Pressure rise when filling sealed tanks
Pressure rise when filling sealed tanks
(OP)
Hello
I hope someone can help me with this. It's a simple question but I'm not from a classical Chem. Eng. background so could do with a pointer. If you have a sealed tank of volume V and you fill it to some proportion x (0-1) at a rate Q L/min, how can I determine the final pressure in the tank as a function of time (or fill level)? It seems like it should be easy but I've not done this before and I don't know where to start. Can I take pV^n = constant and substitute the time-dependent vessel volume V(t) for the volume term?
The pressures are low by process engineering standards and the fluid is water based to let's assume compressibility is zero.
In an extension to this, if the contents are reacting and undergo a small volume change on reaction, how can I build this in to the calculation?
Many thanks.
I hope someone can help me with this. It's a simple question but I'm not from a classical Chem. Eng. background so could do with a pointer. If you have a sealed tank of volume V and you fill it to some proportion x (0-1) at a rate Q L/min, how can I determine the final pressure in the tank as a function of time (or fill level)? It seems like it should be easy but I've not done this before and I don't know where to start. Can I take pV^n = constant and substitute the time-dependent vessel volume V(t) for the volume term?
The pressures are low by process engineering standards and the fluid is water based to let's assume compressibility is zero.
In an extension to this, if the contents are reacting and undergo a small volume change on reaction, how can I build this in to the calculation?
Many thanks.





RE: Pressure rise when filling sealed tanks
That is pretty basic, but will give you a start point. If the gas and liquid combine to create more gas or less gas then just add or subtract that from V2 to find your new pressure.
The gas compressibility won't be ideal, but as a first order number you wont be far off. If you know the compressibility factor then you can add this in as well.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Pressure rise when filling sealed tanks
RE: Pressure rise when filling sealed tanks
The tank is isothermal. It is not lagged but there is a temperature control system associated with it. The headspace as the tank fills will be a water/organic mixture.
I think in thje first case the p-V-T relatinship will be adequate, and if I want to account for density changes I can modify the volume expression, I think.
RE: Pressure rise when filling sealed tanks
RE: Pressure rise when filling sealed tanks
I'm using Aspen+ software. I can't run this as a tank or CSTR because the dyanmics of the process are too complex to simulate in Aspen. All I want is a single point VLE calculation for times of interest in the process. i.e., Here is the liquid phase composition, here is the temperature, here is the pressure: what is the vapour phase composition?
RE: Pressure rise when filling sealed tanks
Regards
StoneCold
RE: Pressure rise when filling sealed tanks
For example, the heat of vaporization would be calculable but I don't know how much of the liquid I'm vaporizing, and this heat is lost in the overall heat balance anyway in a real application, were it to exist. The duty isn't much - 1-5 kW - but it does shift the pressure quite a bit.
RE: Pressure rise when filling sealed tanks
"Duty" in specifications or operations is almost always a term to specify "what percent of time is the system operating (at some specific percent of output)?"
thus, a welder may be rated at 300A, but at a 20% duty cycle (one for 1 minute, off for 4 minutes), but be rated at 200A for 100% duty cycle.
You need to look again at the specifications or problem statement: The duty cycle is likely to be "what percent of time the tank is filling?" or "What percent of time the tank is NOT being filled" - thus, the tank vapor is going back to equilibrium and not pressurizing.
RE: Pressure rise when filling sealed tanks
RE: Pressure rise when filling sealed tanks
You know the temperature. You know the range of pressures that the tank will go through as it is filled. So use that information to get your vapor compositions. There is no heat duty, unless you have a reaction going on. Then you have a heat duty related to the reaction kinetics.
Regards
StoneCold
RE: Pressure rise when filling sealed tanks
The initial problem you described was mush simpler that your second desription. IF you are filling the vessel with a liquid that do flash (e.i. have a vapour pressure above the pressure of the vessel at initial condition) then its more complicated than if not.
I think you should call in aconsultant.
Best regards, Morten
RE: Pressure rise when filling sealed tanks
At the starting point there is a liquid in the tank in equilibrium with vapor in the tank. The fluid VLE data specifies the gas composition.
At the end point it is unclear if the fluid in the tank is still at the same composition. In any case the VLE can be specified for this condition also.
One thing could have changed is that one of vapor components could have exceeded it's vapor pressure relative to the fluid and would then condense from the vapor mixture. In this case you have a lot more going on than filling a tank with water.
Without disclosing more of what you're really trying to calculate it will be hard to provide relevant help.
RE: Pressure rise when filling sealed tanks
Beware however that you must choose the right thermo method. The answer can change significantly and is totally dependent on the components in the reactor as well as the T and P.