3 replies to this topic

[Processmen]

Dear All,

How should I determine the vapor composition of an Acid Water mixture in a fixed temperature. what is the procedure to determine the amount of entrained acid to air?

I know the temperature, humidity, acid water system composition, and the volume of the container.

[MrShorty]

A general answer for a general question. You decide which thermodynamic model you want to use, then use that model to perform a bubble point or flash calculation. The "hardest" part of this process might be choosing a thermodynamic model. The simplest model is probably Raoult's law, but it is unlikely to be very accurate. I find that Raoult's law is often a good place to start because most VLE equations (like activity coefficient models) essentially represent "corrections" to Raoult's law.

[Processmen]

I tried it in Aspen HYSYS, but the mixture is sub-cooled and it gave no vapor composition. The acid is entrained into the air through evaporation, How can I determine the rate of evaporation and the vapor composition in this case?

[MrShorty]

I don't know why the simulator didn't want to calculate a vapor composition. I'm not very familiar with simulators, so if this question is more about how to put the question to the simulator, I'm afraid I won't be much help.

If I'm understanding the problem, it still looks to me like a bubble point/flash calculation. Perhaps if we looked at a simpler problem, it would provide a framework for solving the problem:

Simplification 1) What if there were no acid present, so that you had water in contact with air (picture a cup of water or a bottle of water on your desk). How would you calculate the amount of water in the air? If I understand the problem correctly, this is the same problem without the acid. If you can figure oiut how to compute the water content of the vapor, then it's just a matter of figuring out how to extend the calculation to the solution of water and acid.

Simplification 2) What if the bottle contained no air (assume the bottle won't collapse under a vacuum)? How would you calculate the vapor composition? This is particularly useful if we can assume that the air is "inert" so that it doesn't affect the equilibrium of the water and acid.