Flash Vaporization

Commonly known as flash vaporization or flash evaporation is the fractional vapor which happens when a saturated liquid stream undergoes a decrease in pressure by passing via a throttling valve or other throttling device. This course is one of the easiest unit operations. If the throttling valve or device is located at the entry into a pressure vessel therefore the flash evaporation takes place inside the vessel, then the vessel is always known as a flash drum.

Auto-refrigeration

If the saturated liquid is single-element liquid, 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 always known as “auto-refrigeration” and is the basis of most traditional vapor compression refrigeration systems. If the saturated liquid is a multi-element liquid the flashed vapor is richer in the more volatile elements than is the remaining liquid. Left out flash vaporization can bring about a boiling liquid expanding vapor explosion.

Evaporation stages

A specific type of flash vaporization is employed in the desalination of brackish water or ocean water by multi-stage flash distillation. The water is heated and then routed into a decreased pressure flash evaporation "stage" where some of the water flashes into stream. This steam is gradually condensed into salt-free water. The remaining salty liquid form that first stage is introduced into a second flash evaporation stage at a pressure lower than the first stage pressure.

Desalination capacity

Lots of water is flashed into stream which is also subsequently condensed into more salt-free water. This orderly use of multiple flash vaporization stages is continued until the design objectives of the system are met. A large part of the worlds installed desalination capacity uses multi-stage flash distillation. Classically such plants have 24 or more sequential stages o flash evaporation.

The equilibrium flash of a multi-component liquid is also an enthalpic process and may be visualized as a simple distillation process using a single equilibrium stage. It is diverse and more difficult than the flash vaporization of single element liquid. This is because a multi element liquid, calculating the sum of flashed vapor and residual liquid in balance with each other at a given temperature and pressure requires a trial and error iterative solution. Such a calculation is usually referred to as an equilibrium flash calculation. It comprises solving the following Rachford-Rice equation.

Questions:

• Define flash vaporization.
• State the significance of flash vaporization.