vent composition

[steve121]

I am storing diluted ethanol in an atmospheric tank.

Strength in the storage tank may vary up to 50%, transfered on a batch basis.

I need to calculate the vent stream composition when the tank is re-filled. I know the fill rate/vent rate, but am unsure how to work out the composition of the vent stream.

Can anyone point me in the right direction.

Thanks

 

[MikeClay]

At the beginning of the EtOH addition, the vapors will be in equilibrium with the liquid. The easiest way to get the vapor phase EtOH concentration is to use a process simulator with the appropriate thermodynamic package.
1. Set the stream at liquid concentrations and
temperature.
2. Determine the bubble point pressure (Psat).
3. Determine the vapor phase mole fraction (Ysat)
4. Yvent= Ysat x Psat / Ptank

If the incoming stream does not significantly flash, this concentration can be used for the entire addition. Otherwise, the ethanol emitted will be the amount flashed from upstream conditions to the tank condtions.

--Mike--

 

[oil198]

^this can also be done by hand using Raoult's law and a simple volume displacement. But be careful on the volume of flashed ethanol. If you're using a process simulator it will assume that the tank is empty, i.e. no ethanol in the vapor space so you may over predict the volume of flashed liquid.