Thermal stability, atmospheric

The visbreaking process thermally cracks atmospheric or vacuum residues. Conversion is limited by specifications for marine or Industrial fuel-oil stability and by the formation of coke deposits in equipment such as heaters and exchangers. 

Applied to atmospheric residue, its purpose is to produce maximum diesel oil and gasoline cuts while meeting viscosity and thermal stability specifications for industrial fuels. 

At normal pressures (around atmospheric) and up to about 250°C (approaching the limit of thermal stability for most organic compounds), a volatile substance can be defined as one that can be vaporized by heat between ambient temperature (10 to 30°C) and 200 to 250°C. All other substances are nonvolatile. 

Thermal stability of the polymers ranges from 328 to 390°C in N2 atmosphere (10% weightless). 

The as-spun acrylic fibers must be thermally stabilized in order to preserve the molecular structure generated as the fibers are drawn. This is typically performed in air at temperatures between 200 and 400°C [8]. Control of the heating rate is essential, since the stabilization reactions are highly exothermic. Therefore, the time required to adequately stabilize PAN fibers can be several hours, but will depend on the size of the fibers, as well as on the composition of the oxidizing atmosphere. Their are numerous reactions that occur during this stabilization process, including oxidation, nitrile cyclization, and saturated carbon bond dehydration [7]. A summary of several fimctional groups which appear in stabilized PAN fiber can be seen in Fig. 3. 

Polybenzyls polyphenethyls Parylenes (poly-p-xylylene) Fusible, soluble, and stable at 400°C (752°F) low molecular weight. Melt above 520°C (968°F) insoluble capable of forming films poor thermal stability in air stable to 400-525° C (752-977°F) in inert atmosphere. 

In a study of thermal stability and hydrogen sorption characteristics of a series of sorbent tablets composed of hydride-forming metals dispersed in polymers under a 50% hydrogen in argon atmosphere, it was found that tablets of 80% palladium in PTFE, and 80% of 1 5 atom lanthanum-nickel alloy in PTFE could not be used above 247° C because of explosive decomposition of the PTFE. 

These effects undoubtedly increase thermal stability of SENAs (many of these compounds are distilled in vacuo at temperatures higher than 100°C, see Table 3.7). At the same time, SENAs are hydrolytically highly unstable (see Section 3.4.2.2.). Besides, these compounds can undergo spontaneous decomposition for unknown reasons. It is known that acidic impurities facilitate these processes, whereas triethylamine, on the contrary, stabilizes SENAs (191). Hence, SENAs are recommended to be either stored in a refrigerator with full protection from atmospheric moisture or used in situ. 

Generally, fluorene homo- and copolymers show excellent thermal stability the Tdec of many PF exceeds 400°C (according to thermogravimetric analysis (TGA) analysis under inert atmosphere) [224]. 

Because of its high thermal stability compared to that of other hydrides, water does not decompose extensively below 2000 °K. Thus, at one atmosphere and 2500 °K it is only dissociated to the extent of 9 %. Accordingly, it is impossible to study the homogeneous decomposition by classical methods. It is only with the shock tube technique that the rates of pyrolysis of water and heavy water have been measured. 

Thermal stability tests were carried out in inert and reducing atmospheres such as N2 and CO. In both cases the decomposition started at lower temperatures, at about 950 and 800 °C respectively, with formation of red-brown colored reaction products due to the presence of Cu20. 

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