5 replies to this topic

[mavd25]

Im trying to figure how to control the pressure and level of a tank on hysys without adding a gas like N2 to make the pressure steady. The idea is very simple: a tank full of LNG (liquid natural gas), lets say that 90% of the tank is full with lng and the rest is ng (natural gas), so if I wanna take out the lng off the tank to be sent to the regasification sector, how is that I can keep the tank pressure steady?

the only idea that I can think off, its to blow back to the tank part of the regasified lng, but I supose that I would need some sort of NG back up/storage to be added to the LNG tank while the lng is starting to get gasified.

Thanks

Edited by mavd25, 27 January 2010 - 10:50 PM.

[Steve McGahey]

In reality, you would have a few things going on.

(1) Withdrawing gas will reduce the pressure.
(2) Adding LNG (liquid) will increase the pressure.
(3) Adding LNG (gas) will increase the pressure.
(4) All the time, there is thermal interaction with the environment... heat ingress and heat loss.

I suspect that if you model heat transfer to the environment, you'll find that taking gas off the top of your tank will drop the pressure, cause more LNG to vaporise, which will lower the temperature. The lower-than environment temperature means that heat will come in from the environment (through the tank wall), warm up the tank, causing vaporisation of some of the contents, restoring the pressure in the tank.

Have I missed anything significant?

So, you could counter the loss of pressure through points 2, 3 or 4 above.


For what you want to do, I think you are looking at a combination of points 3 and 4, since you are unloading. Regasified gas seems sensible.

Edited by Steve McGahey, 28 January 2010 - 06:51 AM.

[fallah]

Im trying to figure how to control the pressure and level of a tank on hysys without adding a gas like N2 to make the pressure steady. The idea is very simple: a tank full of LNG (liquid natural gas), lets say that 90% of the tank is full with lng and the rest is ng (natural gas), so if I wanna take out the lng off the tank to be sent to the regasification sector, how is that I can keep the tank pressure steady?

the only idea that I can think off, its to blow back to the tank part of the regasified lng, but I supose that I would need some sort of NG back up/storage to be added to the LNG tank while the lng is starting to get gasified.

Thanks

Please attach a simple sketch of the system for better understanding your concern and submitting better assistance.

[Zauberberg]

Try to feed the tank in the same way as it is fed in practice - as equilibrium liquid at a slightly higher pressure than the storage tank pressure. This way, flashing of LNG will always result in generating certain quantity of vapors and positive flow of vapor phase out of the tank. I have done my dynamic simulation in the same way, and it works just fine.

[mavd25]

Try to feed the tank in the same way as it is fed in practice - as equilibrium liquid at a slightly higher pressure than the storage tank pressure. This way, flashing of LNG will always result in generating certain quantity of vapors and positive flow of vapor phase out of the tank. I have done my dynamic simulation in the same way, and it works just fine.

Good idea on using the flash, but what about for a system with accumulations? Once that the ship fills the tank with lng there is no more lng being added to the tank so the tank has a steady pressure, but now if I withdraw the LNG from the tank how can I control the pressure?

(3) Adding LNG (gas) will increase the pressure.
(4) All the time, there is thermal interaction with the environment... heat ingress and heat loss.

In practice is OK to consider that the heat from the enviroment to the tank is enough to maintain the tank pressure?. So on point (3) I should add to the tank part of the regasified LNG of the downstream process?

Edited by mavd25, 29 January 2010 - 09:34 PM.

[Zauberberg]

Add a heat stream to the tank. Depending on tank size, you may end up with 2,000kW heat influx. Even when there is no LNG inflow to the tank, boil-off gas results from heat leaks into the system.