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Vapor Composition in a Closed Tank

Vapor Composition in a Closed Tank

Vapor Composition in a Closed Tank

(OP)
Hello,

I'm trying to figure out the vapor composition in our expansion tank. The solution is a 30% propylene glycol solution (by weight) and it is at a temperature of 110 degrees Celsius. The total pressure in the tank is 26 PSIG.

There is a 5 PSIG nitrogen blanket on the tank too.

My calculation method was to use the vapor pressure of water at 110 degrees C (20.78 PSI) and solve:

20.78/(26+14.7)*100% = 51.05 mol % water
19.7/(26+14.7)*100% = 48.4% nitrogen
100-51.05-48.40 = 0.55 mol% PG

Or is there a better calculation method to figure out the composition of vapor in the headspace of this tank.

Thanks in advance.

RE: Vapor Composition in a Closed Tank

How are you determining that the N2 pad is 5 psig?

I think you have something else contributing to the vapor pressure

http://www.dow.com/PublishedLiterature/dh_0047/090...

The vapor pressure of 30% PG at 110C is about 1000 mmHg abs, that's about 5 psig at sea level. Add your nitrogen (if 5 psig is right) and that should be a total of 10 psig. If you have 26 psig, there's something else you haven't considered.

RE: Vapor Composition in a Closed Tank


Is it possible that the tank with the PG solution was blanketed with 5 psig N2 and then heated to 110oC?

RE: Vapor Composition in a Closed Tank


Or, may be the tank contained N2 at 5 psig and at some lower temperature and then a hot PG solution was pumped in? In short, could it be the N2 pressure increased due to gas heating?

RE: Vapor Composition in a Closed Tank

(OP)
Thanks for the replies. This is an expansion tank so I forgot to mention that some of the pressure increase is due to the volume change of liquid from 20C to 110C. The tank starts with the 5 PSIG pad at 20C, then the solution is heated to 110C.

Thanks.

RE: Vapor Composition in a Closed Tank


As suspected, the total pressure is the sum of the vapor pressure of the solution at 110oC plus that of nitrogen.

If the solution approaches ideality Raoult's law becomes applicable.

RE: Vapor Composition in a Closed Tank

I am not sure I am correct but I will give this a try. Assuming rault's law is valid in this case for both glycol and water, their partial pressures at 110 deg C can be calculated as
p(glycol)=vapor pressure of glycol*mole fraction in solution
p(water)=vapor pressure of water*mole fraction in solution
now
p(nitrogen)=P(total)-p(water)-p(glycol)
By knowing all three partial pressures the mole fractions may be calculated.
p(glycol)=P(total)*y(glycol) and so on

RE: Vapor Composition in a Closed Tank

(OP)
Thanks everyone.

The reasoning behind this is that we had a PSV discharge line freeze that discharges into this tank. The PSV discharges from the tube side of a shell and tube heat exchanger back into the tank. We have a flow switch on this discharge line that has not alarmed and it is working so the PSV has not discharged.

The discharge line is not a dip tube that discharges in the liquid level of the tank (tank is around 30% full before heating the glycol). The thought was the vapor from the 30% glycol is mostly water, it condensed in the discharge line and froze. The line is insulated but not heat traced.

That is the reasoning why I wanted to know if the vapor was mostly water since 30% glycol freezes around 7 degrees F and water is 32 F.

Thanks again for the responses.

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