Pressure Control of LPG off-spec tank

[Jack Nicholson]

Hi.
I have a question. would you please help me?
We have a LPG off-spec tank, during upset condition Propane and Butane is sent to this tank for further reprocessing. Tank Pressure control is done using methane at 30 barg (as illustrated on picture). Here is the question.
When we reprocess off-spec LPG, lots of soluble methane comes into LPG and hence Propane and butane. How could I overcome this problem.
My idea:
1- Stop using methane as blanketing, and control the tank pressure with its own LPG vapor pressure.

 

[MortenA]

If you need 30 barg (e.g. to move the liquid out of the tank) then your propane wont generate enough pressure. At 20ºC the vapour pressure of propane is around 8 barg.

Best regards, Morten

 

[Latexman]

Building on MortenA's observation, IF the liquid out of the tank goes through a large pressure drop where it is injected into the process, this might be a good place to put a flash tank to vent off mostly CH4 to the flare. If not, too bad.

Good Luck,
Latexman

 

[Jack Nicholson]

Dear MortenA.
Methane supply pressure is 30 barg, but we keep tank pressure about 18 barg. Vapor pressure of LPG at 30 C is around 14 barg, so no LPG would vaporize.
Yes, We pump LPG back to the process, and this CH4 would help maintaining enough pump NPSH.

Dear Latexman
The LPG goes back to the process using pump, then enter depropanzier and debutanizer column at (15 barg, 44 C) and (5 barg, 45 C) respectively. Then this streams enter heat exchangers, where temperature lower to -40 C and -6 C respectively.
Subcooled propane and butane at 4 barg and (-40 C, -6 C) enter in storage tank, where the pressure drop to 4 mbarg. So lots of CH4 produce here...
Please see the attached file

 

[MortenA]

If you can live with whatever your vapor pressure at the ambient/fluid temperature is then you dont need the C1 - You mention NPSH yourself. So the LPG (wo C1) will be at its boiling pressure - but there is a static head from the liquid level in your tank to the pump. Maybe this will enough easy enough to check if you have the pump curve and can measure the min. elevation difference. Now you only have to check if the head of your pump is sufficient.

BTW if your off-spec product is C3/C4 only (and no C1 or C2 w.o. the blanket gas) then the vapor pressure _wont_ be 14 barg at 30ºC! Pure propane (the lighter of C3 and C4) has a VP of 10.2 bara at 32.2º:

https://www.engineeringtoolbox.com/propane-butane-mix-d_1043.html

So your VP of 14 barg at 30ºC makes me think that contaian (a lot) of C1&C2)

Best regards, Morten

 

[LittleInch]

Pressure it with Propane instead?

Just reroute a propane gas feed into a compressor and use it for pressurisation gas?

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