Flash evaporation

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.