7 replies to this topic

[sazy101]

Hi,

I was wondering that why does Natural gas is liquefied under atmospheric pressure? why not high pressure like NGL?

Is there any specific reason for this? like energy content...

 

Thank you for answering this question in advance

 

Sazy

[ankur2061]

Sazy,

 

Present technology for LNG at higher temperatures and higher pressures has not matured enough whereas refrigerated LNG at atmospheric pressures is a mature technology. Also at higher pressures LNG can hold more CO2 in it, whereas at atmospheric pressure CO2 will not remain in the LNG. Also containment (storage) costs for Pressurized LNG (PLNG) at the present time are higher than the cost for refrigerated LNG at atmospheric pressure.

 

I have provided a fact sheet on LNG titled 'Frequently Asked Questions (FAQ) about LNG' at the following link:

 

http://www.cheresour...atural-gas-lng/

 

Also refer to the following links for PLNG:

 

http://www.gastechno...ensheng_Lin.pdf

 

http://www.ivt.ntnu...._TIF/050189.pdf

 

Regards,

Ankur.

[Bobby Strain]

Natural gas is liquified at high pressure and subcooled to the temperature for storage and transport at near atmospheric pressure. You should be able to Google LNG process and learn about the process.

 

Bobby

[Pilesar]

   During liquefaction, the natural gas is condensed at high pressures because the heat of vaporization is reduced as the pressure approaches critical and the bulk of the refrigeration duty can occur at a temperature significantly higher than atmospheric storage temperature. Natural gas is generally available at high pressures from a pipeline so the LNG plant does not usually have to deal with feed compression costs. Joule-Thompson cooling as the pressure is reduced also aids in the refrigeration.

   The reason natural gas is liquefied is for storage and transport -- it has to be vaporized to be burned. Where large quanities of LNG are stored, the cryogenic storage tanks would be much more expensive if designed for higher pressure. Transportation by truck or ship is generally designed for low pressures for the same reason. On a small scale when transportation is not needed, sometimes natural gas is liquefied at higher than atmospheric pressure. Storing in pressurized tanks can be competitve to atmospheric storage for small producers if transportation is not an issue. Compressed natural gas is an altermative means of storing very small quantities, but the volume reduction is much greater for the liquid.

[PingPong]

Any fluid can only be liquid if its temperature is below its critical temperature.

 

I was wondering that why does Natural gas is liquefied under atmospheric pressure? why not high pressure like NGL?

NGL is natural gas liquid(s), which is propane and heavier. NGL can be liquid at ambient temperature under pressure.

 

LNG is liquified natural gas, which is mainly methane, and that has a critical temperature that is well below ambient temperature. So to liquify natural gas it has to be cooled below its critical temperature, and it turns out that the most economical solution is to cool it so deep that it can be stored at atmospheric pressure.

[fallah]

PingPong,

 

Agreed with your statement...

 

It is obvious that below critical temperature, to liquify the natural gas, the higher themperature corresponds to higher pressure...

[Pilesar]

   As Bobby Strain mentioned, natural gas is liquefied under different conditions than it is stored. The most thermodynamically economical pressure to liquefy natural gas is at high pressure as close to the critical pressure as possible. In that pressure range, the heat of vaporization is smallest and the bulk of the condensing duty takes place at the highest possible temperature. LNG liquefiers are designed so that condensing takes place at a pressure some margin below the critical pressure in order to handle operating variables. This means that most LNG liquefiers operate between 500 and 600 psi when fed by a natural gas source already at high pressure. If the natural gas feed source is at atmospheric pressure, then the economics usually dictate that the feed be compressed several hundred psi before it is liquefied because the reduced refrigeration costs will offset the feed compression costs. As LNG leaves the liquefier, the pressure is reduced for atmospheric storage.

  It is impossible to design a pressurized storage that can store LNG at ambient temperature; its bubble point temperature will always be below ambient temperature anywhere on earth. So LNG storage design emphasizes keeping heat transfer to a minimum. Contrast this with liquid propane which can be stored in pressurized containers such that its bubble point temperature is higher than its ambient environment.

[PingPong]

LNG can very well be stored at elevated pressure as long as that pressure is below the critical pressure. Like with atmospheric storage, a refrig system is then required to keep the LNG at the temperature corresponding with the storage pressure.

 

Whether LNG is stored at an elevated pressure (but below critical pressure) or at atmospheric pressure, is purely an economic decission. And as it turns out it is most economical to store it at atmospheric pressure and -161 ^oC.

 

It is possible to store NGL's like propane at atmospheric pressure, by adding a refrig system, but in this case it turns out that that is not economical.