Condensate Stabilizer Vacuum Condition
Condensate Stabilizer Vacuum Condition
(OP)
We've been asked to look into the possibility of a condensate stabilizer dropping into a vacuum condition during a shutdown in a very cold Canadian winter. My gut reaction is that this won't happen, but I'm struggling with how to perform this calculation. The volume of the tower and reboiler is fixed, the mass at the time of the shutdown is fixed, and the system simply starts to cool down. The vapour condenses into condensate and the pressure begins to drop due to vapour collapse - much like a amine regenerator. Can anyone provide tips on how to calculate the final pressure during a cooldown? Is it as simple as looking at the vapour pressure of the tower in the equilibrium fluids at an ambient of -30°C?





RE: Condensate Stabilizer Vacuum Condition
RE: Condensate Stabilizer Vacuum Condition
RE: Condensate Stabilizer Vacuum Condition
If you want, just get the process simulator to give you the tray by tray composition and density.
RE: Condensate Stabilizer Vacuum Condition
In case there is a different procedure for shutdown, you might need to consider different overall composition. This again can be calculated by mixing all the product streams at given conditions and flash the resultant stream at the final cooldown temperature.
My guess is - as long as you have a few % of Methane in the column, you won't see any vacuum. Methane boils at -161C and it needs something more harsh than Canadian winter to condense.
http://antwrp.gsfc.nasa.gov/apod/astropix.html
RE: Condensate Stabilizer Vacuum Condition
RE: Condensate Stabilizer Vacuum Condition
RE: Condensate Stabilizer Vacuum Condition
I don't think that detailed stage-to-stage calculations will bring more accurate results. After sufficient time, all liquid phase will drain through the trays and end up in the bottoms anyway, so the column will contain 2 distinct phases - vapor and liquid.
http://antwrp.gsfc.nasa.gov/apod/astropix.html
RE: Condensate Stabilizer Vacuum Condition
RE: Condensate Stabilizer Vacuum Condition
RE: Condensate Stabilizer Vacuum Condition
RE: Condensate Stabilizer Vacuum Condition
It was a wrong assumption to use the flash operation across a valve (J-T). Ambient cooldown is not an isenthalpic process because a part of the process heat is consumed by the ambient surroundings. Also, different things happen within a flowing system than in an isolated (V=const) system.
The most accurate results can be obtained by builiding a dynamic model of the tower (with accurate system volume = the tower + associated piping), then close all SDV's and introduce a negative heat stream that will cause the entire system to cooldown e.g. 5 degC/hour till it reaches -30 degC. The resulting pressure will be automatically calculated by the model. HYSYS can do this for you in a few minutes.
In the attachment, I presented a simplified manual procedure that I have used on a few occasions when resolving similar subjects. The result will not be within +/-0.1 accuracy but it should be in the same ballpark (order of magnitude) as the real number.
The key concept is to follow the sequence of events, and to consider conservation of mass inside the system, and volume of the system.
I hope this helps.
Best regards
http://antwrp.gsfc.nasa.gov/apod/astropix.html
RE: Condensate Stabilizer Vacuum Condition
RE: Condensate Stabilizer Vacuum Condition
You should consider upset situations (those often cause the units to shut down), and what about steam-out? If they use steam out then vacuum is certain.
Best regards,
Morten