Temperatures surface

Surface temperature of exposed equipment may pose the most readily available ignition source in a hydrocarbon facility. Hot surfaces should be insulated, cooled or relocated, when they pose a threat of ignition, to hydrocarbon release areas. Required equipment should be rated to operate below the autoignitiion temperature of the gas or vapor that may be encountered. 

At low temperatures the oxidation reactions on the catalyst are kinetically controlled and the catalyst activity is an important parameter. As the temperature increases, the buildup of heat on the catalyst surface due to the exothermic surface reactions produces ignition and the catalyst surface temperature jumps rapidly to the adiabatic flame temperature of the fuel/air mixture (Fig. 2) [2]. At the adiabatic flame temperature, oxidation reactions on the catalyst are very rapid, and the overall steady state reaction rate is determined by the rate of mass transfer of fuel to the catalytic surface. The bulk gas temperature rises along the reactor because of heat transfer from the hot catalyst substrate and eventually approaches the catalyst surface temperature. 

The steady state temperature of the catalyst surface under mass-transport-limited conditions can exceed the adiabatic flame temperature if the rate of mass transport of fuel to the surface is faster than the rate of heat transport from the surfaee. The ratio of mass diffusivity to heat dilTusivity in a gas is known as the Lewis number. Reactor models [9] show that for gases with a Lewis number close to unity, such as carbon monoxide and methane, the catalyst surface temperature jumps to the adiabatic flame temperature of the fuel/air mixture on ignition. However, for gases with a Lewis number significantly larger than unity the rate of mass transport to the surface is much faster than the rate of heat transport from the surface, and so the wall temperature can exceed the adiabatic gas temperature. The extreme case is 

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Sampling saturated reservoirs with this technique requires special care to attempt to obtain a representative sample, and in any case when the flowing bottom hole pressure is lower than the bubble point, the validity of the sample remains doubtful. Multiple subsurface samples are usually taken by running sample bombs in tandem or performing repeat runs. The samples are checked for consistency by measuring their bubble point pressure at surface temperature. Samples whose bubble point lie within 2% of each other may be sent to the laboratory for PVT analysis. 

For the examination of the applied metallic or ceramic layer, the test object is heated up from the outside The heat applying takes place impulse-like (4ms) by xenon-flash lamps, which are mounted on a rack The surface temperature arises to approx 150 °C Due to the high temperature gradient the warmth diffuses quickly into the material An incorrect layer, e g. due to a delamiation (layer removal) obstructs the heat transfer, so that a higher temperature can be detected with an infrared camera. A complete test of a blade lasts approximatly 5 minutes. This is also done automatically by the system. In illustration 9, a typical delamination is to be recognized. 

With this testing method an evaluation is possible within shortest time, i.e. directly after the heat impulse. The high temperature difference between a delamination and sound material is affected - among other parameters - by the thickness of the layer. Other parameters are size and stage of the delamination Generally, a high surface temperature refers to a small wall thickness and/or layer separation [4],... 

The atoms on the outennost surface of a solid are not necessarily static, particularly as the surface temperature is raised. There has been much theoretical [12, 13] and experimental work (described below) undertaken to investigate surface self-diffiision. These studies have shown that surfaces actually have dynamic, changing stmetures. For example, atoms can diflfiise along a terrace to or from step edges. When atoms diflfiise across a surface, they may move by hopping from one surface site to the next, or by exchanging places with second layer atoms. 

Molecular adsorbates usually cover a substrate with a single layer, after which the surface becomes passive with respect to fiirther adsorption. The actual saturation coverage varies from system to system, and is often detenumed by the strength of the repulsive interactions between neighbouring adsorbates. Some molecules will remain intact upon adsorption, while others will adsorb dissociatively. This is often a frinction of the surface temperature and composition. There are also often multiple adsorption states, in which the stronger, more tightly bound states fill first, and the more weakly bound states fill last. The factors that control adsorbate behaviour depend on the complex interactions between adsorbates and the substrate, and between the adsorbates themselves. 

In this maimer, it can also be seen that molecules will desorb as the surface temperature is raised. This is the phenomenon employed for TPD spectroscopy (see section Al.7.5.4 and section BT25). Note tliat some adsorbates may adsorb and desorb reversibly, i.e. the heats of adsorption and desorption are equal. Other adsorbates, however, will adsorb and desorb via different pathways. 

Adsorbed atoms and molecules can also diflfiise across terraces from one adsorption site to another [33]. On a perfect terrace, adatom diflfiision could be considered as a random walk between adsorption sites, with a diflfiisivity that depends on the barrier height between neighbouring sites and the surface temperature [29]. The diflfiision of adsorbates has been studied with FIM [14], STM [34, 35] and laser-mduced themial desorption [36]. 

This is known as the Planck radiation law. Figure A2.2.3 shows this spectral density fiinction. The surface temperature of a hot body such as a star can be estimated by approximating it by a black body and measuring the frequency at which the maximum emission of radiant energy occurs. It can be shown that the maximum of the Planck spectral density occurs at 2.82. So a measurement of yields an estimate of the... 

Figure A3.9.3. Time-of-flight spectra for Ar scattered from Pt(l 11) at a surface temperature of 100 K [10], Points in the upper plot are actual experimental data. Curve tinough points is a fit to a model in which the bimodal distribution is composed of a sharp, fast moving (lienee short flight time), direct-inelastic (DI) component and a broad, slower moving, trapping-desorption (TD) component. These components are shown...

Figure A3.9.6. Population of the first excited vibrational state (u = 1) versus inverse of surface temperature for NO scattering from an Ag(l 11) surface [28], Curves (a) = 102 kJ moC and (b) E = 9 kJ mor ...

Figure A3.9.11. Dissociation of H2 on the W(100)-c(2 x 2)-Cu surface as a function of incident energy [71]. The steering dominated reaction [102] is evident at low energy, confmned by the absence of a significant surface temperature.

Direct dissociation reactions are affected by surface temperature largely tlirough the motion of the substrate atoms [72]. Motion of the surface atom towards the incoming molecule mcreases the likelihood of (activated) dissociation, while motion away decreases the dissociation probability. For low dissociation probabilities, the net effect is an enliancement of the dissociation by increasing surface temperature, as observed in the system 02/Pt 100]-hex-R0.7° [73]. 

Head-Gordon M, Tuiiy J C, Rettner C T, Muiiins C B and Auerbach D J 1991 On the nature of trapping and desorption at high surface temperatures theory and experiments for the Ar-Pt(lll) system J. Chem. Phys. 94 1516... 

Analysis of the electromagnetic radiation spectrum emanating from the star Sirius shows that = 260 nm. Estimate the surface temperature of Sirius. 

The thermographic sensor is used as a remote sensing radiometer when a reference target is imaged. It is usually necessary to correct for emissivity and atmospheric transmission to determine surface temperature with a reasonable degree of accuracy. 

Panels then move into a cooling device, normally a wheel or rack, where they are held individually and air is circulated between them to remove the majority of heat remaining in the boards after pressing. It is desirable to reduce the average board surface temperature to about 55°C. This temperature is sufficient to complete the cure of adhesive in the core of the board. The heat also helps to redistribute moisture uniformly within the boards, because the board surfaces are drier than the core when the boards come out of the press. Warm boards are normally stacked for several hours to a day to allow for resin cure and moisture equalization. 

Thermal Quenching. Endothermic degradation of the flame retardant results in thermal quenching. The polymer surface temperature is lowered and the rate of pyrolysis is decreased. Metal hydroxides and carbonates act in this way. 

Temperatures of hydrothermal reservoirs vary widely, from aquifers that are only slightly warmer than the ambient surface temperature to those that are 300°C and hotter. The lower temperature resources are much more common. The value of a resource for thermal appHcations increases directiy with its temperature, and in regions having hotter water more extensive use of geothermal resources has been implemented. Resources in remote areas often go unused unless hot enough to be employed in generating electricity. 

Development of molybdenum electrodes in the 1950s permitted the use of electrically assisted melting in regenerative furnaces (81). In the 1990s, approximately one-half of all regenerative tanks ate electrically boosted. Operating practice has shown that effective use of electricity near the back end of the furnace, where the batch is added, can reduce fossil fuel needs. This lowers surface temperature and reduces batch volatilisation. 

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