Once the gas stream has had the acid gases and water vapor removed, it is now classified as dry, sweet gas, and is suitable for even further processing. The next logical step is NGL (Natural Gas Liquids) extraction and fractionation.
These processes are dictated by the profitability metric referred to as the “Frac Spread”. This is calculated as the difference between the revenue from sales of NGL’s contained in the gas stream as liquid and their value if left in the gas pipeline and sold at gas prices.
NGLs are composed of the following components:
- Ethane (C2)
- Propane (C3)
- Butanes (C4) – Both normal and iso-
- Natural Gasoline – mix of pentanes (C5) and heavier molecules (C6 - C9)
Natural Gasoline is used in gasoline blending, specialty solvent production, gasoline blending, feedstock for ethanol production, and as a diluent for syncrude production.
MEI Maverick Engineering has the experience and capabilities to meet your needs for Greenfield and Brownfield NGL projects, including:
- Front-end design packages
- Detailed Engineering & Design
- Procurement, including purchasing, expediting, and inspection services
- Project Controls (Cost Control and Scheduling)
- Total Installed Cost Estimates (per AACE guidelines)
- Construction services & management
- Project Management
- Commissioning & Startup services
NGL Extraction
There are two principle methods for removing NGLs from the natural gas stream are the absorption method and the cryogenic expander process.
Absorption
The absorption method for NGL extraction is extremely similar to absorption for dehydration. The primary difference lies in the use of an absorbing oil in NGL absorption as opposed to glycol in the dehydration process. The absorbing oil has an affinity for the NGLs, similar to the affinity for water demonstrated by glycol. As the natural gas is passed through the absorption tower, it contacts the “lean” absorption oil which absorbs a high volume of the NGLs. The absorption oil is now “rich” in NGLs, and exits the bottom of the tower. The “rich” oil goes to a lean oil still, where the mixture of absorption oil and NGLs is heated to a temperature higher than the boiling point of the NGLs, but below that of the oil. This process yields a recovery of approximately 75% of the butanes and 85-90 % of the natural gasoline from the natural gas feedstock.
The absorption process can be improved to target specific NGLs. The lean oil is cooled through refrigeration prior to feeding to the absorption tower. This refrigerated absorption method can recover propane upwards of 90%, approximately 40% of the ethane, and virtually 100% of the natural gasoline can also be recovered by this method.
Cryogenic Expansion Process
Absorption methods can extract virtually all of the heavier NGLs, however, the lighter hydrocarbons, such as ethane, prove more difficult to recover from the natural gas feedstocks. If it is economic to extract ethane and other lighter hydrocarbons, cryogenic processes are demanded for their high recovery rates. Cryogenic processes consist of lowering the temperature of the gas stream to around -120 degrees Fahrenheit.
There are a number of different ways to chill the gas to these temperatures, but one of the most effective is the turbo expander process. The natural gas stream is cooled by using external refrigerants, followed by an expansion turbine which rapidly expands the chilled gases. This causes the natural gas temperature to drop significantly and rapidly, thus condensing ethane and other hydrocarbons. Methane will remain in its’ gaseous form. Recovery of ethane in the Cryogenic expansion process is approximately 90-95 %. In addition, the expansion turbine is able to convert some of the energy released when the natural gas stream is expanded into recompressing the gaseous methane effluent, thus saving energy costs associated with extracting ethane.
The extraction of NGLs from the natural gas stream produces both cleaner, purer natural gas, as well as the valuable hydrocarbons that are the NGLs themselves. The extracted gas will now serve as a feedstock for LNG (Liquid Natural Gas) production.
NGL Fractionation
Once NGLs have been removed from the natural gas stream, they must be separated into their individual products to be useful. This is accomplished through fractionation. Fractionation works based on the different boiling points of the different hydrocarbons in the NGL stream. A particular fractionator is named for the hydrocarbon that is boiled off. The entire fractionation process is a series of steps, starting with the removal of the lighter NGLs from the stream, usually in the following order:
- Deethanizer - ethane removal
- Depropanizer – propane removal
- Debutanizer – butanes (normal and iso-) removal, leaving the pentanes and heavier hydrocarbons in the NGL stream.
- Butane Splitter or Deisobutanizer - this step separates the iso and normal butanes.
LPG is commonly referred to as “Liquefied Petroleum Gas” and consists of propane, butane(s), or a combination of the two.