Equilibrium??

[Alanator]

I just found this particular forum by accident, even though I have been using eng-tips for several months.

I have a "disagreement" with some colleagues.

We have a 27,000 gallon liquid nitrogen tank. As part of a test I pressurized it to 70 psig, opened the drain valve and monitored the temperature in the liquid line. I closed the drain line a couple of hours later. I left the tank pressurized and opened the drain the next day. I looked at the temperature and could not see any increase. A colleague says the low temperature is caused by liquid flowing across the thermowell and I say it's because the liquid has not equalized. In fact I think the liquid would probably take a couple of weeks to reach saturation temperature. If we fill the tank once or twice a day I'm not sure that it will ever reach the temperature associated with 70 psig.

Any thoughts?

 

[dcasto]

I'll assume you pressured the tank up with nitrogen vapors? When does your nitrogen come from, an air plant or NRU plant?
If a tank that large is not stirred, it can stratify with a colder more dense fluid at the bottom. I've even seen in mixed hydrocarbons a statification. I've even seen non-condensibles come out of solution and raise a storage tank pressure in hydrocarbon service.

 

[Alanator]

There is a pressure-build coil on the tank. A bit of liquid is sent to the coil, vaporized and returned to the top of the tank.

 

[dcasto]

The vaporized nitrogen has a low density and it will not mix. The added pressure on the tank just makes the liquid N2 subcooled for the new pressure. Think about this. If you had a tank of water, it is in equalibrium with water vapor at lets say 90F. There is water vapor above the liquid. Now add N2 to the tank and raise the pressure, the water will not boil or become hotter.

 

[iainuts]

Hi Alanator,
For the sake of argument, let's say your 27,000 gallon LIN tank has an NER (natural evaporation rate) of 0.5%. It may be higher or lower than this due to age or other performance factors, but for a first stab, I’ll use this value.

A 0.5% NER on a 27,000 gallon tank corresponds to a heat leak of roughly 3250 Btu/hr. If LIN is saturated at roughly 20 psig and then pressurized to 70 psig (which results in the fluid subcooling) then assuming the tank is half full (say 14,000 gallons = 94,000 lbm of LIN) the temperature rise rate is aproximately 0.07 degrees F per hour.

If you’re refilling this tank once or twice a day, you’d be hard pressed to measure any significant difference in the temperature over any period of time.