Flash evaporation
Partial vaporization from pressure reduction via throttling.
Flash evaporation, or partial evaporation, occurs when a saturated liquid stream experiences a sudden drop in pressure as it passes through a throttling valve or similar device. This process is one of the simplest unit operations in chemical engineering. When the throttling device is positioned at the entrance to a pressure vessel, the flash evaporation takes place inside that vessel, which is then called a flash drum. For a single-component liquid, such as propane or liquid ammonia, a portion of the liquid immediately flashes into vapor upon the pressure reduction. Both the resulting vapor and the remaining liquid cool to the saturation temperature corresponding to the new, lower pressure. This cooling effect, known as auto-refrigeration, is the principle behind most conventional vapor compression refrigeration systems. The flash evaporation of a single-component liquid is an isenthalpic, or adiabatic, process, and the amount of liquid vaporized can be predicted using a heat balance equation around the throttling device. This type of flash evaporation is employed in multi-stage flash distillation for desalination, where heated water is routed through successive stages at progressively lower pressures, each stage flashing some water into steam that is condensed into fresh water. Such plants typically contain 24 or more sequential stages. In contrast, the equilibrium flash of a multi-component liquid, such as a mixture of propane, isobutane, and normal butane, is more complex. The flashed vapor becomes richer in the more volatile components than the remaining liquid. Calculating the amounts of vapor and liquid in equilibrium requires solving the Rachford-Rice equation through an iterative, trial-and-error method. This multi-component flash is widely used in petroleum refineries, petrochemical plants, and natural gas processing facilities. Uncontrolled flash evaporation can lead to a boiling liquid expanding vapor explosion (BLEVE).
- field
- Chemical engineering / Thermodynamics
- known_for
- Simplest unit operation; basis of multi-stage flash distillation and vapor compression refrigeration
- type
- Physical process
Lore & Background
Flash evaporation occurs when a saturated liquid stream passes through a throttling valve or device, reducing its pressure. If the valve is located at the entry into a pressure vessel, the vessel is called a flash drum. For a single-component liquid, such as propane or liquid ammonia, part of the liquid immediately flashes into vapor, and both vapor and residual liquid cool to the saturation temperature at the reduced pressure—a phenomenon known as auto-refrigeration, which is the basis of most conventional vapor compression refrigeration systems. For a multi-component liquid, the flashed vapor is richer in the more volatile components than the remaining liquid. Uncontrolled flash evaporation can result in a boiling liquid expanding vapor explosion (BLEVE).
Reader's Guide
Flash evaporation is a key process in both single-component and multi-component systems. For single-component liquids, it is an isenthalpic (adiabatic) process, and the fraction vaporized can be calculated using a heat balance equation or, if enthalpy data is unavailable, an approximate equation using specific heat and latent heat. This type of flash evaporation is used in multi-stage flash distillation for desalination, where water is heated and routed through successive reduced-pressure stages, each flashing some water into steam that is condensed into salt-free water. A large part of the world's installed desalination capacity uses this method, typically with 24 or more sequential stages. For multi-component liquids, the equilibrium flash is more complex, requiring a trial-and-error iterative solution of the Rachford-Rice equation to determine the fraction vaporized and the compositions of vapor and liquid phases. The process is distinct from single-component flash and is visualized as a simple distillation using a single equilibrium stage.
Did You Know?
- Flash evaporation of a single-component liquid is an isenthalpic process often referred to as an adiabatic flash.
- The auto-refrigeration effect from flash evaporation is the basis of most conventional vapor compression refrigeration systems.
- Uncontrolled flash evaporation can result in a boiling liquid expanding vapor explosion (BLEVE).
- Multi-stage flash distillation, which uses sequential flash evaporation stages, accounts for a large part of the world's installed desalination capacity.
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