Runaway phenomenon

The basic phenomenon was observed in modeling studies by Bjoreskov and Slinko (1965) that sudden increase in inlet temperature caused a transient drop of the peak temperature. The wrong-way response name was given by Mechta et al (1981) after they experienced the opposite a sudden of inlet temperature resulted in an increase of the peak temperature (which may eventually cause a runaway.) The work used a pseudo-homogeneous reaction model and explained the phenomenon by the different speeds of transient response in gas and solid. The example in the last part of Chapter 7.4 explained the speed difference by the large difference in heat capacity of gas and solid phases. For this a two-phase model is needed and spatial and time changes must be followed. 

Table 3.5 shows that the study of chemical kinetics is critical in successful scale-up of catalytic systems, of gas-phase controlled systems, and of continuous tank stirred reactors (CSTR). For scale-up of batch systems consisting of gas or liquid compounds, chemical kinetics and heat transfer effects must be studied because the combination of these phenomenon determine the conditions for a runaway and thus involve the safety of the operation. 

This vork was also particularly important in that it showed that polymerisation ceased in the absence of ultrasound (Fig. 5.36 at T = 40 °C). Although this phenomenon has not been utilised it does offer the possibility of controlling runaway exothermic reactions. 

The shutdown property of separators is measured by measuring the impedance of a separator while the temperature is linearly increased. Figure 7 shows the actual measurement for Celgard 2325 membrane. The heating rate was around 60 °C/min, and the impedance was measured at 1 kHz. The rise in impedance corresponds to a collapse in pore structure due to melting of the separator. A 1000-fold increase in impedance is necessary for the separator to stop thermal runaway in the battery. The drop in impedance corresponds to opening of the separator due to coalescence of the polymer and/or to penetration of the separator by the electrodes this phenomenon is... 

A phenomenon known as thermal runaway can occur when a large new lithium surface is produced on charge which reacts exothermically with components of the electrolyte. In some cases the temperature is raised sufficiently to melt some of the lithium, which then in turn reacts with more electrolyte, causing a further rise in temperature and eventually the device goes on fire. 

However, several exothermic reactions are characterized by moderate or low values of the B number here, the transition stages from safe to runaway conditions may cover a quite wide range of the parameter values, and the choice of the boundaries for the safe region is very discretional. Hence, not surprisingly, the main discrepancies among the different criteria are found at low B numbers [14, 15]. Moreover, in this case, runaway is a less dramatic phenomenon posing the problem to decide whether a bland explosion still represents a safety issue. In this case, an effective runaway criterion should be more properly determined on the basis of the actual ability of the system to comply with certain levels of temperature and pressure. 

Returning now to the issue of reactor temperature control, we can generally state that reactors with either substantially reversible or endothermic reactions seldom present temperature control problems. Endothermic reactions require that heat be supplied to generate products. Hence, they do not undergo the dangerous phenomenon of runaway because they are self-regulating, that is, an increase in temperature increases the reaction rate, which removes more heat and tends to decrease the temperature. 

As a consequence of this explanation the reaction runaway to total methanation is not a necessary condition for the observed phenomenon. Any simple exothermic two phase reaction in an adiabatic reactor ought to show the same behaviour provided that one phase with a high throughput is used to carry the heat out of the reactor and the flow is suddenly reduced. This will be shown in the following simulation results. Due to problems with the numerical stability of the solution (see Apendix) only a moderate reaction rate will be considered. Reaction parameters are chosen in such a way that in steady state the liquid concentration Cf drops from 4.42 to 3.11 kmol/m3 but the temperature rise is only 3°C (hydrogen in great excess). At t = 0 the uniform flow profile... 

Bilous and Amundson [1] were the first to describe the phenomenon of parametric sensitivity in cooled tubular reactors. This parametric sensitivity was used by Barkelew [2] to develop design criteria for cooled tubular reactors in which first order, second order and product- inhibited reactions take place. He presented diagrams from which for a certain tube diameter dt the required combination of CAO and Tc can be derived to avoid runaway or vice versa. Later van Welsenaere and Froment [3] did the same for first order reactions, but they also used the inflexion points in the reactor temperature T versus relative conversion XA trajectories, which describe the course of the reaction in the tubular reactor. With these trajectories they derived a less conservative criterion. Morbidelli and Varma [4] recently devised a method for single order reactions based on the isoclines in a temperature conversion plot as proposed by Oroskar and Stern [5]. 

Autocatalysis is a distinctive phenomenon while in ordinary catalysis the catalyst re-appears from the reaction apparently untouched, additional amounts of catalyst are actively produced in an autocatalytic cycle. As atoms are not interconverted during chemical reactions, this requires (all) the (elementary or otherwise essential) components of autocatalysts to be extracted from some external reservoir. After all this matter was extracted, some share of it is not introduced in and released as a product but rather retained, thereafter supporting and speeding up the reaction(s) steadily as amounts and possibly also concentrations of autocatalysts increase. At first glance, such a system may appear doomed to undergo runaway dynamics ( explosion ), but, apart from the limited speeds and rates of autocatalyst resupply from the environment there are also other mechanisms which usually limit kinetics even though non-linear behavior (bistability, oscillations) may not be precluded ... 

The amazing thing about Example 2.5.2 is that the system has solutions that reach infinity in finite time. This phenomenon is called blow-up. As the name suggests, it is of physical relevance in models of combustion and other runaway processes. 

Typical for strongly exothermic processes is that at some location in the reactor an extreme temperature occur, frequently named the hot spot. In some processes with very strong exothermic reactions the hot spot temperature can raise beyond permissible limits. This phenomenon is called runaway. An important task in reactor design and operation is thus to limit the hot spot and avoid excessive sensitivity of the reactor performance to variations in the temperature. The value of the temperature at the hot spot is determined mainly by the reaction rate sensitivity to changes in temperature, the heat of reaction potential of the process, and the heat transfer potential of the heat exchanger units employed. A heat exchanger is characterized by the heat transfer coefficient and heat transfer areas. 

The light-off phenomenon is a safe runaway. There is a lot of articles devoted to the prediction of runaway condition in the chemical engineering literature. Villermaux [10] showed that runaway occurs in a batch reactor at a time given by ... 

Consider the phenomenon referred to as spontaneous human combustion (SHC), as publicized from time to time (e.g., as was reported in Arthur C. Clarke s Mysterious Universe, shown on the Discovery Channel, for instance, on October 22,1996, and was mentioned in Charles Dickens Bleak House). If this weird phenomenon does indeed occur, it could instead be referred to as spontaneous ignition, followed by combustion. And if it is at least conceivable for aberrations to occur among the enzyme-catalyzed reactions involved in the metabolism of glucose or other carbohydrates to yield CO2 and H2O, then conceivably there may be a case. Ordinarily, body metabolism reaction rates are miniscule as compared to the direct combustion or combustive oxidation of conventional fuels. If enzyme promoters exist, however, there is the possibility that runaway metabolic processes occur, similar to those in the ignition and further combustion of carbonaceous materials. If so, ample air or oxygen supply would also be required for this extremely unlikely scenario. 

From these equations, it is clear that an increase in temperature reduces the relaxation time. When the dielectric loss factor increases with the increasing temperature, food would experience a phenomenon known as thermal runaway. When frozen food is thawed with higher microwave power, certain area of the food would be overheated while other areas remain much cooler. Hence, it is important to maintain low microwave power for thawing frozen food to have a uniform thawing. 

An additional phenomenon may also be important in the tropical forest C/P interaction. Humic molecules and organic acids actively compete with phosphorus for soil fixation sites. This means that increases in soil carbon density at higher [CO2] may serve to displace phosphate ions from sorption sites and into the soil solution, where they can then be utilized by plants. It is not inconceivable that this effect could give rise to a runaway positive feedback CO,-induced increases in tropical forest plant growth... 

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