Experimental Study and Kinetic Analysis of the Oxidation of Light

Ind. Eng. Chem. Res. 1996, 35, 2127-2136

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Experimental Study and Kinetic Analysis of the Oxidation of Light Hydrocarbon Mixtures Renato Rota,* Francesca Bonini, Massimo Morbidelli, and Sergio Carra` Dipartimento di Chimica Fisica Applicata, Politecnico di Milano, via Mancinelli 7, 20131 Milano, Italy

The combustion of various C1-C2 hydrocarbon mixtures has been experimentally investigated in a continuous perfectly stirred reactor, in a temperature range from 1000 to 1300 K, and at stoichiometric ratio values between 0.5 and 1.5. The concentration values of several molecular species have been measured by GC analysis. The data obtained have been compared with the predictions of two different detailed kinetic models; both fail to predict the experimental trends in various situations. This leads to the conclusion that experimental data based on mixtures of hydrocarbons must be obtained to validate detailed kinetic mechanisms. Introduction Complex reacting systems such as those involving hydrocarbon combustion can be described in a wide range of operating conditions only by detailed kinetic models, which account explicitly for the various molecular and radical species participating in the process (cf., Westbrook and Dryer, 1984). Detailed kinetic mechanisms are rather complex and not unique: several compilations involving hundreds of elementary reactions have been presented in the literature even for the simple case of the combustion of C1C2 hydrocarbons (cf., Dagaut et al., 1991; Kilpinen et al., 1992; Miller and Bowmann, 1989; Tan et al., 1994). The kinetic parameters of the reactions involved in these mechanisms can be estimated both theoretically and experimentally with reference to each single elementary reaction. However, both these procedures are usually expensive and time consuming (Miller and Fisk, 1987). Simpler approaches include the consideration of known parameters of similar reactions (Miller and Bowman, 1989) or the direct fitting of experimental data relative to the overall behavior of various combustion systems (Tan et al., 1994). In other words, each detailed model is tuned to several sets of experimental data, usually involving a single fuel oxidation. Therefore, it is important to produce new sets of kinetically significant data to investigate the behavior of these models under different conditions. In this regard, experimental data relative to hydrocarbon mixture oxidation are not readily available in the literature. For instance, two recent papers (Rota et al., 1994a; Tan et al., 1995) reported a few experimental data concerning the oxidation of methane-ethane mixtures under jet-stirred reactor conditions. However, the former considered only an equimolar mixture of methane and ethane, while the latter investigated only stoichiometric and fuel-lean conditions. Moreover, it is well-known that even the oxidation of a single hydrocarbon (say, methane) involves several other C2 hydrocarbons as sketched in Figure 1. However, their low concentration values do not allow a safe estimation of the interconnecting reaction paths, which exhibit rather different relative importance under different conditions (e.g., at different concentration of reactants). This means that the study of the oxidation of hydrocarbon fuel mixtures should be able to evidence lack of agreement between model * Corresponding author. Fax: +39 2 23993180. Email: [email protected].

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Figure 1. Scheme of C1-C2 hydrocarbon oxidation. Table 1. Composition of Feed Mixtures Used in the Experimental Runsa run I (MER ) 1) II (MER ) 0.33) III (MER ) 3) IV

Φ

[CH4]

[C2H6]

1.5 1.0 0.5 1.5 1.0 0.5 1.5 1.0 0.5 1.5 1.0 0.5

2090 2040 2010 800 820 850 3310 3340 3480 650 700 660

2170 2090 2090 2480 2530 2660 1170 1140 1240 650 700 670

[C2H4]

720 760 730

[C2H2]

[O2]

660 690 660

7000 10500 23100 6200 10800 21400 6300 11300 24100 4500 7500 14300

a Concentration values in ppmv. Φ values are approximated for the sake of reference in the following tables and figures. MER is the methane/ethane ratio in the feed.

predictions and experiments, which are not easily identified when a single fuel oxidation is considered. In this work, we have extended the aforementioned studies (Rota et al., 1994a; Tan et al., 1995), experimentally investigating the oxidation of several fuel mixtures involving methane, ethane, ethylene, and acetylene in a perfectly stirred reactor (PSR). This operates isothermally in the temperature range 1000-1300 K under highly diluted conditions (more than 95%), with stoichiometric ratios ranging between 0.5 and 1.5. The main aim was to produce experimental data under conditions not previously investigated and to deduce qualitative behaviors and trends to be reproduced by detailed kinetic models. These data would be a severe test for testing the reliability of any detailed kinetic mechanism. For the sake of example, we have also compared the experimental results with the predictions of two different detailed kinetic models previously developed to simulate light hydrocarbon combustion (Dagaut et al., 1991; Rota et al., 1994b). It has been found that these models are able to well reproduce the experimental findings under various conditions (usually those involving only methane-ethane mixtures), but they definitely fail to predict even the qualitative trends © 1996 American Chemical Society

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Ind. Eng. Chem. Res., Vol. 35, No. 7, 1996

Table 2. Experimental Results for Run I (MER ) 1)a Φ

T (K)

[CO]

[CO2]

[CH4]

[C2H2]

[C2H4]

[C2H6]

1.5

1260 1250 1180 1100 1020 955 1275 1190 1115 1100 1025 1200 1120 1035

2735 2760 1885 500 0 0 1700 1890 1845 1555 165 965 1385 1630

650 610 250 50 0 40 3665 2450 475 275 55 4495 3475 540

990 950 1500 1875 1965 1980 130 550 1330 1535 2020 30 250 1220

230 210 95 15 0 5 55 85 25 15 0 0 0 0

605 575 935 915 275 35 60 370 675 745 485 10 135 460

90 110 375 1065 1765 2000 15 125 595 740 1620 10 70 680

1.0

0.5

a

Table 5. Experimental Results for the Equimolar Methane-Ethane-Ethylene-Acetylene Mixtures (Run IV)a Φ

T (K)

[CO]

[CO2]

[CH4]

[C2H2]

[C2H4]

[C2H6]

1.5

1265 1185 1110 1095 990 1270 1185 1110 1100 990 1270 1205 1195 1130 1110 1090 1040 990

2390 1930 1165 1000 215 2120 2355 2470 2050 325 355 470 525 980 1175 1535 1975 710

590 355 180 170 105 2685 1820 625 405 115 4595 4285 4105 3730 3205 2945 645 130

250 395 480 495 605 45 115 350 445 600 0 10 15 15 25 90 405 560

375 355 375 385 545 55 80 190 285 515 5 5 15 5 25 50 230 510

265 540 740 760 775 45 105 430 600 770 0 10 10 15 20 95 495 745

25 70 170 190 520 5 20 85 130 450 5 5 5 5 5 15 130 330

1.0

0.5

Concentration values in ppmv.

Table 3. Experimental Results for Run II (MER ) 0.33)a Φ

T (K)

[CO]

[CO2]

[CH4]

[C2H2]

[C2H4]

[C2H6]

1.5

1260 1180 1170 1100 1085 980 1290 1210 1200 1125 1100 1000 1300 1220 1140 1060 1020

3375 3075 2915 1635 1145 0 2410 2680 2760 2980 2515 185 320 625 810 2005 1665

755 505 435 150 115 60 3445 2805 2420 1070 405 50 6010 5705 4440 2675 230

360 515 545 740 750 800 85 190 200 385 630 835 0 0 35 205 660

235 115 90 35 10 0 65 75 55 25 20 0 0 0 0 5 5

320 560 620 1130 1150 275 75 195 210 425 840 605 0 0 50 285 785

30 65 85 425 635 2180 10 25 30 105 370 1925 5 0 20 120 810

1.0

0.5

a

Φ

T (K)

[CO]

[CO2]

[CH4]

[C2H2]

[C2H4]

[C2H6]

1.5

1250 1240 1165 1075 990 1265 1175 1085 985 1270 1200 1190 1120 1105 1020 985

1570 1340 210 0 0 1680 2030 180 0 370 575 670 1375 1740 160 0

230 185 55 40 30 3555 1835 110 35 5200 4415 4205 3320 2370 150 40

2065 2435 3040 3220 3235 130 910 3100 3420 10 235 265 395 815 3080 3430

90 80 5 0 0 0 5 0 0 0 0 0 0 0 0 0

605 670 560 175 10 30 240 385 20 5 65 80 105 195 295 65

155 195 510 950 1100 10 130 730 1155 5 35 45 60 125 610 1145

0.5

a



Table 4. Experimental Results for Run III (MER ) 3)a

1.0

a


evidenced for other situations (namely, the experiments simultaneously involving methane, ethane, ethylene, and acetylene). This is a rather important point since it indicates that a detailed kinetic model may be able to reproduce some experimental results even if some reaction pattern is missing or not correctly reproduced. However, this is quite dangerous since a detailed kinetic model, which is based on elementary radical reactions, is expected to reproduce the experimental behavior in a wide operating window. Moreover, this finding strongly supports the need for new reliable experimental data. Finally, the rate of production analysis (that is, the analysis of the values of the production rate of each compound due to any reaction considered) has been used to support the aforementioned conclusions.

Figure 2. Comparison between data obtained in this work (filled symbols) and those previously reported in the literature by Dagaut and Cathonnet, 1990b (empty symbols; curves show the trends). 1500 ppmv of ethane, Φ ) 1.0, τ ) 0.12 s, P ) 1 atm.

Figure 3. Comparison between data obtained in this work (filled symbols) and those previously reported in the literature by Tan et al., 1995 (empty symbols; curves show the trends). 300 ppmv of ethane, 3000 ppmv of methane, 60000 ppmv of oxygen in nitrogen, τ ) 0.14 s, P ) 1 atm.

Experimental Apparatus The experimental apparatus used in this work has been described earlier in detail (Rota et al., 1994b), and so we review in the following only the most relevant aspects. All the experiments have been performed in a jet-stirred reactor, made of silica to minimize wall catalytic reactions, which can be operated at atmospheric pressure and up to 1300 K. The reactor is a 40 mm diameter sphere located inside an oven, designed to maintain the reaction temperature at the desired

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Figure 4. Experimental methane conversion for methane-ethane mixtures. Symbols are the experimental measurements, while curves show the trends. Experimental conditions: (A) MER ) 0.33 and (B) Φ ) 1.5.

value. As a consequence of the high dilution conditions used in the feed stream, heating due to chemical reactions is negligible. The flow rates of the reactants (fuel, air, and diluting nitrogen) are measured and regulated through massflow controllers, which allow for preparation of mixtures with various compositions. The diluting gas (nitrogen) is fed through a preheating coil and heated at the desired reaction temperature before entering the reactor. The fuel, properly diluted with cold nitrogen, is fed through a capillary tube, to reduce its residence time in the hot zone outside the reactor, thus minimizing pyrolysis reactions. For the same reason, the capillary tube was jacketed by the admission tube of cold air. Reactants and diluting nitrogen (the flow rate of the latter being much larger than that of the first) are mixed at the entrance of the injectors where the residence time is much smaller than that inside of the reactor. Exhausts leave the reactor through four holes located in the upper part of the reactor. The temperature is measured by a chromel-alumel thermocouple located inside the reactor. Sampling of the reacting mixture inside the reactor is realized by means of a sonic quartz probe jacketed

Figure 5. Experimental ethane conversion for methane-ethane mixtures. Legend as in Figure 4.

with cooling water, able to freeze the composition of the sample (Malte and Kramlic, 1980). Sonic conditions at the orifice of the probe (about 40 µm in diameter) were maintained by a leak-free membrane vacuum pump. The sample is fed directly to a single column (Carboxen1000) gas chromatograph, equipped with TCD and FID connected in series. Helium was used as carrier gas. This setup allows one to measure the concentration of several molecular species, that is, CO, CO2, CH4, C2H2, C2H4, and C2H6. We have estimated an error upper bound for these measurements of about 5%. In this work we have investigated various reacting mixtures, with different compositions, stoichiometric ratios, and reaction temperature values. The operating conditions adopted in all experimental runs are summarized in Table 1. The residence time in the reactor was kept constant and equal to 0.1 s (computed at 1100 K), which requires an overall flow rate equal to about 0.24 N m3/h. Note that a variable amount of CO2 (not larger than about 100 ppmv) was always present as an impurity in the feed stream. Experimental Results All the obtained experimental data are summarized in Tables 2-5. It can be noted that the material balance

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Figure 7. Experimental conversion values of methane, ethylene, acetylene, and ethane in the equimolar fuel mixture (run IV in Table 1) and Φ ) 1.0. Symbols are the experimental measurements, while curves show the trends.

Figure 6. Experimental selectivity values for methane-ethane mixtures. Legend as in Figure 4A.

of the carbon atoms is fulfilled in all runs: the maximum error is about 10%. To compare the results of different experimental runs in a systematic way, these will be considered in terms of conversion (ηi) for the ith fuel and selectivity with respect to the carbon atoms (γi) for the ith product. These quantities are defined as follows:

ηi )

out xin i - xi 100 xin i

(1)

(2)

Figure 8. Comparison between experimental data (curves with symbols) and model predictions (curves without symbols) for methane-ethane mixtures with MER ) 0.33 and Φ ) 1.5. (- -) DCB model; (s) MKGH model.

out where xin are the mole fractions of the ith i and xi compound in the feed and in the outstream, respectively. NF is the total number of fuels, and ni is the number of carbon atoms in the molecule of the ith species. A first validation of the experimental procedure has been performed by comparing some results obtained by Dagaut and Cathonnet (1990b) and by Tan et al. (1995) with those produced in our laboratory in similar conditions. The former used 1500 ppmv of ethane as fuel with stoichiometric conditions (that is, Φ ) 1.0), a residence time equal to 0.12 s, a temperature range from 1000 to 1250 K, and atmospheric pressure. The latter

investigated the oxidation of a 0.3% methane-0.03% ethane-6% oxygen mixture in nitrogen with a residence time equal to 0.14 s, a temperature range from 1000 to 1150 K, and atmospheric pressure. The results of such comparisons are shown in Figures 2 and 3. The good agreement obtained is a confirmation of the reproducibility of this kind of data in different laboratories and adds confidence to the experimental results. Let us now first analyze the experiments relative to binary mixtures of fuels. Figure 4 reports methane conversion values as a function of temperature for various stoichiometric ratios (Φ ) (fuel/air)/(fuel/air)St) and methane to ethane ratios (MER). As expected, ηCH4

γi )

- xin ni(xout i i ) 100 NF in out j nj(xj - xj )

∑


Figure 9. Comparison between experimental data (curves with symbols) and model predictions (curves without symbols) of methane conversion for methane-ethane mixtures. (A) DCB model and (B) MKGH model. Left side: MER ) 0.33. Right side: Φ ) 1.5.

increases with temperature and decreases with the stoichiometric ratio, Φ (see Figure 4, part A). More interesting is the effect of the methane-ethane ratio, MER, shown in part B of the same figure. It can be seen that methane conversion is the lowest for MER ) 3, while it is substantially unchanged for mixtures with MER ) 1 and MER ) 0.33. This behavior is consistent with the conclusion that the presence of ethane increases methane reactivity up to a maximum value. Above this value, higher ethane concentrations do not affect methane conversion. These findings are confirmed by the data of Dagaut and Cathonnet (1990a), which indicate that methane is almost inert under the conditions considered here, and by those of Tan et al. (1995) that evidenced the importance of traces of ethane on the oxidation of methane. The ethane conversion in the same experimental runs is shown in Figure 5B. It can be seen that, contrary to the previous case, methane depresses ethane reactivity: the larger the MER, the lower the ethane conversion. In part A of Figure 5 we see that the ethane conversion increases with temperature and decreases with Φ. Note also that in general the ethane conversion is larger than the methane conversion, particularly for low temperature values. This is not obvious since the high concentration of radical species generated by the more reactive ethane could enhance the methane conversion to values close to that of ethane. Figure 6 shows the selectivity of intermediate species (i.e., CO, C2H4, and C2H2) as a function of temperature for various values of Φ and MER ) 0.33. The behavior of these data can be understood by considering the competition between pyrolysis and oxidation reactions, indicated by the simplified kinetic scheme shown in

Figure 1. At low temperature, pyrolysis (that is, paths 1 and 2 in Figure 1) prevails on oxidation (paths 4 and 5), so that the conversion to ethylene, γC2H4, is larger than that to carbon monoxide, γCO. Only under fuellean conditions (Φ ) 0.5) is γCO close to γC2H4 since the excess oxygen favors the oxidation reactions. Since oxidation reactions toward CO (paths 4, 5, and 6) and CO2 (path 8) are more sensitive to temperature than pyrolysis, it follows that as temperature increases γC2H4 decreases. However, γCO depends on the competition between CO formation from hydrocarbon and consumption to give CO2. The net result is that under fuel-rich conditions formation always prevails on consumption and γCO increases with temperature, while the opposite is true for fuel-lean conditions. The behavior of CO at Φ ) 1.0 can then be regarded as a transition condition between fuel-rich (i.e., Φ > 1) and fuel-lean (i.e., Φ < 1) conditions. Accordingly, at stoichiometric conditions the selectivity of CO exhibits a maximum value at intermediate temperature values. For C2H2 we see in Figure 6 that selectivity increases with temperature. This is because at low temperature paths 3 and 6 are sufficiently slow to permit the accumulation of ethylene. Increasing the temperature increases the rate of path 7 but by an amount less than that of path 3, thus leading to larger values of γC2H2. Thus we observe that, contrary to the case of ethylene, for acetylene the rate of the formation reactions through pyrolysis is more sensitive to temperature than that of the consumption reactions by oxidation. To investigate the relative reactivity of all the hydrocarbons considered above, which arise from the interconnected reaction patterns illustrated in Figure 1, it is convenient to compare their conversion in an experi-

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Figure 10. Comparison between experimental data and model predictions of ethane conversion for methane-ethane mixtures. Legend as in Figure 9.

mental run where they are all fed to the reactor simultaneously (run IV). The obtained experimental results are shown in Figure 7 for Φ ) 1.0. In this case the sequence of reactivity (more evident at low temperature) is

( )

ki ) AiTβi exp -

C2H6 > C2H2 > CH4 > C2H4 The position of C2H4 in this ranking is surprising. It means that under these conditions the reactions leading to ethylene production (paths number 1 and 2 in Figure 1) are more effective than those involved in ethylene decomposition to acetylene (path 3) or to CO (path 6). These experimental results constitute a severe test for the reliability of kinetic models, as discussed in the next section. Comparison between Experimental Data and Detailed Model Predictions To simulate the behavior of the combustion process in the perfectly stirred reactor, we have used the computer code PSR (Glarborg et al., 1990), based on the CHEMKIN subroutine library (Kee et al., 1989), which solves the system of NS nonlinear algebraic equations

QoutCout - QinCin i i - riV ) 0

reactions and 50 species, and including nitrogen chemistry. For both the kinetic models examined, the forward reaction rate constants have been computed through the modified Arrhenius expression:

(3)

where NS represents the number of chemical species involved and isothermal conditions are assumed. The chemistry of the system has been described using two different detailed kinetic models: (1) the one proposed by Dagaut et al. (1991) (DCB mechanism), involving 200 reactions and 42 species and (2) the one discussed by Rota et al. (1994) (MKGH mechanism), involving 226

Ei RT

(4)

The backward reaction rate constants have been computed from the forward ones using the equilibrium constant values obtained from the thermodynamic data base CHEMKIN (Kee et al., 1990), complemented by the data reported by Burcat (1984) for species not included in this data base. The DCB mechanism was originally formulated to reproduce experimental data obtained from combustion of light hydrocarbons and should be able to describe the oxidation of different fuels up to C3. The MKGH mechanism derives from the kinetic scheme proposed by Kilpinen et al. (1992), suitably modified to improve the agreement between PSR experimental data and model predictions with respect to acetylene. Note that both these models contain reactions involving all the compounds considered here, that is, methane, ethane, ethylene, and acetylene. A typical comparison between experimental results and model predictions is shown in Figure 8, in the case of a methane-ethane mixture with Φ ) 1.5 and MER ) 0.33. We can see that both the detailed kinetic models provide a good prediction. Moreover, the conversion and selectivity values predicted by the two models are not significantly different. However, these conclusions cannot be generalized, because different reactions may prevail when different operating conditions are considered.


Figure 11. Comparison between experimental data (curves with symbols) and model predictions (curves without symbols) of intermediate selectivity values for methane-ethane mixtures. MER ) 0.33. Left side: DCB model. Right side: MKGH model.

Let us now proceed to compare model predictions with the experimental findings discussed in the previous section. Figure 9 shows methane conversion values as a function of temperature for various methane-ethane mixtures. Even if quite large differences between model predictions and experimental data are evident, it can be seen that the qualitative trends are correctly reproduced by both models. In particular, methane conversion, ηCH4, not only increases with temperature and decreases with the stoichiometric ratio, Φ, but also attains its lowest values at MER ) 3 while it remains almost unchanged for MER ) 1.0 and 0.33. This means that the effect of ethane on the methane conversion is qualitatively well predicted by both models. Similar conclusions arise from the ethane conversion values shown in Figure 10. Also, the depressing effect of methane on ethane conversion is qualitatively well predicted by the two models. On the other hand, Figure 11 evidences that the predictions of the two models are not always similar; for instance, the MKGH model overpredicts acetylene selectivity for the low temperature values. This conclusion is confirmed by the results shown in Figure 12 relative to the case where four hydrocarbons are simultaneously fed to the reactor. In this case the behavior of the two models is in fact quite different. While the MKGH model predicts almost the same conversion for

all the fuels with a sort of on/off switch around 1050 K, the DCB model gives rather different conversion values for the four fuels in the low temperature range. Here the experimental data differ from the results of both models and seem to exhibit a somewhat intermediate behavior. Both models predict complete conversion at temperature values around 1200 K, in agreement with the experimental findings. Moreover, the reactivity sequence found experimentally (C2H6 > C2H2 > CH4 > C2H4) is not respected by the two models. The DCB model predicts in fact

C2H2 > C2H6 > CH4 > C2H4 while the MKGH model predicts

C2H6 > C2H4 > C2H2 > CH4 Thus, under the operating conditions considered here, the DCB model overestimates the C2H2 reactivity, while the MKGH model overestimates that of C2H4. Some insights about the behavior of these models under these conditions can be obtained from a rate of production analysis carried out at two temperature values: 1300 and 1000 K. It should be stressed that the results of the analysis reported in the following are representative

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Figure 13. Reaction paths for DCB (top) and MKGH (bottom) kinetic mechanisms at 1300 K. Arrow dimensions roughly indicate the relative importance of the different patterns.

C2H2 f C2H3 f CH2O f HCO f CO while the MKGH model also involves the following reaction patterns:

C2H2 f HCCO f CO Figure 12. Comparison between conversion experimental data (curves with symbols) and model predictions (curves without symbols) for the equimolar fuel mixture (run IV in Table 1 with Φ ) 1.0).

only for the conditions investigated, that is, those of run IV with Φ ) 1 (see Table 1). By analyzing the production and consumption rates of different fuels and intermediate products, the reaction patterns reported in Figures 13 and 14 have been produced. The arrow dimensions roughly indicate the relative importance of the various reaction paths. It can be seen that at high temperature (Figure 13) the two mechanisms exhibit a rather similar behavior. Methane is consumed through the reaction sequence

CH4 f CH3 f CH2O f HCO f CO while the ethane oxidation proceeds through C2 hydrocarbon intermediates leading to the kinetic path

C2H6 f C2H5 f C2H4 f C2H3 f C2H2 f ... f CO No direct interchange between CH4 and C2 hydrocarbons is evidenced. However, under these conditions the conversion of all the fuels is almost complete, and the differences between the predictions of the two models are hard to detect. Different conclusions are reached by analyzing the results at low temperature (Figure 14). Here the two models behave in fact in a different way. The DCB model predicts that acetylene is consumed preferentially through the path

C2H2 f CH2 f CO2 It is apparent that the only effective path of the DCB model is too fast, thus leading to acetylene conversion values, ηC2H2, larger than the experimental ones (as seen in Figure 12A). On the other hand, both mechanisms involve the same reaction pattern for C2H4 consumption and production, but with different relative values. The DCB model predicts an overall production rate of ethylene larger than its consumption rate, thus leading to negative conversion values, in agreement with those measured experimentally. The opposite is true for the MKGH model, whose ethylene consumption rate is higher than the production one. On the whole it can be concluded that the submechanism of the DCB model relative to C2H2 requires appropriate modifications, as well as that involving C2H4 for the MKGH model. However, this is only an example of the results that can be obtained through this kind of analysis. A deeper investigation, leading to a modification of the kinetic mechanisms considered, is out of the scope of this paper. Conclusions In this work, experimental data have been reported in operating windows not previously investigated in the literature, particularly referring to mixtures of fuels. This should allow a reliable estimation of the interconnecting reaction paths among C1 and C2 hydrocarbons, which exhibit rather different relative importances at


appropriate modifications, as well as that involving C2H4 for the MKGH model. In conclusion, an improvement of the description of combustion processes through detailed kinetic models could be pursued by looking first for a qualitative agreement between well-established experimental trends and model predictions under conditions involving various ratios between reactant concentrations. This would enable one to evidence reaction patterns not correctly considered in a kinetic model. With this regard, the experimental results discussed in this work would help such an improvement. Acknowledgment The help of C. Benedetti, R. Fiandaca, M. Maconi, and M. Rossi in developing this work, as well as the financial support of ENEL-CRTN, Pisa (Italy), is gratefully acknowledged. Nomenclature

Figure 14. Reaction paths for DCB (top) and MKGH (bottom) kinetic mechanisms at 1000 K. Arrow dimensions roughly indicate the relative importance of the different patterns.

different concentrations of reactants. The study of the oxidation of hydrocarbon fuel mixtures has been able to evidence lack of agreement between the predictions of two currently available detailed kinetic models and experiments, which are not identified when a single fuel oxidation is considered. Moreover, several replications of experiments performed in other laboratories have been performed; this adds confidence to the experimental results presented and provides data for use in the improvement of detailed kinetic models. Thus summarizing, the main findings of this work are as follows: (1) methane conversion is strongly influenced by ethane concentration; in particular, the presence of ethane increases the methane reactivity up to a maximum value. Above this value, higher ethane concentrations do not affect methane conversion; (2) methane depresses ethane reactivity: the larger the amount of methane, the lower the ethane conversion; (3) in general, ethane conversion is larger than that of methane when they are fed together, particularly for low temperature values; (4) intermediate species (i.e., CO, C2H4, and C2H2) show a competition between pyrolysis and oxidation reactions and are more or less sensitive to temperature depending on the compound considered. This leads to different trends of the intermediate selectivity with temperature; (5) the sequence of relative reactivity of C1 and C2 hydrocarbons fed together differs from the expected one; under these conditions the reactions leading to ethylene production are more effective than those involved in ethylene decomposition to acetylene or to CO; (6) the detailed kinetic models considered fail to predict the relative reactivity sequence found experimentally when methane is fed together with ethane, ethylene, and acetylene. Production rate analysis indicates that the submechanism of the DCB model relative to C2H2 requires

Ai ) pre-exponential factor of the ith reaction, cm3 mol s K Ci ) concentration of the ith species, mol/cm3 Ei ) activation energy of the ith reaction, J/mol ki ) reaction rate constant of the ith reaction, cm3 mol s MER ) methane/ethane molar ratio in the feed ni ) number of carbon atom in the ith molecule NF ) number of fuels NS ) number of chemical species Q ) volumetric flow rate entering the reactor, cm3/s R ) ideal gas constant ri ) rate of production of the ith compound, mol/(cm3 s) T ) temperature, K V ) reactor volume, cm3 xi ) mole fraction of the ith species Greek Letters βi ) temperature exponent of the ith reaction γi ) selectivity of the ith product defined by eq 2 ηi ) conversion of the ith reagent defined by eq 1 Φ ) stoichiometric ratio, (fuel/air)/(fuel/air)St τ ) residence time, s Superscripts in ) inlet out ) outlet

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Received for review November 14, 1995 Accepted April 16, 1996X IE950689U

X Abstract published in Advance ACS Abstracts, June 1, 1996.

Experimental Study and Kinetic Analysis of the Oxidation of Light

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