Evacuating Pentane Line Like Refrigerant 1

[RJB32482]

We are looking at evacuating an underground pentane line by pulling a vacuum on it like evacuating refrigerant from a compressor system. I am looking at what vacuum we have to pull on it to get it to "vaporize" and get pulled from the system. So what do I look at for pentane. My thought was the vapor pressure of pentane at ambient conditions. So if the vapor pressure at 60 F is 10 psia, then would we need to pull at least 4.7 psi of vacuum on the pipeline to get it evacuated?

Or how would I compare it to the refrigerant (vapor pressures of both)? So if the vapor pressure of the refrigerant is lower than pentane, then we could pull the same vacuum and be able to get the pentane out of the pipe.

Any thoughts on this method. We are going to steam the pipe also to get the pentane out of the creases in the carbon steel pipe to do hot work on it.

Thanks.

 

[dcasto]

Pulling a vaccum on the line as a vapor recovery is good and it will work, just be prepared to get slugs of liquids in during the operation. Also. it will take longer than you think. The pentane will not just jump to the vapor phase without heat added. That heat will come from the ground and you'll be waiting on the the heat transfer rate of the earth to the pipe.

Be prepared for liquids even after you've pulled a vaccum because you'll be doing a batch distillation of your pentane and I can not believe you don't have some C6 and heaviers that will be left behind. If the pipe diameter will handle it, go to -10 psig.

 

[Montemayor]

RJB:

You fail to tell us what pentane phase is in the line prior to pulling a vacuum. I have to assume that the line is filled only with gaseous pentane -- NO LIQUID.

You also fail to tell us the scope of why you are evacuating. I have to assume you're doing this for maintenance. Consequently, you will need to sweep the line with air to evacuate the last remnants of pentane vapor out of the line prior to opening it up to the atmosphere.

You state you are trying to "vaporize" the pentane by pulling a vacuum. This is flawed chemical engineering. If you have liquid pentane in the line, you will be creating a "refrigerant evaporator" effect: the liquid will partially vaporize and the remaining portion will get colder and colder as the vacuum increases. You will be there for ages, waiting for all the liquid to vaporize without imposing a heat load on the cold interior. I don't believe that is what you want to do.

If you have liquid pentane in the line, drain it as much as you can. Then suject the line to a sweep of hot gas or steam in order to vaporize and evacuate the remaining pentane under slight pressure. Once you establish pure, dilute pentane gas in the line, then subject it to a vacuum with a subsequent purge/sweep of air. Sniff (test) the evacuated vapors to make sure no pentane remains behind prior to opening the line and working on it.

That's the way I would do it.