Flash evaporation (orpartial evaporation) is the partial vapor that occurs when asaturated liquid stream undergoes a reduction in pressure by passing through athrottling valve or other throttling device. This process is one of the simplestunit operations. If the throttling valve or device is located at the entry into apressure vessel so that the flash evaporation occurs within the vessel, then the vessel is often referred to as aflash drum.^[1][2]

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

If the saturated liquid is a multi-component liquid (for example, a mixture ofpropane,isobutane and normalbutane), the flashed vapor is richer in the morevolatile components than is the remaining liquid.

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

The flash evaporation of a single-component liquid is anisenthalpic process and is often referred to as anadiabatic flash. The following equation, derived from a simple heat balance around the throttling valve or device, is used to predict how much of a single-component liquid is vaporized.

${\displaystyle X=\frac{H_u^L-H_d^L}{H_d^V-H_d^L}}$^[3]

where:
${\displaystyle X}$	=   weight ratio of vaporized liquid / total mass
${\displaystyle H_u^L}$	=  upstream liquid enthalpy at upstream temperature and pressure, J/kg
${\displaystyle H_d^V}$	=  flashed vapor enthalpy at downstream pressure and corresponding saturation     temperature, J/kg
${\displaystyle H_d^L}$	=  residual liquid enthalpy at downstream pressure and corresponding saturation     temperature, J/kg

If the enthalpy data required for the above equation is unavailable, then the following equation may be used.

${\displaystyle X=\frac{c_p(T_u-T_d)}{H_v}}$

where:
${\displaystyle X}$	=  weight fraction vaporized
${\displaystyle c_p}$	=  liquidspecific heat at upstream temperature and pressure, J/(kg °C)
${\displaystyle T_u}$	=  upstream liquid temperature, °C
${\displaystyle T_d}$	=  liquidsaturation temperature corresponding to the downstream pressure, °C
${\displaystyle H_v}$	=  liquidheat of vaporization at downstream pressure and corresponding saturation     temperature, J/kg

Here, the words "upstream" and "downstream" refer to before and after the liquid passes through the throttling valve or device.

This type of flash evaporation is used in thedesalination ofbrackish water or ocean water by "Multi-Stage Flash Distillation." The water is heated and then routed into a reduced-pressure flash evaporation "stage" where some of the water flashes into steam. This steam is subsequently condensed into salt-free water. The residual salty liquid from that first stage is introduced into a second flash evaporation stage at a pressure lower than the first stage pressure. More water is flashed into steam which is also subsequently condensed into more salt-free water. This sequential use of multiple flash evaporation stages is continued until the design objectives of the system are met. A large part of the world's installed desalination capacity uses multi-stage flash distillation. Typically such plants have 24 or more sequential stages of flash evaporation.

Theequilibrium flash of a multi-component liquid may be visualized as a simpledistillation process using a singleequilibrium stage. It is very different and more complex than the flash evaporation of single-component liquid. For a multi-component liquid, calculating the amounts of flashed vapor and residual liquid in equilibrium with each other at a given temperature and pressure requires a trial-and-erroriterative solution. Such a calculation is commonly referred to as an equilibrium flash calculation. It involves solving theRachford-Rice equation:^[4][5][6][7]

${\displaystyle \sum _i\frac{z_i(K_i-1)}{1+\beta (K_i-1)}=0}$

where:

• z_i is the mole fraction of componenti in the feed liquid (assumed to be known);
• β is the fraction of feed that is vaporised;
• K_i is the equilibrium constant of componenti.

The equilibrium constantsK_i are in general functions of many parameters, though the most important is arguably temperature; they are defined as:

${\displaystyle y_i=K_i{x}_i}$

where:

• x_i is the mole fraction of componenti in liquid phase;
• y_i is the mole fraction of componenti in gas phase.

Once the Rachford-Rice equation has been solved forβ, the compositionsx_i andy_i can be immediately calculated as:

The Rachford-Rice equation can have multiple solutions forβ, at most one of which guarantees that allx_i andy_i will be positive. In particular, if there is only oneβ for which:

then thatβ is the solution; if there are multiple suchβ's, it means that eitherK_max<1 orK_min>1, indicating respectively that no gas phase can be sustained (and thereforeβ=0) or conversely that no liquid phase can exist (and thereforeβ=1).

It is possible to useNewton's method for solving the above water equation, but there is a risk of converging to the wrong value ofβ; it is important to initialise the solver to a sensible initial value, such as (β_max+β_min)/2 (which is however not sufficient: Newton's method makes no guarantees on stability), or, alternatively, use a bracketing solver such as thebisection method or theBrent method, which are guaranteed to converge but can be slower.

The equilibrium flash of multi-component liquids is very widely utilized inpetroleum refineries,petrochemical andchemical plants andnatural gas processing plants.

Spray drying is sometimes seen as a form of flash evaporation. However, although it is a form of liquid evaporation, it is quite different from flash evaporation.

In spray drying, aslurry of very small solids is rapidly dried by suspension in a hot gas. The slurry is firstatomized into very small liquid droplets which are then sprayed into a stream of hot dry air. The liquid rapidly evaporates leaving behind dry powder or dry solid granules. The dry powder or solid granules are recovered from the exhaust air by usingcyclones,bag filters orelectrostatic precipitators.

Natural flash vaporization or flash deposition may occur duringearthquakes resulting in deposition of minerals held insupersaturated solutions, sometimes even valuableore in the case of auriferous, gold-bearing, waters. This results when blocks of rock are rapidly pulled and pushed away from each other byjog faults.^[8]

• Evaporator
• Vapor–liquid separator
• Multi-stage flash distillation

• Vapor and Flash Steam Animation, photos and technical explanation of the difference between Flash Steam and Vaporized fraction.
• Flash Steam Tutorial The benefits of recovering flash steam, how it is done and typical applications.
• Water Desalination Technologies in the Middle East and Western Asia
• Discussion of spray drying
• Flash evaporation program online Flash distillation of the hydrocarbon compounds.

• Gas-liquid separation
• Evaporators

• Articles with short description
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