Pulling vacuum from vessel with n-butane

[Mike4chemic]

Hello,
Before performing maintenance at our facilities we need to completely evacuate 2 heat -exchangers from shell side with interconection pipelines from liquid N-butane.
At the beginning we are draining all liquid from the HEs.
Due to layout accessibility there are several of liquid residue at low points of the HEs.
At this stage we are planning to start pulling vacuum from the top of the HEs with a dry vacuum pump.
My question is how long would it take to achieve vacuum of 1 psia? Is it possible? and how cold will the HEs get?. Is it reasonable to get as low as butane saturated temperature at 1 psia? (-67 °F)
Or can we expect that the liquid will evaporate sooner (at higher pressure)?
The vessels volume is around 50 m^3 (1765 ft^3) and we estimate to have 0.5 m^3 (17.65 ft^3) of liquid left in it after draining procedure.
Vacuum pump flow rate is 245 ACFM at 24"HgV and the pump can pull up to 29.5" HgV
Initial conditions after liquid drain are 2.4 bara @ 25 °C
As the HEs are insulated, we can also assume adiabatic process .

I will appreciate any help

Regards, Mike

 

[Mike4chemic]

Dear Colleagues,

Are there any recommendation?

 

[sshep]

Hey Mike,

I know you realize it is possible to theoretically get this system pretty cold by vaporizing butane in the manner you are describing. Every bit of butane vaporized takes that same amount of heat from the liquid and metal it leaves behind. Using your calculated amount of liquid and the mass of the metal, you can calculate what the final temperature could approach assuming a uniform system temperature (i.e. good conduction of heat from the metal mass to the point of vaporization).

Because the vaporization becomes limited by heat ingress into the system, if the exchanger mass is small enough it could take awhile to vaporize the last pockets of liquid- it could also get enough to cause brittle fracture near the end. In proylene, etc refrigerant systems, hot gas of the same composition is sometimes put back ito the system to help vaporize the last pockets of liquid without adding contamination (like steam/water which might freeze later). For other equipment hot N2, steam, etc, put into the system provides heat to the metal which will then conduct to the liquid pockets and vaporize the last butane.

Anyway, the input of heat into the system is the only way to make sure you get the last liquid out quickly and at reasonable temperature if there is alot of butane to vaporize. There is probably a reason why you are not doing this (steam-out or hot N2 drying), but we are not sure why that is.

best wishes always,
sshep