Bottled nitrogen cooling due to JT effect 3

[PetroBob]

We are letting nitrogen down from high pressure cylinders at 4250 psig / 29,300 kPag, to just above atmospheric. The nitrogen will be used to purge piping and equipment. I understand that the nitrogen will get very cold due to JT cooling.

HYSYS gives very alarming temperatures; eg if bottles are very cold on midwinter day at -22degF / -30degC, then HYSYS predicts the temp of gas once pressure is reduced to atmospheric is -133degF / -92degC. If nitrogen bottles are +50F/+10degC then HYSYS gives -34F/-37C outlet, for which low temp carbon steel is adequate.

I understand that we can take some credit for heat retaining in the metal of cylinders and piping (per previous Eng-tips thread

http://www.eng-tips.com/viewthread.cfm?qid=78680).

However I believe we may still need some heating system so as not to have a metal embrittlement issue.

What solutions are most commonly used to prevent excessive cooling of pressurized gas bottles and downstream piping and equipment? Eg heating jacket on bottle, heat tracing of discharge piping, extra low temp piping?

 

[MortenA]

Sorry what a mess, i started at 0ºC! So at the end its farly close (-88ºC)

 

[gaswell]

Morten,
I appreciate your efforts to validate the results,

with standard models you should be able to replicate
(within the tolerance allowed by different databases)
the values which I calculated with Prode Properties
(see my previous post)

results for
Tin -30C Pin 4250 Psig Pout 0 Psig
calculated Tout with different standard
(not extended) models

Peng Robinson -91.95 C
Soave Redlich Kwong -85.47 C
Lee Kesler (plocker) -88.377 C

a table with experimental values gives about -89.4 C (dH=0)
so all these results are reasonably close
(even SRK if you consider the error vs. total dT)

proceeding from a graph you may exepct an error of 2-5 degrees
but what's the problem ?

I still remain of the opinion that for this application (N2 cylinders)
it's better to ask the supplier,
it's not a special application,
they produce so many N2 cylinders that sure they have all the specs. )

With the limits discussed above (no heat transfer) a simulation may appear a interesting exercise but nothing more.

 

[MortenA]

I agree thats its an exersice (but somewhat interesting). I, however, dont always trust vendors. They want to sell you stuff. t least the calaculation will tell you that you have to sharpen your pensil.

 

[MortenA]

Oh, and you still owe me that reference

 

[PetroBob]

Thanks Gaswell, MortenA and sshep for all the interest. Your check from first principles against simulators like HYSYS is very interesting.

Unfortunately, in this instance, there are times when the gas may be supplied via a high pressure gas cylinder mounted on a vehicle, therefore the client is saying that a heating system for the gas bottles is not a consistently feasible option. We are looking at either low temp (cryogenic) valves and piping and/or two stage heating between a dual regulator/control valve system, either taking credit for natural ambient heat (in the building), or more likely, from heat tracing, or if absolutely necessary, some kind of heater for the gas.

 

[sshep]

Hey PetroBob,

What is the flowrate and temperature target? The first thing to do is to calculate the duty.

My gut feel is the duty will be small, but heat transfer poor, especially if the line ices. An inline electric heater is obvious, but maybe it can be done with fins or extra metal surface to increase the effective heat transfer area.

Best wishes,
Sean