Thermal runaway
Thermal runaway (engl. Runaway ) refers to the overheating of a exothermic chemical reaction or a technical apparatus due to a self-reinforcing heat -producing process. Going through often leads to fire or explosion and, as a result, causes the apparatus to be destroyed by excess pressure (bursting).
Chemical reaction engineering
Exothermic chemical reactions must be controlled by cooling. The cooling must be carried out in a dynamic equilibrium with the heat generation of the chemical reaction in such a way that only enough heat is dissipated that, on the one hand, the reaction does not overheat and, on the other hand, enough heat remains in the system for the reaction to proceed. A temperature increase of 10 ° C accelerates every chemical reaction by a factor of 2 to 3 (so-called RGT rule ).
Thermal runaway can happen if
- the cooling fails or the power is too low.
- the heat transfer from the chemical mixture is too low. This can lead to the effect that the cooling only reaches outer areas due to poor heat transport from the interior of the reaction mass. This is often the result of insufficient mixing of the reactants during the reaction.
- the chemical reaction is accelerated by impurities. This can be, for example, contact with the cooling liquid (water can accelerate some reactions) or materials from the reactor wall, which can sometimes even have a catalytic effect.
The original thermal runaway can subsequently be reinforced by
- Decomposition reactions of the reactants, whose products in turn can be reactive,
- Defects in seals and reactor jacket, which lead to contact with other reactive materials,
- Polymerization (and increase in viscosity ) in the reaction mixture, see also Trommsdorff effect .
Protective measures are
- a generous design of the cooling system
- Devices for extinguishing a fire
- Dosing devices for reaction inhibitors
- Work with sufficient dilution or provide an emergency dilution
Thermal stability of operating points
The risk of thermal runaway arises particularly when a reactor is operated at the unstable operating point . A reactor generally has three possible operating points at which the amount of heat dissipated by cooling corresponds to the amount generated by the exothermic reaction.
The stable operating point at low temperature is characterized by self-regulation, i.e. i.e. that the reactor reaches it by itself. At temperatures below this point, more heat is generated by the reaction than is dissipated by the cooling, which causes the reaction mass to heat up. At temperatures above the dissipated heat is higher than the heat of reaction and the reaction mass cools down.
The unstable operating point is characterized by the fact that the reactor constantly tends to leave it. At lower temperatures, the cooling is stronger than the heat of reaction and the reactor strives to return to the stable operating point. At higher temperatures, on the other hand, the cooling is no longer sufficient to dissipate the heat of reaction and the reactor threatens to run down.
The heat release curve is S-shaped, since in the area of high conversions only a finite amount of heat can be released due to the limited convertible mass, even at any high temperature. Above the unstable operating point there is a further intersection between the heat generation curve and the heat dissipation line. This is a second stable operating point at a higher temperature and a higher specific product output .
The choice of the desired operating point of the reactor will fall on a stable point if possible. In exceptional cases, it may also be necessary to select an unstable operating point. However, the unstable operating point can only be maintained through constant control interventions.
A rising inlet temperature of the coolant or a deterioration in the heat transfer, e.g. B. by fouling , the heat dissipation line can shift to the right or become flatter. At first, this shifts the operating point only imperceptibly to slightly higher temperatures, until finally the point at which the point of intersection between the heat generation curve and the heat dissipation line disappears. This is feared, as the reactor then changes to the upper unstable operating point in a short time without warning - the reactor goes through.
electronics
Lithium-ion batteries
If there is a local short circuit of the internal electrodes in a lithium-ion battery with liquid, solid or bound electrolyte ( lithium polymer battery ), for example due to contamination of the separator by trapped foreign particles or mechanical damage, the short-circuit current can through the internal resistance heats up the immediate vicinity of the damaged area to such an extent that the surrounding areas are also affected. This process expands and releases the energy stored in the accumulator in a short time. Lithium cobalt dioxide batteries are particularly at risk . Such thermal runaways have been blamed as the cause of the more frequent fires in laptop batteries in the past. The causes were probably manufacturing errors in connection with fluctuations in the operating temperature.
With more recent developments, the risk of fire is almost eliminated by changing battery chemistry ( LiFePO 4 ) or by improving the cell membrane, for example ceramic coatings (see Li-Tec Battery ).
transistor
Overheating a transistor increases the current permeability, which can lead to a further increase in current and can heat it up even further. This self-reinforcing process can lead to self-destruction.
In the case of a power MOSFET , when the temperature is on, the drain-source forward resistance increases with increasing temperature, which causes an increasing power loss in the barrier layer. In the event of insufficient cooling, the power loss given off in the form of heat can no longer be sufficiently dissipated, which further increases the forward resistance. This ultimately leads to the destruction of the component.
Electrical engineering
With oil-cooled power transformers, there is a risk of thermal runaway due to contamination (mostly due to water in the hygroscopic cooling oils). The dielectric loss factor increases, which can lead to heating up to explosion.
literature
- R. Gygax: Chemical reaction technology for safety, In: Mettler Toledo publication. No. 00724386, ( source , br.mt.com)
Individual evidence
- ↑ See Störfall Griesheim 1993 at Hoechst
- ↑ Panasonic, December 18, 2009: Panasonic Starts Mass-Production of High-Capacity 3.1 Ah Lithium-ion Battery , added on February 11, 2012
- ↑ etz, VDE-VErlag: Risk of explosion in transformers due to "thermal runaway" , website, accessed on July 1, 2012
- ↑ etz, issue 7/2010, VDE-Verlag: Risk of explosion in transformers due to "thermal runaway" , PDF article, accessed on July 1, 2012