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[Orion Jones]

I have a relief valve on an 98% Sulfuric acid storage tank. When it is heated by fire the H2SO4 will boil and give off a vapor?  What is this vapor?

 

Is it H20 and SO3    or is it just H2SO4 vapor?  

 

How do I determine the density of the H2SO4 vapor?

[fseipel]

The sulfuric-acid water azeotrope is at 98.4 wt%.  Typically H2SO4 above 98% is fortified with SO3, that can attack even PVDF.  The boiling point is around 330 deg C.  If it isn't fortified, you can just read the H2SO4/water concentration off a VLE diagram such as that in link below.  Typically a desiccant dryer is fitted in vent, so there won't be a relief valve.  I would expect the vent sizing to be based upon maximum fill rate rather than fire sizing. See http://www.generalch...nical-data.html for most physical properties including VLE.  At 98% in liquid phase, you'll have about 80% H2SO4 in vapor phase, this assumes no SO3 or oleum fortification and atmospheric pressure. Density may be calculated based upon vapor density of the pure components.