In this episode of the Oil 101 podcast series, we will discuss Natural Gas Liquids.
Raw natural gas produced at the wellhead contains various heavier hydrocarbons that can be liquefied into condensate and removed from the natural gas stream.
These are called natural gas liquids or NGLs
In this 4-minute podcast, we will discuss:
- What are NGLs
- NGL extraction
- Lead absorption method
- Cryogenic processes
Listen to Oil 101 – What Are NGLs? below:
Thanks for listening to the EKT Interactive Oil and Gas Podcast Network.
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- Upstream vs Downstream
What are NGLs?
The processed natural gas used by consumers is composed almost entirely of methane.
But raw natural gas produced at the wellhead also contains various heavier hydrocarbons that can be liquefied into condensate and removed from the natural gas stream.
These are called natural gas liquids or NGLs and include ethane, propane, butane, pentane and pentanes plus.
NGLs have higher values as separate products, making it economical to remove them from the gas stream.
While the chemical composition of NGLs is similar, their applications vary widely. For example, ethane is used to produce ethylene, which is turned into plastics. Propane, in contrast, is burned for heating.
NGLs are also often used as feedstocks for petrochemical plants. Butane is usually further processed in refineries and added to gasoline.
For reporting purposes, reserves associated with NGLs are typically combined with oil reserves, not natural gas.
Producers often target liquids-rich production during periods of higher oil prices, as the value of NGLs is influenced by oil prices.
The US is the largest producer of NGLs in the world, reaching nearly 3.3 million barrels per day in 2015.
There are two principle techniques for removing NGLs from the natural gas stream – the lean absorption method and cryogenic processes.
Lean Absorption Method
Most NGLs can be removed by passing the raw natural gas through an absorption tower.
The natural gas is brought in contact with a pool of gas-absorbing oil that catches the heavier hydrocarbons but allows the methane to move through.
The mixture of absorbing oil and NGLs is fed into an oil still. Here it is heated to a temperature above the boiling point of the NGLs, but below that of the oil, to separate them.
While absorption methods can extract almost all of the heavier NGLs, the lighter hydrocarbons, such as ethane, are often more difficult to recover. In some cases, it is economic to simply leave the lighter NGLs in the natural gas stream.
But if needed, cryogenic processes can be used to extract ethane and other lighter hydrocarbons.
The cryogenic process consists of dropping the temperature of the gas stream to around minus 120 degrees Fahrenheit using a turbo-expander.
At this temperature, methane remains a gas while the heavier hydrocarbons become liquid and can be removed.
The collected NGLs are then separated by passing them through a series of progressively warmer distillation columns, starting with a demethanizer.
Each column is controlled to a temperature that will vaporize a specific hydrocarbon from the remaining liquid.