Cryogenic Separation Of Natural Gases -- Methane, Ethane, Propane, Butane
From Audubon Companies:
Cryogenic separation is a processing operation that is commonly used to extract natural gas liquids (NGLs) from raw natural gas. Unlike absorption methods, which are used primarily for extracting heavier NGLs, cryogenic processes are used to separate out lighter hydrocarbons such as ethane, propane, and butane from an incoming stream.
During the process, refrigerants are used to cool the gas stream to very low temperatures. Expansion turbines then push the gas through a narrow pipe into an expansion chamber. When the gas enters the chamber, its vapor pressure drops, causing it to cool even further (typically to around -120 degrees Fahrenheit). At this temperature, methane (an extremely light hydrocarbon) stays in the stream, while the other heavier hydrocarbons condense into liquid form and flow out.
To separate these NGLs from one another, the liquid mixture is then passed through additional chambers where it is progressively warmed. Each NGL has its own unique boiling point and by controlling the temperatures within the different chambers, each specific hydrocarbon can be systematically removed and collected.
With ethane recovery rates from the gas stream ranging from 90 – 95%, cryogenic separation is typically one of the most efficient methods of NGL extraction. The expansion turbine used in the process can also be configured to harness some of the energy released during the expansion process to recompress the gaseous methane effluent, which ultimately results in lower energy costs.
Aside from propane, which is a very important standalone fuel, NGLs are most commonly used as petrochemical feedstock and as blending and processing components in a wide range of refining processes.
Lots of words above. Bottom line:
- natural gas (all that stuff one sees being flared) comes up with the crude oil
- on site, the natural gas (all that stuff one sees being flared) is separated from the crude oil
- that natural gas, instead of being flared, is brought to a very, very, very low temperature
- at that very, very, very low temperature, methane remains as a gas, and continues to flow (either to be flared or to be captured)
- meanwhile, all the other "gases" become liquid and settle out and piped elsewhere
- these NGLs are piped through progressively warmer chambers, and the various "liquids" separate out into pure ethane, propane, and butane
You are progressing, but still have some misconceptions. At the well, you have three phases coming up: oil (liquid hydrocarbons), "wet gas" (methane plus NGLs), and water.
ReplyDeleteIf natural gas is being flared, it is almost ALWAYS flared at the well. This means the "wet gas" is flared. So you lose BOTH the methane and the more valuable NGLs.
At a natural gas processing plant, fractional refrigeration is used to separate the methane from the various NGLs. However, it is generally illegal to flare the methane at a processing plant. In rare cases (e.g. Canada, Permian), it actually might make temporary sense to flare methane (and ethane), while recovering the more valuable NGLs. This could occur if the price for natural gas was negative, because of a lack of transport pipeline space. NGLs are worth more (and have limited ability to transport by rail/truck/barge) so they will almost always not have negative pricing. But you still can't flare at NGL plants. (I personally think it should be allowed as it would allow at least collecting more oil/NGLs. But in any case, it would be very rare, much rarer than wellsite flaring, which can occur just because of lack of gathering pipes.)
Got it, thanks.
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