Pyrolysis of ethane

The mechanism proposed by Rice and Herzfeld for the pyrolysis of ethane is as follows  

The chain carriers are the radicals C2H5 and H. The free radical CHj does not propagate the chain reaction, since it is formed in step (1), deactivates in step (2), and is not to be regenerated. The stationary conditions for CH3, C2H5 and H lead to the following set of differential equations  

[C2Hg]-2 5[H][C2H,] = 0 from which the concentration of hydrogen atoms is 

This expression can be inserted into eq. (12.34), leading, after some mathematical rearrangement, to 

The constant k is very small since the initiation reaction has a very high activation energy. The terms involving 3/2 3 are therefore very small in comparison with k-jcjk.ik and therefore 

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. 

In the range 800 to 1000°K ethane decomposes principally into ethylene + H2 accompanied by smaller amounts of CH4 and traces of higher hydrocarbons, the latter being principally CsHs and CaHe. The over-all reaction is thus 

A simplified Rice-Herzfeld scheme for this system is 

Note The equilibrium constants for the molecular reactions have been obtained from the NBS tables (data at 900°K), whereas the equilibrium constants for the free radical reactions were obtained by extrapolating the data at 300°K. The constants obtained by the two methods are generally within 15 per cent of each other. At 900 K the data are much less sensitive to errors in AH/. An error in H/ of 2 Kcal will result in an error in Xeq of a factor of 3. An error in / of 2 cal/mole-°C will result in an error in Keq of a factor of 2.7. 

From the equilibrium constants, it is possible to get fairly accurate values (with some simple algebraic approximations) of the equilibrium concentrations of the various species. These are given in Table XIII.9 for two different initial pressures of C2H6. One of the most striking fea- 

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When a reaction has many participants, which may be the case even of apparently simple processes like pyrolysis of ethane or synthesis of methanol, a factorial or other experimental design can be made and the data subjected to a re.spon.se. suiface analysis (Davies, Design and Analysis of Industrial Experiments, Oliver Boyd, 1954). A quadratic of this type for the variables X, Xo, and X3 is... 

The reaction is highly endothermic, so it is favored at higher temperatures and lower pressures. Superheated steam is used to reduce the partial pressure of the reacting hydrocarbons (in this reaction, ethane). Superheated steam also reduces carbon deposits that are formed by the pyrolysis of hydrocarbons at high temperatures. For example, pyrolysis of ethane produces carbon and hydrogen ... 

Pyrolysis of ethane is an important industrial process. The following data were obtained at 1150 K and 60 bar ... 

Table 8.1 shows the stochastic model solution for the petrochemical system. The solution indicated the selection of 22 processes with a slightly different configuration and production capacities from the deterministic case, Table 4.2 in Chapter 4. For example, acetic acid was produced by direct oxidation of n-butylenes instead of the air oxidation of acetaldehyde. Furthermore, ethylene was produced by pyrolysis of ethane instead of steam cracking of ethane-propane (50-50 wt%). These changes, as well as the different production capacities obtained, illustrate the effect of the uncertainty in process yield, raw material and product prices, and lower product... 

The pyrolysis of ethane proceeds with an activation energy of about 300 kJ/mol. How much faster is the decomposition at 650°C than at 500°C ... 

In steam cracking the homogeneous pyrolysis of ethane to produce ethylene... 

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

Determine the reaction order for pyrolysis of ethane under these conditions. 

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 ... 

Production of Ethylene by Pyrolysis of Ethane in an Isothermal Tubular Reactor... 

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

Thermal cracking investigations date back more than 100 years, and pyrolysis has been practiced commercially with coal (for coke production) even longer. Ethylene and propylene are obtained primarily by pyrolysis of ethane and heavier hydrocarbons. Significant amounts of butadiene and BTXs (benzene, toluene, and xylenes) are also produced in this manner. In addition, the following are produced and can be recovered if economic conditions permit acetylene, isoprene, styrene, and hydrogen. 

Ethylene is produced in large quantities in many countries by the thermal pyrolysis of ethane with the generalised stoichiometry ... 

The lines for the free energy of formation of ethane and ethylene cross at about lOOOK. Relative to ethane, ethylene becomes favoured at this and higher temperatures. The successful pyrolysis of ethane thus takes place at temperatures over lOOOK. 

The cross over point of the free energy curves of heptane and ethylene occur at a lower temperature than that of ethane, namely at about 800K. Pyrolysis of naphtha to ethylene can therefore be practiced at lower temperatures than that required for the pyrolysis of ethane. 

Similar methods have been employed in the photolysis of acetone in the presence of H2/ the photolysis of light and heavy acetone mixtures, the pyrolysis of ethane,- and the decomposition of C2H6Br in the presence of radioactive BrS, HBr, and DBr. ... 

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. 

Figure 6. Surfaces formed on tubular reactors during pyrolysis of ethane at 800°C. (Top left,) Stainless steel 410—Surface A, (predominant deposit) (top right,) stainless steel 410—Surface A, (less frequent deposit) (middle left,) stainless steel 410—Surface B (middle right stainless steel 410—Surface C (bottom left,) Hastelloy X—Surface A (bottom right)...

Initiation reactions. These comprise the initial split of the parent molecule into radicals. In the pyrolysis of ethane, for example, the C-C bond is weaker than the C-H bonds, so that the molecule is split into two methyl radicals ... 

In the uninhibited pyrolysis of ethane a certain amount of butane is formed, the rate of its formation being roughly or i of that of the CH4 formation butane is formed from the combination of C2H5 radicals. Even at the lowest NO pressures used in our experiments, no C4H10 could be detected. Evidently the NO markedly cuts down the C2H5 concentration, so that the rate of the bimolecular combination reaction is very strongly reduced. 

To demonstrate the utility of LCR and the interaction between various modeling elements, semantic relationships, and supporting methods, let us first consider the pyrolysis of ethane forming principally ethylene and hydrogen. Although this example is relatively simple, it highlights the functionality of LCR and underscores various issues that require resolution for computer implementation to be successful. 

The identification of various free-radical pathways and underlying elementary reactions involved in the pyrolysis of ethane are evaluated by applying the procedure, FIND-ALL-PATHWAYS, to the substrates, and associating with that call the designated instance of reaction-environment and the group of composite operators to be used (e.g., K, K, K b-initio - The call to FIND-ALL-PATHWAYS is shown below ... 

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... 

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