Editing Flash evaporation (section)

{{Short description|Partial vapor due to reduction in pressure}}
[[Image:Vap-Liq Separator.png|thumb|right|246px|A typical flash drum]]
'''Flash evaporation''' (or '''partial evaporation''') is the partial vapor that occurs when a [[Boiling point#Saturation temperature and pressure|saturated liquid]] stream undergoes a reduction in pressure by passing through a [[thermal expansion valve|throttling valve]] or other throttling device. This process is one of the simplest [[unit operation]]s. If the throttling valve or device is located at the entry into a [[pressure vessel]] so that the flash evaporation occurs within the vessel, then the vessel is often referred to as a [[vapor–liquid separator|flash drum]].<ref>{{cite book|author=Stanley M. Walas|title=Chemical Process Equipment:Selection and Design|publisher=Butterworth-Heinemann|year=1988|isbn=0-409-90131-8 }}</ref><ref>{{cite book|author=Gas Processing Suppliers Association (GPSA)|title=Engineering Data Book|edition=10th Edition, Vol. 1|publisher=Gas Processing Suppliers Association, [[Tulsa]], [[Oklahoma]]|date=1987}}</ref>

If the saturated liquid is a single-component liquid (for example,  [[propane]] or liquid [[ammonia]]), a part of the liquid immediately "flashes" into vapor. Both the vapor and the residual liquid are cooled to the [[saturation temperature]] of the liquid at the reduced pressure. This is often referred to as "auto-refrigeration" and is the basis of most conventional [[vapor compression refrigeration]] systems.

If the saturated liquid is a multi-component liquid (for example, a mixture of [[propane]], [[isobutane]] and normal [[butane]]), the flashed vapor is richer in the more [[relative volatility|volatile]] components than is the remaining liquid.

Uncontrolled flash evaporation can result in a boiling liquid expanding vapor explosion ([[Boiling liquid expanding vapor explosion|BLEVE]]).