Flash evaporation

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Flash evaporation

Flash (or partial) evaporation
Evaporation

Evaporation is the slow vaporization of a liquid and the reverse of condensation. A type of phase transition, it is the process by which molecules in a liquid State of matter spontaneously become gaseous ....
is the partial vaporization

Vaporization

Vaporization of an element or compound is a phase transition from the liquid phase to gas phase. There are two types of vaporization: evaporation and boiling....
that occurs when a saturated liquid

Boiling point

The boiling point of a liquid is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid....
stream undergoes a reduction in pressure by passing through a throttling valve

Thermal expansion valve

A thermostatic expansion valve is a component in refrigeration and air conditioning systems that controls the amount of superheat at the outlet of the evaporator....
or other throttling device. This process is one of the simplest unit operation

Unit operation

In chemical engineering and related fields, a unit operation is a basic step in a process. For example in milk processing, homogenization, pasteurization, chilling, and packaging are each unit operations which are connected to create the overall process....
s. If the throttling valve or device is located at the entry into a pressure vessel

Pressure vessel

A pressure vessel is a closed container designed to hold gases or liquids at a pressure different from the ambient pressure.The pressure differential is potentially dangerous and many fatal accidents have occurred in the history of their development and operation....
so that the flash evaporation occurs within the vessel, then the vessel is often referred to as a flash drum

Vapor-liquid separator

A vapor-liquid separator is a vertical vessel used in several industrial applications to separate a vapor-liquid mixture. Gravity causes the liquid to settle to the bottom of the vessel, where it is withdrawn....
.

If the saturated liquid is a single-component liquid (for example, liquid propane

Propane

Propane is a three-carbon alkane, normally a gas, but compressible to a transportable liquid. It is derived from other petroleum products during oil or natural gas processing....
or liquid ammonia

Ammonia

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Encyclopedia

Vaporization

Boiling point

Thermal expansion valve

Unit operation

Pressure vessel

Vapor-liquid separator

Propane

Ammonia

Ammonia is a chemical compound with the chemical formula nitrogenhydrogen. It is normally encountered as a gas with a characteristic pungent odor....
), 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

Propane

Propane is a three-carbon alkane, normally a gas, but compressible to a transportable liquid. It is derived from other petroleum products during oil or natural gas processing....
, isobutane

Isobutane

Isobutane, also known as methylpropane or 2-methylpropane, is an alkane, isomeric with butane. Recent concerns with depletion of the ozone layer by freon gases have led to increased use of isobutane as a gas for refrigeration systems, especially in domestic refrigerators and freezers, and as a propellant in aerosol sprays....
and normal butane

Butane

Butane, also called n-butane, is the unbranched alkane with four carbon atoms, CH3CH2CH2CH3....
), the flashed vapor is richer in the more volatile

Volatility

Volatility is the measure of the state of instability.*For volatility in chemistry, see Volatility .*For volatility in finance, see Volatility ....
components than is the remaining liquid.

Flash evaporation of a single-component liquid

The flash evaporation of a single-component liquid is an isenthalpic (i.e., constant enthalpy

Enthalpy

In thermodynamics and chemistry, the enthalpy is a quotient or description of thermodynamic potential of a system, which can be used to calculate the heat transfer during a quasistatic process taking place in a closed system thermodynamic system under constant pressure....
) process and is often referred to as an adiabatic
Adiabatic process

In thermodynamics, an adiabatic process or an isocaloric process is a thermodynamic process in which no heat is transferred to or from the working fluid....
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.

X = 100 ( H_u^L – H_d^L ) ÷ ( H_d^V – H_d^L )

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

X = 100 · c_p ( T_u – T_d ) ÷ H_v

( Note: 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 the desalination

Desalination

Desalination, desalinization, or desalinisation refers to any of several processes that remove excess sodium chloride and other minerals from water....
of brackish water or ocean water by "Multi-Stage Flash

Multi-Stage Flash

Multi-stage flash distillation is a desalination process that distillation sea water by flashing a portion of the water into steam in multiple stages of what are essentially regenerative heat exchangers....
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.

Equilibrium flash of a multi-component liquid

The equilibrium flash of a multi-component liquid may be visualized as a simple distillation

Distillation

Distillation is a method of separation process mixtures based on differences in their Volatility in a boiling liquid mixture. Distillation is a unit operation, or a physical separation process, and not a chemical reaction....
process using a single equilibrium 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-error iterative

Iterative method

In computational mathematics, an iterative method attempts to solve a problem by finding successive approximations to the solution starting from an initial guess....
solution. Such a calculation is commonly referred to as an equilibrium flash calculation. It involves solving the Rachford-Rice equation:

where:

z_i is the mole fraction of component i in the feed liquid (assumed to be known);
ß is the fraction of feed that is vaporised;
K_i is the equilibrium constant of component i.

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

where:

x_i is the mole fraction of component i in liquid phase;
y_i is the mole fraction of component i in gas phase.

Once the Rachford-Rice equation has been solved for ß, the compositions x_i and y_i can be immediately calculated as:

The Rachford-Rice equation can have multiple solutions for ß, at most one of which guarantees that all x_i and y_i will be positive. In particular, if there is only one ß for which:

then that ß is the solution; if there are multiple such ßs, it means that either K_max<1 or K_min>1, indicating respectively that no gas phase can be sustained (and therefore ß=0) or conversely that no liquid phase can exist (and therefore ß=1).

It is possible to use Newton's method
Newton's method

In numerical analysis, Newton's method is perhaps the best known method for finding successively better approximations to the zeroes of a Real number-valued function ....
for solving the above Rachford-Rice equation, but there is a risk of converging to the wrong value of ß; it is important to initialise the solver to a sensible initial value, such as (ß_max+ß_min)/2 (which is however not sufficient: Newton's method makes no guarantees on stability), or, alternatively, use a bracketing solver such as the bisection method

Bisection method

In mathematics, the bisection method is a root-finding algorithm which repeatedly divides an Interval in half and then selects the subinterval in which a root exists....
or the Brent method, which are guaranteed to converge but can be slower.

The equilibrium flash of multi-component liquids is very widely utilized in petroleum refineries

Oil refinery

An oil refinery is an industrial process plant where crude oil is processed and refined into more useful petroleum products, such as gasoline, diesel fuel, asphalt, heating oil, kerosene, and liquefied petroleum gas....
, petrochemical

Petrochemical

Petrochemicals are chemical products made from raw materials of petroleum or other hydrocarbon origin. Although some of the chemical compounds that originate from petroleum may also be derived from coal and natural gas, petroleum is the major source....
and chemical plant

Chemical plant

A chemical plant is an industry Industrial process factory that manufactures chemicals, usually on a large scale. The general objective of a chemical plant is to create new material wealth via the chemical or biological transformation and or separation of materials....
s and natural gas processing

Natural gas processing

Natural gas processing plants, or fractionators, are used to purify the raw natural gas extracted from underground Natural gas field and brought up to the surface by Oil well....
plants.

Spray drying

Spray drying is a commonly used method of drying a liquid feed through a hot gas. Typically, this hot gas is air, but sensitive materials such as Medication, and solvents like ethanol require oxygen-free drying and nitrogen gas is used instead....
is the rapid drying of a slurry

Slurry

A slurry is, in general, a thick suspension of solids in a liquid and may be:* A mixture of water and cement to form concrete* A mixture of water, thickening agent#weapon use, and oxidizers used as an water gel...
of very small solids suspended in a liquid. The slurry is first atomized

Atomization

*In atomic spectroscopy, atomization stands for the conversion of a vaporized sample into atomic components. Liquid samples are first nebulized, the fine mist is transported into the atomization source , where the solvent evaporates and the analyte is vaporized, then atomized....
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 using cyclones, bag filters

Filtration

Filtration is a mechanical or physical operation which is used for the separation of solids from fluids by interposing a medium to fluid flow through which the fluid can pass, but the solids in the fluid are retained....
or electrostatic precipitator

Electrostatic precipitator

An electrostatic precipitator , or electrostatic air cleaner is a particulate collection device that removes particles from a flowing gas using the force of an induced electrostatic charge....
s.

A brief explanation of spray drying has been included here because some readers may consider spray drying to be a form of flash evaporation. However, although it is a form of liquid evaporation, it is quite different from flash evaporation.

External links

Animation, photos and technical explanation of the difference between Flash Steam and Vapor.
The benefits of recovering flash steam, how it is done and typical applications.
in the Middle East and Western Asia
Flash distillation of the hydrocarbon compounds.

Flash evaporation

Flash evaporation of a single-component liquid

Equilibrium flash of a multi-component liquid

Spray drying

See also

External links