4 replies to this topic

[Lowball]

Hello all,

 

I have whats seems like a simple question:

 

I need to calculate how much Ammonium hydroxide will evaporate in a sealed tank. The tank is about 5 liters, and I need to know how the concentration will vary over time with different tank levels. The tank is pressurized with air at 30psig. The solution is 20:1 NH4OH:DI H2O. My inclination is that there will be very little if any change since the tank is sealed and pressurized.

 

I would appreciate any help to get going in the right direction!

 

Thank you!

[Art Montemayor]

If your tank is sealed, then what goes in can't come out.

It nothing can come out, the chemicals inside will remain the same in quantity and composition - IF there is no chemical, biological reaction, or decomposition taking place (which I assume is the case).  The tank level should not vary, so you haven't explained all the details of what you propose.  Are you extracting or pumping solution out of the tank over time?

[MrShorty]

Am I confused, or is the real question about the equilibrium between NH4OH and NH3?

 

NH4OH exists in equilibrium with NH3 -- NH4OH -> NH3 + H2O. Considering the high concentration of NH4OH indicated (90+%), assuming no auxiliary acid to keep the NH3 dissolved, it seems like there will be an appreciable amount of NH3 in the headspace. Maybe that is what you are asking -- how to perform this flash calculation?

 

According to Perry's, a 90+% solution of NH3 in H2O has a vapor pressure of 60 - 200 psia or higher, depending on temperature.

[Lowball]

Sorry for the poorly worded question. I know nothing will leave the tank if it is sealed, What I do want to know is how much if any of the mixture will evaporate into the headspace with varying tank levels, ie. if the 5 liter tank has 1 liter, 2 liters etc. filled. And sorry I incorrectly stated the concentration its the other way around 1:20 NH4OH:DI H2O.

 

The bigger question I am trying to answer is: How long can the tank sit unused before the concentration crosses a threshold.

 

 

Thanks again!

[MrShorty]

What I do want to know is how much if any of the mixture will evaporate into the headspace with varying tank levels, ie. if the 5 liter tank has 1 liter, 2 liters etc. filled

That still sounds like you want to do a flash calculation. It should be easy enough to get a good estimate using the data in Perry's (or whatever equation or source you want to use to estimate the partial pressures of NH3 and H2O over the solution). Considering the low pressures expected, I doubt the there will be enough material in the headspace to appreciably change the liquid composition (except maybe when the tank is near empty).

 

Can the tank "breathe" from/to the air source that is used to maintain the pressure? Perhaps this is part of your question -- will there be any NH3 lost to the air source?

 

For the 2nd half of that question -- as Art indicated, if it really is sealed and there are no reactions going on -- then the liquid in the tank should be stable indefinitely. Perhaps it is a question of how comfortable you are with Art's assumptions -- Any chance the O2 in the air will oxidize the NH3 over long enough periods of time? Will the NH3 react -- however slowly -- with the tank's construction materials? Any trace components that could potentially participate in any reactions?