Thermal runaway
Thermal runaway describes a process that is accelerated by increased temperature, in turn releasing energy that further increases temperature. Thermal runaway occurs in situations where an increase in temperature changes the conditions in a way that causes a further increase in temperature, often leading to a destructive result. It is a kind of uncontrolled positive feedback.
In
Chemical engineering
Chemical reactions involving thermal runaway are also called thermal explosions in
Chemical reactions are either endothermic or exothermic, as expressed by their change in
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
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. This is because the amount of reaction scales with the cube of the size of the vessel (V ∝ r³), but the heat transfer area scales with the square of the size (A ∝ r²), so that the heat production-to-area ratio scales with the size (V/A ∝ r). Consequently, reactions that easily cool fast enough in the laboratory can dangerously self-heat at ton scale. In 2007, this kind of erroneous procedure caused an explosion of a 2,400 U.S. gallons (9,100 L)-reactor used to metalate methylcyclopentadiene with metallic sodium, causing the loss of four lives and parts of the reactor being flung 400 feet (120 m) away.[3][4] Thus, industrial scale reactions prone to thermal runaway are preferably controlled by the addition of one reagent at a rate corresponding to the available cooling capacity.
Some laboratory reactions must be run under extreme cooling, because they are very prone to hazardous thermal runaway. For example, in Swern oxidation, the formation of sulfonium chloride must be performed in a cooled system (−30 °C), because at room temperature the reaction undergoes explosive thermal runaway.[4]
Microwave heating
Electrical engineering
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.g. electrical explosion or fire). 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.g. transistors, diodes, or
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
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.
Semiconductors
Bipolar junction transistors (BJTs)
One rule of thumb to avoid thermal runaway is to keep the operating point of a BJT so that Vce ≤ 1/2Vcc
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. This in turn causes the thermal feedback transistor to turn on at a slightly lower voltage, reducing the crossover bias voltage, and so reducing the heat dissipated by the output transistors.
If multiple BJT transistors are connected in parallel (which is typical in high current applications), a current hogging problem can occur. Special measures must be taken to control this characteristic vulnerability of BJTs.
In power transistors (which effectively consist of many small transistors in parallel), current hogging can occur between different parts of the transistor itself, with one part of the transistor becoming more hot than the others. This is called
Power MOSFETs
Power
Metal oxide varistors (MOVs)
Metal oxide varistors typically develop lower resistance as they heat up. If connected directly across an AC or DC power bus (a common usage for protection against voltage spikes), a MOV which has developed a lowered trigger voltage can slide into catastrophic thermal runaway, possibly culminating in a small explosion or fire.[6] To prevent this possibility, fault current is typically limited by a thermal fuse, circuit breaker, or other current limiting device.
Tantalum capacitors
However, if the energy dissipated at the failure point is high enough, a self-sustaining
Therefore, tantalum capacitors can be freely deployed in small-signal circuits, but application in high-power circuits must be carefully designed to avoid thermal runaway failures.
Digital logic
The
Batteries
When handled improperly, or if manufactured defectively, some
Astrophysics
Runaway thermonuclear reactions can occur in stars when nuclear fusion is ignited in conditions under which the gravitational pressure exerted by overlying layers of the star greatly exceeds thermal pressure, a situation that makes possible rapid increases in temperature through gravitational compression. Such a scenario may arise in stars containing degenerate matter, in which electron degeneracy pressure rather than normal thermal pressure does most of the work of supporting the star against gravity, and in stars undergoing implosion. In all cases, the imbalance arises prior to fusion ignition; otherwise, the fusion reactions would be naturally regulated to counteract temperature changes and stabilize the star. When thermal pressure is in equilibrium with overlying pressure, a star will respond to the increase in temperature and thermal pressure due to initiation of a new exothermic reaction by expanding and cooling. A runaway reaction is only possible when this response is inhibited.
Helium flashes in red giant stars
When stars in the 0.8–2.0
Novae
A
X-ray bursts
Analogous to the process leading to novae, degenerate matter can also accumulate on the surface of a neutron star that is accreting gas from a close companion. If a sufficiently thick layer of hydrogen accumulates, ignition of runaway hydrogen fusion can then lead to an X-ray burst. As with novae, such bursts tend to repeat and may also be triggered by helium or even carbon fusion.[20][21] It has been proposed that in the case of "superbursts", runaway breakup of accumulated heavy nuclei into iron group nuclei via photodissociation rather than nuclear fusion could contribute the majority of the energy of the burst.[21]
Type Ia supernovae
A type Ia supernova results from runaway carbon fusion in the core of a carbon-oxygen white dwarf star. If a white dwarf, which is composed almost entirely of degenerate matter, can gain mass from a companion, the increasing temperature and density of material in its core will ignite carbon fusion if the star's mass approaches the Chandrasekhar limit. This leads to an explosion that completely disrupts the star. Luminosity increases by a factor of greater than 5 billion. One way to gain the additional mass would be by accreting gas from a giant star (or even main sequence) companion.[22] A second and apparently more common mechanism to generate the same type of explosion is the merger of two white dwarfs.[22][23]
Pair-instability supernovae
A pair-instability supernova is believed to result from runaway oxygen fusion in the core of a massive, 130–250 solar mass, low to moderate metallicity star.[24] According to theory, in such a star, a large but relatively low density core of nonfusing oxygen builds up, with its weight supported by the pressure of gamma rays produced by the extreme temperature. As the core heats further, the gamma rays eventually begin to pass the energy threshold needed for collision-induced decay into electron-positron pairs, a process called pair production. This causes a drop in the pressure within the core, leading it to contract and heat further, causing more pair production, a further pressure drop, and so on. The core starts to undergo gravitational collapse. At some point this ignites runaway oxygen fusion, releasing enough energy to obliterate the star. These explosions are rare, perhaps about one per 100,000 supernovae.
Comparison to nonrunaway supernovae
Not all supernovae are triggered by runaway nuclear fusion.
See also
- Cascading failure
- Frank-Kamenetskii theory
- Safety of lithium-ion batteries
- Boeing 787 Dreamliner battery problems
- UPS Flight 6(a 2010 jet crash related to lithium-ion batteries in the cargo)
- Plug-in electric vehicle fire incidents
References
- ^ "The explosion at the Dow chemical factory, King's Lynn 27 June 1976" (PDF). Health & Safety Executive. March 1977. Archived from the original (PDF) on 10 January 2018. Retrieved 9 January 2018.
- ISBN 978-0-7506-4883-7. Archivedfrom the original on 2023-09-09. Retrieved 2016-11-05.
- ^ Lowe, Derek (2009-09-18). "175 Times. And Then the Catastrophe". Corante. Archived from the original on 2015-03-20. Retrieved 16 April 2016.
- ^ a b Lowe, Derek (2008-04-30). "How Not To Do It: Diazomethane". Science Translational Magazine. American Association for the Advancement of Science. Archived from the original on 2022-01-15. Retrieved 16 April 2016.
- .
- ^ Brown, Kenneth (March 2004). "Metal Oxide Varistor Degradation". IAEI Magazine. Archived from the original on 2011-07-19. Retrieved 2011-03-30.
- doi:10.1016/S0026-2714(02)00034-3. Archived from the original(PDF) on 2010-09-23.
- ^ "AMD Athlon64 "Venice"". LostCircuits. May 2, 2005. Archived from the original on 2007-05-02. Retrieved 2007-06-03.
- PMID 25919582.
- CNN Money. Archivedfrom the original on November 9, 2020. Retrieved August 3, 2020.
- ^ "PC Notebook Computer Batteries Recalled Due to Fire and Burn Hazard" (Press release). U.S. Consumer Product Safety Commission. Archived from the original on 2013-01-08.
- ^ "Lenovo and IBM Announce Recall of ThinkPad Notebook Computer Batteries Due to Fire Hazard" (Press release). U.S. Consumer Product Safety Commission. 2006-09-28. Archived from the original on 2013-01-08. Retrieved 2018-06-27.
- ^ "Dell laptop explodes at Japanese conference". The Inquirer. 21 June 2006. Archived from the original on 2006-08-15. Retrieved 2006-08-15.
{{cite web}}
: CS1 maint: unfit URL (link) - ^ "Hazardous Materials Accident Brief — Cargo Fire Involving Lithium-Ion Batteries, Memphis, Tennessee, August 7, 2004". National Transportation Safety Board. September 26, 2005. Archived from the original on 2012-10-07. Retrieved 2013-01-26.
- ^ Taylor, David. "The End Of The Sun". The Life And Death Of Stars. Archived from the original on 2019-05-22. Retrieved 2015-05-24.
- ^ Pols, Onno (September 2009). "Chapter 9: Post-main sequence evolution through helium burning" (PDF). Stellar Structure and Evolution (lecture notes). Archived from the original (PDF) on 2019-05-20. Retrieved 2015-05-24.
- from the original on 2023-09-09. Retrieved 2018-11-04.
- PhysOrg. Archivedfrom the original on 13 September 2019. Retrieved 15 August 2010.
- S2CID 17055772.
- S2CID 14089038.
- ^ S2CID 121603976.
- ^ S2CID 38997016.
- ^ "NASA's Chandra Reveals Origin of Key Cosmic Explosions". Chandra X-ray Observatory web site. Harvard-Smithsonian Center for Astrophysics. 17 February 2010. Archived from the original on 11 April 2012. Retrieved 28 March 2012.
- S2CID 4336232.
External links
- Safetycenter.navy.mil: Thermal runaway at the Library of Congress Web Archives (archived 2004-02-23)