LPG railcar unloading and vapor liquid equilibrium

[shighfill]

I am working on a system to unload LPG from railcars to spheres using a compressor (compress the vapor from the sphere into the railcar to push the liquid up the diptube and into the same sphere). My client is concerned about the rate of condensation of the compressed vapors in the railcar. My gut tells me the rate of condensation is small in comparison to the rate of liquid transfer from the railcar. But it'd be nice to have some calcs to back that up. Ignoring heat loss from the vapor phase to the railcar walls, it seems the condensation rate would be dependent on the heat transfer from the hotter vapor to the cooler liquid. This should be through natural convection inside the railcar. I am having a heck of a time finding a correlation for the natural convection heat transfer coefficient from vapor to liquid. Has anybody had any experience with a system like this? Any feel/knowledge for rate of condensation vs. rate of liquid transfer, any advice on how I might calc this, any advice on correlation for heat transfer coefficient in this situation. Thanks in advance!

 

[BenThayer]

no experience with this but we did the exact same thing with ammonia vapor from bullets to railcars.

condensation in the railcar was not an issue since we were just increasing the pressure in the car about 5-10 psig higher than it was already to overcome hydraulics. plus you get some small benefit from the heat of compression.

we used some Corken compressors that looked alot like these:

http://www.corken.com/products/Lpg_compressors/lpg_compressors.html

we had more trouble with condensation in the lines to the railcars and the compressors had some type of inlet float valve that proved inadequate and i would highly recommend tracing that line especially if you are in a relatively cold climate.

 

[Montemayor]

I've done this and had some bad experience(s). I've still got photos of what was left of the poor Corken compressor when the liquid ammonia got into the cylinder(s) - just pieces of cast iron. Lucky for all of us, the compressor was located remotely and separated from the railcar and human traffic.

This method works. However, you must ensure that you have 100% well-designed, well-checked P&IDs, physical equipment and piping installation, and all critical instrumentation. Pay particular attention to the piping and low spots or wrong slopes on lines. Above everything else, protect the compressor from liquid slugs - either from the vessels or from line condensation. And locate your compressor remotely away from humans.

 

[shighfill]

Thanks for the fast responses! BenThayer, I've been looking at that Corken site quite a bit. I'm glad to hear your info about the inadequacy of their inlet float valve...I saw that and thought of the cold winter climate and how that just wouldn't work. And we do plan to trace and insulate the vapor lines to and from the compressor, as well as put in a properly sized inlet KO drum, probably including a steam coil to vaporize any condensate in the winter.

Montemayor, in your experiences (good and bad), did you notice much effect of condensation in the railcar on the liquid transfer?

 

[BenThayer]

i think one reason the floats were inadequate is because the ammonia vapor could condense within the system and within the compressor, etc.

we had a relatively large KO but we fought the condensation quite a bit.

our site also had the disadvantage of the equipment being out of service for significant periods of time and then used only occasionally when a railcar came back or needed to be off-loaded.

we were primarily a producer and not a user.

i hope my comments regarding the corkens was not construed as a slam against the equipment. i think the machines are fine but you have to be very careful in how you arrange your piping and your procedures.