Reactor periodic phenomena

As outlined in Section 2.2.4.1, impurities and catalysts may decrease To significantly a decrease of 100°C is not unusual. The material of construction of the sample cup may act as a catalyst, resulting in surface-induced decomposition which may even be promoted by the sample/surface area ratio in the DSC cup. Therefore, it is important to check if the substance is catalyzed during the DSC experiment and if such catalysis is representative of process conditions. Frequently, substances that are sensitive to catalysis are handled in passivated glass-lined reactors, receptacles, or containers. Another phenomenon to recognize is autocatalytic decomposition. Substances that are susceptible to autocatalytic decomposition have an induction period prior to initiation of rapid decomposition. The same holds for substances that contain inhibitors, which can be depleted. 

Over time, original surface reactants will eventually become depleted and secondary emissions of by-products should be reduced. Many studies have shown that ozone uptake on indoor surfaces tends to decrease with continued exposure, a phenomenon known as aging (Morrison and NazarofF, 2000 Morrison et al., 1998 Reiss et al, 1995a Sabersky, Sinema and Shair, 1973 Simmons and Colbeck, 1990). Further, there is evidence that secondary emission rates also decrease with time. Morrison and Nazaroff (2002) showed that secondary aldehydes on new carpet fibers, in a fixed-bed reactor, could be depleted in a day however, the reactivity of whole carpet was not substantially decreased over the relatively short time periods studied. Wang and Morrison (2006) showed that carpet in older homes... 

A period of particularly rapid accumulation of carbon from CH4 was observed during the initial 5 min at 873 K. This effect (Fig. 2d) is favored by higher CH4 pressure. This phenomenon is not observed at 973 K or 1073 K. Buoyancy is observed as a weight loss at the moment when CH4 enters the reactor and is thus excluded as a potential explanation for the rapid weight increase. Calculations show that the Ni/C ratio is very close to 1 at the end of this initial period (at 4.5 bar) indicating that a rapid saturation of the nickel phase could possibly be of importance. 

The high internal surface of the catalyst may be reduced due to sintering (Crowe and Lee, 1971) or fusion of the surface area. This decay usually occurs at high temperatures and/or long operating periods of the reactor. As a result of this phenomenon the activity of the catalyst declines. The influence of this phenomenon decreases when there is traces of metallic oxides in the catalyst. 

A related oscillatory phenomenon is that in which the concentration of one or more reactants, fed to a flow reactor, is varied in time. Such forced periodic feed oscillations during oxidation reactions have now been studied by a number of authors. It is found that not only can conversion be increased but the selectivity of certain parallel reactions can be improved, which may be of value in industrial applications. Cutlip and Abdul-Karem and Jain ... 

This will appear as a series of long periods of quiescence interrupted by short-lived bursts. This phenomenon is called intermittency and is a signature of turbulence. It is a well-known phenomenon that occurs, for instance, for mass flow in trickle flow reactors at flow velocities where flow becomes turbulent. 

In case plugging and/pr fouling is a problem, trickle flow clearly has a disadvantage over slurry reactors. A well known phenomenon is the pressure drop built-up during operation of a trickle flow hydrodesulfurization reactor limiting the operating period. Similar problems do not occur in slurry reactors. 

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