High-pressure Inclusion discussion

The inclusion of reactions to represent the low-temperature chemistry in a detailed model for n-butane oxidation at high pressures, that is appropriate to temperatures down to about 600 K began in 1986 [225]. At the present time, models which include around 500 species and more than 2000 reversible reactions to represent alkane isomers up to heptane, are in use [219] and still larger schemes are under development [220]. Progress in the validation and application of these models, and kinetic representations for propane and propene oxidation, are discussed in the next subsection. Modelling of the low-temperature combustion of ethene has also been undertaken more recently [20]. 

Baddour [26] retained the above model equations after checking for the influence of heat and mass transfer effects. The maximum temperature difference between gas and catalyst was computed to be 2.3°C at the top of the reactor, where the rate is a maximum. The difference at the outlet is 0.4°C. This confirms previous calculations by Kjaer [120]. The inclusion of axial dispersion, which will be discussed in a later section, altered the steady-state temperature profile by less than O.S°C. Internal transport effects would only have to be accounted for with particles having a diameter larger than 6 mm, which are used in some high-capacity modern converters to keep the pressure drop low. Dyson and Simon [121] have published expressions for the effectiveness factor as a function of the pressure, temperature and conversion, using Nielsen s experimental data for the true rate of reaction [119]. At 300 atm and 480°C the effectiveness factor would be 0.44 at a conversion of 10 percent and 0.80 at a conversion of 50 percent. 

Inclusion of selenium in a book on fluid metals is justified by its status as a borderline metal at high temperatures and pressures. Also, as we have discussed in chapter 2, the dramatically different molecular structure of the low-temperature liquid and the vapor means that selenium is an excellent example of a system with strongly state-dependent interactions. Structural evolution of the liquid as it is heated to the region of the critical point must inevitably lead to interesting changes in the physical properties. 

Volume defects consist of inclusions or precipitates of a second phase material or voids. Voids can be formed by vacancy clusters or from the nucleation of bubbles from dissolved gases or from components with high vapor pressures. Such defects can range in size from microscopic to gross. Bear in mind that not all such defects are unwanted. Many are purposely introduced into the final solid to tailor certain electrical, optical, and magnetic properties, or to serve as strengthening mechanisms. These topics are discussed in later chapters. 

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