Posted by AnnMarie Fauske on Serious incidents can result in death, injury, capital loss, and business interruptions.
Chemical engineering[ edit ] Thermal runaway is also called thermal explosion in chemical engineeringor runaway reaction in organic chemistry. It is a process by which an exothermic reaction goes out of control: 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 hydrocrackinghydrogenationalkylation SN2oxidationmetalation 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 Runaway reactions unwanted exothermic side reaction s that begin at higher temperatures, following an initial accidental overheating of the reaction Runaway reactions. Failure of the mixer can result in localized heating, which initiates thermal runaway.
Similarly, in flow reactorslocalized 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. Inthis 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. Almost such reports are available to view as of January Microwave heating[ edit ] 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. Electrical engineering[ edit ] 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. 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.Runaway Reactions Reactive chemical hazards can lead to runaway reactions with a catastrophic consequence.
The hazards in such system are seldom the characteristics of the chemical by itself but are highly dependent on the process conditions and modes of operations.
A General Strategy for the Safer Scale-Up of Batch and Semi-Batch Reactions -with Richard Kwasny, Ph.D., Senior Consulting Engineer, Fauske & Associates, LLC. Thermal runaway incidents continue to occur in batch production facilities in the chemical and pharmaceutical industries.
Serious incidents can result in death, injury, capital loss, and business interruptions. Preventing Runaway Reactions thermal stability criteria [1, 4] As a guideline, three levels are sufficient to characterize the severity and probability of a runaway reaction, as shown in the Table.
A runaway reaction is frequently going to the worst case contingency when sizing the relief devices for your reactor systems.
The problem is, these are extremely involved calculations that require specialized data, and chemical engineering and modeling expertise. How to Prevent Runaway Reactions August Phenol-formaldehyde reactions are common industrial processes. The reaction of phenol or substituted phenol with an aldehyde, such as formaldehyde, in the presence of an acidic or basic catalyst is used to prepare phenolic resins.
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 other words, "thermal runaway" describes a process which is accelerated by increased temperature, in turn releasing energy that further increases temperature.