Everything about Flash Evaporation totally explained
Flash (or partial) evaporation is the partial
vaporization that occurs when a
saturated liquid stream undergoes a reduction in pressure by passing through a
throttling valve or other throttling device. This process is one of the simplest
unit operations. 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
flash drum.
If the saturated liquid is a single-component liquid (for example, liquid
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
volatile components than is the remaining liquid.
Flash evaporation of a single-component liquid
The flash evaporation of a single-component liquid is an
isenthalpic (for example, constant
enthalpy) process and is often referred to as an
adiabatic 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 )
»
Newton's method (also known as the Newton-Raphson method) is an efficient iterative algorithm for solving the above Rachford Rice equation. Alternatively, an
Excel spread sheet and the Excel Solver function can be used.
The equilibrium flash of multi-component liquids is very widely utilized in
petroleum refineries,
petrochemical and
chemical plants and
natural gas processing plants.
(See
mole fraction for a definition of that terminology).
Spray drying
Spray drying is the rapid drying of a
slurry of very small solids suspended in a liquid. The slurry is first
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 or
electrostatic precipitators.
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's a form of liquid evaporation, it's quite different from flash evaporation.
Further Information
Get more info on 'Flash Evaporation'.
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