Energy Fuels 2009, 23, 5287–5289 Published on Web 09/17/2009
: DOI:10.1021/ef900815j
Lumped Kinetic Modeling of the Oxidation of Isocetane (2,2,4,4,6,8,8Heptamethylnonane) in a Jet-Stirred Reactor (JSR) E. Ranzi,* A. Frassoldati T. Faravelli, and A. Cuoci Dipartimento di Chimica, Materiali e Ingegneria Chimica Politecnico di Milano Received July 30, 2009. Revised Manuscript Received September 9, 2009 Introduction In a recent publication Dagaut and Hadj-Ali1 discussed the kinetics of oxidation of isocetane (2,2,4,4,6,8,8-heptamethylnonane). They carried out experiments in a jet-stirred reactor at 10 atm and a constant residence time of 1 s, over the temperature range of 770-1070 K, and for variable equivalence ratios (0.5, 1, and 2). These data were also modeled using two different computer-generated semidetailed kinetic reaction mechanisms: the POLIMI Scheme2 and the Nancy Scheme.3 Dagaut and Hadj-Ali1 presented comparisons between experimental and computational results, using both the mechanisms. Unfortunately, the CHEMKIN simulations they performed with the POLIMI scheme were affected by an error because of convergence problems.4 On the basis of these completely erroneous simulations (Figure 1a), they concluded that the model proposed by Agosta et al.2 poorly represents the experimental results. They also attributed this erroneous behavior to the lumping procedures and to the strong simplifications. Figure 1b shows the correct comparisons between model predictions and experimental data, when the proper convergence is attained. The agreement is quite good, the reactivity of the model is in line with the measurements, as the profiles of the main oxidation products (CO, CO2 and H2O) testify. Lumped Kinetic Model The POLIMI kinetic model was already discussed in previous papers2,5 and it is also available on the web [http://www.chem. polimi.it/creckmodeling/]. The overall set of primary propagation reactions of decomposition and oxidation of isocetane (2,2,4,4, 6,8,8-heptamethyl-nonane) are automatically obtained by using the MAMOXþ program.6 The detailed kinetic scheme involves 78 new intermediate radicals: 8 alkyl and 8 peroxy radicals, 31 alkyl hydroperoxyl, and 31 peroxy alkylhydroperoxy radicals. The complexity of this scheme becomes more evident when considering the need to properly describe the successive reactions of a large number of intermediate species. In fact, 9 branched pentadecene isomers are obtained by demethylation reactions of alkyl radicals or via decomposition reactions of the 31 alkyl hydroperoxy radicals. Similarly, 3 alkenes, 8 hydroperoxides,
Figure 1. Isocetane oxidation at Φ=1. Panel a: Erroneous simulation (solid lines).1 Panel b: Correct simulation with the POLIMI model (dashed lines).
17 cyclic ethers, and 25 ketohydroperoxides retaining the initial carbon skeleton of the isocetane fuel are also formed. Therefore, it is convenient to use a semi-detailed approach, directly lumping in the initial stage the different isomers and considering in this way only one equivalent olefin and only one cyclic-ether. The lumping rule, that is, the definition of the isomer mixture, is based on the results of the detailed mechanism. The overall initial distribution of the different primary products is carried out on the basis of the detailed model in a wide range of temperature and pressure conditions. The initial selectivity of the primary products (i.e., moles of product/100 moles decomposed at conversion approaching zero) is predicted by solving the linear system of continuity equations for all of the intermediate radicals (steadystate approximation). While the details of this reduction approach are reported in Ranzi et al.,7 Figure 2 shows the resulting selectivities of the primary products of isocetane oxidation as a function of the temperature at 1 atm. Thus, at 800 K the initial selectivity to form decomposition products is ∼30-35%, cyclic ethers still are ∼30-35%, β-scission products are ∼20%. Parent
*To whom correspondence should be addressed. E-mail: eliseo.
[email protected]. Phone: 39 02 2399 3250. (1) Dagaut, P.; Hadj-Ali, K. Energy Fuels 2009, 23, 2389–2395. (2) Agosta, A.; Cernansky, N. P.; Miller, D. L.; Faravelli, T.; Ranzi, E. Exp. Therm. Fluid Sci. 2004, 28 (7), 701–708. (3) Battin-Leclerc, F.; Bounaceur, F.; C^ ome, G. M.; Fournet, R.; Glaude, P. A.; Scacchi, G.; Warth, V. EXGAS-ALKANES:A Software for the Automatic Generation of Mechanisms for the Oxidation of Alkanes; CNRS: 2004. (4) Dagaut, P. Personal communication. (5) Ranzi, E.; Dente, M.; Goldaniga, A.; Bozzano, G.; Faravelli, T. Prog. Energy Combust. Sci. 2001, 27, 99–139. (6) Ranzi, E.; Faravelli, T.; Gaffuri, P.; Garavaglia, E.; Goldaniga, A. Ind. Eng. Chem. Res. 1997, 36 (8), 3336–3344. r 2009 American Chemical Society
(7) Ranzi, E.; Faravelli, T.; Gaffuri, P.; Sogaro, A.; D’Anna, A.; Ciajolo, A. Combust. Flame 1997, 108, 24–42.
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Energy Fuels 2009, 23, 5287–5289
: DOI:10.1021/ef900815j remaining radicals (QOOHI16I) follow the low temperature mechanism with the formation of unstable ketohydroperoxide components (OQOOHI16). From this figure, the limited role of these branching reactions is clear. The low temperature mechanism should then be active only at temperatures lower than 750 K, in agreement with the indications of Dagaut and Hadj-Ali.1 Table 1 summarizes the lumped kinetic scheme of isocetane. The kinetic parameters of the lumped reactions are estimated by minimizing the differences between primary selectivities predicted with the lumped and detailed kinetic models in the whole range of investigated temperatures and pressures.
alkenes, branching and homolysis products sum up to the remaining ∼15%. The large selectivity of the cyclic-ether fraction in the intermediate temperature region is clearly evident. Following the approach already discussed in the analysis of isooctane oxidation,7 it is convenient to distinguish two different lumped alkyl-hydroperoxy radicals to take into account that tertiary radicals (QOOHI16T) go toward homolysis reactions, while the
Comparisons with Experimental Measurements of Dagaut and Hadj-Ali1 Figure 3 shows the comparisons between model predictions and experimental data1 obtained for isocetane oxidation in rich, stoichiometric, and lean conditions. A quite good agreement is generally observed for major species, as well as for minor components. The experimental reactivity is not significantly influenced by the stoichiometry. On the contrary, the equivalent ratio affects the predicted isocetane conversion. The reactivity is under-predicted in rich conditions, while a better agreement is obtained in stoichiometric and lean conditions. As already observed by Dagaut and Hadj-Ali,1 the availability of these new experimental data could allow
Figure 2. Initial selectivity of primary products of isocetane oxidation at 1 atm. Comparison between predictions of detailed (points) and lumped models (lines).
Figure 3. Isocetane oxidation at 10 bar and Φ = 0.5, 1, and 1.5. Mole fractions vs JSR temperature [K]. Comparisons between experimental data1 (points) and model predictions (lines).
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: DOI:10.1021/ef900815j
Table 1. Lumped Kinetic Model of Isocetane2 [Units: L, mol, s, cal] A RI16 f 0.3375C4H8 þ 0.5975C8H16 þ 0.065C3H6 þ 0.18C4H9 þ 0.125C4H9O2 þ 0.065C5H11 þ 0.13CH3 þ 0.5RI8 þ 0.5C8H16 O2 þ RI16 f HO2 þ C16H32 O2 þ RI16 f RI16OO RI16OO f RI16 þ O2 RI16OO f QOOHI16I QOOHI16I f RI16OO QOOHI16I f OH þ 0.5CH2O þ 0.5C4H8O þ 0.5C3H6 þ 0.475C4H8 þ 1.2C8H16 þ 0.25C2H4 QOOHI16I f cy-C16H32O þ OH QOOHI16I f HO2 þ C16H32 QOOHI16I þ O2 f OOQOOHI16I OOQOOHI16I f QOOHI16I þ O2 OOQOOHI16I f OQOOHI16 þ OH OQOOHI16 f OH þ 0.5C3H5O þ 0.5C2H3O þ 0.25CH2O þ 0.25C2H4O þ 0.25C3H6O þ 0.25C4H8O þ 0.5625C4H8 þ 0.25C3H6 þ C8H16 RI16OO f QOOHI16T QOOHI16T f RI16OO QOOHI16T f OH þ 0.5CH2O þ 0.5C4H8O þ 0.5C3H6 þ 0.475C4H8 þ 1.2C8H16 þ 0.25C2H4 QOOHI16T f cy-C8H16O þ OH þ C8H16 QOOHI16T f HO2 þ C16H32 QOOHI16T þ O2 f OOQOOHI16T OOQOOHI16T f QOOHI16T þ O2 OOQOOHI16T f HO2 þ C4H8O þ 0.25C4H8 þ C3H6O þ C8H16 OOQOOHI16T f OH þ CH2O þ C3H6O þ 0.79C4H8O þ 0.21C3H6O þ 0.0525C4H8 þ C8H16
E 14
1.0 10
29 000
2.5 108 2.0 109 6.3 1013 1.0 1012 1.2 1012 1.0 1012
3500 0 30 800 24 000 20 500 21 500
3.0 1010 1.5 1011 2.0 109 2.0 1013 2.0 1010 1.0 1016
14 500 21 000 0 30 500 24 000 41 000
1.3 1012 26 500 1.2 1011 19 500 2.0 1012 22 500 2.0 1010 1.5 1011 2.0 109 6.3 1013 2.5 109
Figure 5. Stanford shock tube experiments11,12 Panel a: CH3 radical formation from isooctane. Comparisons between model predictions (line) and experimental measurements (symbols). Panel b: Variation of peak XOH vs reflected shock temperature. Comparisons between model predictions (circles) and experimental measurements (triangles).
15 000 21 000 0 30 000 22 500
similar butadiene overpredictions were also observed with the “detailed” kinetic scheme of LLNL.9 Specific investigations should be addressed both from the experimental and modeling point of view to clarify this aspect. To complete the picture, Figure 4 shows the predicted isocetane conversion in a wide temperature range, at 10 bar and 1 s of residence times. Predicted results show that the effect of low temperature mechanism (TOT0810) becomes evident at temperatures lower than ∼800 K. Model predictions indicate a significant reactivity of the system in the temperature range 600-700 K. This reactivity is primarily dictated by the lumped isomerization reactions of peroxy radicals. Mainly this low temperature mechanism could greatly benefit from experimental verification, being of interest in the engine fields.
1.5 1012 27 500
Reliability of the Lumped Kinetic Model
Figure 4. Isocetane conversion in the JSR at 10 atm and 1 s contact time. Model prediction (TOT0810, solid lines) and high temperature model (HT0811, dashed lines).
Over several years, the lumped POLIMI kinetic scheme5-8 was validated in a wide range of pyrolysis and combustion conditions, and we are involved in a continuous work of refinements and extensions toward new fields of application of the model. Because of the lumping approach, this kinetic model allows to face also the kinetics of real liquid fuels where complex mixtures of large hydrocarbons are involved.10 Isocetane is only one of these examples, where a reasonable agreement is obtained in a predictive way. The reliability of the lumped model has also been recently verified by the Stanford research team in shock tube experiments.11,12 Figure 5 shows that the model is able to satisfactory predict methyl radical profiles in the oxidation of isooctane, as well as the OH radical history in n-dodecane combustion.
a further refinement of the kinetic model. Despite the lumping approach and the severe simplifications, the resulting agreement with several intermediate species (e.g., CO, CH2O, methane, and isobutene) is noteworthy, mainly considering that the mechanism was only built on predictive basis. While selectivities of major species are well predicted, over predictions of propylene and corresponding under predictions of ethylene indicate that the stoichiometry of the primary lumped reactions of Table 1 could be better refined on the basis of these experimental data. Some apparent deviations mainly at Φ = 2 also derive from the underestimation of reactivity. Relevant deviations are present for minor species such as acetylene and mainly butadiene. It is possible to note that similar discrepancies with experimental data obtained in the same JSR were already observed with large n-alkane oxidation.8 Possibly, this disagreement cannot be entirely referred to the lumping and simplifications because very
Acknowledgment. The authors acknowledge the direct and fair cooperation of Philippe Dagaut. (9) Curran, H. J.; Gaffuri, P; Pitz, W. J.; Westbrook, C. K. Combust. Flame 1998, 114, 149–177. (10) Ranzi, E. Energy Fuels 2006, 20 (3), 1024–1032. (11) Vasu, S. S.; Davidson, D. F.; Hong, Z.; Vasudevan, V.; Hanson, R. K. Proc. Combust. Inst. 2009, 32, 173–180. (12) Davidson, D. F.; Oehlschlaeger, M. A.; Hanson, R. K. Proc. Combust. Inst. 2007, 31, 321–328.
(8) Ranzi, E.; Frassoldati, A.; Granata, S.; Faravelli, T. Ind. Eng. Chem. Res. 2005, 44 (14), 5170–5183.
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