LPG Rail Tank Cars Emptying

[shvet]

Good day, forum

Please direct me in case of this topic has been discussed somewhere before.

The question is concerned mobile tanks for liquified gases (rail cars, ISO tanks, trucks), let say propane and pentane. They have no device at the bottom of tank and are filled/emptied via a device located upside of the shell. Loading/unloading device has a pipe oriented downside which protrudes to the very bottom of shell and through which a tank is loaded/unloaded. The problem is actually this pipe does not provide complete unloading and there is dead space and some amount of liquid stays.

Someone can say that dead volume is negligible, but practice says that actual losses are 0.3-0.5 tonnes per a railcar which are quite painful for commerce.

Please guide me - are there dedicated devices for liquified gas (e.g. propane) and near products (e.g. pentane) for complete emptying? May be some external electric heaters?

Links to documents of Vendors/Manufacturers would be appreciated.

 

[LittleInch]

Can you attach a sketch or drawing showing what you mean?

I think you're going to struggle to get that out unless you vapourise it, but that might take a long time.

Are they really losses? How is the LPG measured and paid for? That's where you find out if what is left in the rail is truly "lost" or just there for the next time around.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[3DDave]

This one has bottom valves:

https://www.vtg.com/fileadmin/vtg/d...tter/Fluessiggas-Kesselwagen-LPG-G47.117D.pdf

Other than that, the amount mentioned is a small fraction of the car capacity and isn't lost, just shuttled. That amount does have a cost for fuel to move it back and forth, but that transport cost doesn't seem to be the original problem.

 

[1503-44]

Its just like a pipeline. After initial fill, the entire volume always remains inside the pipe. Where's the pain? You only pay a little bit more for transporting it back to the source and that is not paid for by rail carriers, but by the gas customers.

 

[shvet]

Can you attach a sketch or drawing showing what you mean?

https://www.saferack.com/natural-gas-lpg/

Are they really losses?

They are. Let say a purchaser bought 32.7 tonnes of propane, but can unload only 32.2 tonnes and 0.5 tonnes will be returned to a producer. This losses were payed as they were weighed by a producer but can not be refunded as they can not be weighed by someone (purchaser, producer, seller or a third party).

How is the LPG measured and paid for?

By railroad scale weighing before/after loading/unloading at a producer site. And no - a producer loads an emptied, cleaned, dried, and inerted rail tank.

That's where you find out if what is left in the rail is truly "lost" or just there for the next time around.

There is no way to obtain proven weighing of that kind of losses.

This one has bottom valves:

https://www.vtg.com/fileadmin/vtg/dokumente/waggon...

Other than that, the amount mentioned is a small fraction of the car capacity and isn't lost, just shuttled. That amount does have a cost for fuel to move it back and forth, but that transport cost doesn't seem to be the original problem.

1/ A purchaser has what it has got. A purchaser can not change rail tank design unless it is bought out.
2/ Are you talking about cash flow or profit? Yes - these losses have negligible impact on cash flow. No - these losses have major/painfull impact on profit.

Its just like a pipeline. After initial fill, the entire volume always remains inside the pipe. Where's the pain? You only pay a little bit more for transporting it back to the source and that is not paid for by rail carriers, but by the gas customers.

The pain is that initial fill of pipeline is payed for by an operator (not a purchaser) and this amount is accounted as operator's assets (not purchaser's losses) as it can be measured, accounted and sold.

@all
Actually I do not understand what all of you are exactly talking about. My question is rather simple, obvious and particular and I can not understand what has gone wrong.

 

[3DDave]

Hmm. It is usually the case that when a problem such as you describe comes up and no commercial solution is already in wide use that there is no solution, at least not one you wanted.

The people who would do this are the ones who make the tanks - but you aren't on the phone to them.

The other people who would know this are the producers who you say empty the tanks completely; there is no chance they are letting a half ton of LPG evaporate through the open man hole cover.

I think that if your margins are such that 0.25% is a problem, get out now. I'd be selling all my shares and writing a new resume.

 

[LittleInch]

Thanks for the info. I assume you are dealing with ambient temperature pressurised task cars?

The only way I can see easily is to vent the gas off and reliquify it.

What's the motive force to get the liquid out? Pressure in the tank?

What happens when you get down to the last bit? Do you end up with a mix of vapour and liquid? Could you then divert the vapour into a different vessel?

But that last little bit of product must go somewhere when it leaves you. Who is getting those dregs at the bottom? The producer?

Follow the commercial trail if you can.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

And positive gas pressure keeps air out of the tank.

 

[shvet]

I assume you are dealing with ambient temperature pressurised task cars?

Yes. I am talking about a conventional industrial LPG terminal. You will find one of them in all big ports.

The only way I can see easily is to vent the gas off and reliquify it.

This is the idea of the topic.

What's the motive force to get the liquid out? Pressure in the tank?

Tanks are unloaded by a compressor. A valve manifold provides rearrangement of compressor piping that way it can evacuate vapors from tank. But near vacuum is unacceptable because of risk of leaking.
Anyway evacuation by compressor can not provide sufficient emptying because of near adiabatic evaporation of LPG and some sorts of LPG do not evaporate during winter season.

What happens when you get down to the last bit? Do you end up with a mix of vapour and liquid?

After a tank is unloaded there is some vapors (compressed at 1-2 barg) and a liquified gas (into dead volume).

Could you then divert the vapour into a different vessel?

Only into flare or vent knockdown drum.

But that last little bit of product must go somewhere when it leaves you. Who is getting those dregs at the bottom? The producer?

A producer or a third party depressurizes tank, inspects, cleans, dries, inertes and certifies as ready for loading.
During depressuring the dead volume is vaporized and swept out to a flare or a vent stack. A producer does not weigh a rail tanks cars because of a rail scale can not provide so high proven weighing accuracy.

Moreover - keep in mind that usually there are other parties (exporter, importer, retailer, broker etc.) between producer and purchaser.

 

[3DDave]

Here's the answer: Add bottom valves.

Since you won't do the math on figuring out how much energy will be required to vaporize that amount of LPG and compare that to the value of the LPG and the cost to add electrical heating features to the tank car, this will be the cheapest option.

Apparently it's 428000 J/kg or 428000 Watt-seconds/kg to vaporize LPG.

You have 1000 kg, so that's 120 kWatt-hours of energy.

How much does that cost? Probably $10 US.

LPG price ~$0.40/liter; density is roughly 0.5 kg/liter so you are looking at 2000 liters or $800.

Wow - that looks like a great idea. Definitely enough money to add heaters.

But then I wonder why electricity is not used near LPG handling operations.

You probably want this to happen in 5 minutes per tank, max. So you need to supply 1.4 MW of power into that volume. Of course a lot of that power will be conducted into the surrounding metal, so bump it to 2.0 MW power supply. Roughly 2000 Amperes at 1000 Volts will do fine. Per car, if they are being unloaded simultaneously.

Or you can add water to the tank to displace the LPG from the bottom and let the supplier do the cleaning; I'm sure water separation / dewatering is an available process step.

Or require shipment in cars that have bottom valves or trade the cars that are on hand for ones with bottom valves and not need to build a high-power delivery system in the middle of a potentially explosive situation in a petroleum gas handling facility.

 

[3DDave]

https://www.utlx.com/build-a-tank-car/3-heatersystem.html

https://www.utlx.com/build-a-tank-car/10-bottomvalve.html

https://www.utlx.com/build-a-tank-car/9-fittingsflange.html

Use the wrong equipment, get unwanted results.

 

[shvet]

Del

 

[3DDave]

I'm now more interested in who gave you this problem to solve. They clearly don't expect you to succeed as a simple calculation (what engineers do) shows that it is impractical.

The water is only used during the emptying process - you assured me the LPG supplier emptied and cleaned the tank. If that is not actually true then that's a rule change.

It cannot be against industry practice. The company you took the image from makes bottom feed/bottom drain equipment for LPG.

If you have an engineering calculation then it can be checked to see if it is right. I found a company that makes LPG vaporizers - their 500 kg/hour unit takes 63 kWs which translates to 126 kW-hr per 1000 kg; very close to what I calculated.

Do the work, show your work. Maybe you are in Russia? Yeah, I already know that.