The difference between LPG, NGL & LNG

This stuff is dangerous. It is REALLY scary that people in the plant don't know what it is. Let me describe them in order of increasing energy density:

CNG--Compressed natural gas, is gas used for motor fuel compressed to a high pressure (usually around 3,000 psig) and is primarily methane (C1, but can have some C2)

LNG--Liquefied natural gas, is gas that has been compressed at very low temperatures and is transported as a liquid. Primarily methane.

LPG--Liquefied Petroleum Gas, gas that is liquid near room temperature and atmospheric pressure. This product has very little methane and butane and is primarily propane (C3)

NGL--Natural Gas Liquids, heavier hydrocarbons that have been extracted from field gas for use as a feedstock in chemical plants. Depending on prices for Propane and Ethane, NGL may or may not contain these products (as propane and ethane prices fluctuate the plants bypass, partially bypass, or use the de-propanizer and de-ethanizer vessels). The combined mix of stuff is a liquid at ambient temperature and moderate pressure.


David Simpson, PE
MuleShoe Engineering
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

"It is always a poor idea to ask your Bridge Club for medical advice or a collection of geek engineers for legal advice"

Thanks for your reply. The facilite that I worked in is NGL recovery unit. However, in the past it was called as LPG recovery unit. I am confused and I need more clarfication about the difference between them.

Here is the quickest way to get Z

T = Temp in deg F
P = Pressure in psia
G = specific gravity
Criticle Temp (CT)= 169.01+314.001*G
Criticle Press (CP) = 708.75-57.5*G
Reduced P (PR) = P/CP
reduced Temp (RT) =(460+T)/CT
Z = 1 + (0.0703 * PR / TR) * (1 - 6 / TR ^ 2)


For gases with less than 5% N2 and CO2 and pressures under 1500 psis.

Dcasto,
Were you in the wrong thread?

David

yeah, I hit a key and didn't realize it and jumper here.

LPG is normally a commercial product ready to sell. You can sell it for a posted price.

NGL's is a mixture of many components and it is not commerically sold. The price for it is set by it's composition and by the commerical products mixed in it. NGL may have 50% LPG, 20% commercial Butane, 10% iso butane, and 20% gasoline, so the price is the weighted average.

Let me explain the difference between LPG and NGL while giving a bit of historical perspective. LPG originated as a mixed component stream isolated in a refinery or gas plant which contained primarily propane (C3) and was typically isolated from butane, particularly isobutane in a de-propanizer distillation column. The butane and heavier mix was often used to blend into mogas. A significant fraction of the LPG was still n-butane, but almost all of the heavier components were removed. The resulting liquid propane product was sold for use as fuel in heaters and engines designed to run on propane (e.g. forklifts).

LPG was collected by chilling a gas stream that was rich in propane and butanes. Normally the chilling was accomplished by using either a lean oil absorber with ammonia refrigeration, or with propane refrigeration which could lower the temperature to minus 40 deg F. After recovery, the propane vapor pressure mostly determined the pressure the LPG liquid would exert, so most propane (LPG) storage is designed to meet this pressure. At normal ambient temperatures, LPG has a pressure of approximately 140 psia, but at high ambient temperatures over 100 deg F, the pressure can exceed 200 psia. Up through the 1950s, this was the main product used.

With the growth of the petrochemical industry in the 1960s, the demand for ethane for ethane crackers to make ethylene soared. (Ethylene is used to make plastics and polymers.) In order to extract and collect ethane, gas plants were designed and built to extract ethane from natural gas. These plants typically used turboexpander plants where the ethane-rich natural gas was cooled during expansion in an expansion turbine to provide the cooling, followed by compression of the lean gas back to pipeline delivery pressure (typically in excess of 1000 psig) in the compressor section of the turboexpander. These gas plants could chill the gas to temperature below minus 80 deg F, and remove the bulk of the ethane.

The recovered NGL was generally more than 50 mol% ethane and exerted much higher vapor pressures than LPG, because ethane at normal ambient temperatures has a vapor pressure of approximately 580 to 620 psia. Special NGL pipelines and gathering and storage was designed to transport and store the higher pressure NGL product. After transportation, the NGL is typically fractionated in a de-ethanizer to remove the ethane, followed by a de-propanizer to remove the propane from the butane and heavier. In some cases, the butanes are fractionated in a de-isobutanizer to separate i-butane from n-butane. The vast majority of the ethane transported in these systems is used in ethane crackers to make ethylene, and most of these crackers are in either Texas or Louisiana. The longest NGL pipelines extend up into Wyoming from Texas.

If your facility was originally designed to handle LPG, and now is handling NGL, you need to be very careful that the facility vessels and piping can handle the higher pressure NGL. The most likely problem could occur if you have propane storage bullets (vessels) designed with wall thicknesses and heads for the lower pressure LPG. In LPG plants, these storage bullets are large and expensive, so they used minimum wall thicknesses etc. If you fill these vessels with a high fraction ethane NGL, you could get a catastrophic failure. Be forewarned!

zinfan, you history is "outta date" At first they used lean oil to isolate LPG, not refrigeration. Lean oil plants were used to remove ethane at rates up to 50% well into the 60's.

LPG is anywhere from a mixture of C4 and C3's at 1% to 40% C4. The RVP is from 140 to 208, depending on contracts.

Ethane has no vapour pressure, its a gas at 100F. Plants can now extract over 90% of the ethane.

We can recover 20% of the ethane and put the mixture in our propane bullets.

If you like to join GPA section A where all the natural gas experts hang out, come on in.

Response to dcasto:

First, I would like to say that I was motivated to respond by a possible safety hazard; namely the storage of potentially high vapor pressure NGL in storage bullets designed for a lower pressure LPG product. If you read the questioner response above, he states:
"The facilite that I worked in is NGL recovery unit. However, in the past it was called as LPG recovery unit. I am confused and I need more clarfication about the difference between them. "

If the facility was originally designed to handle LPG, which typically involve normal operating pressures of only around 200 psi, he should be aware that some (high ethane content) NGL can have a much higher vapor pressure, and hence pose a possible hazard.

I found an online link to an NGL pipeline spec, which shows the vapor pressure specification for the NGL mix (Y grade) in that pipeline is 600 PSI.

I've put 50 psia vapour pressure product in that line. The 600 psia is the maximum value. I've messed up and put 800 psia stuff too (not for long).

There had better not be a safety issue. If the plant doesn't have overpressure protection, they are in a world of hurt.

bottomline, what they called it in the past was wrong in todays world. So what. Their plant makes a mixture and they may or may not make spec products. We do not know. If they make spec products like LPG, HD-5 propane, field grade Butanes or spec C4 and spec iC4 or even 15 pound gasoline, then they won't be recovering ethane unless they get a dethanizer and a way to get that ethane (or EP mix) out.