7 replies to this topic

[antator]

Hi,

I'm currently working on a project where I've come across a boil off gas system on an ethylene tank. From my background reading I believe it is a pretty standard setup, simply put the boil off gas from the tank is compressed, cooled and then flashed. I was wondering if anyone could confirm the reasoning for such a setup over a simple heat exchanger cooling system. The only reasoning I could come up with was that the heat transfer coefficient to a liquid would be significantly higher than that to a gas and thus by compressing the gas to a liquid you result in a much more efficient process, can any confirm this is correct?

[antator]

Just a thought, would this setup have been chosen to take advantage of the vapour-compression refrigeration cycle? If so I assume that this would be more efficient than direct cooling.

[Zauberberg]

Antator,

We would appreciate any sketch showing the arrangement of your system. You can do this in a matter of minutes.

Normaly, BOG compressors discharge back to the process (LNG plants) where the gas is mixed with a stream flashed from the final LNG flash drum (CoP process), and subsequently re-condensed. For storage or transporting facilities (LNG tankers), normally there is a packaged, closed-loop reefrigeration system (usually N2) that condenses the BOG.

http://www.maritime-...ion systems.pdf
http://gasbilen.eu/u...ne_jakobsen.pdf

[antator]

I apologise for my poorly phased initial question, let me rephrase and hopefully answer my own question with the aid of a very basis block flow diagram (attached).

 Ethylene BOG Cycle.pdf   17.32KB   206 downloads

What I was initially trying to ask was why would we bother with the compression stage before cooling the stream and then flashing the stream - the alternative being to just condense the boil off ethylene?

The reason I came up with in the end (after reading around and asking a few questions in the office) was that normally on site the only coolant you have available would be cooling water and air, both of which would obviously not be sufficient to condense ethylene. However, by taking advantage of the vapour-compression refrigeration cycle and the Joule-Thompson effect (as shown in the BFD) you can achieve much lower temperatures, and hence condense the boil off ethylene.

[Zauberberg]

Something that would make the problem even easier for understanding is if you create Ethylene saturation curve (boiling/condensation pressure at various temperature levels). There you can see that condensing Ethylene at higher temperatures calls for more compression Horsepower (higher discharge pressure), and vice versa. Normally, in LNG plants Ethylene is condensed by Propane loop. If Propane or other heavier refrigerant is not available, you have to think of something else. Flash drum shown on the sketch has the purpose to remove any non-condensables or Methane from the liquid Ethylene stream flowing back to the tank (think of it as of a kind of pressure letdown system). It's a common way of doing things.

I hope this helps you out.

[antator]

Thanks, that would explain why the cooler uses propylene as a refrigerant then

[ethylene]

Thanks, that would explain why the cooler uses propylene as a refrigerant then

Hi antator

Are you working on ethylene plant project?
Using propylene as refrigerant for ethylene BOG recovery, I guess propylene source is from process plant refrigerant compression system. Otherwise standalone propylene refrigerant cycle is necessary.
Choice of refrigerant will depend condensing temperature and pressure, then results in economical and practical optmised solution.

[antator]

Thanks, that would explain why the cooler uses propylene as a refrigerant then

Hi antator

Are you working on ethylene plant project?
Using propylene as refrigerant for ethylene BOG recovery, I guess propylene source is from process plant refrigerant compression system. Otherwise standalone propylene refrigerant cycle is necessary.
Choice of refrigerant will depend condensing temperature and pressure, then results in economical and practical optmised solution.

Both ethylene and propylene are being produced in a cracker and being stored in the tank farm. The propylene being used in the ethylene BOG system is taken from the propylene BOG system liquid return to the propylene tank, and being returned to the propylene BOG system in the vapour phase. The propylene BOG system then has it's own refrigeration package for the cooling step in that system - Best to explain by a diagram I know, but I don't have one to hand unfortunately.