Flash Evaporation - Flash Evaporation of A Single-component Liquid

Flash Evaporation of A Single-component Liquid

The flash evaporation of a single-component liquid is an isenthalpic 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.

where:
	= weight fraction vapourized
	= upstream liquid enthalpy at upstream temperature and pressure, J/kg
	= flashed vapor enthalpy at downstream pressure and corresponding saturation temperature, J/kg
	= 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.

where:
	= weight fraction vapourized
	= liquid specific heat at upstream temperature and pressure, J/(kg °C)
	= upstream liquid temperature, °C
	= liquid saturation temperature corresponding to the downstream pressure, °C
	= liquid heat of vaporization at downstream pressure and corresponding saturation temperature, J/kg

Here, 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 desalination of brackish water or ocean water by "Multi-Stage Flash 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.