Pyrolysis ethane

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,... 

In ethane pyrolysis assume that the chain termination step is the adsorption of ethyl radicals... 

Pyrolysis of Ethane Pyrolysis of hydrocarbons such as C2H6 is important in the petrochemical process industry. The overall reaction can be written as... 

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,... 

Here we assume that reaction (R13) is at its high-pressure limit because ethane pyrolysis as an industrial process typically is carried out at high pressure. In the ethane molecule, the C-C bond is weaker than the C-H bonds and the reaction... 

The methyl radical CH3 is comparatively unreactive and is only of secondary importance as chain carrier. The major chain-propagating steps in ethane pyrolysis are... 

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. 

P. Dagaut, J.-C. Boettner, and M. Cathonnet. Ethane Pyrolysis and Oxidation A Kinetic Modeling Study. Int. J. Chem. Kinet., 22 641-664,1990. 

Process Coke via calcium carbide Natural gas partial oombustioa Arc namraJ gas. Arc coal Ethane pyrolysis C, cutextra> don... 

The key features of the thermodynamics of ethane pyrolysis are illustrated in Figure 2.1, which shows the free energy relationship of ethane to the product ethylene and other compounds of interest over a range of ten ratures. This graph illustrates several points which are central to the technology and production economics of ethylene production ... 

Table XIII. 10. Values op Some of the Rate Constants for the Reactions Occurring in the Ethane Pyrolysis...

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-... 

Figure 9. Examples with more than one type of coke. (Top left) Butadiene—560°C (2nd position), incoloy coupon (top right) ethylene— 500°C (3rd position), incoloy coupon (middle left) acetylene—800°C (alonized tube) (middle right) acetylene—780°C (4th position), alonized coupon (bottom left) butadiene—460°C (1st position), alonized coupon (bottom right) ethane pyrolysis—800°C, SS 304 tube.

Ethane pyrolysis at 800°C in SS 304 reactor fluffy (or gas phase) and ribbon cokes. 

Deposition and Gasification of Coke During Ethane Pyrolysis... 

Considerable information was obtained for ethane pyrolysis relative to coke deposition on and to decoking from the inner walls of a tubular reactor. Both phenomena are affected significantly by the materials of construction (Incoloy 800, stainless steel 304, stainless steel 410, Hastelloy X, or Vycor glass) of the pyrolysis tube and often by their past history. Based on results with a scanning electron microscope, several types of coke were formed. Cokes that formed on metal tubes contained metal particles. The energy of activation for coke formation is about 65 kcal/g mol. 

Surfaces B and C for the Incoloy 800 reactor were quite different from the platelets and crystallites formed in the stainless steel 304 and 410 reactors. The metal concentrations for Surfaces A, B, and C varied rather erratically compared with the bulk metal (see Table III). Clearly, further investigations are needed to learn about coke deposits formed during ethane pyrolysis in Incoloy 800 reactors. 

Ethane Pyrolysis, MS dissertation, Purdue University, West Lafayette, IN, 1976. 

In the ethane pyrolysis the H atoms react with ethane with the abstraction of a hydrogen atom and the formation of an ethyl radical, viz. 

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... 

The relative magnitudes of the radical concentrations depend on the relative rates of the chain-propagating steps. For example, these steps in the ethane pyrolysis are... 

The kinetics of the ethane pyrolysis were first investigated by Pease , who used a flow system and worked at 650 °C. He found the main products to be ethylene and hydrogen, the overall reaction being represented approximately by the equation... 

The chain-propagating steps in the ethane pyrolysis are undoubtedly... 

The scheme of reactions proposed for the ethane pyrolysis provides a satisfactory interpretation of the overall kinetic behaviour. If it is assumed that reaction (1) is in its first-order region, and that the rate of reaction (3) is k3[C2H5][C2H6], ... 

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. 

Some such studies on the ethane pyrolysis in the presence of nitric oxide have been made by Pratt > who has been particularly concerned with the kinetics of formation of nitrogen, hydrogen, methane and butane. Work has recently been carried out at the University of Ottawa on the pyrolyses of ethane (Esser and Laidler ) and of acetaldehyde (Schuchmann and Laidler ). The technique of gas-liquid chromatography has been used for the most part to analyze the reaction... 

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 CH3 radicals are present in significant amounts in the acetaldehyde pyrolysis, being chain-carriers in the ethane pyrolysis, on the other hand, the CH3 radical concentration will be low, so that not much CH3NO will be formed. 

Acetonitrile, CH3CN, is formed in the ethane pyrolysis, but not in the acetaldehyde pyrolysis. These facts are easily explained if the main source of CH3CN is... 

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