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Organic Vapor Concentration In Head Space of Tank

Organic Vapor Concentration In Head Space of Tank

Organic Vapor Concentration In Head Space of Tank

My command of thermodynamics has failed me and I am almost embarrassed to ask, but after a few hours of banging my head I figured I'd ask for help.

I am simply trying to estimate the concentration of an organic vapor (CS2 in this case) in the head space of a nitrogen inerted tank at atmospheric conditions and while under pressure.

At atmospheric conditions, CS2 boils at 46*C.

At 20*C it has a vapor pressure of 5.8 PSIA.

Assuming CS2 is charged to an nitrogen inerted vessel, at 20*C and 14.7 PSIA, is the concentration (molar ratio) in the head space simply 5.8/14.7 = 0.39?

If the vessel is then pressured to 50 psig, is the concentration in the head space then 5.8/(50+14.7)=0.09?

Thank you in advance. Please forgive my ignorance.

RE: Organic Vapor Concentration In Head Space of Tank

Yes, you'd be right there - concentrations in both cases would be mole fraction of CS2. This estimate assumes CS2 still behaves as an ideal gas / liquid at these temperatures.

If you need higher accuracy, then work out the vapor phase partial molar fugacity for CS2 at this temperature / pressure and apply this correction to the mole fraction you get from the ideal gas case (ideal case mole fraction / vapor phase partial molar fugacity).

RE: Organic Vapor Concentration In Head Space of Tank


Thank you for the reply!

I am suspicious that because CS2 vapor is heavier than air, if under static conditions, if there would be a significantly higher concentration near the liquid level than in the head space at the top of the vessel. While transferring into the vessel though I think I can assume the head space is pretty well mixed.

Thank you again George.

RE: Organic Vapor Concentration In Head Space of Tank

myrdale ......

You are aware that, for various practical and safety considerations, CS2 is normally stored in tanks under water ?


From an international standard:


5.1 Storage

5.1.1 General — Due to high vapour pressure of carbon disulphide,
bulk storage should always be under water and at or below atmospheric
temperature. Tanks or drums should be sprayed with water in hot
weather to keep the temperature low and the vapour pressure down.

5,1*2 Type of Construction — Structures in which carbon disulphide is
stored should be non-combustible and either open type or well
ventilated at floor level.

5.1-3 Isolation — Buildings for the storage of carbon disulphide should
be isolated from other operations and buildings.

5.1.4 Ventilation — Carbon disulphide vapour is 2.63 times as heavy as
air, therefore, special down draft or lateral type ventilation should be
provided for room where carbon disulphide is stored or used. Although
the threshold limit value for carbon disulphide is accepted as 20 ppm
( see 4.1 ) as an added precaution, the concentration of carbon disulphide
vapour in the work room should be maintained below 15 ppm. Care
should be taken to ensure that the same air is not recirculated in the
room and the vapour is drawn away from the operators' faces. The
ventilators of carbon disulphide storage rooms should be fitted with two
layers of the fine wire mesh of non -corroding metal.

There are both state and federal regulations that address the storage of this dangerous liquid.


Tell us more details about your proposed storage tank.

Tell us where the tank will be located ..


Sr. Process Engineer

RE: Organic Vapor Concentration In Head Space of Tank


Thank you for the additional feed back. In our case this vessel has been in operation for +10 years. Our original vendor was involved in the design and installation of the vessel. When we switched vendors a few years ago, they audited our storage tank and were satisfied with the design and operation.

As for my original question, I have been looking back through the emissions modeling we had completed for this vessel and for other vessels. I see some of the calculations but not all of them. I primarily wanted to understand how the concentration of a solvent in the head space of a vessel was estimated.

Despite my original searches prior to posting, I have since found a very similar question to mine on the boards from 2005.


RE: Organic Vapor Concentration In Head Space of Tank

Close to the liquid level, would agree that in the transient case, you'd have a higher concentration, but in a short while, depending on molecular diffusion characteristics of CS2 in N2, this should even out to be the mole fraction calculated throughout.

One way to check if CS2 apporaches ideal behaviour in this operating range is to get the values for compressibility factor - I'd say if it is less than 0.9 in this range, you might want to make the effort to obtain the partial molar fagacity factor.

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