Pollard. 28 Finally, the primary reaction consuming C2H4OOH is: C2HI4OOH ~ C2H40 + OH
(4)
The reaction sequence (2, 4) is important because it produces ethene oxide, the primary hydrocarbon intermediate observed. Also, it converts HO2 to the more reactive OH radical, and thus accelerates the overall rate of oxidation. At temperatures near 700 K, OH radicals can react with ethene in two ways: abstraction and addition: C2H4 + OH ~ C2Ha + H20
(5)
C2H4 + OH --~ C2H4OH
(6)
For these reactions we used the rate expressions recently obtained by Tully29'3~ (As = 4.8 x 1012 cm3/mol-s; Ea = 5.9 kcal/mol); (A~ = 1.63 • 1012 cmZ/mol-s; Ea = -0.69 kcal/mol). The CzH4OH adduct formed by the addition of OH can dissociate back to reactants or undergo further addition with 02: C2H40H + 02--~ O2C2HI4OH
(7)
The resulting adduct decomposes to formaldehyde and OH: O2C2H4OH--* CH20 + CH20 + OH
(8)
The reaction sequence 6 followed by 7 and 8 is known as the Waddington Mechanism. 31'32 This path is the primary formation route for formaldehyde. It is also a chain carrying path since the OH is regenerated. Other important abstraction reactions involving ethene are: C2H4 + CHa--~ C2Ha + CH4
(9)
C2H4 + CHaO -~ C2Ha + CH3OH
(10)
At intermediate temperatures the vinyl radical reacts with 02 to give formaldehyde and formyl:
C2H3 + O2---~ HCHO + HCO
205 (11)
For this reaction, we use the rate of Slagle et al. 17 (A = 4 • 1012 cma/mol-s; Ea = -0.25 kcal/mol). Since ethene oxide is the major hydrocarbon intermediate in the reaction at these temperatures, it was necessary to incorporate reactions forming and consuming this species. As mentioned earlier, the main reaction path forming ethene oxide results from HO2 addition to ethene. There are also additional steps that form ethene oxide. These are disproportionation reactions which are included in the mechanism: C2H4 + CH302 ~ C2H40 + CH30
(12)
C2H4 + C2H502---> C2H40 + C2H50
(13)
Ethene oxide is consumed by three routes: decomposition, isomerization, and H-atom abstraction. Each of these paths is shown below: C2H40 ~ CH3 + CHO
(14)
C2H40 --~ CH3CHO
(15)
C2H40 + OH ~ C2HaO + H20
(16)
For Reactions 14 and 15, Baldwin et al. 2~ estimated the rate constants at 480~ C. We used these values along with our own estimates for the activation energies to estimate the A factors. The primary radical abstracting H atoms is the OH radical. The abstraction path removes an H atom but keeps the C---O---C structure intact. For Reaction 16, we assigned the same rate parameters as CzH4 + OH. The C2H30 radical formed in Reaction 16 is consumed by two isomerization reactions: C2H30 ~ CH3CO
(17)
C2I-I30 ~ CH2CHO
(18)
We used Ea = 14 kcal/mol for each of these reactions based on the study of Lorenz and Zellner. 21 The ratio of the rate constants for these two reactions was obtained by Baldwin et al. 2~ On the basis of this information we assigned A17 = 8.5 • 1014 cma/mol-s and Als = 1 • 1014 cma/moi-s. One of the primary chain terminating steps in the mechanism is the self reaction to HI02. Results and Discussion Comparisons of the calculated and experimental species concentration profiles are presented in Figs. 1-3. The results for the lower temperature case 696 K are presented in Figs. 1 and 2. The results for the higher temperature case, 718 K, are presented in Fig. 3.
206
REACTION KINETICS 14
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The majority of the fuel consumption at both temperatures occurs by the reaction of ethene with HOz and OH. Reactions are included in the mechanism for both the addition paths involving these radicals and H-atom abstraction by these radicals. At the temperatures investigated in this study, the addition paths dominate the process. At 696 K, OH addition to ethene followed by subsequent reactions accounts for about 55% of the fuel consumed. The OH abstraction path accounts only for about 8%. HO2 addition accounts for approximately 37% of the fuel consumed. At 718 K, the OH abstraction path is more important, accounting for about 14% of the fuel consumption. H atom abstraction by HOe was not found to be important in this temperature range. Likewise, consumption of the fuel by the pyrolysis step, C2H4 + M ~ Cell3 + H + M
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FIc. 2. Concentrations major intermediate species for an initial temperature of 696 K. Curves indicate computed results using numerical model; symbols represent experimental results. is not important at these temperatures due to its very high activation energy (107.5 kcal/mol). For the 696 K case, the calculated fuel consumption becomes faster than that measured in the experiment after about 200 s (Fig. 1), resulting in more fuel being consumed. Since the model predicts a higher degree of fuel consumption, the predicted reaction intermediates and products are affected. In order to isolate the intermediate and product chemistry from the fuel effect, the species concentrations for the 696 K case are plotted against percent fuel consumed (Fig. 2). This approach allows for a comparison of modeling and experimental results on a more common basis from which we can evaluate how well the model predicts quantitatively the underlying intermediate and product chemistry. For the 718 case, the calculated and experimental fuel disappearence profiles are in good agreement (Fig. 3). Thus, for this case, normalization of
207
MODELING OF ETHENE OXIDATION
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the results on the basis of fuel consumed is not really required. The fact that the 718 K case performs better on a time plot than the 696 K case indicates that an adjustment in the activation energies of one or more of the fuel consuming reactions or chain branching reactions may be required. The |ormation of the major intermediates and products in the reaction results primarily from either the HO2 or OH addition paths. The reaction between HO2 and ethene (Reaction 2) determines the amount of ethene oxide produced in the calculation. As seen in Figs. 1-3, for both temperature, cases the ethene oxide concentrations attained during the calculations are in very good agreement with the measured concentration throughout the course of the reaction. Ethene oxide is consumed primarily by the Reactions 14, 15, and 16. Of these three, Reaction 16 is most important followed by the decomposition path (Reaction 14). The isomerization step (Reac-
(20)
Very little ketene was formed in the calculation and ketene was not measured in the experiments. Reaction 17 is the major consumer of CzH30. The acetyl radical CH3CO formed in this reaction is a major source of methyl radicals, CH3CO + M ~ CH3 + CO + M
300
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tion 15) is the least important. Little acetaldehyde was formed in the calculations, and no acetaldehyde was measured in the experiments. The C2H30 radical produced in Reaction 16 undergoes two isomerization reactions. Reaction 18 ultimately leads to ketene by the reaction,
(21)
along with the direct decomposition reaction of C2H40 (Reaction 14). The relatively large amounts of both methane and methanol observed in the calculations and the experiments indicate sul)stantial levels of CH3 in the reaction. There are three principal reactions forming methane at these temperatures. All involve H-atom abstraction by CH3: CH3 + HEO2---->CH4 + t l O z
(22)
CH3 + CzH4 ~ CH4 + C2H3
(23)
CH3 + CHzO ~ CH4 + HCO
(24)
At 696 K, the reaction with hydrogen peroxide (Reaction 22) accounts for about half of the methane formation. Reactions 23 and 24 account for about 25% each. Measurements of the H202 concentration were not made in the experiment. However, the model predicted substantial levels formed in the reaction. The calculated maximum H202 concentration ranged from 0.6% to 1.2%. At 718 K, Reaction 23 is the most important produeer of methane, accounting for about 85% of CH4. At this temperature, Reaction 22 accounts only for about 2% of the CH4. Methanol is formed by H-atom abstraction by methoxy radicals: CH20 + CH30 ~ CH3OH + HCO
(25)
C2H4 + CH30---> CH3OH + C2H3
(26)
Reaction 25 is the most important path at 696 K while reaction with the fuel is most important at 718 K. Some CHaOH is also formed from the reaction of methylperoxy radic',ds: CH3Oz + CH302 CHzO + CHaOH + 02
(27)
REACTION KINETICS
208
A comparison between the calculated and measured concentration profiles of methane and methanol is shown in Figs. 1-3. From Figs. 2 and 3, at both temperatures, the experimental and predicted methane concentrations are in good agreement in the early stages of the reaction (up to approximately 35% fuel consumed). In the latter stages of the reaction, the methane concentration is overpredicted by the model. At 696 K (Fig. 2), the predicted maximum concentration of methane is 2.5 times higher than the measured amount. At 718 K, the predicted maximum is higher by a factor of 1.8. The over prediction of methane in the latter stages of the reaction seems to indicate that methane is not being consumed fast enough. With methanol, the calculated species concentrations are slightly underpredicted by about 20% for both temperature cases. Since a primary step forming methanol is Reaction 25, the underprediction of methanol may be due to only the underprediction of formaldehyde, and not as a result of methanol chemistry. The model correctly predicts the trends in the species profiles for methane and methanol with temperature. Both model and experiment indicate a decrease in the methanol yield and an increase in the methane yield with increasing temperature. The reactions between Oil and ethene (Reactions 5, 6) determine the amount of formaldehyde produced in the calculation. As mentioned earlier, OH addition (Reaction 6) dominates at these temperatures. Following the addition of 0-2 (Reaction 7) the adduct decomposes, yielding two formaldehyde molecules and another OH (Reaction 8). This reaction path accounts for about 90% of the formaldehyde produced. The OH abstraction path also leads to formaldehyde. Hydrogen atom abstraction by OH forms C2H3 (Reaction 5) which reacts mainly with O2 to give CH20 and HCO (Reaction 11). We also consider alternative products from the reaction of 0.2 and C.2H3: C,2tt3 + O2 ~ C2H.2 + HO2
(28)
This minor reaction path is included to account for the small amount of acetylene observed in the 718 K case. Formaldehyde is consumed by reactions with Ott and HO2: CH20 + OH ~ HCO + H20
(29)
CH20 + HO2 ~ HCO + H202
(30)
Comparison of the measured and calculated species concentrations in Figs. 2 and 3, shows that the CH20 concentrations are underpredicted. The calculated maximum CH20 concentration is about 20% low for the 696 K, and about 50% low for the 718
K case. Although, the overall concentrations are lower, the shapes of the calculated profiles follow the experimental profiles very well. The predicted peak formaldehyde occurs earlier in the reaction for the 696 K case. In the 718 case, the predicted and experimental peak CH20 concentrations are coincident with respect to the reaction time. The underprediction of CH20 by the model and the sensitivity of this underprediction to temperature indicates that the rate parameters used for Reactions 6, 7, and 8 may need re-evaluation. Small amounts of propene (0.08%) and propene oxide (0.015%) were observed experimentally in the products. Reactions forming these species were incorporated in the mechanism. Propene is formed from the n-propyl radical which is formed by the addition of CH 3 to ethene: Ctt3 + C2H4 ~ nC3It7
(31)
nC3H7 + 02 ~ C3H6 + HO2
(32)
Propene oxide is formed by the reaction of HO2 with propene: C3H6 + HO2 ~ C3H60 + OH
(33)
Both propene and propene oxide were underpredieted by about a factor of 5, Although these are very minor products, adjustment of the rate parameters for some of the above reactions or adding new reaction paths to form these species may be required to improve the predictions. Finally, the model and experiments show that carbon monoxide is the most abundant product formed from ethene oxidation at these temperatures. CO is formed mainly by the oxidation of the formyl radical: HCO + 02 ~ CO + HO2
(34)
and to a lesser extent by decomposition of the acetyl radical (Reaction 21). One of the characteristics of low and intermediate temperature hydrocarbon oxidation chemistry is that there is not complete conversion of CO to CO2. Consequently, the large heat release that accompanies this conversion, which is observed in high temperature chemistry, is absent at these lower temperatures. At the temperatures of this study, there was partial conversion of the CO to CO2. This occurred by the reaction of CO with OH and HOz: CO + OH ~ CO2 + II
(35)
CO + H O 2 ~ COg + OH
(36)
Reaction 35 is the primary, reaction forming COz at these conditions. Its relative importance increases
MODELING OF E T H E N E OXIDATION with temperature. At 696 K, Reaction 35 accounts for 65% of the CO2, while at 718 K, it is responsible for 73% of the CO2. As seen in Figs. 1-3, both the general shape and the absolute concentrations of the experimental CO data are very well predicted by the model for each temperature case. The calculated results for CO2 indicate a slower initial rate of CO2 formation relative to the data for both temperature cases. One possibility for increasing the calculated CO2 levels in the initial stages of the reaction is to have an early route for CO2 formation, which does not proceed through the CO paths. Such a reaction path has been considered before in low and intermediate temperature modeling studies, ez'z6 This path involves O2 addition to acetyl radicals followed by hydrogenation and decomposition directly to CO2. These reactions are also included in the present mechanism for ethene. It may be necessary to adjust the rates of these reactions to increase the importance of this path and to increase the CO2 production in the early stages of the reaction.
Conclusions
A detailed chemical kinetic mechanism for ethere oxidation has been developed and used to model reactions in a static reactor at temperatures near 700 K. The modeling calculations identified some of the key reaction steps at the present conditions. The formation of the majority of the reaction intermediates results from two main reaction paths. The addition of HOz to ethene leads to ethene oxide, which becomes the source of methyl radicals. The methyl radicals lead to methane and methanol. The addition of OH to ethene leads to formaldehyde, which is subsequently converted to CO and COz. Formaldehyde also results from the OH abstraction path via the vinyl radical. At these temperatures OH abstraction is less important than OH addition. However, the relative importance of the abstraction path increases with temperature.
Acknowledgments The authors would like to thank Dr. Frank Tully of Sandia National Laboratories for the use of his experimentally determined rate expression prior to its publication. The computational modeling portions of this work were supported by the U.S. Department of Energy, Division of Energy Conversion and U t i l i z a t i o n T e c h n o l o g i e s and w e r e performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore National Laboratory Contract No. W-7405-ENG-48 and by Drexel University Contract No. DE-FG0487AL44658. The experimental work was supported
209
by the U.S. Army Research Office under Contract No. DAAG29-85-K-0253; Project No. 22437-EG. REFERENCES 1. WESTBROOK, C. K., DRYER, F. L., AND SCHUC, K. P.: Comb. Flame 52, 299 (1983). 2. WESTBROOK,C. K., THoRYrON, M. M., Prrz, W. J., AND MALTE, P. C.: Twenty-Second Symposium (International) on Combustion, p. 863, The Combustion Institute, 1989. 3. CATHONNET,M., GMLLARD, F., BOETrNER, J. C., CAMBRAY, P., KARMED, D. AND BELLET, J. C.: Twentieth Symposium (International) on Combustion, p. 819, The Combustion Institute, 1985. 4. DAGAUT, P., CATHONNET, M., BOETTNER, J. C., AND GAILLARD, F.: Combst. Flame 71, 295 (1988). 5. SCHUr K. P., SANTORO, R. J., DRYER, F. L., AND GLASSMAN,I.: WSSCI Paper 78-42, Spring Meeting of The Western States Section/The Combustion Institute, 1978. 6. THORNTON, M. M., MALTa, P. C. AND CRrUI'ENDEN, A. L.: WSSCI Paper 85-35, Fall Meeting of the Western States Section/The Combustion Institute, 1985. 7. WILK, R. D., MILLER, D. L. AND CERNANSKY, N. P.: WSSCI Paper 86-6, Joint Spring Meeting of the Canadian and Western States Section/The Combustion Institute, 1986. 8. HOFFMAN, J. S. AND LITZINGER, T. A.: ESSCI Paper 88-20, Fall Meeting of the Eastern States Section/The Combustion Institute, 1988. 9. BALDWIN, R. R., SIMMONS, R. F., AND WALKER, R. W.: Trans. Faraday Soc. 62, 2486 (1966). 10. BALDWIN, R. R. AND WALKER, R. W.: Eighteenth Symposium (International) on Combustion, p. 819, The Combustion Institute, 1981. 11. BALDWIN,R. R., HOPKINS, D. E., MALCOLM, D. G. AND WALKER, R. W.: Oxidation Communications 6, 231 (1984). 12. BALDWIN,R. R., DEAN, C. E., AND WALKER, R. W.: J. Chem. Soc., Faraday Trans. II 82, 1445 (1986). 13. MORRIS, E. D., STEDMAN, D. H., AND NIKI, H.: J. Amer. Chem. Soc. 93, 3570 (1971). 14. ATKINSON, R. AND ASCHMANN, S. M.: Int. J. Chem. Kin. 16, 1175 (1984). 15. BARTELS, M., HOYERMANN, K., SIEVERT, R.: Nineteenth Symposium (International) on Combustion, p. 61, The Combustion Institute, 1983. 16. SLaGLE, I. R., FENG, Q., AND GUTMAN, D.: J. Phys. Chem. 88, 3648 (1984). 17. SLAGLE, I. R., PARK, J-Y, HEAVEN, M. C. ~AND GUTMAN, D.: J. Amer. Chem. Soc. 106, 4356 (1984). 18. CROCCO, L., GLASSMAN,I., AND SMITH, I. E.: J. Chem. Phys. 31, 506 (1959). 19. BENSON, S. W.: J. Chem. Phys. 40, 1, 1964.
210
REACTION KINETICS
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