Pyrolysis neopentane

To illustrate the concepts of determining, non-determining and negligible processes, the mechanism of the pyrolysis of neopentane will be discussed briefly here. Neopentane pyrolysis has been chosen because it has been studied by various techniques batch reactor [105— 108], continuous flow stirred tank reactor [74, 109], tubular reactor [110], very low pressure pyrolysis [111], wall-less reactor [112, 113], non-quasi-stationary state pyrolysis [114, 115], single pulse shock tube [93, 116] amongst others, and over a large range of temperature, from... 

A reaction mechanism is a set of elementary processes. In most programs, an elementary process is described by a list of the following type Reaction label/Reactant 1/Reactant 2/Product 1/Product 2/Pre-expo-nential factor/Activation energy. For example, C6me and co-workers [182] describe the initiation process of the neopentane pyrolysis as... 

If the particular reaction studied is the unimolecular decomposition of a free radical, such as (3), then the use of a trap will enable the effective concentration of the radical to be measured. A radical trap will indicate the presence or absence of a free radical reaction and may sometimes provide evidence for a partly or entirely molecular reaction. Rate data for free radical reactions are derived assuming the occurrence of a steady state concentration of radicals. The time required to produce a steady state concentration of methyl radicals in the pyrolysis of AcH is shown for various temperatures in Fig. 1. Realistic values for rate coeflBcients may be obtained only if the time of product formation is long compared to the time to achieve the steady state concentrations of the radicals concerned. Thus deductions from the results from the bromination of isobutane , neopentane , and toluene have been criticised on the grounds that a steady state concentra-... 

As can be seen, H2S is a positive catalyst in the pyrolysis of neopentane but a negative catalyst in the pyrolysis of ethane. The experimental data are fitted nicely by the theoretical curves obtained by the kinetic treatment of the cycles taken from refs. 31 and 32. This kinetic treatment is reproduced in ref. 3, pp. 155-158. In brief, YH is a positive catalyst when the coupling rate constant 21 is larger than the coupling rate constant of the coupled main cycle. 

Figures 2 and 3 show the variations in the experimental selectivities as a function of the space time for the pyrolysis of neopentane in a stirred flow reactor at a temperature of 1008 K and at a neopentane pressure of 2.19 x 10 Pa.

Establish a reaction scheme for the pyrolysis of neopentane starting with the results of the analysis of the selectivities shown in Figures 2 and 3. 

Figure 4 shows the variations of log rQ as a function of log Pq at two temperatures, rQ is the rate of the pyrolysis reaction of neopentane in a batch reactor, extrapolated to zero conversion, and Pq is the initial partial pressure of neopentane in the reactor. Law (58) can be written ... 

Figure 4 Pyrolysis of neopentane in a batch reactor reaction order at conversion equal to zero.

ARIS R., Introduction to the Analysis of Chemical Reactors, Prentice-Hall (1965). BARONNET F., DZIERZYNSKI M., C6ME G.M., MARTIN R., NICLAUSE M., The pyrolysis of neopentane at small extents of reaction, Int. J. Chem. Kin., Ill, 197 (1971). C6ME G.M., The Use of Computers in the Analysis and Simulation of Complex Reactions, in "Modern Methods in Kinetics", Comprehensive Chemical Kinetics, Vol. 24, Elsevier (1983). 

In what follows, the principal characteristics of straight chain reactions will be discussed for the case of the pyrolysis of neopentane, which will then allow the kinetic rules fi i Y, which are useful for the generation of reaction mechanisms to be discussed. 

The pyrolysis of neopentane experimental facts a) Experimental studies... 

This reaction has been studied using batch reactors, perfectly stirred continuous reactors, tubular continuous reactors, BENSON type reactors, wall-less reactors and shock tubes. The reaction has been carried out at temperatures between 700 and 1300 K, at pressures of 0.1 Pa to 10 Pa and at reaction times of 10 s to 10 s. The effects of the nature and of the area of the reactor walls as well as those of various additives have also been studied. The diversity of the studies carried out by a dozen teams throughout the world, the particularly widespread range of operating conditions (600 K for the temperature, which represents 11 orders of magnitude for the rate of initiation, 8 orders of magnitude for the pressure and reaction duration) make the pyrolysis of neopentane into a model radical reaction. 

The rate of pyrolysis of neopentane is strongly decreased by the addition of propene or of isobutene. The reaction is therefore believed to be strongly auto-inhibited by the isobutene formed, this is expressed in particular, by a current order of reaction which varies with the extent of the reaction and is much higher than the order at conversion equal to zero. 

The inhibition mechanism of the pyrolysis of neopentane in the presence of isobutene can be used to explain the auto-inhibition of the pyrolysis of neopentane by the isobutene formed during the reaction. 

The primary mechanism of the pyrolysis of neopentane can be generalized to the case of an organic substance pH possessing a labile H atom. The mechanism proposed by GOLDFINGER, LETORT and NICLAUSE is the following ... 

The law for the pyrolysis of neopentane is again found, but it also applies to the pyrolysis of ethanal and numerous other organic substances. 

The comparison of reactions (52) and (92) allows the deep-seated origin of the difference between the mechanisms of pyrolysis of neopentane (eqns 48 to 54) and of ethane (eqns 90 to 94), at "low" temperatures (lower than 800 K) to be understood. 

The calculation of the kinetic law from the complete mechanism is very complicated and it appears reasonable to treat the limiting case already discussed in the 4.4 dealing with the inhibition of the pyrolysis of neopentane by isobutene. The following hypotheses can be made ... 

Figure 1. Acceleration, by HCl, of methane formation in the pyrolysis of neopentane. iqo mm...

Figure 2. Influence of initial concentration of HCl on the initial rate of formation of CH (or i-C Hg) in the pyrolysis of neopentane. -p neo-csHit iQQ mmHg T = 480°C.

Results which have been obtained with H S suggest that a free radical HS. is more reactive than a methyl free radical when abstracting an hydrogen atom from a neopentane molecule. A detailed investigation of HCl and HBr influence upon neopentane pyrolysis is to be published shortly (2). 

The inhibition of neopentane pyrolysis by various alkenes, including propene (5, 18), and the inhibition and self-inhibition of this pyrolysis by isobutene (5, ) have been interpreted on... 

We have studied the pyrolysis of 2-2 dimethyl-propane (neopentane) in the gaseous phase at about 500 C and at small extents of reaction. The kinetic parameters obtained from these investigations in a continuous stirred reactor have been compared to the values previously determined from batch experiments (12 to 19). 

According to previous studies (14, 1 5, 18), the pyrolysis of neopentane yH would imply the main primary stoichiometric equation ... 

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