Nitrogen blanket conservation

[twr]

There are two soybean oil silos getting filled from a railcar ~once a week. We keep a Nitrogen blanket on the silos in case the oil is held for a few weeks, even though that doesn't happen often. (Blanket maintained at 6-7" WC with pressure/vac relief valve)
Our nitrogen generator seems undersized for the application - we need high purity nitrogen on some of our other applications, and the purity drops significantly when we're emptying the railcar into the silos.

Nitrogen is added to maintain a blanket on the railcar.

Now with all that background - the question:
Has anyone got experience taking the Nitrogen from the silo and putting that back on the railcar? In effect it would be a closed loop, trading soybean oil for Nitrogen gas. I'm thinking about automatic shut-off valves and pretty big lines to persuade the gas into the railcar rather than venting to atmosphere as we do now.

 

[dcasto]

Why not? Or if nothing else, take the N2 and run it through your N2 generator (I'll assume its a PSA). You'll need to purge the lines or have valves on them, but I'd guess you do that too.

We unload propane and butane by taking the vapors off the tank to pressure up and displace the liquid from the rail car.

 

[ash9144]

Pretty common to vent the car to the tank. That will save on nitrogen use and balance the two vessels. Probably not important for oil transfers but with liquids with flash potential it is valuable.

 

[TD2K]

Doesn't seem anything unusual other than the pressures are lower than I'm more familar seeing during loading operations.

Do the rail cars come in already inerted with nitrogen? If they contain air, that's going to defeat the purpose of your proposed line.

 

[JimCasey]

That's even done in some gas stations. THere is a bellows on the filler nozzle, and the displaced vapors are allowed to flow back to the storage tank. My point being, it is a widely accepted and developed technology.

 

[MJCronin]

twr,

The vapor vent configuration you envision is commonly used with dangerous and/or volitile liquids. Offloading can be by pressure or by pumping, each has it's own advantages/disadvantages

As an example, see Figure #2 of this link:

http://www.oxy.com/OXYCHEM/products...ompressor Unloading of Tankcars (510-103).pdf

Most importantly, if you offload by pressure, ensure that your recieving tank has an emergency pressure vent. At the end of liquid delivery, there is commonly a pressure surge that can destroy a marginal tank.

My opinion only...


-MJC

 

[MJCronin]

twr,

Oh yeah....

Are these "silos" that you describe filled with oil,... are they really "storage tanks" ?

Is the bottom of the container flat, tapered, rounded or what ?

(The term silo is typically used for containers that store bulk solids, powders, etc)

-MJC

 

[twr]

We call them silos - they are vertical storage tanks, sloped bottom to the outlet pipe.
Thanks for all the input - seemed to make sense. I think there will be a common header & shut-off valve to the railcar. The silos still have the 3" breather valve to maintain low pressure - just need to make sure that the pressure drops through the line back to the railcar keep everything flowing correct.

 

[BenThayer]

line size and pressure drop limitations may prevent this but there is not anything inherently wrong with what you are proposing.

make sure your breather conservation valve can handle ALL credible scenarios.