Final Stages NGL Separation

1. Final Stages NGL Separation Fractionation Unit Overview 3 Towers: Deethanized, Depropanizer, and Debutanizer

2. Deethanizer • NGL’s are fed to this unit from cryogenic unit. • Remember natural gas liquids contains an ethane, propane, butane all mixture. • The first column is the deethanizer.  • Why? Because ethane will be removed from the other productsand will exit out the top of this tower.  • How? NGL’s enter the deethanizer tower where they will be heated up by a reboiler just enough to vaporize ethane. • The reboilertransfers heat via steam or hot oil; dependant on the plant configuration.(Usually hot oil) • Reboiler heats the NGL’s just enough to vaporize ethane allowing it to travel up the tower.The ethane vapor leaves the top of this tower to a condenser.

3. Finished Ethane Product The condenser uses cooling water for the removal of heat from the ethane causing it to condense. Liquid ethane flows to the reflux drum.  Where some ethane is sent back to the column as reflux while some leaves the system as product.  Again, the purpose of reflux is to improve product purity by prohibiting other products from leaving with the ethane.

4. Depropanizer • What doesn’t get vaporized is going to travel out of the bottom of the deethanizer tower.  • Propane and Butane are still present in the NGL stream • This botttomfeed travels to the Depropanizervessel.  • The depropanizeroperates in the same manner as the previous tower. (higher temperature) • Areboilersupplies just enough heat to vaporize propane from the NGL mixture. • Propane escapes from the top as vapor and travels to an Air Cooled heat exchanger.

5. Finished Propane Product The Air Cooled heat exchanger is another form of a condenser used to liquefy propane vapors into a liquid state. This air cooled heat exchanger utelizes a fan to move air across the tubes cooling the propane vapors and allowing them to condense. The condensed vapors (liquid) flow to a reflux drum where some product is sent back to depropanizer as reflux while some is sent out as product.

6. Debutanizer • The remaining NGL stream leaving the bottom of the Depropanizer contains butane and natural gasoline which will be separated by the Debutanizer tower.  • The butane leaves the top of this column and natural gasoline exits the bottom.  • The NGL steam is heated in the debutanizer by a reboiler. • Butane vaporizes and travels up and out the top of this column. • Butanes vapors enter an air cooled heat exchanger.

7. Finished Butane Product • This heat exchanger condenses the butane; causing it to liquefy. • The liquid butane product flows to the reflux drum where some product can sent back to the column as reflux and some can be sent out of the system as product.  • Butane product can be further refined in some plants. • It can be sold as butane product or it can be to a butane splitter tower or deisobutanizer. • Which separates butane from isobutene in a similar fasion. 

8. Natural Gasoline • The Bottom’s product of the debutanizer is natural gasoline. • Natural Gasoline is a product of this unit as well.    • Our discussion today has centered around the process of fractionation.  • Remember: Fractionation columns are very similar to distillation towers. The key difference being that the vaporization points within a fraction tower are much tighter than the vaporization points in a distillation tower.

9. Reminder The towers on this page are the Dethanizer, Depropanizer and Debutanizer.  The Dethanizer has ethane leaving the top, the Depropanizerhas propane going exiting the top and the Debutanizer has butane leaving the top.  So, each tower is named for what product is exiting it as an overhead product and not a bottoms product.