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Kinetic effects in thermal explosion with oscillating ambient conditions
View Preview. Learn more Check out. Abstract A procedure is presented herein to calculate the critical temperature for the thermal explosion hereafter symbolized as T c of an arbitrary quantity of thermally unstable liquid placed in an arbitrary container, by measuring experimentally the rate of heat transfer from the container as well as the rate of heat generation of the liquid sample in temperature increments of 1. Citing Literature. Volume 14 , Issue 1 January Pages Related Information. Close Figure Viewer.
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Browse All Figures Return to Figure. Previous Figure Next Figure. There are also concerns regarding global warming that a global average increase of 3—4 degrees Celsius above the preindustrial baseline could lead to a further unchecked increase in surface temperatures.
Critical Temperatures for the Thermal Explosion of Chemicals
For example, releases of methane , a greenhouse gas more potent than CO 2 , from wetlands , melting permafrost and continental margin seabed clathrate deposits could be subject to positive feedback. Thermal runaway is also called thermal explosion in chemical engineering , or runaway reaction in organic chemistry. It is a process by which an exothermic reaction goes out of control: the reaction rate increases due to an increase in temperature, causing a further increase in temperature and hence a further rapid increase in the reaction rate.
This has contributed to industrial chemical accidents , most notably the Texas City disaster from overheated ammonium nitrate in a ship's hold, and the explosion of zoalene , in a drier, at King's Lynn. Chain branching is an additional positive feedback mechanism which may also cause temperature to skyrocket because of rapidly increasing reaction rate. Chemical reactions are either endothermic or exothermic, as expressed by their change in enthalpy. Many reactions are highly exothermic, so many industrial-scale and oil refinery processes have some level of risk of thermal runaway.
These include hydrocracking , hydrogenation , alkylation S N 2 , oxidation , metalation and nucleophilic aromatic substitution. For example, oxidation of cyclohexane into cyclohexanol and cyclohexanone and ortho-xylene into phthalic anhydride have led to catastrophic explosions when reaction control failed. Thermal runaway may result from unwanted exothermic side reaction s that begin at higher temperatures, following an initial accidental overheating of the reaction mixture. This scenario was behind the Seveso disaster , where thermal runaway heated a reaction to temperatures such that in addition to the intended 2,4,5- trichlorophenol , poisonous 2,3,7,8-tetrachlorodibenzo-p-dioxin was also produced, and was vented into the environment after the reactor's rupture disk burst.
Thermal runaway is most often caused by failure of the reactor vessel's cooling system. Failure of the mixer can result in localized heating, which initiates thermal runaway. Similarly, in flow reactors , localized insufficient mixing causes hotspots to form, wherein thermal runaway conditions occur, which causes violent blowouts of reactor contents and catalysts.
Incorrect equipment component installation is also a common cause. Many chemical production facilities are designed with high-volume emergency venting, a measure to limit the extent of injury and property damage when such accidents occur. At large scale, it is unsafe to "charge all reagents and mix", as is done in laboratory scale.
Consequently, reactions that easily cool fast enough in the laboratory can dangerously self-heat at ton scale. In , this kind of erroneous procedure caused an explosion of a 2, U. Some laboratory reactions must be run under extreme cooling, because they are very prone to hazardous thermal runaway. The UK Chemical Reaction Hazards Forum  publishes analysis of previously-unreported chemical accidents to assist the education of the scientific and engineering community, with the aim of preventing similar occurrences elsewhere.
Almost such reports are available to view as of January Microwaves are used for heating of various materials in cooking and various industrial processes. The rate of heating of the material depends on the energy absorption, which depends on the dielectric constant of the material. The dependence of dielectric constant on temperature varies for different materials; some materials display significant increase with increasing temperature.
This behavior, when the material gets exposed to microwaves, leads to selective local overheating, as the warmer areas are better able to accept further energy than the colder areas—potentially dangerous especially for thermal insulators, where the heat exchange between the hot spots and the rest of the material is slow.
These materials are called thermal runaway materials. This phenomenon occurs in some ceramics. Some electronic components develop lower resistances or lower triggering voltages for nonlinear resistances as their internal temperature increases. If circuit conditions cause markedly increased current flow in these situations, increased power dissipation may raise the temperature further by Joule heating.
A vicious circle or positive feedback effect of thermal runaway can cause failure, sometimes in a spectacular fashion e.
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To prevent these hazards, well-designed electronic systems typically incorporate current limiting protection, such as thermal fuses, circuit breakers, or PTC current limiters. To handle larger currents, circuit designers may connect multiple lower-capacity devices e. This technique can work well, but is susceptible to a phenomenon called current hogging , in which the current is not shared equally across all devices.
Typically, one device may have a slightly lower resistance, and thus draws more current, heating it more than its sibling devices, causing its resistance to drop further. The electrical load ends up funneling into a single device, which then rapidly fails. Thus, an array of devices may end up no more robust than its weakest component. The current-hogging effect can be reduced by carefully matching the characteristics of each paralleled device, or by using other design techniques to balance the electrical load.
However, maintaining load balance under extreme conditions may not be straightforward. Devices with an intrinsic positive temperature coefficient PTC of electrical resistance are less prone to current hogging, but thermal runaway can still occur because of poor heat sinking or other problems. Many electronic circuits contain special provisions to prevent thermal runaway. This is most often seen in transistor biasing arrangements for high-power output stages.
However, when equipment is used above its designed ambient temperature, thermal runaway can still occur in some cases. This occasionally causes equipment failures in hot environments, or when air cooling vents are blocked. This can lead to thermal runaway phenomena within internal regions of the semiconductor junction ; the resistance decreases in the regions which become heated above this threshold, allowing more current to flow through the overheated regions, in turn causing yet more heating in comparison with the surrounding regions, which leads to further temperature increase and resistance decrease.
This leads to the phenomenon of current crowding and formation of current filaments similar to current hogging, but within a single device , and is one of the underlying causes of many semiconductor junction failures. Leakage current increases significantly in bipolar transistors especially germanium -based bipolar transistors as they increase in temperature. Depending on the design of the circuit, this increase in leakage current can increase the current flowing through a transistor and thus the power dissipation , causing a further increase in collector-to-emitter leakage current.
Thermal runaway - Wikipedia
This is frequently seen in a push—pull stage of a class AB amplifier. If the pull-up and pull-down transistors are biased to have minimal crossover distortion at room temperature , and the biasing is not temperature-compensated, then as the temperature rises both transistors will be increasingly biased on, causing current and power to further increase, and eventually destroying one or both devices. Another practice is to mount a thermal feedback sensing transistor or other device on the heat sink, to control the crossover bias voltage.
As the output transistors heat up, so does the thermal feedback transistor. It is worth noting that as convective heat loss parameter increases sodium droplet explosion time significantly increases. Figure 6 reveals the effects of modified radiative heat loss parameter on sodium droplet explosion time for typical sensitized, Arrhenius and bimolecular reaction rates. Significant increment in sodium droplet explosion time is notice as modified radiative heat loss parameter increases.
In the same vein, Figure 7 shows the effects of modified radiative heat loss parameter on sodium droplet explosion time for typical sensitized, Arrhenius and bimolecular reaction rates. It is observed that increase in modified radiative heat loss parameter causes a significant reduction in sodium droplet explosion time.
Conclusions It is pertinent to note that previous studies on ignition or thermal explosion single sodium droplet were characterized only with constant reaction rate and thermal conductivity. In this study, the problems of thermal explosion of single sodium droplet have been extended to permit a more general reaction rates and variable thermal conductivity in the presence of both convective and radiative heat losses.
The new model was considered under physically reasonable assumptions in order to obtain information which is useful for safety measures in nuclear reactor facilities. The mathematical formulation involves a highly non-linear ordinary differential equation. The entire dynamical picture of the model is qualitatively uncovered by exploiting numerical techniques. Numerical results were presented in Tables and Figures for sensitized, Arrhenius and bimolecular reaction rates respectively.
The effects of varying pertinent dimensionless parameters were also reported. Tabular and graphical results revealed that the delay before explosion and reduction in critical temperature depend significantly on the inclusion of variable thermal conductivity and heat loss mechanisms, which is of great importance for nuclear facilities safety.
In addition, contribution of generic reaction rates allows study on sensitized and bimolecular reaction rates which were not considered in the previous studies.