HCl-Air-Water Vapor Equilibrium Question

[KernOily]

Hi guys. I have a dumb mechanical engineer question. I am trying to determine the composition of the gas mixture that is present in the vapor space above the liquid inside an atmospheric storage tank. The tank contains 35.2% HCl. So I fired up my trusty simulator to run a flash calc to determine the composition, and it is 85.2 mole % air, 3.9% H20, and 10.9% HCl vapor at 115° F and 0 psig. OK, nice. Now the question is, since HCl is about 2x heavier than equilibrium saturated air (air-water vapor mixture), will this gas mixture segregate into layers over time? If so, how do I determine the time?

I need the vapor phase composition to support the sizing of the PVRV for the tank and also for the tank vent vapor scrubber. Never fails, I shoulda been a ChE. Life is a cruel teacher; it makes you take the exam before you've even had a chance to study. Thanks guys!

 

[Compositepro]

Gas mixtures never separate due to molecular weight under normal gravity. Molecular motion keeps everything stirred-up. They can be slow to mix themselves, but will never separate.

 

[Latexman]

What are the credible scenarios? Breathing out while filling? Breathing in while pumping out? Fire? Any other? If it's breathing scenarios only, you do not need compositions using API 2000 methodology. Formulas or tables give sizing capacity in volumetric flow of air at standard conditions.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[KernOily]

Thanks for the great replies guys.

The API 2000 tables are based on hexane. This particular vapor phase is far far away from Hexane so I did not feel I could defend a decision to use the tables vs. actually running the calcs.