Flash
evaporationEvaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....
is the partial vapor that occurs when a
saturated liquidThe boiling point of an element or a substance 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 valveA thermal expansion valve is a component in refrigeration and air conditioning systems that controls the amount of refrigerant flow into the evaporator thereby controlling the superheating at the outlet of the evaporator...
or other throttling device. This process is one of the simplest
unit operationIn chemical engineering and related fields, a unit operation is a basic step in a process.Unit operation involves bringing a physical change such as separation, crystallization, evaporation, filtration etc.. For example in milk processing, homogenization, pasteurization, chilling, and packaging are...
s. If the throttling valve or device is located at the entry into a
pressure vesselA pressure vessel is a closed container designed to hold gases or liquids at a pressure substantially different from the ambient pressure.The pressure differential is dangerous and many fatal accidents have occurred in the history of their development and operation. Consequently, their design,...
so that the flash evaporation occurs within the vessel, then the vessel is often referred to as a
flash drumA vapor–liquid separator is a device used in several industrial applications to separate a vapor–liquid mixture.For the common variety, gravity is utilized in a vertical vessel to cause the liquid to settle to the bottom of the vessel, where it is withdrawn....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
propanePropane is a three-carbon alkane with the molecular formula , normally a gas, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used as a fuel for engines, oxy-gas torches, barbecues, portable stoves, and residential central...
or liquid
ammoniaAmmonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
), 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
propanePropane is a three-carbon alkane with the molecular formula , normally a gas, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used as a fuel for engines, oxy-gas torches, barbecues, portable stoves, and residential central...
,
isobutaneIsobutane, also known as methylpropane, is an isomer of butane. It is the simplest alkane with a tertiary carbon. 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 normal
butaneButane is a gas with the formula C4H10 that is an alkane with four carbon atoms. The term may refer to any of two structural isomers, or to a mixture of them: in the IUPAC nomenclature, however, butane refers only to the unbranched n-butane isomer; the other one being called "methylpropane" or...
), the flashed vapor is richer in the more
volatileRelative volatility is a measure comparing the vapor pressures of the components in a liquid mixture of chemicals. This quantity is widely used in designing large industrial distillation processes. In effect, it indicates the ease or difficulty of using distillation to separate the more volatile...
components than is the remaining liquid.
Uncontrolled flash evaporation can result in a boiling liquid expanding vapor explosion (BLEVE).
Flash evaporation of a single-component liquid
The flash evaporation of a single-component liquid is an isenthalpic (i.e., constant
enthalpyEnthalpy is a measure of the total energy of a thermodynamic system. It includes the internal energy, which is the energy required to create a system, and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure.Enthalpy is a...
) process and is often referred to as an
adiabaticIn thermodynamics, an adiabatic process or an isocaloric process is a thermodynamic process in which the net heat transfer to or from the working fluid is zero. Such a process can occur if the container of the system has thermally-insulated walls or the process happens in an extremely short time,...
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 ( HuL – HdL ) ÷ ( HdV – HdL )
| where: |
|
| X |
= weight percent vaporized |
| HuL |
= upstream liquid enthalpy at upstream temperature and pressure, J/kg |
HdV
|
= flashed vapor enthalpy at downstream pressure and corresponding saturation
temperature, J/kg |
HdL
|
= 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.
- X = 100 · cp ( Tu – Td ) ÷ Hv
| where: |
|
| X |
= weight percent vaporized |
| cp |
= liquid specific heat at upstream temperature and pressure, J/(kg °C) |
| Tu |
= upstream liquid temperature, °C |
| Td |
= liquid saturation temperature corresponding to the downstream pressure, °C |
Hv
|
= liquid heat of vaporization at downstream pressure and corresponding saturation
temperature, J/kg |
( 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
desalinationDesalination, desalinization, or desalinisation refers to any of several processes that remove some amount of salt and other minerals from saline water...
of brackish water or ocean water by "
Multi-Stage FlashMulti-stage flash distillation is a water desalination process that distills sea water by flashing a portion of the water into steam in multiple stages of what are essentially countercurrent 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
distillationDistillation is a method of separating mixtures based on differences in volatilities of components 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
iterativeIn computational mathematics, an iterative method is a mathematical procedure that generates a sequence of improving approximate solutions for a class of problems. A specific implementation of an iterative method, including the termination criteria, is an algorithm of the iterative method...
solution. Such a calculation is commonly referred to as an equilibrium flash calculation. It involves solving the Rachford-Rice equation:
where:
- zi is the mole fraction of component i in the feed liquid (assumed to be known);
- β is the fraction of feed that is vaporised;
- Ki 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:
- xi is the mole fraction of component i in liquid phase;
- yi is the mole fraction of component i in gas phase.
Once the Rachford-Rice equation has been solved for β, the compositions x
i and y
i can be immediately calculated as:
The Rachford-Rice equation can have multiple solutions for β, at most one of which guarantees that all x
i and y
i will be positive. In particular, if there is only one β for which:
then that β is the solution; if there are multiple such βs, it means that either K
max<1 or K
min>1, indicating respectively that no gas phase can be sustained (and therefore β=0) or conversely that no liquid phase can exist (and therefore β=1).
It is possible to use
Newton's methodIn numerical analysis, Newton's method , named after Isaac Newton and Joseph Raphson, is a method for finding successively better approximations to the roots of a real-valued function. The algorithm is first in the class of Householder's methods, succeeded by Halley's method...
for solving the above water equation, but there is a risk of converging to the wrong value of β; 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 the
bisection methodThe bisection method in mathematics is a root-finding method which repeatedly bisects an interval and then selects a subinterval in which a root must lie for further processing. It is a very simple and robust method, but it is also relatively slow...
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 refineriesAn oil refinery or petroleum refinery is an industrial process plant where crude oil is processed and refined into more useful petroleum products, such as gasoline, diesel fuel, asphalt base, heating oil, kerosene, and liquefied petroleum gas...
,
petrochemicalPetrochemicals are chemical products derived from petroleum. Some chemical compounds made from petroleum are also obtained from other fossil fuels, such as coal or natural gas, or renewable sources such as corn or sugar cane....
and
chemical plantA chemical plant is an industrial process plant 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. Chemical plants use special equipment,...
s and
natural gas processingNatural-gas processing is a complex industrial process designed to clean raw natural gas by separating impurities and various non-methane hydrocarbons and fluids to produce what is known as pipeline quality dry natural gas.-Background:...
plants.
Contrast with spray drying
Spray dryingSpray drying is a method of producing a dry powder from a liquid or slurry by rapidly drying with a hot gas. This is the preferred method of drying of many thermally-sensitive materials such as foods and pharmaceuticals. A consistent particle size distribution is a reason for spray drying some...
is sometimes seen a form of flash evaporation. However, although it is a form of liquid evaporation, it is quite different from flash evaporation.
In spray drying, a
slurryA slurry is, in general, a thick suspension of solids in a liquid.-Examples of slurries:Examples of slurries include:* Lahars* A mixture of water and cement to form concrete* A mixture of water, gelling agent, and oxidizers used as an explosive...
of very small solids is rapidly dried by suspension in a hot gas. The slurry is first
atomizedAtomization or Atomizer may refer to:* The conversion of a vaporized sample into atomic components in atomic spectroscopy* An apparatus using an atomizer nozzle* Atomizer Geyser, a cone geyser in Yellowstone National Park...
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 filtersFiltration is commonly the mechanical or physical operation which is used for the separation of solids from fluids by interposing a medium through which only the fluid can pass...
or
electrostatic precipitatorAn 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.
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