Localized hot-spots

Heterogeneous catalysts are more common. However, they degrade and need replacement. If contaminants in the feed material or recycle shorten catalyst life, then extra separation to remove these contaminants before the feed enters the reactor might be justified. If the cataylst is sensitive to extreme conditions, such as high temperature, then some measures can help to avoid local hot spots and extend catalyst life ... 

The autoclave is not the only component of an LDPE plant which may be exposed to a decomposition. Local hot spots in a secondary compressor may initiate a decomposition reaction consequendy it is necessary to protect these units from serious overpressure by pressure relieving devices and to release the products of the decomposition reactions safely. The problem of the aerial decomposition referred to eadier has been largely overcome by rapidly quenching the decomposition products as they enter the vent stack. 

Two methods of preventing overheating are available either general ventilation to provide fairly even conditions over the whole space at working level or local ventilation to give spot cooling for localized hot spots. 

In addition to these bulk temperature effects, which can be readily predicted, local hot spots can develop at the point of reaction. This phenomenon is aggravated in this system because the recycle make-up... 

It is remarkable that, except for local hot-spots such as around industrial sites, mining areas and volcanoes, the elemental compositions of atmospheric dust in similar locations, such as remote or rural or urban are relatively constant over the world. This suggests either common sources, or a dominant source, or good mixing and transport of the dust around the globe. In fact all three factors have a role in determining the uniformity. Because of the consistent composition it is possible to estimate the median concentrations of the elements in atmospheric dusts in similar, but widely separated, locations. These estimates are given in columns 2 to 7 in Table n. The concentrations of the elements in the atmospheric dust are expressed as mass per volume of air. For remote locations (columns 2 to 5) the concentrations are in ng m 3, whereas for rural and urban areas (columns 6 and 7) the elemental concentrations are in xg m-3. 

An interesting way to retard catalyst deactivation is to expose the reaction mixture to ultrasound. Ultrasound treatment of the mixture creates local hot spots, which lead to the formation of cavitation bubbles. These cavitation bubbles bombard the solid, dirty surface leading to the removal of carbonaceous deposits [38]. The ultrasound source can be inside the reactor vessel (ultrasound stick) or ultrasound generators can be placed in contact with the wall of the reactor. Both designs work in practice, and the catalyst lifetime can be essentially prolonged, leading to process intensification. The effects of ultrasound are discussed in detail in a review article [39]. 

Addition of appropriate nucleation sites on the heated surface or a localized hot spot. 

In a subsequent paper [32], however, Berlan himself cast doubt on the existence of nonthermal effects, attributing the observed rate increases to localized hot-spots in the reaction mixture or to superheating of the solvent above its boiling point. He also mentioned the difficulty of measuring the temperature accurately in MW cavities. Furthermore, kinetic studies by Raner et al. [33], showed that the Diels-Alder reaction of 3 with 23 (Scheme 4.12) occurred at virtually the same rate under MW and conventional heating at the same temperature. 

A thermal explosion is the third type of chemical explosion. In this case, no reaction front is present, and it is therefore called a homogenous explosion. Initially, the material has a uniform temperature distribution. If the temperature in the bulk material is sufficiently high so that the rate of heat generation from the reaction exceeds the heat removal, then self-heating begins. The bulk temperature will increase at an increasing rate, and local hot spots may develop as the thermal runaway proceeds. The runaway reaction can lead to overpressurization and possible explosive rupture of the vessel. 

Since the reaction is quite exothermic, the mixture must be well stirred to avoid developing any local hot spots which could lead to explosions. Although no difficulties were encountered in either the submitters or checkers laboratories, it is well to keep in mind that 2,4,6-trimethylpyrylium perchlorate is potentially hazardous hence due precaution should be exercised at all times. 

Detect both a localized hot spot and low level temperature increases along their entire length. 

Truly isothermal operation of a tubular reactor may not be feasible in practice because of large enthalpies of reaction or poor heat transfer characteristics. Nor is it always desirable, as, for example, in the case of a reversible exothermic reaction (see Sect. 3.2.4). In an exothermic catalytic reaction, it may be necessary to provide adequate means for heat transfer to prevent the development of local hot-spots on which coking may occur and reduce the catalyst activity. An excessive temperature rise may also cause the catalyst particles to sinter, thereby reducing their surface area and causing an irreversible decrease in catalytic activity. 

In experiments run over a number of cycles, the activity was observed to increase after the first cycle, unlike the y-A Os counterpart which deactivated. Using BN, no Pt sintering occurred and this was ascribed to the high thermal conductivity of BN, ensuring that no local hot-spots were formed. On the basis of XPS, the locus of Pt particle attachment was proposed to be surface boron oxide impurities. Taylor and Pollard have compared the activities of silica (194 m g ) and boron nitride (7 m g ) supported vanadium oxide catalysts for propane oxidation. The use of boron nitride was reported to significantly... 

Because propellants are constantly subjected to abnormally high temperatures in various parts of the propulsion system during operation, high thermal stability is desirable. Decomposition of the propellant at temperatures experienced in the combustion chamber cooling jacket, the injector, and/or the gas film on the combustion chamber wall, can cause undesirable product deposition (resulting in local "hot spots and burnout), explosion in the cooling jacket and/or injector, undesired reaction chains in the combustion chamber, etc. 

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