Flash Distillation

Table of Contents

What is Flash Distillation?

Flash distillation is a single stage separation process used to separate a liquid stream with multiple contents into separate components [1]. A liquid stream is passed into the separator at a constant pressure and is then heated causing the more volatile components in the liquid stream to enter the vapor phase while the less volatile components remain in the liquid phase [2]. The remaining liquid stream is then passed through a valve and into a flash drum. This results in a large drop in pressure resulting in some of the liquid vaporizing [2]. The flash drums are often very large which gives a distinct separation in the vapor phase at the top of the drum and the liquid phase at the bottom of the drum. This allows for the vapor to be extracted at the top and for the liquid to be drained at the bottom [2]. Flash distillation is a single stage operation and is good for simple separations between components of separate volatilities with Raoult's Law being a fundamental concept along with vapor-liquid equilibrium. Figure 1 below shows how the compositions of a binary mixture are affected when the components are heated and have different volatilities. Inlet liquid streams with only two components are considered a binary mixture while streams with more than two components are called multi component. Packing material can also be used in flash distillation to allow for more effective separation between the components in different phases. The packing material in the flash drum should be able to absorb the respective gas and liquid components to allow for trace amounts of a component to exit the rising vapor phase and join the descending condensed phase going down the flash drum [5]. However since flash distillation with a flash drum is only a single stage operation, the effectiveness of packing material would be low due to the limited contact the material has with the rising vapor compared to that in a multi stage distillation column.

How to Build a Flash Distillation Unit

Building a flash distillation unit first requires a heater to raise the temperature of the liquid mixture feed. There then needs to be a valve allowing the warmed liquid feed to pass into a large flash drum which is just a large vessel to enable the pressure drop and separation process. Valves can also be added at the two separate outgoing product streams to collect the vapor and liquid streams. A pump may also be required at the start to continuously move the liquid mixture feed [1].

Advantages and Disadvantages to Flash Distillation

Industry Applications

Flash distillations ability to easily separate vapors from liquids makes it a common unit operation in various industries. Petroleum refinery is a big industry that utilizes flash distillation to separate different hydrocarbons based on their volatilities. Crude oil distillation involves heating up crude oil to a specific temperature which enables easy hydrocarbon separation due to the different volatilities of the components. Other processes in the petroleum refining industry that use flash distillation include hydrotreating, alkylation, and steam cracking. Similarly to oil refinery, flash distillation can also be used for natural gas processing to separate gases such as methane, ethane, and nitrogen. Acid gas treatment is also a common use of flash distillation which aids in sulfur removal from gases. Sulfur is removed through a low pressure flash drum which causes light gases such as methane and ethane to vaporize first leaving a sulfur rich stream to be later treated[3]. The process can be illustrated by Figure 1 below.