Vacuum Distillation Unit - Part 3 - Petroleum Refining Article Series - Chemo Concept

In the previous article, We talked about the atmospheric distillation unit which separates desalted crude oil into various petroleum fractions. These fractions are further separated into their own separate unit to get pure hydrocarbons or to get hydrocarbon of the specifies range. 

In the atmospheric distillation unit, the Separation of the heavier fraction is not very efficient and economical. To separate heavier fraction into a different fraction we require more energy input. 

Two major solutions can be used to separate heavier fractions: 

1. Increasing temperature: Heavier fraction can be separated by providing extra heat in the furnace which increases the temperature of crude oil and separates heavier fractions. There is one major drawback of this method, By increasing the temperature of crude oil, there are chances of cracking of different fractions. 
2. Decreasing pressure: By reducing the pressure, We can decrease the boiling point of the components. So by reducing the pressure of the system we can separate heavier components into fractions by application of heat without cracking of components. 

Suggested Read: A Beginner's Guide of Distillation

What is Vacuum Distillation

Vacuum distillation is a distillation operation carried out in a vacuum (pressure less than atmospheric pressure) for materials that have a high boiling point (very low vapour pressures) or materials that may degrade near to their boiling point. 

Vacuum Distillation Operation

Distillation is carried out at lower pressures (about 25 to 40 mm of Hg abs). If required pressure can be further decreased by the addition of steam at the furnace inlet and bottom of the distillation unit. 

The addition of steam is very beneficial because it not only increases the velocity of petroleum fraction in the furnace tube and also minimizes coke formation in the furnace. Also, steam decreases the partial pressure of petroleum fraction which makes the separation more efficient.

Factors affect Vacuum Distillation Unit Design and Operation

Three major parameters affect distillation operation

1. Boiling range of the feed 
2. Pressure inside column
3. Furnace outlet temperature

Vacuum distillation is generally carried out for atmospheric column bottoms and separated into light gas oil and heavy gas oil. Also, light ends and heavy bottoms are produced in the vacuum distillation unit.  

The furnace outlet temperature would be 380 to 450°C. Depending on the requirement different outlet temperature can be achieved by changing the steam flow rate in the column or furnace. There are three configurations: 

1. Dry Operation: In this operation, No steam is added in the inlet of the furnace and at the bottom of the column to get the maximum outlet temperature of petroleum fractions. 
2. Wet operation: In this operation, Steam is added at the inlet of the furnace and at the bottom of the column to get the minimum outlet temperature of petroleum fractions.
3. Damp Operation: In this operation, Steam is only added to the furnace inlet to get the temperature in between temperature achieved by dry operation and wet operation. 

By lowering the pressure inside a column, Separation can be improved. To lower the pressure inside column equipment such as steam ejector and barometric condenser or vacuum pump and surface condenser is used.

Conclusion

In this post, we talked about vacuum distillation in petroleum refining. We discussed What is vacuum distillation, vacuum distillation operation and factors affecting the vacuum distillation unit such as the boiling point range of the distillation unit, the outlet temperature of the furnace and operating pressure of the column. 

Further Also Check Out: (Previous Article on This Series) 

• Crude Oil Desalting
• Atmospheric Distillation Unit