Ethane pyrolysis mechanism

For a system containing a larger number of atoms, the general picture of the potential energy surface and the transition state also applies. For example, in the second reaction step in the mechanism of ethane pyrolysis in Section 6.1.2,... 

The reaction mechanism for this process is fairly well established and may serve as a more complex example of a chain reaction. The initiation step in ethane pyrolysis is thermal dissociation of the ethane molecule,... 

Consequently the overall reaction is of order 0.5 in [C2H6]. Equation 13.42 only holds for a low conversion at higher conversions of ethane into ethylene and hydrogen, subsequent reactions of these components would have to be considered. Ethane pyrolysis is another example that global parameters can be inferred from knowledge of the detailed chemistry. Global rate measurements can then be used to verify the proposed mechanism. 

The enumeration of all the possible reactions involving radicals and molecules in the ethane system would be a tedious task, but one is not really justified in accepting a mechanism for the ethane pyrolysis until such an exhaustive inquiry has been completed. On the other hand, at our present stage of knowledge, the detailed investigation is impracticable if not impossible. The Rice-Herzfeld principle presents about as practical and complete a guide as is at present warranted for the economical discussion of hydrocarbon reactions. However, even this scheme for the ethane pyrolysis [Eq. (XIII. 10.4)] has been considerably shortened in the discussion already presented [Eq. (XIII.10.5)], and we may now go back and look at some of the reactions which have been neglected in the latter, simplified chain. 

The kinetic scheme for the low-temperature photolysis is almost hopeless at our present state of knowledge of the elementary steps involving Clio and CII3CO radicals. The scheme is even more complicated than that for the ethane pyrolysis, and as noted earlier, the products are certainly more complicated. It is interesting to note that, where the products are simple because of a long chain, the kinetics become extremely sensitive to walls and impurities. On the other hand, at lower temperatures at which chains arc shorter and the reaction is not so sensitive to walls, etc., the chemical complexity of the products becomes important and the investigations just as difficult. With all the work that has been done on CHsC HO (pyrolysis and photolysis), the elementary mechanism is known with some assurance only at the higher temperatures, and even here the initiation processes are subject to ciuestion. The evidence for three-... 

The general treatment the results of which are summarized in Table 11 has proved very valuable in deciding on possible mechanisms to fit a given set of experimental results. It must always be borne in mind, however, that Table 11 only includes extreme cases, and that intermediate situations are possible. It will be seen later, for example, that in the ethane pyrolysis the predominant termination step is... 

Since the conversion of C2H5 into C2H4 + H is indeed the slow step in the ethane pyrolysis, the occurrence of this reaction does explain the non-zero rates at maximal inhibition and the increase in rate at high NO concentrations. On the other hand, for reactions like the acetaldehyde pyrolysis the g - P transition is not rate limiting, and the Norrish-Pratt mechanism then gives no explanation for the behavior. Also, the Norrish-Pratt mechanisms as originally written down do not explain the large amounts of products such as H2O, N2 and N2O that are found in the ethane pyrolysis. 

The fact that there is a significant increase in the rate of methane formation shows that the NO is providing some entirely different mechanism for CH4 production. In this connection, it is interesting that in the ethane pyrolysis these is no HCN, which is formed in significant amounts in the acetaldehyde decomposition. A likely source of HCN is... 

The K-butane pyrolysis is analysed here as an initial, simple example of a pyrolysis reaction mechanism. It is important to note that the pyrolysis reactions of small hydrocarbons are fundamental to the proper understanding of the whole process. In fact, the pyrolysis mechanism displays a typical hierarchical structure and the small hydrocarbons must be analysed first. Fig. 1 shows the main and minor products from K-butane decomposition, under isothermal conditions, at 1,093 K and 1 atm. Ethylene, propylene and methane are the main products, while only trace amount of butenes, ethane, benzene and cyclopenta-diene are observed. These model predictions have been confirmed and validated by several experimental measurements (Dente and Ranzi, 1983). 

This example illustrates how to apply the QSSA to a flow reactor. We are interested in determining the effluent concentration from the reactor and in demonstrating the use of the QSSA to simplify the design calculations. Ethane pyrolysis to produce ethylene and hydrogen also generates methane as an unwanted reaction product. The overall stoichiometry for the process is not a simple balance of ethane and the products. The following mechanism and their kinetics have been proposed for ethane pyrolysis 22]... 

In recent years, most workers have adopted an abridged version of the Rice-Herzfeld mechanism(1-5) for ethane pyrolysis... 

The thermal decomposition of pure ethane is a much studied reaction, and the mechanism is well defined. For purposes of comparison with the oxygen reactions, the ethane pyrolysis is redescribed below. 

Matheu, D.M., Grenda, J.M. A systematically generated, pressure-dependent mechanism for high-conversion ethane pyrolysis. 1. Pathways to the minor products. J. Phys. Chem. A 109, 5332-5342 (2005a)... 

Taylor in 1925 demonstrated that hydrogen atoms generated by the mercury sensitized photodecomposition of hydrogen gas add to ethylene to form ethyl radicals, which were proposed to react with H2 to give the observed ethane and another hydrogen atom. Evidence that polymerization could occur by free radical reactions was found by Taylor and Jones in 1930, by the observation that ethyl radicals formed by the gas phase pyrolysis of diethylmercury or tetraethyllead initiated the polymerization of ethylene, and this process was extended to the solution phase by Cramer. The mechanism of equation (37) (with participation by a third body) was presented for the reaction, - which is in accord with current views, and the mechanism of equation (38) was shown for disproportionation. Staudinger in 1932 wrote a mechanism for free radical polymerization of styrene,but just as did Rice and Rice (equation 32), showed the radical attack on the most substituted carbon (anti-Markovnikov attack). The correct orientation was shown by Flory in 1937. In 1935, O.K. Rice and Sickman reported that ethylene polymerization was also induced by methyl radicals generated from thermolysis of azomethane. 

Kassel67 proposed an activation energy of 44 kcal. for the reaction on the basis of an uncertain mechanism for CH4 pyrolysis, and Bawn3 reported that ethane was not formed when methylene was (presumably) produced in the presence of methane at 300°C. by reaction of Na vapor with CH2Br2. 

However, many reactions, although their mechanism may be quite complex, do conform to simple first or second-order rate equations. This is because the rate of the overall reaction is limited by just one of the elementary reactions which is then said to be rate-determining. The kinetics of the overall reaction thus reflect the kinetics of this particular step. An example is the pyrolysis of ethane(4> which is important industrially as a source of ethylene01 (see also Section 1.7.1 Example 1.4). The main overall reaction is ... 

Question. In Problem 6.12 where the mechanism for the pyrolysis of ethanal is... 

The steam pyrolysis of LPG follows the same pathway of that for ethane, namely by a complex branching chain free radical mechanism. This can be divided into initiation, chain propagation and chain termination reactions. This gives rise to a large number of intermediates and products. As with ethane, products of higher carbon number than the feed are formed. 

Historically, of the various hydrocarbon pyrolyses, that of ethane has been the object of the greatest amount of investigation. As a consequence more is known about it than about the pyrolysis of any other hydrocarbon. Even more important, sufficient thermal and kinetic data are now available so that the rates (and activation energies) of various chain mechanisms can be calculated with about order of magnitude reliability. 

If we consider in retrospect the work on the pyrolysis of ethane, we are struck by the fact that, while it has produced much controversy and much travail and has been a great stimulus to further work, very little if any quantitative data of interest have come from it. On the contrary, all of the best available data on the steps in the proposed mechanism have come from quite different studies on the behavior of free radicals. And in fact, even at present the best use one can make of the data on this pyrolysis is to check them qualitatively against a proposed mechanism. It is quite doubtful that they can be used to predict individual rate constants with any reliability. 

The thermal decompositions (pyrolyses) of hydrocarbons other than the cyclic ones invariably occur by complex mechanisms involving the participation of free radicals the processes are usually chain reactions. In spite of this, many of the decompositions show simple kinetics with integral reaction orders, and this led to the conclusion by the earlier workers that the mechanisms are simple. Ethane, for example, under the usual conditions of a pyrolysis experiment, decomposes by a first-order reaction mainly into ethylene and hydrogen, and the mechanism was thought to involve the direct split of the ethane molecule. Rice et however, showed that free radicals are certainly involved in this and other reactions, and this conclusion has been supported by much later work. An important advance was made in 1934 when Rice and Herzfeld showed how complex mechanisms can lead to simple overall kinetics. They proposed specific mechanisms in a number of cases most of these have required modification on the basis of more recent work, but the principles suggested by Rice and Herzfeld are still very useful. 

Although methane is the simplest hydrocarbon, the elucidation of the mechanism of its pyrolysis has proved a matter of considerable difficulty. The reaction proceeds in a very different way from the other paraffin pyrolyses a carbon-carbon single bond is much weaker than a carbon-hydrogen bond so that C-C bond ruptures are important in all of the other pyrolyses. In the methane pyrolysis C-C bond rupture becomes gradually more important as product e.g. ethane) accumulates, so that the character of the process changes as reaction proceeds. 

Most of the generated reactions were eliminated during the construction of the mechanism. The primary tool was the comparison of reaction rates for parallel reaction channels. This approach required careful planning of the order of generation of the reaction types. The program was used for the pyrolysis of C1-C4 hydrocarbons. Mechanisms for ethane, propane and butane contained 15, 49 and 76 species and 18, 115 and 179 reactions, respectively. These mechanisms were compared with schemes which had been proposed by human experts. Most of the reactions were identical, and, in general, the program proposed a superset of those presented in the literature. 

Thus, the crucial difference in the proposed pathways is the emergence of C2H4 or C2H6 as the mechanistic indicator. In fact ethane, C2Hg, is the observed product. Based on this and on other data Speckman and Wendt proposed the reaction pathway for the pyrolysis of EtjAs shown in Scheme 1. Their observations are consistent with the data from the low-pressure pyrolysis of Et3As obtained by Jensen and coworkers. Both these groups results are in conflict with observations of Melas and collaborators , who interpreted their thermolysis study of AsEts interms of a / -elimination mechanism. 

The initiation step in the classic mechanism for the pyrolysis of ethane involves the formation of methyl radicals, but of more relevance to catalysis by metal oxides is the role of O ions in generating these radicals. Bohme and Fehsenfeld [Ref. 11] have shown that the reaction... 

To illustrate the simplest changes in kinetics that can be observed as reaction conditions are changed, consider the simple chain mechanism for the pyrolysis of ethane. The initiation reaction must produce active species by breaking a bond in the ethane... 

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