This is as good a place to start as any…..
NGL stands for Natural Gas Liquids. That does not mean that all natural gas liquids come from natural gas. Far from it.
Natural Gas Liquid refers to the lighter, condensable hydrocarbon fractions of the hydrocarbon stream that you are looking at. ”Lighter” hydrocarbons are those with less carbon molecules. Methane is the lightest because it has one carbon and 4 hydrogen atoms. Hydrocarbons chain together and become quite long, some containing 20 or more carbon atoms.
So….lighter means less carbon atoms. It also mean lower boiling points. This will be important later, because these liquids are processed and separated by boiling.
Ok….so what is really in the list of “NGLs” ? Here is a reference list so you can get a feel for it.
|Methane||164–160 °C, -263-256 °F||Gas||CH4|
|Ethane||-89 °C, -127 °F||Gas or NGL||X||CH2H6|
|Propane||-42 °C, -43 °F||NGL||X||Y||CH3H8|
|IsoButane||-9 °C, 8-16 °F||NGL||Nat Gasoline||X||Y||CH4H10|
|Butane||-1 °C, 30-34 °F||NGL||Nat Gasoline||X||Y||CH4H10|
|Pentane||36 °C, 97 °F||NGL||Nat Gasoline||X||Y||CH5H12|
|Hexane||68-69 °C, 155-156 °F||NGL||Nat Gasoline||X||Y||CH6H14|
|Other Heavier (C7+||>70 °C, >157 °F||NGL||Nat Gasoline||X||Y||CH6H14|
I could go on for a while with these definitions, but you get the point. If you are interested, you can find more of this in numerous places on the net.
The first entry on the list, methane, is NOT an NGL. Methane can not be condensed under normal processes, because the boiling point is so low. That is ok, because it is normally used for heating fuel.
The first condensable hydrocarbon is ethane. The boiling point is low, but ethane has value by itself for the chemical industry. Ethane is very interesting, because it can wind up used for fuel OR it can be used as chemical feed. It usually has more value as chemical feed, but that is not always the case.
Methane and ethane are sometimes referred to as “dry gas” or “natural gas” because these two hydrocarbons make up over 95% of the gas that is delivered to city gates for use as fuel.
You will also notice these hydrocarbons are classified as X or Y grades. Y grade is a common term in the industry for the “easily” condensables. X and Y grade hydrocarbons can be stored in a liquid state under pressure. Y grade can be stored at a pressure much less than X grade, which makes it easier to move in a liquid state.
Propane is used for both fuel and for chemical feed, because it is shipped via pipelines / rail / trucks to retail market hubs ( fuel ) and can be cracked ( chemical ). It all depends on the price and availability.
Isobutane and butane are interesting, because they are the same chemical formula. However, isobutane boils more closely to propane. “Iso” is used in refinery processes to make gasoline as well as other chemical industry processes. Butane is used to either make more iso or to blend straight into gasoline. The gasoline blending of butane depends a great deal on vapor pressure requirements of the finished product.
Most natural gasoline ( see the table column ) does not get much heavier than C6 ( hexane ). However, small amounts will be present. In certain formations for natural gasoline from gas processing there might be some other compounds like benzene. This may seem odd, because benzene is usually associated with refinery processes. However, in areas of high geothermal activity, samples will show benzene because the geothermal heat caused the benzene reaction to occur.
Terminology that Means the Same Thing
Terminology that Does NOT Mean the Same Thing!
You may hear some people refer to these heavier hydrocarbons in the C5 to C7 range as “light naptha”. The heavier gasoline is also referred to as naptha. Naptha is generally between C5 and C12. It does not occur in great quantities in natural gas sourced liquids, but is found more in crude oil or as an intermediate product in a refinery.
Different terminology can tell you the background of the person you are talking to. Most refinery people will refer to these heavier components and “gasoline” or “naptha” or “light straight run”. Most gas processing people will just refer to it as “C5+”. Folks up in the northern tier of states in the US and the Canadians may refer to it as Condensate.
LPG is Not NGL!
LPG stands for Liquified Petroleum Gas. It contains propane, isobutane and/or butane.
NGL stands for Natural Gas Liquids. That is everything from ethane thru C5+.
As a final comment, there are some compounds that occur in natural gases that should be mentioned now because they lower the value of the gas and gas products. These are typically removed during scrubbing, compression, dehydration, treating, or at the gas processing plant. Below is a partial list of some of the molecules in natural gas streams that are removed from the gas streams to increase the value. The boiling points are important. That will be discussed in the next post!
|Helium||−268.93 °C, −452.07 °F||He|
|Hydrogen||-252.87 °C, -423.17 °F||H2|
|Nitrogen||-195.79 °C, -320.33 °F||N2|
|Hydrogen Sulfide||-60 °C, -76 °F||H2S|
|Carbon Dioxide||-57 °C, -70 °F||CO2|
|Water||100 °C, 212 °F||H2O|