Page 61 - Separation process engineering

Chapter 2. Flash Distillation

2.1 Basic Method of Flash Distillation

One of the simplest separation processes commonly employed is flash distillation. In this process, part of
a liquid feed stream vaporizes in a flash chamber or part of a vapor feed condenses, and the vapor and
liquid in equilibrium with each other are separated. The more volatile component will be more
concentrated in the vapor. Usually a large degree of separation is not achieved; however, in some cases,

such as the desalination of seawater, complete separation results.
The equipment needed for flash distillation is shown in Figure 2-1 for a liquid feed. The fluid is
pressurized and heated and is then passed through a throttling valve or nozzle into the flash drum. Because
of the large drop in pressure, part of the fluid vaporizes. The vapor is taken off overhead, while the liquid
drains to the bottom of the drum, where it is withdrawn. A demister or entrainment eliminator is often
employed to prevent liquid droplets from being entrained in the vapor. The system is called “flash”
distillation because the vaporization is extremely rapid after the feed enters the drum. Because of the
intimate contact between liquid and vapor, the system in the flash chamber is very close to an equilibrium

stage. Figure 2-1 shows a vertical flash drum, but horizontal drums are also common. Partial
condensation is similar to Figure 2-1, except the vapor is cooled before entering the drum.
Figure 2-1. Flash distillation system

The designer of a flash system needs to know the pressure and temperature of the flash drum, the size of

the drum, and the liquid and vapor compositions and flow rates. He or she also wishes to know the
pressure, temperature, and flow rate of the feed entering the drum. In addition, he or she needs to know
how much the original feed has to be pressurized and heated. The pressures must be chosen so that at the
feed pressure, p , the feed is below its boiling point and remains liquid, while at the pressure of the flash
F
drum, p drum , the feed is above its boiling point and some of it vaporizes. Because the energy for

vaporization comes from the hot feed, T > T drum , if the feed is already hot and/or the pressure of the flash
F
drum is quite low, the pump and heater shown in Figure 2-1 may not be needed.

The designer has six degrees of freedom to work with for a binary separation. Usually, the original feed
specifications take up four of these degrees of freedom:
Feed flow rate, F

Feed composition, z (mole fraction of the more volatile component)