Quench


Quench

A quench refers to a rapid cooling. In polymer chemistry and materials science, quenching is used to prevent low-temperature processes such as phase transformations from occurring by only providing a narrow window of time in which the reaction is both thermodynamically favorable and kinetically accessible. For instance, it can reduce crystallinity and thereby increase toughness of both alloys and plastics (produced through polymerization).

In metallurgy, it is most commonly used to harden steel by introducing martensite, in which case the steel must be rapidly cooled through its eutectoid point, the temperature at which austenite becomes unstable. In steel alloyed with metals such as nickel and manganese, the eutectoid temperature becomes much lower, but the kinetic barriers to phase transformation remain the same. This allows quenching to start at a lower temperature, making the process much easier. High speed steel also has added tungsten, which serves to raise kinetic barriers and give the illusion that the material has been cooled more rapidly than it really has. Even cooling such alloys slowly in air has most of the desired effects of quenching.

Extremely rapid cooling can prevent the formation of all crystal structure, resulting in amorphous metal or "metallic glass".

Role of quenching in scrubbing

In pollution scrubbers, sometimes hot exhaust gas is quenched, or cooled by water sprays, before entering the scrubber proper. Hot gases (those above ambient temperature) are often cooled to near the saturation level.

If not cooled, the hot gas stream can evaporate a large portion of the scrubbing liquor, adversely affecting collection efficiency and damaging scrubber internal parts. If the gases entering the scrubber are too hot, some liquid droplets may evaporate before they have a chance to contact pollutants in the exhaust stream, and others may evaporate after contact, causing captured particles to become reentrained. In some cases, quenching can actually save money.

Cooling the gases reduces the temperature and, therefore, the volume of gases,permitting the use of less expensive construction materials and a smaller scrubber vessel and fan.

A quenching system can be as simple as spraying liquid into the duct just preceding the main scrubbing vessel, or it can be a separate chamber (or tower) with its own spray system identical to a spray tower.

Quenchers are designed using the same principles as scrubbers. Increasing the gas-liquid contact in them increases their operational efficiency. Small liquid droplets cool the exhaust stream more quickly than large droplets because they evaporate more easily. Therefore, less liquid is required. However, in most scrubbing systems, approximately one-and-a-half to two and- a-half times the theoretical evaporation demand is required to ensure proper cooling ("Industrial Gas Cleaning Institute 1975"). Evaporation also depends on time; it does not occur instantaneously.

Therefore, the quencher should be sized to allow for an adequate exhaust stream residence time. Normal residence times range from 0.15 to 0.25 seconds for gases under 540°C (1000°F) to 0.2 to 0.3 seconds for gases hotter than 540°C ("Schifftner 1979").

Quenching with recirculated scrubber liquor could potentially reduce overall scrubber performance, since recycled liquid usually contains a high level of suspended and dissolved solids. As the liquid droplets evaporate, these solids could become reentrained in the exhaust gas stream. To help reduce this problem, clean makeup water can be added directly to the quench system rather than adding all makeup water to a common sump. [ [http://www.epa.gov/apti/ US EPA Air Pollution Training Institute] developed in collaboration with North Carolina State University, College of Engineering (NCSU)]

References

External links


Wikimedia Foundation. 2010.

Synonyms:

Look at other dictionaries:

  • quench — quench·able; quench·able·ness; quench; quench·er; quench·less; quench·less·ly; quench·less·ness; un·quench·ably; …   English syllables

  • Quench — Quench, v. t. [imp. & p. p. {Quenched}; p. pr. & vb. n. {Quenching}.] [OE. quenchen, AS. cwencan in [=a]cwencan, to extinguish utterly, causative of cwincan, [=a]cwincan, to decrease, disappear; cf. AS. cw[=i]nan, [=a]cw[=i]nan, to waste or… …   The Collaborative International Dictionary of English

  • quench — [kwentʃ] v [T] formal [: Old English; Origin: cwencan] 1.) quench your thirst to stop yourself feeling thirsty, by drinking something ▪ We stopped at a small bar to quench our thirst. 2.) quench a fire/flames to stop a fire from burning ▪ a… …   Dictionary of contemporary English

  • Quench — Quench, v. i. To become extinguished; to go out; to become calm or cool. [R.] [1913 Webster] Dost thou think in time She will not quench! Shak. [1913 Webster] …   The Collaborative International Dictionary of English

  • quench — [kwench] vt. [ME quenchen < OE cwencan, to extinguish, caus. of cwincan, to go out, akin to Fris kwinka, MHG verquinen, to pass away < IE base * gwey , to complain, weep] 1. to extinguish; put out [to quench fire with water] 2. to overcome; …   English World dictionary

  • Quench — Genre musical Trance, Techno, House Années d activité 1993 2003 Quench est un duo australien spécialisé dans la musique techno, formé du compositeur et producteur Cj Dolan et du producteur Sean Quinn. Leur titre Dreams, paru en 1993 et… …   Wikipédia en Français

  • quench — ● quench nom masculin (anglais quench) Brusque refroidissement d un produit pétrolier au cours d un procédé de raffinage, ayant pour but de stopper la réaction de craquage ou d en contrôler l exothermicité …   Encyclopédie Universelle

  • quench — [ kwentʃ ] verb transitive 1. ) if you quench your THIRST, you drink something so that you do not feel thirsty anymore a ) used about other types of feelings or needs: Books have traditionally quenched a child s thirst for knowledge. 2. ) OLD… …   Usage of the words and phrases in modern English

  • quench — (v.) O.E. acwencan to quench (of fire, light), from P.Gmc. *cwandjan, probably a causative form of root of O.E. cwincan to go out, be extinguished, O.Fris. kwinka. Related: Quenched; quenching …   Etymology dictionary

  • quench — [v1] destroy, extinguish annihilate, check, choke, crush, dampen, decimate, demolish, destruct, dismantle, douse, end, kill, knock down, moisten, put down, put out, quash*, quell, raze, ruin, shatter, smother, snuff out*, stifle, suppress, wreck; …   New thesaurus

  • quench — index allay, assuage, cancel, deter, discourage, extinguish, inhibit, quash, repress …   Law dictionary