6 replies to this topic

[pavanayi]

Matt,
There have been a few threads on the same lines in this forum.
Go through and see if it helps you.

Evaporation

[MrShorty]

I'm unsure of what value of Molecular Weight and vapor pressure to use; the weighed average of the mixture

Assuming the equation can be applied to mixtures (which I'm not sure of either), these questions would seem to be simple VLE questions. To fix concepts, you could easily use Raoult's law to calculate the bubble point pressure of the mixture. It wouldn't be very accurate because NH3/H2O is very non-ideal, but it would help you set up the spreadsheet for how to do it. You could then figure out a more robust model (maybe an activity coefficient model??) for dealing with the deviations from Raoult's law in this system. That would give you the bubble point pressure, and then the vapor composition.

Your source for the equation claims that the molecular is supposed to be the average molecular weight of the liquid. Oftentimes the trouble with getting equations like this over the internet is that there isn't enough discussion of how the equation is derived to really understand it (I would think you would use the average molecular weight of the vapor rather than the liquid). In this case, though, I don't think it is going to make a big difference what molecular weight you use. You know the molecular weight of the mixture (whether vapor or liquid) is going to be between the molecular weight of H2O (18) and NH3 (17), so there isn't much room for error in the molecular weight, which would then introduce a small error in your evaporation rate. Where you're only looking for an approximation, pick something and put it in, or do the calculation at both extremes of 17 and 18 and see how much maximum error the molecular weight introduces.

or the pure values of just ammonia.

NIST (webbook.nist.gov) has data for the properties of pure NH3.

[MrShorty]

I'd be happy to figure an activity coefficient model, but are we correct in assuming that the ammonia mixture is really at equilibrium?

The referenced equation calls for a vapor pressure at pool temperature. Even for a pure compound, vapor pressure is an equilibrium property. Whenever someone asks me for the "vapor pressure" of a pure compound without further qualification, I assume they mean the vapor pressure of the fluid at equilibrium. Extending to liquid mixtures, I would assume they mean "bubble point" pressure, again at equilibrium.

I can't say whether there is something built into the equation in question to account for the fact that the pool might not be exactly at equilibrium. As presented, I assume that the input parameter "vapor pressure" is to be calculated at equilibrium. If the equation requires something other than "equilibrium vapor pressure", someone more familiar with the equation will have to comment.