Confined pyrolysis

Michels R, Landais P, Elie M, Gerard L, Mansuy L (1992) Evaluation of factors influencing the thermal maturation of organic matter during confined pyrolysis experiments. Am Chem Soc Division Fuel Chem Preprints 37 1588-1594 Nelson RA (1985) Geologic analysis of naturally fractured reservoirs. Gulf, Houston, 320 PP... 

During pyrolysis to silver (via silver dicyanamide), initial heating must be slow to avoid explosion [1], High intensity illumination will also cause explosive decomposition of a confined sample [2], Safety precautions for preparation and subsequent use of the explosive salt are detailed [3],... 

Equation (48) e ees with experimental results in some circumstances. This does not mean the mechanism is necessarily correct. Other mechanisms may be compatible with the experimental data and this mechanism may not be compatible with experiment if the physical conditions (temperature and pressure etc.) are changed. Edelson and Allara [15] discuss this point with reference to the application of the steady state approximation to propane pyrolysis. It must be remembered that a laboratory study is often confined to a narrow range of conditions, whereas an industrial reactor often has to accommodate large changes in concentrations, temperature and pressure. Thus, a successful kinetic model must allow for these conditions even if the chemistry it portrays is not strictly correct. One major problem with any kinetic model, whatever its degree of reality, is the evaluation of the rate cofficients (or model parameters). This requires careful numerical analysis of experimental data it is particularly important to identify those parameters to which the model predictions are most sensitive. 

Homolytic substitution has been little studied, and work has been confined to the reaction of dibenzofuran with carboxymethyl radicals produced from acetyl peroxide or di-tcrt-butyl peroxide in boiling acetic acid or by pyrolysis of chloroacetylpolyglycolic acids. The method of analysis of the resultant mixture of 1- (55%), 4- (30%), and 3-dibenzofuranacetic acid (15%) was crude, but the results were in accord with simple HMO calculations. The amount of the 1-substituted product is perhaps surprising in view of the steric hindrance at this position. 

In kinetically limited models, the pyrolysis rate is no longer calculated solely from a heat balance at the pyrolysis front. Instead, the rate at which the condensed-phase is volatilized depends on its temperature. This gives a local volumetric reaction rate (kg/m3-s) by assuming that all volatiles escape instantaneously to the exterior gas-phase with no internal resistance, the fuel mass flux is obtained by integrating this volumetric reaction rate in depth. One consequence is that the pyrolysis reaction is distributed spatially rather than confined to a thin front as with heat transfer limited models and the thickness of the pyrolysis front is controlled by decomposition kinetics and heat transfer rates. For a pyrolysis reaction with high activation energy or for very high heat transfer rates, the pyrolysis zone becomes thin, and kinetically limited models tend toward heat transfer limited models. 

The following discussion is confined to the radiant coil where pyrolysis reactions occur. 

Examples of this type of study are the pyrolysis of hydrocarbons ana other organic compounds carried out by Szwarc and coworkers. By confining the total extent of reaction to a few per cent, it was found possible to study the primary reactions occurring in what is normally a very complicated system. Thus in the gas-phase pyrolysis of toluene the stages of the reaction may be represented by... 

Pyrolysis to Si3N4 and Si3N4-SiC. Precursor polymers are converted to ceramics by pyrolysis. The efficiency of conversion depends on several factors. Aside from simple stoichiometric efficiency, volatility losses, entrapment of impurities, and side reactions (such as oxidation and hydrolysis) can contribute to the apparent ceramic yield. Because of their high volatility, precursors such as the solid disilazanes previously described are unsuitable for conversion to ceramics, unless pyrolysis is carried out in a confined space. In unconfined pyrolysis, polymers of low molecular weight (<1000) are even less suitable as ceramic precursors. 

Behar F. and Hatcher P. G. (1995) Artificial coalification of a fossil wood from brown coal by confined system pyrolysis. Energy Fuels 9, 984-994. 

Monthioux M. (1988) Expected mechanisms in nature and in confined-system pyrolysis. Fuel 61, 843-847. 

We confine our discussion to the arsenic precursors and their pyrolysis. The reader is referred to the technical literature for the MOCVD preparation of GaAs . 

As noted above, the use of TMAH/thermochemolysis for the structural characterization of different humic materials have been applied in two different forms pyrolysis of the humic materials in the presence of TMAH or a confined reaction of the humic materials with the TMAH in a sealed glass ampoule at lower temperatures. Both methods release the same types of compunds. 

Free radical elimination reactions in solution are rather rare instead they tend to be confined to the pyrolysis of polyhalides and primary monohalides as illustrated above. 

Michels R., Landais P., Philp R.P., Torkelson B.E. (1995) Influence of pressure and the presence of water on the evolution of the residual kerogen during confined, hydrous pyrolysis, and high-pressure hydrous pyrolysis of Woodford Shale. Energy Fuels 9, 204—15. 

Professor Hurd s interests, although confined to organic chemistry, were most eclectic. His own graduate work had involved him in nitrogen chemistry, and he maintained a lively research interest in that field. He was also deeply interested in thermal transformations— his treatise on the pyrolysis of carbon compounds had been recently published—and in reaction mechanisms, such as that of the acid-catalyzed dehydration of pentoses. At that time, Hurd also taught the special, graduate course in... 

In order to maintain state-specific populations, low pressures are required. If pressures are raised to the point where collisions are frequent, then specific effects of laser excitation are lost and intermolecular energy transfer heats the entire sample. Reactions induced in this fashion then yield products characteristic of thermal processes. A particularly interesting aspect of this pyrolysis technique is the homogeneous nature of the reaction. Since heating is confined to the region near the laser beam, heterogeneous effects due to hot walls in a conventional pyrolysis reactor are avoided. 

One of the great issues in the field of silicon clusters is to understand their photoluminescence (PL) and finally to tune the PL emission by controlling the synthetic parameters. The last two chapters deal with this problem. In experiments described by F. Huisken et al. in Chapter 22, thin films of size-separated Si nanoparticles were produced by SiLL pyrolysis in a gas-flow reactor and molecular beam apparatus. The PL varies with the size of the crystalline core, in perfect agreement with the quantum confinement model. In order to observe an intense PL, the nanocrystals must be perfectly passivated. In experiments described by S. Veprek and D. Azinovic in Chapter 23, nanocrystalline silicon was prepared by CVD of SiH4 diluted by H2 and post-oxidized for surface passivation. The mechanism of the PL of such samples includes energy transfer to hole centers within the passivated surface. Impurities within the nanocrystalline material are often responsible for erroneous interpretation of PL phenomena. 

During the last few years, we have studied silicon nanocrystals produced by CO2 laser pyrolysis of silane and we have been able to show that, in these experiments, the PL characteristics can be unambiguously explained in terms of quantum confinement effects.However, to observe the photoluminescence with the naked eye, we had to wait a few hours or even a few days. It appeared that the silicon nanocrystals were passivated by natural aerial oxidation and that, with time, the photoluminescence became more and more intense. 

CO2 laser pyrolysis of silane in a gas flow reactor and the extraction of the resulting silicon nanoparticles into a cluster beam apparatus has been shown to offer an excellent means for the production of homogeneous films of size-separated quantum dots. Their photoluminescence varies with the size of the crystalline core. All observations are in perfect agreement with the quantum confinement model, that is, the photoluminescence is the result of the recombination of the electron-hole pair created by the absorption of a UV photon. Other mechanisms involving defects or surface states are not operative in our samples. 

The porous interior of the pellet provides a resistance to mass transfer, i.e., confines the volatiles, which increases reactive fragment concentrations. This promotes polymerization and condensation reactions which form the greater char fraction and condensed aromatics (Table III and Figure V) than reported for conventional pyrolysis. 

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