Pentane Pressure Rise 4

[Mike4chemic]

Hi,

We transfer a liquid pentane at ambient temperature from the track to the storage tank(volume 50m3).
Since the storage tank is a pressure vessel, the filling procedure is executed without an atmospheric vent.

The question is:

1.Does pentane's filling will cause a pressure rise into the storage tank? (liquid volume increased, vapor volume decreased..)
2. Is it correct that part of pentane's vapor will be condensed due to pressure rising ?
The main assumption is that the tank's free volume contains only a pentane's vapor without non-condensable gases.

Thanks in advance
bmicky

 

[Montemayor]

Come on, Mikey! A Chemcial Engineer that doesn't consider the vapor pressure of a condensable fluid is going to have a lot of problems down the road. You and I should know that the pressure in the system is the Pentane's vapor pressure. You've said that there are no non-condensables present. Therefore, at 100 ^oF ambient temperature the pressure in the system (railcar and storage) will be 15.575 psia - according to the NGPSA Databook.

The pressure will rise only if the ambient temperature rises - assuming, as you say, no non-condensables. Why should the vapor pressure rise if the temperature doesn't? The answer is that it doesn't rise - unless you've filled the system with 100% liquid pentane and no vapor space.

This is basic Smith & VanNess Thermo. and should be easily understood.

 

[Latexman]

I know you said no non-condensables were present, but I'd recommend you have a "Plan B" in mind to deal with inerts, if they build-up to a level that causes problems.

Good luck,
Latexman

 

[Montemayor]

Good comment and excellent tip Latex.

I treated the query strictly as it was presented - which is very close to being theoretical - and I agree with your pragmatic approach regarding the possibility of higher pressure (& possible pressure relief incidents) existing due to non-condensables. To avoid taking this into consideration is, in my opinion too simplistic. The existance of low boilers as dissolved impurities is yet another possibility. Designing or planning on the basis of pure vapor pressure is not being realistic.

 

[Latexman]

Thank you, Mr. Montemayor. The reason I brought inerts up is it's an issue that may not affect this application on a day-to-day normal operation basis, but it is very real on start-up, shutdown, and when Mr. Murphy pays you a visit. I have designed, built, started-up, run, and repaired several tanks, including a few that "ride the bubble". With flammables, like Pentane, you MUST purge, pad, and inert the tank prior to introducing the material. Well, the first time you fill the blasted thing, those inerts have to go somewhere! If you don't provide for that in the design, your co-workers start doubting you. Then, later on (hopefully, much later), when your successor's successor has to shutdown and inspect the tank, you have to go through similar and the same motions - MT the tank, purge, pad, and inert, test for Pentane, if ok - admit air, ventilate with air, inspect, repair and test - if needed, wash out, dry, purge, pad, and inert the tank prior to re-introducing the material. So, if you are good (and lucky), the inerts vent will not be used frequently, but it will be used!



Good luck,
Latexman

 

[Mike4chemic]

Gentlemen, Thank you very much. The issue is clear.
Now,I would like to change a scenario.
For example:

We transfer a liquid pentane at 100 F from the track to the storage tank(volume 50m3)by the pump with flow rate 15 m3/hr. The initial pentane's volume is 30 m3. The pressure in the vessel is 30 psia.
As far as I understand, the partial pressure of non-condensable will be 30 psia- 15.6 psia = 14.4 psia
Since the initial tank's free space is 20 m3, after 1 hr the free space will be 5 m3 (20 m3-15 m3).
The non-condensable's pressure rising (after 1 hr) will be:
14.4 psia * 20/5 = 57.6 psia.
Therefore, the total pressure in the tank after 1 hr will be 57.6 psia + 15.6 psia ( pentane's pressure at 100 F)=73.2 PSIA.

Is it correct?

Thanks in advance
bmicky

 

[dcasto]

pretty close. Remember that the tanks "void" volume has pentane in the vapor phase and that 15m3 will condense an fill the tank slightly more too. Do your calculations on a mass basis, not volumetric.

Now were is the "heat removal" system needed to condense those C5 vapors, what about all the heat you put into the C5's through the pump. On small systems or ones realtively large changes. Think about the end where the pentanes come from, some pentanes must vaporize to fill the void, where is the energy coming from to make C5 vapors?

Just some thoughts for you to ponder, and trust me, they will bite you some day.