A wide range kinetic modeling study of alkene oxidation

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1 A wide range kinetic modeling study of alkene oxidation M. Mehl 1, T. Faravelli 1, E. Ranzi 1, A. Ciajolo 2, A. D'Anna 3, A. Tregrossi 2 1. CMIC-Politecnico di Milano ITALY 2. Istituto Ricerche sulla Combustione CNR, Napoli - ITALY 3. Dipartimento di Ingegneria Chimica - Università Federico II, Napoli ITALY 1. Introduction The proper comprehension of the chemistry and mainly of the low temperature oxidation mechanisms is fundamental for the study and prediction of autoignition phenomena in engines. The efficiency of SI engines is thermodynamically conditioned by the operating compression ratio which is limited by critical knock conditions. For this reason the capability of predict knock occurrence in SI engines and autoignition in HCCI ones through the kinetic characterization of gasoline behavior, can be a first important step for a better gasoline formulation, higher engine performances and lower pollutants emissions. Gasoline is a complex mixture of hydrocarbons where alkenes may represent a relevant fraction. Even if they play an important role in combustion kinetics, there are few experimental data and little kinetic modeling in this field if compared with the oxidation of linear and branched alkanes that has been extensively studied. This work presents a first attempt to the modeling of oxidation and combustion of large alkenes. Particular attention is devoted to 1-pentene, chosen as a reference species of alkenes. The main features of the model are presented and some comparisons with experimental data are also shown. 2. The Kinetic Scheme The kinetic scheme here discussed is based on a hierarchical concept and modularity. The core of the model is constituted by a detailed sub-mechanism for C 1 -C 4 species, while the characteristic modular structure allows the introduction of different reaction subsystems, necessary to describe the oxidation and the pyrolysis of higher hydrocarbons in the different conditions or the formation of specific pollutants. Consequently, progressive extensions of the main oxidation mechanism is achieved by introducing new reaction sub-schemes. Assuming analogy rules for similar reactions, only a few fundamental kinetic parameters are needed to extend the scheme to heavier species. These intrinsic rate parameters define the main classes of primary propagation reactions, appropriate to the different temperature ranges The main peculiarity of this kind of schemes is that they cover a wide range of operating conditions of hydrocarbon oxidation, particularly the high and low temperature regions. For these reasons this work presents just some relevant features of the low temperature module of alkenes, while a detailed discussions of the model can be found elsewhere [1]. 3. Low Temperature Reactivity of Alkenes The importance of the low temperature oxidation mechanism of alkenes is the clear result of their presence in all the combustion processes of saturated species. Once again the mechanism involve the initial formation of alkenyl radicals and their successive oxidations with the possible degenerate branching path with peroxide and ketohydroperoxide species. Two major differences can be highlighted in respect of alkane oxidation mechanism. The first is the formation of very stable allylic radicals, while the latter is the possibility to add propagating radicals on the double bond. A very simplified reaction scheme is reported in figure 1. This standard scheme includes the low, intermediate and high temperature reactions. IV5.1

2 29th Meeting on Combustion As already mentioned, one of the reasons for the lower reactivity of alkenes, when compared to alkanes, is the formation of allyl radicals. As a consequence of their relative stability, these radicals reach high concentrations and are available for oxygen addition and/or recombination reactions. Moreover, the O2 addition reactions need to overcome this stability and also the corresponding allyl-peroxy radicals could decompose in a favored way. On these bases, the ceiling temperature is expected to shift at lower temperatures with the result of a limited importance of the low temperature mechanism. This mechanism always proceeds via internal isomerization, oxygen addition and finally ketohydroperoxides formation, similarly to the alkane oxidation path. The other reaction relates to radical addition on the double bond. These reactions mainly involve H, OH or HO2 radicals, respectively leading to the formation of alkyl radicals, hydroxyalkyl or hydroperoxyalkyl radicals which further contribute to the low temperature reactivity. In agreement with previous kinetic analysis [2,3], below 65K the foremost reaction pathway involves the formation of hydroxyalkyl radicals and the successive oxygen addition. Due the high electronegativity of oxygen, the net result of this O 2 addition is the final decomposition to form OH radical and two aldehydes, via Waddington mechanism: OH OH + OH + O 2 OO -OH The successive reactions of alkyl and hydroperoxyalkyl radicals obtained via H and HO 2 additions are already discussed and considered in the oxidation mechanism of alkanes. Radical addition reactions prevail on the corresponding H-abstraction reactions mainly for light alkenes, such as ethylene and propylene. The limited reactivity of propylene, when compared to heavier alkenes, is mainly due to the difficulty of the isomerization of the allyl peroxy radical through the internal abstraction of the vinyl H-atoms. As already observed by Prabu [4], above butenes and pentenes H abstraction reactions, mainly if not only limited to the allyl H-atoms, prevail on the corresponding radical addition reactions. R + H Q (C n H 2n ) + HO 2 QOOH -H +OH O + O S β-dec. products QOH HO 2 + D n SOO CH 2 CHO + C k H 2k O OOQOH HO 2 + QOH OH + Ethers OH + RCHO DOOH Waddington Mechanism + D k OODOOH ODOOH + OH (branching) Fig. 1 Main oxidation pathways of alkenes

3 Italian Section of the Combustion Institute In this reaction scheme Q represents the alkene (C n H 2n ) and S the parent radical while D k identifies the conjugated dialkene with k C-atoms (C k H 2k-2 ) and R is the alkyl radical. It should also be noted that the large amount of resonantly stabilized species acts as radical scavengers so reducing the reactivity of the system. 4. Results and Discussion This kinetic scheme has been validated on the basis of different experimental data concerning the oxidation of 1 pentene both in the low and high temperature range. In order to validate the scheme in a broad range of operating conditions different type of experimental measurements have been taken into account: pressurized flow reactors (PFR) data [4], Rapid Compression Machine (RCM) data [3] as well as data obtained in a Jet Stirred Flow Reactors (JSFR)[5]. These results are also compared with similar data obtained with n-pentane in order to better highlight the reactivity of alkenes Pressurized Flow Reactor (Drexel University) A first set of comparisons concerns pressurized flow reactor experiments [4]. These data provide information in a range of temperature between 65K and 8K. The reactivity of a 1- pentene/o 2 /N 2 mixture is analysed in a flow reactor in controlled cool down (CCD) experiments and a reactivity curve is obtained with the main oxidation products at different temperatures and constant residence time (~15-2 ms). Another set of experimental data is obtained by sampling the composition of the reacting mixture at different positions in the reactor providing information about composition vs. residence time at Constant Inlet Temperature (CIT). Molar Fraction Time [s] Fig. 2 Main oxidation products from 1-Pentene oxidation in PFR ( C 5 H 1 : red, CH 2 O: blue, CO: green, CO 2 : light blue): a) CCD experiments and simulations; b) CIT experiments and simulations. The experimental data clearly show a reactivity curve with a NTC region and 1-pentene conversion peaks at about 7K. The kinetic scheme is able to reproduce this experimental behavior, as shown in figure 2. Model predictions of water, formaldehyde and carbon monoxide, major oxidation products, well agree with the experimental measurements. Figure 3 shows the time evolution of the mixture at 673K at constant inlet temperature (CIT). Model prediction well reproduce the main features of low temperature oxidation of 1-pentene, both in terms of overall reactivity and in terms of product selectivity, also in these conditions. IV5.3

4 29th Meeting on Combustion 4.2. Rapid Compression Machine (CNRS Lille) Rapid compression machine experiments allow to study the induction time of an air-fuel mixture in conditions similar to the ones usually applied in internal combustion engines. Minetti and coworkers studied the autoignition behavior of 1-pentene in RCM and compare the ignition delay times of 1-pentene and n-pentane in a range of temperature between 6K and 9K at about 7atm [3]. They reported both the total ignition time and also the low temperature ignition time, if detected. Simulations show a NTC region between 74K and 8K in agreement with experimental measurements. The typical ignition time of 1-pentene in this region are about 3-5 ms, while cool flames can be observed, with characteristic times of 1-3 ms, up to about 8 K. Autoignitio Delay [ms] Fig. 3 Autoignition delay times of 1-pentene in RCM at 7atm [3]. 5. Jet Stirred Flow Reactor (Naples University) A further set of experimental data refers to the oscillating oxidation of 1-pentene in a JSFR. At low temperatures, the consumption of 1-pentene occurs trough slow reactions. If inlet temperature increases, the temperature rises with the formation of cool flames, due to the accumulation and decomposition of hydroperoxide species. Characteristic oscillations in reactor temperature are obtained for this reason. Panel a of figure 5 reports the typical oscillations of the reactor temperature in the NTC region, at about 62 K. At inlet temperatures higher than 635 K, both experimental and model evaluations evidence damped oscillations and the system converges to a steady solution. Model predictions reproduce the experimental observations in more than a qualitative way. 683 a) b) Time [s] 1 2 Fig. 5 Oscillating oxidation of 1-pentene in JSFR. Red lines: Model predictions; blue lines: experimental measurements [5]

5 Italian Section of the Combustion Institute 6. n-pentane and 1-Pentene Comparisons A comparison between n-pentane and 1-pentene reactivity allows to focus some interesting features of alkenes reactivity. The following simulations refer to RCM and ST conditions. In the low temperature region, 1- pentene autoignition is slower than the corresponding one of n-pentane. On the contrary, at higher temperatures, the reactivity of alkenes increases and their autoignition becomes faster. The predicted results are reported in panel a of Fig. 6, and fully confirmed also by the experimental measurements of Minetti et al. [3]. Similar effects have been also experimentally detected by Wilk et al. [6], who discussed the behavior of propane and propene in static reactors. Panel b of the same figure shows that, according to model predictions, at very high temperature (T>15 K) once again the reactivity of alkenes turn to be slightly lower in respect of alkanes, at both the investigated pressures. The same trend has been also verified for C4 and C3 species. This high temperature behavior could be explained on the basis of the H/C ratio of the different fuels. Therefore the differences vanishes with heavier fuels. Autoignitio Delay [s] a) b).1.1 1e e /T [1/K] Fig. 6 Comparisons of ignition delay times of 1-pentene (red) and n-pentane (blue): a) RCM at 7 atm (see conditions of Fig.2 [3], b) Stoichiometric fuel-air in Shock Tube at 1 and 2 atm (solid and dashed lines). Similarly, the lower reactivity of alkenes at low temperatures is less relevant when the number of C-atoms in the hydrocarbon chain increases. This trend is coherent with the octane number which rapidly decreases for the heavier fuels. Due to the lower reactivity of alkenes at low temperatures, the octane number of pentenes and hexenes is higher than the ON of the corresponding alkanes. Moreover, alkenes are characterized by a high octane sensitivity, that is a a parameter defined as the difference between octane numbers in different standard engine conditions (RON and MON) [7]. The slow reactivity in the low temperature range reduce the knock propensity of such compounds, particularly in RON conditions, where operating temperatures are lower. This characteristic of light alkenes make them interesting candidates as additives for the formulation of new gasolines. 7. Conclusions This work presents some preliminary results mainly related to the modeling of the low temperature oxidation of 1-pentene. The main features of this mechanism are analysed focusing on the central role of allyl radical and OH radical additions. Experimental results and model predictions of 1-pentene oxidation have been reported in a wide range of operating conditions from 6 up to 16K and from 1.5 to 7 atm. IV5.5

6 29th Meeting on Combustion 1 Octane Number Hydrocarbon chain lenght Fig. 7 Octane number of alkanes (solid lines) and alkenes (dashed lines) in RON (blue) and MON (red) conditions [7] The general good agreement both in terms of reactivity and selectivity confirms the validity of the model. The reactivity of 1-pentene and n-pentane have been also compared in order to better understand the role of alkenes in internal combustion engines. Alkenes are less reactive than parent alkane in the NTC region but they more rapidly ignite at high temperatures. On similar basis it is possible to explain the high octane number sensitivity of small alkenes. 8. Acknokledgments Authors gratefully acknowledge the financial support of Enitecnologie, S. Donato (Milano) 9. References 1. E. Ranzi, M. Dente, A. Goldaniga, G. Bozzano, T. Faravelli, Prog. Energy Combust. Sci. 27:99 (21). 2. S. Touchard, R. Fournet, P.A. Glaude, V. Warth, F. Battin-Leclerc, G. Vanhove, M. Ribaucour, R. Minetti Proc. Comb. Institute 3 (25) R. Minetti, A. Roubaud, E. Therssen, M. Ribaucour, And L. R. Sochet, Combust. Flame, 118: (1999). 4. Prabhu S. K., Bhat R. K., Miller D. L., Cernansky N. P., Combust. Flame, 14:377 (1996). 5. P.F. Corsaro, Autoignizione di 1-pentene, ciclopentano e ciclopentene in un reattore continuo a perfetta miscelazione, Tesi di laurea. Università Federico II, R.D. Wilk, B.P. Cernansky, W.J. Pitz, C.K. Westbrook, Combust. Flame 77 (1989) ASTM data series DS 4B: Physical Constants of Hydrocarbon and Non-Hydrocarbon Compounds. American Petroleum Institute, Philadelphia (1988).

INFLUENCE OF THE POSITION OF THE DOUBLE BOND ON THE AUTOIGNITION OF LINEAR ALKENES AT LOW TEMPERATURE

INFLUENCE OF THE POSITION OF THE DOUBLE BOND ON THE AUTOIGNITION OF LINEAR ALKENES AT LOW TEMPERATURE R. BOUNACEUR, V. WARTH, B. SIRJEAN, P.A. GLAUDE, R. FOURNET, F. BATTIN-LECLERC * Département de Chimie-Physique