14 replies to this topic

[Joerg]

Dear all,

 

I just got a project for a LNG vaporizer on an LNG carrier. The ship runs with a duel fuel engine, means with high pressure gas instead of diesel. My problem is that my supplier of the vaporizer is not 100% that sure liquid converts into gas due to the high pressure.

The prozess is as follow.

- Main tank with LNG

- a high pressure pump increase the pressure to 320bar into the vaporizer.

- the input temperature of the LNG into the vaporizer is -140C degree

- the output temperature of the vaporizer +45C degree and 320bar

Does someone know if the LNG is still in liquid form at the outlet of the vaporizer or if it changes into vapor?

 

Thanks in advance

Joerg

 

 

 

[curious_cat]

The critical Temperature of Methane is about -82 C 

 

At +45 C it has to be a vapor (rather,  a gas) IMHO. Pressure does not matter. 

 

Technically you seem to be going from a high pressure liquid to a supercritical fluid; not a conventional (below critical point) Liquid to Vapor transition. Not sure what complexity this entails in the vaporizer design, if any.

Edited by curious_cat, 02 December 2013 - 06:40 AM.

[curious_cat]

As an aside, I thought LNG carriers used BOG to run in gas mode. Is it worth vaporising your liquid cargo and using that to fire the engines instead of diesel?

 

Just curious of the operating strategy. I've no idea.

[PingPong]

I agree with curious_cat that at 45 ^oC the gas will be above its critical temperature. Even if it contains some ethane and small amounts of propane and heavier.

 

My problem is that my supplier of the vaporizer is not 100% that sure liquid converts into gas due to the high pressure.

It seems to me that your real problem is: you cannot be sure that the vaporizer has been properly sized if the vendor does not know what he is doing. How can they have calculated the right U-value it they don't know whether there is gas, or liquid, or dense phase coming out of the vaporizer?

[curious_cat]

My problem is that my supplier of the vaporizer is not 100% that sure liquid converts into gas due to the high pressure.

It seems to me that your real problem is: you cannot be sure that the vaporizer has been properly sized if the vendor does not know what he is doing. How can they have calculated the right U-value it they don't know whether there is gas, or liquid, or dense phase coming out of the vaporizer?

 

 

@pingpong:

 

Do you know if the conventional HTC correlations for reboilers etc. work for this case of a supercritical phase transition? 

 

This isn't nucleate nor film boiling, is it? Technically, is this even boiling? I'd be curious to know if the design considerations are different here or not. 

[Joerg]

Thanks for your fats reply.

 

The BOG in this project is kust used for the auxiliaries around, the main engine is using the LNG from the main tank, but I'm not the technical expert of such kind of systems, I just got the order for the vaporizers....

 

 

Thx

Joerg

[curious_cat]

The BOG in this project is kust used for the auxiliaries around, the main engine is using the LNG from the main tank, but I'm not the technical expert of such kind of systems, I just got the order for the vaporizers....

 

Interesting! I wonder what the economics are. 

[PingPong]

Do you know if the conventional HTC correlations for reboilers etc. work for this case of a supercritical phase transition? 

 

This isn't nucleate nor film boiling, is it? Technically, is this even boiling?

In my opinion there is no real phase transition and there is no boiling either.

The fluid gradually expands about a factor two in volume when going from -140 to +45 ^oC at 320 bar. Other properties also change gradually, not suddenly at a specific temperature.

 

 

It depends on the exact composition of the natural gas how the phase diagram looks. John Campbell (the one from the books) gives a diagram for a typical natural gas (80% C1, 8% C2, 12% C3+):

 

 

As can be seen, for a gas mixture it is not really the Critical Point that is determining the phase transitions. If the pressure is below the Cricondenbar, but above the critical temperature, there can still be liquid (+gas).

At 320 bar I am sure we will be above the Cricondenbar, but at 45 ^oC we may still be below the Cricondentherm, so according to Campbell's phase definitions, we could be in dense phase, not gas phase, at the oulet of the "vaporizer". What we call it does not matter though. There is only a gradual change of all properties from -140^oC/320bar to +45^oC/320bar.

[Bobby Strain]

And I am sure that the pressure is not 320 bar! Maybe 320 kpa

 

Bobby

[Joerg]

 

Do you know if the conventional HTC correlations for reboilers etc. work for this case of a supercritical phase transition? 

 

This isn't nucleate nor film boiling, is it? Technically, is this even boiling?

In my opinion there is no real phase transition and there is no boiling either.

The fluid gradually expands about a factor two in volume when going from -140 to +45 ^oC at 320 bar. Other properties also change gradually, not suddenly at a specific temperature.

 

 

It depends on the exact composition of the natural gas how the phase diagram looks. John Campbell (the one from the books) gives a diagram for a typical natural gas (80% C1, 8% C2, 12% C3+):

 

 

As can be seen, for a gas mixture it is not really the Critical Point that is determining the phase transitions. If the pressure is below the Cricondenbar, but above the critical temperature, there can still be liquid (+gas).

At 320 bar I am sure we will be above the Cricondenbar, but at 45 ^oC we may still be below the Cricondentherm, so according to Campbell's phase definitions, we could be in dense phase, not gas phase, at the oulet of the "vaporizer". What we call it does not matter though. There is only a gradual change of all properties from -140^oC/320bar to +45^oC/320bar.

 

Thanks for your great expalnation. A similar statement I got from the manufacturer of the vaporizer.

[Joerg]

And I am sure that the pressure is not 320 bar! Maybe 320 kpa

 

Bobby

 

Sure it will be 320 bar operation pressure. Duel fuel ship engines run with such high pressure.

Joerg

[Bobby Strain]

Joerg,

       So, shame on me for my assumption. I guess Wartzilla thought this to be extreme, too. So they developed a dual-fuel marine engine to operate with 10 bar pressure.

     It seems that most of these systems for fuel delivery are delivered as a complete system. So, it should be a simple matter for you to specify the duty requirements for your heater application and send it to the various vendors. And, of course, there is no phase change in such a high pressure system. And the fluid properties should be well established.

 

Bobby

[PingPong]

I suspect the required gas pressure depends on where the gas is injected in the engine system.

 

Wärtsillä seems to inject the gas in the air line upstream the inlet valve of the cylinder, where the air pressure is low.

 

If however the vendor of Joerg's diesel engine injects the gas inside the cylinder, just before TDC, a much higher pressure is required, not only to enter the cylinder but also to achieve rapid turbulence/mixing with the air.

Would be nice if we knew the vendor and type number of that engine so that we all understand what we are talking about.

[Bobby Strain]

PingPong,

       I did a bit of research. Most of these engines use LNG fuel at 300+ bar. You can Google the subject and find lots of stuff.

 

Bobby

[curious_cat]

PingPong,

       I did a bit of research. Most of these engines use LNG fuel at 300+ bar. You can Google the subject and find lots of stuff.

 

Bobby

 

Why do they? I read a bit but I still cannot figure out the fundamental reason. Is it thermodynamics or practical concerns?

 

All that jacketed high pressure pipe with special oil isolated valves etc. sounds like a lot of complexity.