Cooldown on LNG Tank

[zwoll]

Is it acceptable to use liquid nitrogen for cooling down LNG tank? Then later on introduce LNG to tank? How it will affect mixing Liquid Nitrogen and LNG in the storage tank.
API 625 says LNG is utilized for cool down operation. Appreciate any help!

 

[georgeverghese]

Liquid nitrogen boils at -196degC, while LNG IBP is at approx -160degC at 1atm. So, after cooldown with LN2, once tank temp has risen to say -190degC or higher, but still less than -160degC, you can be sure that there is no more LN2 in the tank.

 

[chicopee]

Introducing N2 into LNG will lower the BTU content of pure LNG providing both are thoroughly mixed as gases before being used as a fuel. Same thing applies to natural gas which is mixed with air during winters by gas companies when there limited supply and the gas air mixture would be used for heating and process applications.

 

[IRstuff]

Since the LN2 will eventually evaporate from the LNG, you will need to bleed off the nitrogen gas. Unless you have a bountiful supply of LN2, there will be a cost factor, although blowing off LNG as a gas is probably a worse atmospheric impact.

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[Compositepro]

There essentially will not be any LN2 in the LNG, as georgev points out. The N2 will all be in the vapor space. This will all have to be vented off eventually and may result in over-pressuring the tank, if not vented.

What pressure does the tank operate at? Where does vented gas go?